QUANTITATION USING BENZENE IN GAS-CHROMATOGRAPHY CHEMICAL IONIZATION MASS-SPECTROMETRY

Dilute mixtures of benzene in helium provide abundant C6H6.+ ions, which selectively react by charge transfer under chemical ionization (CI) conditions with unsaturated compounds in complex hydrocarbon mixtures and do not react with alkanes or cycloalkanes. The charge-transfer spectra from the ion/molecule reactions of C6H6.+ are very simple, containing essentially only M.+ ions from the samples. Relative molar sensitivities for olefins and alkylbenzenes in benzene chemical ionization mass spectrometry (CIMS) are essentially constant within +/- 10-15%, independent of molecular weight in the range of 100-200 Da, molecular structure, the degree of substitution, or ionization energy. The relative molar sensitivities in benzene CIMS show much loss variation with molecular structure than relative molar sensitivities in low-voltage electron ionization mass spectrometry (LV-EIMS). GC/CIMS with benzene as the charge-transfer reagent gas allows quantitative analyses of the aromatic and olefinic components in gasolines or other complex hydrocarbon mixtures without the necessity of calibration curves for the individual components.