The Vehicle Cabin Atmosphere Monitor (VCAM) is a miniature gas chromatograph-mass spectrometer (GCMS) that is scheduled for a one-year test run in the International Space Station (ISS) beginning in 2008. Later versions are expected to be used to monitor the air in the Orion crew exploration vehicle and in habitats on the Moon and Mars. With each successive environment in which the instrument will be used, it will be designed to detect a wider assortment of chemicals.

Above: The development unit for VCAM's preconcentrator/gas-chromatograph assembly. The base is 30.5 cm by 10.2 cm (about 12 inches by 4 inches). Left: The development unit for VCAM's Paul ion trap. DH = detector housing, H = heater bulb, IC = ionizer connections, PT = Paul trap, VF = vacuum flange. The vacuum flange is less than 15 cm (about 6 inches) in diameter.

During the ISS trial, it will operate autonomously, checking the air routinely once a day and reporting the results to the ground crew via the ISS EXPRESS Rack. It can also be activated by the ground crew at any time as events warrant. If there is a chemical spill, for example, the VCAM can be used to determine the hazard level in the cabin and to ascertain when the spill has been sufficiently cleaned up.

VCAM consists of three parts: a small preconcentrator, a gas-chromatograph (GC) column (wrapped around the copper-colored cylinder in the above photo), and a type of mass spectrometer (MS) called a radio-frequency (rf) ion trap. A turbo pump with a backing pump draws the gas through the GC and MS.

Preconcentrator

Cabin air is swept over the preconcentrator for several minutes. The preconcentrator behaves like activated charcoal and adsorbs all the organics from the air it contacts. The air flow is then shut off and the preconcentrator is heated in a low flow of helium, a process which desorbs the organic material and allows it to be fed into the GC column. This system increases the sensitivity of the instrument by about a factor of 10.

Gas Chromatograph

If the complex mixture of gases that constitute the cabin atmosphere were presented to the mass spectrometer all at once, the instrument would be overwhelmed and unable to identify the individual gases. So a gas chromatograph is needed to sort the gases and present them one at a time (or sometimes two or three at a time) to the MS.

The GC consists of a column whose walls are lined with an "active phase" material. As gases pass through the column, they stick (adsorb) and unstick (desorb) to the column walls several hundred times. The amount of time a gas sample spends in the GC column (its "elution time") can total up to about 20 minutes, depending on the temperature and the particular active-phase material lining the column walls.

Each class of chemical (such as alcohols, aldehydes, ketones, aromatics, etc.) tends to adsorb and desorb at different rates, so each class spends a characteristic amount of elution time in the column before exiting. Thus the gases, all mixed together when they entered the GC, emerge one class at a time.

The VCAM's GC column is a long, narrow tube that is wrapped around the circumference of a spool. If stretched out, the tube would measure 10 meters (almost 33 feet) long. Yet its outside diameter is only 400 microns (about 0.016 inch), and the hole in its center, through which the gases travel, has a diameter of only 100 microns (about 0.004 inch).

Once the gases are separated, the MS is able to provide positive identification of each one it encounters.

Mass spectrometer

Schematic diagram of the Paul ion trap mass spectrometer and electronics. The electron filament for the ionizing electrons (e) is denoted by F; the pulser which switches the filament bias voltage is MS1; that for switching the high voltage to the ion detector is MS2; the electron lens elements are L; the trap end caps are E; the trap ring is R; and the detector is D. The radiofrequency trapping potential is indicated as M.

Like the TGA, the VCAM's mass spectrometer identifies chemicals from their "fractionation" or "fragmentation" patterns. Here is how it works:

• The output of the gas chromatograph (neutral analyte) fills the trap's central area (indicated on the diagram by a cloud of dots).

• An electron beam enters the same area (through the opening to the left of the gas in the diagram) and ionizes the gases--that is, the electrons break apart the electrically neutral gas molecules and turn the fragments into atoms carrying a positive charge. All of the ions below a certain energy level are trapped inside a dynamic electric field, which is generated by the radio frequency (rf) voltage M.

• The amplitude of the radio signal (i.e., the rf voltage) is ramped up, exciting the ions and increasing their energy. As their energy levels increase, the ions escape the field in order of their masses (through the opening to the right of the gas in the diagram). The lighter ions are ejected first, then heavier and heavier ions are ejected until all escape. The ejected ions strike a detector, which records the number of ions of each level of mass. Based on this fractionation information along with the time each class of chemicals spent in the GC (their elution time), the instrument's computer identifies the ions and determines the source molecules and their concentration.

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 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 suffers. Resolution also depends on the spectral purity and stability of the rf trapping voltage.

Goals

VCAM's sensitivity is currently at about 15,000 parts per trillion (ppt). Its developers hope to achieve about 500 ppt. In addition to atmospheric gases, AEMC is working on methods to enable the instrument to analyze chemicals in drinking water.

AEMC seeks 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 gas chromatograph and all electronics.