(For chromatographic devices, another common measure of device capability is the number of theoretical plates, N, which is related to the resolution by R = (N/5.55)^1/2. The number of theoretical plates is often reported as a constant.)

This general expression for P_inf is easily extended to devices for which multiple parameters can be varied—e.g., for two parameters, x and y:

where it is assumed here that the resolution for y does not depend upon the value of x.

For ion mobility spectrometry (IMS), the output is the intensity and the variable parameter (x) is the drift time. The critical quantities, then, are the resolution of the drift time and the precision of the intensity measurements. The resolution of the drift time quantifies the degree of separation required to distinguish different peaks. At drift time t, the resolution is given by R(t) = t/w_h(t), where w_h(t) is the width of the peak at half maximum intensity, as shown in Figure A-1. (See, for example, Asbury and Hill, 1999; Matz, Tornatore, and Hill, 2001; Clemmer and Jarrold, 1997; Dugourd, Hudgins, Clemmer, and Jarrold, 1997.) The literature supports the assumption that the drift time resolution is constant, R(t) ≡ R. Matz et al. report that R is about 30 for commercial IMS instruments and use R = 36 in their calculations. Asbury and Hill report that for a typical IMS device, the number of theoretical plates rarely exceeds N = 5,000, which corresponds to R = 30.

The number of intensity values that can be reported, S, is also constant. Fetterolf and Yost use S = 2¹² for mass spectrometry. For IMS, the value of S appears to be less well defined. Often a 12-bit analog-to-digital converter is used, which would imply S = 2¹² at first glance. However, in practice the number of reproducible intensity values is much less, perhaps as low as 2⁴ (Knapp 2003).

Front Matter (R1-R14)

Executive Summary (1-8)

1 Background and Overview (9-14)

2 Mass Spectrometry for Trace Detection of Threat Agents (15-28)

3 Strategy for Improving Trace Detection Capabilities (29-32)

Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer (33-38)

Appendix B: Biographies of Committee Members (39-42)