Quantification of Nanoparticles via Flow Injection Analysis Using Asymmetric Flow Field-flow Fractionation (AF4)

To determine the number density of fine precipitates in steels by asymmetric flow field-flow fractionation (AF4) with inductively coupled plasma-mass spectrometry (ICP-MS), an analysis method employing flow injection was investigated. For accurate calibration, matrix matching was performed by mixing the standard solution and AF4 carrier solution in front of a nebulizer. Two surfactants were used for AF4 separation; it was found that the appropriate selection of surfactants based on their acidity constant is essential to avoid salt precipitation. In addition, the effect of the AF4 retention time on recovery was investigated. A long retention time led to the adsorption and aggregation of the samples in the AF4 separation channel. Results showed that an AF4 retention time within 20 min facilitated superior recovery. Moreover, five types of AuNPs were analyzed via AF4-ICP-MS and quantified using flow injection analysis. Good analytical performance was achieved for all AuNPs and the recoveries exceeded 93%, and the coefficient of variation was within 5%. The effect of particle size on the recovery was not confirmed. Furthermore, the developed flow injection analysis for AF4-ICP-MS was applied to evaluate niobium carbide (NbC) precipitates in steels. The number density of nanometer-sized NbC was quantified to be within 10(13) to 10(14) particles per 1 g of Fe. It was quantitatively confirmed that the long-duration heat treatment led to an increase in the number density of nanometer-sized NbC. Hence, this method can be useful for quantitatively analyzing the size and number density of nanoprecipitates in steels.