Kenneth L. Busch

In the ideal (off) world, we would assemble and operate our mass spectrometers in geosynchronous orbit. With ultralow pressure and infinite pumping capacity right outside the laboratory window, a few solar panels providing the power needed to operate the instrument (which is really minimal outside the power needed to operate vacuum pumps), and no terrestrial distractions, the only remaining impediment would be the wait that may be needed for appearance of a service engineer. The constrained transport chain for getting samples from Earth to the orbiting instrument might prove to be a blessing in disguise, serving to discourage submission of casual or repetitive samples, and focusing attention on properly prepared and validated samples. Should the return visit to Earth for the annual ASMS meeting become problematic, ASMS webcasts could prove useful.

Such a scenario may be premature but it is not unrealistic. The location of mass spectrometers off-planet is not limited to the Viking instrument resident on the surface of Mars. Mass spectrometers have sampled the composition of other planetary atmospheres, comets, and interplanetary space itself. The need to design such instruments to meet the constraints of space and power, and to ensure robustness, has informed the development of the newest generation of mobile mass spectrometers for the rest of us. As with all design that pushes to extremes, it is a clear understanding of the basics that catalyzes progress. Note that the basic studies in extraterrestrial mass spectrometry (MS) extend back to the 1950s and 1960s. The designers of such instruments also had unique approaches to creating and maintaining a vacuum in the instrument. Creating a vacuum may be as simple as opening a port to space in transit. Maintaining a vacuum during descent through a planetary atmosphere requires careful consideration of the pressures that may be encountered, the composition of the gases encountered (these were in fact what was to be measured), and gas conductances within the system and through its ports. Perhaps the design would consider a control parameter involving pressure measurements taken on-site, with the readings fed into a system that makes decisions on the fly. These complex topics deserve a more detailed exposition, which will appear in this column eventually. Until then, and to return to Earth orbit, interested readers might learn about the Wake Shield Facility (described at http://www.svec.uh.edu/wsfp.html), which takes advantage of the excellent vacuum available in the wake of the space shuttle.

Earlier columns in "Mass Spectrometry Forum" covered general topics of vacuum systems (as well as our sometimes confusing uses of the terms vacuum and pressure), and the operation of high vacuum pumps (1,2). Here, to keep things simple, we will call any pressure below 1 atmosphere (760 Torr) a vacuum. There are subsidiary terms of rough vacuum, low vacuum, high vacuum, and ultrahigh vacuum, with such terms corresponding, in order, to lower and lower pressures. The pressure in interstellar space, by the way, is about 10^-16 Torr, and the pressure within interplanetary space higher (depending upon where you are). These are isotropic gas pressures, and the fact that the interstellar pressure is so low is one reason why radiation pressure can be used to exert a force upon solar sails.

The focus of this column is the measurement of pressure in a mass spectrometer, located somewhere on the surface of planet Earth (±5 km) (3). The continued growth and diversification of MS should refocus our attention on the attainment of vacuum and the accurate measurement of pressures. At the heart of MS is the ability to create ions, to move them around, to differentiate them by their mass-to-charge ratios, and to detect them. For years, and certainly through the dominant era for electron ionization (EI) and chemical ionization (CI) sources, we thought of the MS instrument as under high vacuum from source through to the detector. We also came to know the vacuum pumping system as a high-cost, high-maintenance part of the instrument. Certainly, pumping systems evolved from the crude apparatus first used by Aston in his parabola mass spectrographs, but the consistent general model was a high-vacuum diffusion pump (or two) for the main system to achieve a high vacuum, with associated backing pumps (the rough pumps) that also could be plumbed into source interfaces for separators or direct insertion probes. Most pumps (including diffusion pumps and rotary vane backing pumps) transport gas molecules against a pressure gradient, so the ultimate exhaust for the backing pumps should be a hood vented to outside. The advent of reliable turbomolecular pumps did not shift the basic design of the pumping systems in our instruments, but added certain advantages of speed and pump placement. The basic goal still was to move the gas molecules from inside the system to outside of it.