Trace background impurities

The first quotation is of use to those studying kinetics—one of the first signs that a radical process is going on is a rate variance from one day to the next, when background levels of trace radical initiators are at different levels. The second stresses the importance of removing trace radical impurities in synthesis. Very pure metal hydrides (or other complexes) may show a surprising stability provided they are free of all radical contaminants. 

The control experiment in pure supporting electrolyte (dotted lines in Fig. 13.2) shows a sharp faradaic current spike, which is mainly due to pseudocapacitive contributions (adsorption of (bi)sulfate and rearrangement of the double layer) plus oxidation of adsorbed Hupd (dotted lines in Fig. 13.2a), but no measurable increase in the CO2 partial pressure (m/z = 44 current) above the background level (dotted lines in Fig. 13.2b). Therefore, a measurable adsorption of trace impurities from the base electrolyte can be ruled out on the time scale of our experiments. Moreover, this experiment also demonstrates the advantage of mass spectrometric transient measurements compared with faradaic current measurements, since the initial reaction signal is not obscured by pseudocapacitive effects and the related faradaic current spike. 

For difficult separations, multiple extractions are frequently carried out, although in many cases the background is also coextracted. Using multiple extractions, polar interferences may sometimes be transferred from the aqueous into organic solvents that can dissolve minute amounts of water. This problem cannot be eliminated by simple presaturation of the extraction solvent but only by washing the extract with small amounts of water (58). Another relevant issue to be considered in trace residue analysis concerns the purity of the organic solvents, since they can introduce solvent impurities into the sample extract. Therefore, the need for high solvent purification should not be overlooked in some applications. 

Rinse sections in high-quality distilled water to remove all traces of chloride ions and other impurities that might contaminate the silver-enhancing solution. Some commercial enhancement solutions are reported to be resistant to contamination, the user should evaluate this carefully, especially if high levels of nonspecific background are encountered... 

If no fluorophore exists in a given area of the thin-layer plate, then no emission signal can be obtained. The fluorescence of the sample is then an absolute quantity relative to this zero signal and proportional to the number of emitting species present in the sample. However, in practice, the adsorbent does contain trace amounts of fluorescent impurities and thus background noise is observed, but usually at a lower level than experienced in absorption measurements. The low background noise is an important factor in the high sensitivity of fluorescence. 

The application of PIXE (Proton Induced X-Ray Emission) and micro-PIXE to quantitative analysis of impurities in polyethylene are described. The equipment, sources of background which affect the sensitivity and the precision of PIXE measurements are discussed for both thick and thin targets. A number of applications of micro-PIXE to the identification and location of trace elements in the "Trees" found in the polymer insulation of HV cables are presented. 

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