The role of stationary phase selection on performance for explosives analysis using GC-ECD

Gas chromatography with electron capture detection (GC-ECD) analysis of explosive-related nitro organic compounds was performed using four different column stationary phases with the focus being on their impact on analyte stability and transfer efficiency during analysis. All four columns used were 6 m x 0.53 mm, and the four stationary phases were a 1.0-microm thick 5% phenyl siloxane/95% methyl siloxane non-polar phase, a 1.5-microm thick 5% phenyl siloxane/95% methyl siloxane non-polar phase optimized for explosives analysis, an intermediate polarity 0.5-microm thick trifluoropropylmethyl siloxane phase, and a proprietary intermediate polarity 0.5-microm thick phase. Although all exhibited similar recovery (as defined as the detector signal per injected mass) when new, the intermediate polarity phases maintained higher sample recovery over the course of analyzing hundreds of samples than the non-polar phases, particularly for the nitramines hexahydro-1,3,5-trinitro-1,3,5-triazine and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, for which a 7x and 3x decrease in recovery were observed, and the nitrate esters nitroglycerin and pentaerythritol tetranitrate, for which a 7x and 11x decrease in recovery were observed. For most other explosive-related compounds, the differences in recovery were between 1.5x and 3x over the same course. Although the detailed chemical formulation of the stationary phases have not been disclosed by their manufacturers, we attribute the observed differences in performance to the stability of their passivation chemistries with regard to other mobile-phase compounds contained in complex field samples. Although these effects have been qualitatively noted in the past and in response, maintenance procedures have been developed to help account for this behavior, the analyst's preference is to use an explosives analysis method that does not require these time-consuming measures. Our desire to prolong this maintenance interval provided the motivation for the assessment reported in this paper. From our assessment, we conclude that manufacturers of GC columns should focus more attention on the stationary phase and passivation chemistries that can lead to the development of a column that is better able to maintain passivation against explosive compound degradation; and users intending to perform large numbers of analyses using GC-ECD should make this a consideration when selecting a column.