Compound‐specific δ18O analyses of neutral sugars in soils using gas chromatography–pyrolysis–isotope ratio mass spectrometry: problems,possible solutions and a first application

Although gas chromatography–pyrolysis–isotope ratio mass spectrometry (GC‐Py‐IRMS) has allowed us to make online compound‐specific δ¹⁸O measurements for about the last ten years, this technique has hardly been applied. We tested different pyrolysis reactor designs using standards (vanillin, ethylvanillin, a fatty acid methyl ester and alkanes) in order to optimize the GC‐Py‐IRMS δ¹⁸O measurements. The method was then applied to methylboronic acid (MBA) sugar derivatives (pentoses, 6‐deoxyhexoses and hexoses). Plant‐ and microbial‐derived monosaccharides were extracted hydrolytically from litter and topsoils before derivatization. The measured δ¹⁸O values of samples and co‐analyzed reference material were first drift‐corrected by use of regularly discharged pulses of CO reference gas. Secondly, they were corrected for the amount dependence of the δ¹⁸O values. Thirdly, the δ¹⁸O values were calibrated using the reference material (principle of ‘Identical Treatment’), and, finally, a correction was applied by taking the hydrolytically introduced and water‐exchangeable oxygen atoms into account. Our results suggest that the δ¹⁸O values of plant‐derived monosaccharides in litter reflect the climatic conditions of the last year, whereas δ¹⁸O values of the respective topsoils reflect the averaged climate signal of the last decades or even centuries. This demonstrates the high potential of the method for palaeoclimate reconstructions. Copyright © 2009 John Wiley & Sons, Ltd.