Absolute isotopic abundance ratios and the atomic weight of a reference sample of silicon

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.