Secondary Ion Mass Spectrometry
Data
Unlike other techniques that detect electrons, light or energy, SIMS detects ions (charged particles) and is able to detect elements from hydrogen to uranium and a wide range of molecular ions up to a mass in the range of 105 atomic mass units (amu). These are separated in the analyser based on their mass/charge (m/z), which directly correlates with the atomic mass units.
For example, Cu has two natural isotopes, one with a mass of 63 amu and the other with a mass of 65 amu. So we see two peaks for Cu+ at m/z 63 and m/z 65. The ratio of the intensities of the two signals is the same as their isotopic abundance.
Ionisation of a sample will be different for each atom and molecule depending on their ionisation potential (Ki). Some elements and molecules ionise better than others. For example, if we consider a sample with an equal concentration of Cu and Zn, the ionisation potential for Cu = 750 kJ/mol whereas Zn = 900 kJ/mol. Consequently, Cu is more likely to be ionised than Zn and will result in more Cu+ ions being detected than Zn+ ions despite these elements being present at the same concentration.
Ionisation efficiency is impacted by a number of factors, particularly the chemical environment of the atoms of interest. Consequently, quantification with SIMS is complicated and requires matrix-matched standards for all ions.
In some newer instruments large complex molecular ions can be selected from the first mass analyser and sent to a second mass analyser to be fragmented so they can be identified more precisely.