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Determination of E/N Influence on K Values Within the Low Field Region of Ion Mobility Spectrometry Brian C. Hauck, William F. Siems, Charles Steve Harden, Vincent M. McHugh, and Herbert H. Hill, Jr. J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12331 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 13, 2017
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The Journal of Physical Chemistry
Determination of E/N Influence on K0 Values within the Low Field Regime of Ion Mobility Spectrometry
Brian C. Hauck,1, a) William F. Siems,1 Charles S. Harden,2, b) Vincent M. McHugh,3 Herbert H. Hill Jr.1,*
1. Washington State University, Department of Chemistry, 305 Fulmer Hall, Pullman, WA 99164 2. LEIDOS – U.S. Army ECBC Operations, P.O. Box 68, Gunpowder, MD 21010 3. U.S. Army Edgewood Chemical Biological Center, Aberdeen Proving Ground, MD 21010
Submitted to Journal of Physical Chemistry February 2017 a) b)
Current Affiliation: ORISE Postdoctoral Fellow - U.S. Army Edgewood Chemical Biological Center, Aberdeen Proving Ground, MD 21010 Current Affiliation: Science and Technology Corporation (STC) – U.S. Army Edgewood Chemical Biological Center Operations, Gunpowder, MD 21010
*Correspondence: Herbert H. Hill, Jr. (email:
[email protected], fax: 509-335-8867)
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ABSTRACT The established theory of ion motion within weak electric fields predicts that reduced ion mobility (K0) remains constant as a function of the ratio of electric field strength to drift gas number density (E/N). However, upon increasing the accuracy and precision of K0 value measurements during a previous study, a new relationship was seen in which the K0 values of ions decreased as a function of increasing E/N at field strengths below 4 Td. Here the effect of E/N on the K0 value of an ion has been investigated in order to validate the reality of the phenomenon and determine its cause. The pertinent measurements of voltage and drift time were verified in order to ensure the authenticity of the trend and that it was not a result of a systematic error in parametric measurements. The trend was also replicated on a separate ion mobility spectrometer drift tube in order to further validate its authenticity. As a result, the theory of ion motion within weak electric fields should be revised to reflect the behavior seen here.
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The Journal of Physical Chemistry
1. INTRODUCTION Ion mobility spectrometry (IMS) is an analytical technique used for the separation and identification of ions within an applied electric field and is established as a versatile detector for chemical warfare agents,1,2 explosives,3,4 and narcotics.5,6 The operation principles of IMS have been previously described in detail7,8 but in the simplest form, the drift velocity (vd) of an ion within the drift tube is achieved through a balance between the energy gained by an applied electric field (E) and the energy lost by the collisions of the ion with a drift gas neutral. An ion’s mobility (K) value is a proportionality constant between vd and E where
vd L2 K= = E Vt d
(1)
L is the distance traveled by the ion while under the influence of the electric field, V is the voltage drop across L, and td is the drift time of the ion across L. K is normalized to standard temperature and pressure using the drift gas temperature (T) and pressure (P) to obtain the ion’s reduced mobility (K0) value and allow for comparison of values across instrumentation.8
L2 K0 = Vt d
273.15 P T 760
(2)
An ion’s K0 value is dependent on the ratio of electric field intensity to drift gas number density (E/N) of the instrument. At constant pressure and temperature, the current mobility theory only takes into account E and N to sufficiently define and predict the behavior of the ion within the electric field. Low electric field intensities are defined by conditions in which the ion receives less energy from the electric field than from the thermal energy of the buffer gas, as shown in Equation (3), and the K0 value is predicted to remain essentially constant.
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