Characterizing engineered nanoparticles (ENPs) in complex environmental matrices remains a challenging task. This work presents a two-dimensional size analysis method by combining differential mobility analyzer (DMA) and single-particle inductively coupled plasma-mass spectrometry (spICP-MS) with a new atomizer (ATM)-enabled sample introduction that is relatively easy to operate. The tailing of electrical mobility size distributions was solved by heating the aerosol flow, where water-shelled gold nanoparticles (AuNPs) were dehydrated, effectively eliminating the tailing. The improved method has a good sizing performance and can resolve the size fractions of mixed 30 nm and 50 nm AuNPs. It can reliably analyze 7.8 × 105 to 1.9 × 107 # of 50 nm AuNPs (or 4.1 × 105 to 107 # NPs/mL, equivalent to 0.6 to 14.3 μg Au/L) with a linear response and a limit of detection of 7.8 × 105 # AuNPs (equivalent to 4.1 × 105 # AuNPs/mL) that is relevant to NP concentrations in surface water and wastewater samples. The potential of this method to analyze environmental samples was demonstrated by characterizing AuNPs and silver nanoparticles (AgNPs) spiked in wastewater, where both NPs were revealed to form heteroaggregates with colloids existing in wastewater. The method can even directly analyze nanosized Ag particles inherent in the wastewater before adding external AgNPs. The result indicates that ATM-DMA-spICP-MS is a relatively simple two-dimensional size analysis method that has a great potential to characterize heteroaggregated NPs in aqueous environmental samples.
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