A small mass spectrometer utilizing a miniature field ionization source is now undergoing development. It is designed for use in a variety of applications in which there are requirements for a lightweight, low-power-consumption instrument that can analyze the masses of a wide variety of molecules and ions. The device can operate without need for a high-vacuum, carrier-gas feed radioactive ionizing source, or thermal ionizer. This mass spectrometer can operate either in the natural vacuum of outer space or on Earth at any ambient pressure below 50 torr (below about 6.7 kPa) — a partial vacuum that can easily be reached by use of a small sampling pump. This mass spectrometer also has a large dynamic range — from singly charged small gas ions to deoxyribonucleic acid (DNA) fragments larger than 104 atomic mass units — with sensitivity adequate for detecting some molecules and ions at relative abundances of less than one part per billion.

A Field Ionizer and a Rotating-Field Mass Spectrometer are integrated into a single instrument that has a mass <1 kg and a power consumption <5 W.
This instrument (see figure) includes a field ionizer integrated with a rotating-field mass spectrometer (RFMS). The field ionizer effects ionization of a type characterized as “soft” in the art because it does not fragment molecules or initiate avalanche arcing. What makes the “soft” ionization mode possible is that the distance between the ionizing electrodes is less than mean free path for ions at the maximum anticipated operating pressure, so that the ionizer always operates on the non-breakdown side of the applicable Paschen curve (a standard plot of breakdown potential on the ordinate and pressure electrode separation on the abscissa).

The field ionizer in this instrument is fabricated by micromachining a submicron-thick membrane out of an electrically nonconductive substrate, coating the membrane on both sides to form electrodes, then micromachining small holes through the electrodes and membrane. Because of the submicron electrode separation, even a potential of only 1 V applied between the electrodes gives rise to an electric field with a strength of in excess of a megavolt per meter — strong enough to ionize any gas molecules passing through the holes.

An accelerator grid and an electrostatic deflector focus the ions from the field ionizer into the rotating-field cell of the RFMS. The potentials applied to the electrodes of the cell to generate the rotating electric field typically range from 1 to 13 V. The ions travel in well-defined helices within this cell, after which they are collected in a Faraday cup. The mass of most of the molecules reaching the Faraday cup decreases with increasing frequency of rotation of the electric field in the cell. Therefore, the frequency of rotation of the electric field is made to vary in order to scan through a desired range of ion masses: For example, lightweight gas molecules are scanned at frequencies in the megahertz range, while DNA and other large organic molecules are scanned at kilohertz frequencies.

The current of accelerated ions is attenuated by collisions between these ions and the much slower (thermal) background gas molecules. In a typical case of operation at 5 Torr, an initial ion-beam current of about 10 nA would be attenuated to about 40 pA. However, the instrument could still afford adequate sensitivity because the electric current of ions collected by the Faraday cup is read by use of an electrometer that can resolve a current of the order of a femtoampere. In certain cases of low vacuum (10–5 Torr), a channel electron multiplier (CEM) plate could also be utilized in a single ion detection mode.

This work was done by Frank Hartley and Steven Smith of Caltech for NASA’s Jet Propulsion Laboratory.

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:

Innovative Technology Assets Management

JPL

Mail Stop 202-233

4800 Oak Grove Drive

Pasadena, CA 91109-8099

(818) 354-2240

E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-30245, volume and number of this NASA Tech Briefs issue, and the page number.

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This Brief includes a Technical Support Package (TSP).
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Low-Pressure, Field-Ionizing Mass Spectrometer

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    Overview

    The document is a Technical Support Package for the Low-Pressure, Field-Ionizing Mass Spectrometer (NPO-30245), developed by NASA's Jet Propulsion Laboratory (JPL). It is part of NASA Tech Briefs, which disseminate information about aerospace-related technological advancements that have potential applications beyond their original context.

    The Low-Pressure, Field-Ionizing Mass Spectrometer represents a significant development in mass spectrometry technology, which is crucial for analyzing the composition of various materials, including gases and liquids. This technology is particularly relevant in fields such as space exploration, environmental monitoring, and materials science, where precise measurements of chemical compositions are essential.

    The document emphasizes the importance of compliance with U.S. export regulations, indicating that the information may contain proprietary data and is subject to export control laws. This highlights the sensitive nature of the technology and the need for responsible dissemination of information.

    Additionally, the Technical Support Package serves as a resource for individuals and organizations interested in the commercial applications of this technology. It encourages collaboration and innovation through the NASA Innovative Partnerships Program, which aims to foster partnerships between NASA and private entities to promote the use of NASA-developed technologies in commercial sectors.

    For further inquiries or detailed information regarding the research and technology related to the Low-Pressure, Field-Ionizing Mass Spectrometer, the document provides contact details for the Innovative Technology Assets Management team at JPL. This team can assist with additional resources and information, facilitating access to the advancements made in this area.

    Overall, the document serves as a comprehensive overview of the Low-Pressure, Field-Ionizing Mass Spectrometer, its potential applications, and the collaborative opportunities available through NASA's programs. It underscores NASA's commitment to sharing technological advancements that can benefit a wide range of industries and contribute to scientific progress.