The negative alkali flame detector response

Abstract

Under certain conditions, the alkali flame detector will give negative response (inverted peaks) for organic compounds containing halogen, nitrogen, or other elements. With a modification of the alkali flame detector particularly suited for such a study, the range and magnitude of negative response was defined in terms of alkali bead bore, hydrogen flow, carrier gas flow, electrode height, and other parameters. Organic compounds containing chlorine, bromine, iodine, and nitrogen, were used as test substances; an organophosphate was studied under the same conditions for purposes of comparison. Cl, Br, I, N, and C can show either positive or negative response and each can be distinguished from the others by proper choice of parameters. Both halides and carbon compounds show stronger response in the negative mode than in the positive mode. The use of two different carrier gases, nitrogen and helium, did not cause significant differences, except in its effect on the response of nitrogen compounds. In general, a large bead bore, a high hydrogen supply and/or a low carrier gas flow favor negative response. These are also the conditions which establish a large area of contact between flame and alkali surface and, consequently, a large background current. In order to obtain reproducible response of adequate magnitude, the Rb 2SO 4 salt surface must be smooth and homogenous. The described detector functions can be used for qualitative microanalysis of hetero-organic compounds by gas—liquid chromatography.