Flow injection systems procedures

Tecator [20] has described a flow injection system for the determination of nitrate and nitrite in 2 mol/1 potassium chloride extracts of soil samples. Nitrate is reduced to nitrite with a copperised cadmium reductor and this nitrite is determined by a standard spectrophotometric procedure in which the soil sample extract containing nitrate is injected into a carrier stream. Upon the addition of acidic sulfanilamide a diazo compound is formed which then reacts with N-(l-naphthyl)ethylcncdiamine dihydrochloride provided from a second merging stream. A purple azo dye is formed, the intensity of which is proportional to the sum of the nitrate and the nitrite concentration. Nitrite in the original sample is determined by direct spectrophotometry of the soil extract without cadmium reduction. 

A more complex biosensor for acetylcholine has been developed by Larsson et al. [154]. Three enzymes, AChE, ChOX, and HPR, have been coimmobilized in an Os-based redox polymer on solid graphite electrodes. After a careful optimization of the immobilization procedure, the biosensor, inserted into a flow cell of very small volume, was integrated into a flow injection system, and some samples of microdialysate, taken from rat brains before and after stimulation with KCl, were analysed. Even if a clear increase in signal could be noted, it was not possible to distinguish whether it was due to an increase in choline or in acetylcholine, since the biosensor responded to both metabolites. 

Flow injection procedures are very useful for performing trace analyses in highly concentrated salt solutions. Fang and Welz [270] showed that the flow rate of the carrier solution can be significantly lower than the aspiration rate of the nebulizer. This allows even higher sensitivities than with normal sample delivery can be obtained. Despite the small volumes of sample solution, the precision and the detection limits are practically identical with the values obtained with continuous sample nebulization. The volume, the form of the loop (single loop, knotted reactor, etc.) and the type and length of the transfer line between the flow injection system and the nebulizer considerably influence the precision and detection limits that are attainable. 

The spectrophotometric determination of cadmium in natural waters involving in-line formation of an aqueous dithizone suspension [121] illustrates another application of in-line suspension addition. Solid dithizone reagent was packed in a mini-column through which a surfactant (Triton X-100) stream was allowed to flow. The emerging suspension formed was added by confluence to the main analytical channel of a flow-injection system. With this innovation, good sensitivity was achieved without the need for an analyte separation/concentration LLE step and the entire procedure was carried out in the aqueous phase. 

Sakai, T., Piao, S., Teshima, N. et al. (2004) Flow injection system with in-line Winkler s procedure using 16-way valve and spectrofluorimetric determination of dissolved oxygen in environmental waters. Talanta, 63 (4), 893-898. 

Most biosensors described in the literature for the determination of urea are potentiometric based on NH4 or HCOj" sensitive electrodes [181, 182]. Osaka and co-workers constructed a highly sensitive and rapid flow injection system for urea analysis with a composite film of electropolymerised inactive PPy and a polyion complex [183]. Pandey and co-workers fabricated a urea biosensor based on immobilised urease on the tip of an ammonia gas electrode (diameter 10 pm) made from a PPy film coated onto a platinum wire [170]. The enzymatic response was achieved in the wide range of 0.001-0.05 M with a stability of more than 32 days. Cho and co-workers [184] developed a procedure for urea determination by crosslinking urease onto PANI-Nafion composite electrodes, which could sense the ammonium ions efficiently. Such a urea biosensor has a detection limit of about 0.5 pM and a response time of 40 seconds. 

Slllca-lmmoblllzed brown alga Pilayella littoralis Online metal preconcentration procedure in a flow-injection system for determination of Al, Co, Cu, and Fe in lake water samples by (ICP-OES) [70]... 

Several instrument manufacturers supply flame photometers designed specifically for the determination of sodium, potassium, lithium, and sometimes calcium in blood serum, urine, and other biological fluids. Single-channel and multichannel (two to four channels) instruments are available for these determinations. In the multichannel instruments, each channel can be used to determine a separate element without an internal standard, or one of the channels can be reserved for an internal standard such as lithium. The ratios of the signals from the other channels to the signal of the lithium channel are then taken to compensate for flame noise and noise from fluctuations in reagent flow rate. Flame photometers such as these have been coupled with flow injection systems to automate the sample-introduction process (see Section 33B-3). Typical precisions for flow-injection-analysis-based flame photometric determinations of lithium, sodium, and potassium in serum are on the order of a few percent or less. Automated flow injection procedures require l/KIO the amount of sample and 1/10 the time of batch procedures. -... 

Another procedure for the determination of TOC and its fractions in industrial effluent samples has been recently introduced [129]. A flow injection system using a gas-liquid transfer microreactor is developed, and adapted to a turbidimetric spectrophotometer. Samples are decomposed into glass vials in a microwave oven and a fraction of the CO2 is injected into a carrier gas and pumped to a glass microreactor. With minor modifications, the system allows the determination of different carbon fractions. The advantages of the proposed procedure are simplicity, low volume of samples and reagents, high frequency of determinations, and low cost. The dynamic range is 20-800 mg C/L, and the calculated LOD is 17 mg C/L. 

Cabero et al. [80] developed a method based on the conversion of cyclamate to cyclo-hexylamine and the subsequent reaction with l,2-naphthoquinone-4-sulfonate, yielding a spectrophotometrically active derivative, which is detected at 480 nm thus, other sweeteners, such as saccharin or aspartame, do not interfere in these determinations. The hydrolysis step is performed batchwise by treatment of cyclamate with hydrogen peroxide and hydrochloric acid, while the cyclohexylamine derivatization is carried out in the flow injection system (Figure 24.9). Rocha et al. [81] reported a flow system based on multicommutation for fast and clean determination of cyclamate. The procedure exploits the reaction of cyclamate with nitrite in an acidic medium and the spectrophotometric determination of the excess of nitrite by iodometry. The flow system was designed with a set of solenoid micropumps to minimize reagent consumption and waste generation (Figure 24.10). 

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