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- Posts: 184
- Joined: Thu May 07, 2009 7:32 pm
Here is an interesting note related to my other post.
We have some pyrethroid pesticide standards that arrived as 100 ug/mL methanol ampoules from AccuStandard. I did the dilutions to 1 ug/mL using methanol again.
When I run them with an ammonium formate ACN:water mobile phase, I get one or two very clear [M + MeOH + H]+ peaks eluting at an earlier retention time, followed by [M + NH4]+ eluting later. This is the case for flucythrinate and deltamethrin but not permethrin. Will test more this afternoon.
With an ammonium acetate MeOH:water mobile phase, the methanol adduct ion is also visible but it co-elutes.
It would seem to suggest to me that many of these molecules form very strongly bound methanol adducts or clusters (perhaps through a mechanism akin to acetal formation?), strong enough to spend 3 minutes on a column and separate from their parent compounds rather than lose MeOH. But when eluting with MeOH they equilibrate rapidly enough to elute as one peak. I would hazard a guess that if I made my dilutions in ACN rather than MeOH, during storage they would probably equilibrate back to the parent compound, but that would take some testing.
Has anyone else experienced this? Methanol in the injection solvent, ACN mobile phase, and MeOH adducts are still visible on MS?
We have some pyrethroid pesticide standards that arrived as 100 ug/mL methanol ampoules from AccuStandard. I did the dilutions to 1 ug/mL using methanol again.
When I run them with an ammonium formate ACN:water mobile phase, I get one or two very clear [M + MeOH + H]+ peaks eluting at an earlier retention time, followed by [M + NH4]+ eluting later. This is the case for flucythrinate and deltamethrin but not permethrin. Will test more this afternoon.
With an ammonium acetate MeOH:water mobile phase, the methanol adduct ion is also visible but it co-elutes.
It would seem to suggest to me that many of these molecules form very strongly bound methanol adducts or clusters (perhaps through a mechanism akin to acetal formation?), strong enough to spend 3 minutes on a column and separate from their parent compounds rather than lose MeOH. But when eluting with MeOH they equilibrate rapidly enough to elute as one peak. I would hazard a guess that if I made my dilutions in ACN rather than MeOH, during storage they would probably equilibrate back to the parent compound, but that would take some testing.
Has anyone else experienced this? Methanol in the injection solvent, ACN mobile phase, and MeOH adducts are still visible on MS?
