Arsenic trace analysis

Arsenic, determination by x-ray emission spectrography, 328 enhancement effect on, 190 trace analysis by x-ray emission spectrography, 228, 229... 

Low column recovery can also be caused by an excessively large surface area of the stationary phase in relation to the quantity of analyte, which is a common tendency in trace analysis. For example, the recovery of arsenic in the separation of arsenic(V) acid, its methylated derivatives MMA, DMA, trimethylarsine oxide (TMAO), and arsenic(lll) acid was 101-104 % in extracts from frog tissue in which arsenic was found at the milligram per kilogram level, and only 20 % in extracts from fish tissue containing arsenic in nanogram per kilogram amounts [169]. 

In the trace analysis of rubidium- and caesium arsenates, arsenic is distilled off as arsine [116]. In the trace analysis of high purity cadmium, the matrix can be separated by distillation at 630 C [117. By heating aluminium with ethyl bromide the metal is converted into ethylaluminium bromide, a liquid which boils at I30 C under reduced pressure [118]. 

To better understand the step-wise approach for method development and validation, it is necessary to give examples. They are taken from organic and inorganic trace analysis of environmental matrices. Figure 2.2 illustrates the steps for the validation of the analytical procedure for the determination of polychlorobiphenyls (PCB) in industrially contaminated soil. Figure 2.3 shows the steps necessary to validate the determination of trace elements and particularly arsenic in a fish tissue. Each step of the procedure will provide the necessary information so that the next step can be done with confidence. In practice, the analyst will develop a procedure to quantify all primary and secondary method characteristics as defined in section 2.1.4. 

No peaks could be detected for samples with arsenic concentrations below 100 ppm (Thurow et al., 1998), since the concentration of the analyzed compound in the head-space is to low due to low vapor pressure. Thus headspace analysis is suitable for rapid screening procedures but can not be used for trace analysis. The reproducibility of the headspace results was about 2%. 

It has been found that hair contains trace metals (e.g., Cu, Au, Ce, Na) in ratios which are typical for a particular individual, and activation analysis can be used to identify hair from a particular person. This application achieved public notice when it was found that hair from Napoleon had a relatively large amount of arsenic, indicating that some time prior to his death he had received large doses of arsenic. Through analysis of the hair of the Swedish king Erik XIV (who died suddenly in 1577 after a meal of pea-soup) it has been foimd that he must have received lethal amounts of arsenic as well as large amounts of mercury. The latter is assumed to have been taken into his body through the use of a mercury compound for treatment of an old wound. 

For trace analysis, the main ceramic elements of interest are Zn, Pb, Cu, Bi, Sb, Sn, Ag, As, Mn, Cr, Se, and Hg. Many of these are environmentally important. In certain cases the detection limits of flame AAS are inadequate, so that hydride generation for antimony, selenium, arsenic and bismuth, cold vapor for mercury, and graphite furnace AAS for lead and cadmium are required. A variation of AAS is atomic fluorescence, and this is used to achieve the detection limits needed for Hg and Se in environmental samples. Microwave digestion techniques for sample preparation are becoming more common, where, unlike fusion, there is no risk of loss of volatile elements from unfired samples and fewer reagents are... 

Radioactivity was a discovery that was soon brought into the arsenal of analytical chemistry. It is interesting to note that, as in some other fields, radiation from radioactive species was examined for analytical purposes in a variety of different ways at a time when they had little practical importance, because the radiation detectors were primitive and the field of potential applications limited owing to the small number of natural radionuclides. After the production of artificial radionuclides had started the importance of the earlier radiochemical methods increased rapidly and this field became one of the most powerful in trace analysis. 

An AA spectrometer is also available with a graphite furnace and vapor generation accessories for the trace analysis of lead, antimony, arsenic, and mercury at parts-per-billion levels. AA is used for quantitative analysis of these metals in polymers as well as finished formulations. It has been used to determine the elemental composition of catalysts and plastic additives, polymer formulations, and composite materials. Samples may be rapidly acid digested prior to analysis using a microwave oven or similar techniques. Microwave furnaces are also available for dry ashing. 

PVC compositions are typically analyzed as an indication of correct formulation or to deconstruct competitive compounds. Occasionally, analysis is required by specification, such as the level (usually zero detectable) of vinyl chloride monomer (VCM). The latter is carried out by headspace gas chromatography (GC) per ASTM D 3749 for resins, and ASTM D 4443 for compounds. Such analyses are routinely done by suppliers. The formulator in the field will usually send samples to a testing laboratory. This is also the case with trace analysis, often for levels of lead, cadmium, arsenic, or mercury. 

D.S. Bushee, I.S. Krull, PR. Demko, and S.B. Smith, Jr. Trace analysis and speciation for arsenic anions by HPLC-hydride generation-inductively coupled plasma emission spectroscopy. J. Liquid Chrom., 7(5), 8671 (1984). 

Zinc smelters use x-ray fluorescence spectrometry to analyze for zinc and many other metals in concentrates, calcines, residues, and trace elements precipitated from solution, such as arsenic, antimony, selenium, tellurium, and tin. X-ray analysis is also used for quaUtative and semiquantitative analysis. Electrolytic smelters rely heavily on AAS and polarography for solutions, residues, and environmental samples. 

Landsberger S, Swift G, and Neuhoff J (1990) Nondestructive determination of arsenic in urine by epithermal neutron activation analysis and Compton suppression. Biol Trace Elem Res 26-27 27-32. 

Stoeppler M, Vahter M (1994) Arsenic. In Herber RFM, Stoeppler M, eds. Trace element analysis in biological specimens. Elsevier, Amsterdam. 

The ability of metal ions to form complexes with formazans is utilized to determine these ions either directly (for low valent reducing ions) or indirectly in the presence of a reducing agent. Among others, molybdenum(VI) and vanadium(V) have been determined using this method.442,443 Indirect methods have been reported for the analyses of substances that do not reduce tetrazolium salts. Examples include arsenic in nickel ores436 and traces of selenium.437 A method for the extraction and analysis of a number of metal ternary ion association complexes has been described.444 - 448... 

Nixon277 compared atomic absorption spectroscopy, flame photometry, mass spectroscopy, and neutron activation analysis as methods for the determination of some 21 trace elements (<100 ppm) in hard dental tissue and dental plaque silver, aluminum, arsenic, gold, barium, chromium, copper, fluoride, iron, lithium, manganese, molybdenum, nickel, lead, rubidium, antimony, selenium, tin, strontium, vanadium, and zinc. Brunelle 278) also described procedures for the determination of about 20 elements in soil using a combination of atomic absorption spectroscopy and neutron activation analysis. 

Lester et al. [24] have described a robotic system for the analysis of arsenic and selenium in human urine samples which demonstrates how robotics has been used to integrate sample preparations and instrument analysis of a biological matrix for trace elements. The robot is used to control the ashing, digestion, sample injection and operation of a hydride system and atomic absorption instrument, including the instrument calibration. The system, which routinely analyses both As and Se at ppb levels, is estimated to require only... 

Usually, samples are presented for analysis as liquids. Thus, solid samples must be dissolved. Analytical or ultra-high-purity grade reagents must be used for dissolution to prevent contamination at trace levels. Certain volatile metals (e.g. cadmium, lead and zinc) may be lost when dry ashing, and volatile chlorides (e.g. arsenic and chromium) lost upon wet digestion. It is particularly easy to lose mercury during sample preparation. Appropriate steps must be taken in the choice of method of dissolution, acids and conditions (e.g. whether to use reflux conditions) to prevent such losses. 

The application of high-sensitivity ICP-MS detectors coupled to HPLC has enabled the detection of trace arsenic compounds present in marine animals. Thus, arsenocholine has been reported as a trace constituent (<0.1% of the total arsenic) in fish, molluscs, and crustaceans (37) and was found to be present in appreciable quantities (up to 15%) in some tissues of a marine turtle (110). Earlier reports (46,47) of appreciable concentrations of arsenocholine in some marine animals appear to have been in error (32). Phosphatidylarsenocholine 45 was identified as a trace constituent of lobster digestive gland following hydrolysis of the lipids and detection of GPAC in the hydrolysate by HPLC/ICP-MS analysis (70). It might result from the substitution of choline with arsenocholine in enzyme systems for the biogenesis of phosphatidylcholine (111). 

The use of activation analysis in criminal investigations (forensic activation analysis) is also well established. The basic idea here is to match the trace-element distributions found in bullets, paint, oil, and so on found at the scene of a crime with the trace-element distributions in objects found with criminal suspects. Such identification is rapid and nondestructive (allowing the actual evidence to be presented in court). Moreover, the probability of its correctness can be ascertained quantitatively. Other prominent examples of the use of forensic activation analysis involve confirmation of the notion that Napoleon was poisoned (by finding significant amounts of arsenic in hair from his head) and the finding that the activation analysis of the wipe samples taken from a suspect s hand can reveal not only if he or she has fired a gun recently but also the type of gun and ammunition used. 

Thomas and Sniatecki [51] also performed an analysis of trace amounts of arsenic species in natural waters using hydride generation IPC-ICP-MS. Six arsenic species were determined with detection limits in the range 1.0-3.0 fig l-1 and total arsenic was determined using hydride generation by atomic fluorescence detection. It was found that the predominant species present in bottled mineral water samples was always As(V) with very low levels of As(III). The authors described how the system required . .. further work using special chromatographic software. .. to improve the quantitative measurement at a natural level. ... 

Veal, D.J. (1966) Nondestructive activation analysis of crude oils for arsenic to one part per billion, and simultaneous determination of five other trace elements. Analytical Chemistry, 38(8), 1080-83. 

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