An efficient and stable sample preparation and calibration strategy for nanoparticle analysis using laser ablation single particle-ICP-MS

Abstract

Nanoparticles are used in various fields, such as material manufacturing, catalysis and medicine, due to their unique physical and chemical properties. Accurate characterization of nanoparticles is essential for manufacturing purposes as well as for assessing their impact on the environment and human health. To achieve this, single particle inductively coupled plasma mass spectrometry (sp-ICP-MS) has become an essential analytical technique for nanoparticle analysis. It can also be used with laser ablation as a sampling method to overcome challenges related to introducing nanoparticles in liquid suspension. Similar to conventional sp-ICP-MS, laser ablation sp-ICP-MS requires standards for signal calibration, which is challenging as the availability of standard reference materials is limited for all different kinds of nanoparticles. In this work, nanoparticles embedded in polymer thin films are analyzed using laser ablation sp-ICP-MS, whereby the laser is used to sample and transport the intact particles to the plasma. For creating a calibration for mass and size investigations, defined amounts of the element of interest were introduced into the ICP-MS by quantitatively ablating polymer thin film spiked with a defined amount of liquid element standard with different laser spot sizes. The method was developed and optimized using gold nanoparticles with certified sizes that were analyzed using a quadrupole-ICP-MS in single-element mode. The nanoparticles were sized using the proposed calibration approach with a deviation of ≤2.5% from the certified diameter value. Using the calibration approach, a limit of detection for gold of 3 × 10⁻⁷ ng was calculated, which translates to a particle size of approximately 15.5 nm, comparable to values in the literature for liquid-suspension-based approaches. Multi-element nanoparticles in the form of gadolinium-doped cerium oxide (GDC) nanoparticles with two elements of interest were analyzed using an ICP-TOFMS utilizing the thin-film-based calibration approach. Comparative measurements of the material confirmed the investigated sizes and composition of the particles. This developed alternative approach circumvents the need for certified particulate standard materials by using in-house-produced spiked polymer thin films as storage-stable calibration standards. Moreover, changing the laser spot size makes it straightforward to alter the number of particles introduced into the ICP-MS.