Pii: s0003-2670(02)00628-

Analytica Chimica Acta 468 (2002) 119–131 Fábio R.P. Rocha , Boaventura F. Reis , Elias A.G. Zagatto , José L.F.C. Lima , Rui A.S. Lapa , João L.M. Santos a Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, P.O. Box 96, Piracicaba, 13400-970 SP, Brazil b Faculdade de Farmácia, Universidade do Porto, Rua Anibal Cunha 164, Porto 4050-047, Portugal Received 2 May 2002; received in revised form 25 June 2002; accepted 4 July 2002 Abstract
Multicommutation refers to flow systems designed with discrete computer-controlled commutators resulting in flow net- works in which all the steps involved in sample processing can be independently implemented. The flow systems can bere-configured by the control software, presenting thus increased versatility, potential for automation and for minimization ofboth reagent consumption and waste generation. The main objective herein is to review the concept of multicommutation inorder to permit a proper evaluation of the characteristics and potentialities of the related flow systems, to assist methodologicalimplementation and to discuss similarities with other existing strategies. Implementation of tandem streams, controlled dilu-tions, wide-range determinations, sequential determinations, titrations and in-line separation/concentration are emphasized.
2002 Elsevier Science B.V. All rights reserved.
Keywords: Multicommutation; Flow analysis; Tandem streams; Automation 1. Introduction
tion by the environment (and vice versa) is avoided.
Other features inherent to flow analysis are the low Since the inception of flow analysis in the earlier consumption of sample and reagents, the partial and fifties flow systems have proved to be excellent reproducible development of the involved steps, that tools for solution handling and consequently for car- opens the possibility of reaction kinetics exploitation, rying out methods related to wet chemical analysis.
the recording of a transient signal, etc. As a rule, the aqueous sample is introduced into the In the earlier air-segmented flow analyzers, a sam- analytical path and processed inside it under repro- pling arm was used to select either the sample or the ducible conditions. The manifold can thus be regarded wash (carrier) solution to be aspirated towards the as a closed laboratory where the sample contamina- analytical path This task can be considered as thebeginning of commutation in flow analysis. However,little evolution in the commutation concept was noted ∗ Corresponding author. Tel.: +55-19-429-4650; during the development of the segmented-flow analyz- ers. In fact, chemical processing was not altered from E-mail address: [email protected] (E.A.G. Zagatto).
1 Present address: Instituto de Qu´ımica, Universidade de São sample to sample and no active devices or feedback mechanisms were present in the flow system.
0003-2670/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 3 - 2 6 7 0 ( 0 2 ) 0 0 6 2 8 - 1 F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131 Commutation became more evident in relation to unsegmented flow-analyzers conceived in the 1970sSteps others than single sample introduction,such as multiple injections, stream splitting, addition/removal of manifold components and stream redirec-ting were efficiently accomplished by using speciallydesigned commutators. This led to the efficient imple-mentation of merging zones, zone sampling, stopped-flow, procedures exploiting differential kinetics, simultane-ous determination, etc. A comprehensive review ofcommutation in flow-injection analysis was presentedin 1986 The single commutators, usually sliding bars or rotary valves, were only able to linked commutationbecause they operated in two resting positions thusoriginating two states. Therefore, in spite of the above-mentioned capabilities, the related systems lacked ver-satility. The drawback was circumvented by designingthe manifold using several commutators with discreteoperation This led to a drastic improvement insystem performance, as 2n states of commutation(n = number of active devices) could be establishedand exploited. The related flow systems, often namedmulticommuted ones, present potentialities to be usedas general-purpose systems, as they rely on the con- Fig. 1. Representation of some mechanical commutations in flow cepts of different flow-analyzers (segmented-flow, analysis. (a–d) Dashed lines represent the flow paths after com- flow-injection, sequential injection) and are compat- ible with streams of different characteristics (seg-mented, unsegmented, monosegmented, tandem).
Although, some potentialities of the flow systems The different configurations in illustrate me- exploiting multicommutation were recently outlined chanical commutations in flow analysis. In addition, the related concepts, adherence with the already flow rate modifications and stream reversals are also proposed flow-analyzers and guidelines for system de- relevant examples usually accomplished without man- sign were not emphasized. The main objective herein is therefore to review the concept of multicommuta- The simplest mechanical commutation involves tion in order to permit a proper evaluation of the char- acteristics and potentialities of the related flow sys- easily accomplished by using three-way valves or tems, to assist methodological implementation and to portions of more elaborated valves The strategy present the similarities with other existing strategies.
has been exploited for intermittent addition of flowingstreams. In this way, a washing stream can be added(after achievement of the analytical signal in 2. The concept of commutation
order to reduce washing time, thus improving samplethroughput , a confluent stream can Commutation is defined as “(1) a passing from one be added only when the processed sample is passing state to another, (2) the act of giving on thing for an- through a confluence point, allowing reduction of the other, (3) the act of substituting, (4) in electricity: a reagent consumption Moreover, the stopped-flow change of direction of a current by a commutator” approach can be implemented in a similar manner.
F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131 The sample carrier stream of a single-line flow system expanded in a pronounced manner by taking advan- is allowed to recycle, and the processing sample is tage of discrete devices with independent operation.
stopped inside the analytical path. This permits long This is the essence of the multicommuted systems.
residence times to be efficiently attained in order to Their inherent potentialities, also in relation to the improve sensitivity and/or to permit kinetic measure- exploitation of feedback mechanisms, are discussed ments On the other hand, this simple mechanical commutation permits to feed the analytical path withstacks of different zones (The flow-set upis designed in such a way that either stream IN1 or 3. The concept of multicommutation
stream IN2 is directed towards the outlet. The strat-egy is often used in sandwich techniques A flow system can be regarded as a compart- sequential injections Expansion of this strategy ment with a number of inlets (samples, reagents, permits also the establishment of tandem streams, a keyboard commands) and several outlets (results, novel approach for handling flowing solutions. Even recycled solutions, wastes). When commutation with a single device, fast successive commutations is not so characteristic, such as in the classical (multicommutation) can be performed, originating a segmented-flow-analyzers, influence of outlet on the binary string constituted by slugs of the involved solu- inlet parameters and vice versa, as well as real-time tions. An analogous strategy permits the achievement modifications in the sample processing, are not easily accomplished. With a single commutation, only few Other kind of mechanical commutation involves of the potentialities inherent to flow analysis are feasi- the exchange of manifold components (that ble These capabilities are expanded in relation to is a powerful tool for achieving different sample the most advanced systems, the multicommuted ones.
residence times. In fact, two different mean sample A multicommuted system can be then considered residence times can be provided for every assayed as an analytical network that involves the actuation of sample depending on whether a short or a long re- n active devices (or n operations with a single device) actor is placed in the analytical path. The approach on a single sample allowing the establishment of up to is beneficial mainly for implementing simultaneous 2n states. It presents several in- and outlets parame- determinations involving kinetic discrimination or ters that are often interdependent. The analytical steps wide-range determinations Another possibility required for sample processing can be defined through is to trap the processing sample inside an incubation the control software, being eventually (in feedback coil in order to get increased residence times without exploiting systems) real-time modified. In short, mul- ticommutation is inherent to flow systems that may An usual strategy for mechanical commutation in- present several states; the sample under processing volves two inlet and two outlet streams and offers the is usually submitted to different operations, such as possibility of selecting different paths between them splitting, slicing, trapping, mixing in tandem, under (The configuration can be settled by using different conditions. Commutation is also responsi- six-port valves or sliding-bar injectors. As different ble for additions of components (including sampling components can be placed in the paths between the loops) to the analytical path and/or stream re-directing.
commuting sites, the strategy is very attractive for More elaborated systems usually comprise more implementing loop-based injection ion-exchange active devices. Therefore, presence of several valves involving addition/removal of minicolumns mul- in the system has been erroneously taken as an indica- tisite detection leaping filters etc.
tor of a multicommuted system. This is not a sine qua The above-mentioned potentialities are expanded by non condition, as there are multicommuted systems replicating the unity configurations and/or including with a single valve flow systems with several feedback mechanisms. When the unity configurations commutators where the aspect of multicommutation are replicated in a single commutation unity, only two is not highlighted t is important to emphasize states can be established for the system, and linked that the configuration inherent to sequential injection commutation is concerned. System versatility can be analysis is considered here as an expansion of the F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131 can be introduced into the manifold by rapidly andsequentially switching the commutators, usuallycomputer-controlled valves. This unique stream canbe seen as a set of neighboring solution slugs thatundergo fast mixing while being transported throughthe analytical path.
For constant total volumes of sample and reagent, mixing is improved by decreasing the aliquot volumesand increasing the number of slugs. Insertion of n pairsof sample/reagent slugs results in 2n − 1 interfaceswhere mixing occurs by axial dispersion. In contrast tomost flow systems, sample/reagent interaction starts inthe sampling step, thus increasing the mean residence Fig. 2. Establishment of a tandem stream. S1, S2: miscible solu- time without affecting sampling rate.
tions, V: three-way solenoid valve, R: reactor coil, D: detector.
Tandem stream was initially exploited for the deve- lopment of an improved single-line system for spec- commutation concept, although multicommutation is trophotometric multiparametric analysis of natural waters into which several plugs of differentreagent solutions were sequentially introduced. Theingenious strategy involved three-way computer-cont- 4. Tandem streams
rolled solenoid valves and permitted the addition of di-fferent One of the main potentialities of multicommutation is the establishment of a tandem stream ( A similar strategy named tandem injection was A number of aliquots of different miscible solutions adopted for sample dilution prior to sequential ICP– Fig. 3. Implementation of tandem streams: (a) carrier flowing towards the analytical path; (b) insertion of a sample aliquot; (c) insertionof a reagent aliquot in tandem with the sample; (d) insertion of a sample aliquot in tandem with the reagent. Vi: commutators; C: carrier;R: reagent; D: detector. For didactic purposes, dispersion at the liquid interfaces (as shown in sample/reagent mixing wasomitted.
F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131 OES or ICP–MS Several sample plugs were 5. Flow setups
introduced into a diluent solution yielding a tandemstream that was directed towards the inlet of the Multicommutation is usually accomplished by tak- nebuliser. Under good mixing conditions, an almost ing advantage of valves, timing devices and other steady situation was attained. Undulations on the artifacts for improving system performance. These recorded tracing were reported, and procedures for devices can be operated in a passive or active manner, minimizing them were discussed. Another way to and external timing is often exploited for versatility implement a tandem stream is to converge different flowing solutions towards a rapidly and sequentiallyswitching three-way valve. This is particularly attrac- tive when the tandem stream feeds the sampling loopof a flow-injection system the holding coil In a multicommuted flow-analyzer comprising only passive devices, sample processing is defined pre- Drawbacks caused by differences in refractive viously to its introduction into the analytical path.
index in spectrophotometric measurements can be In spite of this, several time-dependent analytical readily minimized in flow-systems with tandem procedures can be efficiently carried out, as the sys- streams, as verified in the determination of pindolol tem presents inherent timing. The discrete devices in pharmaceutical preparations and ethanol in are thus used mainly to establish tandem streams, alcoholic beverages Multicommuted flow sys- merging zones, addition/removal of components, etc.
tems with tandem streams were also proposed for in order to improve figures of merit such as reagent improving sample/reagent mixing in the spectropho- consumption and sample throughput. Versatility is tometric determination of ascorbic acid and improved because the devices are usually indepen- clomipramine in pharmaceutical preparations, dently operated. Implementation of zone sampling is and glycerol in alcoholic fermentation juices a good example to illustrate this feature: with linked Other profitable examples are presented in the next mechanical commutation, only one sample aliquot per injection can be re-sampled whereas a number of The previously described approach has been dif- aliquots per injection can be obtained by exploiting ferently named. Israel et al. termed their proposal as tandem injection The term binary sampling Tandem streams are also compatible with flow net- was adopted in the first article of the series about works based exclusively on passive devices. In this multicommutation expression multi-insertion way, the intermittent addition of reagents was ex- principle was adopted when a procedure for the de- ploited for the determination of nickel with dimethyl- termination of nitrate and nitrite in water, fertilizer glyoxime. Different reagent plugs were inserted in and food samples was proposed However, the tandem with several sample aliquots, allowing the authors termed a similar strategy as tandem flow when establishment of a stream comprising the solutions proposing a procedure involving gas diffusion for required for iron masking, nickel oxidation and devel- determination of chlorine A similar approach opment of the color-forming reaction A similar named pulsed flow the insertion of strategy involving immobilization of the oxidizing small aliquots of different solutions at a frequency typically 1.0–1.5 Hz. Solutions are pressurized against The potentialities of the analyzers are expanded by nozzles in order to attain instantaneous turbulence taking advantage of external timing. In this way, the that contributes to improve mixing and reduce axial different steps of sample processing (addition of spe- dispersion. The authors coined the term time-division cific reagents for sequential determinations, multiple multiplex technique for an approach that consisted stop periods, cascade dilutions, etc.) can be indepen- in using computer-controlled solenoid valves for dently implemented. Another possibility is to provide creating concentration profiles exploited for poten- via keyboard the information needed for processing tiometric titration of calcium and spectrophotometric different samples in a diverse and specific way as in F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131 Incorporation of feedback mechanisms in the Dilution is inherent to flow systems being dictated control software expands the performance of multi- by parameters such as sample volume and analytical commuted flow-analyzers. With active devices strate- path length. Moreover, the dilution degree can be gically placed in the manifold, sample processing increased by exploiting zone sampling split can be modified according to the preliminary mea- zones for example. In this way, multicommuted surements. In this way, parameters such as sample systems have been proposed to implement controlled residence time or sample dispersion can be easily dilutions in order to expand the concentration range of modified, as demonstrated in the wide range determi- procedures, thus avoiding outlier samples. In this con- text, a flow system was proposed for controlled dilu- monitored signal is acceptable for providing the ana- tions of plant digests aiming the direct determination lytical result, another measurement can be performed of calcium by flame atomic absorption spectrometry and potassium by flame atomic emission spectrometry A similar strategy can be exploited in designing in- The diluent aliquots were intercalated in tan- telligent systems, such as the flow-injection system dem with the sample plugs, and each sample/diluent proposed for the analysis of saline waters by ICP–OES volumetric ratio corresponded to a different dilution Matrix interferences caused by differences in degree. The approach allowed up to 40-fold dilution salinity between sample and reference solutions were without affecting the analytical precision.
circumvented by considering a prior measurement that An alternative approach was proposed for the fluo- allowed the proper selection of the amount of sodium rimetric determination of folic acid in pharmaceutical chloride to be added to the reference solutions.
Self-optimized flow systems are also feasible, as employed to change the sample volume in order to the operational conditions can be systematically varied provide variable degrees of sample dilution, thus ex- according to a previously defined algorithm in order tending the linear response range. Samples were firstly to attain the best analytical response Moreover, processed in a condition of medium dispersion and discrete devices can be exploited for implementing the analytical signal was evaluated by the control soft- variations in flow rates, stop intervals, flow reversal, ware that decided if the analyte could be quantified etc. They may also be useful for specific applications or if the sample should be re-processed with higher involving localization/processing of a flowing sample or lower dispersion. In this way, linear response was obtained within 0.100 and 40.0 mg l−1, yielding a pro- discussion of this possibility is outside the scope of cedure able to monitor tablet dissolutions.
An automated multicommuted flow system was proposed for expanding the linear range in spectropho-tometric procedures, aiming the determination of cal- 6. Applications
cium in different samples ive sample-processingconditions were previously defined, corresponding to As science is recurrent, it is very difficult to pre- dispersion coefficients within 2.15 and 754, achieved cisely define when multicommutation was conceived.
by changing the sample volume (10–500 ␮l), the an- In fact, several contributions involving different in- alytical path length (225–425 cm) and by exploiting dividually operated discrete devices a number zone sampling at different portions of the dispersed of linked three-way valves several commutators zone. The criterion for accepting a given measurement establishment of tandem streams etc.
or to call for sample reanalysis was similar to that were proposed before the term multicommutation was previously described. Quantification was achieved in coined. However, the applications highlighted below up to three trials, and the R.S.D. were estimated as are restrict to those where reference to multicommu- <0.8% regardless of the sample processing condi- tation is explicit and/or its main characteristics are tions. Linear response within 0.250 and 1000 mg l−1 allowed the direct determination of calcium in waters, F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131 plant digests, antacids, fertilizers and calcareous was proportional to zinc concentration due to the rocks. Multicommutation was also exploited to ex- limited extension of the copper reaction whereas the tend the linear response range for the turbidimetric following analytical signal reflected the concentration determination of sulfate in plant digests the of both species. Intermittent reagent addition allowed fluorimetric determination of isoniazide in pharma- also the design of a flow system for the determina- ceutical preparations Also, a similar approach tion of iron and aluminum in plant digests, using allowed implementing programmable isotope dilu- tions for ICP–MS Spikes were generated from The strategy was expanded when a multicommuted a single enriched 112Cd solution. About 30 sam- flow system was proposed for spectrophotometric ples could be run per hour consuming 33–168 pg of determination of total nitrogen and phosphorus in biological materials involving intermittent additions A flow system designed in the closed-loop configu- of several reagents and requiring increased resi- ration was proposed for implementing successive dilu- dence times The manifold was designed with tions required for expanding the concentration range in a single channel into which sodium salicylate and the chloride determination in parenteral solutions sodium hypochlorite or ammonium heptamolybdate The sample zone was continuously recycled through and ascorbic acid were added by proper switching the flow cell, allowing several analytical signals to be of strategically positioned solenoid valves. Moreover, obtained for the same sample injection. After obtain- the alkaline and acidic conditions required for the ing the first analytical signal, the front and tail portions different determinations were efficiently established.
of the sample zone were removed and replaced by the Although an ammonium salt was used for the deter- reagent solution, by the action of a three-way solenoid mination of phosphorus, the analytical signal corre- valve. Thus, the sample dispersion and the reagent sponding to nitrogen was not hindered by carryover availability were both increased. The process was re- effects, emphasizing the capability of the strategy in peated several times, resulting in signals correspond- dealing with addition/removal of reagents. The sam- ing to different dispersion degrees, allowing achieving ple residence time was increased by decreasing the linear response up to 10 g l−1, with R.S.D. <1.8%.
rotation speed of the computer-controlled pump afterthe sampling step. A multicommuted flow system was also employed for the sequential potentiometricdetermination of free and total cyanide exploiting gas Discrete commutators with independent computer- control allow the intermittent addition of different The potentialities of multicommutation for sequen- reagents, thus expanding the potentiality for sequen- tial determinations are expanded by association with tial determinations without changing the manifold multichannel detectors. Spectrophotometric multi- structure. The strategy can be illustrated by the mul- determinations using a low selectivity chromogenic tiparametric analyses of alloys Aliquots of the reagent (4,2-pyridylazoresorcinol) were implemented reagents required for iron and chromium determina- by exploiting measurements at different pH values, tion were introduced in tandem with sample slugs addition of masking agents and kinetic/spectral dis- permitting the sequential determination.
crimination for the sequential determination of iron, Multicommutation was also exploited for simul- copper, zinc and nickel ver, a flow-system taneous determination of copper and zinc in plant with intermittent addition of different selective chro- digests using Zincon as chromogenic reagent mogenic reagents was proposed for the determination The analytes were in-line complexed with cyanide of iron, copper, zinc, calcium and magnesium in and the different rates of reaction with formaldehyde multivitaminic preparations Sample/reagent in- allowed implementing the kinetic discrimination. A teractions were improved due to the establishment of three-way solenoid valve was used for managing the sample splitting, yielding two sample aliquots to be Multicommutation permits different tasks to be processed under different conditions. The analytical simultaneously accomplished for improving system signal corresponding to the shorter residence time performance and analytical characteristics. In this F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131 sense, a flow system was proposed for the inorganic Multicommutation in connection with the stopped- speciation of nitrogen in waters For ammonium flow approach was exploited for the spectrophotomet- determination, the processing sample was trapped in- ric determination of creatinine in urine using picric side a coiled reactor in order to improve the reaction acid as reagent Design of the flow manifold al- development. Simultaneously, another sample aliquot lowed improving analyte conversion rate and in-line was processed for nitrate and nitrite. In this way, sample dilution was achieved through zone sampling.
60 determinations could be performed per hour with In this way, matrix effects were minimized and the a 120 s residence time for ammonium, enough for dynamic response range matched the expected crea- achieving a 95% conversion rate. Other application tinine concentrations in the samples. As a high blank in this context refers to the sequential determination value was concerned, an ingenious strategy permitted of anions in natural waters The sample acted the correction of the analytical signal. The sample as carrier solution into which several plugs of the aliquot was split and one portion was retained inside a required chromogenic reagents were introduced. The warm reactor (37 ◦C) in order to enhance the reaction system allowed also in-line concentration into an development, whereas the other was straightforward anion-exchange resin minicolumn simultaneously to directed towards detection for evaluation of the blank the direct determination of the analytes in order to per- mit species at lower concentration to be determined.
The beneficial effects of implementing zone Multicommutation allows also implementing pro- trapping with multicommutation were illustrated in cedures for accuracy assessment. The analyte is de- relation to an improved procedure for the spectropho- termined by two different methods with the same tometric determination of phoshate in natural waters flow network and the mean of the obtained results Further, simultaneous trapping of multiple sam- is inherently more reliable. In this way, a multi- ple zones were utilized to permit an increase in the commuted flow manifold was proposed for chloride sample residence time without impairing the sampling determination in natural waters with different sample rate in a flow system designed to spectrophotomet- matrixes based on the spectrophotometric proce- ric determination of boron in plant digests A dure using mercury(II) thiocyanate and the turbidi- four-way solenoid valve directed the sample zones to metric method using silver nitrate Moreover, three similar reaction coils allowing a mean residence in-line addition/recovery tests were implemented time of 200 s with a sampling rate of 65 determina- for every assayed sample in order to detect matrix tions per hour. The favorable characteristics of the approach were also highlighted in the determina-tion of ammonium and phosphate in natural waters using an electronically operated sliding bar commu-tator coupled to three-way solenoid valves The For improving sensitivity in analytical procedures sample zones were simultaneously processed and a based on relatively slow reactions, the system is de- sampling rate of 56 determinations per hour was at- signed to provide an increased sample processing time tained even for a 160 s residence time. Recently, the without impairing other features such as sampling rate strategy was applied to the spectrophotometric deter- and sample dispersion. In this regard, the stopped- mination of amiloride hydrochloride in pharmaceutic flow approach typically involves monitoring preparations. With two channels able to accommo- of the processed sample after stopping it inside the date stopped sample zones, the sampling rate and the flow-through detector presents unique advantages.
mean sample residence times were 30 h−1 and 60 s, Also, other flow setups allowing sample trapping in- respectively. It is interesting to comment that these side specific portions of the manifold have been pro- figures of merit are modifiable at will depending on posed Alternatively, increasing the sample processing time can be efficiently accomplished by Multicommutation has also been exploited to fa- exploiting segmentation The potentialities of cilitate the introduction of sample, reagent and air these approaches are expanded with multicommuta- bubbles into a monosegmented flow system The approach was demonstrated in the spectrophotometric F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131 determination of manganese in soybean based on pe- used as carrier stream and the reference signal was riodate oxidation of Mn(II) to permanganate in acidic estimated during its passage through the electrode.
medium. Improved sensitivity was attained by setting The volume of the sample zone was fixed during the a 5 min sample residence time and immersing the entire titration procedure, thus avoiding matrix vari- main reactor inside a 47 ◦C water bath. The reaction ations that could affect the electrode response; the was completed before detection and a sampling rate titrant volume was continuously varied therefore a of 50 determinations per hour was achieved because diluent solution was added to keep the total volume four sample zones were simultaneously processed.
of the processed sample. With monosegmentation, the A similar strategy was adopted in a flow system for mixing conditions were improved without excessive spectrophotometric determination of l(+)-lactate in sample dispersion. The search process was analogous silage material using lactate oxidase and a crude to the molar ratio method for determining complex extract containing peroxidase The system pro- stoichiometry. At every step, the sample zone was vided a very long sample residence time (17 min) and monitored and the signal evaluated in order to decide permitted 16 samples to be processed per hour.
if the titrant volumetric fraction should be increasedor decreased. For achieving the end-point, the signal should be within the pre-selected interval defined asthe mean value ± three-fold the standard deviation of Titrations are classical procedures often required the measurements performed on the buffered carrier in analytical laboratories in view of the achievement stream. The procedure yielded precise (R.S.D. = 1%) of intrinsically reliable results. As they are generally and reliable results even for titrations of dilute weak tedious and time-consuming, several mechanized pro- acids. A similar algorithm was further applied for the cedures have been proposed, including those carried determination of ascorbic acid in juices and soft drinks out in a flow basis However, most of them can- involving titration with 2,6-diclorophenol-indophenol not be rigorously considered as titrations, because a tation and tandem streams were also exploited to True titration procedures, without requiring cal- develop a procedure for acid–base titrations based on ibration, have been developed by exploiting multi- commuted flow setups. In this context, the binary An ingenious approach involving one three-way search process was proposed for end-point detec- valve that managed sample and the titrand inlet as a tion in spectrophotometric titrations It relies on tandem stream was proposed and applied to acid–base the evaluation of the least volumetric fraction of the titrant causing a measurable change in the color ofan indicator. Signals related to sample zones com- 6.5. In-line concentration/separation prising only the sample or the titrant were consideredas reference. For end-point detection, the sample and Procedures for in-line concentration are usually titrant volumetric fractions were varied by maintain- time-consuming, therefore impairing the sampling ing the total volume of the sample zone, similarly to rate. The drawback can be circumvented by using a the continuous variation method. Feedback mecha- multicommuted flow manifold, as illustrated in the de- nisms assisted the decision about whether sample or termination of cadmium, nickel and lead in foodstuffs titrant volumetric fraction should be increased in or- and plant materials by ICP–OES sorption der to converge to the end-point. The procedure was and elution were simultaneously carried out by using illustrated by the acid–base titration in the presence three ion-exchange columns managed by a four-way of phenolphthalein. End-point was reached in up to valve, allowing a sampling rate of 90 determinations/h 3 min and less than 2 ml of titrant was consumed.
to be attained even for a 120 s loading time.
The binary search process was further applied to Multicommuted flow-systems were also associated the potentiometric acid–base titration, employing a to an electrothermal atomic absorption spectrome- tubular polymeric membrane hydrogen ion-selective ter with a tungsten-coil atomizer, aiming to perform electrode A diluted buffer solution (pH 7.0) was in-line concentration and separation Analyte F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131 retention/elution, system washing and column con- sampling strategies such as tandem streams, sandwich ditioning were independently accomplished by using sampling, monosegmented flow and sequential injec- discrete commutators. The potentialities were illus- tion analysis were implemented without changing the trated by in-line concentration of lead in a Chelex-100 resin minicolumn matrix separation by sorp- Minimization of the reagent consumption is one tion of metal complexes in fullerene (C60) aiming of the favorable characteristics of the multicommuted the determination of cadmium, lead and nickel in flow systems, especially those exploiting tandem water samples A similar flow setup was pro- streams. A considerable reduction in the reagent con- posed for in-line separation-concentration of copper sumption and waste generation is the most direct in the analyses of unpolluted seawater by graphite alternative towards the development of green analyt- furnace atomic absorption spectrometry with a W-Rh ical procedures This can be exemplified by the enzymatic determination of glucose in soft drinks and A flow system designed with discrete commutators sugarcane juices where the required amounts of per- (three-way solenoid valves) was employed for in-line oxidase e glucose oxidase were 85% lower in relation matrix separation and concentration of copper, cad- to the analogous batch procedure The reagent mium, lead, bismuth and selenium from seawater aim- consumption and consequent generation of effluent ing their determination by electrothermal vaporization volumes associated with different flow-based proce- ICP–MS In-line resin conditioning, concentra- dures were critically compared in relation to the de- tion, column washing and elution were independently termination of cabaryl with p-aminophenol The performed, and a sampling rate of 22 determinations/h multicommuted system required a reagent amount similar to that of a sequential-injection system, and ca.
A multicommuted flow system was employed for 27-fold lower than that of a typical flow-injection sys- overcoming the interference of lead and copper on the tem. However, other analytical figures of merit such spectrophotometric determination of cadmium as sampling rate and detection limit were enhanced Interfering species were separated by electrolytic de- in comparison with the sequential injection system.
position and cadmium was in-line concentrated as In addition, improved sensitivity in the spectrophoto- chloro-complexes in an anion-exchange resin mini- metric determination of iron in waters in a sequential column. In this way, up to 50 mg l−1 Pb2+ and injection system was achieved by exploiting tandem 250 mg l−1 Cu2+ did not interfere in the determina- streams The potential of the flow-based proce- tion performed at a sampling rate of 20 h−1 with a dures, including the multicommuted ones, to follow detection limit estimated as 0.23 ␮g l−1.
the general tendency towards a greener analyticalchemistry was recently emphasized System optimization is usually time-consuming, and can be automatically carried out by exploiting the In contrast to conventional flow-systems, multi- independent control of the discrete commutating and commuted systems exploiting tandem streams permit propelling devices, as demonstrated for chloride de- the use of a single channel for handling the dif- ferent involved solutions This feature allowed flow-rates, sample/reagent volumes and mean avail- exploitation of gravity to propel the solutions able time for reaction development were real-time aiming the achievement of a pulseless flow and a re- modified according to a simplex algorithm. The opti- duced maintenance program. The performance of the mization process was efficient even for sample batches system was demonstrated in the spectrophotometric with pronounced variability in analyte concentrations.
determination of chloride in natural waters with a The versatility of multicommutation was exploited sampling rate of 160 determinations/h and reduced to develop a flow system able to detect and circum- reagent consumption. Gravity was also the driving vent potential sources of inaccuracy Feedback force in the turbidimetric determination of sulphate mechanisms were included in the control software in plant and animal tissues By exploiting a aiming the real time characterization of the sources computer-controlled six-way solenoid valve, several of inaccuracy and adoption of corrective actions for F.R.P. Rocha et al. / Analytica Chimica Acta 468 (2002) 119–131 every assayed sample. Specific situations where the of the already existing modalities and not a novel one.
analytical results were more susceptible to inaccuracy The growth of proposals in conexion with multicom- such as partial overlap of sample and reagent zones, mutation is then clearly to be increased.
improper matrix matching, incomplete masking as In view of the presence of discrete active devices well as lessening of the efficiency of solid-phase in the analytical path, systems exploiting feedback mechanisms will probably be more and more de- Multicommuted flow systems with detection by signed especially with regard to intelligent systems, chemiluminescence were proposed for determination polyvalent systems, total analytical systems, novel of phenols in natural waters lactic acid in procedures for in-line solvent extraction and sample yoghurts The former procedure was based on preparation, implementation of several methods per the oxidation of phenols by permanganate in acidic analyte aiming accuracy assessment, detecting and cir- medium after in-line concentration on a XAD-4 resin cumventing sources of inaccuracy, etc. The tendency minicolumn. A detection limit of 5 ng l−1 was at- towards miniaturization will certainly be increased.
tained with a sampling rate of 12 determinations/h.
This aspect, together with the more rational utiliza- The procedure for lactic acid was based on reaction tion of reagents by multicommuted systems, will lead with lactate oxidase yielding hydrogen peroxide that to a reduction in sample and reagent consumption. As reacted with luminol in alkaline medium producing a consequence, a decreased waste generation is fore- the chemiluminescence. The sampling rate was esti- seen, matching the present tendency towards Green mated as 55 determinations/h for samples containing within 10–125 mg l−1 l(+)-lactate. All required stepswere efficiently implemented with discrete commu-tators and the procedures presented a considerable Acknowledgements
reduction in the reagent consumption.
Multicommuted flow systems were designed for Partial support from FAPESP, CNPq and CAPES/ICCTI the independent management of the solutions aiming in-line eletrodissolution of alloys. In this context, thedirect determination of aluminium, copper and zincin the solid sample by FAAS was implemented References
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