Rhenium filaments

Carburization of rhenium filaments has been used to optimize Th and Pa ionization efficiency for TIMS analysis on single filaments (Esat 1995). ReC has a greater work function than Re metal, and elemental oxidation state is maintained in the reduced or metal state by the presence of carbon in the filament. Using this method and a mass spectrometer with improved ion optics, Esat (1995) was able to improve Th transmission and ionization efficiency by about a factor of 30 over conventional methods. Using more conventional mass spectrometry, Murrell et al. (personal communication) were able to improve ionization efficiency for Pa and Th by a factor of 5-10 over conventional graphite sandwich loads on Re filaments (Goldstein et al. 1989 Pickett et al. 1994). For Pa analysis, one drawback is that Pa and U ionization commonly overlap using this... 

The El source is formed by a tungsten or rhenium filament, an anode, an ion repeller, a magnet, and a series of lenses for extracting, focusing and accelerating the ions formed (Figure 2.2). 

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. 

An electron impact (El) ion source uses an electron beam, usually generated from a rhenium filament, to ionize gas-phase atoms or molecules. Electrons from the beam (usually 70 eV) knock an electron from a bond of the atoms or molecules creating fragments and molecular ions [366,534,535]. Several factors contribute to the popularity of El ionization in environmental analyses such as stability, ease of operation, simple construction, precise beam intensity control, relatively high efficiency of ionization, and narrow kinetic energy spread of the ions formed. 

The beam of ionizing electrons is produced by thermionic emission from a resis-tively heated metal wire ox filament typically made of rhenium or tungsten. The filament reaches up to 2000 °C during operation. Some reduction of the working temperature without loss of electron emission (1-10 mA mm ) can be achieved by use of thoriated iridium or thoriated rhenium filaments. [22] There is a wide variety of filaments available from different manufactures working almost equally well, e.g., the filament can be a straight wire, a ribbon, or a small coil (Fig. 5.9). 

Lead Analysis by TEMS. Lead analyses are made using the gel technique described by Cameron et al. (10). Lead ions are thermally produced at rhenium filament temperatures between 1100-1300°C, depending on the sample size. Ten ng of sample allows an analysis of 10 runs with 200 sweeps each across the masses of interest. 

The uranium is separated, after dissolving the sample as described for lead, by extraction with tributyl phosphate (TBP) from 4M nitric acid. After the organic phase is scrubbed with 4M nitric acid, the uranium is back-extracted into distilled water and evaporated to dryness. The uranium is loaded on a rhenium filament for analysis by dissolving the purified sample in a small volume of 0.05M nitric acid. 

The wet chemical procedure consisted of refluxing first with nitric acid until the initial reaction subsided. Then hydrofluoric acid was added to the flask, and the reflux continued until the reaction was complete. The resulting solution containing the inorganic lead was evaporated to near dryness, diluted to a suitable lead concentration with dilute nitric acid, and loaded onto rhenium filaments for M.S. 

Mass spectrometer -for platinum-group metals [PLATINUM-GROUP METALS] (Vol 19) -rhenium filaments for [RHENIUM AND RHENIUM COMPOUNDS] (Vol21)... 

Bias corrections determined from analysis of standards are applied to the samples under test. Use of such an average bias correction can be viewed only as an approximation to the truth so many factors contribute to bias that it is impossible to control them all. For example, as previously stated, the work function of a rhenium filament is determined by which crystal face is involved One way of loading samples on filaments is through use of single resin beads [56,57]. The beads are 100-200 xm in diameter, which is about the size of rhenium crystallites in a poly crystalline filament [17]. Clearly the work function applicable to the analysis in question may or may not be that operative when instrument calibration was carried out. Another parameter difficult to control in real-world conditions is sample purity, which also affects bias. It is impossible to purify all samples to the same degree, and contaminants adversely affect ionization efficiency low efficiency means higher filament temperatures, which in turn mean a different bias correction. These are only two of sundry variables that can affect ionization efficiency. 

McNamara, K. M. and Gleason, K. K. (1993), Comparison of tantalum and rhenium filaments in diamond CVD using selective carbon-13 labeling. J. Electrochem. Soc., 140(2) L22-L24. 

A modification implies desorbing the sample from a heated rhenium filament near the electronic beam. This method is called desorption electron ionization (DEI). 

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... 

Table II shows quite reasonable results for natural abundance isotope ratios it must be noted that Cr Fe and Mg are frequently observed as interferences Heating a clean but unloaded filament gives rise to Cr and Fe ions implying that these two elements are contaminants of the rhenium filament ribbon Heating a filament loaded with silica gel and phosphoric acid gives rise to Mg ions as well as those from Cr and Fe while only Cr and Fe ions are seen when silica gel alone is on the filament This implies a Mg contamination of the acid Finally inhibition of one metal s emmission by another has been seen previously (5) The previously reported inhibition of iron by zinc and of cadmium by iron is seen in this work when the metals of Table II are contaminated with the appropriate ions ...

A variation of the method for determining ° Pb- ° Pb dates of whole single zircons by TIMS analysis that eliminates the need for zircon dissolution and chemical separation of lead was introduced by Kober (1986), with a slight, yet important, modification described a year later (Kober, 1987). In the modified method, usually referred to as the zircon evaporation method, a single zircon crystal is placed into a folded side rhenium filament (the evaporation filament), which is positioned opposite of a blank rhenium ionization filament. The evaporation filament is heated for a short time to evaporate lead onto the target ionization filament. The current to the evaporation filament is then mrned off and the ionization filament is heated until Pb ionization begins and lead isotopic ratios are measured in the normal fashion. Current to the ionization filament... 

The Sr/ Sr ratios of calcite and dolomite in 12 samples were determined after washing the samples with distilled water to remove the pore salts that result from drying. The calcite samples were then reacted with dilute acetic acid and the dolomite samples with 0.1 HCl, and analysed using an automated Finnigan 261 mass spectrometer equipped with nine Faraday collectors. All analyses were performed in the static multicollector mode using rhenium filaments. Correction for isotope fractionation during the analysis was made by normalization to Sr/ Sr = 0.1194. The mean standard error of mass spectrometer performance was 0.00003 forNBS-987. 

Another simple means of creating ions is a surface ionization source. This works effectively for species having low ionization energies, which in this work include atomic silicon and atomic transition metals. Typically, a rhenium filament resistively heated to about 2200 K is used. Silane or the vapors of a transition metal complex or salt are directed at the filament, where decomposition and ionization occur. It is generally believed that the electronic state distribution of the ions formed is in equilibrium at the filament temperature. This generally produces ground-state ions, e.g., exclusively Si+( P), with a distribution of spin-orbit levels associated with the filament temperature. 

A transport efficiency of greater than 80% has been achieved by a rhenium filament ETV system with a glass housing geometry. Optimum sensitivity is obtained with a moderate heating rate and it is dependent on the carrier gas flow. 

An El ion source uses an electron beam, usually gener- 163,234,235 ated from a rhenium filament, to ionize gas-phase atoms or molecules... 

Because the ion production is higher at higher values of the filament work function and heating temperature, the rhenium filament heated to 2200°C will produce more positive ion cirrrent than tungsten filament healed to 2100°C. 

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