Microcoulometric detector methods

Positive identification of low-ppb (pg/L) levels of endosulfan in human blood has been achieved by GC equipped with a microcoulometric detector (GC/MC) (Griffith and Blanke 1974). Although GC/MC is specific and nearly as sensitive as GC/ECD for detecting endosulfan in blood, GC/MC is more difficult to operate. Both isomers of endosulfan can be measured in blood using a method described by Guardino et al. (1996). According to the authors, endosulfan can be recovered and measured with an approximate limit of quantitation (LOQ) of 0.2 pg/L (sub-ppb). 

GC/ECD and GC equipped with a microcoulometric detector have been used to determine heptachlor and heptachlor epoxide in a variety of human tissues, including the liver, brain, adrenals, lungs, heart, kidneys, spleen, and pancreas (Curley et al. 1969 Klemmer et al. 1977 Radomski et al. 1968). Details of a sample preparation method were not reported for GC equipped with a microcoulometric detector (Curley et al. 1969). Sample preparation steps for GC/ECD include homogenization, extraction with petroleum ether or hexane, usually followed by a clean-up procedure (Klemmer et al. 1977 Radomski et al. 1968). Recovery, sensitivity, and precision data were not reported (Curley et al. 1969 Klemmer et al. 1977 Radomski et al. 1968). 

Various procedures described by Crippen and Smith (5) are available for identifying components in a chromatogram. Because of the limitations mentioned above, the choice of method was restricted. Peak identifications for this sample relied on cleanup and preliminary separation by column adsorption chromatography with activated alumina and Florisil, use of the chlorine- and sulfur-specific microcoulometric detector, and parallel injections onto three or more columns containing liquid substrates varying widely in polarity. 

Most published methods are for analysis of crops and soil residues of the intact acaricides. Extraction has been done by stripping, blender or soxhlet. Extraction solvents have included petroleum ether, benzene, carbon tetrachloride, acetonitrile, diethyl ether, methanol and hexane/acetone. Clean-up steps have em -ployed liquid/liquid partitioning and adsorption on activated charcoal, activated charcoal/Florisil, Florisil, alumina and silica gel. Burke (14) reported that CB is not completely recovered from Florisil. Horn and coworkers (7) found that no clean-up was necessary when analyzing dog urine for CB using a Schecter-Haller procedure. For detection of residues, the colorimetric and UV methods have been replaced by gas chromatographic methods employing microcoulometric or electron capture detectors. 

Some techniques may offer selective screening as well as specificity —e.g., microcoulometric methods described by Coulson et al. (24). This technique consists of a combination of gas chromatography, combustion, and continuous coulometric titration for chlorine or sulfur. The development of the flame photometric detector offers a similar potential for the selective screening and specificity of pesticides which contain phosphorus or sulfur (25). Even so, one or more tests in addition to the initial... 

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