Distinguishing two isomeric mephedrone substitutes with selective reagent ionisation mass spectrometry (SRI-MS)

Detecting Hexafluoroisopropanol Using Soft Chemical Ionization Mass Spectrometry and Analytical Applications to Exhaled Breath

Here we explore the potential use of proton transfer reaction/selective reagent ion-time-of-flight-mass spectrometry (PTR/SRI-ToF-MS) to monitor hexafluoroisopropanol (HFIP) in breath. Investigations of the reagent ions H₃O⁺, NO⁺, and O₂^+• are reported using dry (relative humidity (rH) ≈ 0%) and humid (rH ≈ 100%)) nitrogen gas containing traces of HFIP, i.e., divorced from the complex chemical environment of exhaled breath. HFIP shows no observable reaction with H₃O⁺ and NO⁺, but it does react efficiently with O₂^+• via dissociative charge transfer resulting in CHF₂⁺, CF₃⁺, C₂HF₂O⁺, and C₂H₂F₃O⁺. A minor competing hydride abstraction channel results in C₃HF₆O⁺ + HO₂^• and, following an elimination of HF, C₃F₅O⁺. There are two issues associated with the use of the three dominant product ions of HFIP, CHF₂⁺, CF₃⁺, and C₂H₂F₃O⁺, to monitor it in breath. One is that CHF₂⁺ and CF₃⁺ also result from the reaction of O₂^+• with the more abundant sevoflurane. The second is the facile reaction of these product ions with water, which reduces analytical sensitivity to detect HFIP in humid breath. To overcome the first issue, C₂H₂F₃O⁺ is the ion marker for HFIP. The second issue is surmounted by using a Nafion tube to reduce the breath sample's humidity prior to its introduction into drift tube. The success of this approach is illustrated by comparing the product ion signals either in dry or humid nitrogen gas flows and with or without the use of the Nafion tube, and practically from the analysis of a postoperative exhaled breath sample from a patient volunteer.

Real-time differentiation of ethylbenzene and the xylenes using selected ion flow tube mass spectrometry

RATIONALE

Monitoring of isomeric analytes using mass spectrometry usually requires a time-consuming chromatographic separation of the analytes before analysis. Selected ion flow tube mass spectrometry (SIFT-MS) can provide rapid direct analysis of ethylbenzene and xylene by utilizing the different reaction chemistry of the isomers. O₂ ⁺ yields the same product ions from each isomer but in different ratios. OH^- yields different product ions.

METHODS

The reaction chemistry of C₈ H₁₀ with the reagent ion OH^- generated from a microwave discharge of moist air in a commercial SIFT-MS instrument was utilized in this study. The product ion from OH^- yielded ions at different masses for each isomer. To gain an understanding of how the product ion from ethylbenzene of HO₂ ^- was generated, a theoretical study of the potential reaction surface was undertaken that accounted for the experimental observations.

RESULTS

Measurements of OH^- with ethylbenzene showed the product ion to be HO₂ ^- at m/z 33. The reaction of OH^- with xylene yielded the major product ion at m/z 105, C₈ H₉ ^- . HO₂ ^- also underwent a slow secondary reaction with CO₂ and O₂ present from air in the samples. These findings were supported by calculations of the potential energy surface for the reactions. Measurements made on a certified gas mixture of ethylbenzene and xylene in the concentration range up to 5000 ppbv gave a linear response for each analyte.

CONCLUSIONS

A fast, efficient method was developed for monitoring xylene and ethylbenzene in a mixture without the need for chromatographic separation before analysis using SIFT-MS.

Selective Reagent Ion Mass Spectrometric Investigations of the Nitroanilines

This paper presents an investigation of proton and charge transfer reactions to 2-, 3- and 4-nitroanilines (C₆H₆N₂O₂) involving the reagent ions H₃O⁺·(H₂O)_n (n = 0, 1 and 2) and O₂⁺, respectively, as a function of reduced electric field (60-240 Td), using Selective Reagent Ion-Time-of-Flight-Mass Spectrometry (SRI-ToF-MS). To aid in the interpretation of the H₃O⁺·(H₂O)_n experimental data, the proton affinities and gas-phase basicities for the three nitroaniline isomers have been determined using density functional theory. These calculations show that proton transfer from both the H₃O⁺ and H₃O⁺·H₂O reagent ions to the nitroanilines will be exoergic and hence efficient, with the reactions proceeding at the collisional rate. For proton transfer from H₃O⁺ to the NO₂ sites, the exoergicities are 171 kJ mol^-1 (1.8 eV), 147 kJ mol^-1 (1.5 eV) and 194 kJ mol^-1 (2.0 eV) for 2-, 3- and 4-nitroanilines, respectively. Electron transfer from all three of the nitroanilines is also significantly exothermic by approximately 4 eV. Although a substantial transfer of energy occurs during the ion/molecule reactions, the processes are found to predominantly proceed via non-dissociative pathways over a large reduced electric field range. Only at relatively high reduced electric fields (> 180 Td) is dissociative proton and charge transfer observed. Differences in fragment product ions and their intensities provide a means to distinguish the isomers, with proton transfer distinguishing 2-nitroaniline (2-NA) from 3- and 4-NA, and charge transfer distinguishing 4-NA from 2- and 3-NA, thereby providing a means to enhance selectivity using SRI-ToF-MS.

Compendium of the Reactions of H3O+ With Selected Ketones of Relevance to Breath Analysis Using Proton Transfer Reaction Mass Spectrometry

Soft chemical ionization mass spectrometric techniques, such as proton transfer reaction mass spectrometry (PTR-MS), are often used in breath analysis, being particularly powerful for real-time measurements. To ascertain the type and concentration of volatiles in exhaled breath clearly assignable product ions resulting from these volatiles need to be determined. This is difficult for compounds where isomers are common, and one important class of breath volatiles where this occurs are ketones. Here we present a series of extensive measurements on the reactions of H₃O⁺ with a selection of ketones using PTR-MS. Of particular interest is to determine if ketone isomers can be distinguished without the need for pre-separation by manipulating the ion chemistry through changes in the reduced electric field. An additional issue for breath analysis is that the product ion distributions for these breath volatiles are usually determined from direct PTR-MS measurements of the compounds under the normal operating conditions of the instruments. Generally, no account is made for the effects on the ion-molecule reactions by the introduction of humid air samples or increased CO₂ concentrations into the drift tubes of these analytical devices resulting from breath. Therefore, another motivation of this study is to determine the effects, if any, on the product ion distributions under the humid conditions associated with breath sampling. However, the ultimate objective for this study is to provide a valuable database of use to other researchers in the field of breath analysis to aid in analysis and quantification of trace amounts of ketones in human breath. Here we present a comprehensive compendium of the product ion distributions as a function of the reduced electric field for the reactions of H₃O⁺. (H₂O)_n (n = 0 and 1) with nineteen ketones under normal and humid (100% relative humidity for 37 °C) PTR-MS conditions. The ketones selected for inclusion in this compendium are (in order of increasing molecular weight): 2-butanone; 2-pentanone; 3-pentanone; 2-hexanone; 3-hexanone; 2-heptanone; 3-heptanone; 4-heptanone; 3-octanone; 2-nonanone; 3-nonanone; 2-decanone; 3-decanone; cyclohexanone; 3-methyl-2-butanone; 3-methyl-2-pentanone; 2-methyl-3-pentanone; 2-methyl-3-hexanone; and 2-methyl-3-heptanone.

PTR-MS and GC-MS as complementary techniques for analysis of volatiles: A tutorial review

This tutorial review is a critical commentary on the combined use of two instrumental analytical techniques, namely GC-MS and PTR-MS. The first mention of such an analytical approach likely appeared after the year 2000 and despite many advantages, it has not been applied very often. Therefore, the aim of this article is to elaborate on the concept of their combined use and to provide a curse tutorial for those considering taking such an approach. The issue of complementarity was raised in a broad sense of this term. Special emphasis was placed on indicating the possibilities of complementary utilization of GC-MS and PTR-MS and presenting the advantages and disadvantages as well as the current application of these techniques when used together.

Methods in plant foliar volatile organic compounds research

Plants are a major atmospheric source of volatile organic compounds (VOCs). These secondary metabolic products protect plants from high-temperature stress, mediate in plant-plant and plant-insect communication, and affect our climate globally. The main challenges in plant foliar VOC research are accurate sampling, the inherent reactivity of some VOC compounds that makes them hard to detect directly, and their low concentrations. Plant VOC research relies on analytical techniques for trace gas analysis, usually based on gas chromatography and soft chemical ionization mass spectrometry. Until now, these techniques (especially the latter one) have been developed and used primarily by physicists and analytical scientists, who have used them in a wide range of scientific research areas (e.g., aroma, disease biomarkers, hazardous compound detection, atmospheric chemistry). The interdisciplinary nature of plant foliar VOC research has recently attracted the attention of biologists, bringing them into the field of applied environmental analytical sciences. In this paper, we review the sampling methods and available analytical techniques used in plant foliar VOC research to provide a comprehensive resource that will allow biologists moving into the field to choose the most appropriate approach for their studies.

Monoterpene separation by coupling proton transfer reaction time-of-flight mass spectrometry with fastGC

Proton transfer reaction mass spectrometry (PTR-MS) is a well-established technique for real-time analysis of volatile organic compounds (VOCs). Although it is extremely sensitive (with sensitivities of up to 4500 cps/ppbv, limits of detection <1 pptv and the response times of approximately 100 ms), the selectivity of PTR-MS is still somewhat limited, as isomers cannot be separated. Recently, selectivity-enhancing measures, such as manipulation of drift tube parameters (reduced electric field strength) and using primary ions other than H3O(+), such as NO(+) and O2 (+), have been introduced. However, monoterpenes, which belong to the most important plant VOCs, still cannot be distinguished so more traditional technologies, such as gas chromatography mass spectrometry (GC-MS), have to be utilised. GC-MS is very time consuming (up to 1 h) and cannot be used for real-time analysis. Here, we introduce a sensitive, near-to-real-time method for plant monoterpene research-PTR-MS coupled with fastGC. We successfully separated and identified six of the most abundant monoterpenes in plant studies (α- and β-pinenes, limonene, 3-carene, camphene and myrcene) in less than 80 s, using both standards and conifer branch enclosures (Norway spruce, Scots pine and black pine). Five monoterpenes usually present in Norway spruce samples with a high abundance were separated even when the compound concentrations were diluted to 20 ppbv. Thus, fastGC-PTR-ToF-MS was shown to be an adequate one-instrument solution for plant monoterpene research.

Selective reagent ionisation-time of flight-mass spectrometry: a rapid technology for the novel analysis of blends of new psychoactive substances

In this study we demonstrate the potential of selective reagent ionisation-time of flight-mass spectrometry for the rapid and selective identification of a popular new psychoactive substance blend called 'synthacaine', a mixture that is supposed to imitate the sensory and intoxicating effects of cocaine. Reactions with H3O(+) result in protonated parent molecules which can be tentatively assigned to benzocaine and methiopropamine. However, by comparing the product ion branching ratios obtained at two reduced electric field values (90 and 170 Td) for two reagent ions (H3O(+) and NO(+)) to those of the pure chemicals, we show that identification is possible with a much higher level of confidence then when relying solely on the m/z of protonated parent molecules. A rapid and highly selective analytical identification of the constituents of a recreational drug is particularly crucial to medical personnel for the prompt medical treatment of overdoses, toxic effects or allergic reactions.

Headspace analysis of new psychoactive substances using a Selective Reagent Ionisation-Time of Flight-Mass Spectrometer

The rapid expansion in the number and use of new psychoactive substances presents a significant analytical challenge because highly sensitive instrumentation capable of detecting a broad range of chemical compounds in real-time with a low rate of false positives is required. A Selective Reagent Ionisation-Time of Flight-Mass Spectrometry (SRI-ToF-MS) instrument is capable of meeting all of these requirements. With its high mass resolution (up to m/Δm of 8000), the application of variations in reduced electric field strength (E/N) and use of different reagent ions, the ambiguity of a nominal (monoisotopic) m/z is reduced and hence the identification of chemicals in a complex chemical environment with a high level of confidence is enabled. In this study we report the use of a SRI-ToF-MS instrument to investigate the reactions of H₃O⁺, O₂⁺, NO⁺ and Kr⁺ with 10 readily available (at the time of purchase) new psychoactive substances, namely 4-fluoroamphetamine, methiopropamine, ethcathinone, 4-methylethcathinone, N-ethylbuphedrone, ethylphenidate, 5-MeO-DALT, dimethocaine, 5-(2-aminopropyl)benzofuran and nitracaine. In particular, the dependence of product ion branching ratios on the reduced electric field strength for all reagent ions was investigated and is reported here. The results reported represent a significant amount of new data which will be of use for the development of drug detection techniques suitable for real world scenarios.