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I'm pretty confused about mass resolution. The resolution of my instrument is 1000 and I have two masses at 85.0639 and 85.0651. So, the difference is 0.0012. I know that (M1-M2)=M/R. But, I'm not sure if my spectrometer will be able to separate these ions.

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    $\begingroup$ It is not very clear what those "1000" means. As resolution is usually meant the smallest difference distinguished by device reading. E.g. the classical, old school 2-arm mechanical scales had resolution 0.0001 g. $\endgroup$
    – Poutnik
    Jan 3, 2023 at 11:07
  • $\begingroup$ My bad, I meant mass resolution. $\endgroup$
    – user130197
    Jan 3, 2023 at 11:20
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    $\begingroup$ From your Q it seems 1/1000 is the relative resolution of the device, so for relative mass 85, resolution is 0.085 > 0.0012 so it seems you are out of luck. // If it is wrong, that it is probably your fault due bad description of scenario. $\endgroup$
    – Poutnik
    Jan 3, 2023 at 11:55
  • $\begingroup$ related/helpful: Resolution in Mass Spectrometry $\endgroup$
    – uhoh
    Jan 4, 2023 at 2:28

1 Answer 1

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The mass resolution $R$ of a mass spectrometer is defined as

$$R = \frac{m}{\Delta{}m} $$

with $\Delta{}m$ the mass difference between two masses still be resolved, $\Delta{}m = m_1 - m_2$. Thus, if your spectrometer specs detail out $R = 1000$, then you record masses of $m_1 = 100.0$ and $m_2 = 100.1$ as two separate signals.*

In your example, $m_1 = 85.0639$ and $m_2 = 85.0651$, or a $|\Delta{}m| = 0.0012$. So you would need a resolution of

$$ R = \frac{85.0639}{0.0012} \approx 70887$$

to record these masses as separate signals. This number leans more toward a high-res spectrometer (e.g., to substitute a combustion analysis), than the simpler bench-tops routinely hyphenated at the exit of a HPLC.

* This omits how you define a peak in mass spectrometry. For simplification, this back on the envelope computation assumes zero-width of the signals and no partial overlap.

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    $\begingroup$ +1, There is a non-ending argument about the terms spectroscopy vs. spectrometry :-) Your mass spectroscopy reminds me of my undergraduate analytical chemistry teacher held me outside the class and said that I was the only person in class who wrote mass spectroscopy in the exam. Why did you write that? It is mass spectrometry! I still see very established chemists arguing about it. $\endgroup$
    – AChem
    Jan 3, 2023 at 14:46
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    $\begingroup$ @AChem The inconsistency (twice spectroscopy, twice spectrometry) is fixed. With "The term mass spectroscope continued to be used even though the direct illumination of a phosphor screen was replaced by indirect measurements with an oscilloscope." from wikipedia, I agree, since its recordings no longer rely on emitted/captured electromagnetic radiation, MS is better off as spectrometry. Google Ngram $\endgroup$
    – Buttonwood
    Jan 3, 2023 at 21:12

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