Friday, March 22, 2013

High mass resolution mass spectrometry

Mass spectrometry is the dark art of separating objects by mass.  The name comes from the alchemal days of photographic plate detectors; just like a prism separates white light into a spectrum of colors, a magnet can separate a beam of ions into their component masses, which will then form an image on a plate.

These days, electronic counting systems have replaced chemical emulsion ion detectors, but the name lives on.  In the case of atomic and molecular charged particles, the masses are not continuously distributed, like the energy distribution of white light.  Rather, different ions have discrete masses.  To a first approximation, the nominal mass of an atom (or an atomic ion, if the atom is charged) is simply the sum of its protons and neutrons.  Thus, an atom with 26 protons and 30 neutrons (Iron fifty-six, abbreviated by scientists as “56Fe”) has a nominal mass of 56. 

The whole point of mass spectrometry is to separate things with different masses.  So, for example, most mass spectrometers can separate 56Fe, with 26 protons 30 neutrons, from 54Fe, which also has 26 protons, but only has 28 neutrons. The ability to distinguish atoms of the same element with different mass- isotopes- is one of the main uses of mass spectrometers.  In this case, separating 54Fe from 56Fe requires a mass resolution of 1 part in 28.  The mass resolution, defined by IUPAC as M/ΔM = 56/2 = 28.  This is quite low. It is about nine times worse than what is needed to separate 240Pu from 239Pu, for example. And even then, a mass resolution of 240 is still generally considered low.  There is no formal definition of high and low mass resolution.  However, as a general rule, mass spectrometers which can only measure the nominal masses of inorganic ions are generally known as low mass resolution instruments.

Note the use of the word ‘nominal’ when describing ionic masses so far.  As it turns out, exact masses are not the same was nominal masses.  For one thing, protons and neutrons do not have the same mass; their mass differs by about one part in a thousand.  More importantly, combining them into nuclei changes some of their mass into energy via Einstein’s famous equation, E=mc2.  This ‘binding energy’ makes the nucleus lighter than its component protons and neutrons, and different nuclei have different binding energies, and therefore different exact masses.  So, for example, the mass resolution required to resolve a molecule of hydrogen, 1H2, from deuterium (2H) atom (a hydrogen atom with a neutron in its nucleus) is about 1350.  As a good working definition, high mass resolution is mass resolution high enough to resolve ions with the same nominal mass (called “isobaric interferences” by mass spectrometrists) as a result of the small differences in real mass caused by their binding energy.

The trouble with this definition, of course, is that the mass resolution required to separate isobaric interferences, depends on what they are. For example, see the isobaric interferences in figure 1.

Figure 1. SHRIMP mass spectrum of atomic and molecular peaks at mass 56 in San Carlos Olivine (Mg2SiO4 with Fe and Ca substituting for Mg). Green and purple: nominal M/dM = 5000; orange and blue: nominal M/DM = 15000; green and orange: Faraday cup; blue and purple, electron multiplier.

This figure contains mass scans taken on both high (~5000) and higher (~15000) mass resolution.  The mass resolution required depends on the interference.  For example, the resolution required to resolve 28Si2 from 40Ca16O is more than ten times higher (~15000) than the mass resolution required to resolve 56Fe from 24Mg16O2 (~1500). Note that increasing mass resolution also decreases signal intensity.

The easiest way to avoid interferences is to not create them in the first place.  This can be done by chemists who purify the element whose isotopes they wish to measure, or by material scientists who make pure compounds without trace elements (such as Ca in the above figure).

Another technique is to use an ionization source that doesn’t produce many molecular ions.  So high mass resolution is most useful when a compositionally complex material is ionized using a method that creates all sorts of complex species.

This is why high mass resolution mass spectrometry is popular in geological SIMS analyses.  Minerals generally contain a wide variety of minor and trace elements, and the SIMS ionization produces all sorts of molecular fragments.  So being able to resolve species based on their mass defects is extremely useful.

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