Fast Switching of CO<sub>3</sub><sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> and O<sub>2</sub><sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> Reactant
Ions in Dopant-Assisted
Negative Photoionization Ion Mobility Spectrometry for Explosives
Detection
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Abstract
Ion
mobility spectrometry (IMS) has become the most deployed technique
for on-site detection of trace explosives, and the reactant ions generated
in the ionization source are tightly related to the performances of
IMS. Combination of multiform reactant ions would provide more information
and is in favor of correct identification of explosives. Fast switchable
CO<sub>3</sub><sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> and O<sub>2</sub><sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> reactant ions were realized in a dopant-assisted
negative photoionization ion mobility spectrometer (DANP-IMS). The
switching could be achieved in less than 2 s by simply changing the
gas flow direction. Up to 88% of the total reactant ions were CO<sub>3</sub><sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> in the bidirectional mode, and 89% of that were O<sub>2</sub><sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> in
the unidirectional mode. The characteristics of combination of CO<sub>3</sub><sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> and O<sub>2</sub><sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> were demonstrated by the detection of explosives,
including 2,4,6-trinitrotoluene (TNT), cyclo-1,3,5-trimethylene-2,4,6-trinitramine
(RDX), ammonium nitrate fuel oil (ANFO), and black powder (BP). For
TNT, RDX, and BP, product ions with different reduced mobility values
(<i>K</i><sub>0</sub>) were observed with CO<sub>3</sub><sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> and
O<sub>2</sub><sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub>, respectively, which is a benefit for the accurate identification.
For ANFO, the same product ions with <i>K</i><sub>0</sub> of 2.07 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> were generated, but improved peak-to-peak resolution as well as
sensitivity were achieved with CO<sub>3</sub><sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub>. Moreover, an improved peak-to-peak
resolution was also obtained for BP with CO<sub>3</sub><sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub>, while the better sensitivity
was obtained with O<sub>2</sub><sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub>