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Ion mobility spectrometry rediscovered Security personnel and researchers alike are rediscovering the power of ion mobility spectrometry in the wake of the events of September 11, 2001, and the dawn of proteomics. Katie Cottingham
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xcept among a small number of intrepid researchers, ion mobility spectrometry (IMS) has received little respect in the analytical community. For years, ion mobility spectrometers were perceived as a niche instrument for the detection of explosives, narcotics, and chemical warfare agents by security and military forces. Experts say that the technique also wasn’t well understood, although IMS has existed since the 1970s. Only recently have researchers gained a better understanding of ion behavior at atmospheric pressure, which may be opening the door to a revolution of sorts. “We could be on the cusp of something big here. . . . We may be entering an era where we’re interested in more than just mass. We’re interested in shape, not just as a research curiosity, but as a routine part of chemistry and biochemistry,” says Gary Eiceman of New Mexico State University. Indeed, more chemists and biologists are taking another look at IMS because of its unique ability to separate ions on the basis of size, mass, and charge. One benefit of this technique is that it can help scientists distinguish between isomeric species—a particularly important feature for proteomics research. Flexibility, speed, ease of maintenance, and low detection limits are luring users to IMS. Results for volatile
compounds present at parts-per-billion or parts-per-trillion levels can be in hand within seconds, and company representatives say that all commercially
available IMS instruments are easy to maintain. Consumables are not required for some units. Many companies offer handheld devices, which can be easily
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transported into the field. Tables 1 and 2 list several IMS instruments. These lists are meant not to be comprehensive; they provide general information about the types of ion mobility spectrometers currently available. Where possible, information about one handheld and one benchtop instrument is provided for each company.
Buy it or build it? Demand for IMS instruments is high, particularly because of the increased vigilance after 9-11. Eiceman says there are now ~10,000 IMS units in airports making ~10 million measurements each year, and military units around the world are using more than 50,000 handheld devices.
resentatives. Occasionally, companies will send an instrument to a professor for evaluation, but they count few academic scientists among their customers at present. As Rachel Kohn of Smiths Detection explains, “A number of university groups that are doing IMS research basically put together their own instrumentation because they are trying to do something different, and they are looking for novel features that we may not have on a standard instrument.” Commercial instruments also have not yet caught up to the resolution capabilities of machines created by academic researchers, according to Herb Hill of Washington State University. Companies report resolving power val-
We may be entering an era where we’re interested in more than just mass. We’re interested in shape, not just as a research curiosity, but as a routine part of chemistry and biochemistry.
Although ion mobility spectrometers are best known for security applications, their use in other areas is on the rise. For example, IMS instruments are finding new homes in industrial analytical R&D and in pharmaceutical companies for cleaning validation. IMS devices are also used for stack monitoring and the detection of contaminants in industrial process streams. Ion mobility spectrometers are even aboard the International Space Station, where they monitor the air for volatile organic compounds. Most IMS instruments are currently designed to be “black boxes”, though a few machines can be programmed by the user to detect additional known substances by running standards. This makes ion mobility spectrometers very easy for non-chemists to use. But academic researchers are still not lining up in droves to purchase commercial machines, according to company rep436 A
ues of up to 60, but, Hill says, “There are several of us in the academic field that have produced instruments that have resolving powers of up to 150.” Thus, as scientists enter the field, they discover that they typically must build their own ion mobility spectrometers, which Eiceman says is “part of the joy and frustration” of this research.
Commercial IMS instruments IMS is often compared to time-of-flight MS, but it can also be thought of as gaseous electrophoresis. A sample is ionized, and the ions are gated into a drift tube at ambient pressure. While in the drift tube, the ions encounter an electric field, which pulls them toward a detector. The ions travel at different rates through a countercurrent drift gas, typically N2 or clean dry air, depending on their masses, sizes, and charges. Small ions have high mobility and reach the
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detector ahead of larger ions with lower mobility. The instrument operates in negative- or positive-ion mode. Nearly all of the commercially available IMS instruments handle gas samples, and a handful of them analyze solid or liquid samples, which are introduced into the machine via a filter or a swab. The samples are then thermally desorbed to obtain a gas-phase sample. Although several research groups use a variety of ionization sources in homebuilt IMS instruments, most off-theshelf instruments use 63Ni, which Eiceman says is costly in financial terms as well as time, bookkeeping, paperwork. “It makes it hard to use the technology,” he explains. “We’re all looking for a small, low-power ion source that replicates 63Ni with comparable gas-phase chemistry, without radioactivity.” Current alternative ionization sources for commercial instruments include tritium, laser, corona discharge, and UV photoionization. Drift tube lengths and diameters also vary among commercial IMS devices, and some companies closely guard these parameters. According to Hill, maximizing IMS performance by adjusting drift tube dimensions is a complex issue. A larger diameter and longer length typically increase resolution, but the signal decreases. “Thus, it is the old analytical trade-off. You can sacrifice sensitivity for increased resolution or sacrifice resolution for increased sensitivity,” says Hill. Resolution itself is a complicated parameter to pin down for IMS instruments, however. As Eiceman says, “Resolving power in ion mobility spectrometry is one of the great mysteries of life.” IMS separates ions not only by mass, but also by shape, so mass spectrometric definitions of resolution are not appropriate. For example, he points out that isomers—ions with the same masses but different shapes—can be separated with IMS, but using a mass spectrometric definition of resolving power (m/∆m) gives an answer of infinity. A commonly used definition of resolving power for IMS, according to both Hill and Eiceman, is the drift time divided by the peak width at half-height, which is reported by most companies in Tables 1 and 2.
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Table 1. Selected benchtop IMS instruments.1 Product
IMS 5000
UVIMS-MCC-Portable
Itemiser3
AirSentry
IONSCAN 400B
Company
Draeger Safety AG & Co., KgaA Revalstraße 1 23560 Lübeck Germany +49 451 882 0
G.A.S. TechnologiezentrumDortmund Joseph-von-Fraunhofer Str.13 D-44227 Dortmund Germany +49 231 9742 288
GE Ion Track 205 Lowell St. Wilmington, MA 01887 978-658-3767
Molecular Analytics, Inc. 14550 A York Rd. Sparks, MD 21152 410-472-7300
Smiths Detection 30 Technology Dr. Warren, NJ 07059 908-222-9100
URL
www.draeger.com
www.gas-dortmund.com www.geindustrial.com/ ge-interlogix/iontrack
www.ionpro.com
www.smithsdetection. com
Price (U.S.D.)
Depends on options
$22,500
$46,400, which includes on-site training
$18,000–47,000, depending on options
Not available
Sample phases handled
Gas
Gas
Gas, liquid, or solid
Gas
Liquid, solid
Resolving power (drift time/peak width at halfheight)
50
30–60
Not available
22.5 for 200-µs gating pulse
~44 for 200-µs gating pulse
Limit of detection
0.1–1000 ppb
Low ppm to low ppb, depending on substance
