Fields requiring sensitive analysis

In relation to the field of heterogeneous catalysis, an advantage of PL is that solid samples can be easily investigated. Other assets include its high sensitivity (often samples of less than a few tens of mg only are needed), the short analysis times required, and the different types of data, as discussed in the previous sections, which can be obtained from the investigation of the spectral and temporal profiles of both the excitation and emission processes (Anpo and Che, 1999). 

Table I. Fields Requiring More Sensitive Analysis...

The field of forensic science, scientific analysis used within the legal system, is very wide-ranging narcotics and explosives are amongst the areas of analysis covered. Many different techniques are used within the field to identify samples found at a crime scene or seized by police. Techniques used must fulfil certain requirements sensitive enough to deal with often very small or trace samples, provide clear, characteristic results which easily distinguish different materials and minimal sample size and preparation. The latter point is one which Raman spectroscopy can easily fulfil, as the technique is both nondestructive and noninvasive. No sampling is required, which means that further analysis is possible should it be necessary, for example in the case of an appeal. Raman spectroscopy fulfils the other requirements as it is both sensitive and provides characteristic spectra for different substances. 

The existence and properties of individual molecules, atoms, and ions in the gaseous state provide a fundamental basis for elemental spectrometric analytical determinations. Classical techniques based on these principles have been utilized for decades for the major, minor, and trace analysis of substances in virtually every field of science. The enhancement of techniques based on these properties has recently resulted in technology that exhibits ultratrace analysis sensitivity (sub-part-per-million detectability) and allows expansion of the scope of analysis to the rarer and more exotic elements. A basic understanding of some of the fundamental atomic properties of the elements that are used for mass spectrometric analysis is required to fully utilize this technology. 

Much the same problem exists in the materials analysis field today. The scientific literature contains numerous references to materials of 5-9 s and 6-9 s purity. Such statements are misleading and of questionable validity because the estimates are in many cases derived from resistivity measurements supplemented by emission spectrographic analyses (with a sensitivity of only 1-10 ppm for most elements). To establish that a sample of material contained less than 1 ppm total impurities would require analyses for all elements present by techniques with sensitivities and accuracies in the range 1-20 ppb. 

Solid-surface room-temperature phosphorescence (RTF) is a relatively new technique which has been used for organic trace analysis in several fields. However, the fundamental interactions needed for RTF are only partly understood. To clarify some of the interactions required for strong RTF, organic compounds adsorbed on several surfaces are being studied. Fluorescence quantum yield values, phosphorescence quantum yield values, and phosphorescence lifetime values were obtained for model compounds adsorbed on sodiiun acetate-sodium chloride mixtures and on a-cyclodextrin-sodium chloride mixtures. With the data obtained, the triplet formation efficiency and some of the rate constants related to the luminescence processes were calculated. This information clarified several of the interactions responsible for RTF from organic compounds adsorbed on sodium acetate-sodium chloride and a-cyclodextrin-sodium chloride mixtures. Work with silica gel chromatoplates has involved studying the effects of moisture, gases, and various solvents on the fluorescence and phosphorescence intensities. The net result of the study has been to improve the experimental conditions for enhanced sensitivity and selectivity in solid-surface luminescence analysis. 

Immunoassays designed for environmental applications are mostly sold as some variation of the ELISA format. ELISA-like formats dominate the field because they are inexpensive and because they provide high sensitivity and precision without requiring complex instrumentation. The basic ELISA format supports both field and laboratory-based applications but is limited by multiple steps and inadequate sensitivity for some applications, excessive variability and sometimes long analysis times. Some of the other formats discussed in this article may replace the ELISA for selected applications however, because many laboratories are familiar with the ELISA technology, there will be a significant delay before alternative formats are widely accepted. 

Table 5.8 gives an indication of the range of elements that may be determined. Most procedures will require an analyte concentration of 10-3 mol dm 3 or more, although with special conditions, notably potentiometric end-point detection, the sensitivity may be extended to 1(H mol dm 3. The analysis of mixtures of metal ions necessitates masking and demasking, pH adjustments and selective separation procedures. Areas of application are spread throughout the chemical field from water treatment and the analysis of refined food and petroleum products to the assay of minerals and alloys. Table 5.10 gives some selected examples. 

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