Sensitivity Constraints of Extractive Electrospray for a Model System and Secondary Organic Aerosol

The quantification of an aerosol chemical composition is complicated by the uncertainty in the sensitivity of each species detected. Soft-ionization response factors can vary widely from molecule to molecule. Here, we have employed a method to separate molecules by their volatility through systematic evaporation with a thermal denuder (TD). The fraction remaining after evaporation is compared between an extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF) and a scanning mobility particle sizer (SMPS), which provides a comparison between a quantified mass loss by the SMPS and the signal loss in the EESI-TOF. The sensitivity of the EESI-TOF is determined for both a simplified complex mixture (PEG-300) and also for a complex mixture of a-pinene secondary organic aerosol (SOA). For PEG-300, separation is possible on a molecule-by-molecule level with the TD and provides insights into the molecule dependent sensitivity of the EESI-TOF, showing a higher sensitivity toward the most volatile molecule. For a-pinene SOA, sensitivity determination for specific classes is possible because of the number of molecular formula observed by the EESI-TOF. These classes are separated by their volatility and are broken down into monomers (O-3-5,O-6-7,O-8+), dimers (O4-7,8+), and higher order oligomers (e.g., trimers and tetramers). Here, we show that the EESI-TOF initially measures 60.1% monomers, 32.7% dimers, and 7.2% trimers and tetramers in a-pinene SOA, but after sensitivity correction, the distribution of SOA is 37.4% monomers, 56.1% dimers, and 6.4% trimers and tetramers. These results provide a path forward for the quantification of aerosol components with the EESI-TOF in other applications and potentially for atmospheric measurements.

Automated summary

This study employs a method to separate molecules by their volatility and compares the remaining fraction after evaporation using EESI-TOF and SMPS, revealing the molecule-dependent sensitivity of EESI-TOF. Testing PEG-300 and α-pinene SOA, it is found that EESI-TOF has higher sensitivity towards more volatile molecules. This research provides new insights for the quantification of aerosol components with EESI-TOF in atmospheric measurements and other applications.