Interlaboratory comparison of centrifugal ultrafiltration with ICP-MS detection in a first-step towards methods to screen for nanomaterial release during certification of drinking water contact materials

A key requirement for evaluating the safety of nano-enabled water treatment devices is measuring concentrations of insoluble nanomaterials released from devices into water that may be ingested by consumers. Therefore, there is a need for simple technique that uses commonly available commercial laboratory techniques to discriminate between nanoparticles and dissolved by-products of the nanomaterial (e.g., ionic metals). Such capabilities would enable screening for particulate or dissolved metals released into water from nanomaterial-containing drinking water contact materials (e.g., paint coatings) or devices (e.g., filters). This multi-laboratory study sought to investigate the use of relatively inexpensive centrifugal ultrafilters to separate nanoparticulate from ionic metal in combination with inductively-coupled plasma mass spectrometry (ICP-MS) detection. The accuracy, precision, and reproducibility for the proposed method were assessed using mixtures of nanoparticulate and ionic gold (Au) in a standard and widely utilized model water matrix (NSF International Standard 53/61). Concentrations for both ionic and nanoparticulate gold based upon measurements of Au mass in the initial solutions and Au permeating the centrifugal ultrafilters. Results across different solution compositions and different participating labs showed that ionic and nanoparticulate Au could be consistently discriminated with ppb concentrations typically resulting in <10 % error. A mass balance was not achieved because nanoparticles were retained on membranes embedded in plastic holders inside the centrifuge tubes, and the entire apparatus could not be acid and/or microwave digested. This was a minor limitation considering the ultrafiltration method is a screening tool, and gold concentration in the permeate indicates the presence of ionic metal rather than nanoforms. With further development, this approach could prove to be an effective tool in screening for nanomaterial release from water-system or device materials as part of third-party certification processes of drinking water compatible products.