Comparative evaluation of filtration and imaging properties of analytical filters for microplastic capture and analysis

Pollution by microplastics (MPs) is a growing problem that is now well-recognized, as concerning levels of MPs have been found in drinking water, food, and even human tissues. Given the evolving understanding of their toxicological effects on human health, MPs are an area of concern requiring further study. Consequently, there is a need for greater understanding of the performance characteristics of common MP analytical methods and where possible, for standardizing methods and reporting practices. Here, we report our work comparing filtration and imaging properties of five analytical filters suitable for MP capture and analysis. We compared track-etched polycarbonate with (PCTEG) and without gold coating (PCTE), polytetrafluoroethylene (PTFE), porous silicon (PSi), and gold-coated microslit silicon nitride membranes (MSSN-Au). Four of the filter types had a nominal 1.0 μm cut-off, except for PCTEG which had a 0.8 nominal cut-off. We examined the ultrastructure of each membrane type by electron microscopy to understand how their physical properties influence filtration and imaging performance. We compared clean water filtration rates and timed volume passage for each filter in comparison to its porosity and working surface area. We further compared optical microscopy imaging properties for each filter with model MP samples in both bright-field and fluorescent modes with accompanying Nile Red staining. In terms of absolute and surface area-normalized flow rates, our measurements ranked the filters in order of MSSN-Au > PTFE > PCTE > PCTEG > PSi. Similarly, we found MSSN-Au filters compared favorably in terms of optical microscopy performance. Collectively, these data will aid practitioners when choosing analytical filters for MP surveillance and testing.

Characterization of Na+-coupled glutamate/aspartate transport by a rat brain astrocyte line expressing GLAST and EAAC1

D-aspartate (D-Asp) uptake by suspensions of cerebral rat brain astrocytes (RBA) maintained in long-term culture was studied as a means of characterizing function and regulation of Glutamate/Aspartate (Glu/Asp) transporter isoforms in the cells. A-asp influx is Na+-dependent with Km = 5 microm and Vmax = 0.7 nmoles x min(-1) x mg protein-1. Influx is sigmoidal as f[Na+] with Na+Km approximately 12 microm and Hill coefficient of 1.9. The cells establish steady-state D-Asp gradients >3,000-fold. Phorbol ester (PMA) enhances uptake, and gradients near 6,000-fold are achieved due to a 2-fold increase in Vmax, with no change in Km. At initial [D-Asp] = 10 microm, RBA take up more than 90% of total D-Asp, and extracellular levels are reduced to levels below 1 microm. Ionophores that dissipate the Delta(mu)Na+ inhibit gradient formation. Genistein (GEN, 100 microm), a PTK inhibitor, causes a 40% decrease in d-Asp. Inactive analogs of PMA (4alpha-PMA) and GEN (daidzein) have no detectable effect, although the stimulatory PMA response still occurs when GEN is present. Further specificity of action is indicated by the fact that PMA has no effect on Na+-coupled ALA uptake, but GEN is stimulatory. d-Asp uptake is strongly inhibited by serine-O-sulfate (S-O-S), threohydroxy-aspartate (THA), L-Asp, and L-Glu, but not by D-Glu, kainic acid (KA), or dihydrokainate (DHK), an inhibition pattern characteristic of GLAST and EAAC1 transporter isoforms. mRNA for both isoforms was detected by RT-PCR, and Western blotting with appropriate antibodies shows that both proteins are expressed in these cells.