Broader peaks in SIM mode than in SCAN.

[saioa]

Hello!
Recently I have started working with a GC-MS and for my surprice when I use a SIM mode the analytes tail quite much while in SCAN mode I would say they do not tail. Is this normal? Do you have any idea of the problem and how I could solve it?
Thanks in advance for your comments.
Saioa

[JI2002]

In theory, peak shape has nothing to do with scan or sim. However, because the sensitivity by using SIM is much higher than using SCAN, you could see the tailing in SIM which you couldn't see in SCAN.

[saioa]

Yes, I know it has nothing to do with the mode, that is why I was very surprice to see that in SCAN (of course higher concentrations) the peak width is about 0.2-0.3 min while in SIM the width is at least 0.5 min and therefore my analytes are not well resolve. Do you think it is my own settings in the GC-MS?

[Kostas Petritis]

Saioa,

JI2002 already explained you the likely reason for what you see... Take your SIM chromatogram and trace a line parallel to the x axis in about half of the peak width... Would the upper part look more like what you obtain with your SCAN?

[chhubert]

Did your "peak" contain only one single compound, as you was talking about unresolved analyte peaks in SIM? If there was two (or more) analytes almost co-eluting, I guess the peak broadening in SIM was due to peak merging. You'd better check whether adequate no. of data points being acquired in each peak. Maybe you've set a relative long dwell (or scan) time in SIM mode. Just for information, a chromatographic peak should contain about 10-15 points for it to be quantitated.

[saioa]

Hello!
I will try to check the dwell time. Probably I have some settings in my programs which are not completely appropiate for SIM...maybe it is the dwell time as suggested. Thank you very much for the information you gave me.
Saioa