Application of the O 2-doped ECD to isomer differentiation
Campbell, James Allen
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The addition of oxygen to the nitrogen carrier gas of a constant current electron capture detector (BCD) is shown to provide increased sensitivity and isomer distinction for environmentally important compounds such as polychlorinated biphenyls, polycyclic aromatic amines and hydroxides with appropriate EC-enhancing tags, chloroanthracenes and chlorophenanthrenes, methylanthracenes and methylphenanthrenes, and 2,3,7-trichlorodibenzo-p-dioxin. In most cases, the isomers of these particular compounds can be distinguished from the other isomers based solely on the measured response enhancements. The oxygen doped BCD has been applied to compound identification where only partial resolution with capillary column gas chromatography is obtained. In addition, in instances where several compounds coelute and have the exact same retention times, the mole fraction of each component in the unresolved peak can be determined. Atmospheric pressure ionization mass spectrometry (APIMS) gives an indication of the ions formed with and without the presence of oxygen in the source. For the chloroanthracenes, methylanthracenes, and methylphenanthrenes, actual oxygen incorporation is observed when oxygen is present. In contrast, for the polycyclic aromatic amines derivatized with trifluoroacetic anhydride (TFAA), the reaction with oxygen to produce an induced response involves a charge transfer between the oxygen anion and the analyte molecule. Several TFAA derivatives of the aminoanthracenes and aminophenanthrenes were examined using electron impact mass spectrometry and chemical ionization mass spectrometry. Negative chemical ionization mass spectrometry with methane and isobutane as the reagent gases was evaluated as a method of isomer differentiation of these compounds and shows considerable promise. In order to improve the precision involved in the measurement of response enhancements, two methods with parallel and series arrangement of the ECDs were utilized. The parallel arrangement with dual columns represents a significant improvement in the reproducibility of response enhancement measurements. The series detector arrangement seems to be confusing in view of possible additional reactions in the detectors. The addition of ethyl chloride to an electron capture detector increased the response of anthracene and similar molecules with a low normal BCD response and those that react with the gaseous electron through a resonance type of reaction mechanism. The addition of ethyl chloride to the detector does not significantly increase the baseline frequency, in contrast to the addition of oxygen. The negative ions formed in this reaction have been identified.