# Resolution in mass

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.

• 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. Jan 3, 2023 at 11:07
• My bad, I meant mass resolution.
– user130197
Jan 3, 2023 at 11:20
• 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. Jan 3, 2023 at 11:55
• related/helpful: Resolution in Mass Spectrometry
– uhoh
Jan 4, 2023 at 2:28

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.

• +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. Jan 3, 2023 at 14:46
• @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 Jan 3, 2023 at 21:12