SIMS technique

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The Ion Microprobe employs Secondary Ion Mass Spectrometry (SIMS) for the chemical analysis of small volumes of material.

In SIMS, the sample's surface is bombarded under vacuum with a finely focused beam of primary ions (Cs+, O+, O- or Ar+). The collision cascade results in the ejection and ionisation of atoms and molecules from the sample's surface layers. These secondary ions are accelerated into a double-focusing mass spectrometer where they are separated according to their energy and mass/charge ratio before being detected.

There are three basic modes of operation:

1. Point Analysis: A focused stationary beam is used to determine the composition at a point. The spot size is generally between 1 and 25um but is dependent on the application.
2. Depth profiling: Scanning the primary ions over the surface of the sample causes the surface layers to be slowly eroded, revealing the compositional change with depth.
3. Imaging: The distribution of elements or isotopes may be imaged with a spatial resolution of about ~1um over areas up to 500um2.

The Ion Microprobe can quantitatively analyse nearly all elements in the periodic table from H to U, but there are differences in sensitivity exceeding four orders of magnitude. Even so, for many elements the detection limits are in the p.p.b. range. The instrument may also be used to determine the isotopic ratios of elements (e.g. C, O, Si, B, Li, S) to a precision better than 0.5 per mil.

Relative ion yields

You can view a graph of relative ion yields, available via PDF:

Elemental Analysis

SIMS analysis allows the detection of virtually all the elements in the periodic table, often with ng/g levels of detection. The technique is particularly suited to the analysis of volatiles such as H,C,S,F and Cl in volcanic systems, Rare Earth Elements in any mineral, H in Nominally Anhydrous Minerals (NAMs) and Mg/Ca, Sr/Ca ratios used in palaeoclimate studies.

Isotopic Analysis

The isotopic ratio of many elements may be accurately determined by SIMS if the concentration and ionisation efficiency allows. Typically isotopic ratios of H, B, Li, C, O, Si, S can readily be determined at a spatial resolution of 5-20 µm.

Depth Profiles

As the ion beam slowly erodes the atomic layers of the sample, the isotopic signal is recorded with a depth resolution of 20-30 nm. Applications are mainly concerned with the study of diffusion rates of isotopes, the composition of thin coatings and films, and dopant measurements in semiconductors.

Geochronology

In situ U/Pb dating using high spatial resolution SIMS is a powerful approach to obtain geologically meaningful ages from complexly zoned minerals such as zircon, apatite, sphene and monazite.

Images

Images may be acquired by various methods (e.g. Direct Scanning Ion Image, multiple spot analysis, depth profiles). For many applications, the best technique is multiple spot analyses and contouring the data according to concentration or isotopic abundance.