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MEETING NEWS Reports from the
Mass Spectrometry in Homeland Security Workshop—Knoxville, Tenn.
Sensitive mass detectors for national security applications The instrumentation for rapid identifica- that can directly measure charge with very high sensitivity. The collection eletion of explosives, explosive residues, ments, which are produced by standard munitions, and sensitive nuclear materials is based on ion mobility spectrometry lithographic techniques, can be as small (IMS) or isotope ratio MS (IRMS). Teams of researchers led by M. Bonner Denton of the University of Arizona, Dave Koppenaal of the Pacific Northwest National Laboratory, Chris Gresham of the Sandia National Laboratory, and Gary Hieftje of Indiana University are developing ion detectors with improved capabilities that will enhance IMS and IRMS for these analyses. The new ion detectors draw on technologies developed for visible CCDs, near-IR devices, and IR multiplexer arrays used for night vision and Depiction of an array of micro-Faraday cups. in astronomy. as 10 µm and closely spaced (10 µm Detection of IR radiation requires apart) in large numbers. The small physmaterials with lower band gaps than siliical size of the collection elements and con devices can offer. However, these materials are not suitable for the required the associated measurement circuitry results in a dramatic reduction in the readout electronics, so IR focal plane deinput capacitance and allows the accutectors are often constructed by placing IR-active materials on top of silicon read- mulation of charge on femtofarad capacitors. This improves the sensitivity of out multiplexers. These “stacked sysFaraday-type detectors from ~6000 tems” can read extremely low levels of ions/s (which can be achieved in the charge and have exceptionally low readmost expensive electrometers) to only a out noise and low dark currents, says few ions. Thus, signals from a very low Denton. Combining Faraday ion detecflux of ions can be reproducibly and tion with the technologies developed for quantitatively measured. Furthermore, CCDs and IR multiplexers has resulted the device inherently has a dynamic in the micro-Faraday finger array, which range as large as 8 orders of magnitude. is capable of sensitive charge detection In IMS, which is used primarily to and has ultralow noise characteristics. identify explosives and chemical warfare The micro-Faraday finger array represents a step toward direct detection of agents, the compounds of interest are ionized and separated by mobility, which single ions. A linear array of extremely is based on ion mass, size, and charge. high impedance electrometer amplifiers The separated ions are detected at the have been integrated with very small end of a drift tube, and a drift time is collection elements to produce an array
associated with each separated ion packet. The new ion detector improves detection limits, dynamic range, and resolution; an instrument that uses the improved detector can detect dangerous compounds at levels hundreds of times more sensitive than those of current IMS instrumentation, says Denton. In addition, the quantitative measurements can be better because the detector offers a combination of high sensitivity with a large dynamic range. IRMS can fingerprint and track nuclear materials and munitions. It uses simultaneous detection to eliminate much of the correlated noise effects caused by parts of the mass spectrometer, including the ionization source, controlling electronics, and detection system, says Denton. If a detector collects two or more signals of interest simultaneously, then the multiplicative noise effect can be reduced or eliminated because the signals were subjected to the same variability. The new microarray can detect many more elements simultaneously than previous instruments could, and it has high sensitivity, which permits isotopic analyses with a precision approaching 0.001% RSD in a rapid analysis time, he says. While much of the research by Denton and his colleagues is aimed at improving national security applications such as explosive residue monitors at airport screening terminals and military chemical warfare agent monitors, they are also interested in developing these instruments for routine analysis. a —Steve Miller with contributions from M. Bonner Denton
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