Craig:

You may find this link useful:

http://link.springer.com/article/10.1007/BF01576174

Best

Syed

Might look here too:

http://xylitol.org/articles-about-xylitol/xylitol/

The organization might be able to help as well. Most of the studies are using 13C markers, but they must know what "normal" values are to understand the isotope mass balance to make sense of their data.

Hope this helps!

Birch (bark) like most terrestrial plants is a C3-plant which as far as natural 13C abundance of its carbohydrates (incl. cellulose) and biogenically related compounds is concerned means one can expect to observe delta13C values of -28 to -22 ‰ v. VPDB.  These figures correspond to a 13C abundance range of 1.08045 to 1.08705 atom%, respectively.  For cellulose in raw cotton we observed a delta13C range of -27.77 to -23.96 ‰ while for beet sugar we observed d13C values ranging from -27.63 to -25.38 ‰ v. VPDB.

Weber, Gensler & Schmidt (1997) published a paper on 13C isotopic linkage between glucose and biogenically related plant products (sadly not xylitol); Isotopes Environ. Health  Stud., 33, 151-155.  Another paper by H.L. Schmidt reports on 13C distribution of various plant metabolites in Solanum tuberosum; Gleixner, Scrimgeour, Schmidt and Viola (1998), Planta, 207, 241-245.

Site-specific natural isotope fractionation studied by nuclear magnetic resonance (SNIF-NMR) provides isotopic criteria that characterize a biochemical transformation such as fermentation and enable isotopic ratios measured in end products to be correlated with those of their precursors.

1)       Natural Abundance Isotopic Fractionation in the Fermentation Reaction: Influence of the Nature of the Yeast

 Claude Vallet et al., Bio-organic Chemistry 24(4):319-330 · December 1996

 2)     Ethanol fermentation from biomass resources: current state and prospects Appl Microbiol Biotechnol (2006) 69:

 Hope this helps!