How mass analyzer resolution changes across mass ranges - and how to monitor it!

Due to the dispute between Guugle and Virginia Tech that resulted in - finally, I guess - the loss of my email, I also lost a lot of files linked on this blog that were still in that Googl drive. 

I'm hoping to find them at some point and put them back up. If you do see a link that is down you could try leaving a comments (it's hard to get through spam) or email me at LCMS methods@gmail.com (remove the space).

In basically all mass analyzers, we set the mass resolution at a particular point in the m/z range. The easiest example is always this one (you can click to expand) 

Note that at 558.3010 the resolution is nearly 50,000 arbitrary units (not arbitrary) and at 1273.0643 we've decreased to 31,700 resolution. This is an Orbitrap analyzer and this is how it goes. I've got boring slides on why (which...probably....have a dead link here as well....on the list now)

A funny thing happened around 2013 when every new Orbitrap magically had a lot higher resolution when marketing people thought "we should start reporting mass resolution at 200 m/z instead of 400 m/z!" Because you get more resolution as you keep going. On a GC-Orbi that can scan down to like 30 m/z you can get close to 1 million resolution! So the scan above was probably 60,000 or 70,000 resolution in the method (at m/z of 200) and this is image above is what you get at higher m/z.

What's less predictable are today's Time of Flight (TOF) analyzers! And this is funny because if they were purely just TOFs they'd be exactly the same resolution all the time. Ions go from point A to point B and that resolution is derived solely from the distance between point A and point B. This is, at static temperature, the same length.

However, in all of today's instruments they're not just TOFs. They're all basically hybrids in one way or another. They've got quads at the front of them and/or quads and traps and quads and traps and ion mobility devices and more quads and traps. Throw in a mirror or 7 (so your TOF tube isn't 25 feet tall....) and you've got a lot to consider here. 

The net result is that you've probably got to get those ions and accelerate them into the final TOF mass analyzer. And sometimes you've got to accumulate them and accelerate them. If all the ions were starting at the same spot at the same time - yes - you get the same resolution across the whole mass range. But if you've got a time of flight effect before you start the actual TOF analysis (through your quads and traps and IMS, etc.,) then you may need to make some compromises and every hardware configuration and vendor is a little different.

Let's take the one I know best right now, which is the TIMSTOFs. My first TIMSTOF had a reported resolution of 60,000. Which would be great across the whole mass range. 

But you don't get that. This image is stuffed into the supplemental of this paper somewhere. 

This is the very best separation I could get on a TIMSTOF Flex in 2020. This involved manually tuning the mirrors and tuning in the now-retired(?) OTOF control software. The trick is to manually override the mass that you're tuning on, but if you go too far one direction you can totally f' up the other side. I think this was tuning on 622 instead of the default (at the time) of 1222. So....you can ...just get that TOF resolution that you need to do TMT quan. 

DISCLAIMER - If your TIMSTOF is in a basement with water and HVAC issues, you won't maintain this resolution for very long at all. Not even 2 days. And that's the very very best separation I ever got. Realistically I could not resolve these peaks at 50% base height on normal experiments. 

The TIMSTOFs have gotten higher resolution. They'll get 80,000 at 1222 during install. I've taken some looks and - nope - I still can't reliably separate the 0.006 Da of the TMT reporter ions.

Okay - so that was a lot of words for - Orbitraps decrease as mass increases, TOFs have an ideal spot and you can tune them in one way or another to optimize that spot, but there are consequences. The best I can tell, they don't seem to change in a quadrupole - and I forget what it's like in an ion trap.

Time for a blast from the past blog post! Check this out! What if you could easily - intra-experiment - regulate your mass resolution on any instrument to see if it is what you need when you need it? This paper details simple reagents that could be used as pre-run or even intra-run controls!