Active control of selectivity in organic acid analysis by gas chromatography

A new method that allows organic acid selectivity to be dynamically controlled during gas chromatography (GC) is presented. It employs dual in-series stainless steel columns, each coated with a pH-adjusted water stationary phase. The first is a 2 m column coated with a pH 11.4 phase that is connected to a second 11 m column coated with a pH 2.2 phase. In this arrangement, organic acids within sample mixtures are trapped on the first column, while the remaining non-ionizable components continue to separate and elute in the system. Later, by injecting a volatile formic acid solution, the trapped acids are released in-situ to the second column for separation and analysis as desired. The method provides good reproducibility with analyte retention times in consecutive trials yielding an average RSD of 1.9%. Further, depending on column temperature, analytes can be readily retained for periods investigated up to about 30 min without significant deterioration in peak shape. This feature provides considerable control over analyte selectivity and resolution compared to conventional separations. Further, by adding a third conventional GC column in-series, both typical hydrocarbon and enhanced organic acid separations are made possible. The method is applied to the analysis of complex mixtures and matrix interference is found to be significantly minimized. Results indicate that this approach offers beneficial advantages for the selective GC analysis of such acidic analytes.