Knudsen Effusion Mass Spectrometers

Molecular weight from vapor pressures Molecular weight from P-V-T measurements Double oven effusion with mass spectrometer Knudsen effusion with mass spectrometer Knudsen effusion with mass spectrometer Velocity distribution analysis Velocity distribution analysis... 

Figure 3. Cross-sectional view of UHV target-mass spectrometer Knudsen effusion apparatus...

Hastie [131] coupled for the first time a quadrupole mass spectrometer with a Knudsen cell. One of the quadrupole mass spectrometer - Knudsen cell systems used at our laboratory is shown in Fig. 4. The system has been developed to study small alkali metal clusters under equilibrium conditions (see Sect. 3.2). Broad-band photoionization by a 1 kW Hg/Xe lamp is used for the first time in Knudsen effusion mass spectrometry to reduce fragmentation. Other quadrupole mass spectrometer - Knudsen cell systems have for example been developed by Hilpert [132, 133], Fraser and Rammensee [134], Plante [135], Ono et al. [136], Kematick et al. [137], and Edwards et al. [138]. Cryogenic pumping is used in the device by Hilpert to reduce mercury background ion intensities for the study of amalgams [132], The instruments described in Refs. 134,135 use a chopper to modulate the molecular beam from the Knudsen cell. Interfering background ion intensities can, thereby, be subtracted. The apparatus developed by the authors of Refs. 137, 138 renders possible the simultaneous application of Knudsen effusion mass spectrometry and the mass-... 

Vapor pressures of phases in these systems were measured by the Knudsen effusion technique. Use of mass spectrometer-target collection apparatus to perform thermodynamic studies is discussed. The prominent sublimation reactions for these phases below 2000 K was shown to involve formation of elemental plutonium vapor. Thermodynamic properties determined in this study were correlated with corresponding values obtained from theoretical predictions and from previous measurements on analogous intermetallics. 

Vapor pressures were determined by using the Knudsen effusion technique. Effusion rates through and orifice contained in each sample cell were measured as a function of temperature by use of a mass spectrometer/target collection... 

The vaporization of Pr203, Nd203, Sm203, and EU2O3 at temperatures ranging from 1950° to 2350°K. has been studied by Panish 149, 150), who analyzed the species effusing from a Knudsen effusion cell with a time-of-flight mass spectrometer. 

Figure 1 shows a magnetic-type sector field mass spectrometer coupled with a Knudsen cell. The most important part of the instrument is the Knudsen cell. It can be heated up to temperatures above 2500 K. The temperatures are measured with an optical pyrometer or a thermocouple. There would be thermodynamic equilibrium in the Knudsen cell if it were closed. However, real Knudsen cells have an effusion orifice (typical diameter 0.1 to 1 mm) through which a small fraction of the molecules effuse without practically disturbing the equilibrium in the cell. A molecular beam representing the equilibrium vapor in... 

Fur] Knudsen cell effusion method with a mass spectrometer 1600°C xv < 0.3, xcr 0.2, liquid phase... 

Through careful consideration of the inner shape of the cell, effusion orifice, and surface area of the metallic sample, near-equilibrium conditions are attained between the condensed phases and the vapor phase while the orifice continuously samples the vapor by effusion. The distribution of the effusing vapor is defined by the shape of the orifice, and typically only a small solid angle of the distribution is selected to form a molecule beam that is analyzed with a mass spectrometer. A critical, but often overlooked, issue is correctly defining the thermodynamic system that is actually measured [7,8]. In a Knudsen cell, the boundary of the thermodynamic system is the inner surface of the cell, and thus the alloy sample, cell material, and vapor are all part of the equilibrium state being measured (alloy + cell material + vapor). All additional components and phases introduced by the container need to be included in the subsequent analysis and in the use of the measured data (the same is true for all experimental thermodynamic measurements made in the past and to be made in the future). In addition to components and phases, the temperature and chemical composition of the system need to be determined. Temperature is a particularly critical measurement in thermodynamics and will be discussed in detail. 

FIGURE 48.2 Knudsen effusion mass spectrometer magnetic sector instrument with key components indicated. 

The central KEMS equation can be derived now that the Knudsen cell vapor source and mass spectrometer have been described. This follows directly from the vapor flux in the molecular beam selected from the distribution of material effusing from the Knudsen cell (molecular beam flux equation) and the definition of the ionization cross section (Equation 48.18). However, in accordance with the aim of identifying factors that affect the measured ion intensity and that are unrelated to sample temperature and composition, it useful to rewrite Equation 48.18 in terms of the number of ions produced per second in the elementary volume dv in the region defined by the intersection of the molecular and electron beams, ni(E) [71,80] (this is prior to the formation of the ion beam) ... 

Stickney, M.J., Chandrasekhariah, M.S., Gingerich, K.A. (1988) Twin-chamber Knudsen-effusion-cell mass spectrometer for thermodynamic studies of alloys. High Temperatures-High Pressures, 20, 627-635. 

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