The use in conjunction with gas chromatography of 3-nitrophthalic anhydride and semicarbazide as reagents for selectively subtracting alcohols and carbonyl compounds, respectively, from mixtures containing these compounds was investigated. Each reagent, when deposited on a diatomaceous earth support and heated as a short plug in the injection block of a gas chromatograph, either removed or markedly reduced the peaks due to a large number of compounds bearing these functional groups. These short precolumns were amenable to operation with glass porous layer open tubular analytical columns under temperature-programmed conditions for compounds covering a moderate range of boiling points without seriously affecting the performance of the columns. The performance of the above reagents was compared with a number of other compounds cited in the literature as specific abstractors, viz., boric acid for alcohols, free fatty acid phase and o-dianisidine for aldehydes and benzidine for aldehydes and ketones. These compounds were tested both as short-heated precolumns and also when coated on lengths of porous layer open tubular columns in the manner of normal liquid phases.