Thermo LTQ Unstable Ion Beam?

[mburleson]

Hello,

I am attempting to tune a Thermo LTQ, but cannot get a stable signal for the calibrant solution - based on the spectrum view. The source seems to be okay, there's some spray that can be seen at higher flow rates, and the issue persists with our other source (from a fully operational TSQ). Is there anyone that can offer any insight? The image below shows that the signal is so unstable, the gain couldn't be calibrated.

[JMB]

Tuning solution,

1) Composition ??

2) Concentration ??

3) Mobile Phase ??

4) Flow Rate ??

5) Scan rate, m/z xxx - yyy, z secs??; +ve ion mode ??

6) What do you see, AND what do you NOT see ??

Regards,
JMB

[lmh]

There's a lot that could be causing trouble, but I'm not sure that this is anything to do with the calibrant or spray chamber at all. So far as I remember (it's a while since I've used a Thermo trap instrument), the electron multiplier gain calibration relies on simply watching electronic noise on the multiplier. It assumes single random events triggered by one electron, and looks at what size "pulse" of current you actually get, after the multiplier multiplies it. In effect, it's looking at noise in the absence of any chemical signal.

Since it's a prerequisite for anything else working at all, it might be that you're not seeing a stable beam because the final detection has gone wrong, not because there is no stable beam.

Since the calibrant is injected from a syringe at low flow-rate, I wouldn't expect to see anything visually in the spray chamber.

The options that occur to me are (1) your EM has failed; (2) there's something fundamentally wrong with the vacuum; (3) there's something nasty happening in the electronics. Is the high vacuum gauge reading something sensible? What was the last known EM voltage, was it huge (indicating imminent failure)?

I assume the instrument has been kept under vacuum at all times. If you attempt to calibrate these when they're not fully dry, they will choose a stupidly low voltage because they're actually seeing water inside their insides, so they don't need to amplify much to get the required "noise" (again, if I recall; I'm going back to the LTQ's predecessor for that).

[mburleson]

Tuning solution,

1) Composition ??

2) Concentration ??

3) Mobile Phase ??

4) Flow Rate ??

5) Scan rate, m/z xxx - yyy, z secs??; +ve ion mode ??

6) What do you see, AND what do you NOT see ??

Regards,
JMB

The tuning solution consists of 20 ppm caffeine and 0.001% Ultramark 1621 in an aqueous solution of acetonitrile (50%), methanol (25%) and acetic acid (1%). The calibrant normally has 1 ppm MRFA, but mine is homemade while we wait on the manufactured one.

The flow rate was high - 20 uL/min just to see anything. Scan rate was "normal" over a range of 150 - 2000 m/z, with a faster injection time of 10 ms, and positive ion mode.

The video below is what I see. No stable signal of the expected masses, but bouncing around.
https://drive.google.com/file/d/1wsMEuq ... sp=sharing

[mburleson]

There's a lot that could be causing trouble, but I'm not sure that this is anything to do with the calibrant or spray chamber at all. So far as I remember (it's a while since I've used a Thermo trap instrument), the electron multiplier gain calibration relies on simply watching electronic noise on the multiplier. It assumes single random events triggered by one electron, and looks at what size "pulse" of current you actually get, after the multiplier multiplies it. In effect, it's looking at noise in the absence of any chemical signal.

Since it's a prerequisite for anything else working at all, it might be that you're not seeing a stable beam because the final detection has gone wrong, not because there is no stable beam.

Since the calibrant is injected from a syringe at low flow-rate, I wouldn't expect to see anything visually in the spray chamber.

The options that occur to me are (1) your EM has failed; (2) there's something fundamentally wrong with the vacuum; (3) there's something nasty happening in the electronics. Is the high vacuum gauge reading something sensible? What was the last known EM voltage, was it huge (indicating imminent failure)?

I assume the instrument has been kept under vacuum at all times. If you attempt to calibrate these when they're not fully dry, they will choose a stupidly low voltage because they're actually seeing water inside their insides, so they don't need to amplify much to get the required "noise" (again, if I recall; I'm going back to the LTQ's predecessor for that).

Thank you for such a detailed response! So, if the electron multipler gain calibration relies on watching electronic noise, would it be worth zeroing it and trying again?

I'm honestly thinking it is something with the EM. It hovered around -1020 to -1053 for multipler 1, and -1043 to -1051 for multipler 2. Sadly, those are the only values I know because I'm the only one willing to both learn and work on it Would it be worth going closer to the extremes? I know they can go from -2500 to 0, but not sure if I need to go closer to 0 or farther away.
I currently see this:
https://drive.google.com/file/d/1wsMEuq ... sp=sharing

The vacuum is sensible and both gauges are showing green on the feedback (1.4 - 1.8 I believe). It sadly hasn't been stored under vacuum as I've worked on it. It has since this latest clearing in my schedule, but I had to replace the turbo controller because I'd leave it pumping down overnight but come in to it off the next morning.

[lmh]

How long was it not under vacuum? I don't know how well EM's survive a long period of exposure to air, and it will definitely need leaving pumped down for quite a few days before it will be properly dry inside. Caveat: I'm not a Thermo engineer, and you'd do much better if you could find one! My experience relates to the previous instrument, the DecaXP, which was a 3d trap with only one detector, not the linear with two.

[mburleson]

How long was it not under vacuum? I don't know how well EM's survive a long period of exposure to air, and it will definitely need leaving pumped down for quite a few days before it will be properly dry inside. Caveat: I'm not a Thermo engineer, and you'd do much better if you could find one! My experience relates to the previous instrument, the DecaXP, which was a 3d trap with only one detector, not the linear with two.

It was a good bit - weeks, sadly. I'll email around and see what I can find about an engineer. Thermo doesn't seem to like my institution due to its location and charge a $5k travel fee. Is there a way to assess the EM? I know how to on our TSQ, but not this one.