Spectrometers vacuum

Apart from the application of XPS in catalysis, the study of corrosion mechanisms and corrosion products is a major area of application. Special attention must be devoted to artifacts arising from X-ray irradiation. For example, reduction of metal oxides (e. g. CuO -> CU2O) can occur, loosely bound water or hydrates can be desorbed in the spectrometer vacuum, and hydroxides can decompose. Thorough investigations are supported by other surface-analytical and/or microscopic techniques, e.g. AFM, which is becoming increasingly important. 

Vacuum systems are integral parts of any mass spectrometer, but vacuum technology definitely is a field of its own. [251-255] Thus, the discussion of mass spectrometer vacuum systems will be restricted to the very basics. 

If both MeOH and EtOH are present in the spectrometer vacuum system, then the reactions come to equilibrium, and the equilibrium constant can be determined, exactly as for proton transfer equilibria. Via a thermochemical cycle, the relative hydrogen bond strengths of the two ions present can be determined from that equilibrium constant and the conjugate acidities of the two bare ions involved, according to Equation (16),... 

Microprocessor controlled quadrupole mass spectrometer Vacuum, 32,1982,163-168... 

Stainless steel surfaces swabbed with trichloethane immediately before insertion in the XPS spectrometer, consistently showed evidence of chloride retention in their Cl(2p) spectra, even after exposure to a spectrometer vacuum of 10 Pa for 2A hours at 3A-A0°C. Figure 5(a) shows such a spectrum. From the Cl(2p) spectral envelope, two sets of Cl (2p 3/2-1/2) doublets were derived by deconvolution. The more intense doublet "1" is assigned to organically-bound chlorine, being close in energy to chlorine bound in some aliphatic hydrocarbons. The Cl(2p 3/2) line labelled "2" corresponds closely to the binding energies. . ... 

An insulator, on the other hand, has an ill-defined Fermi level which does not equilibrate with the spectrometer. Instead, the vacuum level of the insulator (E ) aligns with the local electrostatic potential surrounding its surface. An insulator more than a micron thick (which is the case for most catalyst samples analyzed by XPS) will not be within the local potential of the metal sample holder. The insulator will be separated from the spectrometer vacuum level (EJ) by some voltage (Vp) (30). This voltage will depend on the geometry of the sample holder and on the energy and flux of electrons from the x-ray source, the flood gun, the sample itself, and all other sources within the chamber. The potential Vp cannot be reliably measured. 

Figure 2 represents the collision energy balance in the case of a conducting sample. As the electron kinetic energy is measured with reference to the spectrometer vacuum level, this balance must be expressed as... 

XPS experiments to study the electronic state of catalysts were carried out on an ESCA-3 VC electron spectrometer. Vacuums in tho analyzer and preparation chambers were 1-2 10 and 5 10 Pa, respectively, XPS spectra were calibrated according to the Cis line whose binding energy was taken to be = 285.0 eV (ref. 4). 

Table 1 lists a number of ionization sources which produce ions at either atmospheric pressure or under vacuum conditions. For atmospheric pressure ionization sources a suitable interface is required which allows a controlled leak of ions into the vacuum region of the mass spectrometer. Vacuum ionization techniques likewise require a controlled leak, or mechanical introduction, of neutral molecules into the vacuum chamber, followed by ionization. 

Fig. 2.1 Mass spectrometer vacuum system. The atomizer and furnace support are shown in the loading position. Movement of a furnace along a rail is produced by means of a guide pin. (Reproduced from [11], with permission.)...

Figure 2.2 Schematic layout of the Fabry-Perot cavity spectrometer. Vacuum seals and return springs for the micrometer drive and electrical connections for the piezoelectric actuator are not shown...

GC/MS with capillary columns has been the gold standard for more than 20 years, but LC/MS has become a complementary method due to the success in interface development with atmospheric pressure ionisation (API) for low molecular weight compounds and the appHcation to biopolymers. For many areas of analytical chemistry, LC/MS has become indispensible due to its advantages over GC/MS for polar and thermolabile analytes. A Hmiting factor for LC/MS has been the incompatibility between the hquid eluting from the LC and the mass spectrometer vacuum. This could be overcome in electrospray ionisation with the use of a nebuliser gas ( ion spray ) or additional heated drying gas ( turbo ion spray ) (70, 71]. Due to its high sensitivity and selectivity, APl-MS has become a standard tool for the stracture elucidation of analytes from complex mixtures. 

Atmospheric pressure ionization (API) techniques encompass a range of techniques in which ionization occurs external to the mass spectrometer vacuum. Ionization can be achieved by a variety of methods, including photoionization, corona discharge at the tip of a needle, or by the use of radioisotopes such as Ni. 

Thus far the only difference between the UV and El detectors is that the observed signal for the former is a measure of transmitted intensity I, while that for the latter is a measure of the absorbed intensity — but it turns out that is of little consequence for the present purpose. Thus, for example, a fluorescence detector records a signal that is a measure of the absorbed intensity of the exciting radiation like the El case, but it is also a concentration dependent detector like the simple UV absorption detector. The difference between the two types arises rather in the relationship between the analyte concentration delivered by the mobile phase (c ) and that within the absorption cell or El source in the former case c = c (see equation [4.2]), but the situation is very different in the El case where the mass spectrometer vacuum pumps continuously remove the analyte from the El source. In fact c ei represents an instantaneous steady state value, a compromise between the flow rate of A into the source and the pumping rate out of the source here instantaneous means simply that the establishment of the steady state value c gi occurs on a timescale appreciably shorter than that of the chromatographic peak. Then at this steady state ... 

Although the majority of MALDI-MS experiments have followed the originators (Tanaka 1988 Karas 1987, 1988) by inserting the matrix-analyte mixture into the high vacuum chamber of the mass spectrometer before irradiation, development of MALDI sources that operate at atmospheric pressure has been described (Laiko 2000 Moyer 2002). Such methods obviously require efficient interfaces between the atmospheric pressure ion source and the mass spectrometer vacuum as for the more usual API sources (Section 5.3.3). Currently this approach does not appear to have been exploited for trace quantitative analyses of small molecules. 

Figure 5.15 (a) Sketch of a gas curtain interface for API-MS coupling (Buckley 1974, 1974a French 1977). The ultra-dry gas (N2) curtain separates the ionization chamber (atmospheric pressure) from the orifice leading to the skimmer cone and thence to the mass spectrometer vacuum, (b) Sketch of an API-MS interface based on a heated glass capillary that connects the atmospheric pressure source to the low vacuum region preceding the sampling cone (Figure 5.17). In both cases an electric field E helps direct the ions into the sampling orifice. Reproduced from Bruins, Mass Spectrom. Revs. 10, 53 (1991), with permission of John Wiley Sons, Ltd.

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