Procedure limits, trace analysis

Similar to the analytical procedure for trace analysis in high purity GaAs wafers after matrix separation, discussed previously,52 the volatilization of Ga and As has been performed via their chlorides in a stream of aqua regia vapours (at 210 °C) using nitrogen as the carrier gas for trace/matrix separation.58 The recoveries of Cr, Mn, Fe, Ni, Co, Cu, Zn, Ag, Cd, Ba and Pb determined after a nearly quantitative volatilization of matrix elements (99.8 %) were found to be between 94 and 101 % (except for Ag and Cr with 80 %). The concentrations of impurities measured by ICP-DRC-MS (Elan 6100 DRC, PerkinElmer Sciex) after matrix separation were compared with ICP-SFMS (Element 2, Thermo Fisher Scientific) and total reflection X-ray fluorescence analysis (TXRF Phillips). The limits of detection obtained for trace elements in GaAs were in the low ngg-1 range and below.58... 

Attempts to optimize polymer-based electrodes for trace analysis have started very recently. Ceresa et al. have reported an ion-selective electrode optimized for the determination of Pb2+ in drinking water. The detection limit was 0.7 ppb (3 x 10 9M) which is somewhat poorer than the best LOD reported so far for Pb2+-selective electrodes [10] but the former was optimized for ruggedness and response time rather than LOD. Nevertheless, the obtained LOD was still adequate for the targeted application since it was about 20-fold lower than the 15 ppb action limit for Pb2+ in drinking water imposed by the USA EPA [79]. The authors used ICPMS as a reference method and obtained excellent correlation for samples of concentration 3nM. It was shown that the calibration procedure required ca. 10 min for stable readings in micromolar to nanomolar concentration levels. Moreover, the authors... 

Comparison of various analytical procedures used in trace analysis needs comparable conditions of determination and the same procedure for detection limit calculation. In some special cases the calculated limit of detection for pure solutions can differ by one order of magnitude in analysis of biological objects or food, by two orders in analysis of simple inorganic materials, or by as much as three orders for very complex objects such as mineral or geological items [13]. 

Sensitivity is mainly a limiting factor in trace analysis. It is evident that the method applied should be sensitive enough so that the concentration of substance to be determined is accessible (limit of detection and determination) or the colonies of microbes countable. It must also deliver a measurable difference for a small change in content. When the sensitivity of the method of final detection is a limiting factor the analyst may have several possibilities which all will influence the selection, optimisation and validation of the other steps of the procedure. He may ... 

Kotz etal. (1972, Decomposition of biological materials for the determination of extremely low contents of trace elements in limited amounts with nitric acid under pressure in a Teflon tube) Hartstein et al. (1973, Novel wet-digestion procedure for trace-metal analysis of coal by atomic absorption) Jackson etal. (1978), Automated digestion and extraction apparatus for use in the determination of trace metals in foodstuffs) Campos etal. (1990, Combustion and volatilization of solid samples for direct atomic absorption spectrometry using silica or nickel tube furnace atomizers) Erber et al. (1994, The Wickbold combustion method for the determination of mercury under statistical aspects) and Woit-tiez and Sloof (1994, Sampling and sample preparation). 

A direct reader ICP excels at the rapid analysis of multi-element samples. Common sample types analyzed by ICP include trace elements in polymers, wear metals in oils, and numerous one-of-a-kind catalysts. ICP instruments are limited to the analysis of liquids only. Solid samples require some sort of dissolution procedure prior to analysis. The final volume of solution should be at least 25 mL. The solvent can be either water, usually containing 10% acid, or a suitable organic solvent such as xylene. ICP offers good detection limits and a wide linear range for most elements. With a direct reading instrument multi-element analysis is extremely fast. 

ICP-AES is normally preferred for trace analysis because of its generally better detection limits, freedom from chemical matrix effects, and multi-element capability. When available, it is used instead of AAS. Like AAS, hydride and mercury vapor attachments can be used to enhance the detection limits of the elements listed above. As an alternative to normal sample preparation or microwave procedures, laser ablation is creeping in as an alternative approach. The same pressed discs used for XRE are ablated by an IR or UV laser in a chamber that replaces the spray chamber used normally. The sample preparation takes minutes rather than days or hours, eliminates reagents (even pure reagents are less pure than ceramic powders), and is less susceptible to external contamination. 

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