Why are Count ratios of Cd111/Cd114 and 78Se/Se80 not consistent with their natural abundances?


1. we recently run Cd and Se along with  other EI ( As, Co, Hg, Ni, Pb, V, Ag, Ba, Cr, Cu, Li, Mo, Sb, Sn and TI) with internal standard Sc, Ge, Te, Bi, In and Lu . Aicd diluent is 5% HNO3 (v/v) and 0.5% HCl(v/v). He mode is used 

2. we found the count ratio (CPS)of Cd111/Cd114 is around 0.08-0.09 ( see the below picture). for example: at 0.1 ng/mLconcentration CPS ratio=62.33/814.70=0.08. However, the natural abundance Cd111 is 12.80%, and 28.75% for Cd114. so the CPS ratio from natural abundance should be 0.4. There is big difference between them. does is mean there is serious interference happening? if so, what contributed to this interference? 

3. The same thing happened to Se78 and se80. the CPS ratio (Se78/Se80 )from natural abundance should be 0.5. but our signals gave us 1.6-1.7. ( 27.33/16=1.7 at concentration of 4ng/mL). is there interference happening ? if so, does this interference come from 78Kr which possibly is present in Ar or something else? our Ar is highly pure. 

Thank yoiu

best regards, 

Di Xu

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