Chromatography Forum

Dear all,

I've been using LC-MS for some time now, but unfortunately, I have no mentor to ask all the questions in mind so I will really appreciate your expertise.
My main concern is the hardware and keeping it clean and undamaged. There have been a few gas shortages and I would love to know the best practices for a situation of this kind.

The instrument in question is QTRAP. I've been told that if there's no gas flow I should monitor the system vacuum and if it falls below a certain level there's a risk of instrument contamination.

Why would the system vacuum keep increasing over time if there is no gas flow? The normal operating pressure of the instrument is 2x10^-5 Torr (system running) / 0.6x10^-5 Torr (system idle). I assume the decrease in vacuum when running is caused by the increased gas flow (compared to idle)?

Now, the normal (idle) pressure being 0,6x10^-5 Torr, what would be a harmfully low pressure for this system that could cause contamination?
How long, approximately, is it safe to have the instrument with full vacuum in case of gas shortage? The LC would be detached at this point, of course.

Thank you for reading and I hope you’ll find the time to share your knowledge.
Stay healthy, everyone!

In standby there is a sweep of gas between the inner and outer cones that keeps particulates and other air born contaminates out of the MS. I would suspect that if the vacuum begins to be better(lower ultimate pressure) then it would mean that the inner orifice is becoming clogged, which also happens if the samples are really dirty. I don't know that it would mean that the internals are becoming really dirty though, unless there is a lot of contaminates in the lab air.

Possibly your instrument has a gate between the high vacuum section and the splitter cones.

My PE Elan 6000 is an ICP-MS but it has analogous behaviour. When at idle under vacuum, there is a gate that actually shuts off access to the MS high vacuum section and vacuum drops to 0.9E-6. When operating, the plasma the gate opens and then the final vacuum rises to 2.5E-5.

Sagwa wrote:
Dear all,

I've been using LC-MS for some time now, but unfortunately, I have no mentor to ask all the questions in mind so I will really appreciate your expertise.
My main concern is the hardware and keeping it clean and undamaged. There have been a few gas shortages and I would love to know the best practices for a situation of this kind.

The instrument in question is QTRAP. I've been told that if there's no gas flow I should monitor the system vacuum and if it falls below a certain level there's a risk of instrument contamination.

Why would the system vacuum keep increasing over time if there is no gas flow? The normal operating pressure of the instrument is 2x10^-5 Torr (system running) / 0.6x10^-5 Torr (system idle). I assume the decrease in vacuum when running is caused by the increased gas flow (compared to idle)?

Now, the normal (idle) pressure being 0,6x10^-5 Torr, what would be a harmfully low pressure for this system that could cause contamination?
How long, approximately, is it safe to have the instrument with full vacuum in case of gas shortage? The LC would be detached at this point, of course.

Thank you for reading and I hope you’ll find the time to share your knowledge.
Stay healthy, everyone!

You have to make sure your curtain gas is run as high as possible without degrading the signal or overwhelm the pumping down function. Also if the pressure seems better, a user here suggested that the curtain plate or orifice is becoming clogged. The spray location is important as well as to not prematurely degrade the orifice.

The system goes into idle mode when you don't run anything.