Introduction

Adaptations of technology for astronomy are making dramatic impacts on many other areas of science and technology. This is probably most apparent in the field of modern chemical analysis. Low light level optical spectroscopies in the UV-visible and near IR regions (125 to 1100 nm) have advanced dramatically through the use of modern CCD and CID focal plane arrays. In atomic emission spectroscopy the venerable direct reader, using discrete photomultiplier tubes (PMTs), has been replaced by focal plane

arrays, often providing continuous wavelength coverage at higher resolution and improved sensitivity. Dispersive Raman spectroscopy has capitalized on the sensitivity and multiplex advantage of arrays, thus few scanning PMT based instruments are still employed. Although infrared focal plane arrays are gaining niche markets, total system costs have thus far limited their widespread adoption.

Capacitive Trans Impedance Amplifiers (CTIAs) are often employed as readout circuits in hybrid focal plane infrared arrays. This readout scheme has several desirable characteristics including a high degree of linearity and system gain determined by feedback capacitances, as well as the ability to provide high sensitivity for very small charge packets.

The ability to detect ions is of foremost importance in a wide number of chemical instruments. Mass spectrometers generally use a form of ion multiplier or, in cases where extreme accuracy is required (as in isotope ratio analysis), a Faraday cup or plate. Ion multipliers come in many configurations, but all utilize the principle of ion to electron conversion, which is accelerated onto a dynode to yield multiple secondary electrons. These secondary electron "packets" are subsequently accelerated into the next dynode (see Fig. 1). The process is repeated until a large, easily measured charge packet or steady state current is produced.

Ion Beam

Ion Beam

Multiplier Dynodes (each at less negative voltage)

Electrons AV

Current to Amplifier

Electrons AV

Conversion Dynode (at high negative voltage)

Figure 1. Ion multipliers use avalanche electron multiplication to increase current measured by 106 or more. However, this technique does not work at atmospheric pressure.

Although this approach is successful in many applications, limitations exist which prevent the detection of extremely large ions (the ion lands on the conversion electrode without ejecting an electron capable of causing secondary emission). Another possible difficulty is that the process does not provide the required precision and stability because the efficiency of the electron ejection process is influenced by ion mass and/or energy. Additionally, these charge multiplication techniques are not easily configured into large linear arrays suitable for focal plane mass spectrometers such as those using the Mattauch-Herzog geometry.

Over the last five years we have made significant progress implementing mass spectrometer detector arrays using concepts adapted from CTIA preamplifier array technology [1,2,3] These detectors demonstrate all the desirable characteristics of Faraday type detectors, with sensitivities approaching those of multiplier detectors. However they are easily fabricated into long linear arrays - providing an important multiplex advantage for focal plane geometry mass spectrometers.

In this manuscript, similar detector technologies are evaluated for their compatibility with Ion Mobility Spectrometry (IMS). IMS is widely utilized for the detection of chemical warfare agents, hazardous chemicals explosives, and even for elucidation of large biological molecules (see

Ion Mobility Spectrometer

IMS is a techaique that is being employed to solve problems where portable instrumentation and rugged ness is necessary

I New Instruments Demand Lower Detection Limits

2 Must Operate through a Wide Range of Temperatures

3 Must Operate at Atmospheric Pressure

4 High S/N Ratio

Figure 2. Ion mobility spectrometry is used today for a wide variety of ultra-trace field and laboratory analyses. One of the most common applications is screening for illicit explosives.

This technique, shown in Figures 3 and 4, involves ionizing the molecules of interest or a reagent gas, which subsequently transfers charge to analyte molecules. An ion shutter or gate pulses out a packet of ionized molecules accelerating them down a drift region over which a potential gradient is applied. The ions are collected as a function of arrival time at a Faraday plate or electrode.

Ion Mobility Spectrometer

IMS is a techaique that is being employed to solve problems where portable instrumentation and rugged ness is necessary

I New Instruments Demand Lower Detection Limits

2 Must Operate through a Wide Range of Temperatures

3 Must Operate at Atmospheric Pressure

4 High S/N Ratio

Figure 2. Ion mobility spectrometry is used today for a wide variety of ultra-trace field and laboratory analyses. One of the most common applications is screening for illicit explosives.

Figure 3. An ion mobility spectrometer consists of an ionization region where ions of analyte are generated, an ion shutter or gate to create a "pulse" of ions, a drift tube where different types of molecules are separated into discrete "packets," and a Faraday plate which collects the charge from individual ions.

Figure 3. An ion mobility spectrometer consists of an ionization region where ions of analyte are generated, an ion shutter or gate to create a "pulse" of ions, a drift tube where different types of molecules are separated into discrete "packets," and a Faraday plate which collects the charge from individual ions.

Figure 4. "Small" molecules migrate down the drift region faster than "big" molecules forming "packets" of ions of a single species.

Although this technique is similar to time of flight mass spectrometry, additional separation mechanisms are at work because the drift tube is usually operated at or near atmospheric pressure with a flow of drift gas (often air or nitrogen) from the detector end of the drift tube toward the ionization end. Hence the diffusion coefficient of the ion is also important. A more appropriate analogy of the process is atmospheric pressure ion viscometry. The equation in Fig. 5 relates the ion mobility (K) to the other operational parameters.

Relationship of Ion Mobility to Molecular Terms

Drift Velocity vd= KE

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