Thermal surface ionization source

Thermal ionization mass spectrometry (TIMS) is one of the oldest mass spectrometric techniqnes, first applied by Dempster in 1918.The thermal emission of positivly charged ions emitted from a salt on a heated snrface was first observed by Gehrcke and Reichenheim 12 years before. The thermal snrface ionization source is a very simple ion source and operates under high vacnum conditions. TIMS is mostly useful for elements with relatively low ionization energy - in 

A main characteristic of TIMS is that positively or negatively charged ions of the analyte are also formed and nsed for mass spectrometric analysis. In negative thermal ionization mass spectrometry (NTIMS) elements or molecules with a relatively high electron affinity 2 eV) can be 

In general, ions generated by thermal ionization possess low initial energies (0.1-0.2eV), therefore mostly single magnetic sector field mass spectrometers are used for ion separation. 

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Figure 2.26 a) Experimental arrangement of a single-filament thermal surface ionization source b) Schematic diagram of a double-filament arrangement for thermal surface ionization. (H. Kienitz (ed.), Massenspektrometric (1968), Verlag Chemie, Weinheim. Reproduced by permission of Wiley-VCH.)... 

Thermal surface ionization source on a hot filament thermal surface ionization M+, MX" high < 1( 4 Pa single-focusing SFMS isotope ratios isotope dilution geochronology... 

The first thermal ionization source was developed by Dempster in 1918. The solid material to be analyzed is applied to a hot metal filament and ions are produced by thermal surface ionization at a temperature of 2000 °C. A conunonly used thermal ionization source is the three-filament ion source, developed in 1953 by Inghram and Chupka . This ionization source consists of two parallel filament strips for the sample and an ionization filament in a plane perpendicular to and between the other two filaments. Fig. 4 shows a sectional view of this kind of ion... 

The crucial parameter for the resolution in fast-beam laser experiments is the kinetic energy spread of the ions. For low-energy beams ( < 100 keV) it is determined by the ion source and typically of the order 1 eV, if the acceleration voltage is well stabilized. To meet this value, plasma sources have to be operated with special care with regard to the pressure and potential distribution within the source. Surface ionization sources may almost reach the thermal energy spread of fcT, but this requires very homogeneous and clean surfaces. 

Thermal ionization is based on the production of atomic or molecular ions at the hot surface of a metal filament [95,96]. In this ionization source, the sample is deposited on a metal filament (W, Pt or Re) and an electric current is used to heat the metal to a high temperature. The ions are formed by electron transfer from the atom to the filament for positive species or from the filament to the atom for negative species. The analysed sample can be fixed to the filament by depositing drops of the sample solution on the filament surface followed by evaporation of the solvent to complete dryness, or by using electrodeposition methods. 

Single, double and triple filaments have been broadly used in thermal ionization sources. In a single filament source, the evaporation and ionization process of the sample are carried out on the same filament surface. Using a double filament source, the sample is placed on one filament used for the evaporation while the second filament is left free for ionization. In this way, it is possible to set the sample evaporation rate and ionization temperature independently, thus separating the evaporation from the ionization process. This is interesting when the vapour pressure of the studied elements reaches high values before a suitable ionization temperature can be achieved. A triple filament source can be useful to obtain a direct comparison of two different samples under the same source conditions. 

Large molecules, such as proteins and polymers, do not have the thermal stability to vaporize without decomposing. Desorption ionization sources permit the direct ionization of solids, facilitating the analysis of large molecules. There are several types of desorption sources in which solid samples are adsorbed or placed on a support and then ionized by bombardment with ions or photons. Desorption Cl, one form of desorption ionization, has aheady been described. The important technique of secondary ion mass spectrometry (SIMS) is used for surface analysis as well as characterization of large molecules SIMS is covered in detail in Chapter 14, Section 14.4. Several other important desorption sources are described subsequently. 

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