These days, I am working on peptides. While preparing the standard, I have been facing no detection of peptides in UPLCMSMS analysis. Actual case:

Case 1: Water + peptide STD working solution + IS, precipitation/dilution with ACN or MeOH or dilution with water if required = well detected with good intensity

Case 2: Plasma + Peptide STD working solution + IS, precipitation ACN or MeOH or dilution with acidic water (0.1% TFA in water or water only)] if required = not detected

In my case, the peptides I used are well-soluble in water (10 mg/mL), ACN, and MeOH.