Sensitivity chromatography

Liquid chromatography, having a resolving power generally less than that of gas phase chromatography, is often employed when the latter cannot be used, as in the case of samples containing heat-sensitive or low vapor-pressure compounds. 

Note that in liquid phase chromatography there are no detectors that are both sensitive and universal, that is, which respond linearly to solute concentration regardless of its chemical nature. In fact, the refractometer detects all solutes but it is not very sensitive its response depends evidently on the difference in refractive indices between solvent and solute whereas absorption and UV fluorescence methods respond only to aromatics, an advantage in numerous applications. Unfortunately, their coefficient of response (in ultraviolet, absorptivity is the term used) is highly variable among individual components. 

Thin-layer chromatography (tic) (16) is frequently used. The procedure allows for rapid screening for most dmgs of abuse using simple, inexpensive technology. A drawback to tic, however, is that the technique is not especially sensitive and low levels of dmgs may be missed. 

Methods for iodine deterrnination in foods using colorimetry (95,96), ion-selective electrodes (94,97), micro acid digestion methods (98), and gas chromatography (99) suffer some limitations such as potential interferences, possibHity of contamination, and loss during analysis. More recendy neutron activation analysis, which is probably the most sensitive analytical technique for determining iodine, has also been used (100—102). 

Acetylene has a low solubiHty in Hquid oxygen. Excessive concentrations can lead to separation of soHd acetylene and produce accumulations that, once initiated, can decompose violently, detonating other oxidizable materials. Acetylene is monitored routinely when individual hydrocarbons are determined by gas chromatography, but one of the wet classical methods may be more convenient. These use the unique reaction of acetylene with Ilosvay s reagent (monovalent copper solution). The resulting brick-red copper acetyHde may be estimated colorimetricaHy or volumetricaHy with good sensitivity (30). 

A practical method for low level perchlorate analysis employs ion chromatography. The unsuppressed method using a conductivity detector has a lower detectable limit of about 10 ppm. A suppression technique, which suppresses the conductivity of the electrolyte but not the separated ions, can further improve sensitivity (110,111). Additionally, ion chromatography can be coupled with indirect photometric detection and appHed to the analysis of perchlorates (112). 

Cyclic Peroxides. CycHc diperoxides (4) and triperoxides (5) are soHds and the low molecular weight compounds are shock-sensitive and explosive (151). The melting points of some characteristic compounds of this type are given in Table 5. They can be reduced to carbonyl compounds and alcohols with zinc and alkaH, zinc and acetic acid, aluminum amalgam, Grignard reagents, and warm acidified iodides (44,122). They are more difficult to analyze by titration with acidified iodides than the acycHc peroxides and have been sucessfuUy analyzed by gas chromatography (112). 

Analytical and Test Methods. Measurement of the sohdification point using a highly sensitive thermometer and of APHA color by comparison of molten samples to APHA standards is straightforward. Specific impurities are measured by gas chromatography. A nonaqueous titration is used to determine phthahc acid content. 

Achiral Columns Together with Chiral Mobile Phases. Ligand-exchange chromatography for chiral separation has been introduced (59), and has been appHed to the resolution of several a-amino acids. Prior derivatization is sometimes necessary. Preparative resolutions are possible, but the method is sensitive to small variations in the mobile phase and sometimes gives poor reproducibiUty. 

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