Ion extraction and focusing

It has been suggested from Langmuir probe,fluorescence and pressure measurements that the gas beam downstream of the skimmer cone does not attain the qualities of a true molecular beam for a typical plasma sampling interface. A disturbance or shockwave caused by the presence of the skimmer tip or even formed inside the skimmer tip may be responsible for disturbing molecular beam formation. The intensity and dimensions of the gas beam formed 

Inductively Coupled Plasma Mass Spectrometry Handbook 

or gate, valve (open position) Expansion 

Expansion rotary pump Intennediate turbo pump 

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The ion extraction and focus electrode arrangement comprises two pairs of electrodes located between the ionization chamber and the mass separator. Their function... 

Figure 21 and Table 3 summarize the necessary DC-sources applied to the structures of the chip. Presently all DC-sources are programmable and variable in the range of-230 to +230 V. This flexibility is essential to determine the optimal parameter setting during system optimization. Once it is known many of these sources can be replaced by fixed ones. Still, due to production tolerances and long term drift effects the sources for ion extraction and focus (t/IFO), energy filter (Usl, Uso), and MCP (UHV) will have to remain variable for optimum device performance. 

Figure 3 Details of the differential pumping, ion extraction, and focusing for the Nu Instruments Nu Plasma.

Once the ions have entered the skimmer tip, it is necessary to extract and focus them into the analyser by subjecting the charged ions to constant electric fields. In order to construct an elfective ion optical array, it is necessary to calculate the path followed by the ions in the electrostatic fields. We can resort to a number of mathematical models, such as SIMION, for a better understanding and optimisation of the ion-optical design for ICP-MS and the processes involved [12]. 

At the end of the drift tube, there is an intermediate chamber in which most of the air from the drift tube through a small orifice is pumped away. The ions in the drift tube are extracted and focused by the ion optical lens and finally, in a high vacuum chamber, are detected by a quadrupole mass spectrometer with an ion pulse counting system. The ionic count rates /(HaO" ) and 7(MH" ) are measured in counts per second, which are proportional to the respective densities of these ions. 

FIGURE 1.6 Basic geometry of the time-of-flight mass spectrometer ofWiley and McLaren using two-stage ion extraction and time-lag focusing. (Reprinted with permission from reference 16). 

At the lower pressure side of the skimmer a second expansion step takes place. In older systems, the ions entering through the skimmer were extracted, transferred and focused into the mass analyser region by means of a set of conventional flat lenses. Later, it was determined that a more efficient ion transfer and focusing can be achieved by means of an RF-only quadrupole, hexapole or octapole device (cf. Figure 1). These types of ion transfer and focusing devices are especially in use in systems for LC-MS. In other systems, as used for ICP-MS for instance, ion... 

Before doing this, an overview will be provided of ion extraction and ion focusing optics. 

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]. 

Electron impact (El) ion sources are the simplest type. O2, Ar, or another (most often noble) gas flows through an ionization region similar to that depicted in Eig. 3.30. Electrons from an incandescent filament are accelerated to several tens of eV by means of a grid anode. A 20-100 eV electron impact on a gas atom or molecule typically effects its ionization. An extraction cathode accelerates the ions towards electrostatic focusing lenses and scanning electrodes. 

Typical ion sources employ a noble gas (usually Ar). The ionization process works either by electron impact or within a plasma created by a discharge the ions are then extracted from the region in which they are created. The ions are then accelerated and focused with two or more electrostatic lenses. These ion guns are normally operated to produce ions of 0.5-10 keV energy at currents between 1 and 10 pA (or, for a duoplasmatron, up to 20 pA). The chosen spot size varies between 100 pm and 5 mm in diameter. 

Recent attention has focused on MS for the direct analysis of polymer extracts, using soft ionisation sources to provide enhanced molecular ion signals and less fragment ions, thereby facilitating spectral interpretation. The direct MS analysis of polymer extracts has been accomplished using fast atom bombardment (FAB) [97,98], laser desorption (LD) [97,99], field desorption (FD) [100] and chemical ionisation (Cl) [100]. 

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. 

Selection of a suitable ionisation method is important in the success of mixture analysis by MS/MS, as clearly shown by Chen and Her [23]. Ideally, only molecular ions should be produced for each of the compounds in the mixture. For this reason, the softest ionisation technique is often the best choice in the analysis of mixtures with MS/MS. In addition to softness , selectivity is an important factor in the selection of the ionisation technique. In polymer/additive analysis it is better to choose an ionisation technique which responds preferentially to the analytes over the matrix, because the polymer extract often consists of additives as well as a low-MW polymer matrix (oligomers). Few other reports deal with direct tandem MS analysis of extracts of polymer samples [229,231,232], DCI-MS/MS (B/E linked scan with CID) was used for direct analysis of polymer extracts and solids [69]. In comparison with FAB-MS, much less fragmentation was observed with DCI using NH3 as a reagent gas. The softness and lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Most likely due to higher collision energy, product ion mass spectra acquired with a double-focusing mass spectrometer provided more structural information than the spectra obtained with a triple quadrupole mass spectrometer. 

The choice between the use of solid-state supported extractants and solvent extraction is often made on the basis of the concentration of the desired metal in the aqueous feed. Solvent extraction is usually not effective for treating very dilute feeds because an impracticably large volume of the aqueous phase must be contacted with an organic extractant to achieve concentration of the materials across the circuit. However, solvent extraction is preferred for treating moderately concentrated feeds because most ion-exchange resins and related materials have relatively low metal capacities and very large quantities of resin are required. In this review we will focus on reagents used in solvent extraction because, in the main, the nature of the complexes formed are better understood. 

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. 

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