Ultra-trace determination of bromate in drinking waters by means ofmicrobore column ion chromatography and on-line coupling with

inductively coupled plasma mass spectrometry

Michael Nowak, Andreas Seubert*

Institute of Inorganic Chemistry, University of Hannover, Callinstrasse 9, D-30167 Hannover, Germany

Received 24 June 1997; received in revised form 14 October 1997; accepted 19 October 1997

Abstract

A method for ultra-trace determination of bromate in drinking waters using on-line coupling of ion chromatography (IC) and

inductively coupled plasma mass spectrometry (ICP-MS) has been developed. The method utilizes the microbore column

technique in combination with a self-made high-capacity and high-performance anion-exchanger. The high capacity of the

separation column as well as an optimized elution system based on NH4NO3 allows a direct analysis of almost every water

sample without matrix elimination. Even more, the use of large injection volumes is possible. No trace enrichment is required

because of the sensitivity of ICP-MS detection. The effort on sample pretreatment is therefore reduced to zero. The method

detection limits for bromate in the drinking and mineral waters investigated are in the 50±65 ng/l range, corresponding to

absolute detection limits of 44±58 pg. Retention behaviour of bromate as well as signal-to-noise (SNR) and signal-to-

background (SBR) ratios are dependent on the sample composition. The within-run imprecision of the presented IC-ICP-MS

coupling is 5% at a concentration level of 500 ng/l bromate. The time spent on a complete analysis is 8±15 min, depending on

the bromide content of the sample. Considering sensitivity as well as imprecision and short analysis times, the described IC-

ICP-MS coupling is well suited for precise routine analysis of bromate in drinking waters at sub-mg lÿ1-levels. # 1998

Elsevier Science B.V.

1. Introduction

Bromate is formed during the disinfection process

of drinking water by means of ozonization [1]. It is a

potential carcinogen to rats and mice at mg lÿ1 levels

[2]. Newer toxicological studies [3] have led the

International Agency for Research on Cancer (IARC)

to classify bromate as a group 2B carcinogen to

humans with renal tumor risks at concentrations

>0.05 mg/l. Therefore, the US Environmental Protec-

tion Agency (USEPA) has requested comment on

setting the maximum contaminant level goal for

bromate to zero.

At the moment the USEPA recommends a concen-

tration limit of 10 mg/l for bromate in drinking water

[4], the World Health Organisation (WHO) accepts a

limit of 25 mg/l [5]. The Commission of the European

Communities proposes a concentration limit of 10 mg/

l [6]. Also detection limits <2.5 mg/l as well as

accuracies and precisions of the analytical method

Analytica Chimica Acta 359 (1998) 193±204

*Corresponding author. Tel.: +49 511 762 3174; fax: +49 511

762 2923; E-mail: seubert@mbox.acc.uni-hannover.de

0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.

P I I S 0 0 0 3 - 2 6 7 0 ( 9 7 ) 0 0 6 0 9 - 0

used are recommended to be better than 25% at a

concentration level of 10 mg/l. According to the out-

line of DIN/ISO 15601, detection limits for bromate of

<0.5 mg/l are required.

Bromate is usually analyzed using ion chromato-

graphy (IC) with suppressed conductivity detection

[7,8]. Present methods are based on low-capacity

ion-exchangers. Therefore, injection volume and ionic

strength of the sample are strictly limited to avoid

column overloading. A total removal of interfering

ions like chloride is necessary. In contrast, high-

capacity anion-exchangers allow the determination

of bromate in water samples containing high ionic

strength with minimized effort on sample pretreatment

[9]. However, methods without trace enrichment are

not sensitive enough to meet the aforementioned

requirements. More sophisticated ion chromato-

graphic methods are based on a bromate preconcen-

tration step prior to analysis [10,11]. These techniques

combine low-capacity anion-exchangers with sup-

pressed conductivity detection and are able to detect

bromate at levels down to 1 mg/l. To ensure quantita-

tive enrichment of bromate a total removal of matrix

anions, especially chloride, sulphate and carbonate, is

required. On the other hand an anion like nitrate,

which has a high elution power, cannot be removed

because it does not form an insoluble compound.

Concerning the expensive sample pretreatment and

the sophisticated column switching techniques needed

as well as the time spent on preconcentration and

clean-up steps, these methods are not well-suited for

routine analysis of bromate at mg lÿ1 or sub-mg lÿ1

levels.

Another chromatographic approach is the applica-

tion of reversed-phase-chromatography (RPC) with

direct UV-detection [12]. The reported method is able

to detect bromate at the low mg/l-level without matrix

removal as long as the chloride concentration does not

exceed 50 mg/l. A drawback of this method is the

large sample volume (250 ml), from which the trace

bromate has to be concentrated by means of evapora-

tion.

A sensitive ¯ow-injection analysis (FIA) method

utilizes the oxidation of chloropromazines by bromate

[13,14]. This spectrophotometric technique allows the

determination of bromate at concentration levels

below 1 mg/l, but is disturbed by other disinfection

by-products like nitrite and chlorite when these inter-

fering ions are not removed or separated by ion

chromatography prior to analysis. Such a separation

is realized in the combination of ion-interaction chro-

matography (IIC) with post-column derivatization of

bromate by means of o-dianisidine [15]. The detection

limits of this method are ca. 1 mg/l bromate in deio-

nized water and sample pretreatment is not necessary.

However, o-dianisidine is chemically related to ben-

zidine and, therefore, itself a carcinogen. The deter-

mination of bromate is affected by chlorite due to the

poor resolution of both anions. Other, non-chromato-

graphic methods like pulsed polarography are not

sensitive enough to detect bromate at low mg lÿ1-

levels [16].

Mass spectrometry (MS) has proved to be a quite

sensitive and selective detector for cation as well as

anion chromatography. The simplest technique is the

coupling of IC with inductively coupled plasma mass

spectrometry (IC-ICP-MS), which has been used for

the determination of bromate in several matrices such

as baked goods [17], drinking water [18] or human

urine [19]. A disadvantage of these methods is the

requirement of the same sample pretreatment as used

with conductivity detection when detection limits at

the ng lÿ1 level have to be reached. The main reason

for this is the chromatographic part of the system

which is based on low-capacity anion exchangers and

NaOH-based elution systems. Other combinations are

negative ion mass spectrometry combined with iso-

tope dilution analysis (IC-NTI-IDMS) [20] or electro-

spray ion chromatography tandem mass spectrometry

(IC-ESI-MS-MS) [21], which give excellent detection

limits below 100 ng/l, but are too complex for routine

analysis. In addition, too much time is spent on the

analysis of one sample.

Our aim is to establish a method for the determina-

tion of bromate in drinking water using ion chroma-

tography with mass spectrometric detection suited for

routine analysis. The on-line coupling IC-ICP-MS is

chosen because of its simplicity compared to other

mass spectrometric procedures. The method should

meet the following requirements:

1. Determination of bromate in drinking water at

concentration levels in the 0.1±10 mg/l range.

2. Imprecision better than 25% at sub-mg lÿ1 levels.

3. No sample pretreatment.

4. Time spent on analysis <15 min per sample.

194 M. Nowak, A. Seubert / Analytica Chimica Acta 359 (1998) 193±204

Our strategy is to adjust the ion-chromatographic

part of the combined system with regard to the

requirements of ICP-MS. The optimization of IC is,

therefore, divided into three parts. Part one is the

synthesis and application of high-capacity and high-

performance anion-exchangers. Synthesis is necessary

because these materials are not commercially avail-

able today. High-capacity ion-exchangers allow both

the analysis of water samples containing high ionic

strength (mineral or waste waters) and the use of large

injection volumes >500 ml without overloading the

analytical column [22]. For this reason, no matrix

elimination step is required as long as the detection

system, in this case ICP-MS, is element speci®c. In

view of the sensitivity of ICP-MS detection, a direct

analysis without trace enrichment should be possible.

Part two is the utilization of microbore technology.

These columns with internal diameters of 2 mm, or

less, allow ¯ow rates up to 500 ml/min. An ef®cient

nebulization of the sample is enabled which results in

a higher sensitivity. Also the separation time is

reduced. Part three is the application of a non-alkaline

and non-suppressed elution system. When ICP-MS

detection is applied there is almost no restriction in

choosing the elution system compared to IC using

conductivity or spectrophotometric detection. There-

fore, the eluent can easily be adjusted with respect to

the separation problem.

2. Experimental

2.1. Instrumentation

2.1.1. Chromatography

All chromatographic equipment was delivered from

Metrohm, Herisau, Switzerland. The chromatographic

system consisted of the IC Pump 709 equipped with a

pulse dampener Portmann to minimize pressure pul-

sation. The six-port stainless steel valve of the IC

Separation Center 733 was used for sample injection.

All columns, tubing and ®ttings were made of poly-

ether-ether-ketone (PEEK).

2.1.2. ICP-MS

A Fisons/VG PlasmaQuad Turbo 2� (Fisons, Wins-

ford, Great Britain) in standard con®guration was used

for mass spectrometric detection of bromate. A V-

Groove-Nebulizer was utilized for nebulization of the

eluent. Data were collected using the DOS software of

VG. All further data treatment was done by a self-

written software package. Statistical calculations were

carried out by a spreadsheet program.

2.1.3. Separation column

The packing material of the separation column was

made by functionalization of a self-made polystyrene/

divinylbenzene (PS/DVB) copolymer. This highly

cross-linked copolymer has an average particle size

of 5 mm. The BET surface area is 520 m2/g, the pore

volume 0.44 ml/g and the average pore diameter

3.3 nm. The PS/DVB-Copolymer was functionalized

by chloromethylation using chloromethyloctylether

(CMOE) and AlCl3 as Lewis catalyst [23]. The chlor-

ine was replaced by 2-(dimethylamino)-ethanol in an

SN-reaction. Microbore columns of 100�2 mm i.d.

(Upchurch Scienti®c, Oak Harbor, USA) were ®lled

with this packing material utilizing the down-®ll-

slurry technique. Exchange capacity of the microbore

column for chloride was 150 mmol or 480 mmol/ml

bed volume.

2.1.4. Chromatographic conditions

The two investigated elution systems based on nitric

acid consisted of 100 and 60 mmol/l nitric acid

(65% w/w). The hydrochloric acid eluent

(100 mmol/l) was made from concentrated hydrochlo-

ric acid (37% w/w). Both acids were doubly subboiled

suprapure acids (Riedel de HaeÈn, Seelze, Germany).

All eluents were adjusted to pH 6 with suprapure

ammonia (25% w/w) (Riedel de HaeÈn, Seelze, Ger-

many). In order to avoid eluent contamination with

chloride (which always contains traces of bromide)

and potassium (which forms the plasma species40Ar39K�), the pH was adjusted without direct contact

of a glass electrode. This was done by analyzing the

pH of small samples (5 ml) of the eluent solution

(2.5 l). Ammonia was carefully added to the eluent

solution until the pH of a 5 ml sample was 6. Through-

out all investigations, the eluent ¯ow rate was 0.5 ml/

min and the injection volume was 885 ml in all cases.

Other conditions are as noted in the ®gure captions.

2.1.5. ICP-MS operating conditions

The ICP-MS operating conditions are given in

Table 1. The 79Br isotope was selected for detection

M. Nowak, A. Seubert / Analytica Chimica Acta 359 (1998) 193±204 195

because the plasma species 40Ar2H� interfered with

the isotope 81Br. Data were collected using single-ion

monitoring (SIM) to ensure maximum sensitivity for

bromine, taking 20 mg/l Sr as the internal standard. By

switching to m/z 88 at the beginning and the end of

each analysis, the 88Sr isotope was monitored in order

to correct for instrumental drift during analysis. Using

the operating conditions listed in Table 1, the back-

ground at m/z 79 was 100±150 counts/s whereas at m/z

81 it was 2000±3000 counts/s, respectively. Response

for directly nebulized 100 mg/l bromate solution was

found to be 11000 count/s when monitoring 79Br�.

The calculated sensitivity for bromate was 110 counts/

s per 1 mg/l bromate so that the detection limit for

bromate (3� criterion, ��25 counts/s) was 0.69 mg/l

without any relaunch effect of the separation column.

This effect, which is characteristic of ion-exchangers

and the reason for lower detection limits when using

an analytical column, will be discussed later. The

response for the internal standard Sr on m/z 88 was

found to be 200±220 kcounts/s under optimized con-

ditions.

2.1.6. Reagents

All standard and eluent solutions were made from

deionized water (MilliQ, Millipore, Eschborn, Ger-

many). Sodium bromate, sodium bromide and mono-

bromoacetic acid (Fluka, Buchs, Switzerland) were

used to prepare 1000 mg/l stock solutions, which were

stored at �48C. The standard containing monobro-

moacetic acid was prepared weekly from the recrys-

tallized solid. The stock solutions were diluted daily

for the preparation of working standards and spike

solutions. Eluents were ®ltered through a 0.45 mm

membrane ®lter (Macherey-Nagel, DuÈren, Germany)

and degassed in an ultrasonic bath. All solutions and

eluents were stored in polyethylene (PE) containers.

2.1.7. Sample pretreatment

Drinking and mineral water samples were ®ltered

through a 0.45 mm membrane ®lter (Macherey-Nagel,

DuÈren, Germany), degassed in an ultrasonic bath and

stored in PE-containers.

2.1.8. Properties of investigated water samples

Five mineral waters, a Hannover drinking water and

a Hannover indoor pool water were analyzed. A

summary of the properties of the investigated water

samples is shown in Table 2. In case analysis data

were not available from the manufacturer, the contents

were determined by ICP-AES (cations) and IC

(anions). The water samples were chosen with respect

Table 1

Optimized ICP-MS operating conditions for bromine measure-

ments

Forward power 1700 W

Nebulizer V-Groove

Plasma±Ar 15 l/min

Auxiliary±Ar 1 l/min

Nebulizer±Ar 1.1 l/min

Detection SIM m/z 79

Dwell time 0.653 s

Internal standard 20 mg/l Sr in the eluent

Table 2

Selected anion and cation contents of the investigated water samples. Data are specified by the manufacturer, except where noted. All

concentrations are in mg/l

Sample Na� K� Mg2� Ca2� Clÿ SO2ÿ4 NOÿ3 HCOÿ3

Drinking water 24 3 7 9 50 130 1 Ð c

Indoor pool water 64 a 5 a 7 a 97 a 152 b 161 b 18 b nd d

Muehlenquelle 280 6 51 555 300 1450 0.3 336

Apollinaris 380 30 110 90 100 90 3.6 b 1570

Harzer Grauhof 18 2 9 111 29 62 Ðc 300

Extaler 11 2 55 366 15 891 6 250

Pricca 52 a 6 a 62 a 648 a 300 b 670 b 10 b nd d

a ICP-AES.b IC.c Not specified.d Not determined.

196 M. Nowak, A. Seubert / Analytica Chimica Acta 359 (1998) 193±204

to their different contents of anions as well as cations.

The Hannover drinking water and the pool water had

moderate ion contents according to the German stan-

dards (Trinkwasserverordnung; TrinkwV) [24],

whereas mineral waters in general do not have to

meet the regulations of the TrinkwV. Therefore, the

ion contents (ionic strength) of the mineral waters

chosen were in some case one order of magnitude

higher than of the drinking water investigated.

2.1.9. Calibration and evaluation of detection limits

Calibration was carried out in the mineral water

`Muehlenquelle' because this water had the highest

ionic strength of all investigated waters. Spike con-

centrations (seven in number) in the 50±1000 ng/l

range were chosen and two replicates were prepared

at each concentration level. Spiked water samples

were analyzed within 15 min after preparation. Peak

height was used for quantization of bromate. Method

detection limits (MDL) in `Muehlenquelle' and other

water samples free of bromate were calculated, using

the sensitivity of the calibration in `Muehlenquelle'

and the standard deviation of the background noise

(3� criterion) at the retention time of bromate. The

bromate concentration of an indoor pool water was

determined by standard addition of bromate in the 2±

8 mg/l range. At each spike level, three replicates were

made. In this case, peak area was used for quanti®ca-

tion of bromate for reasons that will be discussed later.

2.1.10. Evaluation of the within-run imprecision

The evaluation of the within-run imprecision was

carried out under optimized chromatographic and

mass spectrometric conditions. A sample of `Mueh-

lenquelle' was spiked with 500 ng/l bromate and

successively analyzed ten times.

3. Results and discussion

3.1. Selection of chromatographic conditions for

on-line-coupling IC-ICP-MS

Common elution systems for the determination of

bromate in water samples using on-line coupled IC-

ICP-MS are based on NaOH in combination with

commercial low-capacity anion-exchangers [17,18].

NaOH is an extremely weak eluent, hence NaOH

concentrations in the 100±180 mmol/l range have to

be used when separation columns with exchange

capacities between 50 and 170 mmol are applied.

Therefore, several problems are caused by NaOH-

based elution systems. Even high-purity qualities of

NaOH always contain impurities of halide ions, espe-

cially chloride and bromide. The latter increases the

background level on m/z 79 during mass spectrometric

measurements. Using the chromatographic conditions

mentioned above, eluent ¯ow rates of 1±1.5 ml/min

are required to ensure acceptable retentions times for

bromate as well as for bromide. Conventional nebu-

lizers like V-Groove or Meinhard reach optimal neb-

ulization conditions at sample ¯ow rates of 1 ml/min

or less. Therefore, the eluent ¯ow rate should not

exceed 1 ml/min. Eluent ¯ow rates <1 ml/min should

lower background noise due to better nebulization.

Another disadvantage is that NaOH-based eluents

have to be suppressed chemically in order to avoid

large amounts of sodium reaching the ICP resulting in

a decreased plasma stability due to the large number of

easily ionizable atoms. Cationic impurities in the

regenerant, usually diluted sulphuric acid, such as

potassium or sodium can pass the ion-exchange mem-

brane of the suppressor and contaminate the eluent.

Chemical suppression of the eluent does not allow the

use of a cationic internal standard because metal

cations are removed during the suppression process.

For this reason, on-line couplings utilizing suppressed

NaOH eluents apply bromide solutions as internal

standard [18]. Using this technique an additional

switching valve is required. Alkaline elution systems

are not well suited for samples containing large quan-

tities of alkali earth metals which are present in most

mineral waters. Calcium or magnesium form insoluble

hydroxides and precipitate on the top of the analytical

column. This results in a slow contamination of the

column as well as in an increased back pressure. The

only way to solve this problem is to remove the alkali

earth cations prior to analysis by the use of H�-

cartridges.

The microbore column technique used in this study

allows eluent ¯ow rates up to 0.5 ml/min. Concerning

the exchange capacity of the column applied

(150 mmol), an elution system with a much higher

eluting power than NaOH had to be established. The

microbore technique offers the possibility of applying

a high-capacity anion-exchanger combined with mod-

M. Nowak, A. Seubert / Analytica Chimica Acta 359 (1998) 193±204 197

erate eluent concentrations and ¯ow rates. Hydrochlo-

ric and nitric acid were chosen as the base of a suitable

elution system. Both acids are available in the highest

purity grades. In this study, further puri®cation was

done by subboiling. Therefore, the contamination of

the eluent with bromide or potassium could be mini-

mized. Other mineral acids like sulphuric or perchlo-

ric acid do not show acceptable selectivities for

monovalent species �SO2ÿ4 � or form ion-pairs with

the exchange sites �ClOÿ4 � so that the dynamic

exchange capacity is lowered. Ammonia was chosen

instead of NaOH to adjust the pH of the eluent because

it was found to contain less chloride (and bromide)

impurities than NaOH. The pH of the elution system

was adjusted at 6 for two reasons. The ®rst is that

alkali earth metals do not precipitate as hydroxides at a

pH of 6. A removal of these ions prior to analysis is

therefore unnecessary. The second reason is that a

cationic internal standard can be used to correct

instrumental drift because there is no need for che-

mical suppression of the eluent. In this case, strontium

was applied as internal standard at a concentration of

20 mg/l.

An elution system based on NH4Cl (pH 6) turned

out to be too weak to separate bromate from bromide

within 15 min. Even at concentrations of 100 mmol/l

and an eluent ¯ow rate of 0.5 ml/min, retention times

were 18 min for bromate and 40 min for bromide,

respectively. Higher NH4Cl concentrations led to a

higher viscosity of the eluent resulting in increased

background noise. A better choice was NH4NO3. An

elution system for the determination of bromate via

IC-ICP-MS based on NH4NO3 has been reported

before but has not been applied to high-performance

anion-exchangers [20]. At a concentration of

100 mMol NH4NO3 (pH 6) and a ¯ow rate of

0.5 ml/min, bromate eluted after 3 min but the back-

ground level was found to be unstable due to inef®-

cient and pulsed nebulization. In this case, the

viscosity of the eluent also seemed to be the reason

for this behaviour. Therefore, the eluent concentration

was lowered to 60 mmol/l NH4NO3 (pH 6). This

elution system showed an excellent resolution of

bromate and bromide at a ¯ow rate of 0.5 ml/min.

The retention times were 5 min for bromate and 8 min

for bromide, respectively. All further studies were

carried out utilizing the optimized NH4NO3 elution

system.

The sample volume could be increased from 300 to

885 ml without a signi®cant loss of column ef®ciency

as well as resolution between bromate and other

potential interfering bromine species. Therefore,

885 ml was chosen as injection volume in order to

lower detection limits.

3.2. Separation of bromate from potential interfering

species

Chloride concentrations in drinking water are at

least three or four orders of magnitude higher than the

concentration of bromate. The determination of bro-

mate in drinking water utilizing anion-exchange chro-

matography in combination with conductivity or

spectrophotometric detection is, therefore, seriously

interfered with chloride. Using an element-speci®c

detection system the determination of bromate may be

interfered with species containing bromine. In this

case, bromide and monobromoacetic acid are poten-

tially interfering species. Bromide is usually present in

drinking waters at sub-ppm levels, monobromoacetic

acid is a disinfection by-product as well as bromate.

Fig. 1 shows a chromatogram of a sample containing

1 mg/l bromate beside 100 mg/l bromide and mono-

bromoacetic acid in deionized water. Both species are

well resolved from bromate but, unfortunately, there is

Fig. 1. Separation of bromate, bromide and monobromoacetic

acid. Chromatographic conditions as described in the text. For mass

spectrometric conditions see Table 1. Sample volume: 585 ml.

Concentrations: 1 mg/l bromate, 100 mg/l bromide and 100 mg/l

monobromoacetic acid.

198 M. Nowak, A. Seubert / Analytica Chimica Acta 359 (1998) 193±204

no selectivity between bromide and monobromoacetic

acid. This means a limitation when a quanti®cation of

all species is needed. A better separation of bromide

and monobromoacetic acid was observed at lower

eluent ¯ow rates where monobromoacetic acid eluted

slightly after bromide. With respect to the aim of this

study, no speciation is required and the eluent ¯ow rate

was adjusted to 0.5 ml/min in order to ensure a fast

determination of bromate.

3.3. Optimization of ICP-MS for detection of 79Br

Bromine has a high ®rst ionization energy (IE1) of

11.8 eV and is incompletely ionized in an Argon

plasma (IE1�15.75 eV) [25]. As a result, there is a

decrease of the detection limits by one or two orders of

magnitude compared to fully ionized elements.

Another problem of bromine detection is the inter-

ference of 81Br with the plasma species 40Ar21H�,

which disturbs monitoring at m/z 81. The 79Br isotope

is interfered with the shoulder of the 40Ar�2 signal,

whereas 40Ar39K� is usually separated from bromine

species during the chromatographic process.

Our attempt was to use the ICP-MS in a standard

con®guration in order to avoid the use of expensive

equipment like ultrasonic nebulizers or high perfor-

mance interfaces [18]. The optimization strategy was

divided into two steps. The ®rst step was to raise the

forward power of the ICP (normally 1350 W) resulting

in a better ionization of bromine. The second step was

to investigate the in¯uence of resolution (m/�m) on

the background level on m/z 79 and on the signal for

the internal standard Sr on m/z 88. Table 3 shows a

comparison of signal-to-noise (SNR) and signal-to-

background (SBR) ratios, respectively, for the eluent

itself and that spiked with 100 mg/l bromate at differ-

ent forward power outputs of the ICP, 1350 and

1700 W. The bromine isotope 79Br was monitored

and all solutions were analyzed directly without uti-

lizing an analytical column. Raising the forward

power, the eluent background level was increased as

was the background noise. The same behaviour was

observed for an eluent solution spiked with 100 mg/l

bromate. The SBR was found to be 115 at 1350 W and

68 at 1700 W, whereas the SNR was 82 at 1350 W and

96 at 1700 W. Considering the fact that the SNR has a

greater in¯uence on detection limits (3� criterion)

than the SBR, a forward power of 1700 W was chosen

for further investigations.

The in¯uence of resolution (m/�m) on the eluent

background level (m/z 79) and the signal of the inter-

nal standard Sr (m/z 88) is shown in Fig. 2. The

background signal obtained at m/z 79 is the sum of

instrumental background, molecular ions with m/z 79

(ArK�, etc.) and the shoulder of the dominant 40Ar�2peak at m/z 80. We used the m/z 88 as monitor for the

decrease of transmission. We assumed that 88Sr� will

show a similar behavior with regard to transmission as79Br�.

Our strategy was to increase resolution as long as

we get a better SBR. The end of the optimization is

reached when both the background (measured at m/z

79) and the analyte signal (measured at m/z 88)

decreased with the same slope. The decrease of inten-

sity on m/z 88 was ca. 20%, but �60% for the back-

ground level on m/z 79 when resolution was enhanced.

The reason for this enhancement in SBR ratio is the

better separation of Ar�2 from the bromine signal (m/z

79).

For further investigations, resolutions of 72.0 (m/z

79) and 79.2 (m/z 88) were chosen because an optimal

ratio between the eluent background level and the

Table 3

Comparision of signal-to-noise (SNR) and signal-to-background ratios (SBR) for monitoring m/z 79 at different forward power outputs of the

ICP. All solutions were analyzed directly without using an analytical column

Forward power 1350 W 1700 W

Eluent background 75 counts sÿ1 164 counts sÿ1

� eluent background noise 9 counts sÿ1 24 counts sÿ1

Eluent spiked with 100 mg/l bromate 8609 counts sÿ1 11 093 counts sÿ1

� noise 105 counts sÿ1 116 counts sÿ1

SBR 115 68

SNR 82 96

M. Nowak, A. Seubert / Analytica Chimica Acta 359 (1998) 193±204 199

intensity of the internal standard was found. It should

be noted that both resolutions belong together for a

given quadrupole adjustment.

3.4. Influence of sample composition on retention of

bromate and background level

While analysing the different water samples it was

found that the retention time of bromate as well as the

background level at m/z 79 were in¯uenced by the

sample composition. In contrast, the retention beha-

viour of bromide was not in¯uenced. Fig. 3(a) shows a

chromatogram of 200 ng/l bromate in deionized water.

The retention time for bromate was 4.75 min. The

background level as well as the background noise were

signi®cantly lower during the negative dead volume

peak (1±3 min) compared to the following part of the

chromatogram. This de¯ection of the background

noise was not observed on m/z 81. Therefore, the

eluent seemed to be contaminated with bromide as

well as potassium (forming 40Ar39K�) and sodium

(forming 40Ar23Na16O�), resulting from impurities in

nitric acid and ammonia. In comparison Fig. 3(b)

shows a chromatogram of `Muehlenquelle' spiked

with 200 ng/l bromate. The bromate concentrations

in `Muehlenquelle' and the other mineral waters

investigated were below the method detection limit

(MDL). `Muehlenquelle' contained the highest anio-

nic matrix of all waters investigated (Table 2). It was

found that bromate was shifted to longer retention

times (5.75 min). Because the detection system is

element-speci®c, there was no dif®culty in reliably

identifying bromate despite the shift of retention time.

Moreover, no relevant bromine containing anionic

species coelutimg with bromate is known to us.

Matrix anions normally act as eluents and, there-

fore, the retention time for bromate should decrease.

The positive injection peak is caused by unretained

cations which form plasma species like 40Ar39K� or40Ar23Na16O�. Compared to Fig. 3(a), the back-

ground level as well as background noise are

decreased until the retention time of bromate. As a

result the SNR is increased. The explanation for this

behaviour is as follows. Before sample loading, the

analytical column is equilibrated with the eluent so

that nitrate is placed on almost all exchange sites.

During the sampling process the nitrate on the top of

the column is removed by matrix anions such as

sulphate or chloride. At the beginning of the elution,

nitrate is bound to the exchange sites which causes a

removal of sulphate and chloride. These anions have a

signi®cantly lower elution power for bromate than for

nitrate. As a consequence the retention time of bro-

mate increases. The better peak shape for bromate in

the mineral water `Muehlenquelle' can be explained in

a similar way. During the transfer of the bromate and

the matrix anions from the sample loop to the top of

the analytical column, the top of the column is mainly

Fig. 2. Influence of resolution (m/�m) on the intensity at m/z 88 (20 mg/l Sr) and on the eluent background level at m/z 79.

200 M. Nowak, A. Seubert / Analytica Chimica Acta 359 (1998) 193±204

converted into a Clÿ- or SO2ÿ4 -form. Because the

anions have a much lower af®nity to quarternary

ammomium functional groups than nitrate, the bro-

mate is compressed into a smaller zone at the column

top than it would have been when the column top was

in the NOÿ3 -form (relaunch effect). Therefore, the

resulting bromate peak in mineral water has a much

better peak shape than in deionized water, because in

this case the column top was in the NOÿ3 -form.

Nitrate is a strong eluting anion for bromate, but

weaker for bromide. Therefore, the bromide in water

samples is retained on the top of the column whereas

sulphate, chloride and bromate are eluted by nitrate.

The eluent background and noise on m/z 79 is lowered

during the elution of these anions. After the elution of

bromide the column is re-equilibrated with nitrate, and

the background level as well as noise are increased due

to contaminations of the eluent. According to this

explanation, the retention behaviour of bromide was

found to be independent of the sample composition.

3.5. Method detection limits for bromate in mineral

and drinking waters

The calibration parameters for the calibration in the

mineral water `Muehlenquelle' are given in Table 4.

Peak height was chosen for quanti®cation because the

MDL were de®ned using the SNR. The noise of the

baseline was easier to determine than the noise of an

area at the retention time of bromate, especially when

measuring the blank solution (unspiked mineral

water). The MDL for bromate in `Muehlenquelle'

was 50 ng/l according to an absolute detection limit

of 44 pg bromate. The MDL using a separation col-

umn was over one order of magnitude lower compared

to that obtained when analyzing a bromate solution

with direct nebulization (0.69 mg/l). The lower MDL

were the result of the relaunch effect, which is char-

acteristic for ion-exchangers [26]. The bromate and

the matrix anions are preconcentrated from the 885 ml

sample volume at the top of the analytical column

when the sample is transferred from the sample loop to

the separation column. This enrichment step results in

lower detection limits compared to direct nebulization

of a solution with the same bromate content. The

bromate concentrations in the other mineral and drink-

ing waters investigated were below the MDL. The

MDL for bromate in these water samples (50±65 ng/l

bromate or 44 to 58 pg) were dependent on the back-

ground noise and decreased with increasing anionic

matrix. The sensitivity of the IC-ICP-MS coupling

described is proved by Fig. 4 which shows calibration

chromatograms of `Muehlenquelle' itself and that

spiked with 100 ng/l bromate.

Fig. 3. Influence of sample composition on retention times for bromate and on signal-to-noise (SNR) and signal-to-background ratios (SBR).

Chromatogram (a): 200 ng/l bromate in deionized water; chromatogram (b): `Muehlenquelle' spiked with 200 ng/l bromate. Chromatographic

conditions as described in the text. For mass spectrometric conditions see Table 1.

M. Nowak, A. Seubert / Analytica Chimica Acta 359 (1998) 193±204 201

3.6. Determination of bromate in an indoor pool

water

Only one sample investigated contained bromate at

concentration levels >65 ng/l. This was a pool water

sample made from Hannover drinking water (Table 2)

by means of disinfection via chlorination and ozoni-

zation. Fig. 5 shows the overlayed chromatograms of

the indoor pool water and the original drinking water.

Unfortunately, the bromate peak is split due to matrix

effects which are not completely understood yet. The

bromate peak is not the result of an interfering species

containing bromide. A diluted sample of the pool

water (1 : 4 w/w with deionized water) gave a single,

well-shaped peak. When spiking the pool water with

bromate the peak-height-ratio of both peaks was found

to be constant, and calibration plots were linear for

evaluating peak area and even for peak height. The

standard addition method was chosen for quantization

of bromate because of the split bromate peak. For the

same reason, the evaluation of peak area was used

instead of peak height. The calibration parameters are

given in Table 5. It is interesting to note that the

pool water was almost free from bromide com-

pared to the original drinking water. This means that

nearly all bromide in the original water must have

been converted to bromate during the disinfection

process.

Table 4

Parameters for the calibration in `Muehlenquelle'. Peak height was used for quantification. The method detection limit (MDL) was calculated

using the threefold standard deviation of background noise at the retention time of bromate (3� criterion). The limit of quantification (LOQ)

was calculated utilising the 10� criterion

Parameter Result

Calibration equation y�0.6515 counts sÿ1/ng lÿ1x

� slope 0.028 counts sÿ1/ng lÿ1

Number of data points 8 (2 replicates each)

Correlation coefficient 0.995

� noise 11 counts sÿ1

Method detection limit [MDL] (3�) 50 ng/l

Limit of quantification [LOQ] (10�) 170 ng/l

Fig. 4. Calibration chromatograms. Chromatogram (a): `Muehlen-

quelle' spiked with 100 ng/l bromate; Chromatogram (b): `Mueh-

lenquelle' without spike. Chromatographic conditions as described

in the text. For mass spectrometric conditions see Table 1.

Fig. 5. Determination of bromate in a Hannover indoor pool water.

Chromatogram (a): Indoor pool water, bromate concentration

1.5 mg/l; Chromatogram (b): Original Hannover drinking water.

Chromatographic conditions as described in the text. For mass

spectrometric conditions see Table 1.

202 M. Nowak, A. Seubert / Analytica Chimica Acta 359 (1998) 193±204

3.7. Evaluation of within-run imprecision

Table 6 shows the results of the within-run impreci-

sion experiments at a concentration level of 500 ng/l

bromate in `Muehlenquelle'. The imprecision was

5.4% for quantization of peak height and 4.4% for

quanti®cation of peak area, whereas retention times

for bromate showed an imprecision of only 0.2%.

Therefore, the method imprecision was mainly caused

by the ICP-MS. Imprecisions in the range of 5% are

usually found in ultra trace determination of metals

using ICP-MS as long as no sophisticated equipment is

utilized [27].

4. Conclusions

The coupling of microbore ion chromatography

utilizing high-capacity and high-performance anion

exchangers with ICP-MS detection allows the deter-

mination of bromate in drinking or related water

samples at concentration levels ranging from 0.05±

10 mg/l. No sample pretreatment such as matrix elim-

ination or trace enrichment is required. Retention

behaviour of bromate as well as SNRs and SBRs

are in¯uenced by the sample composition. None of

the investigated drinking and mineral water samples

contained bromate at concentration levels ca. 65 ng/l

with the exception of a Hannover indoor pool water.

The method detection limits for bromate in the water

samples investigated were in the 50±65 ng/l according

to absolute detection limits of 44±58 pg bromate. The

method shows a within-run imprecision of 5% at a

bromate concentration level of 500 ng/l. The time

spent on complete analysis is 10±15 min depending

on the bromide content of the sample. The presented

on-line coupling therefore meets the requirements for

a routine method and best ful®lls the recommenda-

tions of the Commission of the European Commu-

nities as well as the recommendations for the future

DIN/ISO 15601.

Acknowledgements

The authors would like to thank Dr. Helwig SchaÈfer

of Metrohm AG, Switzerland, for providing the IC

equipment. This study was ®nancially supported by

the Bundesministerium fuÈr Bildung, Wissenschaft,

Forschung und Technologie (BMBF) and the Fonds

der Chemischen Industrie (FCI).

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Table 6

Statistical results of within-run imprecision experiments for `Muehlenquelle' mineral water spiked with 500 ng/L bromate

n Peak height/ (arbitrary units) Peak area/ (arbitrary units) Retention time/ (min0

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