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. -- You received this message because you are subscribed to the Google Groups "spctools-discuss" group. To post to this group, send email to spctools-discuss@googlegroups.com. To unsubscribe from this group, send email to spctools-discuss+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/spctools-discuss?hl=en.
