Ensuring constant ionisation of transition metals using plasma ionisation for accurate mass spec

Question

Mass spectrometry depends on separation of the analyte based on mass to charge (m/z). ICP-MS is used to detect metals, but under the plasma conditions many of these metals could be ionised multiple times, giving them a variety of (m/z) ratios making a difficult to identify peak for any particular metal. Additionally some metals would have m/z ratios that overlap with others. Then it is conceivable that depending on your solution the quantitation of some elements is impracticable. For example if you want to quanitate aluminium (mass ~27) then a solution high in manganese (mass ~54) would make this impracticable, if manganese were twice ionised to give the same mass to charge ratio as a single ionised aluminium ion. How does the ICP-MS ensure atoms are ionised only once, or that only singly ionised species are detected?

This is further complicated if you consider that the elements rarely exist in solution at oxidation states of 0, but rather aqueous ions with varying oxidation states.

Answer 1

Interesting question. Some of the stuff can be justified. As an analytical chemist, we should keep in mind that no technique is perfect or universal, even if the instrument costs thousands or millions of dollars. Mass spectrometer cannot distinguish isobaric species (or ions), for instance, if two ions have identical mass due to a different location of the isotope within the same molecule, the mass spec won't be able to you. However, if there is a small difference in mass at fourth, or even fifth decimal place, modern day mass spectrometers can distinguish them (see e.g., Fourier transform ion cyclotron resonance mass spectrometers). The more money you add, the fancier the mass spec gets, with it comes better and better mass resolution.

Now come to the ion generating source - the plasma, which is at say 10,000 K. Although the name plasma sounds very fancy as the fourth state of matter, and given that its temperature is higher than the surface of the Sun- this temperature is still too low for atoms to ionize like crazy. It is pretty hard to strip off one, then the second and then the third electron from an atom as you are thinking of Al. Therefore in a typical ICP torch, multiple ionization on a single atom is not a major concern. To satisfy yourself, do a Boltzmann distribution calculation for sodium atom at 10,000 K. You will notice that a very small fraction of sodium atoms will be in the excited state.

This is further complicated if you consider that the elements rarely exist in solution at oxidation states of 0, but rather aqueous ions with varying oxidation states.

Once the ions from solution reach the plasma they get reduced to "zero" oxidation state. Consider the plasma as a rich source of free electrons, which are circulating like crazy following the radiofrequency field of the coil. The emission in ICP mainly comes from neutral atoms.