Gas Chromatography-Combustion-Mass Spectrometry with Postcolumn Isotope Dilution for Compound-Independent Quantification: Its Potential to Assess HS-SPME Procedures

A quadrupole GC-MS instrument with an electron ionization (EI) source has been modified to enable application of postcolumn isotope dilution analysis for the standardless quantification of organic compounds injected in the gas chromatograph. Instrumental modifications included the quantitative conversion of the separated compounds into CO2, using a postcolumn combustion furnace, and the subsequent mixing of the gas with a constant flow of 13CO2 diluted in helium. The online measurement of the 12CO2/13CO2 (44/45) ratio in the EI-MS allowed us to obtain quantitative data without resorting to compound-specific standards. Validation of the procedure involved the analysis of standard solutions containing different families of organic compounds (C9−C20 linear hydrocarbons, BTEX and esters) obtaining satisfactory results in all cases in terms of absolute errors (<6%) and precision (<4% RSD). The developed procedure showed excellent linearity over the range assayed (2 orders of magnitude) and adequate detection limits for carbon containing compounds (0.8 pg C s−1). The generic value of this compound-independent calibration approach was assessed by studying the quantitative performance of Head Space-Solid Phase Microextraction (HS-SPME). The proposed compound-independent quantification by EI-MS permits comparison of the performance of different fibers by assessing analyte recoveries with extreme robustness, simplicity, and precision.