Substantial peak size effect on compound-specific δD values analyzed on isotope ratio mass spectrometry

Abstract

Lipid compound-specific hydrogen isotopes (δD) have been widely applied in studies of biogeochemistry and paleoclimatology. δD values of lipid homologues with different chain lengths from a single analysis are often utilized together in order to make full use of the δD information. However, this approach could be potentially impacted by the large analytical error due to the inappropriate amount injected for some homologues. Here, we systematically investigate compound-specific δD deviations with varying contents injected on the isotope ratio mass spectrometry, expressed as amplitudes, using both lab working standards and natural samples. The n-alkane δD values of lab working standards (mixed C21, C25, C27, C29, C31, and C33) could vary by 40‰ to 70‰ when the amplitudes of n-alkanes change from 0.5 V to 10 V. For natural samples (27 n-alkane and 77 fatty acid samples), we have made repeated analyses of the same samples with different homologues targeted for the optimum range. The measured δD values are higher by 20‰ to 40‰ with the amplitudes lower than the optimum range, and lower by 10‰ to 20‰ with higher amplitudes. All the results consistently show higher δD values with decreasing amplitudes, and larger deviations occurring in low amplitude range, implying that special caution should be taken with the δD values measured at low amplitude range. We have attempted amplitude-based correction of lipid δD values, however, this approach should be cautious owing to their large residual errors. Hence, δD values of different homologues in the same samples have to be measured in separate analysis if they could not fall within the optimum range at the same time.