Cromatografía iónica - Metrohm

Ion Chromatography

Dr. Helwig SchäferDr. Markus LäubliDr. Roland Dörig

Metrohm Ltd. CH-9101 HerisauSwitzerland

Helwig Schäfer – Markus Läubli – Roland Dörig

Ion Chromatography

Theory

Columns and Eluents

Metrohm Monograph 8.014.5003

8.732.2003

All Rights Reserved.Printed in Switzerland by Metrohm AG, CH-9101 Herisau – 11.96

Table of Contents

8.732.2003 Monograph «Ion chromatography» I

Table of Contents1 Theory of Ion Chromatography............................................. 1

1.1 Introduction ......................................................................................... 11.2 Ion exchange as a separation mechanism ..................................... 21.3 Chromatographic characteristics .................................................... 4

1.3.1 Retention time and peak width ................................................. 41.3.2 Capacity factor k' ..................................................................... 51.3.3 Selectivity α .............................................................................. 51.3.4 Plate number N ........................................................................ 51.3.5 Resolution R ............................................................................. 61.3.6 Asymmetry factor T .................................................................. 7

1.4 Ion chromatographic separations.................................................... 81.4.1 Dependence of the separation on the column material ........... 81.4.2 Influence of the eluent composition on the separation ............ 91.4.3 Interferences in the chromatogram: system peak.................. 11

1.5 Conductivity detection..................................................................... 121.5.1 General................................................................................... 121.5.2 Direct ion chromatography..................................................... 141.5.3 Ion chromatography with chemical suppression.................... 151.5.4 Comparison in the sensitivities of conductivity

detection with and without chemical suppression.................. 151.5.5 Linearity of the calibration curve............................................. 17

1.6 Optical detection .............................................................................. 181.6.1 General................................................................................... 181.6.2 Direct UV-VIS detection .......................................................... 181.6.3 Indirect UV-VIS detection........................................................ 191.6.4 UV-VIS detection with post-column derivation ....................... 201.6.5 Fluorescence detection.......................................................... 201.6.6 UV-VIS multiwavelength detection.......................................... 20

1.7 Electrochemical detection............................................................... 211.8 Multiple detection ............................................................................. 231.9 Suppressors ...................................................................................... 24

1.8.1 Suppressor reactions ............................................................. 241.8.2 Construction of suppressors .................................................. 25

1.10 Evaluation and calibration .............................................................. 261.10.1 Evaluation of peak area.......................................................... 261.10.2 Evaluation of peak height ....................................................... 261.10.3 Calibration with external standard .......................................... 261.10.4 Calibration with internal standard........................................... 271.10.5 Calibration by standard addition ............................................ 29

1.11 References......................................................................................... 29

Table of Contents

8.732.2003 Monograph «Ion chromatography»II

2 Columns and eluents..................................................................312.1 General hints..................................................................................... 31

2.1.1 Eluents ....................................................................................312.1.2 Protection of separating columns ...........................................312.1.3 Regeneration of columns ........................................................32

2.2 IC Anion Column Hamilton PRP-X100 (6.1005.000)..................... 332.2.1 Column specifications.............................................................332.2.2 General remarks......................................................................332.2.3 Phthalic acid eluent .................................................................34

2.3 IC Anion Column SUPER-SEP (6.1009.000).................................. 352.3.1 Column specifications.............................................................352.3.2 General remarks......................................................................352.3.3 Phthalic acid eluent .................................................................362.3.4 Benzoic acid eluent.................................................................37

2.4 IC Glass Cartridge Metrosep Anion Dual 1 (6.1006.020)............. 382.4.1 Column specifications.............................................................382.4.2 General remarks......................................................................382.4.3 Phthalic acid eluent (without chemical suppression) ..............392.4.4 HCO3

–/CO32– eluent (with chemical suppression)....................40

2.5 IC Anion Column Metrosep Anion Dual 2 (6.1006.100) ............... 412.5.1 Column specifications.............................................................412.5.2 General remarks......................................................................412.5.3 Phthalic acid eluent (without chemical suppression) ..............422.5.4 HCO3


2.6 IC Anion Column Phenomenex STAR ION A300 (6.1005.100) ... 442.6.1 Column specifications.............................................................442.6.2 General remarks......................................................................442.6.3 HCO3


2.7 IC Exclusion Column Hamilton PRP-X300 (6.1005.030).............. 462.7.1 Column specifications.............................................................462.7.2 General remarks......................................................................462.7.3 HClO4 eluent............................................................................47

2.8 IC Cation Column Nucleosil 5SA (6.1007.000) ............................. 482.8.1 Column specifications.............................................................482.8.2 General remarks......................................................................482.8.3 Tartaric acid/citric acid eluent .................................................49

2.9 IC Cation Column Metrosep Cation 1-2 (6.1010.000)................... 502.9.1 Column specifications.............................................................502.9.2 General remarks......................................................................502.9.3 Tartaric acid/dipicolinic acid eluent.........................................51

Index................................................................................................... 53

1.1 Introduction

8.732.2003 Monograph «Ion Chromatography» 1

1 Theory ofIon Chromatography

1.1 Introduction

The term "chromatography" is the general name for a wide range of phys-icochemical separation processes in which the components to be separatedare distributed between a stationary and a mobile phase. The classificationof the various types of chromatography depends on the state of aggregationof these two phases:

Mobil phase Stationary phase

liquid solid

gaseous GLC GSC gas chromatography

liquid LLC LSC (HPLC) liquid chromatography

Since the introduction of high pressure or high performance liquid chroma-tography (HPLC) at the end of the sixties, liquid chromatography has devel-oped into one of the most comprehensive and important methods of mod-ern instrumental analysis. Based on the polarity of the stationary and mobilephases, a distinction is made between the following methods:

1

4

2 3

1 Normal phasechromatography

2 Reversed phasechromatography

3 Ion pair chromatography

4 Ion chromatography

ionicpolarnon-polar

non-polar

polar

ionic

Polarity of themobile phase

Polarity of thestationary phase

1 IC Theory

8.732.2003 Monograph «Ion Chromatography»2

Ion chromatography was first introduced in 1975 and within a short space oftime has developed into an independent analytical technique which todayencompasses all HPLC methods to determine inorganic or organic ions. Thecombination of ion exchange columns and conductivity detection continuesto represent the most important type of ion chromatography, and two differ-ent techniques are used in practice.

In the technique with chemical suppression, the background conductivity issuppressed both chemically and electronically. In contrast, the direct chro-matographic technique (ion chromatography with electronic backgroundsuppression) employs eluents with salts of organic acids in low concentra-tion on ion exchangers of very low capacity to achieve a relatively low back-ground conductivity, which can be suppressed directly by electronic means.The following sections deal with both techniques which have also been im-plemented in the 732/733 Metrohm IC system.

1.2 Ion exchange as a separation mechanism

The vast majority of ion chromatographic separations occur by ion ex-change on stationary phases with charged functional groups. The corre-sponding counter ions are located in the vicinity of the functional groups andcan be exchanged with other ions of the same charge in the mobile phase.For every ion, the exchange process is characterized by a correspondingion exchange equilibrium, which determines the distribution between themobile and stationary phase, for example in the case of an anion A

–:

Estat Amob Astat Emob− + − ↔ − + −

KA

A stat E mob

A mob E stat

=

− × −

− × −

[ ] [ ]

[ ] [ ]

The various ionic components of a sample can thus be separated on thebasis of their different affinities for the stationary phase of the ion exchanger(different equilibrium constants K). The most important group of ion ex-changers is that of the organic materials based on synthetic resins. A co-polymer of styrene and divinylbenzene is frequently employed as the sup-port.

A: sample anionE: eluent anion (counter ion)

1.2 Ion exchange as a separation mechanism


+

(

(

)

)

Cation exchangers are obtained by subsequent sulphonation of this styrene-divinylbenzene resin, anion exchangers by chloromethylation followed byamination.

( )

SO3-

( )

NR3+

In addition to these resins, other polymers and silica gels with chemicallybound phases are also employed. Whereas the classical ion exchange isperformed on macroporous particles of a high exchange capacity with asize of 75 - 250 µm in the batch or column process, in modern ion chroma-tography, ion exchange materials of low capacity with particle sizes of 5 - 10µm are employed. This makes it possible to separate and detect anions andcations efficiently and rapidly with eluents of low concentration.

Styrene-divinylbenzene resin

DivinylbenzeneStyrene

Anion exchangerCation exchanger

1 IC Theory


1.3 Chromatographic characteristics

1.3.1 Retention time and peak width

The elution curve (signal vs time) following a chromatographic separation iscalled a chromatogram. It generally has the following appearance:

t0

tR,1

tR,2

t'R,1

t'R,2

w0.5,1

wb,1 wb,2

w0.5,2

2 σt,1

2 σt,2

The time parameters of the resulting bands (peaks) characterize the chro-matogram and are illustrated in the Figure:

t0 Dead time time needed by the mobile phase to flowthrough the separation system

tR Retention time time needed by an injected substanceuntil its concentration maximum appearsat the end of the separation system

t'R Net retention time = retention time tR – dead time t0

σt Standard deviation half peak width at the inflection points

w0.5 Peak width at half height = 2.354 σt

wb Peak base width = 4 σt

The time parameters t0, tR and t'R can be converted into the dead volume V0,retention volume VR and net retention volume V'R using the constant flowrate. If symmetric elution profiles are found, the shape of the chroma-tographic peaks can be described with sufficient accuracy by a Gaussiancurve. The width of such a Gaussian-shaped peak is determined from thechromatogram as standard deviation σt, width at half height w0.5 or basewidth wb.

Time

Signal

Injection peak

Component 2Component 1



1.3.2 Capacity factor k'

The retention time tR is the qualitative information of a chromatogram. It isconstant for a given component provided the chromatographic conditionsremain unchanged (column, mobile phase, temperature, etc.). For the char-acterization of a substance, it is more convenient to quote the capacity fac-tor k' since, in contrast to the retention times, this is dependent neither onthe flow of the eluent nor on the column length:

kt

t

t t

t

t

tR R R' = =

−= −

'

0

0

0 01

Low values of k' signify that the corresponding ions are eluted in the vicinityof the injection peak and the separation is consequently very poor. Capacityfactors between 1 and 5 are optimum in practice; larger k' values lead onlyto peak broadening, lower detection sensitivity and long analysis times.

1.3.3 Selectivity αα

Two substances are separated only if they have different k' values. A meas-ure of the separation efficiency of a chromatographic system is the selec-tivity αα (also known as relative retention):

( )α = =−

−>

k

k

t t

t tk kR

R

2

1

2 0

1 02 1

'

',

,

' '

1.3.4 Plate number N

An additional useful quantity to characterize a separation system is the platenumber N (number of theoretical plates). A theoretical plate is defined asthat zone of a separation system within which a thermodynamic equilibriumis established between the mean concentration of a component in the sta-tionary phase and its mean concentration in the mobile phase. If a Gaussianpeak shape can be assumed, the plate number N for peaks with a relativelylarge retention time can be calculated as follows with the aid of the pa-rameters retention time and peak width read off from the chromatogram:

Nt t

w

t

wR

t

R R

b=

=

=

σ

2

0 5

2 2

554 16..

1 IC Theory


1.3.5 Resolution R

A measure of the quality of the separation actually found in practice is theresolution R between neighboring peaks:

( ) ( )R

t t

w w

t t

w w

R R

b b

R R=

−

+=

−

+

2 11772 1

1 2

2 1

0 5 1 0 5 2

, ,

, ,

, ,

. , . ,

.

If the peak base widths wb,1 and wb,2 are approximately the same, the reso-lution R signifies the number of times the peak width wb can be fitted into thedistance between the peak maxima. At a resolution of R = 0.5, two maximacan still be perceived separately. For quantitative analysis, a resolution of upto R = 1.5 is desirable; greater values of the resolution lead only to unnec-essarily long analysis times.

The resolution R is dependent on the parameters k2' (capacity factor of thelater eluted substance), selectivity α and plate number N of the column:

RN k

k= ×

−×

+4

1

12

2

αα

'

'

There are thus three possibilities to improve the resolution of two peaks:

• • Altering the k' values:The capacity factors of the individual components are depend-ent essentially on the elution strength of the mobile phase, i.e.changing the concentration and ionic strength of the eluentgives rise to other k' values. Here, however, the capacity of thecolumn and frequently also the detector are limiting factors.

• • Increasing the plate number N:The more plates a column possesses, the greater the resolu-tion, but the retention times are also longer thereby increasingthe analysis time. The plate number increases with increasingefficiency and length of the column. With packing materials ofparticle size 10 µm or greater, the plate number increases withdecreasing flow.

• • Increasing the selectivity α:The most effective possibility to improve the resolution involvesincreasing the relative retention α by using a different columnbetter suited to the separation problem or by changing thecomposition of the eluent. However, this is sometimes extremelydifficult or very time intensive.

If a given column and one particular eluent have to be used, the plate num-ber N is the controlling influence with regard to the resolution and hence thequality of the chromatogram. For a given column length, the plate number Ndepends only on the quality of the packing material and the packing itself.



1.3.6 Asymmetry factor T

The elution of chromatographic signals as Gaussian peaks is often notachieved in practice. An asymmetric peak shape, known as tailing, is oftenfound.

h

cba

c = 0.1 h

The peak asymmetry is quantified by the asymmetry factor (tailing factor)T with a and b being determined at 10% peak height:

Tb

a=

For trouble-free evaluation of the area of a peak, T must be < 2.5, abovethis, the end of the peak can be recognized only with difficulty. Tailing canhave many causes:

• Dead volumeDead volumes between injector and detector lead to peakbroadening and tailing. The asymmetry is more pronouncedwith peaks eluted earlier than with those eluted later, and thetailing increases with the flow.

• Column overloadingIf too much substance has been injected, i.e. the maximumloading of the column has been exceeded, broadened peakswith severe tailing are obtained. Overloading is recognized bya lowering of the capacity factor by more than 10%.

• Chemical effectsThe dominant separation mechanism is adversely influencedby another mechanism, e.g. adsorption phenomena in ion ex-change chromatography. The lower the flow, the more evidentthis tailing.

1 IC Theory


1.4 Ion chromatographic separations

The chromatographic parameters retention time, peak shape, dead time andselectivity are the results of the whole chromatographic system, i.e. altera-tions to the column, the eluent or the flow rate inevitably lead to alterations ofthe chromatographic parameters. This is why test chromatograms are nor-mally included with the separation columns to demonstrate that the separa-tion behavior described can be achieved under exactly defined conditions.

The following sections contain several useful items of information about theoptimization of the column, eluent and flow rate parameters with reference toanalytical problems which may be encountered.

1.4.1 Dependence of the separation on the column material

The separatory behavior of an ion exchange column is basically determinedby the ion exchanger groups bound to the support material (under otherwiseidentical chromatographic conditions). However, interactions of the analyteswith the support material also occur: these are considerably stronger for or-ganic ions than for inorganic ions. For this reason it is a good idea to usecolumns with different support materials to solve an analytical problem.

The following are the main support materials used today for separatory col-umns:

• polystyrene/divinylbenzene copolymers

• polymethacrylate, polyhydroxy alkylmethacrylate

• coated silica gels

The use of suitable combinations of support materials and ion exchangergroups allows optimal conditions to be provided for special separationproblems. Even interferences such as those caused by the eluent or, for ex-ample, by the carbonate content in water samples can be avoided by theselection of a suitable column as is shown in the following illustrations onpage 9. Whereas with the first column the carbonate peak and the chloridepeak overlap, on column 2 a separation of carbonate and chloride is ob-tained. This differing behavior can be traced to the column support material(column 1: polystyrene/divinylbenzene copolymer; column 2: hydroxyethylmethacrylate).

These effects are far stronger in the separation of organic anions. In suchcases it is also a good idea to test separatory columns with different combi-nations of support materials and ion exchanger groups.



1.4.2 Influence of the eluent composition on the separation

During the separation of ions by means of ion exchanger columns smallions are eluted before larger ions and monovalent ions before di- and triva-lent ions; i.e. for anions the following sequence is obtained: fluoride, chlo-ride, nitrite, bromide, nitrate, phosphate, sulfate, arsenate, selenate, etc.When carbonate eluents are used (pH 9…10; mixture of Na2CO3 and Na-HCO3) the ions are eluted in the same sequence. If the pH is shifted towardshigher values then dissociation of phosphate and arsenate occurs: HPO4

2–

to PO43– and HAsO4

2– to AsO43–. There is thus an increase in the retention

time which means that by increasing the Na2CO3 share of the eluent bothphosphate and arsenate can be shifted in ion chromatograms so that, forexample, phosphate is only eluted when sulfate has already been eluted.

Cl–

CO32–

F–

SO42–

NO3–

Cl–

SO42–

CO32–F–

NO3–

Sample: Drinking waterColumn: STAR ION A300

(Polystyrene/Divinyl-benzene Copolymer)

Chloride found: 6.7 mg/L

1 µS/cm

Sample: Drinking waterColumn: Metrosep Anion Dual 1

(Hydroxyethyl meth-acrylate)

Chloride found: 5.7 mg/L

1 µS/cm

1 IC Theory


In principle retention times can be shifted by the addition of ligands to theeluent for the determination of cations. The more selective the ligand, theless the retention times of the other cations are influenced. This effect isused in particular for the separation of the complex-forming metals, the tran-sition metals.

However, ligands can also be used to obtain better separation of alkalimetal ions. The addition of crown ether, for example, leads to a betterseparation of sodium, ammonium and potassium, as can be seen in thefollowing illustrations.

1 µS/cm

Ca2+

Li+

Na+

NH4+

K+

Mg2+

Column: Metrosep Cation 1-2Eluent: 4 mmol/L Tartaric acid

1 mmol/L Dipicolinic acid

Ca2+Li+

Na+

NH4+

K+

Mg2+

Column: Metrosep Cation 1-2Eluent: 4 mmol/L Tartaric acid

1 mmol/L Dipicolinic acid1 mmol/L 18 Crown-6

1 µS/cm



1.4.3 Interferences in the chromatogram: system peak

Ionic eluents are normally used in ion chromatography (see section 1.2);these compete with the analyte ions at the ion exchanger. For these eluentanions, e.g. carbonate, phthalate or benzoate, the same chromatographicconditions apply as for all other ions, i.e. there is also a defined retentiontime for the eluent anions in each chromatographic system.

When the sample is injected into the chromatographic system the eluentflow is interrupted by the sample (water with low analyte contents), i.e. therewill be a negative amount of eluent ions compared with the eluent concen-tration injected. This must lead to a negative peak in the ion chromatogramwith the retention time of the eluent anion. This peak must not be confusedwith the water peak, which is obtained at the start of the chromatogram(dead time).

Such peaks which can be allocated to the eluent anions are actually ob-tained; these are generally known as system peaks. These system peaksnot only appear when the eluent is diluted, but also for each chemical reac-tion of the eluent ions with the sample. Examples of this are alterations to thepH and thus the shift in the dissociation equilibrium or reactions of metalions with the eluent ion (e.g. Ca2+ with CO3

2–).

The system peaks are most apparent in direct ion chromatography as heremeasurements are made at a high conductivity level and small differences inthe ionic strength of the eluent are extremely noticeable.

In contrast, in ion chromatography with chemical suppression measure-ments are carried out at a low conductivity level and the system peak is thusconsiderably smaller. Previously the carbonate system peak has been delib-erately ignored as it lies in the area of the chloride peak on the polysty-rene/divinylbenzene columns which were normally used and almost all sam-ples contain sufficient chloride to obscure the carbonate peak. Only sinceacrylate columns have been used has it been clearly seen that the carbon-ate system peak is present and that this presence can lead to false results(see Fig. page 9).

In particular, in the analysis of natural waters with varying carbonate con-tents the total of carbonate + chloride has usually been determined aschloride.

1 IC Theory


1.5 Conductivity detection

1.5.1 General

As a universal method to determine ionic components, conductivity meas-urement (conductometry) occupies a central position in ion chromatogra-phy. Conductometry is defined as the ability of electrolyte solutions in anelectric field applied between two electrodes to transport current by ion mi-gration. The relationship between the applied voltage U and the current I isgiven by Ohm's law:

RU

I=

U: voltage [ V ]I: current [ A ]

The reciprocal of the ohmic resistance R is the conductance G, which hasthe unit Siemens:

GR

= 1[ S ]

The conductance of an electrolyte solution depends on the electrode sur-face A and the interelectrode distance l. The usual measured variable inconductometry is thus the conductivity κκ:

κ = ×G Kc [ S cm–1 ]

KclA

= [ cm–1 ]

The quotient l/A is known as the cell constant Kc, this can usually not becalculated directly but is determined with calibration solutions. The depend-ence of the electrical conductivity κ on the type and concentration of thedissolved components can be described as follows:

κ =×Λ c eq( )1000

Λ: equivalent conductivity [ S cm2 mol–1 ]

c (eq): equivalent amount-of-substance concentration[ mol/1000 cm3 ]; c(eq) = c × z

c: amount-of-substance concentration [ mol/1000 cm3 ]

z: valency

The conductivity thus increases with increasing electrolyte concentration.This linear relation holds only for dilute solutions, however, since theequivalent conductivity Λ is itself dependent on concentration in accordancewith Kohlrausch's law:



κ = −∞Λ A c eq( )

Λ∞: equivalent conductivity in an infinitely dilute solutionA: constant

The equivalent conductivity Λ∞ is given by the sum of the ionic conductivi-ties Λ∞

+ and Λ∞

– :

Λ Λ Λ∞ ∞+

∞−= −

The ionic conductivities of anions and cations are usually between 35 and80 S cm2 mol–1, the only exceptions being H+ and OH– ions on account oftheir very high mobilities of 350 and 198 S cm2 mol–1, respectively. With theaid of the tabulated ionic conductivities, the conductivity of a pure solutionfor dilute, aqueous electrolyte solutions can be calculated in advance withconsiderable accuracy.

In addition to the ionic species and ionic concentration, the temperature andpolarity of the solvent also influence the electrical conductivity. The tem-perature dependence of 2 – 10 %/°C is very pronounced.

Equivalent ionic conductivities for infinite dilutionin aqueous solutions at 25°C

Anions ΛΛ∞∞– [ S cm2 mol-1 ] Cations ΛΛ∞∞

+ [ S cm2 mol-1 ]

OH– 198 H+ 350

F– 54 Li+ 39

Cl– 76 Na+ 50

Br– 78 K+ 74

I– 77 NH4+ 73

NO2– 72 ½ Mg2+ 53

NO3– 71 ½ Ca2+ 60

HCO3– 45 ½ Sr2+ 59

½ CO32– 72 ½ Ba2+ 64

H2PO4– 33 ½ Zn2+ 53

½ HPO42– 57 ½ Hg2+ 53

1/3 PO43– 69 ½ Cu2+ 55

½ SO42– 80 ½ Pb2+ 71

SCN– 66 ½ Co2+ 53

Acetate 41 1/3 Fe3+ 70

½ Phthalate 38 N(Et)4+ 33

Propionate 36

Benzoate 32

Salicylate 30

1 IC Theory


1.5.2 Direct ion chromatography

In ion chromatography without chemical suppression (direct ion chroma-tography), very high demands are made on the conductivity detector sincethe background conductivity which must be compensated is relatively largein comparison with the measuring signal. Of prime importance for the elec-tronic suppression is the constancy of the background. Since the conductiv-ity is, as mentioned above, greatly dependent on the temperature, an ex-tremely good temperature constancy of the eluent in the conductivitymeasuring cell is required (≤ 0.01°C).

The sensitivity of the conductivity measurement depends on the differencebetween the equivalent ionic conductivities of sample ion (a) and eluent ion(e):

( )∆ Λ Λκ ∝ × −c eq a e( )

Λa > Λe: positive peak

Λa = Λe : no peak

Λa < Λe: negative peak

In anion determinations, salts of phthalic, salicylic or benzoic acid are usu-ally employed since these possess a low equivalent ionic conductivity (seeTable on page 13). If now an anion with a higher equivalent ionic conductiv-ity appears in the detector cell, the conductivity increases and a positivepeak is obtained. On the other hand, negative peaks appear if the equivalentionic conductivity of the sample ion is lower than that of the eluent ion.

An example of this is phosphate that, for example, in 2 mmol/L phthalic acidpH = 5.0 is present as H2PO4

–. Since Λ∞–

(H2PO4–) = 33 is lower than Λ∞

–

(phthalate) = 38, an insensitive negative peak is obtained under these con-ditions (corrective measure: higher pH since Λ∞

– (HPO4

2–) = 57). The sensi-tivity ∆κ for anions is in general 0.1 – 0.5 µS/cm per 1 mg/L (= 1 ppm).

With alkali and alkaline earth cations, chromatography is generally per-formed with dilute acids such as 2 mmol/L HNO3 as eluent. Since the protonhas an exceptionally high equivalent ionic conductivity as a result of its spe-cial migration mechanism, the conductivity sinks drastically as soon as othercations replace the H+ ions. Generally, negative peaks are obtained whichare very sensitive owing to the high ∆Λ value (in general 1 – 10 µS/cm per 1mg/L).

The lower limit of the sensitivity depends on the detector noise, which liesbetween 0.1 – 10 nS/cm. This noise is determined essentially by the qualityof the high pressure pump and the background conductivity. If the detectionlimit is defined as the signal height at a signal/noise ratio S/N = 3, it has thevalue 2 – 50 µg/L for anions and 1 – 10 µg/L for cations.



1.5.3 Ion chromatography with chemical suppression

Chemical suppression is based on the use of the salts of weakly dissociat-ing acids (e.g. NaHCO3) as eluents. These eluents can be eliminated to alarge extent by cation exchange in a post-column reaction according to thefollowing equation.

Na HCO H CONa

H+ −+ →

− +

+ +

3 2 3

The carbonic acid formed as a result of cation exchange is very weakly dis-sociated and thus exhibits a very low residual conductivity. The sample ani-ons undergo a corresponding reaction, shown here with the chloride anionas an example:

Na Cl HClNa

H+ −+ →

− +

+ +

The NaCl is converted to the corresponding free acid by the suppression;this has a considerably higher conductivity than the original salt. Of coursethis only applies for stronger acids; for medium-strong and weak acids theincrease in sensitivity is correspondingly lower. The signal to be measured isthus the sum of Λ∞

– (Cl–) and Λ∞

+ (H+) against a low background conductivity.

For reasons which up to now are not completely clear the calibration func-tion of this system is not linear with the concentration. This means that forcalibration a considerably higher expenditure is required.

1.5.4 Comparison in the sensitivities of conductivitydetection with and without chemical suppression

For anions the relationships mentioned above can be used to determine thefollowing sensitivities for conductivity detection with electronic and chemicalsuppression:

AnionSensitivity with electronicbackground suppression

( )Λ Λsample phthalate− −−

Sensitivity with chemicalsuppression

( )Λ Λsample H− ++

Fluoride 16 404

Chloride 38 426

Nitrate 33 421

½ Sulfate 42 430

Acetate 3 391

1 IC Theory


According to these calculations the sensitivity of conductivity detection withchemical suppression should be higher for anions by a factor of 10 or more.However, when measurements made with the same instrument configurationare compared directly then chemical suppression normally only producessensitivities which are higher by a factor of 2 to 4, as can be seen from thefollowing table.

AnionDetection limit with electronic

background suppressionµµg/L

Detection limit with chemicalsuppression

µµg/L

Chloride 5 2

Nitrite 11 4

Bromide 16 4

Nitrate 16 4

Sulfate 7 4

For cations the sensitivity with direct conductivity detection is already higherthan with chemical suppression. The hydroxides of ammonium and thedoubly charged cations also show a lower dissociation. Thus chemical sup-pression has no advantages for cation chromatography.

CationSensitivity with electronicbackground suppression

( )Λ Λsample H+ +−

Sensitivity with chemicalsuppression

( )Λ Λsample OH+ −+

Lithium – 311 237

Sodium – 300 248

Potassium – 276 272

Ammonium – 277 271

½ Magnesium – 397 251

½ Calcium – 290 258



1.5.5 Linearity of the calibration curve

An important parameter for analytical determination methods is the linearityof the calibration. If chemical suppression is used slightly curved calibrationcurves are usually obtained. In such cases the calibration values must belocated as close as possible to the contents of the sample in order to obtainthe most accurate results. In contrast, calibration curves obtained withoutchemical suppression remain linear for several decimal powers. An exampleof the differing linearity is shown in the following illustration.

Calibration curves for nitrate with and without chemical suppression

0

25

50

75

100

0 5 10 15 20ppm

area

%

non suppressed

suppressed (MSM)

Apart from this curve, which only shows a slight variation from linearity, thelinear working ranges with and without the use of a suppressor differ con-siderably:

Anion Linear working range in µµg/L

without suppressor with suppressor

Chloride 5 … 25’000 5 … 500

Nitrate 15 … 10’000 5 … 500

Sulfate 20 … 10’000 5 … 200

1 IC Theory


1.6 Optical detection

1.6.1 General

In ion chromatography four different types of optical detection have beenused up to now:

• Direct UV-VIS detectionDirect UV-VIS detection is used as an addition to conductivitydetection, e.g. for the determination of ions which absorbstrongly in the UV range (nitrite, nitrate, organic anions) in thepresence of high concentrations of inorganic ions (chloride,phosphate, sulfate), which either have no UV absorption or verylittle.

• Indirect UV-VIS detectionIndirect UV-VIS detection finds universal application in direct ionchromatography for use with eluents with high UV absorption(e.g. phthalate buffers). Ions with no or very low UV activity shownegative peaks and ions with a larger UV activity than the eluentshow positive peaks.

• UV-VIS detection with post-column derivationUV-VIS detection is partly used coupled with post-column deri-vation for the detection of transitional metals (iron, nickel, cop-per, manganese, zinc...).

• Fluorescence detectionIn some special applications fluorescence detection is used forthe highly sensitive detection of fluorescent compounds.

1.6.2 Direct UV-VIS detection

In direct UV-VIS detection the advantage of having different chromophoregroups and thus very different detection sensitivities for the ions is used. Themain area of application lies in the UV range, in which almost all organicions absorb. The more chromophore groups (NO2, NO3, double bonds,phenyl, ketone, ...) which a molecule contains, the better the detection sen-sitivity.

The maximum absorption of the most important functional groups is as fol-lows:

1.6 Optical detection


Functional group λλmax. in nm

Amino 195

Aldehyde 210

Carboxyl 200

Ester 205

Ethylene 190

Ketone 195

Nitro 310

Phenyl 200

The very different detection sensitivity of the inorganic ions can be used withadvantage for matrix reduction, particularly if the analytical samples have ahigh salt content. Chloride, sulfate and phosphate exhibit only very low UVabsorption so that, for example, very low nitrite concentrations can be de-termined in the presence of large amounts of chloride.

In addition the use of a UV detector allows the determination of organic ac-ids which coeluate with sulfate as in this case only the organic acids, andnot however the sulfate, are detected.

1.6.3 Indirect UV-VIS detection

In indirect UV-VIS detection an eluent with as high an absorption as possibleis used so that the analytes reduce the detector signal. As already shown insection 1.5.2, the eluent concentration in the region of a peak is reduced bythe amount of the analyte concentration. This means that at the same UVabsorption of analyte and eluent no detector signal will be received whilelower analyte absorption will result in a negative signal and higher analyteabsorption a positive signal.

Typical eluents for indirect UV-VIS detection are phthalate, benzoate, sali-cylate, various sulfonates and many others. The following table gives theoptimum wavelengths and some detection limits for this type of detection:

Eluent λλmax. in nm Detection limit(mg/L)

Phthalate 300 0.1 … 1

Benzoate 254 0.1 … 1

Citrate 290 0.2 … 2

1 IC Theory


1.6.4 UV-VIS detection with post-column derivation

Post-column derivation should only be used if no other possibility exists; inprinciple all chemical or mechanical manipulations of any sort should beavoided after the separation as such reactions always result in a deteriora-tion in the separation efficiency and in particular of the peak shape.

One of the main uses of post-column derivation is the determination of tran-sition metals in ion chromatography. If direct conductivity detection is nolonger possible (electrical baseline compensation too low, sample with highsalt content) then a photochemical reagent and auxiliary solutions areadded to the eluent stream after the separation column. After passingthrough a reaction spiral the optical absorption of the metal complex ismeasured.

Because of the different locations of the maximum absorption and the inten-sity of the color complexes of the individual metals the detection limits arevery variable. However in some cases any matrix problems present can bereduced by selection of a suitable wavelength.

1.6.5 Fluorescence detection

Direct fluorescence detection has virtually no applications in ion chroma-tography as the analytes are inactive with the exception of a few metal com-plexes with fluorescent ligands.

On the other hand there are several eluents used in IC which are suitable foruse in indirect fluorescence detection (e.g. salicylate).

1.6.6 UV-VIS multiwavelength detection

The use of multiwavelength detectors such as diode arrays or "fast scan-ning" detectors requires a considerably larger expenditure for the necessarycalculations (PC programs) but produces far better interpretations for prob-lematical analyses.

In particular for samples with peaks whose allocation is unknown certainstatements about the structure of the ions under investigation can be madefrom the UV or VIS spectra obtained.

1.7 Electrochemical detection


1.7 Electrochemical detectionElectrochemical detection has occasionally been used with a lot of success.A pre-requirement is that the ions to be determined must be able to be eitherreduced or oxidized. This includes many organic compounds, the transi-tional metals and anions such as nitrite, nitrate, halides, oxohalides, pseu-dohalides, sulfide, sulfite, etc.

There are four different techniques:

Amperometry Measurement of the current at a constantpotential

Coulometry Measurement of the current at a constantpotential but with 100% conversion of theanalyte

Voltammetry Measurement of the current against potential ina defined range of potential

Pulsed amperometry Measurement of the current at constantpotential pulses

Amperometric (including pulsed) and coulometric detectors are most fre-quently used. The cells normally contain three electrodes:

• the working electrode, where the electrochemical reaction occurs,

• the reference electrode for currentless potential measurement and

• the auxiliary electrode as opposite pole to the working electrode.

There are two different types of cell, the thin-layer cell and the wall-jet cell,which are shown schematically in the following illustration.

11 Working electrode (WE)22 Reference electrode (RE)33 Auxiliary electrode (AE)

Thin-layer cell Wall-jet cell

1 IC Theory


The three electrodes are located in very different positions in various typesof thin-layer cell. In the sequence in the direction of eluent flow all possiblecombinations can be found, e.g. WE first, then RE and finally AE or RE first,then WE followed by AE. The arrangement with WE and AE opposite eachother followed by RE seems to be the best.

In the wall-jet cell the WE is always the first electrode with the flow directedvertically onto it. The RE and AE follow in different sequences.

The wall-jet cells are to be preferred to the thin-layer cells because of theirgreater sensitivity; however, after the WE there is a stronger mixing effectand turbulence in the eluent which is a disadvantage if a second detectorhas to be included.

For the working electrodes mainly carbon (graphite or glassy carbon), gold,platinum, silver and very occasionally mercury (voltammetric detector cells)are used as electrode materials. When selecting the electrode material itmust be remembered that every material is also electrochemically reducedand oxidized, i.e. each material provides a different potential window. Theseapplication ranges depend to a great extent on pH, solvent, type of ion andionic concentration of the electrolyte. In the negative potential range themeasuring range is limited by the electrolytic production of hydrogen fromthe protons of the solution and in the positive potential range by the electro-chemical dissolution of the electrode material. The dissolution potential ofthe electrode depends very strongly on the anion of the electrolyte and oncomplex formers in particular. Approximate ranges for the various materialsare given below:

Electrode material Potential range( V vs SCE)

1 mol/L NaOH 1 mol/L HClO4

Glassy carbon –1.4 … 1.0 –0.9 … 1.4

Platinum –1.2 … 0.5 –0.3 … 1.3

Gold –1.2 … 0.8 –0.4 … 1.0

Silver –1.2 … 0.2 –0.5 … 0.4

Mercury –2.6 … 0.0 –1.1 … 0.4

Voltammetric detector cells are usually constructed by the users themselvesor voltammetric flow-through cells are integrated in the existing IC system.

1.8 Multiple detection


1.8 Multiple detection

The use of several detectors during a chromatographic analysis may be ap-propriate for complicated analytical problems. For example, it is a good ideato use a UV detector or ELCD together with the conductivity detector for thedetermination of very small amounts of nitrite in the presence of largeamounts of chloride, as can be seen from the following example of its de-termination in surface water.

For the safe identification of organic anions (e.g. hydroxy, di or tricarboxylicacids) in the presence of inorganic anions the coupling together of a con-ductivity detector with a UV or even a diode array detector is in many casesimportant.

NO2–: 3.6 µg/L

ELCD detection

Cl–: 5528 µg/L

F–

SO42–

NO3–

Conductivity detection

1 IC Theory


1.9 Suppressors

All suppressors have basically the task of improving the detection of theanalyte. In this respect a differentiation must be made between two effectsor criteria in the suppression reaction:

• The high conductivity of the eluent is to be reduced toas low a level as possible.

• In addition the various counter ions of the analyte ionsshould be converted to a single ionic species with ahigher equivalent conductivity.

1.9.1 Suppressor reactions

The basic reaction of such a suppressor in the anionic range has alreadybeen described in section 1.5.3. In the anion analysis sector when the nor-mal sodium carbonate/hydrogen carbonate electrolytes are used the sup-pressor exchanges the sodium ions against protons and this carbonate ionleads to the formation of poorly dissociated carbonic acid. This reactionleads to a reduction in the conductivity of the electrolyte (first criterion).

Na HCO H CONa

H+ −+ →

− +

+ +

3 2 3

In addition all cations in the sample are converted to protons. As the protonhas the largest equivalent conductivity of all cations this reaction provides abetter detection sensitivity (second criterion).

Na Cl HClNa

H+ −+ →

− +

+ +

Careful attention must be given to both criteria when assessing whether theuse of a suppressor makes sense. Such considerations lead to the resultthat in the anionic range the use of suppressors is indicated if carbonateeluents can be used.

On the other hand only the first criterion is fulfilled for the use of suppressorsin cationic analysis (reduction of the eluent conductivity), but not the sec-ond. Then in this case, for example, the suppressor reactions are as followsif nitric acid is used as the eluent and sodium chloride is present as analyte:

Eluent: H NO H ONO

OH+ −+ →

− −

+ −

3 2

3

Analyte: Na Cl Na OHCl

OH+ − + −+ +→

− −

+ −

1.9 Suppressors


It follows that in direct ion chromatography the analyte ion Na+ is measuredagainst H+, i.e. a negative peak will be obtained (difference κ{Na+} –κ{H+} = –300 µS). In contrast in the suppressor reaction the sodium ion is

measured against water and all analyte anions will be converted to hydrox-ide ions (sum of κ{Na+} + κ{OH–} = 248 µS). From these figures it can beseen that normally the use of suppressors is only required when the detectorof the IC system does not permit accurate measurements at higher basicconductivities.

1.9.2 Construction of suppressors

In the initial phases of ion chromatography ion exchanger columns wereused as suppressors. After several hours of operation the columns were ex-hausted and had to be regenerated. A disadvantage of these columns was,apart from the necessary regeneration, a shift in the water peak which de-pended on the degree to which the column was exhausted and the oxidationof nitrite in the suppressor columns.

At the moment both continuously and discontinuously regenerable suppres-sors are in use.

All the continuously regenerable suppressors contain ion exchanger mem-branes or fibers and work on the counter-current principle. The fibers areimmersed in the regeneration agent, e.g. diluted sulfuric acid. The eluentand the analyte flow through the fibers. The exchange of the sodium ions ofthe eluent against protons takes place through the ion exchanger membraneof the fibers.

The membrane suppressors are constructed according to the sandwichprinciple. The eluent and analyte ions flow through a film between two mem-branes and the regeneration solution in spaces above and below thesemembranes.

The advantage of these membrane and fiber suppressors is their continuousregeneration, however their pressure sensitivity and the diffusion of regen-erator ions into the eluent are disadvantages.

In the discontinuously regenerable suppressors, which can also be operatedcompletely automatically, small ion exchanger columns with or without mi-crochannels packed with ion exchangers are used. In this procedure thesuppressor compartments are exchanged automatically after one or a fewchromatograms. The advantages of these suppressors are their high pres-sure stability and their purity.

The self-regenerable suppressors are basically membrane or discontinu-ously regenerable suppressors in which the protons for regeneration areobtained electrochemically by the electrolysis of water.

1 IC Theory


1.10 Evaluation and calibration

1.10.1 Evaluation of peak area

For a chromatogram recorded using a conductivity detector, the area undera substance peak is directly proportional to the amount of substance. Todetermine the peak areas, today electronic integrators or evaluation pro-grams incorporating special algorithms for peak smoothing, peak recordingand consideration of baseline drift are used almost without exception.

Provided the start and end of the peak can be identified reasonably accu-rately, the peak area evaluation provides very good results for medium tohigh concentrations. The peak area determination must particularly be em-ployed in cases where the capacity factors k’ change (e.g. through matrixeffects). The area calculation can be problematic when the peak overlap isextensive, with peaks exhibiting severe tailing and with perceptible detectornoise.

1.10.2 Evaluation of peak height

When the peak shape is constant, the peak height (the distance between thebaseline and the peak maximum) is a quantity which is proportional to thepeak area and which can also be used to evaluate chromatograms. Thepeak height determination is easily performed manually and is thus themethod of choice if the chromatograms are only plotted on a recorder.

A condition for the applicability of the height evaluation is constant k’ values.For reasons of linearity, it is suitable only for low to medium sample concen-trations. The linear range is larger for components eluted later than for thoseeluted rapidly. With excessive peak overlap or a noisy baseline, the peakheight evaluation can be superior to that of the peak area.

1.10.3 Calibration with external standard

The direct comparison of the signal magnitude (peak area or peak height) inan unknown sample with that of a standard solution of the same substanceis by far the most frequently employed method of calibration in ion chroma-tography. It requires the injection of constant volumes under invariablechromatographic conditions. The substance to be analyzed is injected as astandard solution of about the same concentration as that in the sample.This can also be carried out repeatedly at different concentrations (one-point, two-point, multipoint calibration). In a one-point calibration, the sam-ple concentration sought is calculated as follows:

1.10 Evaluation and calibration


c Sc

Sa ast

st= ×

ca : concentraion in analysis sample

cst : concentration of standard solution

Sa : signal of sample

Sst : signal of standard

With two-point and multipoint calibrations, the calibration function specific tothe substance is first plotted. Frequently, this function can be described by acalibration line obtained from the measured points with the aid of a linearregression analysis:

S S m cst st= + ×0

S0 : intercept of the calibration line

m : slope of the calibration line

In this case, the concentration in the analysis sample is calculated from theformula

cS S

maa=

− 0

The signal magnitude of the sample under investigation should lie betweenthat of the lowest and the highest calibration standard since the calibrationline is defined only within the concentration range investigated.

1.10.4 Calibration with internal standard

To take into account errors in the sample pretreatment (e.g. dispensing er-rors) or to determine recovery yields, the calibration with a second sub-stance added to both the external standard and the sample can be used.This component, called an internal standard, should be eluted as near aspossible to the substance to be analyzed yet be completely resolved andhave a similar concentration, response and chemical structure.

Since the sensitivities of the internal standard and the substance under in-vestigation are usually not the same, the correction factor Fx must first bedetermined with ultrapure calibration solutions. Here, a solution of knownconcentration of the substance of interest (cx) and the internal standard (cis)is chromatographed. Fx is obtained from the measured signal heights Sis

and Sx:

1 IC Theory


FS c

S cxis x

x is=

×

×

If the correction factor Fx can be assumed constant in the concentrationrange of interest, the concentration ca of a sample, to which the internalstandard has also been added, can be calculated as follows:

cS

Sc Fa

a

isis x= × ×

1.10.5 Calibration by standard addition

The standard addition method is used in ion chromatography primarily whenmatrix problems occur. The sample solution is spiked with a known quantityof the substance to be determined. The signals of the untreated sample andthe spiked sample solution are measured; the chromatographic conditionsmust be identical. The standard addition can be performed once, twice orseveral times.

In the simplest case with one standard addition, the unknown sample con-centration ca is calculated with the aid of the known, added concentrationdifference ∆c and the measured signal increase ∆S:

c Sc

Sa a= ×∆

∆

With several standard additions, the sample concentration is calculated bylinear regression analysis.

The advantage of the standard addition method lies in its greater reliabilitysince calibration in the sample is performed under actual matrix conditions.Problems are recognized rapidly by non-linear standard addition or shifts inpeak height. Long-term changes in temperature, pressure, etc. are takeninto account by the continual updating of the calibration and have no influ-ence on the measurement results. However, this reliability costs analysistime as calibration must be performed with every sample and not just peri-odically as with the external standard method.

1.11 References


1.11 References

Heinz EngelhardtHochdruck-Flüssigkeits-ChromatographieSpringer-Verlag, Berlin, 1977

Frank C. Smith, Richard C. ChangThe Practice of Ion ChromatographyJohn Wiley & Sons, New York, 1983

Veronika MeyerPraxis der Hochleistungs-FlüssigchromatographieDiesterweg Salle Sauerländer, Frankfurt a.M., 1985

Georg SchwedtChromatographische TrennmethodenThieme Verlag, Stuttgart, 2. Aufl., 1986

D. Frahne, M. Läubli, G. ZimmermannKonduktometrische Detektion zweiwertiger Kationenmit Einsäulen-ICGIT Fachz. Lab. 31, 1167-1169 (1987)

Douglas T. Gjerde, James S. FritzIon ChromatographyDr. Alfred Hüthig Verlag, Heidelberg, 2nd ed., 1987

James G. Tarter (ed.)Ion Chromatography(Chromatographic science; v. 37)Marcel Dekker, Inc., New York, 1987

Georg SchwedtIonen-Chromatographie anorganischer Anionen und Kationenin: Analytiker-Taschenbuch, Band 7, Springer-Verlag, Berlin, 1988

Paul R. Haddad, Peter E. JacksonIon Chromatography, Principles and Applications(Journal of Chromatography Library, volume 46)Elsevier, Amsterdam, 1990

1 IC Theory


Joachim WeissIonenchromatographieVCH, Weinheim, 2. Aufl., 1991

German Bogenschütz, Andreas Wild, Jochen SchäferFortschritte in der Ionenanalytik mit ICLaborPraxis 20, 38-46 (1996)(available as offprint from Metrohm)

Claudia Dengler, Maximilian Kolb, Markus LäubliIonenchromatographische Applikationen mitEinsäulen- und SuppressortechnikGIT Fachz. Lab. 40, 1104-1109 (1996)(available as offprint from Metrohm)

Claudia Dengler, Maximilian Kolb, Markus LäubliMethodenvergleich der Ionenchromatographiemit und ohne chemische SuppressionGIT Fachz. Lab. 40, 609-614 (1996)(available as offprint from Metrohm)

2.1 General hints


2 Columns and eluents

2.1 General hints

2.1.1 Eluents

For the preparation of the eluents one should use chemicals of a purity de-gree of at least "p.a.". For diluting please use high purity water.

Fresh eluents should always be microfiltered (0.45 µm filter) and de-gassed (with N2, He or vacuum). The eluent should be continuously stirredwith a magnetic stirrer, particularly when the recycling procedure is em-ployed or when alkaline eluents are used. For alkaline eluents and eluentswith low buffering capacity one should preferably use CO2 absorbers.

The supply vessel containing the eluent must be closed as tightly as possi-ble to avoid excessive evaporation. This is primarily important with eluentscontaining organic solvents (e.g. acetone), the evaporation of which canlead to drifts in the long term. If work is performed in a very sensitive range,even if one drop of condensate falls back in the eluent this can cause a no-ticeable change in the background conductivity.

Influence of various parameters on anion columns

• Concentration: An increase in the concentration usually leads toshorter retention times and quicker separation, butalso to a higher background conductivity.

• pH: pH alterations lead to shifts in the dissociationequilibrium and thus to changes in the retentiontimes.

• Organic modifiers: Addition of an organic solvent (e.g. methanol, ace-tone, acetonitrile) to aqueous eluents generally in-creases the retention times. The influence is usuallygreater on divalent anions than on monovalentanions.

fperez

Resaltado



Eluent changeWhen the eluent is changed, it must be ensured that no precipitates canbe formed. Solutions used in direct succession must therefore be miscible. Ifthe system has to be rinsed with an organic solution, several solvents withincreasing or decreasing lipophilic character may possibly have to be used(e.g. water ↔ acetone ↔ chloroform).

2.1.2 Protection of separating columns

To protect the column against foreign bodies which could have an adverseinfluence on the separation efficiency, we advise you to subject both theeluents and all samples to microfiltration (0.45 µm filter) and to siphon theeluent through the 6.2821.090 Suction Filter.

To avoid contamination by abrasive particles arising from piston seals of the709 IC Pump, it is advantageous to install an in-line filter between thepump and the 733 IC Separation Center. The optional 6.2821.000 ManufitFilter Unit is eminently suitable for this purpose.

The use of readily interchangeable precolumns serves to protect the actualseparating columns and increase their service life appreciably. The precol-umns available from Metrohm are either actual precolumns or so-calledprecolumn cartridges which are used together with the 6.2821.050 TwinCartridge Holder, the 6.2821.040 Cartridge Head, or the 6.2828.010 Precol-umn Cartridge Holder.

For preserving the column material from pressure drops caused by the in-jector it is recommended to use the optional 6.2620.050 Portmann Pulsa-tion dampener between the 709 IC Pump and the 733 IC Separation Center733.

Always store the separating columns closed when not in use and filled withan eluent in accordance with the manufacturer’s specifications.

2.1.3 Regeneration of columns

If the separation properties of the column have deteriorated, it can be re-generated in accordance with the column manufacturer’s specifications.With the separating columns available from Metrohm, the instructions for re-generation can be found on the leaflet enclosed with every column.

In the case of separating columns with carrier material based on silica, noalkaline solutions may be used for regeneration, otherwise the columnscould be damaged.

Dead volume at the end of a column can be the cause of extreme peakbroadening or splitting (appearance of double peaks). Filling the columnwith glass beads (∅ ≤ 100 µm) frequently improves the separation effi-ciency.

2.2 IC Anion Column Hamilton PRP-X100 (6.1005.000)


2.2 IC Anion Column Hamilton PRP-X100(6.1005.000)

2.2.1 Column specifications

• Column packing Polystyrene/divinylbenzene copolymer with quaternaryammonium groups

• Dimensions 125 × 4.0 mm

• Precolumn 6.1005.020 IC Precolumn cartridge PRP-X100(use together with 6.2821.040 Cartridge head or6.2821.050 Twin cartridge holder)

• pH range 1 ... 13 (above 30°C max. pH = 8)

• Max. flow 8.0 mL/min

• Max. pressure 34 MPa (340 bar)

• Preparation The column is filled with phthalic acid eluent pH = 5.0.

• Storage The column is stored in eluent for short periods(weeks) and in methanol/water (1:4) for longer periods(months).

• Regeneration Wash the column by passing through 0.5 mol/L tar-taric acid or nitric acid (10 mL nitric acid 6 mol/L in 1Lmethanol); flow rate 0.5 mL/min ca. 2 h.

2.2.2 General remarks

• The column can only be used in systems without chemical suppression.

• Sample solutions must always be filtered through a 0.45 µm membranefilter.

• Possibly dilute samples with eluent or use H+ ion exchanger cartridges(6.1012.110) for injection.

• For preserving the column it is recommended to use the 6.1005.020Precolumn cartridge and the 6.2620.050 Pulsation dampener, whichminimizes the pressure shocks caused by the injector.

• For more information on this column, see Application Bulletin No. 265.



2.2.3 Phthalic acid eluent

• Composition 2 mmol/L phthalic acid, 7.6 % acetone, pH 5.0(NaOH); conductivity approx. 160 µS/cm

• Preparation Concentrate solutionAdd 3.323 g phthalic acid, 2 mL NaOH 30 % and10 mL acetone to 950 mL of high purity water.After dilution (ev. with warming up), set the pHvalue to 4.5 with NaOH 1 mol/L and fill up to 1 Lwith high purity water.Ready-to-use eluentMix 100 mL concentrate solution with 75 mLacetone and fill up to 1 L with high purity water.The pH value of this solution is now exactly 5.0.

• Standard chromatogramFlow: 2.0 mL/minInjection volume: 100 µLFull Scale: 4 µS/cmPolarity: +

Peak Component Conc. [mg/L] tR [min]

1 Fluoride 5 1.2

2 Chloride 5 1.8

3 Nitrite 5 2.0

4 Bromide 10 2.7

5 Nitrate 10 3.2

6 Sulfate 10 9.9

System peak ≈14

6

53 4

1

2 1 µS/cm

2.3 IC Anion Column SUPER-SEP (6.1009.000)




• Column packing Polymethacrylate with quaternary ammonium groups


• Precolumn 6.1009.010 IC Anion precolumn SUPER-SEP or6.1005.050 IC Precolumn cartridge PRP-1 (use to-gether with 6.2821.040 Cartridge head or 6.2821.050Twin cartridge holder) Whether or not this precolumncan be used, depends on the sample matrix and hasto be tested in any case.

• pH range 1 ... 13


• Max. pressure 2.5 MPa (25 bar)

• Preparation The column is filled with phthalic acid eluent.

• Storage The column is stored in eluent.

• Regeneration Rinse with 0.1 mol/L HNO3/20 % acetonitrile;flow 0.3 mL/min ca. 24 h.

If this is not sufficient, rinse with the following solutions:Metal impurities: 0.1 mol/L sodium tartrateProtein impurities: 0.1 mol/L NaOH or

20% acetic acidOrganic impurities: 20 % acetonitrile in high purity

water




• Eluents should not contain more than 20% of organic solvents.

• Large concentrations of divalent cations can perturb the base line. Espe-cially for sensitive analyses, they should be removed with a 6.1012.110H+ ion exchanger cartridge.

• The column must be used together with the 6.2620.050 Pulsation damp-ener, which minimizes the pressure shocks caused by the injector.



2.3.3 Phthalic acid eluent

• Composition 2.5 mmol/L phthalic acid, 5 % acetonitrile, pH 4.2(TRIS); conductivity approx. 130 µS/cm

• Preparation Concentrate solutionDissolve 4.15 g phthalic acid in 950 mL high pu-rity water with warming up. Add 10 mL acetoni-trile and adjust the pH value to 3.8 with TRIS(solid). After this fill up to 1 L with high puritywater.Ready-to-use eluentMix 100 mL concentrate with 50 mL acetonitrileand fill up to 1 L with high purity water.



1 Fluoride 5 1.7

2 Chloride 5 2.4

3 Nitrite 5 2.9

4 Bromide 10 3.5

5 Nitrate 10 4.2

6 Sulfate 10 5.8

System peak ≈10

6

43

2

5

11 µS/cm



2.3.4 Benzoic acid eluent

• Composition 3 mmol/L benzoic acid, 2 % acetonitrile, pH 4.65(TRIS); conductivity approx. 120 µS/cm

• Preparation Dissolve 366 mg benzoic acid in 20 mL acetoni-trile and make up to 1 L with high purity water.Set the pH value of the degassed solution to4.65 with TRIS.



1 Acetate 10 1.7

2 Fluoride 5 2.2

3 Formiate 5 2.6

4 Chloride 5 3.4

5 Nitrite 5 4.4

6 Bromide 10 5.4

7 Nitrate 10 6.7

System peak ≈10

8 Sulfate 10 14.3

8 16124

1 µS/cm

2

1

3

4

5

8

76



2.4 IC Glass Cartridge Metrosep Anion Dual 1(6.1006.020)


• Column packing Spheric hydroxyethyl methacrylate with quaternaryammonium groups

• Dimensions Glass cartridge 150 × 3.0 mm(installation in 6.2828.000 Glass Cartridge Holder)

• Precolumn 6.1006.030 Precolumn Cartridge METROSEP AnionDual 1 (installation in 6.2828.010 Precolumn CartridgeHolder)

• pH range 2 ... 12 (>12 only briefly)

• Max. flow 0.7 mL/min (recommended flow 0.5 mL/min)


• Preparation Column and precolumn cartridge are stored in phtha-late eluent and can be used directly for ion chroma-tography without suppression. For ion chromatogra-phy with suppression, the system must be rinsedduring ca. 4 h with the carbonate eluent at a flow rateof 0.5 mL/min.

• Storage The column cartridge is stored for a short period(days) in the closed holder. For long-term storage(weeks) the cartridge is stored in the supplied storagevessel in acetone/water (1:9) in the dark.

• Regeneration Rinse with 0.1 mol/L HNO3 for ca. 2 h at 0.3 mL/min,then switch to rinsing with standard eluent.



• The orthophosphate determination requires an IC system with chemicalsuppression.

• The column is not suited for the determination of low fluoride concentra-tions with acidic eluents.

• The column is not suited for the determination of low nitrite concentrationswith chemical suppression.

• Eluents may contain maximum 10% organic solvents.• The column cartridge must not be rinsed with high purity water.• The column cartridge must be used together with the 6.2620.050 Pulsa-

tion dampener, which minimizes the pressure shocks caused by the in-jector.


2.4 IC Glass Cartridge Metrosep Anion Dual 1 (6.1006.020)


2.4.3 Phthalic acid eluent (without chemical suppression)

• Composition 8 mmol/L phthalic acid, 2 % acetonitrile;pH = 4.1 (TRIS); conductivity approx. 400 µS/cm

• Preparation Concentrate solutionDissolve 13.3 g phthalic acid in 950 mL high pu-rity water with warming up. Add 10 mL acetoni-trile, let the solution cool down and adjust the pHvalue to 4.0 with TRIS (solid). Then fill up to 1 Lwith high purity water.Ready-to-use eluentMix 100 mL concentrate with 20 mL acetonitrileand fill up to 1 L with high purity water.

• Standard chromatogramFlow: 0.5 mL/minInjection volume: 100 µLFull Scale: 10 µS/cmPolarity : +


1 Fluoride 5 3.2

2 Chloride 5 4.5

3 Nitrite 5 5.3

4 Bromide 10 6.2

5 Nitrate 10 7.2

6 Sulfate 10 10.2

System peak ≈17

1 µS/cm

6

54

3

2

1



2.4.4 HCO3–/CO3

2– eluent (with chemical suppression)

• Composition 2.5 mmol/L sodium carbonate,2.4 mmol/L sodium hydrogen carbonate;conductivity after chemical suppressionca. 16 µS/cm

• Preparation Dissolve 265 mg sodium carbonate (water-free)and 201.5 mg sodium hydrogen carbonate inhigh purity water, then fill up to 1 L with high pu-rity water.

• Remarks Depending on the sample organic acids maycause interferences with fluorides (lactate, gly-colate) and with sulfates (tartrate, malate, ma-lonate).



1 Fluoride 2 3.6

2 Chloride 5 5.5

System peak 6.5

3 Nitrite 5 7.1

4 Bromide 10 8.3

5 Nitrate 10 10.0

6 Orthophosphate 10 11.8

7 Sulfate 10 14.6

1 µS/cm

7

6

54

3

2

1

2.5 IC Anion Column Metrosep Anion Dual 2 (6.1006.100)


2.5 IC Anion Column Metrosep Anion Dual 2(6.1006.100)


• Column packing Polymethacrylate with quaternary ammoniumgroups


• Precolumn 6.1005.050 Precolumn cartridge PRP-1 (4 x 20 mm RPphase on polystyrene/divinyl benzene, use togetherwith 6.2821.050 Twin cartridge holder)

• pH range 1 ... 12



• Preparation Columns are filled with borate/gluconate eluent.

• Storage Columns are stored in the used eluent at room tem-perature.

• Regeneration Rinse with 0.1 mol/L HNO3 at 0.3 mL/min during ca.2 h, then rinse with standard eluent; or rinse with elu-ent containing 100 mmol/L of the buffer salt.Organic impurities: Rinse with eluent containing an or-ganic solvent (<20 % vol.).



• The orthophosphate determination requires an IC system with an alkalineeluent.

• Suitable for determination of anions in trace range.• Eluents may contain maximum 20% organic solvents.• No tris-(hydroxymetyl)-aminomethane (TRIS) must be used for setting the

pH value of the phthalic acid eluent.• For preserving the column it is recommended to use the 6.2620.050 Pul-

sation dampener, which minimizes the pressure shocks caused by theinjector.




2.5.3 Phthalic acid eluent (without chemical suppression)

• Composition 5 mmol/L phthalic acid, 2 % acetonitrile; pH =4.5 (NaOH); conductivity approx. 400 µS/cm

• Preparation Concentrate solutionAdd 5 mL NaOH 10 mol/L to 950 mL high puritywater and dissolve 8.31 g phthalic acid in thissolution with warming up. Add 10 mL acetonitrile,let the solution cool down and adjust the pHvalue to 4.3 with NaOH. Then fill up to 1 L withhigh purity water.Ready-to-use eluentMix 100 mL concentrate with 20 mL acetonitrileand fill up to 1 L with high purity water.



1 Fluoride 5 2.3

2 Chloride 5 3.4

3 Nitrite 5 4.1

4 Bromide 10 5.0

5 Nitrate 10 5.9

6 Sulfate 10 9.5

System peak ≈15

1 µS/cm

654

3

2

1

2.5 IC Anion Column Metrosep Anion Dual 2 (6.1006.100)


2.5.4 HCO3–/CO3


• Composition 1.3 mmol/L sodium carbonate,2.0 mmol/L sodium hydrogen carbonate;conductivity after chemical suppression ca.14 µS/cm

• Preparation Dissolve 137.5 mg sodium carbonate (water-free) and 168 mg sodium hydrogen carbonate inhigh purity water, then fill up to 1 L with high pu-rity water.

• Remarks Depending on the sample organic acids maycause interferences with fluorides (lactate, gly-colate) and with sulfates (tartrate, malate, ma-lonate).



1 Fluoride 2 3.2

2 Acetate 2 3.6

3 Formiate 2 4.1

4 Chloride 5 5.8

5 Nitrite 5 8.3

6 Bromide 10 10.5

7 Nitrate 10 13.6


9 Sulfate 10 19.4

1 µS/cm

8

9765

2

4

3

1



2.6 IC Anion Column Phenomenex STAR ION A300(6.1005.100)


• Column packing Polystyrene/divinylbenzene copolymer with quaternaryammonium groups


• pH range 1 ... 12



• Preparation The column is filled with sodium bicarbonate/carbon-ate eluent and can be used directly.

• Storage The column is stored in the used eluent at room tem-perature.

• Regeneration Rinse 30 min with a solution containing 17 mmol/L so-dium bicarbonate and 18 mmol/L sodium carbonate(flow rate 1 mL/min).


• The column can only be used in systems with chemical suppression.

• Limited suitability for the analysis of chloride as carbonates interfere withchloride.


• Eluents must not contain any organic solvents.

• For preserving the column it is recommended to use the 6.2620.050 Pul-sation dampener, which minimizes the pressure shocks caused by theinjector.


2.6 IC Anion Column Phenomenex STAR ION A300 (6.1005.100)


2.6.3 HCO3–/CO3


• Composition 1.8 mmol/L sodium carbonate,1.7 mmol/L sodium hydrogen carbonate;conductivity after chemical suppression ca.14 µS/cm

• Preparation Dissolve 190.5 mg sodium carbonate (water-free) and 142.5 mg sodium hydrogen carbonatein high purity water, then fill up to 1 L with highpurity water.



1 Fluoride 2 1.5

System peak 1.9

2 Chloride 5 2.1

3 Nitrite 5 2.6

4 Bromide 10 3.4

5 Nitrate 10 4.3


7 Sulfate 10 9.3

1 µS/cm

7

6

543

2

1



2.7 IC Exclusion Column Hamilton PRP-X300(6.1005.030)


• Column packing Polystyrene/divinylbenzene copolymer with sul-phonic acid groups


• Precolumn 6.1005.040 IC Precolumn cartridge PRP-X300 (use to-gether with 6.2821.040 Cartridge head or 6.2821.050Twin cartridge holder)

• pH range 1 ... 13



• Preparation The column is filled with 0.5 mol/L H2SO4.


• Regeneration Divalent cations remain on the column and form com-plexes with citrates which give an incorrect citratepeak. It is thus a good idea to inject 100 µL 0.1 mol/LNa2EDTA from time to time. If necessary the columncan be rinsed for 6 h with 0.01 mol/L H2SO4 + 20 %methanol at a flow rate of 0.5 mL/min.




• As monovalent cations are only eluted very late and divalent cations arenot eluted at all these should be removed by a cation exchanger beforethe injection.


2.7 IC Exclusion Column Hamilton PRP-X300 (6.1005.030)


2.7.3 HClO4 eluent

• Composition 1.5 mmol/L perchloric acid;conductivity approx. 300 µS/cm

• Preparation Concentrate solution0.1 mol/L perchloric acid (ready-to-use solution,available on the market).Ready-to-use eluentAdd 15 mL concentrate to 900 mL high puritywater and fill up to 1 L with high purity water.

• Remarks UV detection at 190 nm can also be carried outwith this eluent; this has advantages in the de-termination of tartrate compared with conductiv-ity detection.



1 Tartrate 4 1.8

2 Malate 7.5 2.6

3 Citrate 7.5 3.1

4 Lactate 10 4.2

5 Acetate 25 6.3

6 Succinate 40 8.8

1 µS/cm3

65

2

4

1



2.8 IC Cation Column Nucleosil 5SA (6.1007.000)


• Column packing Spherical silica gel with sulphonic acid groups,Ø = 5 µm


• Precolumn 6.1007.010 IC Precolumn cartridge Nucleosil 5SA(use together with 6.2821.040 Cartridge head or6.2821.050 Twin cartridge holder)

• pH range 2 ... 7

• Preparation The column can be used directly.


• Regeneration – Inject 100 µL c(Na2EDTA) = 0.1 mol/LNote: Do not use any alkaline EDTA solutions!

– Rinse with 30 mL c(HNO3) = 0.1 mol/L at a flowrate of 0.5 mL/min.



• The pH of the standard and sample solutions must lie between 2.5 and3.5. To ensure this the solutions are either made up or diluted with eluentor adjusted to this value with nitric acid (HNO3).


2.8 IC Cation Column Nucleosil 5SA (6.1007.000)


2.8.3 Tartaric acid/citric acid eluent

• Composition 4 mmol/L tartaric acid, 0.5 mmol/L citric acid,3 mmol/L ethylene diamine, 5 % acetone;conductivity approx. 500 µS/cm

• Preparation Dissolve 0.6 g tartaric acid and 105 g citric acidin high purity water. Add 3 mL of a 1 mol/L ethyl-ene diamine solution (6.7 mL ethylene diamineper 100 mL) and 50 mL acetone and fill up to 1L.

• Standard chromatogramFlow: 1.5 mL/minInjection volume: 100 µLFull Scale: 5 µS/cmPolarity: –


1 Nickel 5 2.7

2 Zinc 5 3.4

3 Cobalt 5 4.3

4 Manganese 10 7.5

1 µS/cm

2

31 4



2.9 IC Cation Column Metrosep Cation 1-2(6.1010.000)


• Column packing Spherical silica gel with polybutadiene maleic acidgroups


• Precolumn 6.1010.010 IC Precolumn cartridge Metrosep Cation1-2 (use together with 6.2821.040 Cartridge head or6.2821.050 Twin cartridge holder)

• pH range 2 ... 7

• Preparation The column is filled with eluent and can be used di-rectly.

• Storage The column is stored in eluent (storage temperature+4ºC). Avoid drying up of the column packing!

• Regeneration Organic impurities: Rinse the column with 40 mL highpurity water, 40 mL acetone and 40 mL high puritywater (flow rate 1 mL/min).



• The pH of the standard and sample solutions must lie between 2.5 and3.5. To ensure this the solutions are either made up or diluted with eluentor adjusted to this value with nitric acid (HNO3).

• The eluent can be preserved by addition of 1 % acetonitrile. Higher con-centrations of organic solvents deteriorate the sodium/ammonium sepa-ration.

• Alcohols alter the selectivity of the column material; they should thereforenot be used in eluents.


• All solutions must be stored in plastic containers.

• To obtain a correct sodium determination all contact with glass must beavoided.


2.9 IC Cation Column Metrosep Cation 1-2 (6.1010.000)


2.9.3 Tartaric acid/dipicolinic acid eluent

• Composition 4 mmol/L tartaric acid, 1 mmol/L dipicolinic acid;conductivity approx. 700 µS/cm

• Preparation Dissolve 167 mg dipicolinic acid and 0.6 g tar-taric acid in 100 mL high purity water with warm-ing up, then fill up to 1 L with high purity water.

• Remarks Heavy metal ions contained in the sample solu-tion are complexed by the dipicolinic acid andare eluated in the front peak; as a result they donot interfere with the determination.

• Standard chromatogramFlow: 1.0 mL/minInjection volume: 10 µLFull Scale: 5 µS/cmPolarity: –


1 Lithium 1 2.9

2 Sodium 5 3.4

3 Ammonium 5 3.8

4 Potassium 10 4.7

5 Calcium 10 7.2

6 Magnesium 10 10.6

7 Strontium 20 12.7

8 Barium 20 19.0

1 µS/cm

8

7

6

5

4

3

2

1



Index

8.732.2003 Monograph «Ion chromatography» 53

IndexAAcetate .....................................37,47Acetone .........................................31Addition of ligands ........................10Adsorption.......................................7Alkali cations..................................14Alkali metal ions.............................10Alkaline earth cations ....................14Alkaline eluents..............................31Ammonium....................................51Amount-of-substance

concentration .............................12Amperometry.................................21Anion exchanger .............................3Anions............................................15Asymmetry factor ............................7Auxiliary electrode .........................21

BBackground conductivity...............14Barium ...........................................51Base line drifts...............................26Benzoic acid..................................14Benzoic acid eluent .......................37Bromide.........................34,36,37,39,

....................................40,42,43,45

CCalcium .........................................51Calibration .....................................26Calibration by

standard addition.......................28Calibration curves..........................17Calibration with

external standard .......................26Calibration with

internal standard ........................27Capacity factor .............................5,6Carbonate......................................44Carbonate eluents......................9,24Carbonate in water ..........................8Carbonate system peak ................11Cartridge head ..............................32Cation exchanger ............................3Cations ..........................................16Cell constant..................................12Characteristics.................................4Chemical effects..............................7Chemical suppression ..................15Chloride .........................34,36,37,39,

...............................40,42,43,44,45Chromatogram................................4Chromatographic characteristics....4Chromophore groups ...................18Citrate ............................................47CO2 absorber ................................31Coated silica gels ............................8Cobalt ............................................49Column material ..............................8Column overloading ........................7

Columns and eluents ................... 31Conductance................................ 12Conductometry............................. 12Conductivity.................................. 12Conductivity detection.................. 12Constancy of the background...... 14Construction of suppressors........ 25Correction factor........................... 27Coulometry ................................... 21Counter-current principle ............. 25Crown ether .................................. 10

DDead time....................................... 4Dead volume .......................... 4,7,32Degassing .................................... 31Detection limit.......................... 14,19Detection limit with chemical

suppression .............................. 16Detection limit with electronic

background detection............... 16Detector noise .............................. 14Diode array detectors................... 20Direct ion chromatography........... 14Direct UV-VIS detection................ 18Dispensing error ........................... 27Dissociation equilibrium .......... 11,31Double peaks ............................... 32

EELCD ............................................ 23Electrochemical detection............ 21Electrode material ........................ 22Electrode surface ......................... 12Eluent anions................................ 11Eluent change .............................. 31Eluent composition......................... 9Eluents.......................................... 31Eluents with organic solvents....... 31Equivalent conductivity............ 12,13Equivalent ionic conductivity ... 13,14Errors in sample pretreatment...... 27Evaluation..................................... 26Evaluation programs .................... 26Evaporation .................................. 31External standard ......................... 26

FFast scanning detectors............... 20Filter unit Manufit .......................... 32Fluorescence detection........... 18,20Fluoride ..... 34,36,37,39,40,42,43,45Formiate ....................................... 37

GGaussian curve .............................. 4Glass beads ................................. 32Glassy carbon .............................. 22

HHAMILTON PRP-X100.................. 33HAMILTON PRP-X300.................. 46HClO4 eluent................................. 47HCO3

–/CO32– eluent ........... 40,43,45

High purity water .......................... 31Hydroxyethyl methacrylate ........... 38

IIC Anion Column

HAMILTON PRP-X100 .............. 33IC Anion Column

Metrosep Anion Dual 2 ............. 41IC Anion Column

Phenomenex STAR ION A300 .. 44IC Anion Column

SUPER-SEP .............................. 35IC Cation Column

Metrosep Cation 1-2 ................. 50IC Cation Column

Nucleosil 5SA............................ 48IC Exclusion Column

HAMILTON PRP-X300 .............. 46IC Glass Cartridge

Metrosep Anion Dual 1 ............. 38IC Precolumn Cartridge

Metrosep Anion Dual 1 ............. 38IC Precolumn Cartridge

Metrosep Cation 1-2 ................. 50IC Precolumn Cartridge

Nucleosil 5SA............................ 48IC Precolumn Cartridge

PRP-1................................... 35,41IC Precolumn Cartridge

PRP-X100.................................. 33IC Precolumn Cartridge

PRP-X300.................................. 46IC Precolumn SUPER-SEP........... 35Indirect fluorescence

detection ................................... 20Indirect UV-VIS detection ........ 18,19In-line filter .................................... 32Integrators .................................... 26Interelectrode distance................. 12Interferences in

the chromatogram .................... 11Internal standard .......................... 27Introduction .................................... 1Ion chromatographic

separations ................................. 8Ion chromatography

with chemical suppression ....... 15Ion exchange.................................. 2Ion exchanger columns................ 25Ion exchanger fibers..................... 25Ion exchanger membranes .......... 25Ionic conductivity.......................... 13

Index

8.732.2003 Monograph «Ion chromatography»54

KKohlrausch's law ...........................12

LLactate ..........................................47Liquid chromatography...................1Ligands .........................................10Linear regression...........................27Linear working range.....................17Linearity of calibration curves........17Lipophilic character.......................32Lithium...........................................51

MMagnesium ...................................51Malate............................................47Manganese ...................................49Matrix problems ............................28Matrix reduction.............................19Maximum absorption ....................18Membrane suppressors................25Metrosep Anion Dual 1 .................38Metrosep Anion Dual 2 .................41Metrosep Cation 1-2 .....................50Microfiltration............................31,32Multi-point calibration....................27Multiple detection..........................23Multiwavelength detection ............20

NNegative peaks .............................14Net retention time............................4Net retention volume.......................4Nickel ............................................49Nitrate............................17,34,36,37,

...............................39,40,42,43,45Nitrite ........................19,23,34,36,37,

...............................39,40,42,43,45Noise .............................................14Nucleosil 5SA................................48Number of theoretical plates...........5

OOhm's law......................................12Optical detection...........................18Optimum wavelength ....................19Organic anions..............................23Organic modifiers..........................31Orthophosphate........38,40,41,43,45

PParticle size .....................................3Peak area evaluation.....................26Peak broadening.....................5,7,32Peak detection ..............................26Peak base width..............................4

Peak height evaluation..................26Peak smoothing ............................26Peak width....................................4,5Peak width at half height .................4pH changes...................................31Phenomenex STAR ION A300.......44Phosphate .....................................14Phthalic acid..................................14Phthalic acid eluent ........34,36,39,42Plate number................................5,6Polyhydroxy

alkylmethacrylate .........................8Polymers .........................................3Polymethacrylate...................8,35,41Polystyrene/divinylbenzene

copolymer..........................8,44,46Positive peaks ...............................14Post-column derivation ............18,20Potassium .....................................51Precipitates....................................32Precolumn cartridge holder ..........32Precolumn cartridges....................32Precolumns ...................................32Pulsed amperometry.....................21Protection of

separating columns...................32PRP-1 .......................................35,41PRP-X100 ......................................33PRP-X300 ......................................46Pulsation dampener ......................32

QQuality of the IC Pump..................14

RRecovery rate ................................27Recycling.......................................31Reducibility....................................21Reference electrode......................21References ....................................29Regeneration of columns..............32Relative retention.............................5Resistance.....................................12Resolution .......................................6Retention time .........................4,5,31Retention volume ............................4

SSalicylic acid..................................14Selectivity .....................................5,6Sensitivity of conductivity

measurement.............................14Sensitivity with chemical

suppression...............................16Sensitivity with electronic

background suppression ..........16Separating columns ......................32Shift of dissociation equilibrium ....11

Signal/noise ration.........................14Silica gels .................................48,50Single point calibration..................26Sodium..........................................51Splitting..........................................32Standard addition..........................28Standard deviation ..........................4Standard solution ..........................26STAR ION A300.............................44Stirring ...........................................31Strontium.......................................51Styrene-divinylbenzene resin ..........3Succinate.......................................47Suction filter...................................32Sulfate ........34,36,37,39,40,42,43,45SUPER-SEP...................................35Support material..............................8Suppression ..................................15Suppressor reactions....................24Suppressors..................................24System peak .................................11

TTailing ..............................................7Tartaric acid/citric

acid eluent .................................49Tartaric acid/dipicolinic

acid eluent .................................50Tartrate ..........................................47Temperature dependence ............13Test chromatogram.........................8Theoretical plate..............................5Theory .............................................1Thin-layer cell ................................21Transition metals ......................10,20Twin cartridge holder.....................32

UUV-VIS detection ...........................18UV-VIS detection with

post-column derivation.........18,20UV-VIS multiwavelength

detection....................................20

VValency..........................................12Voltammetric detector cells...........22Voltammetry ..................................21

WWall-jet cell ....................................21Water peak ...............................11,25Working electrode .........................21

ZZinc................................................49

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