The Determination of GC–MS Relative Molar Responses of Some n-Alkanes and their Halogenated Analogs

The dependence of relative response factors on the carbon atom number related to naphthalene has been investigated in homologous series by using gas chromatography-mass spectrometry. Relative responses of some straight chain aliphatic n-alkanes and their halogenated derivatives (chlorine, bromine and iodine) were compared in the experiments. Linear correlations were found between the molecular structures; i.e., the carbon atom number and relative molar response in current homologous series. In conclusion, mass spectrometric detection combined with gas chromatography was less sensitive to n-alkanes than to their derivatives containing a chlorine, bromine or iodine atom. After n-alkanes, mass spectrometric responses increase in the order of 1-chloroalkanes, 1-bromoalkanes and 1-iodoalkanes. These results are in accordance with electron ionization cross section data for n-alkyl-derivatives. The relative molar responses of the individual CH(2) groups are between 0.171 and 0.178 in the homologous series. The increments of chlorine, bromine and iodine atoms to the relative molar responses are 0.081, 0.141 and 0.492, respectively. Based on these results, the addivity rule is valid for both halogen atoms and CH(2) groups in the case of mono-substituted n-haloalkanes. The results of this study show a significant departure from the additivity rule in the case of polyhalogenated alkanes and alkenes. However, the relative molar response can be calculated by means of simultaneously measuring other compounds. Further study is needed about how to influence the relative molar responses as a function of various experimental parameters.