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In February, 2001, the Apollo Space Shuttle carried the world's smallest high-performance mass spectrometer to the International Space Station (ISS), where it stands ready to help astronauts detect and identify external leaks.

Today, JPL is developing even smaller mass spectrometers to monitor the air inside the space station and any future spacecraft that may travel to Mars or other extraterrestrial destinations.

JPL's current Quadrupole Mass Spectrometer array (narrow cylinder) atop its detector (wide cylindrical base). The entire unit is about four inches high.

A mass spectrometer is a device that identifies chemicals by their molecular weight (approximately the combined number of a molecule's protons and neutrons). A typical mass spectrometer is about the size of a four or five-drawer filing cabinet, much too big and heavy for the confines of a spacecraft. So NASA has made an ongoing effort to reduce the size of this valuable instrument, as well as its power requirements.

The Galileo mission to Jupiter and the Cassini mission to Saturn each carry a mass spectrometer about the size of a desktop personal computer. They weigh in the neighborhood of 22 pounds apiece and consume about 25 watts of power.

The instrument aboard the ISS has been reduced to the size of a shoebox. (The actual mass spectrometer is only two inches long - the rest of the volume is taken up by the electronics, battery, ion pumps, and display.) The entire device weighs around five pounds, and uses 15 watts not counting the display. Efforts to miniaturize further will depend on improvements in battery and miniature electronics technology.

How it works: the Quadrupole Mass Filter

JPL is working on versions of the two most popular kinds of mass spectrometer: the quadrupole mass filter, and the radiofrequency (rf) ion trap. The quadrupole type is currently aboard the ISS and the planetary probes, and works like this:

1. A sample of the gas to be analyzed is drawn into the system. (All mass spectrometers analyze gases. If the substance to be analyzed isn't already a gas, it must be converted to a gas - usually by heating - before going through the rest of the procedure.)

2. The gas is bombarded with electrons, which break the electrically neutral molecules apart and turn the fragments into positive ions.

3. The ions are guided into an area bordered by four parallel rods. The rods carry a varying, radiofrequency electrical potential (voltage) which creates a dynamic electric field that affects the paths of the ions. Ions of the correct charge-to-mass ratio are guided down the axis of the rods and strike the detector at the end, generating an electric current. All other ions - those whose masses are too high or too low relative to their charge - are deflected away and strike the rod surfaces or the exit aperture.
   

4. The device's computer, knowing the precise charge and mass of the ions that reached the detector, can identify the fragments and calculate which kind of molecule they came from, thereby identifying the chemical. Then the electric field generated by the rods is changed, and ions of a different charge-to-mass ratio are guided to the detector. Eventually (after less than a sixth of a second) ions of all charge-to-mass ratios in the gas sample can be covered, and all the of corresponding chemicals identified.

The Ion Trap

Miniature Paul Ion Trap

The ion trap mass spectrometer operates on a similar set of principles. But instead of the quadrupole filter's practice of discarding all but one molecule fragment during each test procedure, the ion trap holds all the ions in the sample, then releases them one type at a time to strike the detector and be identified. This enables it to identify multiple substances in each sample.

The ion trap as developed at JPL has higher resolution and sensitivity than the quadrupole filter. Its electronics are simpler, and it consumes only 8 watts of power instead of the quadrupole's 15 watts. However, having been in use longer, the quadrupole is more familiar to many scientists. It also has the virtue of having proven itself in space.

Gas Chromatograph

If you want to identify each substance in a complex mixture of, say, 30 gases, the job will be easier if some device can separate the components of the mixture before you begin your mass-spectral analysis. That's what a gas chromatograph does for a mass spectrometer.

To picture how it works, imagine a large group of children walking down the street. Some are wearing red shirts, others wear blue shirts, and the remainder are dressed in green shirts. They are all mixed together in no particular order.

Then the group enters a toy store. Suppose that for some reason, kids with red shirts tend to spend five minutes in a toy store, while those with blue shirts spend 10 minutes, and those with green shirts spend 12 minutes. If you stand at the store's exit, you'll see all the red-shirted kids file out after five minutes, all the blue-shirted kids after 10 minutes, and all the green-shirted kids after 12 minutes. What began as a disorganized mixture of children is now sorted according to shirt color.

Similarly, when a mixture of gases passes through a chromatograph, they encounter a material to which each class of chemical (such as alcohols, aldehydes, ketones, aromatics, etc.) tends to stick (adsorb) and unstick (desorb) at different rates. The gases, all mixed together when they entered the device, emerge one class at a time.

In typical NASA scenarios, each emerging class is likely to contain just one to four different kinds of molecules (compared to the 30 in our hypothetical original mixture), a much more manageable number for the mass spectrometer to deal with. This leads to an uncomplicated mass spectrum and an accurate identification of the chemicals present in each class.

For more on chromatographs, see Lab-on-a-Chip.

Making it Small

The trade-off in shrinking a quadrupole mass spectrometer can be a loss of sensitivity. While a commercial quadrupole system uses rods that are 20 cm long and 2.0 cm in diameter, JPL's version uses rods that are 2.5 cm long and 0.2 cm in diameter. Smaller rods require smaller apertures - the openings through which the gas enters the rod chamber - to keep the positive ions suitably confined and focused along the axis of the rods. That means, unfortunately, that any given substance must be in relatively higher concentrations for the smaller system to detect them.

JPL gets around this problem by using not four but 16 rods in a precise geometry. An array of four rows, each containing four rods, produces nine paths (quadrupolar-potential areas) through which the ions can travel. It's the equivalent of having nine tiny mass spectrometers instead of one. If the electrical potentials in all nine are configured the same way, the system is nine times more sensitive - almost recovering all the sensitivity lost to miniaturization.

Alternatively, it is also possible to arrange electrical potentials on the rods in such a way that different sections of the array would transmit different mass-to-charge ratios, as if there were several mass spectrometers, each looking for different ions at the same time.

While this array technique has been known for a long time in theory, JPL developed an innovative method of constructing the rods with the required accuracy and holding them to the needed precise tolerance.

Reducing the size of the quadrupolar system puts greater requirements on mechanical alignment, and requires higher rf frequencies on the rods. Generally speaking, if one makes the rods 10 times shorter, the rf frequency needs to be 10 times higher. So JPL's miniature system runs at 10 MHz, while its larger commercial cousins operate at about one MHz.

The ion trap mass spectrometer designed and tested at JPL is about the size of a tennis ball. The ions are formed directly inside the trap, so the trap does not suffer as great a loss in sensitivity as the quadrupole when miniaturized. However, as the trap size gets smaller, the number of ions it can hold decreases because the positively-charged ions repel each other and resist being confined together in too high a density. This results in decreased sensitivity and dynamic range.

An ion trap's mass resolution depends on its hyperbolic electrodes, which need to be machined to very precise tolerances. If these tolerances are not maintained in smaller traps, resolution will suffer.

Goals

AEMC's goals are to reduce the volume, mass, and power requirements of its mass spectrometers to half their present amounts or even less through further miniaturization of the electronics.

It hopes to increase sensitivity to about 100 parts-per-trillion or better through a combination of better spectrometer designs (using the Paul ion trap) and better front-end interfaces (a preconcentrator for the gas chromatograph).

AEMC is also working on methods to enable its mass spectrometer to analyze chemicals in drinking water, as well as in the air.

Other uses

In addition to monitoring spacecraft air and water, the miniature mass spectrometer will add to NASA's arsenal of instruments for exploring the atmospheres of Venus, Jupiter, Saturn, Titan and other bodies. And it could sample molecules in solid material drilled from the surfaces of such bodies as Mars, Europa, or an asteroid.

A miniature mass spectrometer could also be of great use on Earth - to detect explosives, nerve agents, toxic chemicals, PCBs, and CFCs at airports, harbors, embassies, and contamination sites (where it would fit, for example, into holes drilled into the earth to monitor contamination of soil or ground water). It could also be of great use in food and chemical-processing plants, and on each floor of a building to test for a range of target chemicals.