Large-Volume Injection and Assessment of Reference Standards for n-Alkane δD and δ13C Analysis via Gas Chromatography Isotope Ratio Mass Spectrometry

Abstract

Rationale: Compound-specific stable isotope analysis of hydrogen (δD) and carbon (δ13C) in organic compounds is a valuable tool in biogeochemical research. A key limitation of this method is the relatively large amount of sample required to achieve desirable precision. Methods: We developed a large-volume (20 μL) injection method that allows for high throughput analysis of less concentrated samples and tested it for δ13C and δD measurements of n-alkanes. We also conducted a comparison of reference standards and assessed several methods to normalize and correct n-alkane δD and δ13C measurements. Results: The mean precision of the δD method based on 233 environmental n-alkane samples (two to three replications per sample) is 4.0‰ (1σ, estimated from the weighted mean of the pooled unbiased standard deviations) and 0.46‰ (1σ) for δ13C from 37 environmental samples (two to three replications per sample). The evaluation of reference standards shows that the use of n-alkane standards with large offsets in δD values in adjacent n-alkane chains can lead to biases in measurement correction. Conclusions: The large-volume injection method shows good reproducibility of δ13C and δD measurements of n-alkanes and reduces the required sample concentration by about 80%. We propose that for δD measurements, a reference standard set should be used in which each reference standard has a limited range of δD values and no adjacent n-alkane chains, to minimize memory effects.