Interactions of multiple reactant ions with 2,4,6-trinitrotoluene studied by corona discharge ion mobility-mass spectrometry

doi:10.1016/j.ijms.2015.03.002

International Journal of Mass Spectrometry

Volume 380, 1 April 2015, Pages 12-20

https://doi.org/10.1016/j.ijms.2015.03.002 Get rights and content

Highlights

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Interactions of multiple RI generated in CD, with molecules of TNT.
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Reactions of TNT with RI generated from CD operated in reverse gas flow mode.
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Reactions of TNT with RI generated from CD operated in standard gas flow mode.
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We investigated the stability of product ions under various conditions of IMS.

Abstract

In this work we present a study of the interactions of multiple reactant ions (RI) generated in corona discharge (CD) with molecules of 2,4,6-trinitrotoluene (TNT) using the ion mobility orthogonal-acceleration time of flight mass spectrometry (IMS-oaTOF) technique. We have studied response of TNT molecules to different reactant ions (RI) such as O₂⁻, Cl⁻, N₂O₂⁻, NO₃⁻ or NO₃⁻·(HNO₃). The response of TNT to RI results in formation of multitude of species, such as TNT⁻, (TNT-H)⁻ and cluster ions. Additionally, we have investigated the stability of TNT⁻ ions as function of IMS gas temperature and drift field intensity and their conversion to (TNT-H)⁻. Formation of some product ions was assigned to degradation of TNT by neutral reactive species generated by the CD.

Graphical abstract

Introduction

The detection of explosives is an important task in order to ensure the security of the citizens around the world. The ion mobility spectrometer (IMS) is an instrument used in the security area [1], [2], due to its compact design, fast response and ability to operate at atmospheric pressure. IMS is an ion separation technique based on the interactions of the ions with molecules of a buffer gas in weak homogeneous electric field. The gradual expansion of this technique was closely related with development and implementation of new ionisation sources for IMS instruments.

Most conventional ion source used in IMS is the radioactive ⁶³Ni due to its low noise, easy implementation, high reliability and long life time [1]. Main drawback of this source is its radioactivity and the restriction related to it in civil sector. Several non-radioactive ion sources were developed for explosives detection with IMS, such as electro spray ionisation (ESI) [3], secondary electro spray ionisation (SESI) [4], low temperature plasma ionisation (LTPI) [5], distributed plasma ionisation (DPI) [6], dopant assisted photo ionisation (DAPI) [7], [8] and corona discharge (CD) [9], [10].

The CD ionisation source implemented to IMS attracted attention as a nonradioactive alternative to ⁶³Ni. The main drawback of CD source is the need of additional high voltage power supply. This deficiency is on the other hand strongly compensated by its significantly higher signal yield. The major advantage of this ionisation source is its ability to selectively generate different reactant ions (RI).

Different RI allow application of different chemical ionisation schemes for given analyte, which result in different product ions and the sensitivity and selectivity of IMS technique can dramatically increase with properly chosen RI. In the positive polarity, the CD ion source in air is able to selectively generate H₃O⁺·(H₂O)_n or NO⁺·(H₂O)_n RI [11]. The NO⁺·(H₂O)_n reactant ions significantly increase the sensitivity of IMS instrument for detection of monoaromatic compounds. In the negative polarity, used for explosives detection, the situation is more complicated. This is related to neutrals and radicals generated in CD in air and their fast conversion to very stable negative ions (high electron affinity) such as NO₃⁻, N₂O₂⁻, O₃⁻, CO₃⁻, NO₃⁻·HNO₃ and many others [12], [13], [14], [15], [16]. The yield of specific species strongly depends on the parameters of CD, the geometry, gas flow, gas purity, discharge power and temperature. The deployment of reverse gas flow (gas flow is opposite to ions movement in CD gap) also allows the generation of “soft” RI as O₂⁻ and CO₄⁻ [17], [18].

In spite of the fact that CD was implemented to IMS nearly fifteen years ago [9], only a few works deal with implementation of this ionisation source to explosives detection. Tabrizchi and ILbeigi implemented CD operated in positive polarity for explosives detection [19]. In the negative polarity, Tabrizchi and Abedi [20] proposed a curtain gas CD design. This technique has notable sensitivity [21], however the non-attaching gas is required. Another CD-IMS work supported also by MS instrument used mainly NO₃⁻ and NO₃⁻·HNO₃ RI, respectively [22], [23], [24] for explosives detection.

In this work, we will carefully investigate the reactions of multiple RI such as N₂O₂⁻, NO₃⁻, NO₃⁻·HNO₃, NO₂⁻·HNO₃, O₂⁻ and Cl⁻ with the most common explosive 2,4,6-trinitrotoluene (TNT) by the CD-IMS-oaTOFMS instrument. In order to explore possible product ions and their stability, we have investigated the influence of IMS gas temperature, drift field intensity, discharge power, gas flow and TNT concentration.

Section snippets

IMS-oaTOFMS instrument

The IMS interfaced to oaTOFMS instrument was described in detail in [13], [11]. In the present study, the IMS was operated at atmospheric pressure and in the gas temperature range from 305 to 363 K. The length of the drift tube was 8.25 cm and the intensity of the IMS electric field was 495 V cm⁻¹ (if not stated other). In the case of the stability study of the product ions we have varied the drift field intensity from 350 to 543 V cm⁻¹. The Bradbury-Nielsen type of shutter grid (SG) was operated

Results and discussion

The present results are presented in two sections according to the RI applied. In the first section we are going to discuss the product ions of TNT generated in interaction of RI formed in “reverse gas flow mode”, while the second section discusses the product species observed when the CD was operating in “standard gas flow mode”.

Conclusion

In this work we have investigated the response of TNT on multiple RI generated by CD by IMS-MS instrument. Present study shows, that the nature of the product ions detected in the IMS depends not only on the nature of the RI, but also on operational parameters of the drift tube (temperature, electric field intensity) and also considerably on the neutral product (their concentration and composition) formed in the corona discharge.

The interaction of O₂⁻ RI with TNT molecules resulted in the

Acknowledgement

The research was supported by Slovak Research and Development Agency contract number APVV-733-11.

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Interactions of multiple reactant ions with 2,4,6-trinitrotoluene studied by corona discharge ion mobility-mass spectrometry

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

IMS-oaTOFMS instrument

Results and discussion

Conclusion

Acknowledgement

Talanta

Talanta

Int. J. Mass Spectrom.

Int. J. Mass Spectrom.

Int. J. Mass Spectrom.

Int. J. Mass Spectrom.

Int. J. Mass Spectrom.

Int. J. Mass Spectrom.

J. Hazard. Mater.

Int. J. Mass Spectrom.

Talanta

Talanta

Talanta

Int. J. Mass Spectrom.

Talanta

Int. J. Mass Spectrom.

Talanta