Any way to enhance molecular ion for GCMSMS analysis?

[James_Ball]

I am just starting to develop some methods for GCMSMS and am wondering if there is a way to get less initial breakdown of the analyte in the EI source so that you have greater molecular ion to work with going into the collision cell.

I have tried lowering the uamp current to the filaments and am starting to play with reducing from -70ev but not really seeing any shift in abundances. Is there any trick to this or am I just stuck with a lot of fragmentation in the source?

[Loekie]

Cooling the source might help a little or what about CI?

[James_Ball]

Cooling the source might help a little or what about CI?

Well I pushed for CI when ordering the instrument, but ended up getting one without it. Now I wish we had.

Working on a method for Methamphetamine without derivatization, seems I can get a little better sensitivity in SIM than I can with MRM, just more background is present. The molecular ion is probably less than 1% abundance so the molecule breaks down pretty well before hitting the collision cell.

[JMB]

James,

How low have you dropped from 70 eV ?? You may have to go quite low, e.g 20 eV to get a meaningful mol. ion. If you have a solids probe, this would be easy to do stepwise.

Since your M+. is at m/z 149, and the fragments are obviously lower, you could distort the tuning (I assume QQQ) to enhance the low mass end; you don't need sensitivity above ~m/z 175 in either Q1 or Q3.

JMB

[James_Ball]

James,

How low have you dropped from 70 eV ?? You may have to go quite low, e.g 20 eV to get a meaningful mol. ion. If you have a solids probe, this would be easy to do stepwise.

Since your M+. is at m/z 149, and the fragments are obviously lower, you could distort the tuning (I assume QQQ) to enhance the low mass end; you don't need sensitivity above ~m/z 175 in either Q1 or Q3.

JMB

I think I went down to 30ev, I may try lower and see what happens.