By coupling micellar electrokinetic chromatography (MEKC) and inductively-coupled plasma mass spectrometry (ICP-MS), two German chemists have developed a novel method for separating and characterizing nanoparticles and dissolved species.
The characterization of nanoparticles with respect to size and composition is a hot topic in analytical chemistry. Sensitive and rapid methods for nanoparticle characterization in complex matrices are required for exposure, biodistribution, and toxicity studies. ICP-MS is a well established technique that can provide information on composition with respect to elements down to the ultratrace levels. Recently, a special measurement mode, called single particle detection (SP-ICP-MS) has been developed, that enables the determination of the particle size. Unfortunately in this technique dissolved species reveal themselves as a kind of background disturbing the determination of particles. Furthermore, the particle-size determination is limited to particles bigger than about 20 nm due to the limited detection power of the SP-ICP-MS measurement mode.The new method
In order to enhance the separation power and to improve the size limitation obtained by SP-ICP-MS Bastian Franze from the University of Münster and Carsten Engelhard from the University of Siegen initially tried separating nanoparticles with capillary electrophoresis (CE), but were disappointed by the relatively poor resolution obtained. So they added the surfactant sodium dodecyl sulphate (SDS) to the separation buffer, transforming CE into MEKC, and found that this greatly improved the resolution. They report on the capability of this new tool for nanoparticle characterization in a paper in Analytical Chemistry.
Elementogram obtained by MEKC-ICP-MS
Copyright (2014) American Chemical Society
(Reprinted with permission from Anal. Chem,
2014, 86, 5713-5720.)
The advantage of coupling MEKC as a separation module with ICP-MS as the detection device is the clear separation of the two measurement issues: Particle-size separation by MEKC and composition analysis by ICP-MS. Together, the hyphenated technique MEKC-ICP-MS can provide a full characterization of a mixture of nanoparticles as well as the possibility to perform speciation analysis.
The separation principle is related to the electrophoretic mobility that is proportional to the charge-to-size ratio of the analyte. By adding the negatively-charged SDS molecules, which coat the surface of the nanoparticles, the mobility of the particles is enhanced, leading to their movement under an applied electric field in the opposite direction to the electroosmotic flow (EOF). Since larger particles will be surrounded by even mode SDS molecules, they gain a greater negative charge and move faster in the opposite direction. As a consequence, larger particles are more effective in counteracting the EOF than smaller particles and so migrate slower through the capillary. In this way, the German researchers were able to separate a mixture of 5nm, 20nm and 50nm gold nanoparticles in less than 10 minutes.
Since the ICP-MS detection capability in this hyphenated mode is not limited by the ultrafast reading necessary for single particle detection, the researchers were able to analyze a mixture of gold and silver nanoparticles. Furthermore, by simply adding penicillamine to the sample before analysis, free ionic gold and silver species were complexed and could be separated from particular metals.
In order to demonstrate the applicability of this promising method for real world samples, Franze and Engelhard analyzed a colloidal gold preparation sold as a nutritional supplement. Although this preparation was labeled to contain gold nanoparticles, the researchers found that it contained not only NPs around 6nm in size, but also a small amount of gold ions, probably resulting from the disintegration of some of the gold nanoparticles.
The cited study:
Bastian Franze, Carsten Engelhard
, Fast Separation, Characterization, and Speciation of Gold and Silver Nanoparticles and Their Ionic Counterparts with Micellar Electrokinetic Chromatography Coupled to ICP-MS
, Anal. Chem., 86 (2014) 5713-5720. DOI: 10.1021/ac403998e
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The Project on Emerging Nanotechnologies - Consumer Products Inventory European Commission: Review of Environmental Legislation for the Regulatory Control of Nanomaterials Nanoparticle.org: The Information Resource for Particle Technology U.S. EPA: Nanomaterials EPA is Assessing GreenFacts: Nanotechnologies
Related EVISA Resources Journal database: Nanoscience & Nantechnology Link Database: Nanoparticles Brief summary: Tools for elemental speciation analysis Brief summary: ICP-MS - A versatile detection system for speciation analysis Brief summary: CE-ICP-MS for speciation analysis
last time modified: June 23, 2014