ToF-SIMS is a microanalysis technique that characterizes the composition and distribution of the sample surface constituents.44-46 ToF-SIMS involves bombarding the sample surface with a pulsed ion beam (primary ions). As the result of ion impact, the species from the surface are ejected, ionized (secondary ions), and accelerated into a mass spectrometer. These secondary ions are mass-analyzed by measuring their time-of-flight from the sample surface to the detector. Collectively, the ToF-SIMS technique provides spectroscopy for characterization of chemical composition, imaging for the determination of the distribution of chemical species, and depth profiling for thin-film characterization.
Categorized as a static SIMS technique, ToF-SIMS analysis involves the application of an ion beam of intensity <1013 primary ions/cm2 (or defined as sampling equal to or less than 1% of a monolayer) which analyzes the molecules at the top layer of the sample surface. Therefore, the technique is highly surface-sensitive and provides information from the outermost layer of the sample surface. Additionally, static SIMS maximizes the quality and quantity of molecular information obtained by significantly reducing fragmentation of sputtered ions.
By rastering a finely focused ion beam across the sample surface, ToF-SIMS imaging analysis provides a map of surface chemical and elemental distribution. Due to the parallel detection nature of ToF-SIMS, the entire mass spectrum is acquired from every pixel in the image. The mass spectrum and the molecule-specific images can then be reconstructed to determine the composition and distribution of sample surface constituents. The ultimate image spatial resolution is limited by the spot size of the ion beam at ~150 nm in diameter.
A schematic of the ToF-SIMS instrument is given in Figure 13.12. ToF-SIMS analyses were performed on a Physical Electronics TRIFT III ToF-SIMS instrument (Physical Electronics, Eden Prairie, MN, USA), equipped with three primary ion sources: gallium (69Ga+) liquid metal ion gun (LIMG), dual plas-matron oxygen (O+) gun and a cesium (Cs+) ion gun.47 Results presented here were obtained using gallium ions as the primary ion source. The LIMG can be operated at either 25 keV, 600pA mode with a spot size of <150 nm for imaging, or in 15 keV, 3nA mode to achieve high mass resolution (m/Am ~ 8,000 at m/z = 41). A low-energy pulsed electron beam, fired between two ion beam pulses, can be used for charge compensation on the nonconductive samples.
Although widely accepted as a high-sensitivity analytical technique for characterization of surface organic and inorganic compositions, the application of ToF-SIMS to biological materials is still in its relatively early stages. The typical examples are in the areas of characterization of biomolecules on surfaces such as microarrays.48,49 Due to the intrinsic limitation on the working mass range of the technique, the ToF-SIMS has been used primarily to study small molecules. However, efforts have been made to increase the ionization yields and thereby extend the detectable mass range for ToF-SIMS.50,51 Alternatively, using a multivariate statistical approach such as principal component analysis (PCA), several groups are focusing their studies on the identification
FIGURE 13.12 Schematic of the time-of-flight secondary ion mass spectrometry instrument.
of proteins and compounds from complex biological samples.52,53 PCA is a pattern recognition technique that improves the ability to analyze the complex ToF-SIMS data set. However, such a technique provides no specific spatial information.
Molecular imaging by static SIMS has gained considerable interest in the past few years.54,55 The key advantage in this area is the capability of direct identification of molecular components in the biological samples. The most notable example is the identification of cellular sections following freeze fracture, in which extensive chemical information of the fracture surface and cellular sections were determined.56,57
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