chenhongjun But why is 206Pb higher than the total in my data at the same point?Isn't the total signal value 204 + 206 + 207 + 208?
Because, if you use NIST you'll be using the same relative isotopic contributions. For example, if you use 206Pb, for normal Pb isotopic abundances, it'll be relative to 25% of total Pb. But in older zircons, 206Pb can account for more like 90% of Pb (or some much larger value than normal). So you're not comparing apples to apples.
I'll send you an email, and we can look at specific data in more detail.