Segmented Flow Analysis

Apart from segmented flow analysis (SFA) and FIA modalities, SIA, FIA with multicommutation and binary sampling (FIA–MBS), FIA with sandwich sampling (FIASS), and gravity flow-based manifolds are described in the literature [1].

From: Flow Analysis , 2014

SEGMENTED FLOW ANALYSIS

S. Coverly , in Encyclopedia of Analytical Science (Second Edition), 2005

Interface with Other Systems

SFA systems have been used with LC both for sample preparation before chromatography and for postcolumn derivatization. Sample purification by dialysis, filtration, or solvent extraction can be conveniently performed with segmented flow. The low band broadening in segmented streams is advantageous both in precolumn sample preparation to allow lengthy cleanup procedures and postcolumn to maintain peak separation. In these hybrid systems, the output from the SFA manifold is pumped through the LC sample loop, which is actuated by a timer when sample concentration has reached its maximum.

Dialysis and solvent extraction are the most commonly used precolumn techniques. In an SFA unit specially developed for LC sample preparation to ensure solvent compatibility with the mobile phase, the evaporation to dryness module, sample dissolved in a volatile organic solvent is pumped onto a moving PTFE wire; the solvent is evaporated in a stream of hot gas and the residue redissolved in a second solvent compatible with the chromatography system.

Applications of SFA/LC include the determination of drugs in blood serum, pharmaceutical product analysis, and the determination of vitamin A in milk.

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Flow analysis | Segmented Flow Analysis☆

Stephen Coverly , in Encyclopedia of Analytical Science (Third Edition), 2019

Introduction

Segmented flow analysis (SFA) was a pioneering development in laboratory automation. It was conceived in the 1950s by Leonard Skeggs, 1 a researcher in a clinical laboratory, and within a decade of its development, became the dominant technique in automated clinical analysis.

The rapid acceptance of SFA was due to its simplicity. Developed in the age before microprocessor control, when automating even simple procedures required complex mechanical timing and control devices, the low cost and high reliability of a flexible system employing only two moving parts—a peristaltic pump and an autosampler—was a major advance. The versatility of the system led to rapid development of methods and techniques by the manufacturer and by users in research institutions and in industry, and by 1975 almost 8000 papers describing SFA and its applications had been published.

The use of SFA reached its peak in the mid- to late-1980s. Its use in clinical laboratories then declined due to a move toward more highly automated systems requiring less operator involvement and with lower reagent consumption—an important factor for enzymatic reactions. In the industrial field, flow injection analysis (FIA), developed in 1975, replaced SFA in some commercial laboratories, but SFA remains dominant for determinations using long or complex digestion or separation procedures and where low detection limits are needed.

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Historical View

Elias A.G. Zagatto , ... Paul J. Worsfold , in Flow Analysis with Spectrophotometric and Luminometric Detection, 2012

2.2 Segmented Flow Analysis

Segmented flow analysis was conceived in the early 1950's [11] by Skeggs 1 (Fig. 2.2) as a logical consequence of the early developments in flow analysis. Automation at the Veterans Administration Hospital in Cleveland (Ohio, USA) was needed, and Skeggs considered robotic systems at that time too cumbersome to be practical. Therefore, he began to perfect the art of doing chemistry in flowing streams. To this end, his expertise in haemodialysis was also valuable [12].

FIGURE 2.2. The first prototype of a single-channel segmented flow analyser from May 1951.

 Reproduced with permission of Hindawi Publishing Corporation from "R. Stanley, Journal of Automatic Chemistry, 6 (1984) 175".

Segmentation of the main flowing streams by adding air bubbles was the main innovation. It is interesting to note that Skeggs initially tried to develop a flow system without addition of air bubbles, but it was difficult to avoid the inlet of air during replacement of sample and carrier solutions. Skeggs recognised the benefits of adding air bubbles and decided to exploit segmentation. It is however somewhat surprising that air-segmentation was not emphasised in his landmark article "An automatic method for colorimetric analysis" [13] which focussed on urea determination in whole blood or blood serum using in-line dialysis. No comments on the addition and removal of air bubbles appeared in the summary or introductory section of this article, nor in its brief in Chemical Abstracts [14].

The development of air-segmented flow analysis was restricted to a single company (Technicon Corporation Inc.), owner of the main patents until the mid 1970's. The first air-segmented system marked with the AutoAnalyzer®> trade name is shown in Fig. 2.3.

FIGURE 2.3. A first-generation single-channel AutoAnalyzer®> (Technicon Instruments, circa 1957).

 Reproduced with permission of Hindawi Publishing Corporation from "R. Stanley, Journal of Automatic Chemistry, 6 (1984) 175".

The expression continuous flow analysis and the corresponding acronym CFA have often been used to specify this mode of flow analysis, in accordance with official recommendations [15]. A historical survey reveals however that the expression has also been associated with unsegmented flow analysis [16–19]. In the present monograph therefore the expression continuous flow analysis - or CFA - is not used to indicate segmented flow analysis.

Expansion of the original AutoAnalyzer concept resulted in the multi-channel AutoAnalyzer dedicated to simultaneous determinations. For clinical analyses, the multiple, simultaneously recorded peaks can be combined to build up a multiple analysis chart [20], which has been recognised as a very important tool for clinical diagnosis.

The AutoAnalyzer underwent fast development and played a dominant role in the field of routine chemical assays for several decades, especially in relation to clinical chemistry. Nowadays, segmented flow analysis is very important in the context of large-scale analysis and applied worldwide in different fields [21].

In the segmented flow analyser (Fig. 2.4), a sampling arm successively selects the sample or the carrier/wash solution to be aspirated towards the detector, thus establishing the main aqueous stream. A convergent stream of air is thereafter added to promote segmentation.

FIGURE 2.4. Flow diagram of a single-channel segmented flow analyser and the associated recorder tracing.

S/C   =   sample/carrier wash stream; Air   =   air; R   =   reagent; RC  =   coiled reactor; DB   =   de-bubbler; D   =   detector; arrows   =   sites where pumping is applied.

Stream sectioning into a number of small segments reduces broadening of the flowing sample, and plays a beneficial role in the mixing process, reducing intermixing of the sample with the carrier stream and between successive samples.

During sample transport towards the detector, the analytical steps required by the specific application are performed in-line under reproducible conditions. Precise timing is involved; therefore chemical equilibrium is not always reached. Immediately before detection, the flow stream passes through a de-bubbler to remove the air bubbles. Sample passage through the detector results in a change in the monitored signal, which is recorded. The difference between the baseline and the maximum signal is related to the analyte concentration in the sample.

Segmented flow analysis relies on three cornerstone features: sample aspiration, stream segmentation and reproducible conditions due to precise timing.

The maximum signal is associated with the less dilute portion of the flowing sample and is often referred to as the plateau region. The recorded peak shape shows a tendency towards this steady state situation, as well as a slight axial dispersion of the sample zone. As segmentation is involved, a small sample aliquot is enough for the sample to reach the detector with its central portion almost undispersed. All of the above-mentioned features have a positive influence on sample throughput.

The segmented flow analyser is very robust but lacks versatility because its only commuting element is the sampling arm. Moreover, the possibilities for system miniaturisation are limited, because the integrity of the air bubbles cannot be maintained inside very narrow bore tubing. A critical evaluation of segmented-flow analysers is given elsewhere [22].

During the 1960's and 1970's, the design of air-segmented flow systems continued to evolve and different kinds of pumps, tubing, flow-through detectors, and devices for specific in-line operations such as filtration, heating, dialysis, liquid-liquid extraction, ion-exchange and evaporation became commercially available [23]. In this period, more than one hundred million samples were assayed in clinical laboratories using air-segmented flow systems [24] and about 5000 papers were published [25], mostly dealing with methodological developments. However, few conceptual advances were made during this period, and the most significant achievements were: electronic rather than mechanical timing of the sampler, a rapidly moving sampling arm [26], a bubble-gating flow cell through which the air bubbles were allowed to pass [27], and a computer monitored instrument for analytical curve regeneration [28].

After the intensive development of air-segmented flow analysis, some successful analytical procedures without stream segmentation were proposed, mainly in connection with enthalpimetric [29,30], chemiluminometric [31] or spectrophotometric [32,33] detection. In these systems the time domain involved was generally short, thus air addition and removal was not performed. Either the sample or the wash solution was continuously aspirated towards the main channel and measurements were made under an almost steady state situation. Without air-segmentation, however, sample throughput was impaired and sample changing was cumbersome.

It did not take long for several researchers [16,34–40] to realise that insertion of a sample plug into a continuously moving unsegmented carrier stream was beneficial for overcoming some of the problems associated with flow segmentation, the dimensions of the analytical path, sample/reagent consumption and sample throughput. This strategy is considered the essence of flow injection analysis.

The development of chromatography also played an important role in the initial development of flow injection analysis, as pumps, tubing, flow-through detectors and accessories typical of chromatography systems were commercially available at the beginning of the 1970's.

Here, a parallel between the inception of segmented flow and flow injection analysis (section 2.3) can be drawn. Skeggs did not want air bubbles and incidentally realised the improved performance of systems with segmentation; on the other hand, Ruzicka & Hansen did want air bubbles and incidentally realised the improved performance of systems without segmentation [41]. Both were correct.

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Continuous-Flow Extraction

Víctor Cerdà Fernando Maya , in Liquid-Phase Extraction, 2020

Abstract

Flow techniques have no doubt aroused especial interest in relation to many other automatic methodologies of analysis. Ever since segmented flow analysis (SFA) was developed by Skeggs in 1957, flow techniques have been in continuous evolution toward new developments such as those of flow injection analysis (FIA) by Ruzicka and Hansen in 1975; sequential injection analysis (SIA), an alternative to FIA, by Ruzicka and Marshall in 1990; multicommuted flow analysis (MCFIA) by Reis et al. in 1994; and, more recently, multisyringe flow injection analysis (MSFIA) by Cerdà et al. in 1999, lab-on-valve (LOV) by Ruzicka in 2000, multipumping flow systems (MPFS) by Lapa et al. in 2002, and lab-in-a-syringe in 2012. This chapter will be devoted to application of the main automatic flow techniques to liquid-liquid extraction procedures.

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Evolution and Description of the Principal Flow Techniques

Víctor Cerdà , ... Amalia Cerdà , in Flow Analysis, 2014

1.3 Flow injection analysis

FIA technique was developed by J. Ruzicka and E.H. Hansen [6] in 1975. Although at first sight FIA may resemble SFA, FIA is rather different from it in both conceptual and practical terms. Basic components of FIA are virtually the same as those of SFA which include a peristaltic pump to propel the sample and reagents, and a series of plastic tubes (manifold) carrying the liquids to the detector (see Figure 1.3).

FIGURE 1.3. Schematic depiction of a typical FIA system. IV: injection valve, RC: reaction coil.

However, unlike SFA, the sample is not inserted by continuous aspiration; rather, a constant volume of sample is inserted into a stream of carrier liquid via an injection valve (Figure 1.4) for merging with the reagents used by the analytical method applied.

FIGURE 1.4. Manual injection valve typically used in FIA. (For color version of this figure, the reader is referred to the online version of this book.)

Tube lengths and the rotation speed of the peristaltic pump are dictated by the reaction time. Thus, if a long time is required for kinetic reasons, then a long piece of tubing is inserted—usually in coiled form—in order to increase residence times of the sample and reagents in the reactor.

Another difference between SFA and FIA, is that unlike SFA which operates under turbulent flow regime, FIA works in laminar flow, which reduces the likelihood of carryover between successive samples. Also, FIA does not require separation of samples with intervening bubbles, that is to say it uses unsegmented flow.

In Figure 1.5 is shown an FIA manifold used in a theoretical hydrodynamic study involving the injection of a dye. At the bottom is the profile exhibited by the dye plug in its way from the injection point to the detector. The result is an asymmetric peak (Figure 1.6(b) ) exhibiting a χ 2 type distribution. The height and area of the peak are proportional to the concentration of the target species, which facilitates the construction of a calibration curve for its determination in unknown samples. In Figure 1.6(a) are shown the peaks obtained from quadruplicate injections of a series of standards of increasing concentration of analyte.

FIGURE 1.5. Single-channel FIA manifold. IV: injection valve, RC: reaction coil.

FIGURE 1.6. (a) Peaks used to construct a calibration curve. (b) Typical FIA recording.

Table 1.3 summarizes the most salient features of FIA. As can be seen, operating conditions are rather different from those of SFA. One of the most relevant differences is the reduction of sample volume which is reduced from milliliters to microliters. Response times are also substantially shorter and tubing diameters smaller in FIA than in SFA.

Table 1.3. Characteristics of Flow Injection Analysis

Injected sample volume 50–150   μL
Response time 3–60   s
Tubing diameter 0.5   mm
Detection conditions Equilibrium not needed
Throughput About 100 injections/h
Precision 1–2%
Reagent consumption Low
Flushing cycle Unnecessary
Kinetic methods Feasible

While SFA usually requires that the analytical reaction reaches chemical equilibrium, FIA does not. In fact, FIA only requires the extent of reaction to be constant and reproducible, which is facilitated by the high reproducibility provided by the hydrodynamic behavior of the system. Moreover, since FIA uses much thinner tubing and lower flow rates, it consumes samples and reagents much more sparingly than does SFA.

In addition, FIA is much more flexible than SFA allowing the implementation of analytical methodologies unaffordable to the latter, e.g. kinetic methods, stopped-flow methods.

Another major advantage of FIA over SFA is its ease of implementation. In fact, a dedicated manifold can be readily assembled from fairly inexpensive parts, viz. a peristaltic pump, injection valves, flow-cells, Teflon tubing, connectors and available measuring instruments, e.g. spectrophotometers, potentiometers, ammeters, atomic absorption spectrometry equipment. This propitiated a vast expansion of FIA among research laboratories and led to the development of a large number of applications relative to other, more recent techniques within a few years after its inception.

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Foreword

Gary D. Christian , in Flow Analysis with Spectrophotometric and Luminometric Detection, 2012

Flow analysis techniques date to over eighty years ago, but modern analytical flow techniques began in the 1950s with the introduction of segmented flow analysis, followed about two decades later by flow injection analysis. Numerous books have been written over the years on flow analysis in general and flow injection analysis in particular. The most widely used detection systems employ flow cells utilising attenuation or radiation of light. This is the first book to focus on these important detection systems and methods, i.e., spectrophotometry, turbidimetry and nephelometry, and techniques based on fluorescence, chemiluminescence and bioluminescence. It is intended to be complementary to existing monographs.

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Sample Handling

Elias A.G. Zagatto , ... Paul J. Worsfold , in Flow Analysis with Spectrophotometric and Luminometric Detection, 2012

8.4.1 Early Developments

Initial attempts to perform in-line digestion of plant tissues were made in1960 [122]. The Kjeldahl digestion procedure was implemented in a segmented flow analyser by adding the solid sample and the digestion mixture (500   mL   L−1 sulphuric acid   +   3.0   mL   L−1 perchloric acid + 4.8 mL L−1 selenium oxychloride) to one end of a tubular-shaped digestion vessel with a spiral groove on its inner wall. Continuous rotation pushed the mixture towards the opposite end of the vessel and improved mixing conditions. The entire assembly was placed inside a furnace. The outlet flow was handled as in ordinary segmented flow analysis (see 5.1.1). Protein in plant materials was determined in the 0.1–10.0 mg L−1 range at a sampling rate of 10 h−1 and good repeatability of results (r.s.d. <1.5 %) was reported.

The spectrophotometric flow injection determination of chemical oxygen demand (COD) in wastewaters with in-line oxidation [123] was an early application of in-line treatment in unsegmented flow analysis. The sample was inserted into a water carrier stream which merged with an acidic dichromate confluent stream and the combined stream passed through a PTFE coiled reactor at 120 °C. Accurate results were obtained at a rate of 15 h−1; the detection limit and precision were 5 mg L−1 COD and 0.4%, respectively. Another early application was the determination of orthophosphate and polyphosphates, such as diphosphate and triphosphate, with in-line hydrolysis [124]. A strongly acidic solution containing molybdenum(V) and molybdenum(VI) was used as the carrier so that hydrolysis of polyphosphates and colour development of the resultant orthophosphate were simultaneously accomplished. For speeding up the chemical reactions involved, the temperature of the reaction coil was maintained at 140 °C, and a back-pressure coil was required (see 4.3). A sampling frequency of 45 h−1 and r.s.d.3 of <1% were reported. The efficiency of the system was comparable with that of an air-segmented flow system.

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Flow Analysis | Overview☆

E.A.G. Zagatto , ... P.J. Worsfold , in Encyclopedia of Analytical Science (Third Edition), 2016

Dilution/Dispersion

A common feature of flow analysis is that during sample transport towards the detector, the combined effects of dilution and dispersion decrease the analyte concentration and the extent of these processes is very important for system design.

Dilution is the main process for lowering the analyte concentration and lengthening the sample zone, especially in segmented flow analysis. It occurs instantaneously at every confluence point due to mixing of the sample with the confluent stream. The analyte concentration undergoes sudden reduction at each confluence point in proportion to the ratio of carrier-to-confluent stream flow rates. In unsegmented flow analysis, both the reduction in concentration and sample zone lengthening at each confluence point may be less evident due to the sample dispersion occurring downstream.

Dispersion occurs continuously during transport of the sample zone through the analytical path due to the redistribution of material from the sample zone into the carrier stream (and vice versa). It is caused by convective and diffusive mass transport. Convective mass transport is a consequence of the parabolic distribution of the linear velocities of every fluid element whereas diffusive mass transport is dependent on the concentration differences between neighbouring fluid elements and the diffusion coefficients involved. Diffusive mass transport occurs in an isotropic fashion, but only its radial component is relevant as a factor influencing dispersion inside a straight tubular reactor.

Dispersion is an important parameter in unsegmented flow analysis and can be efficiently controlled by varying parameters such as inserted sample volume, reactor geometry and flow rates, in order to attain highly reproducible physical conditions, and hence reproducible analytical results. It is also relevant — although to a lesser extent — in segmented flow analysis, and is caused by the thin layer of solution established on the inner wall of the tubing (see Fig. 3 ).

Different mathematical models have been proposed for a quantitative description of sample dispersion in flow analysis, and the convective-diffusive model has been used most often. It describes the concentration of any fluid element as a function of its spatial and temporal co-ordinates, and allows a good simulation to assist further system design and method development. The model considers the mass balance in an infinitesimal volume of the fluid and the axial and radial concentration gradients, as well as the linear velocities established in a laminar flow pattern. The model has often been adapted in order to consider the presence of reactors other than the straight open tubular reactor (eg, coiled, knotted, packed bead), mixing chambers, mini-columns and other specific devices in the analytical path.

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FLOW INJECTION ANALYSIS | Environmental and Agricultural Applications

M. Miró , W. Frenzel , in Encyclopedia of Analytical Science (Second Edition), 2005

Introduction

Flow techniques are being consolidated as powerful tools for the routine control of parameters in various samples of environmental and agricultural origin. The outstanding features of flow injection analysis (FIA) for analytical applications are its extremely straightforward configuration, easy operation, and low costs. Automated analyses with high sample throughput are attained using relatively simple instrumentation. The microformat of FIA results in considerable reduction of sample and reagent consumption compared to classical wet-chemical methods of analysis. Furthermore, as opposed to air-segmented flow analysis (SFA) methodologies, the reproducible timing and readily controllable dispersion of the analyte zone allows the performance of kinetic measurements without demanding steady-state conditions. Other differential features of FIA in relation to SFA are the injection of smaller sample volumes (of the order of 10–100  μl), the suppression of the washing cycles, the increase of the sampling throughput up to 120 injections h−1, the repeatability improvement, and especially the ease of manifold construction and flexibility. The ready adaptation of batch methods permits FIA to be used in compliance with many official standard methods (namely, US Environmental Protection Agency (EPA), International Standards Organization, and European Norms; see comments in Table 1). Environmental safety is another attribute of FIA. The closed-system chemistry not only decreases contamination of the sample but also prevents technicians coming into contact with hazardous chemicals. Moreover, as a consequence of its inherent miniaturization, a considerable decrease in waste generation, and hence also in costs for waste disposal, is achieved. FIA has also attracted the attention of many researchers owing to its ability to fulfill the current demands in environmental analysis. The fast response of FIA makes the analytical information available in real time, which is especially desirable in monitoring schemes, some of the flow analysers being included in EPA directives. FIA has also been exploited for in-field analysis aiming to obtain high temporal and spatial resolution data without the need for discrete sample collection and storage. This is of great interest whenever offline analysis or excessive sample handling is unacceptable due to the rapid transformation of the target analyte.

Table 1. Analytical features of flowing-stream methods applied to the determination of relevant parameters in natural, tap, wastewaters, and industrial effluents

Analyte Method/reagents/derivatization reaction Detection Working range Particular features
Ammonium Berthelot Spectrophotometry 0.010–1.0   mg   N   l−1 Replacement of phenol by salicylate
Nessler Spectrophotometry 0.050–0.50   mg   N   l−1 Coating of the colloidal product on the tubing walls
GD – boric acid, HCl, or distilled water as acceptors Conductimetry 0.010–0.50   mg   N   l−1 Conductivity changes
GD – pH indicator Spectrophotometry 0.010–1.0   mg   N   l−1 Standard method EN/ISO 11732
ISE containing the nonactin ionophore Potentiometry 0.8–140   mg   N   l−1 Gas-diffusion/use of activated carbon to remove interfering organic compounds
Ion-selective coated wire electrode Potentiometry 0.14–1400   mg   N   l−1 Portable battery-powered analyzer
Clinoptilolite-modified electrode Amperometry 0.28–14   mg   N   l−1 Copper-doped electrode
NaClO-luminol Chemiluminescence 0.05–0.4   mg   N   l−1 Indirect measurement
Hypochlorite-ozone Chemiluminescence 0.0014–1.4   mg   N   l−1 Gas-phase detection
OPA Spectrofluorimetry 0.0004–0.7   mg   N   l−1 Use of nucleophilic species
Nitrite Griess-Ilosvay Spectrophotometry 0.010–0.70   mg   N   l−1 Simultaneous determination of nitrate using a Cu–Cd column/standard method EN/ISO 13395
Griess-Ilosvay/reversed-phase extraction SPS 0.3–30   μg   N   l−1 Solid-phase optrode
Gallocyanine Spectrophotometry 0.003–0.80   mg   N   l−1 Catalytic reaction
Naphthol Green B Spectrophotometry 0.6–60   μg   N   l−1 Catalytic reaction
3-Aminonaphthalene-1,5-disulfonic acid Spectrofluorimetry &gt;14   μg   N   l−1 Formation of an azoic acid salt
3,3′,4,4′-Biphenyltetramine Spectrofluorimetry 9–30   μg   N   l−1 Direct determination
Rhodamine 6G/bromate Spectrofluorimetry 2.4–30   μg   N   l−1 Kinetic method
Iodide Biamperometry 0.02–14   mg   N   l−1 Detection of the tri-iodide generated
Carbon paste electrode modified with a ruthenium polymer Amperometry 0.0007–7   mg   N   l−1 Decrease of the overpotential for nitrite oxidation
Nitrate Griess/Cu–Cd Spectrophotometry 0.060–11.30   mg   N   l−1 Online reduction to nitrite
No reagent Spectrophotometry 0.10–3.10   mg   N   l−1 Direct UV detection-multivariate analysis
Griess/photo-induced reduction Spectrophotometry &gt;0.42   μg   N   l−1 Use of EDTA as activator
Luminol/UV irradiation Chemiluminescence 0.001–1.4   mg   N   l−1 Generation of peroxynitrite species
Tubular ISE composed of a DPNi containing film Potentiometry &gt;3.4   mg   N   l−1 Assessment of various ionic strength modifiers
Total nitrogen Sodium peroxodisulfate/hydrazine/Griess Spectrophotometry 0.20–20   mg   N   l−1 Online microwave-assisted digestion
Alkaline peroxodisulfate/Cd-column/Griess Spectrophotometry 0.03–3.0   mg   N   l−1 Online photo-oxidation
Sodium peroxodisulfate/Cu–Cd column/Griess Spectrophotometry &gt;5.0   μg   N   l−1 Thermal digestion using a capillary tube containing a Pt wire
Orthophosphate Molybdenum blue Spectrophotometry 0.010–0.25   mg   P   l−1 Use of antimony as a catalyst
Molybdenum blue Spectrophotometry &gt;0.2   μg   P   l−1 Solid-phase extraction
Molybdate/vanadate Spectrophotometry 0.15–18   mg   P   l−1 Detection of the yellow complex without reduction
Molybdate/malachite green Spectrophotometry 0.010–0.10   mg   P   l−1 Ion-pair association
Molybdate/rhodamine B Spectrofluorimetry 0.0003–0.095   mg   P   l−1 Quenching of fluorescence
Molybdate/thiamine Spectrofluorimetry 0.007–6.6   mg   P   l−1 Generation of fluorescent thiochrome
Pyruvate oxidase G/luminol Chemiluminescence 0.003–1.0   mg   P   l−1 Generation of hydrogen peroxide via enzymatic reactions
Glassy carbon electrode Amperometry 0.006–0.32   mg   P   l−1 Free of silicate interference
Cobalt wire electrode Potentiometry 3.1–310   mg   P   l−1 Indirect determination
Phosphorus species Sodium peroxodisulfate in acidic medium/molybdate/ascorbic acid Spectrophotometry 0.010–1.0   mg   P   l−1 Determination of dissolved organic phosphorus/online UV-photo-oxidation/standard method EN/ISO 15681
Thermal (or microwave-assisted) digestion in acidic medium/molybdate/tin(II) chloride Spectrophotometry 0.020–2.0   mg   P   l−1 Determination of total dissolved phosphorus previous acidic hydrolysis of condensed phosphates
Molybdenum blue Spectrophotometry/ICP-AES 0.1–200   mg   P   l−1 Serial arrangement of detectors for the determination of orthophosphate and total phosphorus
Silicate Molybdenum blue Spectrophotometry 0.14–1.4   mg Si l−1 Addition of masking agents (tartaric or oxalic acids)/standard method EN/ISO 16264
Molybdate/rhodamine B Spectrophotometry 0.17–2.0   mg Si l−1 Ion-pair association
Molybdate Spectrophotometry 3.5–80   mg Si l−1 Simultaneous determination of orthophosphate and silicate exploiting the differences of reaction rates
0.8–15   mg   P   l−1
Molybdate Spectrophotometry 0.9–30   mg Si l−1 Selection of appropriate acidity and sample segmentation with oxalic acid
0.2–12   mg   P   l−1
Molybdenum blue Spectrophotometry 5.0–50   mg Si l−1 Selection of appropriate concentration of oxalic acid
0.2–7.0   mg   P   l−1
Sulfate Barium chloride in a polyvinyl alcohol medium Turbidimetry 5.0–200   mg   SO4 2−  l−1 Addition of alkaline EDTA to avoid the build-up of precipitate
Barium dimethylsulfonazo-III Spectrophotometry 0.2–14   mg   SO4 2−  l−1 Color-fading reaction
Ferric nitrate Spectrophotometry 10–1000   mg   SO4 2−  l−1 Sample pretreatment with activated charcoal is required to remove interfering organic species
Methylthymol blue (MTB) Spectrophotometry 0.025–1.0   mg   SO4 2−  l−1 Solid-phase extraction/detection of the MTB–Ba complex
Barium chloranilate Spectrophotometry 4.0–100   mg   SO4 2−  l−1 Displacement reaction
BANH-Zr Spectrofluorimetry 1.5–150   mg   SO4 2−  l−1 Comparison of various manifolds
MTB-Zr Spectrophotometry 0.5–20   mg   SO4 2−  l−1 Catalytic reaction
Lead nitrate Potentiometry 100–1000   mg   SO4 2−  l−1 Use of a Pb-ISE
Sulfide Nitroprusside Spectrophotometry 0.5–10   mg   S2−  l−1 No concentrated aggressive reagents are required
Methylene blue Spectrophotometry 0.01–15   mg   S2−  l−1 30-fold sensitivity increase with regard to nitroprusside method
Methylene blue and nitroprusside Spectrophotometry &gt;0.2   μg   S2−  l−1 Gas-diffusion separation/preconcentration using a halted recipient solution
Direct determination on a glassy carbon electrode Amperometry 0.03–24   mg   S2−  l−1 Adaptable to on-board measurements
Sulfide-sensitive electrode Potentiometry 1–1000   mg   S2−  l−1 Tubular electrodes with homogeneous crystalline membranes or treated silver foils
2,6-Dichlorophenol-indophenol modified electrode Amperometry 0.65–32   mg   S2−  l−1 Long-term stability
Chloride Mercury(II)thiocyanate/Fe(III) Spectrophotometry 1.0–1000   mg   Cl−  l−1 Standard method EN/ISO 15682
Hg-EDTA Spectrophotometry &gt;0.2   mg   Cl−  l−1 UV detection
Mercury(II)thiocyanate Spectrophotometry 0.16–2000   mg   Cl−  l−1 Absence of Fe(III)
Ag (or Hg)-chloranilate Spectrophotometry 1.0–20   mg   Cl−  l−1 Use of a solid reagent reactor
AgNO3 Turbidimetry 3.0–30   mg   Cl−  l−1 Recirculation of reagent in a closed system
Chloride-ISE Potentiometry 0.1–10000   mg   Cl−  l−1 Wide dynamic range/standard method EN/ISO 15682
Dedicated Ag-ISE Potentiometry 2.0–90   g   Cl−  l−1 Online titration
Residual chlorine Methyl orange Spectrophotometry 0.1–10   mg   Cl−  l−1 Color fading reaction
4-Nitrophenylhydrazine Spectrophotometry 0.05–40   mg   Cl−  l−1 Generation of azo-dye species (or measurement of decrease of reagent absorbance)
o-Tolidine Spectrophotometry 0.5–5.0   mg   Cl−  l−1 Formation of an unstable hydroquinone
o-Dianisidine Spectrophotometry 0.05–1.30   mg   Cl−  l−1 Gas-diffusion/tandem-flow approach
Gold electrode Amperometry &gt;7.0   μg   Cl−  l−1 Useful for monochloramine determination
Rhodamine 6G Chemiluminescence 0.07–7.0   mg   Cl−  l−1 Electrostatic immobilisation of the reagent
Disproportionation of chlorine into HClO and Cl− Potentiometry 0.7–7.0   mg   Cl−  l−1 Gas-diffusion separation/Cl-ISE
N,N-diethyl-p-phenylenediamine/KI Spectrophotometry 0.1–8.0   mg   Cl−  l−1 Sequential determination of free and combined chlorine
KI/UV detection Spectrophotometry 0.03–3.0   mg   Cl−  l−1 Determination of total chlorine
Fluoride Fluoride-ISE Potentiometry 0.005–50   mg   F   l−1 High selectivity, wide dynamic ranges, and low detection limits
Lanthanum(III)/alizarin complexone Spectrophotometry 0.03–3.5   mg   F   l−1 Sensitivity improvement via addition of cationic surfactants
Zr(IV)-SPADNS Spectrophotometry 0.02–3.5   mg   F   l−1 Inhibitory effect
Lanthanum(III)/alizarin complexone ICP-AES 0.03–1.3   mg   F   l−1 Liquid–liquid extraction/indirect determination
Bromide Phenol red Spectrophotometry 0.16–2.4   mg   Br   l−1 Formation of the bromophenol blue dye
Tetrabase/chloramine T Spectrophotometry 1.0–40   μg   Br   l−1 Catalytic reaction
Permanganate/Pt-electrode Amperometry &gt;0.08   mg   Br   l−1 Gas-diffusion
Iodide Iron(III) thiocyanate Spectrophotometry 0.75–150   μg   I   l−1 Fading effect
Dichromate/KI (receiver) Spectrophotometry &gt;0.2   mg   I   l−1 Gas-diffusion
Ce(IV)/As(III) Spectrophotometry 0.002–0.5   mg   I   l−1 Sandell–Kolthoff's reaction
Sodium nitrite MIP-AES &gt;2.3   μg   I   l−1 Generation of volatile iodine
Cr(VI) FAAS 6.0–220   μg   I   l−1 Indirect method/sorption of formed Cr(III) onto a chelating column
Bromate Chlorpromazine Spectrophotometry 1.0–30.0   μg   BrO3 −  l−1 Masking strategies are required to minimize interference from nitrite and chlorite
Ion-exchanger/ammonium hydroxide ICP-MS 0.06–50   μg   BrO3 −  l−1 Selective elution of bromide and bromate from an activated alumina microcolumn
Chlorate/chlorite KI/HCl Spectrophotometry 0.1–10.1   mg   ClO2 −  l−1 Speciation is possible due to the different acidic requirements for iodide oxidation
0.1–8.3   mg   ClO3 −  l−1
Cyanide Chloramine-T/pyridine-barbituric acid Spectrophotometry/Amperometry 0.002–2.0   mg   CN−  l−1 Gas-diffusion preconcentration/UV-photolysis for total cyanide determination/standard method ISO 14403
Metallic silver-wire electrode Potentiometry 0.05–250   mg   CN−  l−1 Gas-diffusion/use of a non-selective sensor
Luminol/Cu Chemiluminescence 0.005–2.0   mg   CN−  l−1 Luminol oxidation by Cu(CN)4 2−
Surfactants Hypochlorite/rhodamine B Chemiluminescence 0–50   mg   l−1 Determination of amine ethoxylate-based nonionic surfactant
Alizarin fluorine blue Spectrophotometry 0.2–12.0   mg   l−1 Determination of Triton-type surfactants
Bromocresol purple Spectrophotometry &gt;10−6  mol   l−1 Determination of cationic surfactants
Luminol/N-bromosuccinimide (or N-chlorosuccinimide) Chemiluminescence 10−6–10−4  mol   l−1 (CPC, DTAB, CTAB) Decrease of chemiluminescence emission at surfactant concentrations below the critical micelle concentration
Methylene blue (standard method EN/ISO 16265) or rhodamine B Spectrophotometry (or spectrofluorometry) &gt;0.03   mg   l−1 (C12-alkylbenzene sulfonate) Solvent extraction/on-tube detection
DIC GD-Cresol Red or 4-(2′,4′-dinitrophenylazo)-1-naphthol Spectrophotometry 10−6–10−3  mol l−1 Color change of acid–base indicators
GD-NaOH (acceptor stream) Conductimetry &gt;3×10−6  mol l−1 Measurement of signal decrease
GD-H2SO4 (donor stream) QCM 5×10−4 to 2×10−2  mol   l−1 Transient variation of the monitored frequency of QCM on CO2 passage
GD-HNO3 (donor stream)/N2 (recipient stream) FTIR &gt;7.5×10−5  mol   l−1 Gas-phase detection
TOC GD-carbon dioxide detection Spectrophotometry/infrared spectrometry 10−6–10−4  mol   l−1 Photochemical oxidation
GD-carbon dioxide detection BAWIS 10−5–2×10−2  mol   l−1 Wet chemical oxidation
COD Permanganate oxidation Spectrophotometry &gt;5.0   mg   l−1 40   m coil, 100°C
Dichromate oxidation Spectrophotometry 2.0–100   mg   l−1 Microwave-assisted digestion
Cerium(IV) oxidation Spectrophotometry 0.5–130   mg   l−1 20   m coil, 100°C
Dichromate oxidation via microwave-assisted digestion FAAS 25–5000   mg   l−1 Retention of the excess of dichromate on an anionic exchange column prior atomic absorption detection
BOD Trichosporon cutaneum/tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) Spectrofluorimetry 0–110   mg   l−1 Design of a three-layer fiber optic microbial sensor
pH Flat-headed combined glass electrode Potentiometry pH 1.0–14.0 FIA titration
N,N-dioctadecyl-methylamine Potentiometry pH 4.0–11.0 The ionophore was photo-cured on a silver wire
Immobilized indicator Spectrophotometry pH 4.0–10.0 Optosensing method
Thymol blue Spectrophotometry pH 7.6–8.1 (±0.0007   pH units) High temporal resolution
Alkalinity Methyl orange or methyl red Spectrophotometry 10–200   mg   l−1 Online titration
HCl as a titrant Potentiometry No calibration required Determination of partial and intermediate alkalinity

DIC, dissolved inorganic carbon; TOC, total organic carbon; COD, chemical oxygen demand; BOD, biochemical oxygen demand; ISE, ion-selective electrode; SPS, solid-phase spectrophotometry; Tur, turbidimetry; FAAS, flame atomic absorption spectrometry; ICP-AES, inductively coupled plasma atomic emission spectrometry; ICP-MS, inductively coupled plasma mass spectrometry; MIP-AES, microwave-induced plasma atomic emission spectrometry; QCM, quartz crystal microbalance; FTIR, Fourier transform infrared spectrometry; BAWIS, bulk acoustic wave impedance sensor; GD, gas-diffusion; EDTA, ethylenediaminetetraacetic acid; OPA, o-phthaldialdehyde; MTB, methylthymol blue; BANH, biacetylmonoximenicotinylhydrazone; SPADNS, 2-(p-sulfophenylazo)-1,8-dihydroxynaphthalene; Tetrabase, 4,4′-bis(dimethylamino)diphenylmethane; CPC, cetylpyridinium chloride; DTAB, dodecyltrimethylammonium bromide; CTAB, cetyltrimethylammonium bromide; DPNi, tris(4,7-diphenyl-1,10-phenanthroline)nickel(II).

Despite the aforementioned benefits of FIA with regard to manual procedures and precedent flow techniques, the incompatibility of the elastic tubes of peristaltic pumps with concentrated acid or bases and organic solvents usually forces the periodical recalibration of the system or the incorporation of more expensive reagent-resistant tubing. The physical adsorption of organic analytes onto the Tygon tubes is another practical limitation commonly described. Though performance of multiparametric determinations using a single multidetection system or several detectors arranged in series has been reported in several studies, the simultaneous monitoring of various environmental parameters have been rather limited to particular applications. It should be also stressed that the continuous flowing of solutions according to FIA philosophy results in the consumption of carrier, reagent, and sample during the overall analysis protocol.

Sequential injection analysis (SIA) was proposed in 1990 as a new concept for handling solutions alternatively to FIA. Whereas in conventional FIA manifolds the sample volume is inserted into the carrier stream and subsequently merged downstream with reagents, SIA methodology is based on the software-controlled sequential aspiration of precise volumes of sample and reagent, which are afterwards dispensed into the reaction coil by flow reversal. Therefore, a considerable sample and reagent saving is achieved – of the order of 20 times in relation to FIA. The replacement of the FIA injection valve with a rotary selection valve has enabled the incorporation of additional reactors/modules, the performance of backward–forward flow movements, the exploitation of stopped-flow schemes, and the simple development of multiparametric determinations by coupling different detectors and appropriate reagents to the multiposition valve. The use of syringe pumps as liquid drivers has allowed the manipulation of sample and reagent volumes at the low microliter level with high precision. The microanalytical features of SIA, along with the handling of the exact volumes required for the particular assays, have opened new avenues in the monitoring of environmental parameters. For example, atmospheric precipitates are often only present in limited quantities or long collection times are required, so that microsampling is extremely beneficial. The main drawback of SIA for analytical applications is the lower sampling rate achieved in relation to FIA as a consequence of the periodical filling of the liquid driver and the stacking of the entire set of segments in the holding coil. Besides, the sequential and nonsimultaneous propelling of the solutions that takes place in SIA results in longer residence times for proper interdispersion between zones. This shortcoming has been already solved by replacing the multiposition SIA valve with solenoid valves or miniaturized integrated conduits, so that novel flow techniques such as the so-called multicommuted flow injection analysis, multisyringe flow injection analysis (MSFIA), and sequential-injection lab-on-valve have been recently designed. These methodologies, which include the outstanding features of the parent FIA and SIA schemes, have proven effective tools for the miniaturization and automation of environmental assays and also for performing reliable sample pretreatments. Figure 1 illustrates various flowing-stream configurations devised for the spectrophotometric or spectrofluorimetric monitoring of several analytical parameters in aqueous samples.

Figure 1. Schematic illustration of flowing stream manifolds devised for the automatic determination of relevant environmental parameters in water samples using various flow techniques: (A) flow injection analysis, (B) sequential injection analysis, (C) multicommutation flow injection analysis, (D) multisyringe flow injection analysis. The sequential injection-lab-on-valve configuration is depicted in previous article: Flow injection analysis-detection techniques. S, sample; R, reagent; C, carrier; D, detector; IV, injection valve; HC, holding coil; RC, reaction coil; MV, multiposition valve; SV, solenoid valve; SP, syringe pump; MP, multisyringe pump; PP, peristaltic pump; St, stopper; W, waste.

In this article, flowing-stream systems assembled for environmental and agricultural analysis are classified according to their application area, i.e., water, air, soil, and plant analysis. Within each area relevant flow methodologies for the determination of individual analytes are briefly reviewed and specific features of particular methods are outlined. Additional information about the analytical performance of several flow assemblies is listed comprehensibly in the tables. The likelihood of direct introduction and treatment of solid samples in an automated fashion is also highlighted in the bulk of the text. Finally, attention is also paid to the different schemes available for online speciation studies, which are of increasing significance in environmental assays.

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Flow Analysers

Elias A.G. Zagatto , ... Paul J. Worsfold , in Flow Analysis with Spectrophotometric and Luminometric Detection, 2012

5.5.1 The Mono-segmented Flow Analyser

This mode of flow analysis was proposed [128] as a means of easily and efficiently achieving extended sample handling times without excessive sample dispersion. The sample volume is inserted into an unsegmented carrier stream, and two air plugs are added at its ends in order to minimise sample broadening, and hence axial dispersion. The beneficial effects arising from the presence of air plugs at both ends of the sample bolus were already emphasised in 1972, in relation to a chemiluminometric determination of low concentrations of Cr(III) [129].

The approach is particularly attractive for implementing analytical procedures where sensitivity is critical and the chemical reactions involved are relatively slow [130–132], and can be also exploited for accomplishing differential kinetic analysis [133] . As the axial dispersion experienced by the sample is minimal, even for long analytical paths, sample integrity is preserved. Several samples can be simultaneously handled inside the analytical path. Analogously to segmented flow analysis, the air phase is generally removed immediately before sample detection by means of a gas-permeable membrane or a de-bubbler.

This innovation is also useful for attaining low sample dispersion with low injected sample volumes because the air phase hinders axial dispersion. This feature was demonstrated in the potentiometric determination of d-glucose in undiluted whole blood [134].

Only a few air plugs are present in the manifold at any one time; therefore, some processes inherent in unsegmented flow systems such as sample stopping, flow reversal and stream splitting can be exploited. It is also possible to precisely define the instant position of the air plugs, allowing, e.g., the easy implementation of the bubble gating approach [5].

Gaseous samples can also be handled and the spectrophotometric determination of O2 and NO2 in the headspace of packages [135] is a good example of this innovation. The analyte was transported from the gaseous sample towards the liquid segment containing the colour-forming reagent via the thin film established on the tube wall. Sensitivity was enhanced by exploiting flow reversal and/or sample stopping.

The flow system is simplified with flame atomic absorption spectrometric detection [136] because an air plug is not placed at the front of the sample in order not to disturb the steady state of the flame. Consequently, the aqueous sample is inserted into the unsegmented carrier stream with only one air plug positioned after it. Tailing effects are therefore minimised and the sampling rate is significantly improved relative to ordinary flow injection systems with flame atomic absorption spectrometric detection. Moreover, removal of the gaseous phase is not needed.

The use of mono-segmented flow systems for improving liquid–liquid extractions and flow titrations is discussed in Chapter 8.

As stressed in Section 2.3, this innovation is also referred to as segmental flow injection analysis [137]. This broader term includes mono-segmented flow analysers and some specially designed flow systems involving, e.g., injection of gaseous [138] or segmented [131] samples into a continuous flowing unsegmented carrier stream. Conversely, aqueous solutions can be inserted into a segmented stream. With this strategy, fluoride was potentiometrically determined in 0.1 mL of natural water samples at a sampling rate as high as 720 h−1, with the r.s.d. <1% [22]. These examples emphasise that the borderline between segmented and unsegmented flow analysis is not distinct. The potential, limitations and applications of segmental flow injection systems have recently been reviewed [131].

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