Sulfur-free Surfactant for Carbide Nanoparticle Characterization in Steel Using Asymmetric Flow Field–Flow Fractionation Hyphenated Inductively Coupled Plasma–Mass Spectrometry

An asymmetric flow field–flow fractionation (AF4) combined with inductively coupled plasma mass spectrometry (ICP-MS) was applied to measure the concentration and size distribution of nanometer-sized carbides in steel sheets, such as titanium carbides (TiC) and vanadium carbides (VC). Prior to AF4-ICP-MS measurement, TiC and VC nanoparticles in steel were extracted into a solution via selective potentio-static etching by electrolytic dissolution (SPEED) method. The SPEED method enabled the selective dissolution of iron and the carbide nanoparticles were dispersed as primary particles in solution with surfactant. However, sulfur-free surfactant was required in AF4-ICP-MS carrier solutions because sulfur in SDS, generally used to disperse various nanoparticles, causes a spectral interference with titanium and vanadium in ICP-MS analysis. In this study, sulfur-free sodium cholate (SC) was applied as the dispersant of carbide nanoparticles for the SPEED method and AF4 measurements. SC provides a high absolute value of zeta potential on a particle surface and membrane of an AF4 separation channel to prevent particle adsorption on the membrane. Additionally, SC does not generate the spectral interference due to sulfur, in contrast to SDS. Thus, it enabled the sensitive detection of titanium and vanadium in carbide nanoparticles extracted from a steel sheet in AF4-ICP-MS. These results indicate that sulfur-free surfactants are useful for analyzing some precipitates in steels using AF4-ICP-MS.