electrospray ionization (ESI). In MALDI, the sample is mixed with a larger quantity of an organic molecule (the matrix), which is selected for its ability to efficiently absorb radiation from a laser. When the dried sample-matrix mixture is exposed to a laser beam, the matrix absorbs the laser energy and transfers it to the sample, typically forming positive ions with a single charge. In ESI, a sample in a liquid is continuously aerosolized into a fine spray near a needle maintained at high electrical potential, and the droplets take on a charge from the electric field. As the charged droplets evaporate, the charge is transferred to organic molecules in the sample, forming molecular ions that can be separated in the MS.

The uniqueness of mass spectrometry lies in its chemical specificity. Because it directly measures a fundamental property of the target molecule—its molecular weight—it affords a highly specific means of identifying the molecule. By contrast, IMS systems measure a secondary and less specific property of the target molecule—the time it takes for the molecule to drift through a tube filled with a dense gas—and the identity of the molecule is inferred by reference to calibration standards. Since different molecules may have similar drift times, IMS inherently has less chemical specificity than MS.

Since there is no quantitative calibration of airport IMS systems⁷ and no systematic reporting of screened objects and alarms, there are no reliable data with which to properly asses the current IMS instrumentation in terms of probability of false alarms and probability of detection. Instead, another method of comparison is adopted here (described below) that might be used in the future for comparisons of technologies when operational data are unavailable.

The chemical specificity of an instrumental method can be quantitatively estimated on a consistent basis using a metric called “informing power.” In a discussion of the informing power of tandem mass spectrometry (MS/MS), Yost and Fetterolf⁸ use information theory to give figures of merit for chemical resolution of various analysis techniques. The committee has estimated the informing power of IMS using the same method (see Appendix A) and compares it with the previously calculated informing power of MS/MS systems in Table 2-1. The informing power of the tandem QMS/QMS configuration is on the order of 10,000 times greater than that of IMS.

The substitution of an MS/MS analyzer for the analyzer of an ion mobility spectrometer with the same ionization technique would yield an informing power (chemical specificity) increase of approximately 10,000. The higher chemical specificity of MS means that significantly smaller quantities of target molecules can be detected in the presence of relatively large quantities of background molecules. This enables the detection threshold to be lowered without increasing the false alarm rate.

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)