An intercomparison study of four different techniques for measuring the chemical composition of nanoparticles

Abstract. Currently, the complete chemical characterization of nanoparticles (<100 nm) represents an analytical challenge, since these particles are abundant in number but have negligible mass. Several methods for particle-phase characterization have been recently developed to better detect and infer more accurately the sources and fates of ultra-fine particles, but a detailed comparison of different approaches is missing. Here we report on the chemical composition of secondary organic aerosol (SOA) nanoparticles from experimental studies of α-pinene ozonolysis at -50 ºC, -30 ºC, and -10 ºC, and inter-compare the results measured by different techniques. The experiments were performed at the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber at the European Organization for Nuclear Research (CERN). The chemical composition was measured simultaneously by four different techniques: 1) Thermal Desorption-Differential Mobility Analyzer (TD-DMA) coupled to a NO₃- chemical ionization-atmospheric-pressure-interface-time-of-flight (CI-APi-TOF) mass spectrometer, 2) Filter Inlet for Gases and AEROsols (FIGAERO) coupled to an I^- high-resolution time-of-flight chemical-ionization mass spectrometer (HRToF-CIMS), 3) Extractive Electrospray Na⁺ Ionization time-of-flight mass spectrometer (EESI-TOF), and 4) Offline analysis of filters (FILTER) using Ultra-high-performance liquid chromatography (UHPLC) and heated electrospray ionization (HESI) coupled to an Orbitrap high-resolution mass spectrometer (HRMS). Intercomparison was performed by contrasting the observed chemical composition as a function of oxidation state and carbon number, by calculating the volatility and comparing the fraction of volatility classes, and by comparing the thermal desorption behavior (for the thermal desorption techniques: TD-DMA and FIGAERO) and performing positive matrix factorization (PMF) analysis for the thermograms. We found that the methods generally agree on the most important compounds that are found in the nanoparticles. However, they do see different parts of the organic spectrum. We suggest potential explanations for these differences: thermal decomposition, aging, sampling artifacts, etc. We applied PMF analysis and found insights of thermal decomposition in the TD-DMA and the FIGAERO.