Liquid SIMS-TOF

Schematic diagram of the liquid SIMS-TOF. (Reprinted with permission from reference 13).

FIGURE 5.6 Liquid SIMS-TOF mass spectrum of a peptide obtained after 20 ps time delay between the ionizing and extraction pulses. The peaks marked by vertical lines belong to the y-ion series. 

Physical Electronics (Eden Prairie, MN, USA) TRIFTII time-of-flight SIMS (ToF-SIMS) equipped with an liquid metal ion gun. 

In agreement with the NMR results, the bulky alkyl-alkoxysilanes RSi(OC2H5>3 (R = i-C4H9 and n-CgH ) are not able to form polymeric or spherical siloxanes under the conditions used for this study. The observed TOF-SIMS spectra are similar to the El spectra of the pure liquids and, therefore, underline the presence of physically adsorbed species [5,9]. 

Nevertheless, the introduction of time-of-flight (ToF) analysers for SIMS analyses at the beginning of the 1980s, as well as the recent development of liquid ion sources delivering cluster projectiles now permit the analysis of organic materials with high sensitivity and selectivity. Moreover, thanks to its excellent lateral resolution (in the order of micrometres), and its minimal sample preparation, ToF-SIMS has become the reference technique for chemical imaging by mass spectrometry. 

Figure 15.2 shows the schematic representation of a typical ToF-SIMS device. All the system is placed under high vacuum (typically 10 7 torr) to avoid interactions between ions and air molecules. Primary ions are produced by a liquid metal ion gun and then focused on the sample to a spot with a typical size of less than 1 pm. After they impinge the surface, secondary ions are extracted and analysed by the ToF analyser. To synchronize the ToF analyser, the primary ion beam must be in pulsed mode. 

Dye identification is of great interest in textile studies. The classical procedure requires a hydrolysis step and other extraction techniques, followed by identification of the individual compounds present after separation by a chromatographic technique, e.g. high-performance liquid chromatography [Novotna et al. 1999, Szostek et al. 2003]. However, ToF-SIMS can be an alternative method, avoiding the phase of extraction which is always a time consuming and delicate step because of the possible destruction of the molecular structure of the sample [Ferreira et al. 2002]. The development of ToF-SIMS for dye detection has been reported in different studies. 

PHI TRIFT IV ToF-SIMS (Physical Electronics, USA) employs three electrostatic analyzers in the ion path to filter the background and metastable secondary ions. Using liquid metal cluster ion guns (such as Aut ion beam for sputtering of sample surface) increased sensitivity compared to a Ga+ primary ion beam are obtained (www.phi.com). The application of dual primary ion guns is useful for an effective dual beam depth profiling on multi-layered samples. 

For the TOF SIMS analysis, only slides treated with a natural pH HAPS solution were used. These were subsequently extracted with warm and hot water. They were mounted into a grid sample holder for transportation into a VG IX23S time-of-flight (TOF) SIMS instrument operating at a vacuum of < 10 Torr with a microfocused liquid Ga metal ion primary beam source (30 keVx 1.0 nA). For charge compensation, an electron flood gun was used. The working resolution of the spectrometer was determined from a lead phthalocyanine spectrum for Pb+ at mlz = 208 and the molecular ion at mlz = 720, it was 500 and 1000, respectively. 

Figure 8.8 Liquid metal ion source. The primary ion beam is extracted from the Taylor cone of liquid metal. (Reproduced with permission from J.C. Vickerman and D. Briggs, ToF-SIMS Surface Analysis by Mass Spectrometry, IM Publications and SurfaceSpectra, Chichester and Manchester. 2001 IM Publications.)...

The use of separation techniques, such as gel permeation and high pressure liquid chromatography interfaced with sensitive, silicon-specific aas or ICP detectors, has been particularly advantageous for the analysis of silicones in environmental extracts (469,483—486). Supercritical fluid chromatography coupled with various detection devices is effective for the separation of silicone oligomers that have molecular weights less than 3000 Da. Time-of-flight secondary ion mass spectrometry (TOF-sims) is applicable up to 10,000 Da (487). 

Secondary ion mass spectrometry (SIMS), when carried out in the static (i.e., zero damage) mode, is a uniquely powerful tool for the molecular characterization of polymer surfaces. Its molecular specificity also allows the detection and characterization of additives and contaminants, whose presence at the surface can cause major changes in surface behavior, at the submonolayer level. Modern instruments based on time-of-flight (ToF) mass analyzers and liquid-metal ion sources are capable of chemical imaging with a spatial resolution of well below 1 pm. 

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