Thermal ionization filament material

Thermal ionization has three distinct advantages the ability to produce mass spectra free from background interference, the ability to regulate the flow of ions by altering the filament temperature, and the possibility of changing the filament material to obtain a work function matching ionization energies. This flexibility makes thermal ionization a useful technique for the precise measurement of isotope ratios in a variety of substrates. 

Personnel working in some programs at the Los Alamos National Laboratory (LANL) may handle radioactive materials that, under certain circumstances, could be taken into the body. Employees are monitored for such intakes through a series of routine and special bioassay measurements. One such measurement involves a thermal ionization mass spectrometer. In this technique, the metals in a sample are electroplated onto a rhenium filament. This filament is inserted into the ion source of the mass spectrometer and a current is passed through it. The ions of the plutonium isotopes are thus formed and then accelerated through the magnetic held. The number of ions of each isotope are counted and the amount of Pu-239 in the original sample calculated by comparison to a standard. 

The filament material most commonly used in thermal ionization is rhenium. There are several properties that dictate its choice. It has a high enough melting point (3180°C) that it can withstand the temperatures required for efficient ionization (up to about 2200°C). It has the highest work function of any metal with a high enough melting point like all metals, its work function varies with the crystal... 

We have developed a thermal ionization method for use in a standard quadrupole mass spectrometer (1) The method uses a modified solids inlet probe Figure in conjunction with replacable filament assemblies The rhenium filaments are coated with calcium salts precipitated from biological materials in a basic ammonium oxalate solution Calcium is precipitated directly from urine and serum first made basic with ammonium hydroxide fecal samples and aliquots of diet are homogenized with a 9 1 water/nitric acid mixture, centrifuged, made basic and... 

Thermal ionization is useful for elements with ionization potentials less than about 7 electron volts (eV). The thermal ionization process forms positive ions when an analyte is evaporated from a hot metal filament. The filament can be a single filament, where ionization and evaporation occur together, or a double or triple filament, where the sample is evaporated from one filament and the vapor is ionized on a separate filament. Common filament materials include rhenium, tungsten, and tantalum. Whatever the arrangement of the filament, it is always located above the case plate, shown at the top of Fig. 17.1. 

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... 

Fig. 15.5. Thermal ionization efficiency a vs. filament temperature. The curves for Mo reflect the dependence of a on the filament material. The advantage of the Re over the W filament follows fromEqs. 15.1-2. Reproduced from Ref. [7] with permission. John Wiley Sons Ltd, 2008.

Without the bias, formation of cBN (or hard BN) was reported with ECR plasma CVD (293), microwave plasma CVD (275,294), RF plasma CVD with a thermal activation filament (295), and reactive evaporation of B or H3BO3 in an NH3 discharge (296-298). The amount of cBN in an hBN or tBN matrix of these films appears to be snoaUer than that of the films made with bias control. Ion beam deposition from a borazine (B3N3H) plasma (299) or from ionized borazine (264,270,300) was also reported to produce a hard BN material. 

Resonance ionization mass spectrometry as a combination of resonance laser ionization with mass spectrometry can be performed on gas atoms only. Therefore, in RIMS of solid samples, before resonance ionization, a neutral gas has to be produced using several methods known from solid state mass spectrometry. During the evaporation of solid material, e.g., by laser evaporation, thermal evaporation or by sputtering with a primary ion beam, the formation of ions should to be avoided. In RIMS, mostly the thermal evaporation of sample from a heated W or Re filament is applied. 

The measurement of isotope ratios is of considerable importance in several fields of science and medicine. For example, archeologists and geologists use such data to establish the age of artifacts and various types of deposits. Chemists and clinicians use isotopically enriched materials as tracers in various types of studies. 1 he outcome of these studies is based on isotope ratio mea.suremenLs. Historically, isotope ratio measurements were based on thermal atomization and ionization in which the sample was decomposed, atomized, and ionized on one or more elect rically heated filaments. The ions formed in this way were then directed into a double-focusing mass spectrometer, where isotopic ratios were measured. These measurements were time-consuming but quite precise, with relative standard deviations on the order of 0.01 %. Now,... 

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