by
JMB » Tue Mar 31, 2015 9:32 pm
Since you are running GC/MS, I assume that you are using electron impact ionization (EI). This is a very "hard" (high-energy) ionization and will generally give a high degree of fragmentation of most organic molecules; the ion current is therefore spread amongst many m/z values, and none of them may be intense enough for low-level detection.
Dopamine from a crushed fly-head sounds like ultra trace-level analysis. I think that you need to go to Chemical ionization (CI), using initially CH4 as the reagent gas. This is a "soft" (low-energy) ionization process, and will give you primarily intense [M+H]+, lower intensity [M+C2H5]+ and lower still intensity [M+C3H5]+ ions. There will, IN GENERAL, be little to no fragmentation and most of the ion current will be carried by the [M+H]+ signal.
However, I suspect that you will ultimately have to go with electron capture negative ion chemical ionization (ECNICI) for best sensitivity; this will usually give M as a radical anion. Again, this can be performed with CH4 as the reagent gas, and the analyte molecules have to be derivatized with a group of high electron affinity (EA). Typical high EA groups contain halogen atoms (C6F5-, CF3CO-, etc).
There are at least a couple of good ref. books that give comprehensive reviews & detailed procedures for such derivatizations.
One is Blau & King, another is by Knapp et al.
If GC/MS is ultimately unsuccessful, HPLC with fluorescence detection may be required.
Good Luck & let us know if any technique is successful.