LC-TOF MS Poor Ionization

[TBDBITLtof2029]

Hello,

I'm currently working with an Agilent 6230 TOF MS, with an ESI (usually +). Our project is currently working with brain tissue, spinal cord tissue, and blood serum from rats that have been treated with an experimental drug Posiphen, using caffeine as an internal standard.

In our initial stages of TOF analysis, we've had trouble getting clean spectra out of certain trials, and our initial guess is that the samples are not being ionized properly/well. I'm posting one of the spectra here: mainly, the EIC shown above has a hiccup or two in its otherwise smooth results, and we're trying to pinpoint the cause of this hiccup. Anyone have this problem or something like it before?

In other spectra, we've gotten extremely worse curves that don't resemble anything like the control spectra for either caffeine or posiphen in water. Thanks!

[JMB]

Based on your fast chromatography (RT 2.34 min) and busy mass spectrum you could have a co-eluting interference.

Not only does m/z 338 = [M+H]+ for posiphen, but the doublet of signals at m/z 333 and 338 could possibly correspond to [M+NH4]+ and [M+Na]+ for an endogeneous cpd. MW 315 {if you are running for example, an aq. Amm. acetate mobile phase}.

You should generate EIC's at some of the other m/z values, esp. m/z 333, to check for interferences.

Does the small peak at RT 1.95 min correspond to phenserine ??

What does the chromatogram of posiphen std. look like ?

What does the mass spectrum of posiphen std. look like ?