Constraining the response factors of an extractive electrospray ionization mass spectrometer for near-molecular aerosol speciation

Julia Schmale, Yandong Tong
2021
Journal paper

Abstract

Online characterization of aerosol composition at the near-molecular level is key to understanding chemical reaction mechanisms, kinetics, and sources under various atmospheric conditions. The recently developed extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF) is capable of detecting a wide range of organic oxidation products in the particle phase in real time with minimal fragmentation. Quantification can sometimes be hindered by a lack of available commercial standards for aerosol constituents, however. Good correlations between the EESI-TOF and other aerosol speciation techniques have been reported, though no attempts have yet been made to parameterize the EESI-TOF response factor for different chemical species. Here, we report the first parameterization of the EESI-TOF response factor for secondary organic aerosol (SOA) at the near-molecular level based on its elemental composition. SOA was formed by ozonolysis of monoterpene or OH oxidation of aromatics inside an oxidation flow reactor (OFR) using ammonium nitrate as seed particles. A Vocus proton-transfer reaction mass spectrometer (Vocus-PTR) and a high-resolution aerosol mass spectrometer (AMS) were used to determine the gas-phase molecular composition and the particle-phase bulk chemical composition, respectively. The EESI response factors towards bulk SOA coating and the inorganic seed particle core were constrained by intercomparison with the AMS. The highest bulk EESI response factor was observed for SOA produced from 1,3,5-trimethylbenzene, followed by those produced from d-limonene and o-cresol, consistent with previous findings. The near-molecular EESI response factors were derived from intercomparisons with Vocus-PTR measurements and were found to vary from 103 to 106 ion counts s−1 ppb−1, mostly within ±1 order of magnitude of their geometric mean of 104.6 ion counts s−1 ppb−1. For aromatic SOA components, the EESI response factors correlated with molecular weight and oxygen content and inversely correlated with volatility. The near-molecular response factors mostly agreed within a factor of 20 for isomers observed across the aromatics and biogenic systems. Parameterization of the near-molecular response factors based on the measured elemental formulae could reproduce the empirically determined response factor for a single volatile organic compound (VOC) system to within a factor of 5 for the configuration of our mass spectrometers. The results demonstrate that standard-free quantification using the EESI-TOF is possible.

Official source

https://infoscience.epfl.ch/record/289809?ln=en

About this result

This page is automatically generated and may contain information that is not correct, complete, up-to-date, or relevant to your search query. The same applies to every other page on this website. Please make sure to verify the information with EPFL's official sources.

Related concepts

Related publications

Julia Schmale, Yandong Tong
2021
Journal paper

Abstract

Official source

https://infoscience.epfl.ch/record/289809?ln=en

About this result

Related concepts (17)

Related concepts

Related publications (4)

Related publications

Mixed ionic electronic conducting perovskite materials have been receiving considerable attention for the application of oxygen separation membranes. These membranes have potential to be integrated in industrial processes that require pure oxygen and to provide advantages considering economical and environmental aspects. The used perovskite materials can accommodate a high concentration of disordered oxygen vacancies and provide high oxygen permeation flux in the presence of an oxygen partial pressure gradient. However, the materials with high oxygen flux are observed to have low chemical and mechanical stability, originating from oxygen vacancy formation and reduction of the B-site transition metal ion. Based on the fact that the stability of the materials at reducing pO2 is dependent on the redox stability of the B-site cation, a partial substitution of the B-site transition metal with a more stable cation is considered in this thesis to improve the stability of the base perovskite oxide. The approach used for the identification of the suitable material based on the host composition La0.5Sr0.5FeO3-δ (LSF) was to first explore the B-site substituting elements in terms of their effect on the thermal expansion and the isothermal expansion measured during the atmosphere change from air to argon. The elements Al, Cr, Zr, Ga, Ti, Sn, Ta, In, V, and Mg were used for 10 or 20% substitution of Fe. The isothermal expansion of the host material was decreased by substitution, which was attributed to decreased amount of oxygen vacancy formation as it was evidenced later by TGA of chosen compositions. As a result, the compositions substituted with higher valence ions (particularly, 20% Ti4+, LSFTi2, and 10% Ta5+, LSFTa1) were identified as the least expanding materials in isothermal conditions. The effect of substitution was further characterized extensively including structural, mechanical, and oxygen transport properties. The oxygen vacancy (VO••) concentration changes induced by substitution of Fe were reflected in unit cell sizes of heat-treated samples under argon atmosphere. The increase in the unit cell size was attributed to the isothermal expansion due to B-site reduction and lowered to half in case of substituted LSFTi2 and LSFTa1 compared to the host material LSF. Similar trends were observed for interrelated properties such as the coefficient of thermal expansion, isothermal expansion, and the actual amount of oxygen vacancies formed under argon at 900°C, all of which were higher for LSF. The mechanical properties of LSF, LSFTi2, and LSFTa1 were characterized at room temperature. The Young's modulus and the bending strength of the materials were in the same range (141-147 GPa and ∼120 MPa, respectively) while the fracture toughness of the LSF sample was improved by Ti and especially Ta substitution. The fractography of the samples provided evidence that the several LSF samples showed extended cracks possibly related to residual stresses forming during cooling, due to non-equilibrated oxygen stoichiometry across the sample. The potential step measurements showed that the chemical diffusion and the surface exchange coefficients measured during oxidation of the samples were higher than the ones measured during reduction of the samples. The tendency was reversed at lowered pO2 along with the decrease of both coefficients. The influence of B-site substitution on both coefficients was not substantial. The oxygen permeation flux of LSF, LSFTi2, and LSFTa1 was measured under air/argon gradient on planar membranes. The permeation rates of the membranes with thicknesses close to 1 mm were considered to be limited by surface exchange kinetics at temperatures above 875°C. The permeation rate of LSF was decreased (from 0.2 µmolcm-2s-1) to its third by Ti and Ta additions. The permeation rate of Ti and Ta-substituted samples showed a drop around 875°C, keeping a similar activation energy at lower and higher temperature regions. Oxygen vacancy ordering in the perovskite structure was given as a possible explanation, which provided explanation for the slow equilibrium in the lower temperature region. Tubular membranes of LSF and LSFTi2 (0.47 mm and 0.36 mm thick, respectively) and a planar LSFTa1 membrane were characterized using an air/(Ar-CH4) gradient. The measurements conducted using pure Ar on the lean-side provided the possibility to compare the thickness dependence of the permeation. In agreement with the previous observations, the permeation of the materials at and above 900°C was independent of the thickness, therefore, surface exchange limited. Stability under reducing atmospheres was increased by substitution. The LSF membrane failed shortly after the introduction of CH4 to the system, while LSFTi2 survived 5% CH4 and its oxygen permeation was improved substantially by a factor of 14. LSFTa1 membrane was measured with pure CH4 and the permeation was increased by a factor of 9, reaching 0.7 µmolcm-2s-1 at 900°C. The membrane operated stably over 2000h with pure CH4, 1000h. A slow degradation of 3.7% per 1000h was observed at 1000°C, most probably due to cation migration. The compositions identified as a result of B-site substitution screening were effective in improving the stability of the materials without extensive loss of the oxygen permeation rate. Ta-substituted material was shown to be promising with its long-term stability as a partial oxidation membrane.

EPFL2008

Microchip Based Electrospray Mass Spectrometry for Investigation of Neurodegenerative Peptides

Yu Lu

Electrospray ionization mass spectrometry (ESI-MS) is a powerful tool to investigate large molecules. Miniaturized microchips, as a goal of ‘Lab-on-chip’, allow various integrated analysis quickly. The coupling of functional microchips and ESI-MS is capable of providing a sensitive and fast analysis of all products generated in various functional microchips. Beta-amyloid (Aβ) peptides are the main component of senile plaques, a hallmark of Alzheimer’s disease. Copper binding is an important factor to induce soluble Aβ peptides aggregation and generates reactive oxygen species (ROS). By using a sacrificial copper electrode as the anode in positive ionization mode of ESI-MS, copper (I) and (II) complexes were formed on-line in a microchip electrospray emitter. This approach provides direct information on Cu(I)-Aβ16 complexes generated in solution from metallic copper, and tandem mass spectrometry gives evidence that His13 and His14 are involved in the coordination of both Cu(I)- and Cu(II)-Aβ16 complexes. Since soluble Aβ peptide potentially controls copper homeostasis in human body, the damage by excessive ROS may destroy their functions. Two main types of oxidation products of N-terminal truncated Aβ11 and Aβ16 in the presence of ascorbic acid, i.e., oxygen-addition products and N-terminal loss damages were investigated by ESI-MS. The oxidation sites for both of them were firstly well identified together with the influence of pH value and the reaction time on the generation of oxidation products. Alpha-synuclein (α-syn) plays a central role in the pathogenesis of Parkinson's disease. Post-translational modifications (PTMs) of α-syn occur at the C-terminal region and are in very close proximity to the putative metal binding sites. The interactions between the di- and trivalent cations, Cu(II), Pb(II), Fe(II) and Fe(III) and the C-terminal region of α-syn and its phosphorylated forms were carried out using ESI-MS and fluorescence spectroscopy. Dual-sprayer microchip coupled to ESI-MS, successfully ionized aqueous peptide/protein samples due to a cooperative electrospray from the Taylor cone with assisting solution containing an organic solvent and acids. As an effective and quick mixer in the Taylor cone, dual-sprayer microchip was able to gradually change the conformations of large proteins and quickly investigate the fast reactions such as the complexation reaction of Aβ peptide and phospholipid after direct mixing them in the Taylor cone. Droplet microchip was developed and directly interfaced to ESI-MS. Different sizes of droplets were generated and successfully determined by ESI-MS even at a very low sample concentration. Owing to a high feasibility of microchip fabrication, this droplet microchip can also act as an effective micro-mixer capable of introducing multiple samples into one droplet by the introduction of additional micro-channels for aqueous liquids.

EPFL2012

Advances in Structural Elucidation of Small Molecules via High Performance Mass Spectrometry

Konstantin Zhurov

Qualitative and quantitative analyte analysis is omnipresent: from quality control in food, pharmaceutical, biotechnology industries, to new molecular syntheses, detection of explosives and drugs, as well as natural compound mixture analysis, such as crude oils or animal fluids. Of particular interest are analytes associated with human health and energy, both of synthetic and natural origin. Thus, a selection of synthetic analytes related to ongoing efforts in drug design and alternative energy sources, has been made for this Thesis. Additionally, for natural analytes, peptides and crude oil were selected as analytes of choice due to their central role in human health and energy sources, respectively. Mass spectrometry was chosen as the method of choice for analyte investigation (elemental composition and structural feature determination) due to its sensitivity, ability to obtain data on thousands of analytes simultaneously, and use of radical and photo chemistry to probe molecular structure. The thesis is split into three major parts, first investigating synthetic analytes, then peptides and, finally, crude oil fractions. In the first part, use of photoionization is discussed as an alternative to other common analyte ionization methods as most selected synthetic targets do not ionize with the latter. A rough guideline, based on analyte structural composition, is proposed to determine analyte photoionization response. In-source analyte reactions are analysed and used as indicators of various functional group presence. Finally, upper mass boundary on analytes is considered and extended with photoionization. In the second part, peptide-ion/electron and ion/radical anion interactions in tandem mass spectrometry experiments, along with in silico modelling are used to put forth a new mechanistic proposal, which is fundamentally different to all the other mechanistic proposals related to this type of fragmentation process. The proposal involves charge-driven, rather than radical driven peptide radical fragmentation. It is also additionally significantly influenced by the local 3D structure of the peptide. The proposal may be of particular interest when considering radiation damage effects on key biological systems. In the third part, several methods are proposed aimed at enhancing crude oil (and other complex mixture) analysis on a new type of high resolution mass spectrometer, the Orbitrap Elite FTMS, including maximising number of identifiable analytes and evaluating key parameters of mass spectra of complex mixtures. Additionally, a data visualization method is proposed, based on analyte heteroatom content, which creates a graphical representation of all the identified components in the complex mixture and how changes in bulk properties of the sample get reflected on its compositional makeup.

EPFL2014