Time-of-flight secondary ion mass spectrometry (ToF-SIMS)

About this technique

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Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is an analytical technique used to image and record organic and inorganic mass spectral data of solid materials. It is a highly sensitive technique that provides chemical information regarding elemental, isotopic and molecular structure. It involves the analysis of ionised particles that are emitted when the surface is bombarded with an energetic primary ion beam. Emitted particles are accelerated to constant kinetic energy into the time-of-flight chamber, where mass separation is achieved according to mass-to-charge ratio. It is a highly surface sensitive technique, as only the secondary ions generated from the outer 1–2 nm region have enough energy to escape the surface for detection and analysis.

Although not a quantitative technique, ToF-SIMS can provide a qualitative surface chemistry comparison between samples. Using ‘bunched’ instrument settings, high mass resolution spectra can be collected from the surface. By using ‘unbunched’ settings, spatial resolution is optimised (sub-micrometre for Au₁), so that images and chemical maps of the surface can be produced by the rastered beam. If data is saved to RAW format during an acquisition, these images may be produced in retrospect, as a full mass spectrum is saved for every pixel of the rastered area.

A range of samples can be analysed by ToF-SIMS, with the requirement that the sample must be ultra-high vacuum (10^-7Pa) compatible. Films or layers on solid substrates are easily loaded, while powders and mineral slurries can be pressed into indium foil and mounted on silicon for analysis. Small (<20 mm diameter) samples are back-mounted in the 9-sample holder, while larger samples can be front-mounted to the stage plate. A vice holder is also available for mounting samples that require cross-sectional analysis.

Temperature control of samples is available by using the hot/cold stage module. This can range from cooling down to liquid nitrogen temperature to heating up to approximately 200ºC. A unique feature of this particular instrument is the custom-built preparation chamber, where a sample may be subjected to any number of experiments or heating/cooling under vacuum, before introduction into the analysis chamber.

The instrument contains four ion sources. Two of these can be used for both analysis and sputtering (Au and C₆₀), while the caesium and gas guns are purely for sputtering. The Au liquid metal ion gun (LMIG) can be operated using either monatomic primary ions (Au₁) or cluster ions (Au₂ or Au₃). Due to their larger size, use of cluster ions can help to increase the yield of higher mass fragments. Similarly, use of the C₆₀ ion gun can help to increase the yield of much higher mass fragments, potentially into the 1000’s amu range. Hence the C₆₀ is very important to the analysis of polymers, and also when depth profiling organic materials, as the organic information is retained during sputtering.

Operation of the Au or C₆₀ analysis sources in conjunction with any of the sputtering sources can be used to perform depth profiles of materials. Analysis and sputtering phases are alternated to remove and then analyse the underlying layer of the material. If depth profiles are performed with the analysis phase optimised for spatial resolution, the 3-D images of the depth profile may be produced.

Data analysis is an important part of the technique, usually taking longer than the actual acquisition of the data itself. In terms of mass spectra, inorganic components are easily identified based upon their expected mass in the spectra. Organic components are usually not as easily identified, due to the fragmentation of their structures down to C_xH_y, C_xH_yO_z, C_xH_yN_z, etc. fragments. Unlike other forms of mass spectroscopy, it is uncommon to measure a molecular ion (M⁺ or M^-) in ToF-SIMS. While other indicative peaks may be present, organic analysis often comes down to the careful comparison of the ratios of the fragment ions. Hence, ToF-SIMS is often coupled with multivariate data analysis as the amount and complexity of the data can be high. For example, principal component analysis can be used to help characterise differences between absorbed proteins, as well as the relative amount and even the conformation of those proteins.