Joe 😃 First of all, thank you very much for your prompt response and answering the questions that have troubled me for a long time.It turns out that the experimental method we set up determines that we cannot use the gaps method to fit downhole fractionation.
2.One of your 91500 selections is not 91500. NZ010-35 is not 91500.
Yes, I also found this problem, because when I selected the integration interval, I found that there are many U and Pb peaks in 91500, which I thought would affect my data processing results，so in order to avoid selecting these spikes, I did not select the standard range by time, but by the average signal range of U or Pb in 91500, this function is in Tools-Automatic Selections, as shown in the figure https://imgur.com/4XjW9Km，and https://imgur.com/0PQNPo6.
However, selecting the integration interval in this way will cause new problems. For example, as you mentioned, the U content in some samples is just close to the standard sample range and will be mistakenly regarded as 91500.
So I don't know how to avoid being affected by the spike and not mistakenly select the sample as 91500 for the case where there is a spike in the 91500