Element-specific analysis of nanoparticles – detection in complex media

© 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.

Analysis of nanoparticles

© Fraunhofer IGB

Mass spectrum of a titanium dioxide nanoparticle suspension.

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.

© Fraunhofer IGB

Data analysis of a titanium dioxide nanoparticle suspension. Above: Histogram of the raw data shows a Poisson distribution of the rare particle events. Center: Approximation of the Poisson distribution to the normal distribution. Below: Integration of the density distribution provides a continuous cumulative size distribution.

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).

The 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.

Our focus at IGB was on the development of instrumental methods for titanium dioxide nanoparticles in wastewater as well as data analysis and processing without any special software. One critical parameter for the calculation of particle size turned out to be exact determination of the nebulizer efficiency.  

Particle concentrations in the sample that are too high may lead to overlapping particles, which may be misinterpreted as larger particles. The sample must therefore be carefully diluted to an appropriate level to ensure that only one particle is detected per measurement window. If this condition is met, each signal represents one particle, and the signal intensity is correlated with the number of ions and therefore with the particle size.

A method was developed for suspensions of titanium dioxide particles that allows accurate determination of the nebulizer efficiency using particle calibration in the working range of 1–25 µg/l (mass concentration). This allowed particle sizes with a diameter d50 = 333 ± 4 nm to be determined with a relative standard deviation of 1.2 percent. The smallest signal that could be evaluated resulted in a detection limit of 55 nm in ultrapure medium. The method was also used with interference-containing synthetic wastewater matrices.

 

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 and therefore has the potential to be an excellent tool for routine analysis.

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.

References

[1] Petrich, M. (2013) Nanopartikel-Analyse mit ICP-MS, Anorganica 2013

[2] Pace, H. E.; Rogers, N. J.; Jarolimek, C.; Coleman, V. A.; Higgins, C. P.; Ranville, J. F. (2011) Determining transport efficiency for the purpose of counting and sizing nanoparticles via single particle inductively coupled plasma mass spectrometry, Anal. Chem. 83: 9361–9369

[3] Tuoriniemi, J.; Cornelis, G.; Hassellöv, M. (2014) Improving the accuracy of single particle ICPMS for measurement of size distributions and number concentrations of nanoparticles by determining analyte partitioning during nebulisation, J. Anal. At. Spectrom. 29: 743

[4] Dr. Richter (2014) Nanomaterialen in komplexer Matrix. ICP-MS Analyse von Nanomaterialen, Perkin Elmer; Rodgau-Jügesheim