Using dopants in the atmospheric pressure chemical ionization ion source to determine the site of protonation by ion mobility spectrometry.

RATIONALE: Compounds like caffeine metabolites with more than one proton acceptor site can produce a mixture of isomeric protonated ions (protomers) in electrospray ionization and atmospheric pressure chemical ionization (APCI) ion sources. Discrimination between the protomers is of interest as the charge location influences ion structure and chemical and physical properties. METHODS: Protonation of caffeine in an APCI ion source was studied using ion mobility spectrometry. The hydronium ions, H3O+(H2O)n, are the main reactant ions in the APCI ion source. Different dopant gases including NO2, NH3, and CH3NH2 were used to produce new reactant ions NO+, NH4 +, and CH3NH3 +, respectively. Density functional theory was employed to explain the experimental results and calculate the energies of the ionization reactions. RESULTS: The ion mobility spectrum of caffeine showed three peaks. In the presence of NO2 dopant and NO+ reactant ion, caffeine was ionized via charge transfer and formation of M+ ion. As NH3 and CH3NH2 are stronger bases than H2O, the reactant ions NH4 + and CH3NH3 + selectively protonated the more basic site of caffeine, that is, the imidazole nitrogen. Using these dopants, we could attribute the first ion mobility peak to M+ ion, the second peak to the protonation of caffeine at the carbonyl oxygen atom, and the third peak to the protonation of the imidazole nitrogen atom. The calculated collisional cross-sections of M+ and the protomers of caffeine confirmed the peaks' assignment. CONCLUSIONS: The criterion for the selection of an appropriate dopant is that its proton affinity (PA) should be between those of the proton acceptor sites of the molecule studied.

Thermal electron attachment to halogenated silanes in the gas phase.

RATIONALE: Silane derivatives play a crucial role in industrial plasma processes for the fabrication of various electronic devices such as lighting devices, solar cells, and displays. Accurate quantitative data are essential for modeling technological plasmas. This study reports the rate coefficients (k) and activation energies (Ea) for thermal electron attachment to Si2Cl6, Si (CH3)3CHF2, and SiCl (CH3)2Si(CH3)3, which are key parameters for understanding the underlying processes in plasmas. The results obtained for other silane derivatives were also analyzed and discussed. METHODS: The measurements were conducted using the pulsed Townsend technique. In this technique, electrons generated by a laser under an electric field travel to the anode, inducing a charge on it. In the presence of a scavenger gas, electrons are captured, leading to a decrease in the rate of charge increase over time. The kinetic parameters were deduced from the shape of the pulse. The G4 method was used to obtain bond dissociation energies (BDEs). RESULTS: This study determined the kinetic parameters for thermal electron attachment to Si2Cl6, Si (CH3)3CHF2, and SiCl (CH3)2Si(CH3)3 for the first time. The rate coefficients at 298 K were found to be 2.17 ± 0.04 × 10-9cm3s-1, 2.01 ± 0.09 × 10-12cm3s-1, and 8.05 ± 0.07 × 10-12cm3s-1, respectively. The corresponding activation energies were determined to be 0.37 ± 0.04 eV, 0.29 ± 0.03 eV, and 0.21 ± 0.01 eV for Si2Cl6, Si (CH3)3CHF2, and SiCl (CH3)2Si(CH3)3, respectively. The experiment was conducted over the temperature range of 298-378 K. CONCLUSIONS: The findings of this study provide significant new insights into fundamental parameters such as rate coefficients and activation energies for thermal electron capture by chlorinated and fluorinated silane derivatives. These data contribute to advancing our understanding of thermal electron interactions with chlorosilanes, which can be utilized for controlling important species in the plasmas of various modern technologies.

Formation of negative ions from cobalt tricarbonyl nitrosyl Co(CO)3NO clusters.

Electron attachment and corresponding dissociative electron attachment (DEA) to cobalt tricarbonyl nitrosyl (Co(CO)3NO) clusters have been studied by co-expansion with Ar gas into a high vacuum. A monochromatic electron beam was utilized to generate negative ions and the resulting reaction products were identified using mass spectrometry. The ion fragments corresponding to Co(CO)3NO monomers closely resemble results from earlier gas phase experiments and studies conducted on Co(CO)3NO in He nanodroplets. However, contrary to the gas phase or He nanodroplet ion yields, a resonance structure comprising several peaks at energies above ∼4 eV was observed both in the case of molecular clusters [Co(CO)3NO]n- (with n = 1, 2, 3) and clusters comprising DEA fragments. Additionally, the ion yields of numerous other clusters such as ions without nitrosyl ([Co(CO)4]-, [Co2(CO)5]-), clusters consisting of two fragments such as ([Co2(CO)NO]-, [Co2(CO)(NO)2]-, [Co2(CO)2NO]-, [Co2(CO)2(NO)2]-, [Co3(CO)(NO)3]-, [Co3(CO)8(NO)3]-, [Co3(CO)(NO)2]-, [Co3(CO)3(NO)2]-, and [Co3(CO)5(NO)2]-) were recorded. Moreover, NO bond dissociation was confirmed with the [Co(CO)2N]-ion and with N- or O-retaining cluster ions, such as [Co2(CO)(NO)N]-, [Co2(CO)2(NO)N]-, [Co3(CO)2(NO)N]-, [Co3(CO)3(NO)N]- and [Co3(CO)(NO)2N]-, or [Co2(CO)2O]-, [Co2(CO)3O]-, [Co3(CO)3O]-, [Co3(CO)4O]-and [Co3(CO)2(NO)O]- respectively.

Laser Desorption of Explosives from the Surface of Different Real-World Materials Studied Using C2Cl6-Dopant-Assisted Ion Mobility Spectrometry.

A highly efficient and sensitive ion mobility spectrometry (IMS) system with laser desorption sampling was applied for rapid explosive detection using different surface materials. This portable IMS detector, powered by a battery, offers mobility and is suitable for use in the field or combat zones. The laser desorption (LD) sampling of common explosives (Trinitrotoluene-TNT; Dinitrotoluenes-DNTs; Hexogene-RDX; pentaerythritol tetranitrate-PETN; plastic explosives-Compound 4 (C-4) and Semtex) on a wide range of common surface materials, such as metal, ceramic, plastic, glass, drywall, paper, wood, and textiles, was studied. Successful detection was achieved on nearly all surfaces except flammable materials (paper, wood, and textiles). The limit of detection (LOD) was determined for each explosive and specific surface, demonstrating an impressive LOD of 7 ng/mm2 for TNT. RDX, C-4, PETN, and Semtex achieved LOD values of 15 ng/mm2, while DNTs showed an LOD of approximately 50 ng/mm2.

Dissociative Excitation of Nitromethane Induced by Electron Impact in the Ultraviolet - Visible Spectrum.

The excitation of nitromethane (CH3 NO2 ), which is an important propellant and prototypic molecule for large class of explosives, has been investigated by electron impact and subsequent emission of photons in the UV-VIS spectral region between 300 nm and 670 nm. Emission spectrum of nitromethane was recorded at an electron energy of 50 eV. New dissociative excitation channels were observed through the appearance of different CH, CN, NH, OH and NO bands, and the Balmer series of atomic hydrogen. In addition, relative emission cross sections were recorded for the transitions of selected fragments. The emission spectrum was captured with significantly higher resolution in comparison to previous studies.

Effect of ion source polarity and dopants on the detection of auxin plant hormones by ion mobility-mass spectrometry.

Ion mobility spectrometry (IMS) equipped with a corona discharge (CD) ion source was used for measurement of three auxin plant hormones including indole-3-acetic acid (IAA), indole-3-propionic acid (IPA), and indole-3-butyric acid (IBA). The measurements were performed in both positive and negative polarities of the CD ion source. Dopant gases NH3, CCl4, and CHBr3 were used to modify the ionization mechanism. A time-of-flight mass spectrometer (TOFMS) orthogonal to the IMS cell was used for identification of the product ions. Density functional theory was used to rationalize formation of the ions, theoretically. The mixtures of the auxins were analyzed by CD-IMS. The separation performance depended on the ion polarity and the dopants. In the positive polarity without dopants, auxins were ionized via protonation and three distinguished peaks were observed. Application of NH3 dopant resulted in two ionization channels, protonation, and NH4+ attachment leading to peak overlapping. In the negative polarity, two ionization reactions were operative, via deprotonation and O2- attachment. The separation of the monomer peaks was not achieved while the peaks of anionic dimers [2 M-H]- were separated well. The best LOD (4 ng) was obtained in negative polarity with CCl4 dopant. Methylation (esterification) of IAA improved LODs by about one order.

Study of atmospheric pressure chemical ionization of phthalates in air by ion mobility spectrometry/mass spectrometry.

RATIONALE: Phthalates are widely used in consumer products in the chemical industries. Due to their abundance in the milieu, their potentially harmful effect on the environment, human and animal health there is a need for sensitive and fast methods for their detection. METHODS: Positive polarity Corona Discharge Atmospheric Pressure Chemical Ionization (CD-APCI) in the air was applied for ionization of phthalates. The ionization method for the phthalates was studied by atmospheric pressure Ion Mobility Spectrometry (IMS) and hybrid IMS/orthogonal acceleration Time-of-Flight Mass spectrometry (IMS-oaTOF-MS). RESULTS: CD-APCI IMS and MS spectra of selected phthalates (dimethyl phthalate, diethyl phthalate, diethyl isophthalate, diethyl terephthalate, dipropyl phthalate, diisopropyl phthalate, dibutyl phthalate, diisobutyl phthalate, and dibutyl terephthalate) were recorded. In the case of the ortho- and "iso"-isomers exclusively the protonated molecular ions [M + H]+ were detected. In the case of the para- and meta-isomers and regioisomers, APCI resulted in the appearance of hydrated protonated molecular ions [M + H]+ ·(H2 O)0,1,2 . The ion mobilities, collision cross-sections of these ions in air, as well as the limits of detection (LODs) for the phthalate vapors, were determined. In the case of isomeric phthalates, we have demonstrated the potential of the IMS technique for their separation. CONCLUSIONS: The results show that CD-APCI in combination with IMS and IMS-oaTOF-MS is a suitable method for the fast and sensitive detection of phthalates with the potential to separate some isomers.

Low Energy Electron Attachment by Some Chlorosilanes.

In this paper, the rate coefficients (k) and activation energies (Ea) for SiCl4, SiHCl3, and Si(CH3)2(CH2Cl)Cl molecules in the gas phase were measured using the pulsed Townsend technique. The experiment was performed in the temperature range of 298-378 K, and carbon dioxide was used as a buffer gas. The obtained k depended on temperature in accordance with the Arrhenius equation. From the fit to the experimental data points with function described by the Arrhenius equation, the activation energies (Ea) were determined. The obtained k values at 298 K are equal to (5.18 ± 0.22) × 10-10 cm3·s-1, (3.98 ± 1.8) × 10-9 cm3·s-1 and (8.46 ± 0.23) × 10-11 cm3·s-1 and Ea values were equal to 0.25 ± 0.01 eV, 0.20 ± 0.01 eV, and 0.27 ± 0.01 eV for SiHCl3, SiCl4, and Si(CH3)2(CH2Cl)Cl, respectively. The linear relation between rate coefficients and activation energies for chlorosilanes was demonstrated. The DFT/B3LYP level coupled with the 6-31G(d) basis sets method was used for calculations of the geometry change associated with negative ion formation for simple chlorosilanes. The relationship between these changes and the polarizability of the attaching center (αcentre) was found. Additionally, the calculated adiabatic electron affinities (AEA) are related to the αcentre.

Development of Microchip Isotachophoresis Coupled with Ion Mobility Spectrometry and Evaluation of Its Potential for the Analysis of Food, Biological and Pharmaceutical Samples.

An online coupling of microchip isotachophoresis (µITP) with ion mobility spectrometry (IMS) using thermal evaporation interface is reported for the first time. This combination integrates preconcentration power of the µITP followed by unambiguous identification of trace compounds in complex samples by IMS. Short-chain carboxylic acids, chosen as model analytes, were first separated by the µITP in a discontinuous electrolyte system at pH 5-6, and subsequently evaporated at 130 °C during their transfer to the IMS analyzer. Various parameters, affecting the transfer of the separated sample components through the evaporation system, were optimized to minimize dispersion and loss of the analytes as well as to improve sensitivity. The following analytical attributes were obtained for carboxylic acids in the standard solutions: 0.1-0.3 mg L-1 detection limits, 0.4-0.9 mg L-1 quantitation limits, linear calibration range from the quantitation limit to 75 mg L-1, 0.2-0.3% RSD of the IMS response and 98-102% accuracy. The analytical potential of the developed µITP-IMS combination was demonstrated on the analysis of various food, pharmaceutical and biological samples, in which the studied acids are naturally present. These include: apple vinegar, wine, fish sauce, saliva and ear drops. In the real samples, 0.3-0.6% RSD of the IMS response and 93-109% accuracy were obtained.

Ion chemistry of phthalates in selected ion flow tube mass spectrometry: isomeric effects and secondary reactions with water vapour.

Phthalates are widely industrially used and their toxicity is of serious environmental and public health concern. Chemical ionization (CI) analytical techniques offer the potential to detect and monitor traces of phthalate vapours in air or sample headspace in real time. Promising techniques include selected ion flow tube mass spectrometry (SIFT-MS), proton transfer reaction mass spectrometry (PTR-MS) and ion mobility spectrometry (IMS). To facilitate such analyses, reactions of H3O+, O2+ and NO+ reagent ions with phthalate molecules need to be understood. Thus, the ion chemistry of dimethyl phthalate isomers (dimethyl phthalate, DMP - ortho; dimethyl isophthalate, DMIP - meta; dimethyl terephthalate, DMTP - para), diethyl phthalate (DEP), dipropyl phthalate (DPP) and dibutyl phthalate (DBP) was studied by SIFT-MS. Reactions of H3O+, O2+ and NO+ with these phthalate molecules M were found to produce the characteristic primary ion products MH+, M+ and MNO+, respectively. In addition, a dissociation process forming the (M-OR)+ fragment was observed. For phthalates with longer alkyl chains, mainly DPP and DBP, a secondary dissociation channel triggered by the McLafferty rearrangement was also observed. However, this is dominant only for the more energetic O2+ reactions with phthalates, additionally resulting in a recognisable formation of the protonated phthalate anhydride. For the NO+ reagent ions, the McLafferty rearrangement makes only a minor contribution and for H3O+, it was not observed. Experiments on the effect of water vapour on this ion chemistry have shown that protonated DMIP and DMTP efficiently associate with H2O forming the DMIP·H+H2O, DMIP·H+(H2O)2 and DMTP·H+H2O cluster ions, whilst the protonated ortho DMP isomer as well as other ortho phthalates DEP, DPP and DBP does not associate with H2O. The results indicate that the degree of hydration can be used to identify specific phthalate isomers in CI.

Isomer and conformer selective atmospheric pressure chemical ionisation of dimethyl phthalate.

In this work we have studied the ionisation mechanism of Atmospheric Pressure Chemical Ionisation (ACPI) for three isomers of dimethyl phthalate (dimethyl phthalate - DMP (ortho- isomer), dimethyl isophthalate - DMIP (meta) and dimethyl terephthalate - DMTP (para)) using Ion Mobility Spectrometry (IMS) and IMS combined with an orthogonal acceleration Time of Flight Mass Spectrometer (oa-TOF MS). The molecules were chemically ionised using reactant ions H+·(H2O)n (n = 3 and 4). The positive IMS and IMS-oaTOF mass spectra of the isomers showed significant differences in the ion mobilities and in the ion composition. The IMS - oaTOF spectra consisted of clusters of ions M·H+·(H2O)n with different degrees of hydration (n = 0, 1, 2, 3) for different isomers. In the case of the DMP isomer, we have observed almost exclusive formation of M·H+ by proton transfer ionisation, while in the case of DMIP and DMTP, hydrated ions M·H+·(H2O)n (n = 1, 2, 3) and M·H+·(H2O)n (n = 0, 1, 2) respectively were detected, formed via adduct formation reactions. This behaviour was elucidated by differences in ionisation processes. In order to elucidate the ionisation processes we have carried out DFT calculations of the structures and energies of the neutral and protonated and hydrated isomers (for different conformers) and their corresponding proton affinities (PA) and hydration energies.

Study of Atmospheric Pressure Chemical Ionization Mechanism in Corona Discharge Ion Source with and without NH3 Dopant by Ion Mobility Spectrometry combined with Mass Spectrometry: A Theoretical and Experimental Study.

Ionization of 2-nonanone, cyclopentanone, acetophenone, pyridine, and di- tert-butylpyridine (DTBP) in a corona discharge (CD) atmospheric pressure chemical ionization (APCI) ion source was studied using ion mobility (IMS) and time-of-flight mass spectrometry (TOF-MS). The IMS and MS spectra were recorded in the absence and presence of ammonia dopant. Without NH3 dopant, the reactant ion (RI) was H+(H2O) n, n = 3,4, and the MH+(H2O) x clusters were produced as product ions. Modeling of hydration shows that the amount of hydration ( x) depends on basicity of M, temperature and water concentration of drift tube. In the presence of ammonia (NH4+(H2O) n as RI) two kinds of product ions, MH+(H2O) x and MNH4+(H2O) x, were produced, depending on the basicity of M. With NH4+(H2O) n as RI, the product ions of pyridine and DTBP with higher basicity were MH+(H2O) x while cyclopentanone, 2-nonanone, and acetophenone with lower basicity produce MNH4+(H2O) x. To interpret the formation of product ions, the interaction energies of M-H+, H+-NH3, and H+-OH2 in the M-H+-NH3 and M-H+-OH2 and M-H+-M complexes were computed by B3LYP/6-311++G(d,p) method. It was found that for a molecule M with high basicity, the M-H+ interaction is strong leading in weakening of the H+-NH3, and H+-OH2 interactions in the M-H+-NH3 and M-H+-OH2 complexes.

Dissociation of dicyclohexyl phthalate molecule induced by low-energy electron impact.

Experimental investigation of electron ionization (EI) of and electron attachment (EA) onto dicyclohexyl phthalate (DCHP) was carried out using a crossed electron and molecular beam technique. Formation of positive and negative ions by EI and EA with the corresponding dissociation processes was studied and discussed. Due to a low ion yield of the parent positive ion, we were not able to estimate the ionization energy of DCHP. However, we estimated the appearance energies for the protonated phthalate anhydride (m/z 149) to be 10.5 eV and other significant ionic fragments of m/z 249 [DCHP-(R-2H)]+, m/z 167 [DCHP-(2R-3H)]+, and m/z 83 [C6H11]+. The reaction mechanisms of the dissociative ionization process were discussed. In the case of negative ions, we estimated the relative cross sections for a transient negative ion (TNI) and for several detected ions. At low electron energies (close to 0 eV), the TNI of DCHP molecules was the dominant ion, with products of dissociative EA dominating in broad resonances at 7.5 and 8.5 eV.

Quantitative aspects of microchip isotachophoresis for high precision determination of main components in pharmaceuticals.

Although microchip electrophoresis (MCE) is intended to provide reliable quantitative data, so far there is only limited attention paid to these important aspects. This study gives a general overview of key aspects to be followed to reach high-precise determination using isotachophoresis (ITP) on the microchip with conductivity detection. From the application point of view, the procedure for the determination of acetate, a main component in the pharmaceutical preparation buserelin acetate, was developed. Our results document that run-to-run fluctuations in the sample injection volume limit the reproducibility of quantitation based on the external calibration. The use of a suitable internal standard (succinate in this study) improved the repeatability of the precision of acetate determination from six to eight times. The robustness of the procedure was studied in terms of impact of fluctuations in various experimental parameters (driving current, concentration of the leading ions, pH of the leading electrolyte and buffer impurities) on the precision of the ITP determination. The use of computer simulation programs provided means to assess the ITP experiments using well-defined theoretical models. A long-term validity of the calibration curves on two microchips and two MCE equipments was verified. This favors ITP over other microchip electrophoresis techniques, when chip-to-chip or equipment-to-equipment transfer of the analytical method is required. The recovery values in the range of 98-101 % indicate very accurate determination of acetate in buserelin acetate, which is used in the treatment of hormone-dependent tumors. This study showed that microchip ITP is suitable for reliable determination of main components in pharmaceutical preparations.

Direct Liquid Sampling for Corona Discharge Ion Mobility Spectrometry.

We present a new technique suitable for direct liquid sampling and analysis by ion mobility spectrometry (IMS). The technique is based on introduction of a droplet stream to the IMS reaction region. The technique was successfully used to detect explosives dissolved in methanol and oil as well as to analyze amino acids and dipeptides. One of the main advantages of this technique is its ability to analyze liquid samples without the requirement of any special solution.

Acetone and the precursor ligand acetylacetone: distinctly different electron beam induced decomposition?

In focused electron beam induced deposition (FEBID) acetylacetone plays a role as a ligand in metal acetylacetonate complexes. As part of a larger effort to understand the chemical processes in FEBID, the electron-induced reactions of acetylacetone were studied both in condensed layers and in the gas phase and compared to those of acetone. X-ray photoelectron spectroscopy (XPS) shows that the electron-induced decomposition of condensed acetone layers yields a non-volatile hydrocarbon residue while electron irradiation of acetylacetone films produces a non-volatile residue that contains not only much larger amounts of carbon but also significant amounts of oxygen. Electron-stimulated desorption (ESD) and thermal desorption spectrometry (TDS) measurements reveal striking differences in the decay kinetics of the layers. In particular, intact acetylacetone suppresses the desorption of volatile products. Gas-phase studies of dissociative electron attachment and electron impact ionization suggest that this effect cannot be traced back to differences in the initial fragmentation reactions of the isolated molecules but is due to subsequent dissociation processes and to an efficient reaction of released methyl radicals with adjacent acetylacetone molecules. These results could explain the incorporation of large amounts of ligand material in deposits fabricated by FEBID processes using acetylacetonate complexes.

A corona discharge atmospheric pressure chemical ionization source with selective NO(+) formation and its application for monoaromatic VOC detection.

We have developed a new type of corona discharge (CD) for atmospheric pressure chemical ionization (APCI) for application in ion mobility spectrometry (IMS) as well as in mass spectrometry (MS). While the other CD-APCI sources are able to generate H3O(+)·(H2O)n as the major reactant ions in N2 or in zero air, the present CD-APCI source has the ability to generate up to 84% NO(+)·(H2O)n reactant ions in zero air. The change of the working gas from zero air to N2 allows us to change the major reactant ions from NO(+)·(H2O)n to H3O(+)·(H2O)n. In this paper we present the description of the new CD-APCI and discuss the processes associated with the NO(+) formation. The selective formation of NO(+)·(H2O)n reactant ions offers chemical ionization based on these ions which can be of great advantage for some classes of chemicals. We demonstrate here a significant increase in the sensitivity of the IMS-MS instrument for monoaromatic volatile organic compound (VOC) detection upon NO(+)·(H2O)n chemical ionization.

Dissociative electron attachment to hexafluoroacetylacetone and its bidentate metal complexes M(hfac)2; M = Cu, Pd.

Beta-diketones are a versatile class of compounds that can complex almost any metal in the periodic table of elements. Their metal complexes are found to be fairly stable and generally have sufficient vapor pressure for deposition techniques requiring volatile metal sources. Motivated by the potential role of low energy electrons in focused electron beam induced deposition, we have carried out a crossed electron∕molecular beam study on the dissociative electron attachment and non-dissociative electron attachment (NDEA) to hexafluoroacetylacetone (HFAc) and its bidentate metal complexes: bis-hexafluoroacetylacetonate copper(II), Cu(hfac)2 and bis-hexafluoroacetylacetonate palladium(II), Pd(hfac)2. The relative ion yield curves for the native precursor to the ligand as well as its stable, 16 valence electron Pd(II) complex and open shell, 17 valence electron Cu(II) complex, are presented and compared. For HFAc, the loss of HF leads to the dominant anion observed, and while NDEA is only weakly pronounced for Pd(hfac)2 and loss of hfac(-) is the main dissociation channel, [Cu(hfac)2](-) formation from Cu(hfac)2 dominates. A comparison of the ion yield curves and the associated resonances gives insight into the role of the ligand in the attachment process and highlights the influence of the central metal atom.

Absolute cross sections for dissociative electron attachment and dissociative ionization of cobalt tricarbonyl nitrosyl in the energy range from 0 eV to 140 eV.

We report absolute dissociative electron attachment (DEA) and dissociative ionization (DI) cross sections for electron scattering from the focused electron beam induced deposition (FEBID) precursor Co(CO)(3)NO in the incident electron energy range from 0 to 140 eV. We find that DEA leads mainly to single carbonyl loss with a maximum cross section of 4.1 × 10(-16) cm(2), while fragmentation through DI results mainly in the formation of the bare metal cation Co(+) with a maximum cross section close to 4.6 × 10(-16) cm(2) at 70 eV. Though DEA proceeds in a narrow incident electron energy range, this energy range is found to overlap significantly with the expected energy distribution of secondary electrons (SEs) produced in FEBID. The DI process, on the other hand, is operative over a much wider energy range, but the overlap with the expected SE energy distribution, though significant, is found to be mainly in the threshold region of the individual DI processes.

Formic Acid as a Dopant for Atmospheric Pressure Chemical Ionization for Negative Polarity of Ion Mobility Spectrometry and Mass Spectrometry.

Formic acid (FA) is introduced as a potent dopant for atmospheric pressure chemical ionization (APCI) for ion mobility spectrometry (IMS) and mass spectrometry (MS). The mechanism of chemical ionization with the FA dopant was studied in the negative polarity using a corona discharge (CD)-IMS-MS technique in air. Standard reactant ions of the negative polarity present in air are O2-·(CO2)n·(H2O)m (m = 0, 1 and n = 1, 2) clusters. Introduction of the FA dopant resulted in the production of HCOO-·FA reactant ions. The effect of the FA dopant on the APCI of different classes of compounds was investigated, including plant hormones, pesticides, acidic drugs, and explosives. FA dopant APCI resulted in deprotonation and/or adduct ion formation, [M - H]- and [M + HCOO]-, respectively. Supporting density functional theory (DFT) calculations showed that the ionization mechanism depended on the gas-phase acidity of the compounds. FA dopant APCI led to the improvement of detection sensitivity, suppression of fragmentation, and changes in the ion mobilities of the analyte ions for analytes with suitable molecular structures and gas acidity.