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SIMS Theory: Mass Spectra
Mass spectra sample the secondary ions in a preselected mass range by continuously monitoring the ion signal while scanning a range of mass-to-charge (m/z) ratios. The mass analyzer can be either a magnetic sector or a quadrupole. The mass spectrum detects both atomic and molecular ions. Secondary ions containing more than one atom are called molecular ions in SIMS. Note that the term molecular ion also finds use in organic mass spectrometry where it refers to the parent ion before any fragmentation. The mass analyzer must be scanned in small steps to insure that all mass-to-charge (m/z) ratios are sampled. Ten steps per mass unit are common. At higher mass resolution, ten mass increments per peak width adequately define the peak shape. A mass spectrum with a mass range of 100 has at least 1000 data channels for which a reasonable analysis time is 0.1 s per channel. The following figure shows a compressed mass spectrum for a coal fly ash particle. The spectrum clearly shows Li, Be, B, C, O, Na, Mg, Al, Si, K, Ca, Ti, Fe, Zr, Ba, Pb, Th, U. Notable molecular ions include TiO, FeO, BaO, ThO, and UO at m/z 64, 72, 154, 248, and 254, respectively. The ion intensities reflect the isotopic abundances of the elements. For example, silicon isotopes intensities at m/z 28, 29, and 30 parallel the relative silicon natural abundances 92.2 : 4.7 : 3.1. Note the peak for doubly charged Ca++ at m/z 20.
