Why the obtained counts of 206Pb, 207Pb and 208Pb do not correspond natural abundance ?

Hi , 

1. we measured individual counts of 206Pb, 207Pb and 208Pb and found these counts did not correspond natural abundance. And we checked all of the previous records and found this non-correspondence always happens ( almost three years)  no matter what the concentration is. 

natural abundance is 206 Pb: 207Pb: 208Pb=24.1%:22.1%: 52.4%

however, from our raw data( see the below picture), the rough abundance is : 206 Pb: 207Pb: 208Pb=13.0%:11.42%: 52.4%

the standard is named Pb, which means sum. 

2. what makes this difference? is it radioactive decay or interference? 208 Pb has the interference of 207PbH, 207Pb has interference 206PbH. but we don't use hydrogen. we used He Mode and Ar2 Gas. And the diluent is 0.5% HCl and 5% HNO3. we tested Pb with other elements of V, Co, Ni, Cu, As, Cd, Hg.  Internal standards are Bi, Ge, In, Lu, Sc, and Te. 

3. what should we do to solve this problem? use corrective equation if radioactive decay? 

Thank you 

best regards, 

Di Xu

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