Statistical evidence for exchange of oxygen isotopes in holocellulose during long-term storage

A linear regression between oxygen isotope (δ18O) values from holocellulose processed and analyzed in 2000, and 6 years later in 2006, revealed evidence for isotopic exchange of oxygen atoms during the storage period. A statistically significant change in the slope of the regression line indicates a shift in the δ18O values of the stored samples, with most values plotting on one side of a line of equal fractionation. This shift is interpreted as indicating exchange of carbonyl oxygen during storage. A linear regression between δ18O values from the 2000 holocellulose and from alpha-cellulose (α-cellulose) produced in 2006 (from the 2000 holocellulose), yielded a slope that was statistically identical to 1. In this case, a slope of 1 is expected if there was no isotopic exchange during cellulose deposition, holocellulose extraction, and sample analysis. However, the sample variance around the best fit line is high, so it is possible that the slope of 1 is an artifact of a combination of isotope exchange and other non-systematic errors introduced during chemical extraction or analysis. This study indicates exchange of oxygen in holocellulose during 6 years of storage, but evidence for the presence or absence of exchange of oxygen prior to the completion of cellulose extraction is equivocal.