So lid s amp le . th e rma l ioni za tion. ma ss s pec trom e try ha s bee n used to obtai n a bs o lut e va lu es ror t he iso top ic ab und a nce ra t ios of a refe re nce sa m pie or potass iu m. S La nd a rd s of known is ot o pi e c o mp os i· Lio n , pre pare d b y gr av im e tri c all y mixin g nea rl y iSo Lopi ca ll y and c he mi c a ll y pure se parat e d isotopes of "" K a nd " K. we re use d for c a libra t ion . Th e a b solute iso top ic a bund a nce ra t ios a re "" K/ 41 " = 13.8566 ± 0.0063 a nd ""K /41 K = 0.001 7343 ± 0.0000061 whi c h yie ld a to m pe rce nt co mpos iti on s of "!o K = 93.258 1 ± 0.0029 . . w " = 0.01 J 67 ± 0.00004. a nd ." K = 6.7302 ± 0.0029. Th e ca lc ul a te d a to mi c we i!!ht for pot ass iu m is 39.098304 ± 0.000058. Th e indi ca te d un cert a inti es a re o ve ra ll limit s of e rror whi c h arc th e s um o f th e un cer ta int y co mpon e nt s for ra ti o d e te rmin a ti on s a nd th e cu mpon e nt s c ov e r in i! th e effec ts o f known s o urces of po ss ibl e s ys te ma ti c e rror.Key wo rds: A bso lut e ra tius : a to mi c we ii! ht: iso top ic ab u ndance: po ta ss ium : re fe r e nce s ta nd a rd .

Absolute values have been obtained for the isotopic abundance ratios of a reference sample of strontium using solid sample thermal ionization mass spectrometry. Samples of independently known isotopic composition prepared from chemically pure and nearly isotopically pure separated strontium isotopes were used to calibrate the mass spectrometl),. The resulting absolute IlBSrl B "'Sr. B7Srl~\f-.Sr. and B4SrlIl6Sr ratios are 8.3786 ± 0.0033. 0.71034 ± 0.00026. and 0.05655 ± 0.00014 respectively which yields atom percents of: BaSr "'" 82.5845 ± 0.0066. B7Sr "'" 7.0015 ± 0.0026, >