Multicommutation as a powerful new analytical tool

Multicommutation as a powerful new analyticaltoolM. Catala IcardoDepartment of Analytical Chemistry, University of Valencia, Valencia, Spain

J. V. Garcıa MateoDepartment of Chemistry Sciences, University Cardenal Herrera, Valencia, Spain

J. Martınez Calatayud*Department of Analytical Chemistry, University of Valencia, Valencia, Spain

This review presents the state of the art of the

emerging continuous-flow methodology based on

solenoid valves. This uses flow networks to deliver

sample and reagent solutions by controlling the time

of flow through the ON/OFF modes of solenoid

valves and takes advantage of existing flow injection

analysis (FIA) or sequential injection analysis (SIA)

device or manifold configurations. It allows one to

insert a single plug of sample (or reagent) into the

carrier or carrier-reagent stream, mimicking the

approaches of FIA or SIA. In addition to the modes

used in FIA and SIA, the methodology provides a

different mode, based on delivery of a series of

alternating sequential insertions of very small

volumes of sample and reagent. This gives rise to

different hydrodynamic and chemical processes and

new analytical potential. The fundamentals of the

methodology are discussed together with a broad

view and critical survey of the advantages and the

possible trends. # 2002 Published by Elsevier

Science B.V. All rights reserved.

Keywords: Continuous flow; Multicommutation; Tandem flow

1. Introduction

Flow assemblies comprise a variable numberof lines through which the solutions, sampleand reagents are propelled on their way to the

detector. Especially prominent among flowtechniques is flow injection analysis (FIA). Itsinception over 20 years ago brought about dra-matic changes in the way a solution could behandled and dispersion controlled. Its progresshas rested on the development of ingeniousassemblies and devices that allow the repro-ducible insertion of the sample solution into aflowing system.Over time, such devices have evolved from

the hypodermic syringe used by Ruzicka andHansen in the early days of FIA, to the pro-portional injector, to the six-port rotary valve,which is currently the most widely used injec-tion system in FIA. With the six-port rotaryvalve, the sample is loaded and inserted bypushing a mobile element between two fixedpositions, the volume of sample being deter-mined by the length of the sample loop and theinner volume of the rotary valve. In the emer-ging multicommutation mode, the volumes ofsample and other solutions are controlled by thetime during which the stream, flowing at aknown rate, is allowed to circulate, so thatuncertainty in the sample volume dependsmainly on the precision with which the time canbe controlled. With electronic timing devices,the associated error is minimal.The insertion of pre-set volumes of sample by

controlling time was achieved by early FIAresearchers. However, multicommutationmethodology uses solenoid valves as differentialelements and replaces insertion volumes with

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366 trends in analytical chemistry, vol. 21, no. 5, 2002

*Corresponding author. Tel./Fax: +34 96 386 40 62.E-mail: [email protected]

insertion times, thereby expanding the scope oftime-based sampling methods. One otherimmediate result of using solenoid valves is thatthe ensuing assemblies are highly flexible andeasy to alter. In multicommutation assembliesusing three-way solenoid valves, a flow network,comprising a variable number of valves, eachacting as an independent commutator, is estab-lished and is controlled by computer. Such anetwork can be assimilated to an electronic cir-cuit with a variable number of active nodescapable of individually adopting two differentstates: ON and OFF. This permits sample andreagent dispersion to be effectively controlledand opens up new avenues for development inflow analysis. The main foundation of this newmethodology is the use of the solenoid valve(Fig. 1).Solenoid valves have been used previously in

analytical chemistry as ancillary components forvarious purposes involving the handling ofsolutions and gases. Such valves have been usedfrequently in chromatography, for example, forthe insertion of sample and solvents and fordeveloping empirical approaches to mixture-resolution in countercurrent chromatography.Solenoid valves have also been used in gaschromatography, as components of samplemicro-dispensers in synthetic processes, or to

control the inflow and outflow of gas. Themany other applications include the introduc-tion of solutions for determination by massspectrometry, or controlling the flow in refrac-tive-index determinations.In this review, we describe the development,

modes, and uses of a new flow methodologythat uses a manifold including several solenoidvalves, which allow the operator to handle thesolutions involved in the analytical process(i.e., a flow network). The Brazilian groupat Piracicaba, led by B.F. Reis B. F. andE.A.G. Zagatto, who pioneered the develop-ment, has so far been the most active in itsapplication.The emerging flow methodology has been

used by FIA workers as a matter of preference.Its earliest precursor was probably the useof a solenoid valve integrated into an FIAassembly as an ancillary component for specificpurposes (usually, controlling the sample-insertion system). In some cases, a solenoidrather than a valve was used to actuate a pistonvalve (a syringe) [1].There have been several attempts at integrat-

ing one or more valves in FIA assemblies;however, all are much more recent than thosedescribed above. Examples include: the use of asolenoid valve to avoid oscillations in the flowduring sample insertion [2]; the use of a two-way valve as an alternative to other time-basedinjection systems [3]; in the determination ofmagnesium and of copper traces in water sam-ples [4], a series of valves was employed toinsert solid samples into an FIA assembly.Recently, an FIA-related technique calledsequential injection analysis (SIA) was used witha set of three solenoid valves as an interfacebetween a microwave oven for sample digestionand a conventional assembly for the determina-tion of iron [5].

2. The insertion profile in multi-commutation

Three-way solenoid valves allow the design ofactive networks for precise control of sampleFig. 1. Working cycle of a three-way solenoid valve.

trends in analytical chemistry, vol. 21, no. 5, 2002 367

and reagent dispersion, and hence the develop-ment of gradient-based methods that are diffi-cult to implement in FIA.A three-way solenoid valve behaves as a

switch between two states: ON and OFF(Fig. 2). Two of the three valve ports are per-manently connected. While the valve is OFF,the carrier solution is aspirated into the detector;when ON, an electronic pulse of programmablelength allows the sample to be inserted into thecarrier. The volume of sample inserted is pro-portional to the pulse length and can be alteredby changing the profile of the insertionsequence; a single sample segment, or severalsegments of the same or different length, can beintercalated with the carrier solution. The resultis a flexible system that allows the insertion ofvolumes of sample that are variable via soft-ware.In addition, electronic control of the pulse

length ensures reproducible insertion of samplevolumes as low as a few microlitres. Theamount of sample used is that aspirated duringthe time the valve is ON; no flushing, andhence no additional consumption of sample, isrequired. The use of small sample segmentssandwiched between carrier microsegmentsfacilitates thorough mixing of the sample andcarrier solutions, even with large injectedvolumes—which simply require more micro-injections of sample. Finally, solenoid valves areeasily automated.

3. Advantages and disadvantages ofmulticommutation versus ‘‘classical’’FIA

Multicommutation is not simply a more con-venient, effective alternative to classical meth-ods for the insertion of sample and reagentsolutions into a carrier stream. The use of smallnetworks, with nodes consisting of solenoidvalves acting as switchers and controlled viastraightforward software, has dramaticallyexpanded the analytical potential of continuous-flow analysis, which has been enormouslyboosted by FIA methodology in recent years.Multicommutation provides some valuable

advantages, including the following:

(a) Miniaturization of flow assemblies. The smallsize of solenoid valves and electronicinterfaces permits the development ofcompact, integrated systems and of por-table equipment for on-site analyses.

(b) Reduced sample and reagent consumption. Sim-ple multicommutation configurations (seeSection 4.2.) allow the sample andreagents to be micro-dispensed very pre-cisely; volumes of a few microlitres,which correspond to insertion times offractions of a second, can thus be inser-ted very precisely.

(c) Increased reproducibility. Solenoid valvesrequire minimal operator intervention.

Fig. 2. Time-controlled insertion profiles using solenoid valves.


The insertion process (namely, the ON–OFF cycles in each valve of the flowassembly) can be controlled via computersoftware. In this way, multicommutationfacilitates the development of fully auto-matic analytical methods.

(d ) Economy and simplicity. Because solenoidvalves can be switched on and off by asimple electrical pulse (usually 12 V and100 mA), no additional power supply isneeded; a card inserted into the compu-ter’s motherboard governs the wholesystem. Finally, their operation can becontrolled via user-friendly, flexible soft-ware developed in common program-ming languages, such as QuickBasic orTurboPascal.

(e) Flexibility. Multicommutation allowsreactor lengths, inserted sample volumes,and ultimately any variable influencingdispersion profiles, to be changed readilywithout the need physically to alterassemblies. Changes can be made simplyby resetting the duration of the electricalpulses that switch the valves on and off,or by altering their commutationsequence.

( f ) Expanded possibilities for flow analysis. The‘‘electronic control’’ of dispersion, whichresults in more efficient spatial and tem-poral control of dispersion than in FIA,and the increased flexibility of multi-commutation have enabled the reliableimplementation of gradient modes (stop-ped flow, merging zones, zone penetra-tion, zone sampling), automated methodsfor multi-parameter determinations, andcomplex multichannel systems that canbe operated in a simple, effective manner.

However, multicommutation is subject to thefollowing shortcomings:

(a) Restrictions in inserted volumes. With verysmall sample segments and reagent seg-ments, the operation of the propulsionsystem must be synchronized with themicro-insertions; otherwise, pump pulses

introduce irreproducible distortions in thedispersion profiles.

(b) The need to aspirate the sample and reagents.Because three-way valves act as switches,one of their two inflow or outflow posi-tions must invariably be OFF, and circu-lation through the corresponding channelhalted. This requires placing the propul-sion unit behind the valves (although notalways) and aspirating, rather thanpumping, the sample and reagents (seeSection 5).

(c) Limited commercial availability. The scarcityof commercially available equipment(mainly electronic interfaces and soft-ware) for controlling solenoid valvesis reflected in the fact that virtuallyall the reported applications of multi-commutation use laboratory-made hard-ware and software.

4. FIA modes and multicommutation

The flexibility of multicommutation hasresulted in increased simplicity and efficiency incontrolling sample and reagent dispersion rela-tive to other flow methods. In this respect, it isof interest to consider some basic multi-commutation configurations.

4.1. Insertion of discrete solution volumesand mimicking insertion in FIA

The flexibility of multicommutation arises inpart from the fact that it enables the faithfulreproduction of the injection mechanism of thesix-port rotary valve typically used in FIA(Fig. 3). In the loading position, the carriersolution and the sample are circulated throughdifferent channels; the sample volume to beinjected is that contained in the length, L, of thesample loop connecting valves, V2 and V3. Theloop can be filled by using a second propulsionunit (for example, a solenoid pump actuatedautomatically during the loading process toprovide an intermittent, pulsating flow). Duringsample insertion, simultaneous switching of the


three solenoid valves allows the carrier solutionto sweep the sample to the detector.

4.2. Multiparameter determinations basedon efficient discrimination of the sample inthe carrier or carrier-reagent stream

If, as with basic circuits in electronics, a basicor standard configuration were to exist formulticommutation, it would be a serial or par-allel array of solenoid valves acting as sampleand reagent micro-dispensers. The procedure bywhich aliquots of sample and reagents are

sequentially inserted into the sole channel wherechemical reactions are effected in the system –the channel being connected to the propulsion(aspiration) unit – is referred to as binary sam-pling. Fig. 4 shows the assemblies involved and atypical insertion profile.In both systems, valve V1 is intended to aspi-

rate samples and standards, flush the system,and remove any bubbles formed in switchingbetween different solutions. In the serialarrangement, once the system has been filledwith sample solution, each reagent is insertedconsecutively. Reactors L1–L3 provide spatial

Fig. 4. Basic manifolds in multicommutation: C=carrier; S=sample; Rn=reagent; L=Reactor; W=waste; P=pump.

Fig. 3. Multicommutation reproducing the insertion of flow injection analysis (FIA): W=waste; Vn=solenoid valve; P=peristalticor solenoid pump; S=sample; D=detector; C=carrier; T=connector; L=reactor.


discrimination of the different sample–reagent‘‘sandwiches’’. In the parallel arrangement, oneof the inlets of valves V2–V5 is shut, so thereagents must be inserted at intervals, usingsegments of the carrier solution in between. Theinsertion sequence must be programmed insuch a way as to ensure that each solution in thesystem will circulate under constant conditions,unaffected by the other valves in the circuit.The configurations described above immedi-

ately provide apparent advantages, as follows.Sample and reagent consumption are minimal,and the ability to control dispersion individuallyin each dispensed solution permits the determi-nation of different parameters in the samesample, with no overlap between peaks (in the-ory, one peak per inserted reagent). In an FIAassembly, one would have to use a very longreactor to avoid overlap of dispersion profiles inorder to discriminate between peaks.

4.3. Optimization of concentration profiles:flow titrations

One of the most important features of multi-commutation is the high efficiency with whichsolution profiles or concentration gradients canbe established. This facilitates on-line dilutionby up to several orders of magnitude, withoutthe need to use mixing chambers or long coils.The effect can be accomplished by changing theinsertion profiles and sequence via software to‘‘simulate’’ an alteration in the inserted volumesand reactor lengths. One excellent example ofdispersion control and the generation of a lacarte gradients over a wide concentration rangeis provided by pseudo flow titrations. Multi-commutation allows accurate knowledge of the

sample and titrant volumes at any time duringthe process. This is so, provided the volume isproportional to the Fig. 5 insertion time, whichcan be controlled very precisely and altered atwill over a wide range via software for pro-gramming the insertion sequence and time. Thesimplest way of achieving these goals is by usingbinary sampling (see Section 4.2.), where thesample and titrant solutions are dispensed invariable proportions.Fig. 5 illustrates a typical insertion sequence

for a multicommutated titration. In an FIAassembly, this entails using very long or widecoils or mixing chambers capable of providing ahigh dispersion. Thus, reaching the end-pointand completing the titration takes a long time,as a result of the high dispersion on which themethod relies. Also, determining the analyteconcentration occasionally entails the processingof non-linear curves.

4.4. Mimicking SIA

One other example of the high potential ofmulticommutation with solenoid valves is theability to mimic other, more recent, flow tech-niques intended to improve the performance ofunsegmented flow analysis. Such is the case withSIA, which uses a switching valve instead of thetypical injection valve. In this method, accu-rately measured volumes of carrier, sample, andreagents are aspirated by means of a pump thatallows periodic alteration of the flow direction.As a result, inserted solutions lie next to oneanother in the coil. A similar insertion profile,and the ability to reverse the flow directionwhen the sample and reagent reach the detectorflow-cell, can be Fig. 6 accomplished by using

Fig. 5. Insertion sequence for a multicommutation titration.


several solenoid valves, arranged as shown inFig. 6.The insertion of adjacent segments can be

achieved by using the binary sampling approachat some point in the assembly; once the mixturereaches the detector, the flow can be reversedby repeating the sequence depicted in Fig. 6 asmany times as required, without the need tomanipulate the propulsion unit.

5. Analytical applications

The analytical uses of multicommutation aresummarized in two tables below. Table 1 givesthe results obtained in quantitative determina-tions of analytes. Table 1 is divided into sectorsshowing groups of analytes of a similar nature,and the analytical features of the determinationprocedure used with each. Not all applicationsinvolved the determination of a single analyte;thus, Rocks [6] developed an assembly forquantifying several species in clinical samplesusing a multi-analyser to subject samples to aseries of tests.The flexibility of multicommutation allows the

use of flow networks provided with a relativelyhigh number of solenoid valves. This has sim-plified the multi-parameter determination with asingle sample insertion; nitrite, nitrate andammonium can be determined spectro-photometrically using an eight-solenoid valvemanifold [7].More than half the work dealing with multi-

commutation employs flow networks, such asthat depicted in Fig. 4, although sometimes withslight changes.

Plant materials and water have received mostattention, with between 20% and 30%, respec-tively, of the published papers. Most of thework dealing with plant materials is by theresearch group active at Piracicaba, Brazil.Multicommutation allows the use of all kinds

of optical and electrochemical detectors. Morethan half the work involves the use of UV-visi-ble spectroscopy. Also remarkable is the largeproportion (about 20%) of work that employsatomic absorption detection. Although aspira-tion is more popular than propulsion, the‘‘problem’’ caused by placing the pump prior tothe network of valves can be circumvented byconnecting several solenoid valves, acting insome cases as a by-pass, and even by pumpingthe solution to a reservoir from which it isaspirated. These strategies have been employedin cases where the type of detector makesimpossible the use of the aspirating mode (forexample, AAS, ICP-MS, ICP-AES) [8–10].From the data in Table 1, it follows that

reproducibility, sensitivity, limit of detection,and selectivity are (obviously) similar to those ofFIA. These features depend more markedly onthe type of detector and chemistry used than onthe type of assembly employed to handle sam-ples and reagents. However, the reproducibilityis more sensitive to differences in handling pro-cedures. In any case, multicommutation meth-ods are as reliable as their FIA counterparts.Table 2 shows the figures of merit of spec-

trophotometric and potentiometric multi-commutated titrations. The assemblies for theimplementation of titrations in this techniqueare not always flow networks of the same typeas those used in determination of analytes; in

Fig. 6. Multicommutation reproducing the sequential injection analysis (SIA) approach: C=carrier; S=sample; Rn=reagent;L=Reactor; W=waste; P=pump; D=detector; Vn=solenoid valve.


Table 1Analytical determinations in the multicommutation methodology

Analyte Sample matrix Detector Linear range (LOD) RSD,% Sample throughput,h�1 /Samplevolume, ml

Number ofthree-waysolenoidvalves

Ref.

1. Metallic ionsAg/Cd/Hg/Pb/Tl Dogfish muscle;

bovine liver; cornbran; rice flour

ET ICP MS From 0.15 to 41 ng/g, for differentsamples and preparation procedures

5 [16]

As Beer; tomato;mussel; liver paste

ICP AES 10 x detection limit to 100 ppm(beer, 0.5 ng/g; tomato, 2 ng/g;mussel, 50 ng/g; liver, 5 ng/g)

Beer 8;Tomato 11;mussel 9;liver 4

[17]

As(III)/As(V) Potable water PSVI t=20s up to 60ppb; t=5s up to350ppb; t=1s > 300ppb(t=80s 0.5–0.2 ppb)

2.2 (n=8) 4 [18]

Bi(III)/Pb(II) Steel; Al foil ET AAS 1–5 ppb (Bi, 0.25 ppb/Pb, 0.21 ppb) 4–19 �/20 Bi 6; Pb 7 [8]

Ca Water; plantmaterials; milk;pharmaceuticalformulations;fertilizer; rocks

S (575 nm) 0.25–1000 ppm (7 ppb) 0.83 (n=10) 42–68/10–500 5–6 [19]

Ca/K Plant materials AAS/AES Ca, 0–100 ppm; K, 0–10 ppm 1 (100 ppm) Ca 50; K 70 Ca 5; K 4 [9]

Ca/Mn/Zn Plant materials ICP AES [20]

Cd/Pb/Ni Water (bottled,mineral, tap, sea)

ET AAS (Cd, 2.2 ng/l; Pb, 23 ng/l;Pb, 75 ng/l)

Cd <5;Pb <4;Ni <6(n=10)

30/3000 4 [10]

Co Blood ET AAS Up to 50ppb (0.3 ppb) 2.6–3.1(n=10)

[21]

Cr(VI)/NH4+ Water S (Cr: 545 nm) Cr(VI) 0.5–3 ppm (10 ppb);

NH4+ 0.3–15 ppm (7 ppb)

[22]

Cu Plant materials;Foods

AAS (1 ppb) 3 (n=10,21.75 ppb)

48 [23]

Cu Water AAS 2–65 ppb (1.5 ppb) 0.6–2.5 (n=14,4.2–18.8 ppb)

8000 [4]

Cu/Cd/Pb/Bi/Se(IV) Water ET ICP MS 63Cu-65Cu 0.075–0.6 ppb (5–4 ng/l)111Cd 0.025–0.15 ppb (0.2 ng/l)208Pb 0.0125–0.1 ppb (0.3 ng/l)209Bi 0.0025–0.02 ppb (0.06 ng/l)77Se(IV) 0.0125–0.4 ppb (5 ng/l)

>10 (n=6) 5 [24]

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Table 1 (continued)



Ref.

Cu/Zn Plant materials S(620 nm) Cu 0–1 ppm (0.05 ppm);Zn 0–2 ppm (0.04 ppm)

Cu 0.7;Zn 1.7

45/300 5 [25]

Fe Plant materials S (480 nm) 0–10 ppm 0.8 (n=10,6.8 ppm)

220 4 [26]

Fe/Al Plant materials S (540 nm) Fe up to 12ppm (0.2 ppm);Al up to 15 ppm (0.5 ppm)

Fe 1.2;Al 1.8 (n=8)

60 6 [27]

Mn2+ Plant materials S (548 nm) 2.5–40 ppm (1.2 ppm) 0.27 (n=9,17.1 ppm)

50 4 [28]

Ni/Fe/Cr Steel alloys S (Ni: 460 nm,Fe–Cr: 526 nm)

Ni 5–50 ppm; Fe 25–200 ppm;Cr 20–60 ppm

1 Ni 60; Fe–Cr 130 Ni 4;Fe–Cr 5

[29]

2. Inorganic anions

Chlorine Bleach; tablets;water

S (445 nm) 30s preconcentration: 0.05–1.30 ppm(0.05 ppm); 120 s preconcentration:0.02–0.43 ppm (0.43 ppm)

30s,1.5 (n=42,0.72 ppm);120s, 1.8 (n=33,0.23 ppm)

30s,38; 120s, 20 3 [30]

Chloride Parenteral solutions S (480 nm) Up to 10 000 ppm 1.8 (n=5) 5 [31]

Chloride Water; soft drink ISE 10�2–10�4 M 2 (n=10) 2 (+15*) [32]

I2 Water; urine S (464 nm) Up to 10 ppm, b**= 10cm;Up to 25 ppm, b= 5cm;Up to 300 ppm, b= 1 cm

3 [33]

NH3/methylamines Water; air C Methylamines, up to 100 nM;NH3, up to 1000 nM

< 8 [34]

NH4+/PO4

3� Plant materials S (660 nm) N, 25–125 ppm; P, 2.5–12.5 ppm NH4+, 2 (n=8,

73.1 ppm N);PO4

3�, 1.5 (n=8,8.3 ppm P)

80 6 [35]

NO2�/NO3

� Lake and fountainwater; fertilizers;sausage; soils

S (328 nm) NO2�: 0–1.45�10�4 or 0.85�10�4 M

(3.3�10�7 or 1.9�10�7 M);NO3

�: 0–1.45�10�4 M (3.3�10�7 M)

0.4 (n=15,6.4�10�5 M NO3

�)15/1400 1 [36]

NO2�/NO3

�/NH4+ River water S 0.025–1.0 ppm NO2

� (5 ppb);0.10–5.0 ppm NO3

� (15 ppb);0.1–2.0 ppm NH4

+ (25 ppb)

NO2�, 0.32;

NO3�, 0.42;

NH4+, 0.72 (n=20)

60 8 [7]

Phosphorus Water S (635 nm) Up to 2 ppm (0.05 ppm) 2 [37]

SO42� Plant materials T (410 nm) Up to 500 ppm 2 (n=11) 100 / 54

(100–500 ppm)�94 (0–150 ppm)

5 [38]

(continued)

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Table 1 (continued)



Ref.

Chemical OxygenDemand

Water Automatedmicro batchanalyzer

(10ppm) 13/460 5+3* [39]

3. Organic compounds

Amiloride.HCl Pharmaceuticalformulations

S (545 nm) Up to 120 ppm (1 ppm) 2.2 (n=10) 30 2+3** [40]

Carbaryl – S (596 nm) (26 ppb) 0.5 (n=8,5.8 ppm)

70/50 5 [41]

Cephalexin Pharmaceuticalformulations

S(262 nm) Up to 1250 ppm (1.57 ppm) 0.66–2.1 180 2 [42]

Creatinine Urine S 2.9 (n=10,1.26 g/l)

24 3 [43]

Ethanol Alcoholic beverages S (600 nm) 10–50 1.6 (n=11,42.1%)

40/18–296 [44]

Glucose Sugar-cane juice;Soft drinks

S (510 nm) 0.05–0.2% (w/v) 0.3 (n=6,0.1%)

30 4 [45]

Hypoxanthine Sardine; Grunt CH 1 M-3mM 3 [46]

Phenol Water CH (5 ppb) 12–60(withoutpreconcentration)

[47]

Pindolol Pharmaceuticalformulations

S (635 nm) 5–120ppm 1.1 (n=10) 30 5 [48]

p-nitrophenol (1) /salicylic acid (2) /Ibuprofen (3)

S (1), 399nm;(2), 237nm;(3), 296 nm

(1), 0.1–1 mM;(2), 0.1–3 mM;(3), 0.1–3 mM

2–14 1–1* [49]

* Two-way solenoid valve** Path-lengths of the flow cell.S: Spectrophotometer; PSVI: Portable Stripping Voltammetric Instrument; ET ICP MS: Electrothermal-vaporization inductively coupled plasma mass spectrometry; ETAAS: Electrothermal atomic absorption spectrometry; ICP AES: inductively coupled plasma atomic emission spectroscopy; C: Conductimetry; CH: Chemiluminescence;T: Turbidimetry; ISE: Ion-selective electrode

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fact, these insert the sample and reagent in pro-portions variable according to the time thevalves remain ON. The procedure for deter-mining calcium is illustrates this point: in thefirst cycle, 70% of the ON interval is used toinsert the reagent and 30% the sample: in thesecond cycle, both are inserted for identicalperiods: in the third cycle, the proportions ofthe first cycle are inverted. By using a large-enough number of intervals, a flow gradientranging from 100% reagent to 100% sample canbe obtained [11]. The titration efficiency is afunction of the efficiency of sample-reagentmixing in the resulting flow. In this respect, itshould be noted that only one of the five titra-tions reported in four specific papers [12] used amixing-reaction chamber; the others used apiece of coil as reactor and several pumpingpoints to examine the effect of altering the flowrate and its stability. A similar assembly wasused to determine the acidity of vinegar [13]. Inany case, a single pump suffices in practice, evenwhen several channels are used. De Andrade,Poppy and Cascione [14] used a three-valvemodule to titrate HCl with NaOH in themonosegmented flow mode (that is, with abubble behind the sample), using spectro-photometric detection.

6. Trends and conclusions

First of all, users should agree on a commondesignation for a methodology that has so farbeen referred to as a ‘‘multicommutation flowsystem’’, ‘‘multicommutation approach’’, ‘‘bin-ary sampling’’, ‘‘flow networks’’, ‘‘binarysearch’’, ‘‘automated mono-segmented flowsystem’’, ‘‘multi-insertion of small solutionvolumes’’, ‘‘time-division multiplexed tech-nique’’ or even, simply, ‘‘FIA’’. The above-mentioned group at Piracicaba, Brazil, recentlyput forward the name ‘‘Tandem Flow Analysis’’.Although this methodology should naturally

expand by addressing new problems orimproving on existing solutions with the use ofnovel, imaginative flow systems, there will alsobe an inevitable trend to ‘‘transferring’’ pro-cesses that have previously been tackled suc-cessfully with other continuous-flowmethodologies. This will entail the replication(mechanization) and new validation of estab-lished FIA applications such as those involving:(a) the use of solid-phase reactors for pre-concentration, separation, analyte derivatization,in situ preparation of unstable reagents, reagentpurification and enzyme-catalysed reactions; (b)the pretreatment of solid samples (suspensions);

Table 2Spectrophotometric and potentiometric titrations in a multicommutated flow network

Analyte Sample matrix Detector/reactive RSD,% Samplethroughput,h�1/ volume, ml


Ref.

HCl/HAc Vinegar; Coca Cola;lemon soda; Isotonic drink;orange juice

ISE/NaOH 1 (n=9) 5 [50]

H+/OH� Wines S/NaOH-m-cresol indicator 0.7 (n=6) 25/600 5 [12]Ca/ PO4

3� Natural water Ca, ISE/ ethylenebis(oxyethylenenitrilo)tetra-acetic acidP, S (650nm)/molybdenumblue method

Ca, 0.5 (n=10,1mM)

P, 180/100 4* [11]

HCl S (540 nm)/NaOH-phenolphthalein

60/667 3 [51]

HCl S/NaOH 1–10 mM [10]H+ Red and white vinegar S (548 nm)/NaOH-

phenolphthalein1.2–2.1 (n=12) [9]


(c) heterogeneous (liquid–gas) systems, the useof which is bound to expand in the future; and,(d) liquid-liquid extractions.Multicommutation introduces a new hydro-

dynamic concept and thus requires new theore-tical studies concerning the phenomena ofdispersion caused by the use of solenoid valvesfor inserting solutions, and the development offlow networks through which variable sequen-ces of micro-insertions of sample and reagentsare interacting.Multicommutation should lend itself readily to

use in combination with any type of detector,with performance similar to FIA. Also, it mayprovide increased operational reproducibilitywith microsensors.Three-way solenoid valves have so far been

virtually the sole types used, and peristalticpumps have been the main propulsion units(piston and gas pressure pumps have also beenused, but only occasionally). More sophisticatedassemblies are bound to include alternativetypes of propulsion device.It should be noted that the new methodology

is easier to automate. This is bound to promotethe development of portable field equipment foron-site analyses (for example, in hospitals),water supply monitoring, environmental analysis,industrial process control, and similar purposes.In summary, it is not too adventurous to state

that the new methodology constitutes a firmstep towards the development of more flexible,readily automated systems for operation underflow conditions. As indicated by the group atPiracicaba, Brazil: ‘‘Evolution of the commuta-tion concept has lead to the proposal anddevelopment of different generations of flowanalyzers.’’ [15].

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J. Martınez Calatayud is Professor of Analytical Chemistryat the University of Valencia (Spain). He is currently work-ing on continuous-flow methodologies, basically in FIA andtandem-flow. His topics of interest are pharmaceutical andenvironmental samples, mainly surface waters and marinewaters.

J. V. Garcıa Mateo is lecturer in analytical chemistry at theUniversity Cardenal Herrera in Valencia, Spain. His researchwork also deals with flow methodologies and pharmaceu-tical and environmental samples.

M. Catala Icardo is in the Department of Analytical Chem-istry of the University of Valencia, where she has a post-doctoral position working on a European Proposal on theanalytical control of drinking waters.


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Multicommutation as a powerful new analytical tool