Response of a low mass vs a high mass molecule.

[thohry]

In GC/MS, I wonder if a high mass molecule will give higher response than a low mass does. Anyone know this?

Thanks for any ideas.

[Don_Hilton]

Note my response to simonwei in the LC-MS GC-MS and other topics

[chhubert]

You may try tuning your instrument such that the abundance of high mass tune ion (e.g. m/z 512 relative to 69) increases. That may somewhat improve a bit the instrument sensitivity when you monitor the analytes which generate m/z of this range.

[Don_Hilton]

If you are trying to get better sensitivity for higher mass ions, changing tune, as mentioned by chhubert is particularly helpful for quadropole instruments. And that answer prompted me to carry that suggestion a bit further:

If you are using a quadropole or a low resolution TOF (integer mass resolution) addjustment of mass acquisition window may help. For low mass ions, thermal broadening of the mass and issues related to mass defect can be ignored. But, as you get to higher masses, the thermal broadening of the masses can cause a portion of the mass for one ion to fall into the acquisition window for a neighboring mass. As you get into this range, mass defect becomes a factor for sensitivity as well. Because masses of nuclei are not integer values - and differ based on which atom the exact mass of a molecule of one formula will differ from the exact mass of a molecule of a different formula -- which we use in high resolution mass spectrometry to obtain the molecular formula for an ion. In low resolution mass spectrometry, this moves the apex of the mass peak within the acquistion window - and can force the tail of the distribution out one side or the other of the acquisition window - with resulting loss in sensitivity. When the mass is large enough in a low resolution mass spectrometer, the tails fo the energy distribution of the ion fall out both sides of the acquisition window and may even be counted in the abundance of the neighboring masses.

To inscrease sensitivity for unit mass resolution instruments for high mass ions, you can adjust the mass acquisition window to account for the mass defect - and depending on possibility of interference from signals at the neighboring masses, you may be able to widen the window to catch both sides of the energy distribution of the mass peak.

All of this depends on the particular instrument you are using - and the control you have over acquisition parameters.

[lmh]

can I just have a rant about "mass defects"? (no offence to Don_Hilton who is accurate in all he writes)

"Mass Defect" strikes me as a term that should be relegated to history.

There are so many chemical drawing packages around now that give the exact mass of the main isotopologue of any compound that there is absolutely no excuse for not knowing the mass of the thing you are analysing. Mass spectrometers measure mass. In SIM and SRM methods, we have to type the mass of the analyte into the software. It should be very obvious that we should type in the correct mass! There shouldn't be a need for a theoretical discussion to justify typing in the correct mass!

[thohry]

Thank you all.
Don, could you please explain a bit more about thermal broadening effect?

[Don_Hilton]

Ah! P-chem.

Molecules entering an ion do not all have the same energy as they leave the end of the GC column. Because they are molecues in a gas, they have varying motion within the gas: some towards the analyzer section of the mass spectrometer and some towards the repeller plate, some move more quickly across the ionization volume, others move more slowly. (I'll keep it short: details are in the discussion of the Maxwell-Boltzman distribution in your P-chem text.)

And, we use electrons that have come from a heated filament to ionize the molecules in the ion source. These electrons have a distribution of energies as well. As the electrons interact with molecules to expel electons energy is transfered from the electron to the molecule as well - and it is a distribution. (A similar effect works in negative ionization as well.)

Ions of any given mass enter the analyzer section of the mass spectrometer, not with the energy computed between the repeller plate of the ion source and detector, but rather with a distribution of energies around this energy because of these thermal sources. And, as the mechanisim of separation of ions will be affected by the energy of the ions (or velocity - but E = 1/2 * m * V^2), the measured masses will vary according to the energy component added (in a positive or negative direction) by these thermal sources.