Saturday, February 17, 2018

Spectral accuracy of an Orbitrap using Isotope Ratios

There has been this myth out there for years that Orbitraps aren't good at isotope ratio analysis.

Okay, it might not actually be a myth. The original Orbitrap and maybe the Orbitrap XL didn't perform very well against a time of flight (TOF) instrument in a study operated by a TOF manufacturer.

Fast-forward 11 years or something and take a look at this newish study. 

Head to head comparison -- a Q Exactive Plus operating at 140,000 resolution (doesn't appear to utilize the enhanced resolution upgrade) versus an honest to goodness isotope ratio mass spec.

How's it do?

Really well. Honestly, better than I expected despite my borderline obsession with these instruments, with an important caveat or two:

There is a dynamic range where the isotope ratios are spot on. It looks to me from the charts in the paper that if you are above 1e5 counts you're in the clear. Drop below that line and things get wobbly.  Stay above it? And you can just look at the isotopic distribution and count the number of carbons, nitrogens and sulfurs in your molecule without any additional information.

Caveat 2 (I'm adding this one) the mass cutoff of the instrument. Resolution decreases as m/z increases in the Orbitrap so in the low range everything is just incredible, but then you hit the low mass cutoff at 50 m/z....we've got some molecules in the 40 m/z range and it is soul crushing to have to run those compounds on the TSQs...

Is a Q Exactive going to beat a dedicated IRMS instrument? No way. Could you use a QE as a high throughput screening device to determine if compounds needed to be sent off for isotopic determination on an IRMS? Absolutely! As long as it's >50m/z (and in this paper, they never go over 1,400 m/ long as:  50 m/z < your compound < 1400 m/z

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