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Pyrimidal Spike in MS/MS early in chromatogram
Posted: Wed Jan 19, 2005 3:37 pm
by fishin' addict
I'm relatively new to MS/MS and am having an interesting problem with, hopefully, an easy fix. I'm currently operating a Micromass Quattro LC in ESI+ mode analyzing 3 compounds (MRM), aka 3 channels setup for analysis. All dwell times and delays are set to 0.1 min. Early in the run/chromatogram the instrument is exhbiting a spike with a triangular symetry that is 0.1 min wide at the same Tr. I don't believe this peak is ~real~ with regard to the analysis what I'm concerned about is what this may be saying about the inner workings of the instrument. Does anyone have any experience with a situation such as this?
Thanks in advance for any help/discussion!!
Posted: Thu Jan 20, 2005 7:54 pm
by MG
Your dwell time is really 0.1 min? We measure dwell times in ms, and 50ms to 1000ms are typical, depending on how many transitions and the width of the chromatographic peaks. Your's corresponds to 6000ms. I am not saying it is wrong, only unusual. You need to make sure you are getting at least 12 to 15 points across the peaks you are trying to quantitate (chromatographic peaks minimum of 1.2 to 1.5 min wide), or else lower your dwell time.
If 0.1min is your dwell time for a given MRM channel, you would not see the signal change over a smaller increment of time than 0.1 min. To see a "peak", you would need at least two cycles: one for signal to rise and then another for signal to fall to baseline. If I am thinking about this properly, any peak equal to or narrower than 2x your dwell time would appear as a sawtooth or triangular peak equal in width to 2x your dwell time (0.2 min in your case).
The Solution to the Problem
Posted: Fri Jan 21, 2005 3:08 pm
by fishin' addict
MG, yeah I agree that if my settings were in minutes that may be a plausible scenario. Although, I have to apologize I had a chromatogram sitting in front of me and was thinking min. when I wrote the Dwell/Delay comment etc. wrong units, units should have read seconds. Anyhow the peak I was seeing had nothing to do with that information, it was actually just due to momentary Cap/Cone arcing . . . moved the probe a bit to fix it. Thanks for your input and in future posts I'll be sure to proof my units a bit more carefully.