Monolithic Electrostatic Sector for Miniature Mass Spectrometers

NASA's Jet Propulsion Laboratory, Pasadena, California

An improved miniature electrostatic sector has been designed for a miniature double-focusing mass spectrometer (see figure). Miniature mass spectrometers are essential components of high-performance, miniature, low-power instruments that are being developed for use in analyzing chemical compositions of small amounts of substances (e.g., toxic chemicals in the environment) in scientific laboratories, in industrial settings, and in the field.

Precision is critical in the design and fabrication of an electrostatic sector for a mass spectrometer. Especially notable is a very tight tolerance on the radii of the sector rails (the concentric cylindrical electrode surfaces); for this particular electrostatic sector, the nominal inner and outer radii are 29.21 and 30.48 mm, respectively, and the difference between these radii must not vary by more than 10 ?m over the entire arc. In an older design, the two rails were fabricated separately, making it necessary to resort to an elaborate and laborious procedure to align them with each other to satisfy the radius requirement as well as to obtain the required precise alignment between these rails and the other spectrometer components. The older two-piece design also made it difficult to maintain alignment during transport. These difficulties of establishing and maintaining alignment added significantly to the cost of fabrication and limited use to applications in which ruggedness was not needed.

The improved electrostatic sector is inherently rugged and dimensionally stable because it is made from a single piece of a machinable ceramic. The electrodes are made by plating the machined concentric electrode surfaces with nickel to a thickness of 6 µm. Reference surfaces for alignment are machined onto the single piece of ceramic; these surfaces are designed to be positioned on alignment ridges on a mounting plate that is also a precise component of the mass spectrometer. Two screws hold the unitary electrostatic sector in place with the reference surfaces pressing against the alignment ridges. Thus, provided that the concentric electrode surfaces and the reference surfaces are machined within tolerances, alignment of the electrodes with the other components of the mass spectrometer is assured. As a result, the cost of fabrication is no more than about 1/40 of that of the older electrostatic-sector design.

A Mass Spectrometer of Mattauch-Herzog Geometry includes an electrostatic sector plus a magnetic sector that ends in an array detector on a focal plane. The electrostatic sector is made from a single piece of machinable ceramic and has a mass of only 30 g.

This work was done by Mahadeva P. Sinha of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Physical Sciences category. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to