11.4: Other Forms of Atomic Mass Spectrometry
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Although ICP-MS is the most widely used method of atomic mass spectrometry, there are other forms of atomic mass spectrometry, three of which we highlight here.
Spark Source Mass Spectrometry (SSMS)
In SSMS, a solid sample is vaporized using a spark source, as described in Chapter 10.2 for atomic emission. Because the spark is generated in an evacuated housing the interface between the spark source and the mass spectrometer is simpler. Because the spark generates ions with a large distribution of kinetic energies a quadrupole mass analyzer is not practicable; instead, the mass spectrum is recorded using a double-focusing mass analyzer (see Chapter 20 for more details about this type of mass spectrometer). One advantage of the double-focusing mass analyzer is that it is capable or resolving small differences in masses. For example, in ICP-MS the peaks for 56Fe+ and the polyatomic ion 40Ar16O+ overlap, appearing as a single peak. A double-focusing mass analyzer can separate these two ions, which have, respectively, masses of 55.934942 amu and 55.957298 amu.
Glow Discharge Mass Spectrometry (GDMS)
A glow discharge source generates ions in manner similar to that used to generate the emission of photons in a hollow cathode lamp (see Chapter 9.2 for a discussion of the hollow cathode lamp). The sample serves as the cathode in a cell that contains a very low pressure of argon gas. The application of a high voltage pulse between the cathode and an anode that also is in the cell, converts some of the Ar to Ar+ ion, which then collide with the cathode, sputtering some of the solid sample into a mixture of gas-phase atoms and ions, the later of which are drawn into the mass spectrometer for analysis.
Elemental Surface Analysis by Mass Spectrometry
When analyzing a solid sample, we often are interested in how its composition varies either across the surface or as a function of depth. We can gather information across a surface if we can focus the ion source to a small spot and then raster that spot across the surface, and we can gather information as a function of depth if we can use sputter away a portion of the surface. See Chapter 21 for a discussion of two such techniques: secondary ion mass spectometry (SIMS) and laser microprove mass spectrometry.