Detection limit stripping analysis

Adsorptive stripping analysis involves pre-concentration of the analyte, or a derivative of it, by adsorption onto the working electrode, followed by voltanmietric iiieasurement of the surface species. Many species with surface-active properties are measurable at Hg electrodes down to nanoniolar levels and below, with detection limits comparable to those for trace metal detemiination with ASV. 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

Measurement of trace metals, including nickel in seawater can be completed using an in-line system with stripping voltammetry or chronopotentiometry (van den Berg and Achterberg 1994). These methods provide rapid analysis (1-15 minutes) with little sample preparation. The detection limit of these methods for nickel was not stated. Recommended EPA methods for soil sediment, sludge, and solid waste are Methods 7520 (AAS) and 6010 (ICP-AES). Before the widespread use of AAS, colorimetric methods were employed, and a mrmber of colorimetric reagents have been used (Stoeppler 1980). 

To determine ions at mid pg/1 to mg/1 (ppb to ppm) levels with IC, a sample size of 10 to 50/pi is sufficient. To determine ions at lower levels, then a preconcentration or trace enrichment technique has t3rpically to be utilized [20]. With this method, the analytes of interest are preconcentrated on another column in order to "strip" ions from a measured sample volume. This process concentrates the desired species resulting in lower detection limits. However, preconcentration has several disadvantages, compared with a direct method, additional hardware is required. A concentrator column is used to preconcentrate the ions of interest, a sample pump is needed for loading sample, an additional valve is often required for switching the concentrator column in and out-of line with the analytical column and extra time is required for the preconcentration step. It was of interest to explore the development of a high-volume direct-injection IC method that would facilitate trace ion determinations without a separate preconcentration step. This would represent a significantly simpler and more reliable means of trace analysis. 

Sensitivity can be extended by several means. Ultrasonic vibration during electrodeposition lowers the detection limit and enables the analysis of some analytes in difficult matrices.24 In another approach, the detection limit for Fe(III) in seawater is 10 11 M with a catalytic stripping... 

Radiochemical yields are 80-95% for zinc and quantitative for cadmium. The average relative standard deviation was 25% for zinc and better than 10% for cadmium. The detection limit of the method is 50 ppm cadmium in the ash. Analysis of two Illinois coals with unusually high cadmium content (17 and 21 ppm) gave results in good agreement with those obtained by atomic absorption and by anodic stripping voltammetry (4). The recent development and testing of a radiochemical method for the determination of zinc, cadmium, and arsenic in coal and fly ash, by Orvini et al. (14), has already been discussed in the section on arsenic. 

The detection limit of SEV for analysis is greatly extended by its use in conjunction with an electrochemical preconcentration step. This technique, termed dc stripping voltammetry, is discussed in Chapter 24. 

Stripping analysis is the best-known analytical method that incorporates an electrolytic preconcentration step [2-5]. The technique couples the advantages of extremely low detection limits ( 10 10-10-11 M), multielement and speciation capabilities, suitability for on-line and in situ measurements, and low cost. 

Screening methods are available for analysis of benzene in feces and urine (Ghoos et al. 1994) and body fluids (Schuberth 1994). Both employ analysis by capillary GC with an ion trap detector (ITD). Benzene in urine has been determined by trapping benzene stripped from the urine on a Carbotrap tube, followed by thermal desorption GC/flame ionization detection (FID). The detection limit is 50 ng/L and the average recovery is approximately 82% (Ghittori et al. 1993). Benzene in urine has also been determined using headspace analysis with capillary GC/photoionization detection (PID). The detection limit is 40 ng/L (Kok and Ong 1994). 

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