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

Lipid compound-specific hydrogen isotopes (delta D) have been widely applied in studies of biogeochemistry and paleoclimatology. delta D values of lipid homologues with different chain lengths from a single analysis are often utilized together in order to make full use of the delta D 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 delta D 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 delta D values of lab working standards (mixed C-21, C-25, C-27, C-29, C-31, and C-33) could vary by 40 parts per thousand to 70 parts per thousand 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 delta D values are higher by 20 parts per thousand to 40 parts per thousand with the amplitudes lower than the optimum range, and lower by 10 parts per thousand to 20 parts per thousand with higher amplitudes. All the results consistently show higher delta D values with decreasing amplitudes, and larger deviations occurring in low amplitude range, implying that special caution should be taken with the delta D values measured at low amplitude range. We have attempted amplitude-based correction of lipid delta D values, however, this approach should be cautious owing to their large residual errors. Hence, delta D 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.

Automated summary

The study investigated the compound-specific delta D deviations with varying contents injected using isotope ratio mass spectrometry, finding higher delta D values with decreasing amplitudes and larger deviations occurring in low amplitude range. For lab working standards, n-alkane delta D values varied significantly with changes in amplitudes, while repeated analyses of natural samples yielded higher delta D values with lower amplitudes and lower values with higher amplitudes. Amplitude-based correction of lipid delta D values may be attempted, but caution is needed due to large residual errors.