Poor Chromatography on SIM but fine Scan Chromatography.

[MarkRob]

I'm running EPA 8270 and, as the title explains, I'm getting fine chromatography on scan mode but crummy peaks on SIM.

any thoughts?

[jh1]

I'm running EPA 8270 and, as the title explains, I'm getting fine chromatography on scan mode but crummy peaks on SIM.

any thoughts?

What's your SIM dwell time, how many peaks are in a typical SIM window and have you corrected your SIM ions for mass defect?

If your dwell time is set too high you won't get many scans over each peak giving poor peak shape. As far as mass bias goes a peak in scan mode may have a listed mass of 283 but the actual mass in the ion tabulation chart may be 283.2, listing 283 not 283.2 in the SIM window will result in reduced sensitivity.

[MarkRob]

Hi there,

dwell time is 100 and typically I have 8-12 ions per group.

I'm not sure how to do a correction. The manual is very limited when it comes to explaining SIM.

Thanks.

[AICMM]

MarkRob,

You are looking at too many ions per group. Two things are possible. One is that if you really have a 100 ms dwell then you don't get enough data points to define the peak. Second is that the software has shortened the dwell time to accommodate the number of ions and you aren't really sitting at the dwell long enough to pick up the signal.

Simple experiment. Pick out just two ions for one analyte in the group (limit the group to 2 ions) and run the analysis again. This will probably give you excellent sensitivity.

Best regards.

[tangaloomaflyer]

I had a similar problem, my peaks in SIM mode were much broader than the peaks in scan mode. The broad peaks were (I think) caused by the column not being seated in the ion source correctly, and the peaks were broader in SIM due to the increased dwell (I think!).

[Peter Apps]

Another possibility is the width of the window around the target mass for each ion: too narrow and you lose detectability and get oddly spikey peaks.

Peter

[jh1]

Hi there,

dwell time is 100 and typically I have 8-12 ions per group.

I'm not sure how to do a correction. The manual is very limited when it comes to explaining SIM.

Thanks.

I'd recommend shortening the dwell time to 30 for every ion in each group, see how that looks then go from there.

What instrument and software are you using? For Aligent instruments Dwell time must be edited either manually for each ion in the instrument control or by group in data analysis.