Capping the hydroxyl groups (-OH) of α-cellulose to reduce Hy-groscopicity for accurate 18O/16O measurement by EA/Py/IRMS

Obtaining an accurate measurement of O/O at natural abundance level for land plants-derived α-cellulose with the currently popular EA/Py/IRMS (elemental analysis/pyrolysis/isotope ratio mass spectrometry) method is a challenge due to the hygroscopic nature of the exposed hydroxyl groups, as the O/O of adsorbed moisture is usually different from that of the α-cellulose and the relative amount of adsorbed moisture is sample- and relative humidity-dependent. To minimize the hygroscopicity-related measurement error, we capped the hydroxyl groups of α-cellulose by benzylation to various degrees and found that the O/O ratio of α-cellulose increased with the degree of benzyl substitution (DS), consistent with the theoretical prediction that a reduced presence of exposed hydroxyl groups should lead to a more accurate (and therefore more reliable) α-cellulose O/O measurement. We propose the establishment of a moisture adsorption-degree of substitution or percentage of oxygen-O/O ratio equation, based on the measurement of C%, O% and δO of variably capped α-cellulose, so that a robust correction can be made in a plant species- and laboratory conditions-specific manner. Failure to do so will lead to an average underestimate of α-cellulose δO by 3.5 mUr under “average” laboratory conditions.