FIA manifold

In general, the sensitivity of FIA is less than that for conventional methods of analysis for two principal reasons. First, as with chemical kinetic methods, measurements in FIA are made under nonequilibrium conditions when the signal has yet to reach its maximum value. Second, dispersion of the sample as it progresses through the system results in its dilution. As discussed earlier, however, the variables that influence sensitivity are known. As a result the FIA manifold can be designed to optimize the sensitivity of the analysis. 

A sensitive method for the flow injection analysis of Cu + is based on its ability to catalyze the oxidation of di-2-pyridyl ketone hydrazone (DPKH) by atmospheric oxygen. The product of the reaction is fluorescent and can be used to generate a signal when using a fluorometer as a detector. The yield of the reaction is at a maximum when the solution is made basic with NaOH. The fluorescence, however, is greatest in the presence of HCl. Sketch an FIA manifold that will be appropriate for this analysis. 

Figure 3.4 — (A) FIA manifold for the determination of L-lactic acid in milk products P peristaltic pump C carrier S sample V injection valve AD air damper FTC flow-through cell W waste BFB bifurcated fibre LS light source PMT photomultiplier tube R recorder. (B) Cross-section of lactic acid optrode. (Reproduced from [39] with permission of VCH Publishers).

The advantage of the selective adsorption of a particular element oxidation state has been exploited for on-line element preconcentration and speciation analysis of Cr by FAAS. Cespon Romero et al. [21] described an FIA system employing a minicolumn made of a chelating resin containing poly(aminopho-sphonic) acid groups, able to selectively retain Cr(III) ions. An FIA manifold was employed for efficient preconcentration and subsequent elution of Cr(III) with a small volume of 0.5 M HC1. The original sample was also treated with ascorbic acid to reduce Cr(VI) to Cr(III) and total Cr is determined as Cr(III) after appropriate retention and elution. Eluates are introduced into an N20-acetylene flame connected to the column outlet. The concentration of Cr (VI) is obtained by difference. Employing a sample volume of 6.6 mL, LoD for total Cr is 0.2 pg l-1. A study of FI operational variables, interferences, and precision is reported for the analysis of tap, mineral, and river waters. 

Fig. 83. FIA manifold as used in preconcentration work with AAS, ICP-OES and ICP-MS. (Reprinted with permission from Ref. [269].)...

Fig. 49. Response of an FIA manifold provided with a glucose electrode containing polyurethane-immobilized GOD to injection of 25 glucose samples of 1 mmol/1. The measuring frequency was 200/h. (Redrawn from Olsson et al., 1986b).

Scheller et al. (1986a) combined polyurethane-immobilized LOD with an Au/Pd-sputtered carbon electrode. The electrode modification permits H2O2 to be electrochemically oxidized at a potential as low as +450 mV, where interferences by other anodically oxidizable compounds such as NADH and ascorbic acid are largely reduced (Fig. 57). This increased selectivity also enables the measurement of LDH activity. The sensor has been introduced in an FIA manifold. A sample frequency of 200Ai (Fig. 58) and a CV below 1% were obtained with this setup. A platinum electrode with LOD covalently bound to a nylon membrane has been employed for continuous blood lactate determination in an artificial pancreas by Mascini et al. (1985b, 1987). 

Olsson (1988) obtained a pH signal which was linearly dependent on penicillin concentration by using a penicillinase reactor in an FIA manifold. The linearity was due to complete substrate hydrolysis in the reactor and an almost constant buffer capacity of the carrier stream. To exclude disturbances from varying sample pH the penicillin concentration was calculated as the difference between the response with and without hydrolysis. 

Fig, 80. FIA manifold comprising a glucoamylase (GA) reactor and a GOD flow-through electrode for measurement of maltose and starch. (Redrawn from Scheller et al., 1987c). 

The coupled reactions of isomerase, mutarotase and GOD should be applicable to assay fructose, but a sensor using this sequence has not been described to date. An FIA manifold containing an enzyme reactor with glucose isomerase and mutarotase for the determination of fructose has been developed by Olsson (1987). The glucose formed was measured spectrophotometrically. 

Yao (1983) has shown that determination of acetylcholine esterase (AChE) activity is feasible with the above mentioned CME. ACh was hydrolyzed in a reaction vessel arranged in an FIA manifold upstream of the choline electrode. With a sample throughput of 40/h the sensor responded linearly to the enzyme activity over the range of 0.25 to 100 mU. 

The spectrometric assay of iron (II) with 1,10-phenantroline is used in a flow injection analysis (FIA) for iron determination in soil.108 Iron (II) was extracted from soil samples by shaking the soil with an ammonium acetate solution (pH = 3) and the extract used for FIA determination. FIA manifold always contains a step of reduction of iron (III) to iron (II). The system can detect 60 iron samples per hour. It is a fast and reliable system for iron assay in soil. Also, it is very simple to use in every laboratory. The level of iron assay is only of milligram-per-liter magnitude, but a preconcentration step can solve this problem. 

Figure 21.2 Typical FIA manifold for chloride determination where the sample is injected and the reagents are pumped as the carrier stream. (Reprinted by permission of John Wiley and Sons.)...

The simplest way of measuring the dispersion coefficient is to inject a well-defined volume of a dye solution into a colorless carrier stream and to monitor the absorbance of the dispersed dye zone continuously by a colorimeter. To obtain the )max value, the height (i.e., absorbance) of the recorded peak is measured and then compared with the distance between the baseline and the original signal obtained when the cell has been filled with the undiluted dye. Provided that the Beer-Lambert law is obeyed, the ratio of respective absorbances yields a value that describes the FIA manifold, detector, and method of detection. 

Figure 4.1. Single-line FIA manifold for the determination of metal ions by flame atomic absorption spectrometry (AA). The sample (5) is injected into a carrier stream of diluted acid (5 X 0 M sulfuric acid), propelled forward by pump P, and transported to the nebulizer of the AA system, the distance between the injection valve and the AA instrument being reduced as much as possible (length 20 cm) in order to secure limited dispersion of the injected sample.

Figure 4.4. (a) FIA manifold for simultaneous determinations of pH and pCa. 5, point of injection (30 (xL) pH, flowthrough capillary glass electrode FC, flowthrough cell containing a PVC-membrane-based calcium-selective electrode and the common reference electrode, details of which are shown below (b). The carrier solution (A and B) is TRIS buffer of pH 7.4, the connecting tubes (a and b) being made as short as possible. The carrier solution supplied via line B to the tip of the reference electrode is included in order to stabilize the reference electrode junction potential for sera measurements. In (c) is shown the potenti-ometric determination of the ionized calcium content in six serum samples, bracketed by two calibration runs of aqueous calcium standards (0.5-5 mA/), all assays made in triplicate. 

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