Magnet mass analyzer

Traditionally thermal ionization mass spectrometry was the instrument of choice for the isotopic analysis of metals because thermal ionization produced an ion beam with a very small kinetic energy spread ( 0.5 eV). Therefore only a magnetic mass analyzer is needed to resolve one isotope from another. Moreover, ionization of unwanted material, such as atmospheric contaminates, hydrocarbons from pump oil, or production of doubly ionized particles is almost non existent, thus background counts are minimized and signal-to-noise ratio is maximized. 

Fig. 2. Mass spectrometer with photoionization 1—built-in hydrogen lamp 2—vacuum monochromator filled with hydrogen 3—LiF window 4—ionic source container 5—photoionization space with the accelerating grids 6—fluorescent layer for intensity calibration of the incident u.v. light 7—photomultiplier 8—magnetic mass analyzer 9—electron multiplier.

In the geometric separation method, ions having different m/e ratios are separated according to their geometric position at the collecting spot. The magnetic mass analyzer, the quadrupole mass analyzer, and the ion trap are based on these principles of operation. The magnetic mass spectrometer will not be discussed in this chapter as it has not been used for the detection of explosives. 

The technique of MSIB deposition allows deposition of single ion species by passing the ion beam through a magnetic mass analyzer for e/ m selection. Figure 15 shows the general layout of the MSIB technique used for the deposition of... 

Cone-carrying metal foil at its truncated apex. Foil has one or several leaks through which the gas and ions enter the pumping and electrode chamber. 9. Heater and thermocouple wells for temperature control of ion source. 10. Auxiliary electron gun for gas purity determinations. 11-19. Electrodes focusing ion beam into magnetic mass analyzer. Note in later versions of the apparatus the distance from the alpha source to the ion exit slit was shortened, which increases the effective intensity. 

The major components of the mass spectrometer are a gas-handling manifold that enables the gas to be transferred from the ovens to the ion source, the ion source that consists of an electron impact source, magnetic mass analyzer with gap field of 6500 G, four different EM detectors, and an ion pump to ensure low pressure for the EMs. The gas handling system also included a controlled leak valve that could obtain Martian gas and feed the gas directly into the mass spectrometer for analysis. All the tubes in the gas handling system were heated to 35 C to ensure that there is no condensation of water and other volatile vapors on the hardware. 

The cross section measurements were made with an apparatus described previously (1). The instrument consists of a primary mass spectrometer (PMST in tandem with a secondary mass spectrometer (SMS). The PMS is a 1 cm radius of curvature 2500 G permanent magnet mass analyzer. The SMS is a 60° magnetic sector instrument with an 8 in. radius of curvature. The detection of product ions was made by counting ions with a 17 stage electron multiplier. The gas pressure in the reaction cell was measured with an MKS Baratron differential pressure gauge. 

The oldest type of mass analyzers used, going back to the early 1920s, are magnetic mass analyzers. They consist of a curved flight tube located between the poles of an electromagnet, so that the field is perpendicular to the flight direction of the ions. 

By combining a magnetic-sector mass analyzer with an electrostatic analyzer (often termed a double-sector or double-focusing mass spectrometer), a significant improvement in resolution is realized. Such a double-focusing instrument can achieve a resolution of the order of 100,OOO.The double-focusing magnetic mass analyzer utilizes two independent sectors. In addition to the... 

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