The efficiencies of charge exchange reaction in dopant-assisted atmospheric pressure chemical ionization (DA-APCI) and dopant-assisted atmospheric pressure photoionization (DA-APPI) mass spectrometry (MS) were compared by flow injection analysis. Fourteen individual compounds and a commercial mixture of 16 polycyclic aromatic hydrocarbons were chosen as model analytes to cover a wide range of polarities, gas-phase ionization energies, and proton affinities. Chlorobenzene was used as the dopant, and methanol/water (80/20) as the solvent. In both techniques, analytes formed the same ions (radical cations, protonated molecules, and/or fragments). However, in DA-APCI, the relative efficiency of charge exchange versus proton transfer was lower than in DA-APPI. This is suggested to be because in DA-APCI both dopant and solvent clusters can be ionized, and the formed reagent ions can react with the analytes via competing charge exchange and proton transfer reactions. In DA-APPI, on the other hand, the main reagents are dopant-derived radical cations, which favor ionization of analytes via charge exchange. The efficiency of charge exchange in both DA-APPI and DA-APCI was shown to depend heavily on the solvent flow rate, with best efficiency seen at lowest flow rates studied (0.05 and 0.1 mL/min). Both DA-APCI and DA-APPI showed the radical cation of chlorobenzene at 0.05–0.1 mL/min flow rate, but at increasing flow rate, the abundance of chlorobenzene M+. decreased and reagent ion populations deriving from different gas-phase chemistry were recorded. The formation of these reagent ions explains the decreasing ionization efficiency and the differences in charge exchange between the techniques.
Extractive atmospheric pressure photoionization (EAPPI) mass spectrometry was designed for rapid qualitative and quantitative analysis of chemicals in complex matrices. In this method, an ultrasonic nebulization system was applied to sample extraction, nebulization, and vaporization. Mixed with a gaseous dopant, vaporized analytes were ionized through ambient photon-induced ion–molecule reactions, and were mass-analyzed by a high resolution time-of-flight mass spectrometer (TOF-MS). After careful optimization and testing with pure sample solution, EAPPI was successfully applied to the fast screening of capsules, soil, natural products, and viscous compounds. Analysis was completed within a few seconds without the need for preseparation. Moreover, the quantification capability of EAPPI for matrices was evaluated by analyzing six polycyclic aromatic hydrocarbons (PAHs) in soil. The correlation coefficients (R2) for standard curves of all six PAHs were above 0.99, and the detection limits were in the range of 0.16–0.34 ng/mg. In addition, EAPPI could also be used to monitor organic chemical reactions in real time.
The utility of sodium ion adducts produced by matrix-assisted laser desorption ionization for the quantification of analytes with multiple oxygen atoms was evaluated. Uses of homogeneous solid samples and temperature control allowed the acquisition of reproducible spectra. The method resulted in a direct proportionality between the ion abundance ratio I([A?+?Na]+)/I([M?+?Na]+) and the analyte concentration, which could be used as a calibration curve. This was demonstrated for carbohydrates, glycans, and polyether diols with dynamic range exceeding three orders of magnitude.
A potassium-containing hexaazamacrocyclic dication, [M?H?K]2+, is able to add in the gas phase mono- and dicarboxylate anions as well as inorganic anions by forming the corresponding monocharged adducts, the structure of which markedly depends on the basicity of the anion. With anions, such as acetate or fluoride, the neutral hexaazamacrocycle M acts as an acceptor of monosolvated K+ ion. With less basic anions, such as trifluoroacetate or chloride, the protonated hexaazamacrocycle [M?H]+ performs the unusual functions of an acceptor of contact K+/anion pairs.
We demonstrate the first application of laser-induced acoustic desorption (LIAD) and atmospheric pressure photoionization (APPI) as a mass spectrometric method for detecting low-polarity organics. This was accomplished using a Lyman-α (10.2 eV) photon generating microhollow cathode discharge (MHCD) microplasma photon source in conjunction with the addition of a gas-phase molecular dopant. This combination provided a soft desorption and a relatively soft ionization technique. Selected compounds analyzed include α-tocopherol, perylene, cholesterol, phenanthrene, phylloquinone, and squalene. Detectable surface concentrations as low as a few pmol per spot sampled were achievable using test molecules. The combination of LIAD and APPI provided a soft desorption and ionization technique that can allow detection of labile, low-polarity, structurally complex molecules over a wide mass range with minimal fragmentation.
Hydration reactions of deprotonated nucleobases (uracil, thymine, 5-fluorouracil,2-thiouracil, cytosine, adenine, and hypoxanthine) produced by electrospray have been experimentally studied in the gas phase at 10 mbar using a pulsed ion-beam high-pressure mass spectrometer. The thermochemical data, ΔHo, ΔSo, and ΔGo, for the monohydrated systems were determined. The hydration enthalpies were found to be similar for all studied systems and varied between 39.4 and 44.8 kJ/mol. A linear correlation was found between water binding energies in the hydrated complexes and the corresponding acidities of the most acidic site of nucleobases. The structural and energetic aspects of the precursors for the hydrated complexes are discussed in conjunction with available literature data.
Proton transfer reaction mass spectrometry (PTR-MS) has played an important role in the field of real-time monitoring of trace volatile organic compounds (VOCs) due to its advantages such as low limit of detection (LOD) and fast time response. Recently, a new technology of proton extraction reaction mass spectrometry (PER-MS) with negative ions OH– as the reagent ions has also been presented, which can be applied to the detection of VOCs and even inorganic compounds. In this work, we combined the functions of PTR-MS and PER-MS in one instrument, thereby developing a novel technology called dipolar proton transfer reaction mass spectrometry (DP-PTR-MS). The selection of PTR-MS mode and PER-MS mode was achieved in DP-PTR-MS using only water vapor in the ion source and switching the polarity. In this experiment, ketones (denoted by M) were selected as analytes. The ketone (molecular weight denoted by m) was ionized as protonated ketone [M + H]+ [mass-to-charge ratio (m/z) m + 1] in PTR-MS mode and deprotonated ketone [M – H]– (m/z m – 1) in PER-MS mode. By comparing the m/z value of the product ions in the two modes, the molecular weight of the ketone can be positively identified as m. Results showed that whether it is a single ketone sample or a mixed sample of eight kinds of ketones, the molecular weights can be detected with DP-PTR-MS. The newly developed DP-PTR-MS not only maintains the original advantages of PTR-MS and PER-MS in sensitive and rapid detection of ketones, but also can estimate molecular weight of ketones.
In negative electrospray ionization mass spectrometry of 4-nitrobenzyl 4-hydroxybenzoates, a decarboxylation reaction, which was significantly promoted by the presence of a nitro group on the benzyl group, competed with radical elimination reactions. Density functional theory calculations indicated that decarboxylation of deprotonated 4-nitrobenzyl vanillate occurred via a radical route involving homolytic cleavage of the Cbenzyl–O bond to give a triplet ion–neutral complex, followed by decarboxylative coupling.
Complex mixture analysis is a costly and time-consuming task facing researchers with foci as varied as food science and fuel analysis. When faced with the task of quantifying oxygen-rich bio-oil molecules in a complex diesel mixture, we asked whether complex mixtures could be qualitatively and quantitatively analyzed on a single mass spectrometer with mid-range resolving power without the use of lengthy separations. To answer this question, we developed and evaluated a quantitation method that eliminated chromatography steps and expanded the use of quadrupole-time-of-flight mass spectrometry from primarily qualitative to quantitative as well. To account for mixture complexity, the method employed an ionization dopant, targeted tandem mass spectrometry, and an internal standard. This combination of three techniques achieved reliable quantitation of oxygen-rich eugenol in diesel from 300 to 2500 ng/mL with sufficient linearity (R2 = 0.97 ± 0.01) and excellent accuracy (percent error = 0% ± 5). To understand the limitations of the method, it was compared to quantitation attained on a triple quadrupole mass spectrometer, the gold standard for quantitation. The triple quadrupole quantified eugenol from 50 to 2500 ng/mL with stronger linearity (R2 = 0.996 ± 0.003) than the quadrupole-time-of-flight and comparable accuracy (percent error = 4% ± 5). This demonstrates that a quadrupole-time-of-flight can be used for not only qualitative analysis but also targeted quantitation of oxygen-rich lignin molecules in complex mixtures without extensive sample preparation. The rapid and cost-effective method presented here offers new possibilities for bio-oil research, including: (1) allowing for bio-oil studies that demand repetitive analysis as process parameters are changed and (2) making this research accessible to more laboratories.
Previous reports have shown that reactions occurring in the microdroplets formed during electrospray ionization can, under the right conditions, exhibit significantly greater rates than the corresponding bulk solution-phase reactions. The observed acceleration under electrospray ionization could result from a solution-phase, a gas-phase, or an interfacial reaction. This study shows that a gas-phase ion/molecule (or ion/ion) reaction is not responsible for the observed rate enhancement in the particular case of the Fischer indole synthesis. The results show that the accelerated reaction proceeds in the microdroplets, and evidence is provided that an interfacial process is involved.
Particulate matter 2.5 (PM2.5), collected from ambient air in Fukuoka City, was analyzed by gas chromatography combined with multiphoton ionization mass spectrometry using an ultraviolet femtosecond laser (267 nm) as the ionization source. Numerous parent polycyclic aromatic hydrocarbons (PPAHs) were observed in a sample extracted from PM2.5, and their concentrations were determined to be in the range from 30 to 190 pg/m3 for heavy PPAHs. Standard samples of nitrated polycyclic aromatic hydrocarbons (NPAHs) were examined, and the limits of detection were determined to be in the picogram range. The concentration of NPAH adsorbed on PM2.5 in the air was less than 900–1300 pg/m3.
Electron capture dissociation was used to probe the structure, unfolding, and folding of KIX ions in the gas phase. At energies for vibrational activation that were sufficiently high to cause loss of small molecules such as NH3 and H2O by breaking of covalent bonds in about 5% of the KIX (M + nH)n+ ions with n = 7–9, only partial unfolding was observed, consistent with our previous hypothesis that salt bridges play an important role in stabilizing the native solution fold after transfer into the gas phase. Folding of the partially unfolded ions on a timescale of up to 10 s was observed only for (M + nH)n+ ions with n = 9, but not n = 7 and n = 8, which we attribute to differences in the distribution of charges within the (M + nH)n+ ions.
Corona discharge ionization sources are often used in ion mobility spectrometers (IMS) when a non-radioactive ion source with high ion currents is required. Typically, the corona discharge is followed by a reaction region where analyte ions are formed from the reactant ions. In this work, we present a simple yet sufficiently accurate model for predicting the ion current available at the end of this reaction region when operating at reduced pressure as in High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) or most IMS-MS instruments. It yields excellent qualitative agreement with measurement results and is even able to calculate the ion current within an error of 15%. Additional interesting findings of this model are the ion current at the end of the reaction region being independent from the ion current generated by the corona discharge and the ion current in High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) growing quadratically when scaling down the length of the reaction region.
Top-down analyses of protonated insulin cations of charge states of 4+, 5+, or 6+ were performed by exposing the isolated precursor ions to a beam of helium cations with kinetic energy of more than 6 keV, in a technique termed charge transfer dissociation (CTD). The ~100 ms charge transfer reaction resulted in approximately 20% conversion efficiency to other intact charge exchange products (CTnoD), and a range of low abundance fragment ions. To increase backbone and sulfide cleavages, and to provide better structural information than straightforward MS2 CTD, the CTnoD oxidized products were isolated and subjected to collisional activation at the MS3 level. The MS3 CTD/CID reaction effectively broke the disulfide linkages, separated the two chains, and yielded more structurally informative fragment ions within the inter-chain cyclic region. CTD also provided doubly oxidized intact product ions at the MS2 level, and resonance ejection of the singly oxidized product ion revealed that the doubly oxidized product originates directly from the isolated precursor ion and not from consecutive CTD reactions of a singly oxidized intermediate. MS4 experiments were employed to help identify potential radical cations and diradical cations, but the results were negative or inconclusive. Nonetheless, the two-electron oxidation process is a demonstration of the very large potential energy (>20 eV) available through CTD, and is a notable capability for a 3D ion trap platform.
Peptide cation-radical fragment ions of the z-type, [●AXAR+], [●AXAK+], and [●XAR+], where X = A, C, D, E, F, G, H, K, L, M, N, P, Y, and W, were generated by electron transfer dissociation of peptide dications and investigated by MS3-near-ultraviolet photodissociation (UVPD) at 355 nm. Laser-pulse dependence measurements indicated that the ion populations were homogeneous for most X residues except phenylalanine. UVPD resulted in dissociations of backbone CO─NH bonds that were accompanied by hydrogen atom transfer, producing fragment ions of the [yn]+ type. Compared with collision-induced dissociation, UVPD yielded less side-chain dissociations even for residues that are sensitive to radical-induced side-chain bond cleavages. The backbone dissociations are triggered by transitions to second (B) excited electronic states in the peptide ion R-CH●-CONH- chromophores that are resonant with the 355-nm photon energy. Electron promotion increases the polarity of the B excited states, R-CH+-C●(O–)NH-, and steers the reaction to proceed by transfer of protons from proximate acidic Cα and amide nitrogen positions.
Reaction products from the ozonolysis of unsaturated lipids at gas–liquid interfaces have the potential to significantly influence the chemical and physical properties of organic aerosols in the atmosphere. In this study, the gas-phase dissociation behavior of lipid secondary ozonides is investigated using ion-trap mass spectrometry. Secondary ozonides were formed by reaction between a thin film of unsaturated lipids (fatty acid methyl esters or phospholipids) with ozone before being transferred to the gas phase as [M + Na]+ ions by electrospray ionization. Activation of the ionized ozonides was performed by either energetic collisions with helium buffer-gas or laser photolysis, with both processes yielding similar product distributions. Products arising from the decomposition of the ozonides were characterized by their mass-to-charge ratio and subsequent ion-molecule reactions. Product assignments were rationalized as arising from initial homolysis of the ozonide oxygen–oxygen bond with subsequent decomposition of the nascent biradical intermediate. In addition to classic aldehyde and carbonyl oxide-type fragments, carbon-centered radicals were identified with a number of decomposition pathways that indicated facile unimolecular radical migration. These findings reveal that photoactivation of secondary ozonides formed by the reaction of aerosol-bound lipids with tropospheric ozone may initiate radical-mediated chemistry within the particle resulting in surface modification.
This study proposes a new direct and fast method of analysis employing paper spray mass spectrometry (PS-MS). The paper used in the proposed method was modified with molecularly imprinted polymers (MIP) to create a specific site for cocaine analysis in oral fluid. MIP membrane was successfully synthetized and employed. The developed method showed to be linear in a concentration range from LOQ to 100 ng mL–1. The experimental value of LOQ obtained was 1 ng mL–1. The inter-day and intra-day precision and accuracy of the PS-MS method presented values lower than 15%. The total recoveries were also evaluated. The PS-MS method for the analysis of cocaine in oral fluid showed to be very promising and the validation parameters showed a good correlation with the literature.
Active capillary plasma ionization is a highly efficient ambient ionization method. Its general principle of ion formation is closely related to atmospheric pressure chemical ionization (APCI). The method is based on dielectric barrier discharge ionization (DBDI), and can be constructed in the form of a direct flow-through interface to a mass spectrometer. Protonated species ([M + H]+) are predominantly formed, although in some cases radical cations are also observed. We investigated the underlying ionization mechanisms and reaction pathways for the formation of protonated analyte ([M + H]+). We found that ionization occurs in the presence and in the absence of water vapor. Therefore, the mechanism cannot exclusively rely on hydronium clusters, as generally accepted for APCI. Based on isotope labeling experiments, protons were shown to originate from various solvents (other than water) and, to a minor extent, from gaseous impurities and/or self-protonation. By using CO2 instead of air or N2 as plasma gas, additional species like [M + OH]+ and [M ? H]+ were observed. These gas-phase reaction products of CO2 with the analyte (tertiary amines) indicate the presence of a radical-mediated ionization pathway, which proceeds by direct reaction of the ionized plasma gas with the analyte. The proposed reaction pathway is supported with density functional theory (DFT) calculations. These findings add a new ionization pathway leading to the protonated species to those currently known for APCI.
A modified Kendrick Mass Defect (KMD) analysis was applied to the analysis of polycyclic aromatic hydrocarbons (PAHs) and fullerenes in the diffusion flame from a handheld butane torch.
A nonresonant, femtosecond (fs) laser is employed to desorb samples of Victoria blue deposited on stainless steel or indium tin oxide (ITO) slides using either electrospray deposition (ESD) or dried droplet deposition. The use of ESD resulted in uniform films of Victoria blue whereas the dried droplet method resulted in the formation of a ring pattern of the dye. Laser electrospray mass spectrometry (LEMS) measurements of the ESD-prepared films on either substrate were similar and revealed lower average relative standard deviations for measurements within-film (20.9%) and between-films (8.7%) in comparison to dried droplet (75.5% and 40.2%, respectively). The mass spectral response for ESD samples on both substrates was linear (R2?>?0.99), enabling quantitative measurements over the selected range of 7.0?×?10?11 to 2.8?×?10?9 mol, as opposed to the dried droplet samples where quantitation was not possible (R2?=?0.56). The limit of detection was measured to be 210 fmol.
