• By providing a plethora of hyphenated solutions different applications with different requirements on selectivity and sensitivity are being addressed. Using different combinations of the analysis stages, IMS, DMS and QAMS, along with various modes of operation of each exploit the trade-offs between selectivity, sensitivity, and speed. For instance, for explosives detection, the interface of all three stages (patent pending) is designed to provide up to three separate orthogonal detection methods (mobility, charge, and mass) in a single trace explosives detector that can uniquely achieve extremely low false alarm rates with high sensitivity, selectivity, and specificity of detection and deterministic identification of target explosives.

• While currently deployed IMS-based trace detectors are simple, low cost, and operate at atmospheric pressure they exhibit major shortcomings which include, high false alarm rates due to limited chemical specificity, low sensitivity (high rate of false negatives), limited and fixed (predetermined) range of detectable explosives (threat materials), and a need to frequently calibrate the detectors by running standards in the presence of various backgrounds. Because of its powerful analytical capability, a mass spectrometer (MS) avoids most of the problems associated with IMS. The advantages offered by MS-based systems include lower alarm threshold while maintaining low false alarm rates by greater chemical specificity (a factor of 100 better using modest resolution MS).  The uniqueness of mass spectrometry lies in its chemical specificity as it directly measures a fundamental property of the target molecule—its molecular weight—and thus provides a highly specific means of identifying the molecule. Furthermore, MS-based systems provide wider and automatically reconfigurable ranges of detectable chemical threats including explosives, chemical and biological warfare agents.

• IMI’s quadrupole array mass spectrometers offer additional advantages. Their small size and low manufacturing cost make them suitable for large volume field deployment. Their ability to operate at much higher ambient pressures enables them to be packaged in miniature manifolds evacuated by small vacuum pumps.  Such devices have been widely used as residual gas analyzer systems for nearly two decades. Their performance has been recently enhanced under partial sponsorship from the US Department of Homeland Securit

• The QAMS is the core enabling technology that meets the requirements for low cost miniature sensor for fast and unambiguous detection to confirm the presence of explosives such as TATP, RDX, and ANFO, chemical warfare agents, narcotics, and toxic industrial chemicals.  The QAMS is a miniature 4 x 4 array of 1‑millimeter diameter poles that provide nine parallel quadrupoles.  Using an inexpensive manufacturing process based on glass-to-metal seal technology, the 16 poles are aligned and are secured in a glass chassis, which also serves as the vacuum seal.  The length of the mass filter is 30 millimeters, which enables the filter to operate linearly at pressures as high as 1 milliTorr.  The sensor is generally cylindrical and has a height of approximately 1.5 inch and a diameter of approximately 0.5 inch, which reduces the dimensions of the vacuum chamber to a few cubic centimeters.  The small size of the mass filter and the ability of the mass filter to operate at high pressures reduce the vacuum pumping requirements and therefore reduce the overall size and weight of the instrument since vacuum pumps account for a significant percentage of the size and weight of the instrument.  The sensors are driven by equally miniaturized high frequency RF (7 MHz) high voltage (1000 volts peak‑to‑peak) power supplies, which enable operation over a mass range up to 500 atomic mass units (amu) with a mass resolution of 0.5 amu measured at full width half maximum (FWHM).

• When combined with IMS, IMI’s IMS/MS instruments overcome the selectivity limitations of IMS-only systems and the speed limitations of GC‑MS instruments.  With a lower limit of detection, the MS-based instrument provides deterministic detection and identification of traces of compounds.  Increased sensitivity, selectivity and speed qualify an IMS‑MS instrument as a strong alternative for field trace detection.  At such high analysis speeds, the screening cycle time and therefore the throughput are governed by the sample collection and preparation time rather than the analysis time. The use of IMS instruments as front-end filters accomplishes two major goals: (1) the target analytes are pre‑separated, and (2) with the drift gas flowing in the opposite direction of the ion flow, the MS is not exposed to background gasses and hence is kept “clean.” 

• The front-end filters consist of meso-scale IMS and DMS cells. The IMS cell consists of a dissolvation zone, a drift region, and a detector. The dissolvation zone and the drift region are made of a stack assembly of conducting ring electrodes separated by insulating rings. The ions are transported through the cell by applying a constant longitudinal electric field (200 V/cm typically). Downstream of the dissolvation zone is an inlet gate intended to inject discrete packets of ions into the drift tube where they are subject to collisions with a drift gas flowing in the opposite direction. The time of flight of the ions from the inlet gate to the detector depends on characteristics of the molecular ions including weight, size, and geometry. A chromatogram is therefore obtained where the drift times are used for compound identification. In the IMI design, a second gate (outlet gate) is placed near upstream of the detector. Such gate converts the IMS to a filter by allowing only ions with specific drift times to exit the cell and therefore be injected into the next analysis stage.

• In some applications the IMS alone is insufficient. Since the mobility value of IMS instruments is independent of the applied electric field, ions with the same or similar mobility may not be readily separated.  Compared to IMS, DMS (differential mobility spectrometry) or high-field asymmetric-waveform ion mobility spectrometry (FAIMS) instruments use a high‑frequency asymmetric RF waveform applied between two parallel electrodes or metal plates.  This waveform alternates between a short, high-field (approximately 15,000 V/cm) pulse and an opposite polarity, longer, low-field pulse.  At the higher electric field, the mobilities of all ions become dependent on the electric field.  As a result, even ions with the same low-field ion mobilities may often be separated.  Since the ions experience different mobilities during the high and low electric field segments of the waveform, the ions drift towards one plate or the other depending on their charge.  The ion dispersion is stopped by applying a small DC voltage or compensation voltage (CV) to either of the plates.  By scanning the CV over a range of voltages, a differential mobility spectrum is generated.  The DMS apparatus may be set to pass one specific ion continuously (constant CV), making it an ideal continuous source of ions in front of a quadrupole mass spectrometer, which dramatically reduces the background observed.  This ensures that only the threat ions are injected into the vacuum for mass analysis.  The earliest DMS analyzers employed parallel planar electrodes to separate ions produced by atmospheric gas-phase ionization methods.

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QAMS

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