QUANTITATION OF FATTY-ACIDS AND HYDROXY FATTY-ACIDS BY GAS-CHROMATOGRAPHY MASS-SPECTROMETRY - PREDICTIVELY USEFUL CORRELATIONS OF RELATIVE RESPONSE FACTORS WITH EMPIRICAL-FORMULA

Relative response factors (molar sensitivities relative to dibutyl phthalate) for homologous series of n-alkanes (C-12-C-26), trimethylsilyl esters of C-10-C-20 fatty acids (FAs) and bistrimethylsilyl derivatives of C-8-C-16 omega-hydroxy fatty acids (HOFAs) were determined by integration of total ion chromatograms obtained using a conventional gas chromatograph-mass spectrometer in the electron impact ionization mode. For the alkanes up to C-22 and for the FA-TMS derivatives up to C-18 (C-21 including the TMS carbons), response factors increased linearly with total number of carbons (r(2) > 0.99). The slope of this correlation (response factor = 0.0775n - 0.338, where n is the total number of carbon atoms in each analyte) is essentially identical with those determined in earlier studies for smaller molecules not containing silicon. For the HOFA derivatives up to C-14 (C-20 including TMS carbons), the molar response factors also increased linearly with total number of carbons, but for reasons not yet apparent the regression line for these homologs was different (response factor = 0.0330n + 0.456; r(2) > 0.99). Response factors for higher homologs in all three series fell below the regression line for the lower homologs, the deviation increasing with increasing molecular mass and increasing GC retention time. In all three series the upper limit to the linear portion of the correlation corresponded to the homolog whose GC retention time was ca 15 min under the conditions used; a high injection port temperature was found to be the most critical factor in extending the linear range of the correlation.