try this:

http://link.springer.com/article/10.1023%2FA%3A1006245619568

The reprint" NEUTRAL SODIUM FROM COMET HALE-BOPP: A THIRD TYPE OF TAIL"The Astrophysical Journal, 490:L199–L202, 1997 may be of some use.

The two previous readings suggested are excellent as they review nicely our knowledge on Na in comets. Let me point out that if the general scenario of comet formation is correct we should expect the accretion of significant amounts of Na in these bodies, perhaps in neutral form forming part of the interstitial matrix (also in ices?). Its presence could fit our detected overabundance of Na in cometary meteoroids that could be consequence of extensive Na depletion of the inner disk during the early solar system stage. See e.g. our paper: http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004MNRAS.348..802T&db_key=AST&link_type=ARTICLE