Orthogonal in-line microscopy coupled with SP ICP-MS for the quantitative analysis of microparticles

Single particle inductively coupled plasma-mass spectrometry (SP ICP-MS) has become the method of choice for the characterisation of nanoparticles, but its application to micrometre-sized particles remains limited due to size-dependent transport efficiencies (TE). This imposes inherent limitations and prevents the accurate counting of particles as well as the reliable analysis of size distributions. Consequently, the quantitative analysis of entities such as microplastics or cells remains restricted. This study addresses this limitation by developing and introducing an integrated approach combining a new technique named orthogonal in-line microscopy (OIM) with SP ICP-MS. OIM visualises a focal plane within a capillary and uses a dedicated algorithm to count and size particles crossing this plane. The same particle suspension was characterised both optically and via online single particle mass spectrometry to investigate size dependent TE and evaluate correction schemes.

Using polystyrene microplastic particle standards as a model system and a sample capillary smaller than the field-of-view, it was possible to register and analyse all particles crossing the capillary upstream of SP ICP-MS providing an absolute benchmark for subsequent TE studies. Comparing size histograms obtained via OIM and SP ICP-MS, respectively, enabled a quantitative evaluation of TE and provided the basis for a correction scheme for size distributions estimated via SP ICP-MS. Two sample introduction systems, a standard Scott-type double pass spray chamber and a total consumption assembly, were compared, and both demonstrated a significant reduction in TE as sizes increased. For particles as large as 10 μm, TE was reduced 21- to 88-fold, respectively, relative to the expected nanoparticle TE. Furthermore, 20 μm particles were not recovered by SP ICP-MS but could be characterised optically via OIM.

The on-line hyphenation of OIM and SP ICP-MS enabled the analysis of size distributions via two complementary detection regimes. While size distributions in the lower micrometre scale are significantly distorted with stand-alone SP ICP-MS, hyphenating OIM and SP ICP-MS provides the means to recognise and quantify the size dependent TE, as well as a scheme to correct for it. As such, this new hyphenated technique offers a more robust and accurate strategy to count and determine micro-suspensions. Apart from characterising TE drift effects, OIM complements SP ICP-MS by enabling the study of particle shapes as well as particle sizes that are beyond the grasp of SP ICP-MS. Consequently, physical parameters and size ranges that otherwise would be lost via SP ICP-MS can now be conserved to expand single microparticle characterisation and to improve the characterisation of particulates such as microplastics and cells.