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Faster, More Accurate Detection Through Differential Mobility Spectroscopy
By Wes Davis
Advanced ion technology ushers in a new generation of small, inexpensive sensors that in seconds identify chemicals at parts per billion
Differential Mobility Spectroscopy (DMS) is emerging as a leading technology for the on-site detection of trace quantities of chemicals in air, water, or other fluids. It is used to identify key chemical markers in process industries, to monitor indoor air quality — both on earth and in space — in fenceline applications, and, by military and law enforcement agencies to detect chemical warfare agents, explosives, narcotics, etc. This article will discuss the principal characteristics and operation of DMS, how it differs from the older, better-known Ion Mobility Spectroscopy (IMS), and some of the natural applications of DMS.
Ionization Technology Sensors
Both DMS and IMS rely upon ionization chemistry to detect the presence of a given chemical species (Figure 1). Ionization is the process of converting an atom or molecule into its component ions; each chemical species produces a unique set of ions. Ionization sensor sources may be either radioactive—generally nickel (63Ni) or Americium (241Am)—or nonradioactive, e.g., UV, plasma, or carbon nanotube technology. Once the sample is ionized, the objective of both the technologies is to separate and differentiate one ion species from another. After separation, the sensor detects and quantifies one species from another. Finally, the detection and quantification are presented as actionable data.
IMS Basis of Operation
IMS identifies and detects chemicals based on time-offlight (TOF) principles (Figure 2). Essentially, IMS measures the time it takes a certain chemical ion to move through a uniform Rf electric field. Samples are ionized and the sample's ions are "lined up" using a shutter mechanism and then floated into a drift tube. Because it lines up the ions, IMS uses only a very small percentage (~1%) of the sample.
The drift tube contains a homogenous electric field and this field moves the ions down the drift tube. The homogenous electric field duty cycle can be set to more than hundreds of volts per centimeter, depending on the class of compounds targeted. Once set, the field does not change. The ions interact with neutral molecules within the drift tube, resulting in a TOF dependent on ion mass, size, and morphology. Once a target's TOF is established, the identification is based on that known TOF.
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Early IMS systems generally operated in either a positive or negative ion mode. In contrast, recent IMS developments use oscillating negative and positive polarities in the drift tube to capture both negative and positive ion information through sequential operation. This means that a compound's negative and positive ions are determined in different sequential sample events. Quantification is based on the TOF response of an electrometer or Faraday plate.
The strength of IMS is its ability to quickly separate the ions of one chemical species from those of another, typically taking 3—15 ms per measurement. This attribute, coupled with relatively high sensitivity and relatively small size (4—5 in. long), has made IMS a natural fit for detection of explosives, narcotics, and chemical warfare agents, applications in which the overwhelming need was speed of analysis. When time is of the essence, one can't afford to wait 15 minutes for results from a gas chromatograph or deal with a high rate of false positives with less sensitive electrochemical cells. However, because IMS uses TOF, increased selectivity requires a longer drift tube. Longer drift tubes require more power and increase the form factor of resulting devices.
DMS Basis of Operation
DMS identifies and detects chemicals based on a chemical species' ion mobility in low and high electric fields (Figure 3). Samples are ionized and then flowed continuously via a carrier gas, such as air, into the detector area with its parallel plates spaced 0.5 mm apart. Once in the detector area, the ions experience a uniform oscillating asymmetric radio frequency electric field (Rf) which is typically 1 MHz and ranges from 500—1500 V. As applied, the Rf causes a perpendicular motion of the ions, resulting in a zigzag motion. Each ion species will exhibit discrete mobility characteristics.
To make the DMS sensor tunable, a perpendicular DC tuning field, known as the compensation voltage (Cv), is applied. This field is superimposed on the oscillating asymmetrical Rf field and keeps the ions of interest centered between the parallel plates and is detectable simultaneously by both negative and positive electrometers. The electric field conditions required to permit a particular ion to pass though the filter to the detector are specific to each ion species.
The DMS device can be operated in several modes. When functioning as a programmable chemical filter, the compensation voltage is fixed such that only one particular ion species is permitted to reach the detector. The charges received at the detector can be integrated for a selected period of time, improving the SNR and enabling significantly higher sensitivities. Alternatively, when operating in spectrometer mode, the compensation voltage can be scanned across a number of compensation voltages to allow various ions of interest to pass to the detectors.
The resident time for ions in the ion filter region is typically 1—2 ms with a transport gas flow rate of 300 ml/min. This rapid flow rate, coupled with the heating of the sensor (80°C—120°C), minimizes the risk of build-up of any material between the plates. chip. This offers several advantages over traditional IMS:
Sionex Corp. is commercializing DMS technology with its microDMx chip. This offers several advantages over traditional IMS:
- Greater sensitivity through continuous sampling. IMS uses a shutter to enable timing of motion, which results in 99% of the sample being discarded and, thus, lower sensitivity. Typically, DMS is about 10—100 times more sensitive than IMS, detecting in the parts-per-billion to parts-per-trillion range.
- More data for enhanced chemical identification. DMS can simultaneously detect both positive and negative ions, whereas IMS can only detect one or the other. When combined with a range of Rf and Cv combinations that can be switched very rapidly, DMS results in a more data-rich environment and eliminates overlapping peaks, reducing both false positives and false negatives.
- Smaller size. With an overall sensor length of about 1 in. (Figure 4), the microDMx chip is much smaller than IMS implementations. Since IMS relies on differences in TOF for its operation, reducing the size of the drift tube (and the distance of the flight) reduces its resolution.
- Lower cost. DMS hardware is microfabricated, using methods that are capable of mass production. In contrast, IMS fabrication often involves significant manual assembly, adding to the device's cost and complexity.
The microDMx sensor has several features that make it an excellent sensor.
- It is quantitative and has extremely sensitive detection limits, in the parts-per-trillion range;
- It is highly selective since each chemical or group of chemicals has a unique signature in the microDMx spectra due to different chemicals having their own unique differential ion mobility;
- Additionally, the microDMx can simultaneously detect chemical ions in both the positive and negative ion ranges thereby improving its selectivity.
microDMx: Both a Detector and a Filter
The microDMx device can be used in several modes. In the first mode, it functions as a sensor capable of detecting at very low trace levels. One of the advantages of its use as a sensor is that it can be coupled with a number of different "front ends" to enhance even more its sensitivity and selectivity, such as:
- Pre-concentrator/Gas Chromatography
- Pyrolysis/Gas Chromatography The second mode is operating the microDMx as an ion filter. In this case, the faraday detectors are removed so that the the microDMx pre-filters targeted chemical species to allow only those specific ions of interest to pass through for detection in a subsequent device. The benefit of this approach is that many commercially available detectors are enhanced by pre-filtering with microDMx prior to entry into these alternative detectors.
- microDMx/IMS2 — microDMx filters targeted ions to eliminate chemical species of no interest and then allows only the ions of interest to enter into two Ion Mobility Spectrometers to enable simultaneous bipolar ion detection from a common ionization source.
- microDMx/Mass Spectrometer — microDMx filters only targeted ions so that the Mass Spectrometer's signal-to-noise ratio is enhanced due to ion filtering or eliminating unwanted ions.
About the Company
Sionex Corporation makes sensor hardware and software products that it sells to original equipment manufacturers (OEMs) building systems for threat detection (the military and homeland security), medical diagnostics, process control and monitoring markets. The products are based on Sionex's patented microDMx ™ technology, which offers rapid identification of chemicals at minute (parts per trillion) levels in a very small, highly portable form factor. Sionex products are designed to be quickly and easily integrated by OEMs into end-user systems.
Wes Davis is President & CEO of Sionex. Wes joined Sionex in 2004 wi th 25 years of experience leading high technology companies, both public and privately held. Prior to Sionex, Wes was the CEO of Radiant Images, an early stage company with advanced microdisplay technology, where he successfully negotiated financing as well as a strategic partnership with the leader in their targeted market. While at MicroTouch, a manufacturer of touchscreens, and Lasertron, a manufacturer of lasers for the telecommunications market, Wes initiated the sale of both companies generating $275 million in shareholder value. Earlier in his career, he was with Autographix and General Scanning and was a partner with the strategy consulting firm, Braxton Assoc iates. Wes holds a BSE in engineering from Princeton University and an MBA from Harvard.
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