A collection system for dry solid residues from exhaled breath for analysis via atomic force microscopy

Abstract

Exhaled air contains sub-micron droplets of lung liquid, which potentially bear biomarkers of lung diseases. After dehydration they form dry residue particles (DRPs). As a first step in developing techniques to characterize individual DRPs, a new electrostatic collector was designed in which DRPs are charged within a unipolar corona charger, concentrated in a cone funnel, and deposited onto a limited area of a highly oriented pyrolytic graphite surface. The collector captures 80%-90% of DRPs at an optimal flow rate of 0.15 l min. Atomic force microscopy (AFM) revealed flattened round particles 20-50 nm high, with notable protrusions at their surface suggestive of an inhomogeneous internal structure. Exposure to humid air resulted in the DRPs spreading over the surface, with a 50%-200% decrease in their heights and an increase in their lateral dimensions so that their volume decreased by only 10% ± 3%. Exposure to saturated chloroform vapor resulted in drainage of 10%-15% of the DRP volume (presumably lipids), forming collar-shaped rings around each particle but leaving the core size and structure unchanged. AFM measurements combined with laser counter measurements of the DRP concentrations were used to estimate that one liter of air exhaled by volunteers contained less than 100 pg of dry residue material.

Risk assessment of airborne transmission of COVID-19 by asymptomatic individuals under different practical settings

“…Particularly, through mass spectra of particles, the literature has revealed the presence of nonvolatile solutes such as potassium, calcium, and chorine contents in the faceted particles ( Papineni et al, 1997 , and Morozov & Mikheev, 2017 ), which are likely to be generated from the alveolar fluid from the lower respiratory tract ( Johnson & Morawska, 2009 ). In addition, Morozov and Mikheev (2017) suggested that these faceted particles can contain lipids and hydrophobic proteins which are soluble in organic solvents. Noteworthily, such faceted particles could be the residues of droplet evaporation and may correspond to the long-lasting dry particles reported in the experiments of Stadnytskyi et al (2020) .…”

Non-invasive lung disease diagnostics from exhaled microdroplets of lung fluid: perspectives and technical challenges

“…It consisted of a thermostated metal base to which a plastic dumbbellshaped cap is attached with screws. The air is pumped from the cap at a rate of 2.2 l min −1 , while exhaled air is fed in through a tube with an inner diameter of 0.7 mm and accelerates to ∼100 m s −1 , so that 50% of MLFs with a size of 470 nm are captured [23].…”

“…A few reports have described the analytical application of such electro-precipitation procedures to sample airborne particles, which are then imaged under an electron microscope [39] or used to monitor environmental bioaerosols [40,41]. To the best of our knowledge, our recent paper was the first report describing the use of an electrostatic collector to analyse dry residues of MLFs with AFM [23]. The collector allows dry residues to settle on graphite and other conducting substrates with high efficiency because it uses a special electrostatic 'funnel' made of a polymer mesh that directs the charged particles onto a conducting substrate to produce a highly dense deposition suitable for AFM imaging.…”

“…Like filters and impactors, an electrostatic precipitator collects both large and small MLFs. Two major drawbacks of electrostatic collectors should be mentioned: (i) their relatively slow volumetric rate [23]; and (ii) generation of radicals and other reactive oxygen species in the corona discharge, which may chemically modify biomarkers in the collected probes. The latter drawback may be substantially diminished by replacing the corona discharge with electrospray as the means to generate a cloud of small ions [42], although nobody has ever reported the applicability of such a technique to the collection of aerosols and exhaled MLFs.…”

“…Reliable estimates of the amounts of non-volatile materials collected over the course of 10 min are 1-10 ng for most people [23,31]. Obviously, each biomarker will constitute only a small fraction of this amount, and extremely sensitive techniques are required to detect specific biomarkers in the collected probes.…”