Absolute values have been obtained for the isotopic abundance ratios of magnesium, using surface e mi ss ion mas s spectrometry. Samples of known isotopic composition, prepared from nearly pure se parated magnesium isotopes, we re used to calibrate the mass spectrometers. The resulting abso· lute valu es are 25Mg/24Mg = 0.12663 ± 0.00013 and 26Mg/24Mg = 0 .13932 ± 0.00026, yielding an atomic weight (I2C = 12) of 24.30497 ± 0.00044. The indicated uncertainties are. ove.-. all limits of error based on 95 percent confid e nce li mits for th e mean and allowances for effects of known sources of possible sys te matI c error.

Ab so lut e va lu es have been obta in e d for th e iso topi c ab und a nce ratios of a re fe re nce sa mpl e of chromium. us ing s urface e mi ss ion ma ss s pec trom e try. Sa mpl es of known isotopi c compos ition , vrepa re d I rom nea rl y pure se parated c hromium isotop es, were use d to c alibrat e t he mass s pec l rom e tno. Th e res ulting ab solute valu es are C r 5 0/C,-5' = 0.051859 ± 0.000100, Cr'''/C r'' = 0.11 3386 ·± 0.000145 , C r"/C r52 = 0 .028222 ± 0 .000059, yie lding a n atomic we ig ht (C '2= 12) of 51.99612 ± 0.00033. The indi ca ted un ce rt a inties arE' overall limit s of e rro r bas ed on 95 pe rc e nt confidence limit s for th e nwa n and a llo wa nces for e ffec ts of k nown ::io UIT('S uf po ss ibl e sys tl'mali c (' ITOr. Ke y Word s : C hromium . a tomi c w(·ight. is otopi c a bund a nces. absolut e ratios.

\. Abso lute valu es have been obtaine d fo~ the iSQ,top ic a bundance ratios of co mmon , eq ual·atom , and rad,oge lllc lead ,sotOP' C s tandard s usmg sohd·sample ma ss spectrometry. Samples of known 2°'Pbj206Pb ratI o, prepared fro m nearly pure se parated 206Pb and 208Pb so luti ons , were used to calibrate the ma ss s pectrome te rs.

An a bso lute valu e has bee n o btained for the isotopic ab un da nce rati o of te rrestria l rubidium , us in g sa Ud -sa mple the rma l ionizati o n mass spectro me tr y. Samp les of know n i'!otopic composition , prepa re d from nea rly isotopica Ll y pure se parated rubidium isotopes, we re used 10 calibrat~ the mass s pectrom e ters. The res ultin g a bsolute 85 Rb/ 87 Rb ra ti o is 2.59265±0. 00170 whi c h yie lds atom pe rcents of: ·' Rb = 72. 1654±0.0132 and 87 Rb =27.8346 ± 0.0132. Th e a tomic weight calc ulated fro m thi s isotopic co mposilion is 85.46776 ± 0 .00026. The indi cated uncerta inties a re overa ll limits of error ba sed on 95 pe rce nt confide nce limits fo r the m ea n a nd a ll owa nces for the effects of know n so urces of poss ibl e syste m ati c e rror.Key words: absolu te r a ti o; atom ic we ight ; isoto pic abundances; rubidium .

An absolute value is obtained f