Chromatography Forum

Thank you all for the time to read my post.

Everytime we read anything about dwell time of a MS system (GC-MS, LC-MS/MS, and so on..), everyone say that we should have the highest dwell time possible for higher sensitivity. The limit being the points-per-peak, wich should be between 10-15.

But when we try to inject a standard with a higher dwell vs a lower dwell we rarelly see any diference other than less or more points-per-peak.

We had just injected a standard using 60 ms and 6 ms for dwell time and peak intensity is more or less the same.

Can someone tell me litle bit more about?

Thank you very much.

Thank you all for the time to read my post.

Everytime we read anything about dwell time of a MS system (GC-MS, LC-MS/MS, and so on..), everyone say that we should have the highest dwell time possible for higher sensitivity. The limit being the points-per-peak, wich should be between 10-15.

But when we try to inject a standard with a higher dwell vs a lower dwell we rarelly see any diference other than less or more points-per-peak.

We had just injected a standard using 60 ms and 6 ms for dwell time and peak intensity is more or less the same.

Can someone tell me litle bit more about?

Thank you very much.

Obviously, you are not operating at the edge of sensitivity. Try measuring lower amounts of analyte. I would assume you will find a difference between the different dwell times then.

It's not the intensity that increases by measuring longer it's the signal to noise ratio that improves.

Thank you both for your answers.

The amount injected gave us a S/N of 10, so i believe we are working with concentrations low enought.

When we increased the dwell time from 6 ms to 60 ms (10x more), the S/N went up from 10 to 20 (only 2x more).

And even this higher S/N was more to do with a more stable background (average noise) than intensity (from 2,0e3 to 2,3e3).

Is this what people talk about when they ask for higher dwell times, or should we expect more?

Have you ever tried it by yourselfs? If yes, what kind of diferences did you saw?

Thank you very much for your time.

Theoretically the S/n improves with the Square root of the extra time spend. so for yout 10 times longer scan time the s/n improvement could have been 3.16X. The factor of 2 improvement that you saw is probably withing the experimental error.

Iam not expert but when iam setting the dwell time for analytes/method iam looking for "goldean mean".
To high dwell time (for example >60) wont give you enough data points for low concetration peaks ... to low on other hand will raise your noise also...

Thank you all for your answers.

After some reading and some conversations with brand technitians I came to some conclusion that may be usufull for future people with the same doubts.

1) Best dwell time is higly equipment dependent (some new models able to give stable results with 1 ms for screeening aplication, others up to 100 ms).

2) May be diferent for diferent compound.

3) Is all about decreasing noise to improve S/N (sensitivity). There is no increase in signal intensity.

4) Every instrument has an inherent minimum dwell time for data collection below which precision rapidly degrades.
Above this value, S/N doesnt improve by much, you just lose points-per-peak.

Below is a site with several graphs about it that realy mayde me understand in 1 min what was all about:
http://www.basinc.com/library/presentat ... index.html

Hope someone find this post usefull.
Best regards to you all.

That's a great reference. Thank you for sharing.

What I have noticed in both LCMS and GCMS(seems more in GCMS) is that increasing the dwell time makes for more stability of both noise and analyte. If you are in a tuning window doing a repeat scan of a single mass watch what happens with changing dwell. It is not so much that the signal increases but that the average counts of that peak will have a lower RSD from scan to scan, which is what smooths out the baseline noise to improve the s/n in the analytical runs.

Older GC/MS systems like the 5970 always seemed more stable to me than the newer ones, and maybe it is their limited scan speeds (which equals longer dwell) is why. Newer systems are more sensitive and faster scanning but in my usage seem to be less stable than those made 20 years ago.

Sometimes the best solution is just to "sufficient-ize" rather than optimize.

If you set a particular dwell time and it works well for your application, that's great. Adjust if it doesn't.

I was inspired and ran yesterday some experiments to find out that 50 usec was better than 25, or 100.