Why is mass accuracy of mass spectrometers typically measured in ppm?

[Bio X2Y]

Hi,

I have recently posted this question to the BioStar forum (http://biostar.stackexchange.com/questions/8476/why-is-mass-accuracy-of-mass-spectrometers-typically-expressed-in-ppm), but perhaps this group is more suitable.

My first thought is that since mass (or rather m/z) is the thing being measured, accuracy should be measured in absolute units of m/z, i.e. thompsons (Th).

However, in practice, the relative unit ppm seems to be used instead. I find this confusing, since ppm will mean different things at different m/z values.

e.g. (taken from http://courses.chem.indiana.edu/c613/documents/AccurateMassSpectrometryLablecture.ppt)

• 5 ppm @ m/z 300 = ±0.0015 Th
• 5 ppm @ m/z 3000 = ±0.015 Th

When describing the latest-and-greatest new machines, the literature seems to stick with ppm, e.g. "Parts per million mass accuracy on an Orbitrap mass spectometer via lock mass injection in a C-trap".

According to Gross in Mass spectrometry: a textbook: "As mass spectrometers tend to have similar absolute mass accuracies over a comparatively wide range, absolute mass accuracy represents a more meaningful way of stating mass accuracies than the more trendy use of ppm."

So, can someone perhaps shed light on why ppm seems to be preferred?

Even statistical treatments tend to use ppm where I might naively expect to see Th, e.g. Fig. 1 from the Mann lab paper [http://www.ncbi.nlm.nih.gov/pubmed/17164402] graphs the distribution of mass deviations in terms of ppm.

Thanks for your time.

[Matthew Chambers]

For some mass analyzers (at least FT and TOF), resolution decreases with increasing mass. That's not
really the same thing as mass accuracy, but for low intensity peaks (which are often of interest)
will result in less accuracy. It's an easy phenomenon to observe: just look at peak from a profile
FT/TOF spectrum at 200m/z vs. 2000 m/z. Or if you're looking at proteins, at 20,000m/z you can
really tell!

-Matt

On 5/23/2011 9:19 AM, Bio X2Y wrote:
> Hi,
>
> I have recently posted this question to the BioStar forum
> (http://biostar.stackexchange.com/questions/8476/why-is-mass-accuracy-of-mass-spectrometers-typically-expressed-in-ppm),
> but perhaps this group is more suitable.
>

> My first thought is that since mass (or rather /m/z/) is the thing being measured, accuracy should
> be measured in absolute units of /m/z/, i.e. thompsons (/Th/).
>
> However, in practice, the relative unit /ppm/ seems to be used instead. I find this confusing, since
> /ppm/ will mean different things at different /m/z/ values.

>
> e.g. (taken from http://courses.chem.indiana.edu/c613/documents/AccurateMassSpectrometryLablecture.ppt)
>

> * 5 ppm @ m/z 300 = �0.0015 Th
> * 5 ppm @ m/z 3000 = �0.015 Th
>
> When describing the latest-and-greatest new machines, the literature seems to stick with /ppm/, e.g.

> "Parts per million mass accuracy on an Orbitrap mass spectometer via lock mass injection in a C-trap".
>

> According to Gross in /Mass spectrometry: a textbook/: "As mass spectrometers tend to have similar

> absolute mass accuracies over a comparatively wide range, absolute mass accuracy represents a more
> meaningful way of stating mass accuracies than the more trendy use of ppm."
>

> So, can someone perhaps shed light on why /ppm/ seems to be preferred?
>
> Even statistical treatments tend to use /ppm/ where I might naively expect to see /Th/, e.g. Fig. 1

> from the Mann lab paper [http://www.ncbi.nlm.nih.gov/pubmed/17164402] graphs the distribution of

> mass deviations in terms of /ppm/.
>
> Thanks for your time.