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, >

Ab solute va lu es have been obtained for th e isotopic a bund a nce rati os of a re fe re nce sa mpl e of s ilico n us ing e lec tron impa ct mass s pectrom etry. Sa mpl es of known isoto pi c co mpos it ion pre pared fro m nearly isotop ica lly pure se pa rated sili co n isotopes we re used to ca librate th e ma ss s pectrom e te rs. The res ult ing absolute ,sS i/,JOS i ratio = 29.74320 ± 0.00747 and th e 29S i/,,°S i ratio = 1. 50598 ± 0.00086 whi c h yield at om pe rcen ts of '"S i = 92.229:n . 0.00155. " 'Si =,1.()6982 I 0.00124 and :lIISi = :). 10085 I 0.00074. The atomic weight ca lc ulated from thi s isotopic co mpos ition is 28.085526 · 0.000056. Th e indi cated un ce rtainti es are overall limit s of e rror based o n 95 perce nt confidence limit s for th e mea ns a nd a ll owan ces for th e effec ts of kn ow n sources of poss ibl e s yste ma tic e rror. A study of natura l '"S i/,IIIS i rati o variations re ported in th e literature e xtend s tb e es timat ed un ce rt a int y in th e atomic we ight of natural s ili co n to ' 0.000.39.Key word s: Absolut e ratios; a tomi c we ight: iso topi c abundan ce: s ili co n .• Prese nt aJdrt:ss: Tel edyne Isutopcs. Ti monium. Maryland 2 1200.