Chemical Ionization Mass Spectrometry Technique Research Articles

Integrating density functional theory and chemical ionization mass spectrometry techniques to assess benzophenone derivatives in food packaging: implications for enhancing food quality and safety.

Benzophenone and related derivatives are widely used as photoinitiators for food packaging to cure inks or lacquers with ultraviolet (UV) light on cardboard and paper. However, there are concerns about the potential health risks of their migration into food. Knowing the physical and chemical properties of benzophenone and its derivatives could play a significant role in their quantification and analysis using chemical ionization mass spectrometry (CI-MS) methods. These parameters are evaluated using B3LYP/6-311++** density functional theory (DFT) implemented on Gaussian code. Ion-molecule chemistry through the selection of reagent ions, reaction energetics and kinetics, thermodynamic stability, and reactivity of molecules deemed to foster VOC identification and quantification via CI-MS techniques. The VOCs under study are expected to undergo exothermic reactions from H3 O+ , NH4 + , NO+ , and O2 + ions, except endothermic proton transfer from NH4 + to 2-hydroxy-4-methoxybenzophenone and 2,3,4-trihydroxy benzophenone. These compounds possess less proton affinities than NH3 and are least stable in their protonated forms. The DFT computed properties provide the basis for developing reliable and accurate methods to detect and measure the presence of benzophenone and its derivatives in packaging materials and food products.

The terrestrial end-Permian mass extinction in the paleotropics postdates the marine extinction.

The end-Permian mass extinction was the most severe ecological event during the Phanerozoic and has long been presumed contemporaneous across terrestrial and marine realms with global environmental deterioration triggered by the Siberian Traps Large Igneous Province. We present high-precision zircon U-Pb geochronology by the chemical abrasion-isotope dilution-thermal ionization mass spectrometry technique on tuffs from terrestrial to transitional coastal settings in Southwest China, which reveals a protracted collapse of the Cathaysian rainforest beginning after the onset of the end-Permian marine extinction. Integrated with high-resolution geochronology from coeval successions, our results suggest that the terrestrial extinction occurred diachronously with latitude, beginning at high latitudes during the late Changhsingian and progressing to the tropics by the early Induan, spanning a duration of nearly 1 million years. This latitudinal age gradient may have been related to variations in surface warming with more degraded environmental conditions at higher latitudes contributing to higher extinction rates.

Improving the chronostratigraphic framework of the Transvaal Supergroup (South Africa) through in-situ and high-precision U-Pb geochronology

The Transvaal Supergroup, on the Kaapvaal Craton in South Africa, is widely accepted as one of the best-preserved sedimentary archives to constrain planetary-scale environmental changes during the late Archean and early Proterozoic, yet the sedimentation age for certain stratigraphic intervals remains poorly constrained. To improve the temporal control on some of the first-order global changes recorded in these rocks, we carried out U-Pb analyses of detrital zircon populations from several clastic and volcano-clastic sedimentary units of the Transvaal Supergroup. We applied the Chemical Abrasion-Isotope Dilution-Thermal Ionization Mass Spectrometry (CA-ID-TIMS) technique on detrital and volcanic zircon populations that had been previously screened using the Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) technique. We report new maximum depositional age estimates for the Pannetjie (2456.6 ± 7.0 Ma), the Heynskop (2451.5 ± 2.5 Ma), the Makganyene (2423.1 ± 1.0 Ma) and the Hekpoort formations (2248.0 ± 1.1 Ma).A ca. 2.25 Ga-old cluster of LA-ICP-MS analyses in the Makganyene Formation was identified to be spurious, since it was completely removed during the chemical abrasion. Thus, we speculate that circulation of hydrothermal fluids and associated Pb-loss from a radiation-damaged lattice during the emplacement of the much younger Hekpoort Formation or possibly the Ophthalmia Orogeny, recorded in Western Australia, may have reset the U-Pb system of this zircon population. This implies that the accurate maximum depositional age of the Makganyene Formation is ca. 2.42 Ga instead, which denotes the age of the oldest glacial event of global extent during the Paleoproterozoic. Therefore, we suggest that the combination of both dating techniques is essential to ensure accurate maximum depositional age constraints for ancient detrital sedimentary rocks. Additionally, our data provides temporal constraints on a period characterized by major fluctuations in atmospheric oxygen. Finally, and supporting the complex nature of the Paleoproterozoic, linkages between widespread glaciations and atmospheric oxygen fluctuations remain to be explored.

Theoretical Investigation of Charge Transfer from NO+ and O2+ Ions to Wine-Related Volatile Compounds for Mass Spectrometry.

Density-functional theory (DFT) is used to obtain the molecular data essential for predicting the reaction kinetics of chemical-ionization-mass spectrometry (CI-MS), as applied in the analysis of volatile organic compounds (VOCs). We study charge-transfer reactions from NO+ and O2+ reagent ions to VOCs related to cork-taint and off-flavor in wine. We evaluate the collision rate coefficients of ion-molecule reactions by means of collision-based models. Many NO+ and O2+ reactions are known to proceed at or close to their respective collision rates. Factors affecting the collision reaction rates, including electric-dipole moment and polarizability, temperature, and electric field are addressed, targeting the conditions of standard CI-MS techniques. The molecular electric-dipole moment and polarizability are the basic ingredients for the calculation of collision reaction rates in ion-molecule collision-based models. Using quantum-mechanical calculations, we evaluate these quantities for the neutral VOCs. We also investigate the thermodynamic feasibility of the reactions by computing the enthalpy change in these charge-transfer reactions.

Application of Chemical Ionization Mass Spectrometry in Airborne SO2 Observation on Hanseo Beechcraft 1900 D

As the observational constraints of SO2 become a limiting factor for emission inventory and regional and global transport model validations, we present an airborne SO2 observational suite integrated on Beechcraft 1900D (B1900D) operated by Hanseo University. A chemical ionization mass spectrometry (CIMS) technique using SF6− ion chemistry is used to take advantage of fast response (<1 Hz) and the low limit of detection (30 ppt). In this study, we describe the performance of the CIMS system on B1900D along with descriptions on the modified airborne platform. Furthermore, exemplary observational results around point sources such as a steel production facility and a petrochemical industrial facility in the Taeahn Peninsula in South Korea. In addition, an observational result over the Yellow Sea is presented to demonstrate the ability of the instrumentation to capture pollution transport from the continent. Overall, the performance of the airborne CIMS system for SO2 is well demonstrated for future air quality studies.

Modern mass spectrometry in atmospheric sciences: Measurement of volatile organic compounds in the troposphere using proton-transfer-reaction mass spectrometry.

Volatile organic compounds (VOCs) in the troposphere are emitted from a wide variety of natural and man-made sources and resulting in environmental issues such as air pollution and climate change. Proton-transfer-reaction mass spectrometry (PTR-MS), a chemical ionization mass spectrometry technique using H3 O+ reagent ions, allows real-time measurements of various VOCs in air with high sensitivity (parts-per-trillion to parts-per-billion level) and fast time response (<1 s). Modern PTR-MS equipped with time-of-flight mass analyzers can measure hundreds of compounds simultaneously. The applications of PTR-MS have greatly promoted understanding VOC sources and their roles in environmental issues, and therefore, PTR-MS has become a well-established analytical technique for in situ VOC measurements in atmospheric sciences. Here, the principles and implementation of PTR-MS are described. We additionally highlight modern PTR-MS techniques for accurate identification and comprehensive quantification of various VOCs. Finally, recent applications for atmospheric science are presented, using examples from research in biomass burning emissions and chemistry.

Styrene radical cations for chemical ionization mass spectrometry analyses of monoterpene hydrocarbons.

Monoterpene hydrocarbons play an important role in the formation of secondary aerosol particles and in atmospheric chemistry. Thus, there is a demand to measure their individual concentrations in situ in real time. Currently, only the total concentration of monoterpenes C10 H16 can be determined by chemical ionization mass spectrometry techniques using reagent ions H3 O+ , NO+ and (C6 H6 )n +• without gas chromatographic separation. The styrene cation C8 H8 +• was investigated as a reagent for chemical ionization of monoterpenes. The modified selected ion flow drift tube, SIFDT, technique was used to characterize the differences in product ion distributions between α-phellandrene, α-pinene, γ-terpinene, β-pinene, ocimene, sabinene, 3-carene, (R)-limonene, camphene and myrcene. The monoterpene molecular cation C10 H16 +• is the main product (about 90%) for all isomers except (R)-limonene and camphene with an efficient channel of C8 H8 +• C10 H16 adduct formation and γ-terpinene with unexpectedly significant product ions at m/z 134 and 135 corresponding to losses of H2 and H. Utilization of the styrene cation for the ionization of monoterpenes is beneficial due to the very low fragmentation of the product ions. Specific association product ions for camphene and (R)-limonene and fragment product ions for γ-terpinene allow them to be distinguished from other isomers that produce mostly the molecular cation.

Performance of a new coaxial ion–molecule reaction region for low-pressure chemical ionization mass spectrometry with reduced instrument wall interactions

Abstract. Chemical ionization mass spectrometry (CIMS) techniques have become prominent methods for sampling trace gases of relatively low volatility. Such gases are often referred to as being “sticky”, i.e., having measurement artifacts due to interactions between analyte molecules and instrument walls, given their tendency to interact with wall surfaces via absorption or adsorption processes. These surface interactions can impact the precision, accuracy, and detection limits of the measurements. We introduce a low-pressure ion–molecule reaction (IMR) region primarily built for performing iodide-adduct ionization, though other adduct ionization schemes could be employed. The design goals were to improve upon previous low-pressure IMR versions by reducing impacts of wall interactions at low pressure while maintaining sufficient ion–molecule reaction times. Chamber measurements demonstrate that the IMR delay times (i.e., magnitude of wall interactions) for a range of organic molecules spanning 5 orders of magnitude in volatility are 3 to 10 times lower in the new IMR compared to previous versions. Despite these improvements, wall interactions are still present and need to be understood. To that end, we also introduce a conceptual framework for considering instrument wall interactions and a measurement protocol to accurately capture the time dependence of analyte concentrations. This protocol uses short-duration, high-frequency measurements of the total background (i.e., fast zeros) during ambient measurements as well as during calibration factor determinations. This framework and associated terminology applies to any instrument and ionization technique that samples compounds susceptible to wall interactions.

Addition of fast gas chromatography to selected ion flow tube mass spectrometry for analysis of individual monoterpenes in mixtures

Abstract. Soft chemical ionization mass spectrometry (SCI-MS) techniques can be used to accurately quantify volatile organic compounds (VOCs) in air in real time; however, differentiation of isomers still represents a challenge. A suitable pre-separation technique is thus needed, ideally capable of analyses over a few tens of seconds. To this end, a bespoke fast gas chromatography (GC) instrument with an electrically heated 5 m long metallic capillary column was coupled to selected ion flow tube mass spectrometry (SIFT-MS) measurements. To assess the performance of this combination, a case study of monoterpene isomer (C10H16) analyses was carried out. The monoterpenes were quantified by SIFT-MS using H3O+ reagent ions (analyte ions C10H17+, m∕z 137, and C6H9+, m∕z 81) and NO+ reagent ions (analyte ions C10H16+, m∕z 136, and C7H9+, m∕z 93). The combinations of the fragment ion relative intensities obtained using H3O+ and NO+ were shown to be characteristic of the individual monoterpenes. Two non-polar GC columns (Restek Inc.) were tested: the advantage of MXT-1 was shorter retention, whilst the advantage of MXT-Volatiles was better separation. Thus, it is possible to identify components of a monoterpene mixture in less than 45 s using the MXT-1 column and to separate them in less than 180 s using the MXT-Volatiles column. Quality of the separation and the sensitivity of present technique (limit of detection, LOD, ∼16 ppbv) was found to be inferior compared to commercially available fast GC solutions coupled with proton transfer reaction mass spectrometry (PTR-MS, LOD ∼1 ppbv) due to the limited sample flow through the column. However, using combinations of two reagent ions improved identification of monoterpenes not well resolved by the chromatograms. As an illustrative example, the headspace of needle samples of three conifer species was analysed by both reagent ions and with both columns showing that mainly α-pinene, β-pinene and 3-carene were present. The system can thus be used for direct rapid monitoring of monoterpenes above 20 ppbv, such as applications in laboratory studies of monoterpene standards and leaf headspace analysis. Limitation of the sensitivity due to the total sample flow can be improved using a multi-column pre-separation.

Electrostatic Switching and Selection of H3O+, NO+, and O2+• Reagent Ions for Selected Ion Flow-Drift Tube Mass Spectrometric Analyses of Air and Breath.

Soft chemical ionization mass spectrometry techniques, particularly the well-established proton transfer reactionmass spectrometry, PTR-MS, and selected ion flow tube mass spectrometry, SIFT-MS, are widely used for real-time quantification of volatile organic compounds in ambient air and exhaled breath with applications ranging from environmental science to medicine. The most common reagent ions H3O+, NO+, or O2+• can be selected either by quadrupole mass filtering from a discharge ion source, which is relatively inefficient, or by switching the gas/vapor in the ion source, which is relatively slow. The chosen reagent ions are introduced into a flow tube or flow-drift tube reactor where they react with analyte molecules in sample gas. This article describes a new electrostatic reagent ion switching, ERIS, technique by which H3O+, NO+, and O2+• reagent ions, produced simultaneously in three separate gas discharges, can be purified in post-discharge source drift tubes, switched rapidly, and selected for transport into a flow-drift tube reactor. The construction of the device and the ion-molecule chemistry exploited to purify the individual reagent ions are described. The speed and sensitivity of ERIS coupled to a selected ion flow-drift tube mass spectrometry, SIFDT-MS, is demonstrated by the simultaneous quantification of methanol with H3O+, acetone with NO+, and dimethyl sulfide with O2+• reagent ions in single breath exhalations. The present ERIS approach is shown to be preferable to the previously used quadrupole filtering, as it increases analytical sensitivity of the SIFDT-MS instrument while reducing its size and the required number of vacuum pumps.

An electrospray chemical ionization source for real-time measurement of atmospheric organic and inorganic vapors

Abstract. We present an electrospray ion source coupled to an orthogonal continuous-flow atmospheric pressure chemical ionization region. The source can generate intense and stable currents of several specific reagent ions using a range of salt solutions prepared in methanol, thereby providing both an alternative to more common radioactive ion sources and allowing for the generation of reagent ions that are not available in current chemical ionization mass spectrometry (CIMS) techniques, such as alkaline cations. We couple the orthogonal electrospray chemical ionization (ESCI) source to a high-resolution time-of-flight mass spectrometer (HR-ToF-MS), and assess instrument performance through calibrations using nitric acid (HNO3), formic acid (HCOOH), and isoprene epoxydiol (trans-β-IEPOX) gas standards, and through measurements of oxidized organic compounds formed from ozonolysis of α-pinene in a continuous-flow reaction chamber. When using iodide as the reagent ion, the HR-ToF-ESCIMS prototype has a sensitivity of 11, 2.4, and 10 cps pptv−1 per million counts per second (cps) of reagent ions and a detection limit (3σ, 5 s averaging) of 4.9, 12.5, and 1.4 pptv to HNO3, HCOOH, and IEPOX, respectively. These values are comparable to those obtained using an iodide-adduct HR-ToF-CIMS with a radioactive ion source and low-pressure ion–molecule reaction region. Applications to the α-pinene ozonolysis system demonstrates that HR-ToF-ESCIMS can generate multiple reagent ions (e.g., I−, NO3−, acetate, Li+, Na+, K+, and NH4+) having different selectivity to provide a comprehensive molecular description of a complex organic system.

High-precision zircon U–Pb geochronology of astronomically dated volcanic ash beds from the Mediterranean Miocene

Several orbitally tuned Miocene sedimentary sequences around the Mediterranean contain abundant intercalated volcanic ash beds. These sequences provide the rare opportunity to directly compare radioisotopic dating methods with independent and accurate deposition ages derived from astrochronology. We present a large data set (N=16, n=166) of zircon U–Pb dates obtained by chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) techniques for ash beds from an almost continuous orbitally tuned Messinian to Langhian (6.2–15.4 Ma) sedimentary sequence exposed along the Adriatic coast south of Ancona, Italy. We use this unique data set to evaluate (1) the accuracy of zircon U–Pb dates, (2) the significance of initial intermediate daughter product disequilibria for zircon U–Pb geochronology of young rocks, (3) the effect of prolonged pre-eruption zircon crystallization and zircon recycling on U–Pb derived ash bed deposition ages, and (4) discuss the implications for the intercalibration of radioisotope geochronometers and the calibration of the Geologic Time Scale.

Rate Constants and Products of the OH Reaction with Isoprene-Derived Epoxides

Recent laboratory and field work has shown that isoprene-derived epoxides (IEPOX) are crucial intermediates that can explain the existence of a variety of compounds found in ambient secondary organic aerosol (SOA). However, IEPOX species are also able to undergo gas phase oxidation, which competes with the aerosol phase processing of IEPOX. In order to better quantify the atmospheric fate of IEPOX, the gas phase OH reaction rate constants and product formation mechanisms have been determined using a flow tube chemical ionization mass spectrometry technique. The new OH rate constants are generally larger than previous estimations and some features of the product mechanism are well predicted by the Master Chemical Mechanism Version 3.2 (MCM v3.2), while other features are at odds with MCM v3.2. Using a previously proposed kinetic model for the quantitative prediction of the atmospheric fate of IEPOX, it is found that gas phase OH reaction is an even more dominant fate for chemical processing of IEPOX than previously suggested. The present results suggest that aerosol phase processing of IEPOX will be competitive with gas phase OH oxidation only under SOA conditions of high liquid water content and low pH.

SIFT Ion Chemistry Studies Underpinning the Measurement of Volatile Organic Compound Emissions by Vegetation

Chemical Ionization Mass Spectrometry (CI-MS) techniques are appropriate for the determination of volatile organic compound (VOC) emissions from terrestrial vegetation, provided that these compounds have unique analytical ions. In practice, however, many plant volatiles, and mainly those emitted by the isoprenoid and lipoxygenase biochemical pathways, have CI product ions at overlapping m/z values. In this paper, Selected Ion Flow Tube (SIFT) gas phase ion chemistry studies of several series of plant volatile classes using H3O+ and NO+ reagent ions, which have provided the kinetic and mechanistic parameters required for plant volatile quantification by SIFT-MS, are reviewed. The available information has been gathered into tables which allow the reader to decide which analytical ions are best monitored to quantify specific biogenic VOC emissions by SIFT-MS in the presence of interfering compounds. Some typical examples of interferences and how they can be resolved are subsequently given. The reviewed ion chemistry, obtained under SIFT conditions, is also relevant to the analysis of biogenic VOC emissions by Proton Transfer Reaction - Mass Spectrometry (PTR-MS).

Peroxy radical chemistry and OH radical production during the NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;-initiated oxidation of isoprene

Abstract. Peroxy radical reactions (RO2 + RO2) from the NO3-initiated oxidation of isoprene are studied with both gas chromatography and a chemical ionization mass spectrometry technique that allows for more specific speciation of products than in previous studies of this system. We find high nitrate yields (~ 80%), consistent with other studies. We further see evidence of significant hydroxyl radical (OH) formation in this system, which we propose comes from RO2 + HO2 reactions with a yield of ~38–58%. An additional OH source is the second generation oxidation of the nitrooxyhydroperoxide, which produces OH and a dinitrooxyepoxide with a yield of ~35%. The branching ratio of the radical propagating, carbonyl- and alcohol-forming, and organic peroxide-forming channels of the RO2 + RO2 reaction are found to be ~18–38%, ~59–77%, and ~3–4%, respectively. HO2 formation in this system is lower than has been previously assumed. Addition of RO2 to isoprene is suggested as a possible route to the formation of several isoprene C10-organic peroxide compounds (ROOR). The nitrooxy, allylic, and C5 peroxy radicals present in this system exhibit different behavior than the limited suite of peroxy radicals that have been studied to date.

Studies in search of selective detection of isomeric biogenic hexen‐1‐ols and hexanal by flowing afterglow tandem mass spectrometry using [H3O]+ and [NO]+ reagent ions

Plants emit a blend of oxygenated volatile C(6) compounds, known as green leaf volatiles (GLVs), in response to leaf tissue damage related to stress conditions. On-line chemical ionization mass spectrometry (CI-MS) techniques have often been used to study the dynamics of these emissions but they fail to selectively detect some important GLV compounds. A flowing afterglow tandem mass spectrometer (FA-TMS) was used to investigate the feasibility of selective on-line detection of isomeric hexen-1-ols and hexanal. Product ions at m/z 101 and 83 from chemical ionization (CI) of these compounds by [H(3)O](+), and product ions at m/z 100, 99, 83, 82 and 72 from CI by [NO](+), have been subjected to collision-induced dissociation (CID) in the collision cell of the TMS at center-of-mass energies ranging between 0 and 9 eV. CID of product ions at m/z 101 and 83 from CI of GLVs with [H(3)O](+) and of product ions at m/z 83, 82 and 72 from CI of GLVs with [NO](+) resulted in identical fragmentation patterns for all measured compounds, ruling out any selectivity. However, CID of product ions at m/z 100 and 99 from CI by [NO](+) led to CID product ions with abundances differing largely between the compounds, allowing the fast selective detection of 2-hexen-1-ols, 3-hexen-1-ols and hexanal with a chosen accuracy within a well-defined range of relative concentrations. This research illustrates that, in contrast to common CI-MS techniques, FA-TMS allows the selective detection of hexanal in a mixture of hexanal and hexen-1-ols with a chosen accuracy for a well-defined range of relative concentrations and represents a step forward in the search for selective detection of GLVs in CI-TMS.

Kinetics Study of the Aromatic Bicyclic Peroxy Radical + NO Reaction: Overall Rate Constant and Nitrate Product Yield Measurements

The measurements of the overall bicyclic peroxy radical + NO rate constant for the 1,3,5-trimethylbenzene (1,3,5-TMB) system and of the nitrate product yields for the benzene, toluene, p-xylene, and 1,3,5-TMB systems were performed via the turbulent flow chemical ionization mass spectrometry technique. While the overall rate constant was found to be consistent with the value used in the most detailed aromatic oxidation kinetic model (Master Chemical Mechanism, MCM), the nitrate product yields were found to be generally lower than predicted by the MCM and to have a different aromatic species-specific dependence than the MCM predicts.

Comprehensive NO-Dependent Study of the Products of the Oxidation of Atmospherically Relevant Aromatic Compounds

A comprehensive product study, performed via the turbulent flow chemical ionization mass spectrometry (TF-CIMS) technique, of the primary OH-initiated oxidation of many of the atmospherically abundant aromatic compounds was performed. The bicyclic peroxy radical intermediate, a key proposed intermediate species in the Master Chemical Mechanism (MCM) for the atmospheric oxidation of aromatics, was detected in all cases, as were stable bicyclic species. The NO product yield dependences suggest a potential role for bicyclic peroxy radical + HO(2) reactions at high HO(2)/NO ratios, which are postulated to be a possible source of the inconsistencies between previous environmental chamber results and predictions from the MCM for ozone production and OH reactivity. The TF-CIMS product yield results are also compared to previous environment chamber results and to the latest MCM parametrization.

Chemical ionization by [NO]+ and subsequent collision‐induced dissociation for the selective on‐line detection of monoterpenes and linalool

Existing on-line Chemical Ionization Mass Spectrometry (CIMS) techniques for quantification of atmospheric trace gases, such as Biogenic Volatile Organic Compounds (BVOCs), suffer from difficulty in discriminating between isomeric (and more generally isobaric) compounds. Selective detection of these compounds, however, is important because they can affect atmospheric chemistry in different ways, depending on their chemical structure. In this work, Flowing Afterglow Tandem Mass Spectrometry (FATMS) was used to investigate the feasibility of the selective detection of a series of monoterpenes, an oxygenated monoterpene (linalool) and a sesquiterpene (β-caryophyllene). Ions at m/z 137 from [H(3)O](+) chemical ionization of α-pinene, linalool and β-caryophyllene have been subjected to Collision-Induced Dissociation (CID) with Ar in the collision cell of a tandem mass spectrometer at center-of-mass energies ranging between 0 and 8 eV. Similar fragmentation patterns were obtained, demonstrating that this method is not suited for the selective detection of these compounds. However, CID of the ions at m/z 136 produced via [NO](+) chemical ionization of a series of monoterpenes has revealed promising results. Some tracer-product ions for individual compounds or groups of compounds were found, which can be considered as a step forward towards selective on-line monitoring of BVOCs with CIMS techniques.

Investigation of the Role of Bicyclic Peroxy Radicals in the Oxidation Mechanism of Toluene

The products of the primary OH-initiated oxidation of toluene were investigated using the turbulent flow chemical ionization mass spectrometry technique under different oxygen, NO, and initial OH radical concentrations as well as a range of total pressures. The bicyclic peroxy radical intermediate, a key proposed intermediate species in the Master Chemical Mechanism (MCM) for the atmospheric oxidation of toluene, was detected for the first time. The toluene oxidation mechanism was shown to have a strong oxygen concentration dependence, presumably due to the central role of the bicyclic peroxy radical in determining the stable product distribution at atmospheric oxygen concentrations. The results also suggest a potential role for bicyclic peroxy radical + HO(2) reactions at high HO(2)/NO ratios. These reactions are postulated to be a source of the inconsistencies between environmental chamber results and predictions from the MCM.