Two common categories of mass spectrometry are high resolution mass spectrometry (HRMS) or exact mass mass spectrometry and low resolution mass spectrometry (LRMS). Not all mass spectrometers simply mass-to-charge ratios (m/ze) as whole numbers. High resolution mass spectrometers can measure mass so accurately that they can detect the minute differences in (m/ze) between two ions that, on a regular low-resolution instrument, would appear to be identical.
The reason is because each ion has a (m/ze) that is based on the atoms in the ion. In your first year chemistry course you learned that the masses of atoms of the same element can differ because not all atoms of the same element have the same number of neutrons. Hence, the masses that appear in the periodic table are not useful to calculate the exact (m/ze) of an ion because they are average mass for a large number of atoms of an element taking inot account the relative natural abundances of each isotope.
- An atom of 12C has a mass of 12.00000 amu while an atom of 13C has a mass of 13.00335 amu
- An atom of 16O weighs 15.99491 amu while an atom of 17O has a mass of 116.99913 amu
- An atom of 14N weighs 14.00307 amu while an atom of 15N has a mass of 15.00011 amu
- An atom of 1H weighs 1.00782 amu while an atom of 2H has a mass of 2.01410 amu
As a result, on a high resolution mass spectrometer, 2-octanone, C8H16O with atoms of only the most abundant isotopes, has a molecular weight of 128.12018 instead of 128. Naphthalene, C10H8, again with only atoms of only the most abundant isotopes has a molecular weight of 128.06264. Thus a high resolution mass spectrometer can not only distinguish between these two ions it is also able to supply an the molecular formula for the ion because of the unique combination of masses that result.
In reality the resolution required to distinguish the molecular ions for 2-octanone and naphthalene is only 128 / (128.12018-128.06264) ~ 2200, well within the resolution of most mass analyzers. However, being able to discern the formula of the ion is only possible with an accurate mass analyzer capable of knowing the (m/ze) to roughly 1 part in 150,000.
- In LRMS, the molecular weight is determined to the nearest amu. The type of instrument used here is more common because it is less expensive and easier to maintain.
- In HRMS, the molecular weight in amu is determined to several decimal places. That precision allows the molecular formula to be narrowed down to only a few possibilities.
HRMS relies on the fact that the mass of an individual atom does not correspond to an integral number of atomic mass units.
Calculate the high-resolution molecular weights for the following formula.
- C12H20O and C11H16O2
- C6H13N and C5H11N2