National Academies Press: OpenBook

Opportunities to Improve Airport Passenger Screening with Mass Spectrometry (2004)

Chapter: Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer

« Previous: 3 Strategy for Improving Trace Detection Capabilities
Suggested Citation:"Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer." National Research Council. 2004. Opportunities to Improve Airport Passenger Screening with Mass Spectrometry. Washington, DC: The National Academies Press. doi: 10.17226/10996.
×

Appendixes

Suggested Citation:"Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer." National Research Council. 2004. Opportunities to Improve Airport Passenger Screening with Mass Spectrometry. Washington, DC: The National Academies Press. doi: 10.17226/10996.
×

This page intentionally left blank.

Suggested Citation:"Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer." National Research Council. 2004. Opportunities to Improve Airport Passenger Screening with Mass Spectrometry. Washington, DC: The National Academies Press. doi: 10.17226/10996.
×

Appendix A
Estimation of the Informing Power of an Ion Mobility Spectrometer

The idea of informing power comes from information theory (or entropy). H. Kaiser (1978, reprinted from a 1974 publication) provided an introduction to the use of information theory for evaluating methods in analytical chemistry. Fitzgerald and Winefordner (1975) also described the concept and provided applications to molecular absorption, conventional phosphorimetry, and time-resolved phosphorimetry. Fetterolf and Yost (1984) determined the informing power of various tandem mass spectrometry configurations, incuding GC/QMS, QMS/QMS, GC/QMS/QMS (see Table 2-1).

The informing power, Pinf, of a measuring device is the number of bits required to encode the information potentially available from the device. Supposing, for example, that the device can report one of S possible values, the informing power of the device is log2(S). If the device has a parameter x that can be varied over k possible values x1, … ,xk, and the device is capable of reporting S(xi) measurement values at xi, then the informing power is summed over the k settings, producing

Because of the log term, greater gains can typically be achieved by increasing the number of values for x than by increasing S(x).

If the parameter x can be varied continuously, then Pinf can be reformulated by introducing the concept of resolution for the parameter x, defined as R(x) = x/δx, where δx is the smallest distinguishable difference in x for practical purposes. As introduced by Kaiser, the informing power becomes

There might be a number of simplifications to this expression. It might be that S(x) is constant, S(x) ≡ S, for example when S is fixed by characteristics of the detector. Or, the resolution might be constant, R(x) ≡ R. Another common possibility is that δx is constant. If S(x) ≡ S and R(x)≡ R, then

Suggested Citation:"Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer." National Research Council. 2004. Opportunities to Improve Airport Passenger Screening with Mass Spectrometry. Washington, DC: The National Academies Press. doi: 10.17226/10996.
×

FIGURE A-1 The resolution of two peaks in an IMS spectrum depends on the separation of their drift times and the width of the peaks at half intensity. An important quantity that governs whether two different substances are distinguishable by the drift time is the width of the intensity peaks. This width is quantified by wh, the width at half the maximum height of the distribution. Two drift times, td1 and td2, are distinguishable if |td1td2| > 1/2 (wh1 + wh2).

(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 Pinf 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/wh(t), where wh(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).

Suggested Citation:"Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer." National Research Council. 2004. Opportunities to Improve Airport Passenger Screening with Mass Spectrometry. Washington, DC: The National Academies Press. doi: 10.17226/10996.
×

Finally, the value of Pinf depends upon the range of drift times observed. One commercial IMS used in aviation security reports a range of approximately 0 to 15 ms. The literature indicates considerable variation in the drift times observed, depending (as one would expect) on the application and construction of the device (dopant gas, length of drift tube, voltage, gate width, and so on).

The value of Pinf for IMS, then, is approximately

For the comparative calculations in Table 2-1, the value used for Pinf for IMS is 1,000.

REFERENCES

1. Asbury G.R., and H.H. Hill. 1999. Evaluation of ultrahigh resolution ion mobility spectrometry as an analytical device in chromatographic terms, J. Microcolumn Separations 12: 172-178.

2. Clemmer D.E., and M.F. Jarrold. 1997. Ion mobility measurements and their applications to clusters and biomolecules. J. Mass Spectrometry 32: 577-592.

3. Dugourd P.H., R.R. Hudgins, D.E. Clemmer, and M.F. Jarrold. 1997. High-resolution ion mobility measurements, Rev. Sci. Instrum. 68: 1122-1129.

4. Fetterolf D.D., and R.A. Yost. 1984. Added resolution elements for greater informing power in tandem mass spectrometry. Int. J. of Mass Spectrometry and Ion Processes 62: 33-49.

5. Fitzgerald J.J., and J.D. Winefordner. 1975. Information theory and its use to analytical chemistry. Rev. Anal. Chem 2: 299-316.

6. Kaiser, H. 1978. Foundations for the critical discussion of analytical methods. Spectrochimica Acta 33B: 551-576. (Reprinted from a 1974 publication.)

7. Knapp, D.R. 2003. Personal correspondence.

8. Matz, L.M., P.S. Tornatore, and H.H. Hill. 2001. Evaluation of suspected interferents for TNT detection by ion mobility spectrometry. Talanta 54: 171-179.

Suggested Citation:"Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer." National Research Council. 2004. Opportunities to Improve Airport Passenger Screening with Mass Spectrometry. Washington, DC: The National Academies Press. doi: 10.17226/10996.
×

This page intentionally left blank.

Suggested Citation:"Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer." National Research Council. 2004. Opportunities to Improve Airport Passenger Screening with Mass Spectrometry. Washington, DC: The National Academies Press. doi: 10.17226/10996.
×
Page 33
Suggested Citation:"Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer." National Research Council. 2004. Opportunities to Improve Airport Passenger Screening with Mass Spectrometry. Washington, DC: The National Academies Press. doi: 10.17226/10996.
×
Page 34
Suggested Citation:"Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer." National Research Council. 2004. Opportunities to Improve Airport Passenger Screening with Mass Spectrometry. Washington, DC: The National Academies Press. doi: 10.17226/10996.
×
Page 35
Suggested Citation:"Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer." National Research Council. 2004. Opportunities to Improve Airport Passenger Screening with Mass Spectrometry. Washington, DC: The National Academies Press. doi: 10.17226/10996.
×
Page 36
Suggested Citation:"Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer." National Research Council. 2004. Opportunities to Improve Airport Passenger Screening with Mass Spectrometry. Washington, DC: The National Academies Press. doi: 10.17226/10996.
×
Page 37
Suggested Citation:"Appendix A: Estimation of the Informing Power of an Ion Mobility Spectrometer." National Research Council. 2004. Opportunities to Improve Airport Passenger Screening with Mass Spectrometry. Washington, DC: The National Academies Press. doi: 10.17226/10996.
×
Page 38
Next: Appendix B: Biographies of Committee Members »
Opportunities to Improve Airport Passenger Screening with Mass Spectrometry Get This Book
×
Buy Paperback | $29.00 Buy Ebook | $23.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

Protection of the traveling public from terrorist threats involving explosives is a major goal of the Transportation Security Administration (TSA). For 20 years, the TSA (and the Federal Aviation Administration before it) have been investing in technologies to meet that goal. To support that activity, the TSA has asked the NRC to assess a variety of technological opportunities for offering such protection. The NRC is approaching this assignment by issuing a series of reports on chosen technology applications. This is the first of that series and presents an assessment of mass spectrometry for enhanced trace detection (ETD) of chemicals contained in explosives. The report describes limitations of trace detection in general and the current technologies in particular. It then presents a discussion of the potential for mass spectrometry to improve EDT including challenges faced by such a system, recommendations for starting a program to take advantage of mass spectrometry, and recommendations for a phased implementation plan.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!