Estimation of sulfuric acid concentrations using ambient ion composition and concentration data obtained by ion mass spectrometry measurements (APi-TOF)

Sulfuric acid (H2SO4, SA) is the key compound in atmospheric new particle formation. Therefore, it is crucial to observe its concentration with sensitive instrumentation, such as chemical ionisation inlets coupled to Atmospheric Pressure interface Time-of-Flight mass spectrometers (CI-APi-TOF). However, there are environmental conditions and physical reasons when chemical ionisation cannot be used, for example in certain remote places or flight measurements with limitations regarding chemicals. In these cases, it is important to estimate the SA concentration based on ambient ion composition and concentration measurements that are achieved by APi-TOF alone. Here we derive a theoretical expression to estimate SA concentration and validate it with accurate CI-APi-TOF observations. The developed estimate works very well during daytime and with SA concentrations above 2⋅106 cm-3.

Atmospheric pressure chemical ionisation mass spectrometry for the routine analysis of low molecular weight analytes

The routine analysis of low molecular weight analytes by mass spectrometry is often complicated by the lability of the analyte’s functional groups and/or the lack of moieties that can be easily charged. If a molecule is too labile this precludes analysis by techniques such as electron ionisation or chemical ionisation as the analyte will undergo thermal decomposition prior to ionisation as well as spontaneous fragmentation during the ionisation process. If the analyte has a low propensity to form ions in electrospray ionisation (i.e., lacks acidic or basic sites) then often no analyte related ions are observed. In this paper, the robustness and versatility of the established method of atmospheric pressure chemical ionisation is demonstrated for the analysis of low molecular weight analytes. The utility of the technique is demonstrated through the analysis of 30 reference standards of varying functionality, and further by the analysis of 75 synthetic samples which were problematic when analysed by electron or electrospray ionisation. The resulting spectra are dominated by intact molecular species ([M+H]+ and M+ in positive ion mode and [M − H]− and [M + Cl]− in negative ion mode) along with logical neutral losses reminiscent of what you might expect from the analyte’s structure (losses of H2O from alcohols or CO from aldehydes etc). This paper presents atmospheric pressure chemical ionisation as an essential tool for broadening the chemical space of successful analyses for any routine mass spectrometry service laboratory of facility.

Chemical characterisation of benzene oxidation products under high- and low-NO_<i>x</i> conditions using chemical ionisation mass spectrometry

Aromatic hydrocarbons are a class of volatile organic compounds associated with anthropogenic activity and make up a significant fraction of urban volatile organic compound (VOC) emissions that contribute to the formation of secondary organic aerosol (SOA). Benzene is one of the most abundant species emitted from vehicles, biomass burning and industry. An iodide time-of-flight chemical ionisation mass spectrometer (ToF-CIMS) and nitrate ToF-CIMS were deployed at the Jülich Plant Atmosphere Chamber as part of a series of experiments examining benzene oxidation by OH under high- and low-NOx conditions, where a range of organic oxidation products were detected. The nitrate scheme detects many oxidation products with high masses, ranging from intermediate volatile organic compounds (IVOCs) to extremely low volatile organic compounds (ELVOCs), including C12 dimers. In comparison, very few species with C≥6 and O≥8 were detected with the iodide scheme, which detected many more IVOCs and semi-volatile organic compounds (SVOCs) but very few ELVOCs and low volatile organic compounds (LVOCs). A total of 132 and 195 CHO and CHON oxidation products are detected by the iodide ToF-CIMS in the low- and high-NOx experiments respectively. Ring-breaking products make up the dominant fraction of detected signal and 21 and 26 of the products listed in the Master Chemical Mechanism (MCM) were detected. The time series of highly oxidised (O≥6) and ring-retaining oxidation products (C6 and double-bond equivalent = 4) equilibrate quickly, characterised by a square form profile, compared to MCM and ring-breaking products which increase throughout oxidation, exhibiting sawtooth profiles. Under low-NOx conditions, all CHO formulae attributed to radical termination reactions of first-generation benzene products, and first-generation auto-oxidation products are observed. Several N-containing species that are either first-generation benzene products or first-generation auto-oxidation products are also observed under high-NOx conditions. Hierarchical cluster analysis finds four clusters, of which two describe photo-oxidation. Cluster 2 shows a negative dependency on the NO2/NOx ratio, indicating it is sensitive to NO concentration and thus likely to contain NO addition products and alkoxy-derived termination products. This cluster has the highest average carbon oxidation state (OSC‾) and the lowest average carbon number. Where nitrogen is present in a cluster member of cluster 2, the oxygen number is even, as expected for alkoxy-derived products. In contrast, cluster 1 shows no dependency on the NO2/NOx ratio and so is likely to contain more NO2 addition and peroxy-derived termination products. This cluster contains fewer fragmented species, as the average carbon number is higher and OSC‾ lower than cluster 2, and more species with an odd number of oxygen atoms. This suggests that clustering of time series which have features pertaining to distinct chemical regimes, for example, NO2/NOx perturbations, coupled with a priori knowledge, can provide insight into identification of potential functionality.

Volatile atmospheric pressure chemical ionisation mass spectrometry headspace analysis of E. coli and S. aureus

Identifying the characteristics of bacterial species can improve treatment outcomes and mass spectrometry methods have been shown to be capable of identifying biomarkers of bacterial species.

Chemical characterisation of benzene oxidation products under high and low NO_x conditions using chemical ionisation mass spectrometry

Aromatic hydrocarbons are a class of volatile organic compounds associated with anthropogenic activity and make up a significant fraction of urban VOC emissions that contribute to the formation of secondary organic aerosol (SOA). Benzene is one of the most abundant species emitted from vehicles, biomass burning and industry. An iodide time of flight chemical ionisation mass spectrometer (ToF-CIMS) and nitrate ToF-CIMS were deployed at the Jülich plant chamber as part of a series of experiments examining benzene oxidation by OH under high and low NOx conditions, where a range of organic oxidation products were detected. The nitrate scheme detects many oxidation products with high masses ranging from intermediate volatile organic compounds (IVOC) to extremely low volatile organic compounds (ELVOC), including C12 dimers. In comparison, very few species with C≥6 and O≥8 were detected with the iodide scheme, which detected many more IVOC and semi volatile organic compounds (SVOC) but very few ELVOC and low volatile organic compounds (LVOC). 132 and 195 CHO and CHON oxidation products are detected by the iodide ToF-CIMS in the low and high NOx experiments respectively. Ring breaking products make up the dominant fraction of detected signal (89–91 %). 21 and 26 of the products listed in the master chemical mechanism (MCM) were detected and account for 6.4–7.3 % of total signal. The time series of highly oxidised (O≥6) and ring retaining oxidation products (C6 and double bond equivalent = 4) equilibrate quickly characterised by a square form profile, compared to MCM and ring breaking products which increase throughout oxidation exhibiting saw tooth profiles. Under low NOx conditions, all CHO formulae attributed to radical termination reactions of 1st generation benzene products and 1st generation autoxidation products are observed, and one exclusively 2nd generation autoxidation product is also measured (C6H8O8). Several N containing species that are either 1st generation benzene products or 1st generation autoxidation products are also observed under high NOx conditions. Hierarchical cluster analysis finds four cluster of which two describe photo-oxidation. Cluster 2 shows a negative dependency on the NO2/NOx ratio indicating it is sensitive to NO concentration thus likely to contain NO addition products and alkoxy derived termination products. This cluster has the highest average carbon oxidation state (OSc) and the lowest average carbon number and where nitrogen is present in cluster member, the oxygen number is even, as expected for alkoxy derived products. In contrast, cluster 1 shows no dependency on the NO2/NOx ratio and so is likely to contain more NO2 addition and peroxy derived termination products. This cluster contains less fragmented species, as the average carbon number is higher and OSc lower than cluster 2, and more species with an odd number of oxygen atoms. This suggests clustering of time series which have features pertaining to distinct chemical regimes e.g. NO2/NOx perturbations, coupled with a priori knowledge, can provide insight into identification of potential functionality.