Quantification and remote detection of nitro explosives by helium plasma ionization mass spectrometry (HePI-MS) on a modified atmospheric pressure source designed for electrospray ionization

Improvement in the performance of focusing plasma desorption ionization by altering its counter electrode

RATIONALE

Plasma-based ionization sources play a vital role in rapidly analyzing diverse compounds without extensive sample pretreatment. In contrast to other sources, DC voltage-based ionizations are more advantageous due to their high analytical sensitivity and good tandem with commercially available mass spectrometers without extra power supplies. However, their performance is at the risk of high current DC voltage and helium flow rate, which poses significant challenges to practical operation and increased expense.

METHODS

In this work, we propose a novel focusing plasma desorption ionization (FPDI) in which a visible plasma beam is favorably generated between a conducting wire in a polymeric tube and a counter electrode composed of metal mesh and filter paper drilled with holes. A systematic investigation has been conducted on the influences of the geometry of drilled holes in filter paper, applied DC voltage, helium flow rate, and filter paper size. The optimized system is used to analyze various pesticides in fluid foodstuffs.

RESULTS

Compared to metal mesh and conducting paper as the counter electrode for FPDI-MS, combining metal mesh and filter paper drilled holes improved the analysis sensitivity by a factor of more than five. By applying the developed protocol for determining pesticides in complex matrixes such as orange juice and milk, a limit of detection as low as 1.3-3.0 ng mL-1 could be achieved.

CONCLUSIONS

A novel FPDI-MS technique has been developed by combining metal mesh and filter paper drilled with holes as the counter electrode and sample carrier. The corresponding improvement in analysis sensitivity facilitates the future expansion of FPDI-MS applications into different pesticides and other compounds in complex matrixes.

Focusing Plasma Desorption/Ionization Mass Spectrometry

A plasma-based source named focusing plasma desorption/ionization (FPDI) is described, which applies a high direct current voltage between a metal wire inside a polymeric hollow truncated cone and a piece of a one-sided coated conducting paper substrate. The conducting paper acts as both the counter electrode and the sample carrier. Upon the generation of a visible plasma beam, it would directly ionize the samples spotted on the conducting paper substrate or located around the plasma beam. The signal intensity of target analytes in mass spectrometric analysis is dependent highly on whether the conducting paper substrate is grounded or not, the type of conducting paper substrate, the inside diameter of the polymeric hollow truncated cone tip, the metal wire tip-to-polymer tip distance, the polymer tip-to-paper substrate distance, the applied voltage, and the helium flow rate. Based on the experimental observation, a plausible mechanism is proposed for the generation of the plasma beam from FPDI. Compared to the available low-temperature plasma, flowing atmospheric-pressure afterglow, and helium plasma ionization sources, FPDI has demonstrated higher sensitivity and better compatibility with commercial mass spectrometers without any extra power supplies. As a proof of concept, FPDI coupled with a mass spectrometer has also been applied for the discrimination of different brands of gasoline and determination of solid tablets and pesticides with limits of detection in the range of 2.2 to 30.7 ng mL^-1.

Gas phase ion-molecule reactions of nitroaromatic explosive compounds studied by hollow cathode discharge ionization-mass spectrometry

In this study, we have developed a variable pressure operating hollow cathode discharge (HCD) ion source to investigate the gas phase ion-molecule reactions of nitroaromatic explosive compounds. The developed HCD ion source coupled MS system has also been validated as an analytical method to analyze explosives at trace levels. The ion source was designed in such a way that the plasma can be generated alternatively at high pressure (~30 Torr), medium pressure (~5 Torr) and low pressure (~1 Torr) regions. The plasma contains a sufficient amount of reactant ions, electrons and excited species, thus the gaseous analyte molecules were efficiently ionized when they passed through the plasma. In the ion-molecule reactions of the nitroaromatic explosives, the discharge products of NO_x^- (x = 2,3), O₃ and HNO₃ originating from the plasma-excited air were suggested to contribute to the formation of mostly [M - H]^-, [M - NO]^-, [M+NO₃-HNO₂]^- and [M-NO+HNO₃]^- adduct ions at the higher ion source pressures (~5 and 28 Torr) while the electron rich plasma leads to the formation of molecular ion, M^-•, at the lower ion source pressure (~1 Torr). Formation of the hydride-adduct ions of the nitroaromatic compounds reveals the surface-assisted Birch type reduction in the HCD plasma. The variety of spectral patterns in the air-assisted glow discharge would be useful for high through-put detection of TNT and TNT-related explosives. An ambient helium dielectric barrier discharge (DBD) ion source was also used and gave identical mass spectra of the nitroaromatic explosive compounds to those observed by the HCD ion source, but did not give any hydride-adduct ions of the explosive compounds. Ion formation mechanism of these ions is also discussed.

Simultaneous detection and quantification of explosives by a modified hollow cathode discharge ion source

Detection of explosives at trace levels is crucial for security purposes because of increasing worldwide terrorist threats at public places. Previously, a hollow cathode discharge (HCD) ion source has been fabricated for detection of explosives. Recently, the HCD ion source has been modified for a dual pressures operating system and coupled to a linear ion trap MS to analyze explosives simultaneously. Here, trinitrotoluene (TNT), nitroglycerin (NG), pentaerythritol tetranitrate (PETN) and 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) were taken as model explosive compounds and the mass spectra were recorded in the negative mode ionization. At the higher ion source pressure (~28.0-30.0 Torr), NG, PETN and RDX gave adduct ions with the NO₃^- ion while TNT showed the [TNT + NO₃-HNO₂]^- (m/z 242) simultaneously. However, NG and PETN did not give any ion signals at the lower ion source pressure (~0.8-1.0 Torr) while TNT exhibited its molecular ion, [TNT]^-• (m/z 227), as a major ion through electron attachment and RDX showed fragment ions that followed electron capture dissociation concurrently. The modified HCD ion source exhibited better sensitivity in simultaneous detection and quantification of the explosives. The NO₃^- and NO₂^- as reagent ions in the air HCD plasma form stable adduct ions with the NG, PETN and RDX even with TNT at the higher temperature (140-200 °C). The formation of the NO₃^-, NO₂^- in the HCD plasma also causes the formation of [TNT-H]^- (m/z 226) at the higher ion source pressure. The inner metallic surface of the hollow tube assists the Birch reduction type reaction that results in the formation of hydride ion of the TNT, [TNT + H]^- (m/z 228). No significant difference in the spectral pattern for simultaneous and individual measurements for the explosives was observed at the higher ion source pressure. Therefore, it may conclude that the present modified HCD ion source can be used for simultaneous detection and quantification of the explosive compounds at trace and/or ultra-trace levels using air as a carrier gas.

Helium-Plasma-Ionization Mass Spectrometry of Metallocenes and Their Derivatives

Ferrocene and its derivatives and nickelocene undergo facile ionization when exposed directly to the ionizing plasma of a helium-plasma ionization (HePI) source. Mass spectra recorded from such samples under ambient positive-ion-generating conditions show intense peaks for the respective molecular ions [M^+•] and protonated species [(M + H)⁺]. The protonation process occurs most efficiently when traces of water are present in the heated nitrogen used as the "heating gas." In fact, the relative population of the two categories of ions generated in this way can be manipulated by regulating the heating-gas flow. Moreover, rapid and highly efficient gas-phase hydrogen-deuterium exchange (HDX) reactions can be performed in the ion source by passing the heating gas through a vial with D₂O before it reaches the HePI source. Moreover, the ionized species generated in this way can be subjected to in-source CID fragmentation in the QDa-HePI source very efficiently by varying the sampling-cone voltage. By this procedure, ions generated from ferrocene and nickelocene could be stripped so far as to ultimately generate the bare-metal cation. Other typical fragment-ions produced from protonated metallocenes included the M(cp)₁⁺ ions (M = Fe or Ni), by elimination of a cyclopentadiene molecule, or the molecular cation, by loss of a H^• radical. Moreover, H/D exchanges and subsequent tandem mass spectrometric analysis indicated that the central metal core participates in the initial protonation process of ferrocene under HePI conditions. However, in compounds such as ferrocene carboxaldehyde and ferrocene boronic acid, the protonation takes place at the peripheral functional group.

Recent Developments in the Field of Explosive Trace Detection

Explosive trace detection (ETD) technologies play a vital role in maintaining national security. ETD remains an active research area with many analytical techniques in operational use. This review details the latest advances in animal olfactory, ion mobility spectrometry (IMS), and Raman and colorimetric detection methods. Developments in optical, biological, electrochemical, mass, and thermal sensors are also covered in addition to the use of nanomaterials technology. Commercially available systems are presented as examples of current detection capabilities and as benchmarks for improvement. Attention is also drawn to recent collaborative projects involving government, academia, and industry to highlight the emergence of multimodal screening approaches and applications. The objective of the review is to provide a comprehensive overview of ETD by highlighting challenges in ETD and providing an understanding of the principles, advantages, and limitations of each technology and relating this to current systems.

The recent progress of triboelectric nanogenerator-assisted photodetectors

Since 2012, triboelectric nanogenerator (TENG) has attracted significant interest from researchers in the field of energy conversion due to its unique output characteristics of high voltage, pulse and low current. In addition, recent advancements have demonstrated that photodetection platforms based on TENG exhibit great advantages such as being simple, low-cost, portable, with high sensitivity, high response, etc, and are environment friendly. Here, this article provides a comprehensive review on the state-of-the-art photodetectors based on TENG in recent years, and a detailed introduction to the structural design and potential mechanisms. It mainly focuses on self-powered photodetectors (including photodetectors as a load resistance of a TENG and photosensitive materials such as tribo-layer of TENG) and the modulation of photodetectors based on TENG (including utilizing the voltage of TENG as well as triboelectric microplasma). Finally, we put forward some perspectives and outlook, including structure engineering and mechanism guidance, for the future development of simple, high-performance and portable photodetectors based on TENG.

1,4-Benzoquinone as a Highly Efficient Dopant for Enhanced Ionization and Detection of Nitramine Explosives on a Single-Quadrupole Mass Spectrometer Fitted with a Helium-Plasma Ionization (HePI) Source

Previous investigations have evaluated the efficacy of anions such as NO₃^-, Cl^-_, Br^-, CH₃COO^-, and CF₃COO^- as additives to generate or enhance mass spectrometric signals from explosives under plasma ionization conditions. The results of this study demonstrate that for detecting nitramine-class explosives, such as 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), 1,4-benzoquinone (BQ) is a highly effective and efficient dopant. When used in conjunction with ambient-pressure negative-ion helium-plasma ionization (HePI), 1,4-benzoquinone readily captures an electron, forming an abundant molecular anion (m/z 108), which upon exposure to vapors of RDX and HMX generates adduct ions of m/z 330 and 404, respectively. The signal level recorded for RDX upon adduction to the radical anion of 1,4-benzoquinone under our experimental conditions was significantly higher than that realized by chloride adduction using dichloromethane (DCM) as the dopant.

Screening freshness of seafood by measuring trimethylamine (TMA) levels using helium-plasma ionization mass spectrometry (HePI-MS)

Background

Trimethylamine (TMA) is a marker used for monitoring the quality of seafood because it is the primary component of the “fishy” odor.

Methods

The levels of TMA in seafood samples were directly measured by helium-plasma ionization mass spectrometry (HePI-MS). Each sample was directly exposed to the HePI source, and the intensity of the m/z 60 signal for protonated TMA was monitored by a selected-ion-recording (SIR) protocol. Using a set of TMA-spiked water standards, the TMA levels in seafood samples were quantified.

Results

The signal intensity of the m/z 60 ion from shrimp samples maintained at room temperature for 2 days can be attenuated to baseline levels by adding lime juice. The amounts of TMA in samples of salmon and shrimp recovered from some sushi preparations, and in squid samples, were found to be 0.24 μg, 0.16 μg, and 17.2 μg per gram, respectively.

Conclusions

HePI-MS is an efficient technique to screen and monitor the TMA content and assess the quality of seafood.

Recent advances in ambient mass spectrometry of trace explosives

Ambient mass spectrometry has evolved rapidly over the past decade, yielding a plethora of platforms and demonstrating scientific advancements across a range of fields from biological imaging to rapid quality control. These techniques have enabled real-time detection of target analytes in an open environment with no sample preparation and can be coupled to any mass analyzer with an atmospheric pressure interface; capabilities of clear interest to the defense, customs and border control, transportation security, and forensic science communities. This review aims to showcase and critically discuss advances in ambient mass spectrometry for the trace detection of explosives.

Transformation of the gas-phase favored O-protomer of p-aminobenzoic acid to its unfavored N-protomer by ion activation in the presence of water vapor: An ion-mobility mass spectrometry study

An ion-mobility mass spectrometry study showed that the preferred O-protonated form of p-aminobenzoic in the gas phase can be converted to the thermodynamically less favored N-protomer by in-source collision-induced ion activation during the ion transfer process from the atmospheric region to the first vacuum region if the humidity is high in the ion source. Upon the addition of water vapor to the nitrogen gas used to promote the solid analyte to the gas phase under helium-plasma ionization conditions, the intensity of the ion-mobility arrival-time peak for the N-protomer increased dramatically. Evidently, the ion-activation process in the first vacuum region is able to provide the energy required to surmount the barrier to isomerize the O-protomer to the more energetic N-protomer. The transfer of the proton attached to the carbonyl oxygen atom of the O-protomer to the amino group takes place by a water-bridge mechanism. Apparently, the postionization transformations that take place during the transmission of ions from the atmospheric-pressure ion source to the detector, via different physical compartments of low to high vacuum, play an eminent role in determining the population ratios eventually manifested at the detector.

Untrapping Kinetically Trapped Ions: The Role of Water Vapor and Ion-Source Activation Conditions on the Gas-Phase Protomer Ratio of Benzocaine Revealed by Ion-Mobility Mass Spectrometry

The role of water vapor in transforming the thermodynamically preferred species of protonated benzocaine to the less favored protomer was investigated using helium-plasma ionization (HePI) in conjunction with ion-mobility mass spectrometry (IM-MS). The IM arrival-time distribution (ATD) recorded from a neat benzocaine sample desorbed to the gas phase by a stream of dry nitrogen and ionized by HePI showed essentially one peak for the O-protonated species. However, when water vapor was introduced to the enclosed ion source, within a span of about 150 ms the ATD profile changed completely to one dominated by the N-protonated species. Under spray-based ionization conditions, the nature and composition of the solvents have been postulated to play a decisive role in defining the manifested protomer ratios. In reality, the solvent vapors present in the ion source (particularly the ambient humidity) indirectly dictate the gas-phase ratio of the protomers. Evidently, the gas-phase protomer ratio established at the confinement of the ions is readjusted by the ion-activation that takes place during the transmission of ions to the vacuum. Although it has been repeatedly stated that ions can retain a "memory" of their solution structures because they can be kinetically trapped, and thereby represent their solution-based stabilities, we show that the initial airborne ions can undergo significant transformations in the transit through the intermediate vacuum zones between the ion source and the mass detector. In this context, we demonstrate that the kinetically trapped N-protomer of benzocaine can be untrapped by reducing the humidity of the enclosed ion source. Graphical Abstract ᅟ.

Influence of Ionization Source Conditions on the Gas-Phase Protomer Distribution of Anilinium and Related Cations

The gas-phase-ion generation technique and specific ion-source settings of a mass spectrometer influence heavily the protonation processes of molecules and the abundance ratio of the generated protomers. Hitherto that has been attributed primarily to the nature of the solvent and the pH. By utilizing electrospray ionization and ion-mobility mass spectrometry (IM-MS), we demonstrate, even in the seemingly trivial case of protonated aniline, that the protomer ratio strongly depends on the source conditions. Under low in-source ion activation, nearly 100% of the N-protomer of aniline is produced, and it can be subsequently converted to the C-protomer by collisional activation effected by increasing the electrical potential difference between the entrance and exit orifices of the first vacuum region. This activation and transformation process takes place even before the ion is mass-selected and subjected to IM separation. Despite the apparent simplicity of the problem, the preferred protonation site of aniline in the gas phase-the amino group or the aromatic ring-has been a topic of controversy. Our results not only provide unambiguous evidence that ring- and nitrogen-protonated aniline can coexist and be interconverted in the gas phase, but also that the ratio of the protomers depends on the internal energy of the original ion. There are many dynamic ion-transformation and fragmentation processes that take place in the different physical compartments of a Synapt G2 HDMS instrument. Such processes can dramatically change the very identity even of small ions, and therefore should be taken into account when interpreting product-ion mass spectra. Graphical Abstract ᅟ.

Collision-induced dissociation processes of protonated benzoic acid and related compounds: competitive generation of protonated carbon dioxide or protonated benzene

Upon activation in the gas phase, protonated benzoic acid (m/z 123) undergoes fragmentation by several mechanisms. In addition to the predictable water loss followed by a CO loss, the m/z 123 ion more intriguingly eliminates a molecule of benzene to generate protonated carbon dioxide (H - O⁺  ═ C ≡ O, m/z 45), or a molecule of carbon dioxide to yield protonated benzene (m/z 79). Experimental evidence shows that the incipient proton ambulates during the fragmentation processes. For the CO₂ or benzene loss, protonated benzoic acid transfers the charge-imparting proton initially to the ortho position and then to the ipso position to generate a transient species which dissociates to form an ion-neutral complex between benzene and protonated CO₂ . The formation of the m/z 45 ion is not a phenomenon unique to benzoic acid: spectra from protonated isophthalic acid, terephthalic acid, trans-cinnamic acid and some aliphatic acids also displayed a peak for m/z 45. However, the m/z 45 peak is structurally diagnostic only for certain benzene polycarboxylic acids because the spectra of compounds with two carboxyl groups on adjacent ring carbons do not produce a peak at m/z 45. For the m/z 79 ion to be formed, an intramolecular reaction should take place in which protonated CO₂ within the ion-neutral complex acts as the attacking electrophile to transfer a proton to benzene. Copyright © 2017 John Wiley & Sons, Ltd.

Oxidative Ionization Under Certain Negative-Ion Mass Spectrometric Conditions

1,4-Hydroquinone and several other phenolic compounds generate (M - 2) ^-• radical-anions, rather than deprotonated molecules, under certain negative-ion mass spectrometric conditions. In fact, spectra generated under helium-plasma ionization (HePI) conditions from 1,4-hydroquinone and 1,4-benzoquinone (by electron capture) were practically indistinguishable. Because this process involves a net loss of H^• and H⁺, it can be termed oxidative ionization. The superoxide radical-anion (O₂^-•), known to be present in many atmospheric-pressure plasma ion sources operated in the negative mode, plays a critical role in the oxidative ionization process. The presence of a small peak at m/z 142 in the spectrum of 1,4-hydroquinone, but not in that of 1,4-benzoquinone, indicated that the initial step in the oxidative ionization process is the formation of an O₂^-• adduct. On the other hand, under bona fide electrospray ionization (ESI) conditions, 1,4-hydroquinone generates predominantly an (M - 1) ^- ion. It is known that at sufficiently high capillary voltages, corona discharges begin to occur even in an ESI source. At lower ESI capillary voltages, deprotonation predominates; as the capillary voltage is raised, the abundance of O₂^-• present in the plasma increases, and the source in turn increasingly behaves as a composite ESI/APCI source. While maintaining post-ionization ion activation to a minimum (to prevent fragmentation), and monitoring the relative intensities of the m/z 109 (due to deprotonation) and 108 (oxidative ionization) peaks recorded from 1,4-hydroquinone, a semiquantitative estimation of the APCI contribution to the overall ion-generation process can be obtained. Graphical Abstract ᅟ.

Competitive homolytic and heterolytic decomposition pathways of gas-phase negative ions generated from aminobenzoate esters

An alkyl-radical loss and an alkene loss are two competitive fragmentation pathways that deprotonated aminobenzoate esters undergo upon activation under mass spectrometric conditions. For the meta and para isomers, the alkyl-radical loss by a homolytic cleavage of the alkyl-oxygen bond of the ester moiety is the predominant fragmentation pathway, while the contribution from the alkene elimination by a heterolytic pathway is less significant. In contrast, owing to a pronounced charge-mediated ortho effect, the alkene loss becomes the predominant pathway for the ortho isomers of ethyl and higher esters. Results from isotope-labeled compounds confirmed that the alkene loss proceeds by a specific γ-hydrogen transfer mechanism that resembles the McLafferty rearrangement for radical cations. Even for the para compounds, if the alkoxide moiety bears structural motifs required for the elimination of a more stable alkene molecule, the heterolytic pathway becomes the predominant pathway. For example, in the spectrum of deprotonated 2-phenylethyl 4-aminobenzoate, m/z 136 peak is the base peak because the alkene eliminated is styrene. Owing to the fact that all deprotonated aminobenzoate esters, irrespective of the size of the alkoxy group, upon activation fragment to form an m/z 135 ion, aminobenzoate esters in mixtures can be quantified by precursor ion discovery mass spectrometric experiments.

Microwave induced plasma desorption ionization (MIPDI) mass spectrometry for qualitative and quantitative analysis of preservatives in cosmetics

The quantitative and qualitative behaviors of the MIPDI source were systematically studied for the first time in this work.

Direct detection of solid inorganic mercury salts at ambient pressure by electron-capture and reaction-assisted HePI mass spectrometry

Solid HgCl2 is readily detected at ambient conditions by electron capture in a HePI-MS source. The captured electron occupies the empty 6 s orbital of the Hg atom. The resulting radical-anion HgCl2 (-•) can exist as three "flexomers" of different Cl-Hg-Cl angle. The facile in-source formation of HgCl2 (-•) and the adduct [HgCl3](--) is exploited to detect other solid Hg compounds by exposing them to an external chloride source, such as HCl, NaCl, or vapors emanating from solid TiCl3. In situ oxidation of Hg2Cl2 with H2O2 generated signals for HgCl2 (-•) and [HgCl3] (-), suggesting that oxidation makes Hg 6 s orbital available for electron capture.

Direct detection and identification of active pharmaceutical ingredients in intact tablets by helium plasma ionization (HePI) mass spectrometry

A simple modification converts an electrospray ion source to an ambient-pressure helium plasma ionization source without the need of additional expensive hardware. Peaks for active ingredients were observed in the spectra recorded from intact pharmaceutical tablets placed in this source. A flow of heated nitrogen was used to thermally desorb analytes to gas phase. The desorption temperatures were sometimes as low as 50 °C. For example, negative-ion spectra recorded from an aspirin tablet showed peaks at m/z 137 (salicylate anion) and 179 (acetylsalicylate anion) which were absent in the background spectra. The overall ion intensity increased as the desorption gas temperature was elevated. Within the same acquisition experiment, both positive- and negative-ion signals for acetaminophen were recorded from volatiles emanating from Tylenol tablets by switching the polarity of the capillary back and forth. Moreover, different preparations of acetaminophen tablets could be distinguished by their ion-intensity thermograms.