b The effect of changes in operating parameters on the sensitivity of the electron capture detector was determined quantitatively for sec-butyl bromide, a model monofunctional compound of moderate electron affinity.Using a constant direct current voltage as the source of applied potential relatively high detector temperatures and low gas flows yielded optimum results. With a pulsed applied potential, low flows could not b e used and higher detector temperatures only provided a slight increase in sensitivity. The nature of the pulse was not critical as long as the maximum standing current for the detector used was approached. In addition the sensitivities of isomeric butyl monohalides were determined at near optimum conditions. These ranged from 9.3 X 10-11 mole per second for n-butyl chloride to 3.0 X mole per second for tert-butyl iodide. A comparison of all the data obtained showed that affinity for electrons was also dependent upon the hydrocarbon structure of the molecule in addition to the electronegativity of the heteroatom. The halide atom produced a positive center for electron attachment at the alpha position. Surprisingly the branched isomers were found to enhance electron attachment because of the stable free radicals they form after cleavage of the molecule-ion.
This effect was primarily responsiblefor the high sensitivity noted for the tert-butyl halide isomers.INCE the method of electron capture S spectrometry was first described for use in the identification of the components of a mixture resolved by gas chromatography (5) the number of applications of this technique has been disappointingly small in light of the potential orginally predicted for it. To date only some organic compounds have been noted to have a particular affinity for free electrons which permits them to be detected a t sensitivities comparable to or greater than the maximum noted for the highly sensitive flame or argon ionization devices (1, 4, 6). Specifically, the electron capture detector has been utilized only as a very sensitive instrument for determining the presence of minute quantities of highly chlorinated materials such as insecticides (1, d), metal-organic compounds such as the lead alkyls ( 6 ) , certain polycyclic hydrocarbons (7), and Krebs cycle intermediates (4). Its capacity to function as either a qualitative detector or a device for the quantitative analysis of compounds which provide responses somewhat less than those noted with conventional ionization detectors has been largely ignored. Moreover, it has not been fully recognized that in instances where the conventionally sensitive devices respond poorly or in an anomolous manner to the presence of certain compounds, the electron capture detector frequently is the detector of choice where the measurement of trace quantities of these substances is required. For these reasons, and because it was felt that more detailed information concerning the basic characteristics and active mechanisms of this type of detector would be valuable for subsequent investigations, a...
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