How is seleno-DL-methionine determined by HPLC using UV detector?

Answer 1

The HPLC separates your compound from all others, then when it leaves the HPLC column and goes through the UV detector, it will fluoresce under UV light and be detected.

You'll need to run a known sample of your compound through your particular column, so you can determine the time that it will exit. Then for unknowns you can look specifically at that time, and if you see a peak, you've got a good probability that your compound is present. (If there's no peak, you have even better evidence that it's not present).