Characteristic negative response of the helium ionization detector

When ultrapure helium is used as a carrier gas, the helium ionization detector provides a characteristic response, either positive or negative to each gas. The detector response can be made positive for all gases except neon by the inclusion of a few ppM of gaseous additives to the helium carrier gas, but the inclusion of gaseous additives decreases the detector signal. In this study we first investigated the negative response of the helium ionization detector, and then compared the lower and upper detection limits obtained by using ultrapure helium as a carrier gas (yielding positive and negative responses) with those obtained by using helium plus hydrogen as a carrier gas (yielding positive responses). When ultrapure helium was used, the negative response of the detector was found to be linear over a range of about 10/sup 4/ and the detection limits for both positive and negative responses were one to two orders of magnitude better than those obtained when helium plus hydrogen was used as a carrier gas. 8 figures, 1 table.