Chromatography Forum

Hi,

In our toxicological laboratory, we have a GC-MS 5973N. Unfortunately, SIM mode's sensitivity is worse than scan mode's. We don't know where or which the problem is because the autotune is OK.
Anybody can help us?
Thanks!

Post your SIM method parameters (especially dwell time, number of ions).

Hello

SIM mode is more sensitive than Scan...it is all about parameters and calibration.
Have you done dynamic calibration for SIM? Have you got correct ranges for ions?
Without more information we can not help you...it is like swimming in the ocean.

Regards

Tomasz Kubowicz

Thanks for the prompt reply, ions are correct.
We use different methods in SIM (eg tramadol, propoxyphene, fentanyl, LSD, etc), processing the same vial in SCAN mode and SIM mode, abundance or area of the target ion in SIM mode is similar or even lower compared to the same in SCAN mode.
In addition, we use these methods frecuently without problems but now we do not get responses to low concentrations.
So I turned to you for help because if it could be a fail of quadrupole or the ion source's temperature (the temperature ranges between 215 -230 ºC)
Thank you very much for your help
Mayu

You have something set up incorrectly in your SIM experiment. SIM on Agilent systems is anywhere from 10-50 times more sensitive (S/N) than scan mode. Note that on an Agilent system both modes are digital; there really is no such thing as "scan". It should be called "short residence time wide range SIM"!

Make sure your multiplier setting are correct. Make sure you're using the correct tune file. Start with a 10 ms dwell time per ion since there really is very rarely any reason to use a longer dwell (and you can miss thing with too long of a dwell time). Make sure your time windows for the ions are set correctly - it's easy to get your windows off and not know it since you're not in scan mode and can't "see" the big peak you missed.

The scales are different, so don't try to directly compare areas. You need to look at the S/N to see the difference.

hmm I was under the impression the dwell time was a compromise between sensitivity (best at highest dwell) and definition (having enough data points to define the peak) best with lower dwell. That is why you want to have as few ions as possible with as high a dwell as possible whereas it is necessary to reduce the dwell when you have more ions in the window.

Sometimes when you tune the instrument, certain analytes will lose sensitivity despite the majority of all the otherones gaining intensity. If these happens you need to then have the collision energy and q1 and q3 voltages re-optimized for those specific analytes that suffered.

also check to make sure that nobody switched the resoution for those SIM transitions to high resolution, this will result in lower intensity.