ACCURACY AND PRECISION OF GAS-CHROMATOGRAPHY COMBUSTION ISOTOPE RATIO MASS-SPECTROMETRY FOR STABLE CARBON-ISOTOPE RATIO MEASUREMENTS

We evaluated the precision, accuracy and linearity of three commercial gas chromatography/combustion isotope ratio mass spectrometry (GC/C-IRMS) instruments in regard to stable carbon isotope ratio measurements of fatty acids with C-13 enrichment levels ranging from natural abundance to 924 parts per thousand, in comparison to measurements by conventional gas isotope ratio mass spectrometry (GIRMS). Memory effect was also evaluated with a fatty acid mixture containing methyl myristate and methyl stearate at natural abundance levels of C-13 but methyl palmitate at C-13 enrichment levels of 460 or 924 parts per thousand. At natural abundance, the delta(13)C values of fatty acids measured by the GC/C-IRMS instruments were reproducible to within 0.34 parts per thousand (range: 0.11 parts per thousand to 0.32 parts per thousand and accurate to -0.38 +/- 0.40 parts per thousand (mean difference +/- standard deviation; range: -0.03 +/- 0.25 parts per thousand to -0.84 +/- 0.39 parts per thousand). At enrichment levels of C-13 (23 to 994 parts per thousand, the delta(13)C values were reproducible to within 1.48 parts per thousand (range 0.20 parts per thousand to 1.00 parts per thousand and accurate to -1.11+/-3.16 parts per thousand (range: -1.02 +/- 3.23 parts per thousand to +4.56 +/- 13.42 parts per thousand). Stable carbon isotope ratio measurements by all three GC/C-IRMS instruments were found to be linear from -30 to 924 parts per thousand (r(2) > 0.9999; p less than or equal to 0.004) in comparison to those measured by GIRMS. Significant memory effect was found in only one system. Based on our data, the GC/C-IRMS instruments can provide linear, precise and accurate stable carbon isotope ratio measurements and require only pico- and nanogram quantities of samples.