The selection of the standard phase (gas vs. liquid) and the related matrix effect on the direct injection gas chromatographic analysis of VOCs at sub–ppm levels

In this study, the matrix effect on the quantitation of volatile organic compounds (VOCs) at sub–ppm levels has been investigated in relation to the selected standard phase by direct injection gas chromatographic (GC) analysis. To this end, a series of calibration experiments were conducted using both liquid and gas phase standards containing identically a total of 13 target VOCs. Calibration datasets between liquid and gas standards were obtained at the four selected injection volumes, i.e., covering a 1 to 5 and 50 to 500 μL range, respectively. The results indicate that injection volume is a sensitive parameter, as sensitivity tends to decrease with increasing injection volume, especially with liquid standards. Loss of analytes in liquid standard occurred noticeably from ones eluting earlier than solvent used for standard (i.e., methanol). If the extent of such loss is expressed by the percent differences in the response factor (RF) values between two standard types, the results were low or insignificant for valeraldehyde (0.48%), benzene (7.6%), toluene (3.3%), and styrene (4.8%) but generally high for the others (i.e., between 10–80%). The relative sensitivities of VOCs in gaseous standards, if computed by normalization against benzene, generally complied well with those derivable from the literature on flame ionization detectors. In contrast, in case of liquid standards, the use of a small injection volume (≤1 μL) is recommended to maintain the optimal GC performance in light of the matrix effect.