Application of non-stationary phase separation hyphenated with inductively coupled plasma mass spectrometry in the analysis of trace metal-containing nanoparticles in the environment

Engineered metal-containing nanoparticles (MCNs), which have unique physical and chemical properties, are widely used in various fields such as medicine, pharmaceuticals, and microelectronics as well as in daily supplies. These MCNs are inevitably released into the environment during production and use, thus posing a threat to bacterial communities, animals, plants, and human health. There are also abundant natural MCNs in the environment, which play an important role in the environmental cycle of metals. The shape, size, and surface properties of MCNs have a significant impact on their migration, chemical and physical transformation, and biological intake in the environment. Therefore, the analysis and detection of MCNs in the environment should be aimed not only at quantifying their concentration and chemical composition, but also at determining their shape, particle size, and surface charge. In addition, for the detection of MCNs in the environment, challenges due to their low concentrations and the interference from complex environmental matrices must be overcome. A single detection technique is often insufficient for the analysis and detection of MCNs in a complex environment matrix. Therefore, the development of an effective and reliable online hyphenated technique is urgently needed for the separation and detection of MCNs in the environment. Such online hyphenated techniques should be able to eliminate the interference by complex matrices, improve the particle size detection range, and reduce the element detection limit. The online hyphenation of stationary phase-based separation techniques such as liquid chromatography and gel electrophoresis with inductively coupled plasma-mass spectrometry (ICP-MS) can effectively separate MCNs according to their particle size, with low element detection limits. However, these stationary phase-based separation techniques have a shortcoming of the adsorption of nanoparticles on the stationary phase, which leads to blockage of separation channels and low recoveries of nanoparticles. The online hyphenation of a non-stationary phase separation technique with ICP-MS also shows strong nanoparticle separation ability and low element detection limits, so that the problem of colloid blockage in stationary phase-based separation can be resolved. This method is very promising for the rapid and accurate characterization of the particle size distribution and chemical composition of MCNs. However, it cannot provide information about the nanoparticle number concentration of MCNs and the elemental content of a single MCN. In complex environmental samples, pure MCNs cannot be effectively distinguished from MCNs with environmental corona having different thicknesses or pure MCNs adsorbed on/hetero-agglomerated with inorganic/organic colloids. Online coupling single-particle ICP-MS (SP-ICP-MS), an emerging particle detection technique with non-stationary phase separation, can effectively help overcome the above shortcomings. This method can provide information on the hydrodynamic diameter, metal mass-derived diameter, total number concentration, size-dependent number, and size-dependent mass concentration of MCNs. Therefore, it enables comprehensive characterization of MCNs based on a variety of three-dimensional contour plot chromatograms. This review summarizes the separation mechanisms and applicable detectors for three commonly used non-stationary phase separation techniques: hydrodynamic chromatography (HDC), capillary electrophoresis (CE), and field-flow fractionation (FFF). In addition, it focuses on the characteristics and applications of online-coupling non-stationary phase separation with ICP-MS and SP-ICP-MS. Regarding FFF, this review focuses on the separation techniques that are suitable for online coupling with ICP-MS, such as sedimentation FFF and flow FFF (symmetrical flow FFF, asymmetrical flow FFF, and hollow fiber flow FFF). In addition, the characteristics of the online hyphenation of three non-stationary phase separations, HDC, CE, and flow FFF, with ICP-MS are compared, including the separation mechanism, sample volume, analytical time, detection sensitivity, size range, size resolution, recovery, reproducibility, and capability for ion analysis. Finally, this review proposes the prospects for future development of the online hyphenation of non-stationary phase separation techniques with ICP-MS and SP-ICP-MS.