What does Vacuum Gauge indicate for LC-MS/MS?

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

6 posts Page 1 of 1
Dear all,

I am developing a method for determine a group of pharmaceuticals using C18 column. But I just can't make it pass the 10-time repeat. The intensity of some of the compounds decreased dramatically (2 times to 100 times) or just split completely in a random way, without knowing which compounds might behave in this way in the following injection (both native compounds and internal standards), just what I said in my very first post.

It is not a problem of resolution setting, since I set it as "Unit". And I tuned it again by using the same organic solvent as my mobile phase (B) under the same flow rate. It turns out to be better for some compounds. But the problem still existed. One day, I just switched to a new liquid nitrogen as the original one was run out of. Then the vacuum gauge value went to 3.8 (normally it was 3.1). And I did a 5-time repeat whose repeatability was fine for all the compounds. Then The instrument was used by other staff with different methods and target analyses overnight. The next day, I thought yesterday's repeatability seemed good. I could adjust the gradient a little bit to separate the analytes more. But during the adjustment trials, the peak of some analytes either split completely or decreased dramatically in a random way (At this time, the vacuum gauge decreased to 3.4). And then I thought is it a problem with the gradient. So I did a 10-time repeat using yesterday's gradient, and the problem still existed, even though much better than what it appeared at the very beginning I posted my first post here. So I started to think would vacuum possibly be the cause? If so, how? And how could I adjust the value of the vacuum gauge?

Another cause I suspect is when I do the compound tuning (syringe), the TIC is not very stable, went up and down. (Some of them are relatively stable, some of them are not). Actually this is what I really suspect. Maybe I should do compound tuning again with those trouble compounds.

What's your opinions? I appreciate your wisdom a lot. Thanks for your attention.


Best regards,
Kenny
On my ABSciex API3200 I have seen the vacuum change like this and it is because the orifice becomes slightly clogged. The one behind the outer cone. If it clogs too much the peaks will become noisy and reproducibility becomes bad. If you have an ABSciex instrument try cleaning the orifice, of any other brand may have the same problem. A lower number on vacuum usually means that less gas is entering the vacuum chamber which means that less analyte is also reaching the detector.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
On my ABSciex API3200 I have seen the vacuum change like this and it is because the orifice becomes slightly clogged. The one behind the outer cone. If it clogs too much the peaks will become noisy and reproducibility becomes bad. If you have an ABSciex instrument try cleaning the orifice, of any other brand may have the same problem. A lower number on vacuum usually means that less gas is entering the vacuum chamber which means that less analyte is also reaching the detector.


Thanks so much for your reply. The one I am using is exactly ABSciex API3200. But why only one or two of the target analytes abnormally get decreased in intensity or completely split into 2 tiny peaks when the orifice becomes slightly clogged rather than all of the analytes.

And normally the vacuum value that my colleagues used to use is around 3.1. But when I found the reproducibility turned out to be find, the vacuum value is 3.8 which is abnormal for them actually. And is there any other methods or operations that I can do to reach 3.8 again?

Appreciate your attention a lot. Thanks.

Best regards,
Kenny
kennywurb wrote:
James_Ball wrote:
On my ABSciex API3200 I have seen the vacuum change like this and it is because the orifice becomes slightly clogged. The one behind the outer cone. If it clogs too much the peaks will become noisy and reproducibility becomes bad. If you have an ABSciex instrument try cleaning the orifice, of any other brand may have the same problem. A lower number on vacuum usually means that less gas is entering the vacuum chamber which means that less analyte is also reaching the detector.


Thanks so much for your reply. The one I am using is exactly ABSciex API3200. But why only one or two of the target analytes abnormally get decreased in intensity or completely split into 2 tiny peaks when the orifice becomes slightly clogged rather than all of the analytes.

And normally the vacuum value that my colleagues used to use is around 3.1. But when I found the reproducibility turned out to be find, the vacuum value is 3.8 which is abnormal for them actually. And is there any other methods or operations that I can do to reach 3.8 again?

Appreciate your attention a lot. Thanks.

Best regards,
Kenny


Changes in the Collision Gas should be the only adjustment that would cause vacuum pressure to change. Make sure someone else hasn't made adjustments to your method by mistake. Collision gas of 5 normally gives vacuum reading in the 3.1-3.3 range, try other settings. The strange thing is the settings are not increasing with increasing number, so sometimes 4 may have more flow than 5 then flow increases again at 6 ect. I think 6 or 7 might give the 3.8 you had before and it will change the sensitivity of the method also, but depending on how the analyte reacts to the gas it can be either higher or lower. Also each analyte can have a different sensitivity at different collision gas settings, so one setting may make one analyte higher while making another one lower in response. It is trial and error to see what setting maxes out each analyte. Usually I set it to something that gives the best average response between all the analytes instead of making several experiments to try and max out each one.
The past is there to guide us into the future, not to dwell in.
If you vacuum gets worse over your set of runs my guess would be you are not drying your spray well enough.. And when droplets enter the manifold they explode causing vacuum issues...

I don't know Sciex terminology but crank up drying gas temperature, drying gas flow and sheath gas or what ever it is called on a Sciex..
Kind regards
Leadazide
leadazide wrote:
If you vacuum gets worse over your set of runs my guess would be you are not drying your spray well enough.. And when droplets enter the manifold they explode causing vacuum issues...

I don't know Sciex terminology but crank up drying gas temperature, drying gas flow and sheath gas or what ever it is called on a Sciex..



On the Sciex those are GS1 and GS2, (source gas 1 and 2) Those should be in the 50-70 range if you are pumping over 500ul/minute column flow. Also you have the Curtain Gas, which flows between the inner and outer cones to prevent liquids from entering the interface, keep that at 20 if you can while doing analysis, it will make sure only ionized species are drawn into the interface and helps keep out neutrals and liquid droplets.
The past is there to guide us into the future, not to dwell in.
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