My guess is that the problem here comes from the chromatography rather than the MS. In the past we experienced this kind of problems with glutathion as well. It seems that glutathion can create secondary interactions with the column (binding to metal ions etc.) via its disulfide bond, which results in strong retention and decrease or even absence of peaks after a few injections. It is necessary to "condition" the column before running a batch. In our case , we first tried to condition our column (Waters UPLC HSS T3) with albumin but it did not improve. Then I had a strange idea: Since cabbage contains a lot of sulfur containing compounds (glucosinolates), I prepared an extract of cabbage and injected it a few times before running glutathion, and it worked, peak shape was restored! Now, we always run 4-5 injections of a methanolic extract of cabbage (which should be prepared in a way to maintain glucosinolates intact, and possibly concentrate it through SPE) before running GSH/GSSG samples.