Nanoparticle measurement with SP-ICP-MS

Element-specific analysis of nanoparticles – detection in complex media

Scanning electron microscopy image of titanium dioxide nanoparticles in wastewater.
© Fraunhofer IGB
Scanning electron microscopy image of titanium dioxide nanoparticles in wastewater.

Wide use of nanoparticles

Nanoparticles have found a broad range of applications in recent years due to their small size, and can be specifically equipped with new properties so that they can influence the properties of products. Engineered nanoparticles can be found in products such as electronic and optical devices, in paints and varnishes, adhesives and textiles, as well as in contrast agents for medical purposes, cosmetics, food packaging and even in food products. The extensive use of nanoparticles also leads to increased environmental contamination.

 

Labeling requirement for nanoparticles

The cosmetics ordinance that came into force in July 2013 requires that all cosmetic and personal care products containing nanomaterials must be labeled. The assessment of what constitutes a nanomaterial is based on the number distribution. According to this distribution, a nanomaterial must be declared if at least 50 percent of the particles have a size of 1 to 100 nm. Current data from the manufacturers are usually based on a volume distribution (mass distribution). This cannot be translated directly into a number distribution. An appropriate labeling requirement for nanomaterials in foodstuffs came into force in December 2014.

Häufigkeitsverteilung einer Gold-Nanopartikelsuspension.
© Fraunhofer IGB
Frequency distribution of a gold nanoparticle suspension.

Analysis of nanoparticles

Due to the requirements of the legislature, there is an increasing need for a suitable analytical method of characterizing nanoparticles. Current methods use imaging with electron microscopy, such as transmission and scanning electron microscopy (TEM, SEM), or particle measurement based on dynamic or static light scattering (DLS, SLS) or nanoparticle tracking analysis (NTA). With these methods, particles are mainly characterized qualitatively by size distribution, zeta potential, molecular weight and shape. However, these methods are not very selective and are unsuitable for complex polydisperse media, which occur in products such as cosmetics. An element-specific and quantitative method of analyzing nanoparticles directly has not been available until now.

Partikelgrößenverteilung einer Gold-Nanopartikelsuspension.
© Fraunhofer IGB
Particle size distribution of a gold nanoparticle suspension.

New quantitative element-specific methodology

At Fraunhofer IGB a method has recently been established for direct, element-specific and highly sensitive analysis of inorganic particles. Particles are analyzed with inductively coupled plasma mass spectrometry (ICP-MS) in single-particle mode (SP-ICP-MS).

SP-ICP-MS measurement procedure is based on analysis of individual particles. The special feature of this method is that with statistical analysis of the raw data, it is possible to differentiate between the dissolved ionic concentration of the element of interest and the particle concentration.

 

Advantages and prospects

Compared to existing methods, SP-ICP-MS is a rapid procedure with detection limits down to the ultra-trace region. Due to its selective analysis, it is also suitable for complex polydisperse media such as cosmetics and food. Compared to imaging methods, SP-ICP-MS can detect nanoparticles in an element-specific manner, allowing determination alongside other particles. In addition, inorganic cores of coated particles can be analyzed.

The statistical evaluation provides number distributions as required for the labeling according to the cosmetics regulation.

SP-ICP-MS can be used for material characterization and quality control by companies, as well as for monitoring by State Offices of Consumer Protection. It is increasingly developing into the analytical method of choice for investigating the persistence and effects of nanomaterials in the environment.