Secondary Ion Extraction

The Mattauch-Herzoggeometry (Fig. 3.20) enables detection of several masses simultaneously and is, therefore, ideal for scanning instruments [3.49]. Up to five detectors are adjusted mechanically to locations in the detection plane, and thus to masses of interest. Because of this it is possible to detect, e. g., all isotopes of one element simultaneously in a certain mass range. Also fast, sensitive, and precise measurements of the distributions of different isotopes are feasible. This enables calculation of isotope ratios of small particles visible in the image. The only commercial instrument of this type (Cameca Nanosims 50) uses an ion gun of coaxial optical design, and secondary ion extraction the lateral resolution is 50 nm. 

The field strength is scanned by an electromagnet, and the dispersion of adjacent masses (i.e. the resolution) decreases with increasing ion mass. The high secondary ion extraction voltage employed results in efficient transmission of secondary ions from the sample surface to the detector, although it is difficult to analyse samples with surfaces that are fractured or rough. 

The secondary Ion extraction optics should Include an Immersion lens above the sample surface to maximize secondary Ion transmission from the sample surface to the mass spectrometer. An electrostatic analyzer should again be used to filter out high-energy neutral particles and photons from the secondary Ion beam. Energy analysis of the secondary Ions Is generally not necessary, but can be used to vary the amount of fragmentation observed In the mass spectrum (9). 

The secondary ion extraction optics and ion energy filter is of commercial design (Kratos). Also incorporated in the analysis chamber is a scanning electron gun having a... 

In its most elementary form, a SIMS system consists of a source of primary ions, a sample holder, secondary ion extraction optics, a mass spectrometer, and an ion detector, all housed in a UHV compartment. Systems are also equipped with data processing and output systems. A schematic diagram of part of a SIMS system with a quadrupole mass analyzer is shown in Fig. 14.38. The design and operation of mass spectrometers is covered in Chapter 9 and will be only briefly reviewed here. 

Electrostatic deflectors, lenses and secondary ion extraction fields make up the remainder of the ion optical elements within the secondary ion column. These are utilized to ensure that the highest possible fraction of the secondary ions reach the detector. Detectors used in SIMS comprise of Faraday Cups for the measurement of intense signals and Electron Multipliers for pulse counting of less intense signals. Electron Multipliers come in several different designs referred to as Discrete... 

If the samples are of a stable solid form and have spatial dimensions greater than a few millimeters, they can be directly inserted into or affixed (clipped or glued) onto the respective sample stub/platform with no special preparation required. Care must, however, be taken to ensure that any clips used are not in the immediate vicinity of the area to be analyzed. Aside from potentially blocking the primary or secondary ion beams, this can distort the secondary ion extraction field thereby influencing the secondary ion signal intensities recorded. For the same reason, analysis toward the very edge of a sample or around topographical features should be avoided where possible. 

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

Other technique—for example, dynamic secondary ion mass spectrometry or forward recoil spectrometry—that rely on mass differences can use the same type of substitution to provide contrast. However, for hydrocarbon materials these methods attain a depth resolution of approximately 13 nm and 80 nm, respectively. For many problems in complex fluids and in polymers this resolution is too poor to extract critical information. Consequently, neutron reflectivity substantially extends the depth resolution capabilities of these methods and has led, in recent years, to key information not accessible by the other techniques. 

The instrumentation for SSIMS can be divided into two parts (a) the primary ion source in which the primary ions are generated, transported, and focused towards the sample and (b) the mass analyzer in which sputtered secondary ions are extracted, mass separated, and detected. 

Several ion sources are particularly suited for SSIMS. The first produces positive noble gas ions (usually argon) either by electron impact (El) or in a plasma created by a discharge (see Fig. 3.18 in Sect. 3.2.2.). The ions are then extracted from the source region, accelerated to the chosen energy, and focused in an electrostatic ion-optical column. More recently it has been shown that the use of primary polyatomic ions, e. g. SF5, created in FI sources, can enhance the molecular secondary ion yield by several magnitudes [3.4, 3.5]. 

Cesium ions are also sometimes used to enhance the secondary-ion yield of negative elemental ions and that of some polymer fragments [3.6]. They are produced by surface ionization with an extraction technique similar to that of FI sources. 

Applications Early MS work on the analysis of polymer additives has focused on the use of El, Cl, and GC-MS. The major drawback to these methods is that they are limited to thermally stable and relatively volatile compounds and therefore are not suitable for many high-MW polymer additives. This problem has largely been overcome by the development of soft ionisation techniques, such as FAB, FD, LD, etc. and secondary-ion mass spectrometry. These techniques all have shown their potential in the analysis of additives from solvent extract and/or from bulk polymeric material. Although FAB has a reputation of being the most often used soft ionisation method, Johlman el al. [83] have shown that LD is superior to FAB in the analysis of polymer additives, mainly because polymer additives fragment extensively under FAB conditions. 

The bombarding of the specimen surface by the primary beam of high energy ions leads to the ejection (sputtering) of both neutral and charged (+/—) species from the surface. The ejected species may include atoms, clusters of atoms and molecular fragments. The ions enter an extraction lens and the polarity of the applied voltages determines the polarity of the secondary ions that enter the analyser. 

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. 

K. Saito, T. Mitsutani, T. Imai, Y. Matsushita and K. Fukushima, Discriminating the indistinguishable sapwood from heartwood in discolored ancient wood by direct molecular mapping of specific extractives using time of flight secondary ion mass spectrometry, Analytical Chemistry, 80, 1552 1557 (2008). 

SUMNER, L., PATVA, N.L., DIXON, R.A., GENO, P.W., High-performance liquid chromatography/continuous-flow liquid secondary ion mass spectrometry of flavonoid glucosides in leguminous plant extracts, J. Mass Spectrom., 1996,31,472-485. 

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