Pressure Ionization Mass Spectrometry Research Articles

Direct sub-atmospheric pressure ionization mass spectrometry: Evaporation/sublimation-driven ionization is amazing, fundamentally, and practically.

This paper covers direct sub-atmospheric pressure ionization mass spectrometry (MS). The discovery, applications, and mechanistic aspects of novel ionization processes for use in MS that are not based on the high-energy input from voltage, laser, and/or high temperature but on sublimation/evaporation within a region linking a higher to lower pressure and modulated by heat and collisions, are discussed, including how this new reality has guided a series of discoveries, instrument developments, and commercialization. A research focus, inter alia, is on how best to understand, improve, and use these novel ionization processes, which convert volatile and nonvolatile compounds from solids (sublimation) or liquids (evaporation) into gas-phase ions for analysis by MS providing reproducible, accurate, sensitive, and prompt results. Our perception on how these unprecedented versus traditional ionization processes/methods relate to each other, how they can be made to coexist on the same mass spectrometer, and an outlook on new and expanded applications (e.g., clinical, portable, fast, safe, and autonomous) is presented, and is based on ST's Opening lecture presentation at the Nordic Mass spectrometry Conference, Geilo, Norway, January 2023. Focus will be on matrix-assisted ionization (MAI) and solvent-assisted ionization (SAI) MS covering the period from 2010 to 2023; a potential paradigm shift in the making.

Determination of calcium, iron, and selenium in human serum by isotope dilution analysis using nitrogen microwave inductively coupled atmospheric pressure plasma mass spectrometry (MICAP-MS).

In this study, we demonstrate the applicability of nitrogen microwave inductively coupled atmospheric pressure mass spectrometry (MICAP-MS) for Ca, Fe, and Se quantification in human serum using isotope dilution (ID) analysis. The matrix tolerance of MICAP-MS in Na matrix was investigated, revealing that high Na levels can suppress the signal intensity. This suppression is likely due to the plasma loading and the space charge effect. Moreover, 40Ca and 44Ca isotopic fractionation was noted at elevated Na concentration. Nine certified serum samples were analyzed using both external calibration and ID analysis. Overestimation of Cr, Zn, As, and Se was found in the results of external calibration, which might result from C-induced polyatomic interference and signal enhancement, respectively. Further investigations performed with methanol showed a similar enhancement effect for Zn, As, and Se, potentially supporting this assumption. The mass concentrations determined with ID analysis show metrological compatibility with the reference values, indicating that MICAP-MS combined with ID analysis can be a promising method for precise Ca, Fe, and Se determination. Moreover, this combination reduces the influence of matrix effects, broadening the applicability of MICAP-MS for samples with complex matrixes.

Spatio-Temporal Analysis of Anesthetics in Mice by Solid-Phase Microextraction: Dielectric Barrier Discharge Ionization Mass Spectrometry.

Local anesthetics, drugs that only affect a restricted area of the body, are widely used in daily clinical practice. Less studied but equally important is the distribution of local anesthetics inside organisms. Here, we present a rapid in situ testing method of drug distribution in various organs. The temporal and spatial distribution of anesthetics in mice was measured by solid-phase microextraction (SPME), thermal desorption (TD), and dielectric barrier discharge ionization (DBDI) atmospheric pressure mass spectrometry. A coated SPME probe using a tungsten wire as the support covered with a carbonaceous material was prepared by a simple, low-cost flame method. An in-line structure of the inlet allows TD and DBDI to share the same capillary tube, which greatly improves the transmission efficiency. Nine kinds of anesthetics, such as lidocaine and dyclonine, were detected, and the limit of detection was determined to be as low as 13 pg/mL. In addition, the time-dependent distribution of drugs in mice organs was studied. We also found that macromolecules in organisms do not noticeably interfere with the detection. This method is convenient and efficient because it does not require tissue homogenates and allows direct in situ detection. Compared with the conventional analytical methods, this method is simple and rapid, works in situ, and allows microscale analysis of trace analytes in biological organisms with high sensitivity.

Mass Spectrometry-Based Biosensing and Biopsy Technology

Sensitive and accurate detection of biomolecules by multiplexed methods is important for disease diagnosis, drug research, and biochemical analysis. Mass spectrometry has the advantages of high sensitivity, high throughput, and high resolution, making it ideal for biomolecular sensing. As a result of the development of atmospheric pressure mass spectrometry, researchers have been able to use a variety of means to identify target biomolecules and recognize the converted signals by mass spectrometry. In this review, three main approaches and tools are summarized for mass spectrometry sensing and biopsy techniques, including array biosensing, probe/pen-based mass spectrometry, and other biosensor–mass spectrometry coupling techniques. Portability and practicality of relevant mass spectrometry sensing methods are reviewed, together with possible future directions to promote the advancement of mass spectrometry for target identification of biomolecules and rapid detection of real biological samples.

Comprehensive two-dimensional gas chromatography with flow modulator coupled via tube plasma ionization to an atmospheric pressure high-resolution mass spectrometer for the analysis of vermouth volatile profile

The analysis of complex samples is a big analytical challenge due to the vast number of compounds present in these samples as well as the influence matrix components could cause in the methodology. In this way, comprehensive two-dimensional gas chromatography coupled to mass spectrometry (GC × GC–MS) is a very powerful tool to achieve the characterization of complex samples. Nevertheless, due to possible coelutions occurring in these matrices, mixed spectra are generally obtained with electron ionization (EI) which could extremely complicate the identification of the analytes. Thereby, new methodology setups are required to improve the confidence on the identification in non-targeted determinations. Here, we present a high-throughput methodology consisting of GC × GC with flow modulation coupled to high-resolution atmospheric pressure mass spectrometry (HRMS) via a novel tube plasma ion source (TPI). The flow modulator allows to easily automate the GC × GC method compared to traditional cryo-modulators, while the soft ionization provided by TPI helps to preserve the [M]+• or [M+H]+ ions, thus increasing the confidence in the identification. Additionally, the combination of a flow modulation with an atmospheric pressure mass spectrometer significantly improves the sensitivity over flow modulated GC × GC-EI-MS methods because no split is required. This methodology was applied to the analysis of a complex sample such as vermouth where the volatile profile is usually considered by consumers as a product quality indicator since it raises the first sensations produced during its consumption. Using this approach, different classes of compounds were tentatively identified in the sample, including monoterpenes, terpenoids, sesquiterpenoids and carboxylic acid, and carboxylate esters among others, showing the great potential of a GC × GC-TPI-qTOF-MS platform for improving the confidence of the identifications in non-targeted applications.

Gas Dynamic Virtual Nozzle Sprayer for an Introduction of Liquid Samples in Atmospheric Pressure Ionization Mass Spectrometry.

Electrospray may exhibit inadequate ionization efficiency in some applications. In such cases, atmospheric-pressure chemical ionization (APCI) and photoionization (APPI) can be used. Despite a wide application potential, no APCI and APPI sources dedicated to very low sample flow rates exist on the market. Since the ion source performance depends on the transfer of analytes from the liquid to the gas phase, a nebulizer is a critical component of an ion source. Here, we report on the nebulizer with a gas dynamic virtual nozzle (GDVN) and its applicability in APCI at microliter-per-minute flow rates. Nebulizers differing by geometrical parameters were fabricated and characterized regarding the jet breakup regime, droplet size, droplet velocity, and spray angle for liquid flow rates of 0.75-15.0 μL/min. A micro-APCI source with the GDVN nebulizer behaved as a mass-flow-sensitive detector and provided stable and intense analyte signals. Compared to a classical APCI source, an order of magnitude lower detection limit for verapamil was achieved. Mass spectra recorded with the nebulizer in dripping and jetting modes were almost identical and did not differ from normal APCI spectra. Clogging never occurred during the experiments, indicating the high robustness of the nebulizer. Low-flow-rate APCI and APPI sources with a GDVN sprayer promise new applications for low- and medium-polar analytes.

Abstract 529: SNAQ-SEQ™: Use of synthetic internal standards in conjunction with poisson exact test to call variants in contrived circulating tumor DNA specimens

Abstract Background: Targeted next generation sequencing (NGS) analysis of circulating tumor (ct) DNA promises to significantly advance targeted therapy and potentially early diagnosis of cancer. However, targeted NGS has poor accuracy for calling known variants with VAF below 0.5%. The goal of this study from the FDA-Sequencing and Quality Control Phase 2 (SEQC2) consortium was to develop and validate bioinformatic and biostatistical methods that enable incorporation of synthetic competitive internal standards (IS) in targeted NGS analysis of actionable tumor mutations. Methods: A synthetic IS spike-in was designed for each actionable mutation target, suitable for use in NGS following targeted PCR or hybrid-capture enrichment and either with unique molecular index (UMI) or non-UMI library preparation. Contrived ctDNA reference samples developed by the SEQC2 consortium containing actionable mutations at known variant allele fraction were used. An aliquot of each sample was mixed with a mixture of IS. Following Illumina TST170 enrichment and library preparation, each library was sequenced, then native template (NT) sequences were separated from IS sequences bioinformatically. In SNAQ-SEQ™ analysis, Poisson Exact Test (PET) analysis was used to calculate the statistical difference between each sample library NT variant VAF and respective IS variant VAF. Analysis was based on NT variant count and position coverage (i.e., copies recovered in library preparation) and IS count and position coverage. PET performed an exact test of a simple null hypothesis about the ratio between two rate parameters in Poisson distribution. Results: Stochastic sampling effect on IS error detection was minimized by ensuring an IS/NT ratio of 2.5 or greater. Under the specified conditions, in which a minimum of two NT variant observations were required for a call, the IS was able to estimate NGS background error for each sample when a minimum IS:NT ratio of 2.5:1 was used. Without use of IS information, the Illumina pipeline called 73% (41/56) of known TP variants in the 0.1% - 0.3% VAF range. In contrast, SNAQ-SEQ™ analysis (PET analysis of IS and NT information) increased TP detection sensitivity to 86% (48/56), for a 13% increase in sensitivity, with no false positives. Conclusion: Following mixture of contrived ctDNA reference samples with IS, PET analysis enabled calculation of technical error rate, limit of blank, and limit of detection for each variant at each nucleotide position, in each sample. Using this SNAQ-SEQ™ analysis, true positive mutations with variant allele fraction too low for detection by current practice were detected with this method, thereby increasing sensitivity. SNAQ-SEQ™ provides QC that is already standard operating procedure in clinical laboratories for analysis by high pressure liquid chromatography and mass spectrometry. Citation Format: James C. Willey, Erin Crawford, Daniel J. Craig, Joshua Xu, Nathan Haseley, Jennifer Lococo, Tom Morrison. SNAQ-SEQ™: Use of synthetic internal standards in conjunction with poisson exact test to call variants in contrived circulating tumor DNA specimens [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 529.

Detection of Escherichia coli by Combining an Affinity-Based Method with Contactless Atmospheric Pressure Ionization Mass Spectrometry

Escherichia coli are common pathogens, whereas E. coli O157:H7 is the most notorious E. coli strain, owing to its high virulence that can cause serious adverse effects and death. E. coli contains abundant peroxidases. Thus, the presence of E. coli can be determined by mixing E. coli with its substrate such as 3,5,3′,5′ tetramethylbenzidines (TMB) for endogenous peroxidase reactions. Under the presence of a high concentration of E. coli, colorless TMB turned to bluish, owing to the generation of the complexity of TMB and its oxidized TMB. To further reduce the detectable cell concentration, we developed an affinity-based method combined with an endogenous peroxidase reaction and mass spectrometric detection to detect E. coli. Affinity probes (diameter: ~20 µm) modified with maltose were generated for the enrichment of E. coli from sample solutions. E. coli trapped by the affinity probes was reacted with TMB in the presence of hydrogen peroxide for endogenous peroxidase reactions. Contactless atmospheric pressure ionization mass spectrometry was used for the detection of the reaction product, oxidized TMB (TMB cationic radical), to indicate the presence of target bacteria. The results showed that the developed method can be used to rapidly determine the presence of E. coli from a sample solution based on the detection of the TMB cationic radicals. The lowest detectable concentration of our method against E. coli O157:H7 in buffers and in complex juice samples was as low as ~100 cfu mL−1.

Atmospheric Pressure Mass Spectrometry of Single Viruses and Nanoparticles by Nanoelectromechanical Systems.

Mass spectrometry of intact nanoparticles and viruses can serve as a potent characterization tool for material science and biophysics. Inaccessible by widespread commercial techniques, the mass of single nanoparticles and viruses (>10MDa) can be readily measured by nanoelectromechanical systems (NEMS)-based mass spectrometry, where charged and isolated analyte particles are generated by electrospray ionization (ESI) in air and transported onto the NEMS resonator for capture and detection. However, the applicability of NEMS as a practical solution is hindered by their miniscule surface area, which results in poor limit-of-detection and low capture efficiency values. Another hindrance is the necessity to house the NEMS inside complex vacuum systems, which is required in part to focus analytes toward the miniscule detection surface of the NEMS. Here, we overcome both limitations by integrating an ion lens onto the NEMS chip. The ion lens is composed of a polymer layer, which charges up by receiving part of the ions incoming from the ESI tip and consequently starts to focus the analytes toward an open window aligned with the active area of the NEMS electrostatically. With this integrated system, we have detected the mass of gold and polystyrene nanoparticles under ambient conditions and with two orders-of-magnitude improvement in capture efficiency compared to the state-of-the-art. We then applied this technology to obtain the mass spectrum of SARS-CoV-2 and BoHV-1 virions. With the increase in analytical throughput, the simplicity of the overall setup, and the operation capability under ambient conditions, the technique demonstrates that NEMS mass spectrometry can be deployed for mass detection of engineered nanoparticles and biological samples efficiently.

Rapid detection and classification of hongmu by atmospheric pressure ionization mass spectrometry.

Hongmu, a Chinese customary noun representing 29 kinds of wood species such as some Pterocarpus species (abbreviated as spp. hereinafter), Dalbergia spp. and Diospyros spp., is popular among Chinese people due to the furniture made from it. The slow regeneration of hongmu resources led to a decline in production, making hongmu prices high and illegal businesses profit from it. Therefore, it is necessary to identify and distinguish different varieties of hongmu for commercial trade. Herein, a cost-effective and rapid methodology was first developed via atmospheric pressure glow discharge mass spectrometry (APGD-MS) to classify three Dalbergia spp. and three Pterocarpus spp. Meanwhile, principal component analysis (PCA) was further applied to distinguish wood species and six kinds of hongmu extracts were able to be approximately separated into six units. Besides, hongmu could be clearly distinguished from their counterfeits, such as Guibourtia spp., using the method provided here. This method may provide a timely and necessary way for the determination of ingredients and identification of the authenticity of hongmu.

Ambient Mass Spectrometry Imaging of Small Molecules from Cells and Tissues.

New methods to analyze cells and tissues in ambient condition without any harsh chemical fixation or physical freezing and drying are summarized in this report. The first approach, an atmospheric pressure mass spectrometry imaging method, is based on laser ablation in atmospheric pressure assisted by atmospheric plasma and nanomaterials such as nanoparticles and graphene to enhance laser ablation. The second one is based on secondary ion mass spectrometry (SIMS) imaging of live cells in solution capped with single-layer graphene to preserve intact and hydrated biological samples even under ultrahigh vacuum for SIMS bio-imaging in solution.

Nontargeted Screening Using Gas Chromatography-Atmospheric Pressure Ionization Mass Spectrometry: Recent Trends and Emerging Potential.

Gas chromatography–high-resolution mass spectrometry (GC–HRMS) is a powerful nontargeted screening technique that promises to accelerate the identification of environmental pollutants. Currently, most GC–HRMS instruments are equipped with electron ionization (EI), but atmospheric pressure ionization (API) ion sources have attracted renewed interest because: (i) collisional cooling at atmospheric pressure minimizes fragmentation, resulting in an increased yield of molecular ions for elemental composition determination and improved detection limits; (ii) a wide range of sophisticated tandem (ion mobility) mass spectrometers can be easily adapted for operation with GC–API; and (iii) the conditions of an atmospheric pressure ion source can promote structure diagnostic ion–molecule reactions that are otherwise difficult to perform using conventional GC–MS instrumentation. This literature review addresses the merits of GC–API for nontargeted screening while summarizing recent applications using various GC–API techniques. One perceived drawback of GC–API is the paucity of spectral libraries that can be used to guide structure elucidation. Herein, novel data acquisition, deconvolution and spectral prediction tools will be reviewed. With continued development, it is anticipated that API may eventually supplant EI as the de facto GC–MS ion source used to identify unknowns.

High-Spatial Resolution Atmospheric Pressure Mass Spectrometry Imaging Using Fiber Probe Laser Ablation-Dielectric Barrier Discharge Ionization.

Atmospheric pressure mass spectrometry imaging (AP-MSI) is a powerful tool in many fields; however, there are still some difficulties to achieve high spatial resolution for AP-MSI, one of them being the need for a small ablation crater. Here, a fiber probe laser ablation (FPLA) system is introduced that uses an etched optical fiber with a sharp tip (o.d. 200 nm) to deliver ablation laser pulses to a sample surface to ablate materials with high spatial resolution. The tip-to-sample distance was adjusted to ∼10 μm using a micro-actuator having a stepping motor with submicron accuracy. The laser-ablated neutrals were post-ionized using a home-built in-line dielectric barrier discharge source, which can be interfaced to any mass spectrometer with an AP interface. Using MSI on a standard sample with a striped pattern and sections of fingernails treated with the drug methyl green zinc chloride salt, a FPLA-DBDI-MSI spatial resolution of ≈5 μm was demonstrated.

A Tapered Capillary-Based Contactless Atmospheric Pressure Ionization Mass Spectrometry for On-Line Preconcentration and Separation of Small Organics

Capillary electrophoresis (CE) is an effective technique for the separation of different analytes. Moreover, online preconcentration of trace analytes in the capillary for CE analysis has been demonstrated. CE and capillary electrochromatography (CEC) are suitable for the separation of analytes with similar polarities. Given that CE and CEC are only used to separate small-volume samples, sensitive mass spectrometry (MS) is a suitable detection tool for CE and CEC. Contactless atmospheric pressure ionization (C-API) is a continuous flow ion source that only uses a short capillary as the ionization emitter operated at atmospheric pressure for MS analysis. In this study, we demonstrated the feasibility of hyphenating CE/CEC with C-API-MS by using a short and tapered capillary as the interface. The short capillary (a few centimeters) can function as the separation/preconcentration tube and the ionization emitter. This hyphenated technique can be used to analyze small organics within a few minutes. The suitability of using the hyphenated technique for online preconcentration, separation, and quantitative analysis for small organics is demonstrated in this study.

Atmospheric Pressure Mass Spectrometry Imaging Using Laser Ablation, Followed by Dielectric Barrier Discharge Ionization.

Mass spectrometry imaging (MSI) has become a powerful tool in diverse fields, for example, life science, biomaterials, and catalysis, for its ability of in situ and real-time visualization of the location of chemical compounds in samples. Although laser ablation (LA) achieves high spatial resolution in MSI, the ion yield can be very low. We therefore combined an LA system with an ambient ion source for post-ionization and an atmospheric pressure (AP) inlet mass spectrometer to construct a novel AP-MSI platform. A dielectric barrier discharge ionization (DBDI) source is operated in the "active sampling capillary" configuration, can be coupled to any mass spectrometer with an AP interface, and possesses high ion transmission efficiency. This study presents some application examples based on LA-DBDI, a low-cost and flexible strategy for AP-MSI, which does not require any sample pretreatment, and we show MS imaging of endogenous species in a traditional Chinese herbal medicine and of a drug molecule in zebra fish tissue, with a lateral resolution of ≈20 μm.

Formation of the exceptional [M - H]+ cation in atmospheric pressure ionization mass spectrometry analysis of 2-(diphenylsilyl) cyclopropanecarboxylate esters.

In general, ionization of analytes in atmospheric pressure ionization mass spectrometry (API-MS) in positive ion mode results in the formation of protonated molecules ([M + H]+ ) and/or cationized molecules (e.g., [M + Na]+ ). The formation of specific [M - H]+ cations in the API process is of significant interest for further investigation. The ionization processes of 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate esters were investigated using electrospray ionization (ESI)-MS and atmospheric pressure chemical ionization-MS in positive ion mode. Theoretical calculations were carried out with the Gaussian 03 program using the density functional theory (DFT) method at the B3LYP/6-311 + G(2d,p) level. The anomalous [M - H]+ ion and the regular [M + Na]+ ion were both observed using ESI-MS. Interestingly, no [M + H]+ ion was obtained in the ESI-MS analysis, and acidification of the ESI solvent accelerated the formation of [M - H]+ rather than [M + H]+ ion. DFT calculations for the typical methyl 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate (1) indicated that the [1 + H]+ ion can thermodynamically and kinetically undergo facile H2 elimination to generate [1 - H]+ . The favorable formation of [M - H]+ ions in these compounds is attributed to the unique diphenylhydrosilyl group in their structure. The [M + H]+ ion formed easily underwent H2 elimination to produce the [1 - H]+ ion in the API source, and thus, acidification of the ESI solvent apparently accelerates the formation of the [1 - H]+ ion.

Characterization of traditional Chinese medicine by liquid chromatography/atmospheric pressure ionization mass spectrometry

Characterization of traditional Chinese medicine by liquid chromatography/atmospheric pressure ionization mass spectrometry

Wire Desorption Combined with Electrospray Ionization Mass Spectrometry: Direct Analysis of Small Organic and Large Biological Compounds.

A novel atmospheric pressure ionization mass spectrometry based on wire desorption and electrospray ionization (WD-ESI) for direct analysis was developed to characterize chemical compounds with different polarities and thermal stabilities at atmospheric pressure. This technique is a variant of the thermal desorption electrospray ion source developed by Shiea et al. One large improvement is that the heating speed (>500 °C/s) of the thermal desorption in this work is extremely fast, using a self-heating metal wire, with which sample solution can splash from the surface to form small droplets and thus the analytes can be protected from thermal decomposition. With this feature, we have successfully achieved soft ionization of highly polar organic and biological compounds such as aflatoxin, small peptides, and even large proteins from complex matrices. The simple structure and self-cleaning capability of the WD-ESI source make it ideal for on-site screening in various applications such as food safety and biodrug testing, especially when coupled with a transportable mass spectrometer.

Atmospheric pressure ionization mass spectrometry as a tool for structural characterization of lignin.

Lignin occurs in a broad range of forms, e.g., native as the main support for plant walls, and processed, for which its structure depends on the nature of the industrial isolation method, such as in paper production or in biorefineries. Due to the variety of lignin sources, there is no unified agreement on the structure of lignin or even its molecular weight (MW). The focus of this review is on the application of atmospheric pressure ionization methods to lignin analysis by mass spectrometry (MS), namely electrospray ionization (ESI) or direct analysis in real-time (DART). Specific parameters affecting ionization including electrolytes and solvents are discussed. The main challenge for MW determination of lignin is its heteropolymer character as well as the mass range limitations of MS instrumentation. To date, only a few studies have successfully used the mass range above m/z 1500. We present the advantage of ESI in generating multiply charged ions, allowing for a further increase in the mass range of deconvoluted mass spectra. While some methods such as DART do not address the mass range problem, they may serve as excellent imaging tools suitable for structural characterization of lignin. A literature review presents the recent accomplishments in lignin MS analysis by atmospheric pressure ionization techniques. Although significant breakthroughs have been made, it is essential to further improve the operating conditions and validate the methods for a broader range of feedstocks with the results being confirmed using other methods.

Atmospheric Pressure Non-thermal Plasma for Air Purification: Ions and Ionic Reactions Induced by dc+ Corona Discharges in Air Contaminated with Acetone and Methanol

Atmospheric pressure mass spectrometry (APCI-MS) was used to investigate the positive ions in air containing acetone (A), methanol (M) and mixtures thereof (A + M), subjected to +dc corona discharges. The results of experiments with isotopically labelled analogues, perdeuterated acetone Adeu and methanol Mdeu, and relevant thermochemical data found in the literature allowed us to identify the main ionic reactions occurring in single component systems (A or M) and in binary mixtures (A + M). It is concluded that, thanks to its significantly higher proton affinity, A is very efficient in quenching M-derived ions at atmospheric pressure. These conclusions provide a rationale for interpreting the results of a parallel investigation on the reciprocal effects of M and A when treated together in air at atmospheric pressure with +dc corona in a non-thermal plasma reactor developed previously in our laboratory. Specifically, we observed a marked drop in the degradation efficiency of methanol when it was treated in the presence of an equivalent amount of acetone. This effect is attributed to acetone interfering with ion-initiated degradation processes of methanol, and supports the conclusion that ions and ionic reactions are important in dc+ corona induced oxidation of volatile organic pollutants in air.