Differentiation of isomeric compounds by two-stage proton transfer reaction time-of-flight mass spectrometry

We investigated a two-stage ion source for proton transfer reaction (PTR) ionization to achieve more selective mass spectrometric (MS) detection of selected volatile organic compounds (VOCs) than that achieved with commonly used PTR-MS instruments, which are based on single-step PTR ionization with H3O+. The two-stage PTR ion source generated reagent ions other than H3O+ by an initial PTR between H3O+ and a selected VOC, and then a second PTR ionization occurred only for VOCs with proton affinities larger than the affinity of the reagent VOC. Acetone and acetonitrile were useful as reagent VOCs because they provided dominant peaks as a protonated form. Using two-stage PTR-MS, we differentiated isomeric VOCs (for example, ethyl acetate and 1,4-dioxane) by means of differences in their proton affinities; protonated acetone formed the [M + H]+ ion from ethyl acetate but not from 1,4-dioxane. The PTR-MS-derived concentrations agreed quantitatively with those independently determined by Fourier transform infrared spectroscopy (FT-IR) at parts per million by volume (ppmv) levels. In addition, interfering fragment ions formed from alkyl benzenes at m/z 79 (C6H7+) could be distinguished from the m/z 79 ion arising from protonation of benzene, and therefore this method would prevent overestimation of benzene concentrations in air samples in which both benzene and alkyl benzenes are present. This two-stage PTR ionization may be useful for distinguishing various isomeric species, including aldehydes and ketones, if appropriate reagent ions are selected.     

Stopped-flow electrospray ionization mass spectrometry: a new method for studying chemical reaction kinetics in solution

In this work a new mass spectrometry based method for monitoring the kinetics of chemical reactions in solution is described. A stopped-flow mixing instrument is coupled to an electrospray ionization (ESI) mass spectrometer via a novel type of interface. Chemical reactions are initiated by rapid mixing of two reactant solutions. The mixture is instantaneously transferred to a reaction tube where the kinetics can be monitored in real-time by ESI mass spectrometry. With the current setup, a time window from 2.5 to 36 seconds after mixing of the reactants can be monitored. The experimental setup is used to study the kinetics of acetylcholine hydrolysis under alkaline conditions as a function of pH. The intensities of reactant (acetylcholine) and product (choline) ions are monitored simultaneously as a function of time. The reaction is carried out under pseudo-first-order conditions and the intensity-time curves are well described by single exponentials. The rate constants determined from these fits compare favorably with previous data from the literature.     

Solvation in electrospray mass spectrometry: effects on the reaction kinetics of fragmentation mediated by ion-neutral complexes

In electrospray ionization (ESI) on a triple quadrupole mass spectrometer, benzydamine, a molecule with an N,N-dimethylaminopropoxyl side chain, showed a fragmentation pattern in Q1 scans that is dramatically different from the mass-selected collision-induced dissociation (CID) of its MH(+) ion. The N,N-dimethylimmonium ion, which dominates in Q1 scans at higher energies, is only a minor product in all CID spectra. By using a smaller model molecule, N,N,N',N'-tetramethyl-1,3-propanediamine, with the kinetic energy release measured for the corresponding reaction, we have demonstrated that an ion-neutral complex composed of the N,N-dimethylazetidine cation and a neutral counterpart is involved. When the ion-neutral complex intermediate evolves toward elimination to form the immonium ion, the transition state is stabilized by the neutral species. Solvation of the ion-neutral complex, which obstructs the separation of the two partners by the resulting tighter enclosure, facilitates the elimination by enhancing the stabilization of the transition state. Therefore, the prevalence of the immonium ion in Q1 scans was a result of solvation in the ESI source. In CID reactions, where the decomposing ions are mass-selected and thus solvation does not exist, the immonium ion was a minor product, and the separation of the ion-neutral complex became dominant.     

Study on the mechanism of Schiemann reaction by mass spectrometry

Schiemann reaction of 12 aryl diazonium fluoborates was studied by mass spectrometry. The products and by-products of the reactions were identified and the possible pathways were explored. It is shown that Schiemann reaction proceeds by free-radical mechanism.     

Hydroxyproline quantification for the estimation of collagen in tissue using multiple reaction monitoring mass spectrometry

Collagens are highly abundant mammalian proteins that contain a high content of hydroxylated amino acids such as hydroxyproline. We have exploited the high hydroxyproline content of collagen and developed a method for hydroxyproline quantification as a measure of collagen content in muscle samples. The novel method utilizes a highly selective and sensitive method of multiple reaction monitoring (MRM) by mass spectrometry. The analytical method is simple, rapid (5min), convenient (no derivatization), precise (<17% RSD), accurate (90-108%), sensitive (4.88nmol/L) and linear (R(2)>0.999) over three orders of magnitude (5-5000nmol/L).     

Effect of elemental composition of PtPd bimetallic nanoparticles containing an average of 180 atoms on the kinetics of the electrochemical oxygen reduction reaction

PtPd bimetallic nanoparticles containing an average of 180 atoms and composed of seven different Pt:Pd ratios have been prepared within sixth-generation, hydroxyl-terminated, poly(amidoamine) dendrimers. Transmission electron microscopy indicates that the sizes of all seven nanoparticle compositions are within +/-0.2 nm of one another and the calculated size. Single-particle energy-dispersive spectroscopy shows that the elemental composition is determined by the ratio of the Pt and Pd precursor salts used to prepare the nanoparticles. Cyclic voltammetry and rotating disk voltammetry measurements show that the Pt:Pd ratio of the nanoparticles determines their efficiency for the oxygen reduction reaction (ORR). The maximum activity for the ORR occurs at a Pt:Pd ratio of 5:1, which corresponds to a relative mass activity enhancement of 2.4 compared to otherwise identical monometallic Pt nanoparticles.     

Identification and quantification of VOCs by proton transfer reaction time of flight mass spectrometry: An experimental workflow for the optimization of specificity,sensitivity, and accuracy

Proton transfer reaction time of flight mass spectrometry (PTR‐ToF‐MS) is a direct injection MS technique, allowing for the sensitive and real‐time detection, identification, and quantification of volatile organic compounds. When aiming to employ PTR‐ToF‐MS for targeted volatile organic compound analysis, some methodological questions must be addressed, such as the need to correctly identify product ions, or evaluating the quantitation accuracy. This work proposes a workflow for PTR‐ToF‐MS method development, addressing the main issues affecting the reliable identification and quantification of target compounds. We determined the fragmentation patterns of 13 selected compounds (aldehydes, fatty acids, phenols). Experiments were conducted under breath‐relevant conditions (100% humid air), and within an extended range of reduced electric field values (E/N = 48–144 Td), obtained by changing drift tube voltage. Reactivity was inspected using H₃O⁺, NO⁺, and O₂⁺ as primary ions. The results show that a relatively low (<90 Td) E/N often permits to reduce fragmentation enhancing sensitivity and identification capabilities, particularly in the case of aldehydes using NO⁺, where a 4‐fold increase in sensitivity is obtained by means of drift voltage reduction. We developed a novel calibration methodology, relying on diffusion tubes used as gravimetric standards. For each of the tested compounds, it was possible to define suitable conditions whereby experimental error, defined as difference between gravimetric measurements and calculated concentrations, was 8% or lower.     

Rapid white truffle headspace analysis by proton transfer reaction mass spectrometry and comparison with solid-phase microextraction coupled with gas chromatography/mass spectrometry

The gastronomic relevance and high price of white truffle are related mainly to its unique aroma. Here we evaluate, for the first time, the possibility of characterizing in a rapid and non-destructive way the aroma of white truffles based on proton transfer reaction mass spectrometry (PTR-MS). We indicate that anonymous PTR-MS fingerprinting allows sample classification and we also compare qualitatively and quantitatively PTR-MS data with measurements made by solid-phase microextraction gas chromatography (SPME-GC) of the same samples under the same conditions. PTR-MS fragmentation data of truffle-relevant compounds are also published here for the first time. Most of the sulfur-containing compounds detected by GC and relevant for white truffle aroma have a high positive correlation with single PTR-MS peaks. Our work indicates that, after preliminary comparison with GC data, PTR-MS is a new tool for the rapid, quantitative and non-invasive characterization of white truffle by direct headspace injection without any pre-concentration.     

Reaction kinetics in metal hydride reaction beds with improved heat and mass transfer

Periodically operating absorption engines, i.e. absorption heat pumps, heat transformers and refrigerators based on metal-hydrogen reactions, are becoming of great interest in technical applications. In order to keep the material inventory small and to design compact machines, short cycle times are necessary. In this paper, experimental investigations of the technical reaction kinetics of various AB₅ alloys are presented and it is shown that it is equally important to improve both the hydrogen flow and the heat transfer in a reaction bed.     

Degradation kinetics of VX on concrete by secondary ion mass spectrometry

At trace coverages on concrete surfaces, the nerve agent VX (O-ethyl S-2-diisopropylaminoethyl methyl phosphonothiolate) degrades by cleavage of the P-S and S-C bonds, as revealed by periodic secondary ion mass spectrometry (SIMS). The observed kinetics were (pseudo-) first-order, with a half-life of 2-3 h at room temperature. The rate increased with surface pH and temperature, with an apparent second-order constant of k(OH) = 0.64 M(-1) min(-1) at 25 degrees C and an activation energy of 50-60 kJ mol(-1). These values are consistent with a degradation mechanism of alkaline hydrolysis within the adventitious water film on the concrete surface. Degradation of bulk VX on concrete would proceed more slowly.     

New insights on the electrochemical oxidation of ethanol on carbon-supported Pt electrode by a novel electrochemical mass spectrometry configuration

Herein, we demonstrate the versatility and high sensitivity of the meniscus-based approach of a novel on-line electrochemical mass spectrometry (EC-MS) by studying the formation of gaseous/volatile products during ethanol oxidation on carbon-supported Pt electrode. In this way, methane is detected for the first time during ethanol adsorption at this electrode. Accordingly, new insights on the ethanol oxidation reactions are achieved.