The need to conserve space on board spacecraft has inspired a miniaturized mass spectrometer that will be used to protect astronauts from hazardous gases and to make chemical detectors more portable on Earth. A crucial piece of spacecraft safety equipment shrank from a refrigerator-sized rack of electronics to the volume of a shoebox.

One recent engineering triumph has been to shrink a crucial piece of spacecraft safety equipment, a mass spectrometer, from a refrigerator-sized rack of electronics to the volume of a shoebox. In so doing, Principal Investigator Ara Chutjian, leader of the Atomic and Molecular Collisions Group at NASA's Jet Propulsion Laboratory (JPL, Pasadena, California), and his team have developed a high-performance quadrupole mass spectrometer that is the smallest and lightest in the world. Mass spectrometers are essential cargo on crewed space missions for closely monitoring the environment both inside and outside spacecraft for toxins and leaking fluids that could threaten the astronauts' safety.

Chutjian's quadrupole mass spectrometer is the heart of an instrument called the Trace Gas Analyzer (TGA). The TGA measures 10 x 15 x 20 centimeters (4 x 6 x 8 inches), weighs about 2 kilograms (5 pounds), and consumes just 15 watts of power — small enough to be placed on an astronaut's chestpack during spacewalks to monitor the outside environment. Chutjian and his team are working on a version for monitoring the more complex cabin atmosphere as well.

Chutjian's miniature mass spectrometer system was inspired by his efforts to meet former NASA Administrator Dan Goldin's requirement of "faster, better, cheaper. "Says Chutjian, his team "added the corollary 'smaller.'"

Their reason for smaller was twofold. "There's always a call to make flight systems [spacecraft] bigger — bigger cabins and more modules. But we argue that miniaturizing components is another way to add space without adding additional modules, "he says. In addition, "on missions to Mars or the outer planets — either unmanned probes or long-duration crewed flight — mass, volume, and power are all at a premium. You can't carry too much of anything."

Around 1992, Chutjian's team helped put together an ad hoc panel at JPL to list instruments that might be miniaturized. "Since we were all interested in charged particle optics and mass spectrometry, "Chutjian recalled, "we decided that mass spectrometers would be the best candidates."

In fact, there are three mass spectrometers on the International Space Station (ISS). "They are quite large systems, taking up several electronics racks of equipment, "Chutjian noted. "We think all three could be replaced by a single miniature system that would require maybe only a third of an electronics rack."

A Powerful Detector

A mass spectrometer is a powerful tool for detecting and identifying trace amounts of chemicals. Mass spectrometers are sensitive and rugged, making them ideal for use in harsh environments.

Above: The Trace Gas Analyzer, designed to check for ammonia leaks outside the International Space Station and for hydrazine on astronaut space suits or within station airlocks, is a marvel of miniaturization. The entire instrument is about the size of a shoebox and can fit on the astronaut's chestpack for environmental monitoring during extravehicular activities. Credit: NASA. Source: OBPR Space Research Newsletter, March 2003 (Vol. 2, No. 2).

A mass spectrometer functions by drawing in a small sample of the external atmosphere, ionizing (stripping the outer electrons from and thereby giving a charge to) the gaseous components with an electron beam, and then analyzing the mass-to-charge ratio of the ionic species by a radio frequency filtering action. In a quadrupole mass spectrometer, the sample is ionized by a voltage applied to four parallel rods, or poles. By adjusting the ratio of DC voltage to radio frequency amplitude within the instrument, ions of a specific mass-to-charge ratio can be separated from the other charged species. The resulting mass spectrum, which depicts the mass-to-charge ratio and relative abundance of all species of the ions in the sample, allows scientists to determine the identity of compounds in the sample.

Quadrupole mass spectrometers are relatively rugged and can be used for continuous sampling. The challenge for miniaturizing the instrument was maintaining sufficient sensitivity to detect the spacecraft contaminants at their critical levels — a challenge the TGA has met.

Chutjian's TGA was developed for testing the environment outside spacecraft, where there are just a few gases. "We've calibrated [the TGA] to detect seven gases. All the astronaut has to do is read the histogram bars on the instrument display. "The instrument displays a running histogram; the astronaut looks at the chart and notes when a maximum signal is recorded and the time of that spike so that the leak can be detected. The type of gas being sampled can be changed by using switches on the front panel.

NASA is specifically concerned about monitoring for ammonia and hydrazine outside the ISS. Ammonia is used to cool the skin of the space station to maintain a constant temperature within the craft. (The station's exterior can reach temperatures as high as 121 degreesC or 250 degreesF and as low as -157 degrees C or -250 degrees F, depending on exposure to the Sun.) While the station is under construction, with additional modules being added, astronauts must re-route the ammonia lines, which are coupled to one another by quick-connect fittings. Leaks are often found at these couplings, and leaking ammonia can cause the fittings to freeze shut. Astronauts need a way to check that newly connected fittings are not leaking ammonia and thus putting the station's temperature control at risk.

Hydrazine, a component of thruster propellant that is toxic to humans, is always present outside the station. It can adsorb (adhere) to spacesuits and be carried back inside the station after a spacewalk. If the TGA alerts astronauts that it has detected hydrazine outside the ISS or within its airlock, astronauts can take steps to remove it before reentering the station.

The TGA was flown to the ISS aboard Space Shuttle Atlantis in February 2001 and remained aboard the station for 14 months before being returned to Earth. It will be flown to the ISS again when additional space station construction requires quick-connect fittings on ammonia lines to be opened and reconnected.

Wanted: Clean Air

Inside the space station (or any other spacecraft), the air must be continuously monitored for 45 different chemicals, including such toxic gases as carbon monoxide, benzene, and formaldehyde. For this job, Chutjian's team has developed a miniature gas chromatograph to be coupled to the TGA.

A gas chromatograph uses a gas carrier to spread out a sample along a column of special material, designed to separate the sample into various components on the basis of size and chemical reactivity. Each chemical class — such as the alcohols, aromatic compounds, or ketones — exits the column at a slightly different time and directly enters the mass spectrometer, which then identifies specific compounds. For a complex mixture such as spacecraft cabin air, the gas chromatograph's preseparation allows the mass spectrometer to work with a smaller number of molecules at any one time, simplifying the task of identifying specific chemicals.

Chutjian and his team are working to determine the limits of sensitivity of their gas chromatograph–mass spectrometer system and whether it can meet and beat the station's maximum allowable concentration limits by a factor of 10, meaning that the instrument must be able to detect contaminants at concentrations a tenth the maximum allowable in the cabin atmosphere. If the new system can do this, Chutjian believes that it will be a suitable second-generation replacement for the larger systems currently in use.

Ever Smaller

Chutjian has further plans to refine the TGA. After building the mass spectrometer from commercial off-the-shelf parts, the team discovered that the electronics package was about four times bigger than the detector system itself. Chutjian says, "Now the challenge is to move away from continued miniaturization of the mass spectrometer sensor itself, which could compromise instrument sensitivity and resolution, and instead concentrate on reducing the size of the electronics. "The team's first step is to get the power supplies onto a chip. Chutjian estimates that reducing the size of the electronics, which might take two years, will cut the size of the instrument by a factor of three and power consumption by a factor of two.

In Space and On Earth

Miniaturized mass spectrometers could have many uses on Earth as well as in space. Portable instruments could be useful in such applications as environmental field analysis, geological age dating (carbon dating), power plant security (detection of explosives), antiterrorism activities (detection of nerve agents, blister agents, and weapons of mass destruction), and environmental safety (detection of polychlorinated biphenyl [PCB] and perfluorocarbon tracer compounds and, potentially, pesticides).

"At this point, we consider the mass spectrometer to be like film in a camera — to a certain extent, it's simply the detector, "says Chutjian. "The important area of development now is the 'front end' — not only the gas chromatography, but also ionization schemes such as negative ionization, field ionization, nanospray, and zero-energy electron attachment. "He believes that improvements in these areas could lead to factors-of-10 improvements in instrument sensitivity and efficiency.

With new front ends for miniaturized mass spectrometers, Chutjian sees additional uses in space, not only for detection of compounds in air but also in spacecraft drinking water (detecting dissolved volatile organic and inorganic compounds) and, ultimately, for detection of harmful bacteria (at the single-cell level) on surfaces as well.

Aside from human safety issues, NASA may also find uses for mass spectrometers in planetary exploration. NASA will be looking for analysis systems that are low mass, volume, and power for missions to distant planets. Chutjian and his team are hoping to apply their work for human spaceflight to the robotic program — using these miniaturized instruments on robotic probes to study Mars and Venus, as well as for a Europa flyby.

Web Links

Advanced Human Support Technologies -- The OBPR program that sponsored this work reports that Chutjian and his team have published several papers on their work: Chutjian, A., et al. (2000). A miniature quadrupole mass spectrometer array and GC for spaceflight: astronaut EVA and cabin-air monitoring. SAE Technical Paper Series 2000-01-2300. Orient, O.J., and Chutjian, A. (2002). A compact, high-resolution Paul ion trap mass spectrometer with electron-impact ionization. Rev. Sci. Instr. 73, 2157. Orient, O.J., Chutjian, A., and Garkanian, V. (1997). Miniature, high-resolution quadrupole mass spectrometer array. Rev. Sci. Instr., 68, 1393.