NMs in Organic and Particle Rich Waters (High Matrix). Quality Control, Sample Transport, Storage, and Preparation

Samples with a high matrix concentration can be sewage sludge, surface runoff and sometimes river or lake water. Particle extraction from sediment or soil samples is not considered in this section. The matrices discussed here are typically composed of biomass and inorganic particulates. These matrix constituents occur in the nanometer size range and as larger particles. Owing to the high matrix concentration interactions between nanomaterial and matrix constituents increase and particulate matrix components might interfere with the analyte during detection.

It becomes more challenging for the analytical technique to differentiate between nanomaterial and matrix constituent. Hence, the sample preparation has to aim primarily at reducing the matrix concentration by chemical clean up or matrix separation from the analyte. Simple filtration, dilution, centrifugation, or FFF separation is typically not suitable to achieve this aim.

The main problem is that matrix content affects the outcome of the sample preparation steps. During filtration, filter cakes develop quickly and act as a nanomaterial collector that reduces the particle size cutoff. Particle matrix interactions result in heteroaggregates which are removed during centrifugation falsifying the nanomaterial composition in the aqueous media. In a few cases, some detection techniques are able to detect, identify, and quantify nanomaterials in a complex matrix, e.g. sp-ICP-MS enables the detection and quantification of inorganic particulate material in the presence of a certain amount of matrix, which can be adjusted by dilution.

Quality Control, Sample Transport, Storage, and Preparation. A thermodynamic equilibrium does not exist for nanomaterials in water. Consequently, physicochemical properties of nanomaterials, such as size, depend on the surrounding media and may change during any handling/preparation process. During sample transport, storage, and preparation several artifacts may occur, such shift in surface charge which may cause aggregation of the nanomaterials. Therefore, it is substantial to test the effect of transport, storage, and preparation on the nanomaterial, by conducting control experiments with known quantities of defined particles. Particle suspensions may be stabilized after sampling by dilution, addition of stabilizing agents, and cooling but not freezing.

Acknowledgment. AG was supported by the European Union Horizon 2020 framework NANOFASE (grant no. 646002) and the Mistra Environmental Nanosafety program financed by the Swedish Foundation for Strategic Environmental Research (grant no. DIA 2013/48).