Minimizing false positives in ion mobility spectrometry by changing the field or pressure Larry A. Viehland
International Journal for Ion Mobility Spectrometry ISSN 1435-6163 Int. J. Ion Mobil. Spec. DOI 10.1007/s12127-015-0181-0
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Author's personal copy Int. J. Ion Mobil. Spec. DOI 10.1007/s12127-015-0181-0
ORIGINAL RESEARCH
Minimizing false positives in ion mobility spectrometry by changing the field or pressure Larry A. Viehland 1
Received: 28 April 2015 / Revised: 4 June 2015 / Accepted: 5 June 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract A method is proposed for minimizing false positives in ion mobility spectrometry by changing the value of E=N , the ratio of the electrostatic field strength to the gas number density. The feasibility of this method is demonstrated by simulation of the mobilities of 2,4,6-trinitrotolune and 2methyl-3,5-dinitrophenol in helium. The zero-field mobilities of these ions are nearly the same, while their dependences upon E/N are so different that the ions can be distinguished by modest changes in E/N at 473 K and by somewhat larger changes at 300 K. Keywords False positives . Ion mobility spectrometry . Gaseous ion transport . (n,4)-core model potential
overlapping IMS peaks. Using a gas chromatograph as a preIMS separation device or adding chemical dopants can decrease the incidence of false positives [8], as can combining IMS with mass spectrometry [6]. The use of secondary electrospray ionization has been suggested [9] as a further improvement, one that avoids the need for sample separation before analysis or the addition of chemical dopants. The problems with these techniques include capital costs and the need for trained operators. The purposes of this paper are to propose a method by which false positives can be minimized by making rather simple changes in the instrument used, and to demonstrate the feasibility of this method by simulation of the mobilities of two ions that have similar mobilities.
Introduction Background Among the trace detection systems used at airports and in military and policing activities is ion mobility spectrometry (IMS) [10]. Since the consequence of not detecting a security threat might be disastrous, the operating parameters of IMS are chosen to make false negatives unlikely. Unfortunately, this can cause the number of false positives to increase, resulting in substantial costs in time and money [11]. A primary reason for false positives is that the number of chemical species present with similar mobilities can lead to This article is submitted for the special issue on IMS modeling and simulation. * Larry A. Viehland
[email protected] 1
Department of Science, Chatham University, Pittsburgh, PA 15232, USA
It has been known for many years [13] that the average energy of the ion-neutral collisions in an IMS is governed by the gas temperature, T , and the ratio, E=N , of the electric field strength to the gas number density. A rather recent formula [26] for when E/N becomes unimportant is E
σ it is attractive. The ability to consider both long-range attraction and short-range repulsion is an important advantage of using the (12,4)-core potential, in contrast to the hard-sphere
projection method [17]. The parameter n is a measure of the steepness of the repulsive (short-range) potential; n ¼ ∞ for a rigid sphere, it is usually about 8 for the soft potentials that characterize the interactions of small ions and neutrals, but here we use n ¼ 12 as originally suggested by Mason et al. [14]. To determine the other parameters in Eq. (2), we used ChemBio3D [7] to generate a model of the ion bonded to a N2 molecule and then another of the ion along with (but not bonded to) N2. For each, we then used the MOPAC [19] interface from ChemBio3D to find the minimum energy and corresponding structure. We set ε0 equal to the difference in these minimum energies. The value of a was obtained from ChemBio3D by rotating the energy-minimized structure for the non-bonded ion and N2, and setting it equal to the smallest dimension found. Finally, rm was obtained by noting that at large r Eq. (2) becomes V ðrÞ→
c4 ; r4
ð4Þ
where c4 ¼
3 ε0 ðrm aÞ4 : 2
ð5Þ
Since the value of c4 is the same for all ions in the same neutral gas, and since it is known [13] to have the value 5.938 in atomic units when the gas is nitrogen, we can use this value along with ε0 and a to determine rm from Eq. (5). The values for the parameters for each system are given in Table 1. We followed the path outlined above to determine the standard (or reduced) mobilities. Since we are using a simulation and a rather crude estimate of the interaction potential, it is not surprising that the zero-field mobilities are only approximately equal to the experimental values at 473 K. It is possible to systematically vary the parameters (including n ) in the (n,4)-core potential until a set is found for each ion-air system for which the calculated zero-field mobility at 473 K matches the experimental value. This was not done here because adjusting four parameters subject to only two constraints (Eq. 5 and the measured K 0 ) does not yield a unique solution. Instead, we scaled the values with a constant factor chosen so that K 0 ¼ 1:50 cmÇ=Vs for TNT+ in He at E=N ¼ 0, and 1.55 cm²/Vs for DNOC+. The results are shown in Fig. 1, where typical experimental error bars on each Table 1
Parameter values for the (12,4) core potentials
Parameter
TNT+
DNOC+
ε0 (millihartree)
1.219
1.937
a (bohr)
2.143
1.070
rm (bohr)
9.692
7.794
σ (bohr)
8.723
6.931
Author's personal copy Int. J. Ion Mobil. Spec.
1.70
particular that now the DNOC+ mobilities increase more with E=N , since the thermal energy is much smaller than before, while the TNT+ mobilities vary little with E=N , since the thermal energy is slightly below ε0, a small increase in E=N is needed to surmount the mobility maximum, and only at still higher E=N values are the mobilities probing the repulsive part of the potential.
K0
1.60 1.50 1.40
1.30 1.20
Discussion
1.10
E/N
1.00 0
20
40
60
80
100
Fig. 1 Simulated standard mobilities, K 0 in cm2/Vs, of TNT+ (red curve) and DNOC+ (blue points) in He at 473 K, as functions of E=N in Td
set of results have been replaced by a single set of 5 % error bars on the DNOC+ results. What is immediately obvious is that it does not take a very large value of E=N for the TNT+ values to fall outside the error bars and hence for false positives for DNOC+ to no longer occur. The different E=N dependences shown in Fig. 1 can be understood in terms of the well depths given in Table 1. For TNT+ the value of ε0 corresponds to a thermal energy of 384 K, which means that, when the gas temperature is 473 K, the mobility will decrease monotonically from its zero-field value as E=N increases; this is clearly shown in Fig. 1. For DNOC+, however, the well depth corresponds to a thermal energy of 612 K, so the mobility will increase from its zero-field value until the combination of the thermal and field energies is sufficient to reach the mobility maximum; Fig. 1 shows this behavior and that the mobility maximum occurs for E=N ¼ 85 Td. As a confirmation of these arguments, the calculations were repeated for T ¼ 300 K. The results in Fig. 2 are generally the same as in Fig. 1, but clearly one would have to get to much higher values of E=N to minimize false positives for DNOC+ when the air is at 300 K than when it is at 473 K. Note in
This paper proposes that the incidence of false positives in IMS could be reduced by modifying the instrument so that rapid changes in E=N by a factor of 10–20 could be made. Numerical simulations of the mobility of TNT+ ions in air and of DNOC+ ions in air showed that, when the zero-field mobilities were nearly identical, it was possible to distinguish the ions by such changes in E=N . The main reason this works for these ions is the difference in well depths between the two ion-air interactions. When the well depth is larger than or approximately equal to the thermal energy corresponding to the gas temperature in the experiments, the zero-field mobility is most sensitive to the longrange, attractive portion of the ion-neutral interaction, so the mobility will increase as E=N increases. On the other hand, when the well depth is smaller than the thermal energy, the zero-field mobility reflects the short-range, repulsive portion of the interaction, so the mobility decreases with E=N . It should be noted that it may still be possible to distinguish ions with the same zerofield mobility by making changes in E=N even when both mobilities increase or decrease; all that is needed is sufficiently different slopes on graphs of K 0 vs. E=N . It is recommended that experimenters make the changes in their IMS apparatus needed to investigate the extent to which false positives can be minimized by rapid changes in E=N . It is also recommended that both theorists and experimenters pay attention to the magnitude, rather than simply the position, of the potential minima for the ion-neutral interactions probed by IMS.
1.80 K0
1.75
Acknowledgments This work completes and extends work done by Jacqueline L. Wilmot (nee Regan) under the supervision of the author, as part of an undergraduate thesis at Chatham University in 2009. The author is grateful for her interest in the project and her help in getting it started.
1.70 1.65 1.60
Conflict of interest The author declares that he has no conflicts of interest.
1.55 1.50
References
1.45 E/N
1.40 0
20
40
Fig. 2 Same as Fig. 1, at 300 K
60
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