Differences post preventative maintenance.

[Andy F]

Hi All,
We've just had a PM done on a Quantum Access triple quad and something curious has happened. We are analysing Fumonisins in cereals and beer using a Poroshell column and H20/ACN + acetic acid mobile phase.
We monitor 2 transitions for B1 722-528 and 722-352 and 3 transitions for B2 706-512, 706-354, 706-336. Before the PM and as long as I can remember we have quantified using the 722-352 and 706-336 as these gave the biggest responses. After the PM these have flipped with 722-528 and 706-512 giving the largest peak areas. Obviously the PM has changed something, but is it something to be worried about?

Thanks in advance
Andy

[James_Ball]

What was the normal difference in the two peaks for each analyte? Was it 100%/80% or more like 100%/30%? If the first then probably not something to worry about if the second then maybe something has changed a lot.

When a PM is done they should be doing a new tune and mass alignment. If the mass shifted a few hundredths of an AMU after the tune versus the old one then it could be you are no longer hitting the highest sensitivity with your current method settings. It will depend on what resolution you are using for the quads, if you are filtering in the 0.01 to 0.05 AMU range then a shift can cause problems, if you are filtering in the 0.1-0.5 AMU range then maybe not so much.

[Andy F]

Thanks James.
Just looking at the B1 the peak area 722-352 has dropped from 536869 to 77665, but the peak area for 722-528 has gone from 139998 to 293910.
The width is 0.2 amu. We still have enough sensitivity for the method not to create any reporting issues, but other methods the instrument is used for may be a different story.

[James_Ball]

Thanks James.
Just looking at the B1 the peak area 722-352 has dropped from 536869 to 77665, but the peak area for 722-528 has gone from 139998 to 293910.
The width is 0.2 amu. We still have enough sensitivity for the method not to create any reporting issues, but other methods the instrument is used for may be a different story.

You can do a Precursor scan and a Product scan and look at where the mass maximizes and see if the mass axis has shifted slightly. If it has you can tweak the masses you are looking for or widen the amu window to compensate if there has been drift.

Doing Anatoxin samples on a Shimadzu I was seeing a loss in sensitivity and it turned out the mass axis was shifting slightly because of room temperature changes. Once I shifted the mass I was targeting the sensitivity came back.

[lmh]

I really don't know why triple quad manufacturers try to implement resolutions like 0.2amu. It's pointless. The extra selectivity of 0.2 compared to 0.5 or even 0.7 is utterly irrelevant; almost all nearly-isobaric ions will be much closer, probably closer than 0.1amu, so unless you're considering a ToF or orbitrap, they're not going to be resolved. And it just makes it more likely that calibration will become an issue, which it normally shouldn't be on a quadrupole. All I can think is that it's become a distinguishing feature on spec-sheets, and no one wants to say "ours only has 0.5amu resolution".
But as well as the mass accuracy issue, there is the question of tuning optimization on the quadrupoles, and whether they're now favouring high mass over low mass. Again, this can be linked to resolution as resolution is a trade-off with sensitivity, and if it's been optimized a bit too narrow at low mass, that will also cause problems.

[James_Ball]

I really don't know why triple quad manufacturers try to implement resolutions like 0.2amu. It's pointless. The extra selectivity of 0.2 compared to 0.5 or even 0.7 is utterly irrelevant; almost all nearly-isobaric ions will be much closer, probably closer than 0.1amu, so unless you're considering a ToF or orbitrap, they're not going to be resolved. And it just makes it more likely that calibration will become an issue, which it normally shouldn't be on a quadrupole. All I can think is that it's become a distinguishing feature on spec-sheets, and no one wants to say "ours only has 0.5amu resolution".
But as well as the mass accuracy issue, there is the question of tuning optimization on the quadrupoles, and whether they're now favouring high mass over low mass. Again, this can be linked to resolution as resolution is a trade-off with sensitivity, and if it's been optimized a bit too narrow at low mass, that will also cause problems.

This is what threw me off when switching the the Shimadzu. I normally use Unit Resolution, which on the ABSciex was 1amu roughly, but seems Unit Resolution on the Shimadzu is closer to 0.1-0.2amu, and there is still a High Resolution mode.

[Gaetan Glauser]

Hi Andy,

I would be interested to know the cause once you have sorted the issue. It apparently has to do with the second quadrupole (or the collision cell: have you checked if the collision gas flow is ok?), since one transition shows reduced signal and the other increased signal.

Best regards