Chromatography Forum

Theoretically how is TOF resolving power supposed to change with m/z? Based on observed values for calibrant ions (covering 100-3000 m/z), resolving power appears to increase in something like a logarithmic or power fashion. Also wondering why it seems to plateau out at the high mass end (1000-2000)? This question is asked in the context of small molecules - in case matters.

Theoretically how is TOF resolving power supposed to change with m/z? Based on observed values for calibrant ions (covering 100-3000 m/z), resolving power appears to increase in something like a logarithmic or power fashion. Also wondering why it seems to plateau out at the high mass end (1000-2000)? This question is asked in the context of small molecules - in case matters.

In mass spectrometry resolving power (R) is M / Δm, where M is the m/z value of the ion used in the calculation, i.e., m/z 250, and Δm is the difference in the m/z values of two mass spectral peaks of equal intensity that can be separated so the overlay of the base of the two peaks is 10% of the height (10% valley method). Having such a situation is not always possible and an alternative for delta m is the width of the mass spectral peak at 1/2 of its height (FWHM method). The FWNM method of calculating R results in a value of R that is 2x greater than the value obtained with the 10% valley method. Δm is the resolution which is the difference in the m/z values of two mass spectral peaks that can be separated and is usually reported for a given value of R. Many people confuse resolution and resolving power.

A linear TOF mass spectrometer has a constant resolving power throughout the m/z scale. As M gets smaller the difference in the m/z values of ions that can be separated gets smaller. The ability to separate ions of small differences in m/z values is better at the low end of the scale than it is at the high end. Most reflectron TOF instruments operate at a constant resolution (Δm), which means that the resolving power (R) becomes a larger number as you go up the m/z scale.

Are you asking about a linear or reflectron instrument? The size of the ion has no relevance; therefore, small or large molecules are not important.

I am referring to a reflectron TOF. Below is a printout of a cal report for an Agilent oa-TOF. The FWHM (R) increases with m/z, which suggests that resolving power is not constant with m/z.

Your are correct and I am wrong with regard to reflectrons. The resolution (Δm, which is the FWHM column in your posted report) does degrade with incresaing m/z value. What is not constant is the resolving power (R = M/Δm; the Res column in your posted report). If you look carefully you will see that the value in the Res column is obtained by taking the corresponding value in the Actual m/z value column and dividing by the value in the corresponding value in the FWHM column.

In a linear TOF mass spectrometer, I believe the value in the Res would remain constant.

The equation for the tof of an ion in a linear TOF mass spectrometer is:



I hope this is helpful.

Just wondering why dividing the m/z value by the FWHM value doesn't give the Res value exactly as shown? Also, if reflectron TOF's don't shown constant resolving power, is it safe to assume that an orbitrap's resolving power does decrease with 1/sqrt(m/z) as often quoted in articles? Would you agree that for molecules greater than say 900, the TOF is superior in terms of resolving power especially if fast LC is used (requiring fast MS acquisition rates)?

ah, suddenly the thread is orbi-versus-ToF.

It's a different question. Clearly if you want uplc peaks, an orbitrap is unlikely to be fast enough to give you enough measurements across a uplc-width peak (unless you sacrifice resolution by reducing the orbitrap's scan time drastically).

But be careful about global statements like "above mass X ToFs are better than orbitraps". Orbitraps and ToFs are both ongoing techologies; a bad old ToF is going to have a totally awful resolution compared to a modern good ToF, and who knows where orbitrap resolution will go in the future? It's much easier to compare individual instruments than to compare whole technologies.

I cannot answer the question as to why the value obtained from dividing M by Δm is not the same as reported in the Res column of the tune report. You will have to ask Agilent that question.

I agree with Imp, you have changed the thread to be Orbitrap vs TOF. I am not sure what the resolving power spec for the Agilent TOF is. I know that some TOF instruments are being sold with specified R values of 40,000. Bruker has an instrument that claims R = 40,000 and an acquisition rate of 20 spectra per second. The Orbitrap has a resolving power of 10,000 at an acquisition rate of 10 spectra per second and a resolving power of 100,000 at an acquisition rate of 1 spectrum per second. If you had a mass spectral peak that did not give you an unambiguous elemental composition using both of these instruments at the operational rate needed for a UPLC peak, which one would you want to have for the second run of the sample? Waters and ABI/Sciex also have Q-ToF instruments with a 40,000 resolving power; however, you have to remember that all of these resolving powers are at a specified m/z value.

I have a JEOL AccuTOF mass spectrometer. The resolving power determine using ions of PEG (R = M / Δm) at m/z 107 is 4655 (Δm = 0.024); m/z 195 is 5,273 (Δm = 0.037); m/z 327 is 6292 (Δm = 0.052); m/z 503 is 6,371 (Δm = 0.079); and, m/z 943 is 5,753 (Δm = 0.64). To me this is saying the resolving power is constant over this m/z range and the resolution degrades with increasing m/z values.