Pre-separation techniques

In off-line extraction the extracted analytes are collected and isolated independently from any subsequent analytical technique, which is to be employed next. For example, the extracted analyte can be collected in a solvent or on a solid sorbent. The choice of the collection method affects the possibilities for further analysis. The extracts may be used for final direct measurements (i.e. without further separation), e.g. UV and IR analysis. More usually, however, extraction is a pre-separation technique for chromatography, either off-line (the most common mode of SEE) or on-line (e.g. SFE-GC, SFE-LC-FTTR, etc.). The solvents used in extraction may affect subsequent chromatography. 

This approach has been used extensively for amino acid analysis using low-pressure ion-exchange chromatography and post-column ninhydrin reaction. Spraying, dipping and vapour-treatment techniques are well known as post-separation reactions in TLC, but these are considered only briefly since the majority of them are not quantitative. While the problems of pre-separation techniques are quite similar for TLC and HPLC, they differ considerably for post-separation reactions. 

Trends in MALDI-ToFMS research are search for new matrices, solvent-free sample preparation, advances in analyser design (oaToF-MS, Q-ToFMS, QITMS, QIT-ToF, ToF-ToF), and (on-line) coupling to other (pre)separation techniques. 

With the advent of the use of kinematic pre-separators, as described in Section 2.2.2 above, the requirements of the chemical separation have been relaxed. It is no longer necessary to have the highest separation factors from interfering Bi, Po, and actinide radioactivities, so simpler separations which are more specific to the transactinide element being studied can be used. These relaxed separation requirements will allow development of simpler chemical separation techniques, and may lead to a new interest in manually performed chemical separations. 

Trace analysis by IR spectroscopy, involving pre-concentration, separation, and computer techniques has been reported by Hannah et al. (1978). The term trace analysis is used to refer to concentrations in the ppm range up to 1%. In instances where interference is at its minimum, analysis may be performed in a straightforward manner by using difference techniques. However, there are many cases in which analysis may be complicated by the fact that the trace material is structurally similar to the matrix material. Moreover, the presence of other trace compounds is intolerable if their spectra interfere with that of the compound under investigation. In this case it is often necessary to use pre-concentration or separation techniques. This method is illustrated by analyses of aromatic isomers, gasoline additives, drugs, and polymer additives. The different aspects of trace analyses by infrared spectroscopy are discussed by Smith (1986). 

The process was demonstrated in a simulated continuous counter-current chromatographic separation pilot plant. Both the primary method of operation and the pre-pulse technique were demonstrated, with the pre-pulse technique showing improved recoveiy. Using commercial n-paraffin depleted kerosene (Molex Raffinate) feedstock we routinely demonstrated the ability to achieve better than 90% mono-methyl and normal paraffin purity with greater than 70% recovery of mono-methyl paraffins. 

Stable isotopes such as and are an improved alternative to traditional nonconservative chemical tracers, because waters are often uniquely labeled by their isotopic compositions (Sklash and Farvolden, 1979 McDonnell and Kendall, 1992), often allowing the separation of waters from different sources (e.g., new rain versus old pre-storm water). However, smdies have shown that flow paths cannot be identified to a high degree of certainty using 5 0 or data and simple hydrograph separation techniques, because waters within the same flow path can be derived from several different sources (Ogunkoya and Jenkins, 1991). Thus, a number of plausible mnofif mechanisms can be consistent with the isotope data. The need to incorporate flow path dynamics is recognized as... 

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