Abstract
Abstract. Measurement techniques that provide molecular-level information are needed to elucidate the multiphase processes that produce secondary organic aerosol (SOA) species in the atmosphere. Here we demonstrate the application of ion mobility spectrometry-mass spectrometry (IMS–MS) to the simultaneous characterization of the elemental composition and molecular structures of organic species in the gas and particulate phases. Molecular ions of gas-phase organic species are measured online with IMS–MS after ionization with a custom-built nitrate chemical ionization (CI) source. This CI–IMS–MS technique is used to obtain time-resolved measurements (5 min) of highly oxidized organic molecules during the 2013 Southern Oxidant and Aerosol Study (SOAS) ambient field campaign in the forested SE US. The ambient IMS–MS signals are consistent with laboratory IMS–MS spectra obtained from single-component carboxylic acids and multicomponent mixtures of isoprene and monoterpene oxidation products. Mass-mobility correlations in the 2-D IMS–MS space provide a means of identifying ions with similar molecular structures within complex mass spectra and are used to separate and identify monoterpene oxidation products in the ambient data that are produced from different chemical pathways. Water-soluble organic carbon (WSOC) constituents of fine aerosol particles that are not resolvable with standard analytical separation methods, such as liquid chromatography (LC), are shown to be separable with IMS–MS coupled to an electrospray ionization (ESI) source. The capability to use ion mobility to differentiate between isomers is demonstrated for organosulfates derived from the reactive uptake of isomers of isoprene epoxydiols (IEPOX) onto wet acidic sulfate aerosol. Controlled fragmentation of precursor ions by collisionally induced dissociation (CID) in the transfer region between the IMS and the MS is used to validate MS peak assignments, elucidate structures of oligomers, and confirm the presence of the organosulfate functional group.
Highlights
Organic aerosol (OA) species constitute a major fraction of airborne particles globally, comprising 20–90 % of fine particle mass in many regions (Hallquist et al, 2009; Murphy et al, 2006; Zhang et al, 2007) and is either directly emitted into the atmosphere in the particle phase or formed from gas-to-particle conversion processes
We demonstrated the capability of this technique to separate water soluble species and structural isomers of species such as trihydroxybutylsulfate which are not readily separated by other techniques such as liquid chromatography (LC)/MS and gas chromatography (GC)/MS
The use of Ion mobility spectrometry (IMS)–collisionally induced dissociation (CID)-MS to obtain spectra that are analogous to conventional MS/MS spectra is demonstrated
Summary
Organic aerosol (OA) species constitute a major fraction of airborne particles globally, comprising 20–90 % of fine particle mass in many regions (Hallquist et al, 2009; Murphy et al, 2006; Zhang et al, 2007) and is either directly emitted into the atmosphere in the particle phase (primary OA, POA) or formed from gas-to-particle conversion processes (secondary OA, SOA). Despite the increase in mass resolution, UHRMS cannot, resolve structural isomers without a prior separation step and accurate quantification of observed species is complicated by ion suppression/matrix effects (Nozière et al, 2015). Molecular-level identification of individual organic species is often achieved by coupling mass spectrometry with a separation technique such as liquid chromatography (LC) or gas chromatography (GC). GC techniques require heating, which has been shown to decompose some organic species to CO2 or other small molecules (Williams et al, 2016) Recent analytical advances such as the volatility and polarity separator (VAPS) have increased the fraction of WSOC that is amenable to GC/MS analysis without derivatization, but a large portion remains uncharacterized (Martinez et al, 2016). We present measurements of gas-phase species using a custom NO−3 chemical ionization (CI) source coupled to the IMS–TOF. Molecular structures, even within complex high-resolution mass spectra, is highlighted
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