Why I object to the term “ultrahigh resolution mass spectrometry”.

I use our QExactive for untargeted metabolomics in the range m/z 80~1500. Typically with resolution set to the minimum value of 17,500, because the ability to scan quickly is more important than the ability to resolve m/z. The resolution on the Agilent QTOF in the Cancer Research lab is comparable, at ~20,000. I only mention this because it is another “high resolution MS” platform, as I understand the term. I consider these instruments worthy of the “high resolution” qualifier in HRMS because the other instruments I use are all “unit” mass spectrometers that differ in resolution to the QExactive and QTOF, as typically used, by factors in the tens of thousands.

However, as I’m sure you know, the QExactive can go up to resolutions of 140,000. Well, ours can. Newer models may be even more awesome. But the scan speed scales proportionally to the resolution. Consequently, I can’t use that high resolution when I analyse complex biological samples due to the large number of features in the data, the width of the peaks, the need to maintain throughput, & the need to collect multiple MS/MS scans in between full scans.

I recently tweeted a question asking whether the term “ultrahigh resolution mass spectrometers” was useful, when applied to the QExactive, in reference to a paper in ES&T: An Automated Methodology for Non-targeted Compositional Analysis of Small Molecules in High Complexity Environmental Matrices Using Coupled Ultra Performance Liquid Chromatography Orbitrap Mass Spectrometry. I can’t find the resolution used in the LC-MS method in the paper. However, if I assume it was the maximum resolution available on our own instrument that is still only 8 times higher than the value I routinely use and refer to as “high resolution” mass spec.

The seven Tesla Bruker Solarix at the UoA School of Biological Sciences apparently has a maximum resolution of 10 million. That is 71 times higher than the maximum on my QExactive and 571 times higher than the resolution I typically use it at. No one here typically refers to that as “ultrahigh” resolution mass spectrometry. It’s just mass spec. I’m not even sure what you’d use that resolution for, or whether it’s achievable in routine analysis. I think it’s something that protein chemists use to resolve things like post-translational modifications.

The division of analytical techniques into ‘good’, ‘better’ and ‘best’ evolves every day. As an example, a few years ago I was lucky enough to attend a seminar by Peter Derrick where he presented the results of the multi-turn TOF mass spec he had been working on for several years. When the instrument was first reported in 2005 the authors reported resolutions of 350,000 and termed this “very high resolution mass spec”. That resolution is now achieved every day by commercial Orbitrap instruments.

The prefix ‘ultra-‘ indicates extreme or transcendental status of the suffix. If the division between unit resolution and HRMS is in the tens of thousands, I do not think an increase of eight times above that worthy of that prefix. In fact, I reject the need to brand every modest increment in technical capability with an inappropriate adjective. Science evolves and technology that was once cutting edge becomes pedestrian and then obsolete. Science is also beset with jargon. The continuous addition of superlatives to describe the incremental progress in technology is unhelpful to students, who get lost in the (often abused) terminology. It is an impediment to efficient communication of methods between scientists and even more so to the lay public.

Don’t get me started on LC/HPLC/UPLC/UHPLC.


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