Flow injection systems standardization

Fang et al. [661] have described a flow injection system with online ion exchange preconcentration on dual columns for the determination of trace amounts of heavy metal at pg/1 and sub-pg/1 levels by flame atomic absorption spectrometry (Fig. 5.17). The degree of preconcentration ranges from a factor of 50 to 105 for different elements, at a sampling frequency of 60 samples per hour. The detection limits for copper, zinc, lead, and cadmium are 0.07, 0.03, 0.5, and 0.05 pg/1, respectively. Relative standard deviations are 1.2-3.2% at pg/1 levels. The behaviour of the various chelating exchangers used was studied with respect to their preconcentration characteristics, with special emphasis on interferences encountered in the analysis of seawater. 

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. 

Gamborg and Hansen [119] suggested a flow-injection system, in which they mixed enzyme, reagent and sample solution directly in front of a photomultiplier and achieved a lower detection limit of 10 ° M. They faced the problem to obtain stable standard solutions in this low concentration range, which made further improvements impossible. 

Figure 26-18 is a diagram of a dialysis module in which analyte ions or small molecules diffuse from the sample solution through a membrane into a reagent stream, which often contains a species that reacts with the analyte to form a colored product, which can then be determined photometrically. Large molecules, which interfere in the determination, remain in the original stream and are carried to waste. The membrane is supported between two Teflon plates in which complementary channels have been cut to accommodate the two stream flows on opposite sides of the membrane. The transfer of smaller species through the membrane is usually incomplete (often less than 50%). Thus, successful quantitative analysis requires close control of temperature and flow rates for both samples and standards. Such control is easily accomplished in automated flow-injection systems. 

The coupling of a flow injection system to FAAS for potassium resulted in a much smaller standard deviation than if a continuous flow FAAS system was applied. In CRM 408 and even in CRM 409 the latter system worked closer to its limit of determination. 

J.J. Harrow, J. Janata, Heterogeneous samples in flow-injection systems part. 2. standard addition, Anal. Chim. Acta 174 (1985) 123. 

B.F. Reis, M.F. Cine, F.J. Krug, H. Bergamin-Filho, Multipurpose flow-injection system. Part 1. Programmable dilutions and standard additions for plant digests analysis by inductively coupled plasma atomic emission spectrometry, J. Anal. At. Spectrom. 7 (1992) 865. 

In a given environment, different chemical species will have different diffusion coefficients. This parameter (Dm — Eq. 3.4) plays an important role in radial mass transport, thereby influencing sample dispersion [66]. Lower sample dispersion is associated with higher Dm values. Generally, this parameter is not of major concern when designing flow injection systems, as the same Dm value applies to any specific chemical species (analyte) in both the sample and standard solutions. 

Worked example 4. A confluence flow injection system with a very low confluent stream flow rate is designed for the spectrophotometric determination of phosphate in plant digests. The linearity of the analytical calibration graph is good and the recorded absorbance corresponding to a 100.0 mg L 1 P (as phosphate) standard solution is 0.21. Replacing the sample carrier stream by this standard solution (sample infinite volume) yields a steady state situation and the related absorbance is 0.68. The pump is then turned off and an asymptotic increase in absorbance towards 0.95 is observed determine the sensitivity improvement that in principle could be attained simply by increasing the sample volume and the mean sample residence time in the analytical path. 

Flow injection system with merging zones. The sample and titrant are simultaneously injected into convergent carrier streams and the titration is accomplished by adding different standard solutions via the titrant injection port [331] (see also 7.1.1.1). 

E.A.G. Zagatto, A.O. Jacintho, F.J. Krug, B.F. Reis, R.E. Bruns, M.C.U. Araujo, Flow-injection systems with inductively-coupled argon plasma atomic emission spectrometry. Part 2. The generalized standard addition method, Anal. Chim. Acta 145 (1983) 169. 

Note. All detection limits were determined using elemental standards in dilute aqueous solution. All detection limits are based on a 98% confidence level (3 SD). Atomic absorption (Model 5100) detection limits were determined using instrumental parameters optimized for the individual element and EDL where available. ICP emission (Optima 3000) detection limits were obtained under simultaneous multielement conditions with a radial plasma. Detection limits obtained with an axial plasma are typically 5-10 times lower. Cold vapor mercury AA detection limits were determined with a FlAS -400 flow injection system with an amalgamation accessory. Hydride detection limits were determined with an MHS-10 Hydride system. Furnace AA (Model 5100/ZL Zeeman furnace) detection limits were determined using a L vov platform and 50 p.1 sample volumes. ICP-MS (Elan 6100) detection limits were determined using a 3 s integration. 

Flow Injection System. The manifold is shown in Figure 2. The baseline was obtained by pumping a buffer solution through the detection cell (pH meter in millivolt mode). The sample flow rate was always 0.78 ml.min-i. Penicillin standard solutions of 0.1 mM to 65 mM were prepared in the fermentation broth. The diluted samples were injected into the carrier by means of the injection valve which is placed close to the detection cells in order to minimise the delay between injection and detection. The dead volume chosen was 2 ml. The cells are thermostatted (25°C). For measurements, the potential difference between the peak height and the base line was automatically recorded by the computer and the recorder. The potential of the electrode always returned to its base line as soon as a fresh buffer solution come again to the contact of die electrode. 

The use of branched uptake capillaries, coimected to the nebulizer using a T-piece, may be advantageous when a buffer or ionization suppressor is required. In addition to avoiding time-consuming solution preparation, it is also possible to calibrate organic extracts using aqueous standards in this way. The approach may also be extended to couple more complex flow injection systems employing novel chemistries in the same way. 

Figure 5 Flow injection systems used to determine benzophe-none-3 (2-hydroxy-4-methoxybenzophenone or oxybenzone) in sunscreen creams (A) flow injection analysis (FIA) (B) sequential injection analysis (SIA). Reagents (1), (2), (12), (14) peristaltic pump (3) merging point (4) injection valve (5) reaction coil (6) flow cell (7) ethanol (8) autoburette with syringe (9) holding coil (10) eight-channel selector valve (11) sample or standard solutions (13). (Reproduced from Chisvert A, Salvador A, Pascual-Martf MC, and March JG (2001) Fresenius Journal of Analytical Chemistry 369 684-689.)...

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. -... 

In general, in a flow injection system, loading of standard and sample (sequential aliquots) into a sampler is assisted by the use of a pump. As the samples are aspirated, chemicals are simultaneously fed into the system by the pump. Then the samples are loaded by the use of an automatic injection valve. The sample and reagents are moved toward the reaction manifold where the chemical reaction occurs. The reaction product is then loaded into the flow and sent to a chosen/appropriate detector for generation of the analog signal (Ruzicka and Hansen, 1978). 

---