Nanoparticle Size Detection Limits by Single Particle ICP-MS for 40 Elements

The quantification and characterization of natural, engineered, and incidental nano- to micro-size particles are beneficial to assessing a nanomaterial's performance in manufacturing, their fate and transport in the environment, and their potential risk to human health. Single particle inductively coupled plasma mass spectrometry (spICP-MS) can sensitively quantify the amount and size distribution of metallic nanoparticles suspended in aqueous matrices. To accurately obtain the nanoparticle size distribution, it is critical to have knowledge of the size detection limit (denoted as D-min) using spICP-MS for a wide range of elements (other than a few available assessed ones) that have been or will be synthesized into engineered nanoparticles. Herein is described a method to estimate the size detection limit using spICP-MS and then apply it to nanoparticles composed of 40 different elements. The calculated D-min values correspond well for a few of the elements with their detectable sizes that are available in the literature. Assuming each nanoparticle sample is composed of one element, D-min values vary substantially among the 40 elements: Ta, U, Ir, Rh, Th, Ce, and Hf showed the lowest D-min values, <= 10 nm; Bi, W, In, Pb, Pt, Ag, Au, TI, Pd, Y, Ru, Cd, and Sb had D-min in the range of 11-20 nm; D-min values of Co, Sr, Sn, Zr, Ba, Te, Mo, Ni, V, Cu, Cr, Mg, Zn, Fe, Al, Li, and Ti were located at 21-80 nm; and Se, Ca, and Si showed high D-min values, greater than 200 nm. A range of parameters that influence the D-min such as instrument sensitivity, nanopartide density, and background noise, is demonstrated. It is observed that, when the background noise is low, the instrument sensitivity and nanoparticle density dominate the D-min significantly. Approaches for reducing the Dmin, e.g., collision cell technology (CCT) and analyte isotope selection, are also discussed. To validate the D-min estimation approach, size distributions for three engineered nanoparticle samples were obtained using spICP-MS. The use of this confirms that the observed minimum detectable sizes are consistent with the calculated D-min values. Overall, this work identifies the elements and nanoparticles to which current spICP-MS approaches can be applied, in order to enable quantification of very small nanoparticles at low concentrations in aqueous media.