Use of Rapid Reduced Electric Field Switching to Enhance Compound Specificity for Proton Transfer Reaction-Mass Spectrometry.
The high sensitivity of proton transfer reaction-mass spectrometry (PTR-MS) makes it a suitable analytical tool for detecting trace compounds. Its specificity is primarily determined by the accuracy of identifying the m/ z of the product ions specific to a particular compound. However, specificity can be enhanced by changing the product ions (concentrations and types) through modifying the reduced electric field. For current PTR-MS systems, this is not possible for trace compounds that would only be present in the reaction chamber of a PTR-MS for a short time (seconds). For such circumstances, it is necessary to change the reduce electric field swiftly if specificity enhancements are to be achieved. In this paper we demonstrate such a novel approach, which permits any compound that may only be present in the drift tube for seconds to be thoroughly investigated. Specifically, we have developed hardware and software which permits the reaction region's voltages to be rapidly switched at a frequency of 0.1-5 Hz. We show how this technique can be used to provide a higher confidence in the identification of compounds than is possible by keeping to one reduced electric field value through illustrating the detection of explosives. Although demonstrated for homeland security applications, this new technique has applications in other analytical areas and disciplines where rapid changes in a compound's concentration can occur, for example, in the Earth's atmosphere, plant emissions and in breath. Importantly, this adaptation provides a method for improved selectivity without expensive instrumental changes or the need for high mass resolution instruments.
Applications of Direct Injection Soft Chemical Ionisation-Mass Spectrometry for the Detection of Pre-blast Smokeless Powder Organic Additives
AbstractAnalysis of smokeless powders is of interest from forensics and security perspectives. This article reports the detection of smokeless powder organic additives (in their pre-detonation condition), namely the stabiliser diphenylamine and its derivatives 2-nitrodiphenylamine and 4-nitrodiphenylamine, and the additives (used both as stabilisers and plasticisers) methyl centralite and ethyl centralite, by means of swab sampling followed by thermal desorption and direct injection soft chemical ionisation-mass spectrometry. Investigations on the product ions resulting from the reactions of the reagent ions H3O+ and O2+ with additives as a function of reduced electric field are reported. The method was comprehensively evaluated in terms of linearity, sensitivity and precision. For H3O+, the limits of detection (LoD) are in the range of 41–88 pg of additive, for which the accuracy varied between 1.5 and 3.2%, precision varied between 3.7 and 7.3% and linearity showed R2 ≥ 0.9991. For O2+, LoD are in the range of 72 to 1.4 ng, with an accuracy of between 2.8 and 4.9% and a precision between 4.5 and 8.6% and R2 ≥ 0.9914. The validated methodology was applied to the analysis of commercial pre-blast gun powders from different manufacturers.
 Graphical Abstract
Proton transfer reaction mass spectrometry and the unambiguous real-time detection of 2,4,6 trinitrotoluene.
Fears of terrorist attacks have led to the development of various technologies for the real-time detection of explosives, but all suffer from potential ambiguities in the assignment of threat agents. Using proton transfer reaction mass spectrometry (PTR-MS), an unusual bias dependence in the detection sensitivity of 2,4,6 trinitrotoluene (TNT) on the reduced electric field (E/N) has been observed. For protonated TNT, rather than decreasing signal intensity with increasing E/N, which is the more usual sensitivity pattern observed in PTR-MS studies, an anomalous behavior is first observed, whereby the signal intensity initially rises with increasing E/N. We relate this to unexpected ion-molecule chemistry based upon comparisons of measurements taken with related nitroaromatic compounds (1,3,5 trinitrobenzene, 1,3 dinitrobenzene, and 2,4 dinitrotoluene) and electronic structure calculations. This dependence provides an easily measurable signature that can be used to provide a rapid highly selective analytical procedure to minimize false positives for the detection of TNT. This has major implications for Homeland Security and, in addition, has the potential of making instrumentation cost-effective for use in security areas. This study shows that an understanding of fundamental ion-molecule chemistry occurring in low-pressure drift tubes is needed to exploit selectivity and sensitivity for analytical purposes.