Hi @chenhongjun ,
The difference between 2SE and 3SE is that 2SE is two times the standard error of the sample you're looking at, and 3SE is three times that standard error.
In the first image you show, I wouldn't necessarily say that those spikes are beyond 2SD/2SE: I would say that it looks like there are enough of them that you have a bi-modal distribution, and that your sample is a complex mix of the two. Now you could filter these results out, but it would create many short selections between the U spikes that you could then link together to form a "linked selection". You could do this via the Automatic Selections -> 'from channel data' tool to select for a range of U238/Pb206 ratios. However, I think that this would start to approach data manipulation. Others may have a different interpretation. Please feel free to post here if you have anything to add!
Regarding your second image: it does appear that there are different zones within your ablation. Which of these to select/avoid is somewhat subjective unless you define what and why you're rejecting data before adjusting anything. For example, you could select based on concordance, but it looks from your other images that you may not have significant amounts of common Pb present, so concordance may not be appropriate here.
Which brings me to your final images. From what I can see (i.e. the images you've posted) it appears to be a very well defined spread between a common Pb component and a radiogenic component. The intercept of this group is a possible estimate of the 'true' age of your samples. But you cannot (easily) select just for the most radiogenic parts of your samples if they are a complex mixture. It would actually make the nice linear relationship you see when looking at all samples less well define and thus your intercept would be less well determined. I would suggest that you treat all these selections as a whole defining what is happening in your samples, rather than trying to optimise each selection for the most radiogenic components.
Again, others might have more/better advice, so please feel free to add to the discussion here.