Secondary ion mass spectroscopy (SIMS)

About this technique

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Secondary ion mass spectrometry (SIMS) is one of the most sensitive surface analytical techniques available for high analytical precision of in-situ quantitative measurements in solid materials. Its combines high transmission, high mass resolving power, and unique features that make it one of the most versatile and powerful mass spectrometry techniques available.

In SIMS, a high-energy ion beam ablates material from a sample surface and secondary ions are then separated according to mass/charge (m/z) ratio in a mass spectrometer. Although capable of imaging at sub-mm spatial resolution, it is typically operated using a 10–20 mm beam in order to optimise sensitivity. Typical analyses that may be performed are quantification of trace elements in semiconductor or geological materials and spatially resolved, in-situ, stable isotope ratio measurements. The IMS 1280 at the University of Western Australia can detect up to five ion species simultaneously or can detect masses is sequence by cycling the magnetic field of the mass spectrometer.

Although SIMS often requires less sample preparation than other analytical techniques, for in-situ isotope ratio analyses, high-quality sample preparation is mandatory. Sample geometries of less than 7 mm thick and up to 25 mm in diameter can be accommodated. Geological samples are most often cast in vacuum-compatible, 25 mm epoxy plugs, ground, and polished flat. Samples and standards should be cast in the centre 15 mm of the mount and minimal relief should exist between samples and surrounding epoxy. Insulating samples can be accommodated, but should be sputter coated with (typically) 30 nm of gold.

Although SIMS can detect every element in the periodic table, detection limits span more than five orders of magnitude. For species with low ionisation potential or high electron affinity, detection limits are typically sub-ppb. Although quantification of trace elements is one of the most widely utilised strengths of SIMS, secondary ion yields are highly matrix dependent, thus matrix matched standards are mandatory for optimum accuracy. Homogeneously doped standards are desirable but are often unavailable for a given matrix. Thus, ion implants are often used as standards where absolute quantification is required.

Stable isotope ratio analyses are the most common use of SIMS. In order to optimise precision for a given analysis, multicolection is desirable. Multicollection is  possible for single elements from Li to U. D/H analyses are feasible, but magnet switching is required. The following table gives the external precision of isotope ratios that are commonly achieved.

Isotope pair	1sigma	Material
D/H	~3 ‰	organic solids, silicates
10B/11B	<1 ‰	silicates, glass(35 ppm B in NIST glass)
13C/12C	<0.2 ‰	organics
18O/16O	<0.2 ‰	silicates
17O/18O	<0.3 ‰	silicates
33S/32S	<0.2 ‰	sulphides
34S/32S	<0.2 ‰	sulphides
36S/32S	<0.2 ‰	sulphides
56Fe/54Fe	~0.2 ‰	ironoxides (magnetite/hematite), sulphides
57Fe/54Fe	~0.2 ‰	ironoxides (magnetite/hematite), sulphides
87Sr/86Sr	~1 ‰	apatite (1000 ppm Sr)
235U/238U	1-2 ‰	U3O8 standard particles
234U/238U	<10 ‰	U3O8 standard particles
236U/238U	<15 ‰	U3O8 standard particles

Examples of analyses that have been performed using SIMS:

Biological

• C and N isotope analyses of sub-nanogram quantities of bacteria for forensic analysis.
• Sr and O isotope analyses of fossilised animal teeth to reconstruct migration patterns.

Earth and planetary science

• D¹⁷O analyses of meteorites to identify origin.
• D³³S and D³⁶S analyses of sulphide ores to aid in exploration targeting.
• C isotope analyses of ancient organic carbon to examine early life metabolism.
• ¹⁸O/¹⁶O analyses of zircons to discern mantle or crustal influences.

Materials science

• Depth profiling in semiconductors to localise and quantify dopants.