chenhongjun Yes, if you look at the counts, it's obvious that there is more total Pb than just 206Pb. However, that is not how the concentration equation works (unless you use total Pb). The reason that the 206Pb concentration in Table 2 is higher (ignoring the result for Total Pb for just a second) is that the concentration equation assumes when you use NIST612 as your calibrant that 206Pb makes up about 24% of your Pb. When this is incorrect, and 206Pb makes up significantly more of your Pb (80% in our example), the concentration equation gets the wrong result.
You can actually see this if you calculate the different isotopic abundances and look at their ratio. For example, in the Note, the zircon has 0.8 206Pb (proportion of Total Pb). The NIST612 calibrant has 0.24 206Pb (proportion of Total Pb). And we know from our calculations in the Note that the actual concentration of Pb in the zircon is about 6.7 ppm. When you ratio the Pb proportions, and multiply it by the actual concentration, you get the offset we see in Table 2. That is:
0.8 ÷ 0.24 × 6.7 = 22.3 or roughly what we got for Pb concentration based on 206Pb in Table 2.
If our isotopic compositions were the same, we'd get:
0.24 ÷ 0.24 × 6.7 = 6.7 i.e. the correct value.
The little equations above have just stripped out the parts of the concentration equation that deal with converting a CPS value to a ppm value, and just look at the isotopic composition part.
I hope that helps to clarify. If you have more questions, please just let me know.