PM_2.5 Filter Extraction Methods: Implications for Chemical and Toxicological Analyses

Abstract

Toxicology research into the global public health burden of fine particulate matter (PM2.5) exposures frequently requires extraction of PM2.5 from filters. A standardized method for these extractions does not exist, leading to inaccurate inter-laboratory comparisons. It is largely unknown how different filter extraction methods might impact the composition and bioactivity of the resulting samples. We characterized the variation in these metrics by using equal portions of a single PM2.5 filter, with each portion undergoing a different extraction method. Significant differences were observed between extraction methods for concentrations of elements and polycyclic aromatic hydrocarbons (PAHs) of the PM2.5 tested following its preparation for biological response studies. Importantly, the chemical profiles differed from those observed when using standard protocols for chemical characterization of the ambient sample, demonstrating that extraction can alter both chemical component amounts and species profiles of the extracts. The impact of these chemical differences on sensitive endpoints of zebrafish development was investigated. Significant differences in the percent incidence and timing of mortality were associated with PM2.5 extraction method. This research highlights the importance of and rationale for considering extraction method when making inter-laboratory comparisons of PM2.5 toxicology research.

Effect of filter extraction solvents on the measurement of the oxidative potential of airborne PM2.5

“…Three extraction solvents were investigated to represent the typically used protocols for OP assay of PM samples: phosphate buffer (PB), the most common aqueous buffer employed by the Authors (Perrone et al 2019 ; Pietrogrande et al 2018 ; Pietrogrande et al 2019a ; Visentin et al 2016 ) and other researchers (Bates et al 2019 ; Calas et al 2018 ; Fang et al 2015 ; Hedayat et al 2014 ); methanol (Me), the organic solvent frequently used in PM 2.5 toxicology studies , due to its ability to extract hydrophobic and hydrophilic compounds, combined with its low cost and comparatively small blank filter effect (Bein and Wexler 2015 ; Janssen et al 2015 ; Roper et al 2017 ; Roper et al 2019 ; Verma et al 2015 ; Yang et al 2014 ); Gamble’s solution (G), an artificial lung fluid consisting of a mixture of salts (pH: 7.4) widely used for closely simulating the real physiological conditions related to PM exposition. It is typically employed for pulmonary simulation in studies on PM toxicity, since it represents the extra-cellular fluids in the deep lung (Bein and Wexler 2015 ; Collins et al 2015 ; Goix et al 2016 ; Leclercq et al 2017 ; Mukhtar et al 2015 ; Wei et al 2019 ; Wiseman, 2015 ; Xing et al 2019 ; Zeng et al 2019 ).…”

“…methanol (Me), the organic solvent frequently used in PM 2.5 toxicology studies , due to its ability to extract hydrophobic and hydrophilic compounds, combined with its low cost and comparatively small blank filter effect (Bein and Wexler 2015 ; Janssen et al 2015 ; Roper et al 2017 ; Roper et al 2019 ; Verma et al 2015 ; Yang et al 2014 );…”

“…A quarter of filter was extracted for 15 min in an ultrasonic bath using 10 mL of methanol, following the procedure commonly used by other Authors (Bein and Wexler 2015 ; Gao et al 2017 ; Janssen et al 2015 ; Roper et al 2017 ; Roper et al 2019 ; Verma et al 2015 ; Yang et al 2014 ). The extract was then filtered on a PTFE syringe filter (25 mm, 0.22 μm, Kinesis) to remove the suspended solid particles.…”

“…They all require a preliminary solvent extraction to retrieve the PM components from the loaded filter, still conserving, as much as possible, the physical and chemical properties of the complex mixtures present in the atmosphere. This is a critic step, as the major contributors to PM toxicity are trace components, that contribute little to PM mass and may be potentially loss during the extraction process or contaminated by interfering compounds (Bein and Wexler 2015 ; Roper et al 2017 ; Roper et al 2019 ; Simonetti et al 2018 ; Van Winkle et al 2015 ). Although several OP assay procedures are currently in use, to date, no real consensus has emerged towards standardized protocols, including the type of filters used to collect atmospheric PM, solvent and extraction procedures, and OP assay media (Bates et al 2019 ; Calas et al 2017 ; Luo et al 2019 ; Molina et al 2020 ; Roper et al 2015 ; Roper et al 2017 ; Wiseman 2015 ; Yang et al 2014 ).…”

“…In addition, the Gamble’s solution was investigated, as a lung fluid surrogate mixture, that may closely mimic bio-accessibility of particle-bound species in the physiological fluids encountered during PM exposition (Calas et al 2017 ; Collins et al 2015 ; Leclercq et al 2017 ; Luo et al 2019 ; Mukhtar et al 2015 ; Van Winkle et al 2015 ; Wiseman 2015 ; Zeng et al 2019 ). To date, the impact of solvent extraction on PM OP or toxicity has been studied by comparing aqueous solutions vs. organic solvents (i.e., Bein and Wexler 2015 ; Roper et al 2019 ; Yang et al 2014 ), or different simulated lung fluid surrogates (i.e., Calas et al 2017 ; Leclercq et al 2017 ; Van Winkle et al 2015 ). To our knowledge, a comprehensive inter-comparison of the three different extraction liquids has never been conducted.…”