Dwell time for determination of pesticides by GC/MS

[astroguille]

Hi people!

I work on a GC/MSD (6890/5973) for determination of 36 pesticides (organochlorine, pyrethroid and organophosphorus) in fruits, vegetables, water and soil.

The run time is 62 minutes.

Which is the best dwell time for the best data adquisition?

Thanks

[Fat Jonny]

Depends.... how many masses are you monitoring? Are you running or including a scan? The higher your dwell the less number of data points per second you will get and the more jagged your peaks. The lower the more data points per second, but you will get higher noise level.

Why don't you test at different dwell times? For a GCMS method I would start at 50 or 100ms for only a couple of monitored masses.

[astroguille]

Hi

I am monitoring 64 ions. I work in SIM.

I notice that the dwell times are between 20 and 150.

I don't know why these values are so unlike

[Fat Jonny]

That's a lot of ions for one run..... Are you monitoring all 64 at the same time for the whole run? What do your dwell times for the 64 add up to? If they were all 20ms then you could expect one data point every 1.2 seconds, if they were all 150ms, then you would get one data point every 9.6 seconds!! Are your peak shapes ok?

If you have the capability you might want to monitor each mass for only a short time at the approximate retention of your analyte. e.g. monitor each mass for 30 seconds either side of the expected retention time of the analyte. This way at any one time your MS only have a few masses to monitor.

[AICMM]

Astrogulle,

63 ions in how many groups and how many ions in the biggest group? It's a trade off. The more ions you have in a group, the less sensitive all of the ions in the group are because the quad has to scramble from ion to ion to cover the range of ions you assigned. Especially if the mass range of the ions you selected is wide, ie the ions are vastly different molecular weights.

Better to have more groups (since groups are bundled according to time) of fewer ions if you can pull it off. No matter what, 64 ions is a lot of ions.... Perhaps two runs (a pain at 62 minutes) 1/2 the pesticides in one, 1/2 in the other....

Best regards.

[astroguille]

The ions are distributed in 36 groups (the 36 pesticides). The biggest group has 19 ions, and the smallest group has 4 ions.

[sassman]

I would adjust the dwell time to have approximately 10 scans across each peak. Fewer scans will result in poor peak shape and bad data. More scans are not as bad but will result in more noise and decreased sensitivity because your detector does not have enough time to collect a sufficient number of ions.

If your instument has the capability (I've never used an MSD), you can decrease the dwell time for ions which show higher sensitivity and increase the dwell time for ions which show poor sensitivity in order to make the sensitivities more equal. This might be why the times vary between 20 and 150.

[Schmitty]

We do 19 pesticides for an AOAC method in ~20 minutes. There are 3 ions per analyte, using 6 groups across the run. The dwells range from 25 to 75 ms.