I am using a UPLC-ESI-MS (single quad) to quantify a corticosteroid in negative mode. The molecular weight of the compound of interest is 392 but the m/z after scanning with the largest peak is 437. There is no peak in the blank solvent and we cleaned the instrument as well. I am not sure what sort of adduct could be forming for such a big difference. Can someone expert in LCMS guide me?