An approach to estimate the contribution of signal noise to the diameter uncertainty of individual silver nanoparticles and resolution of spICP-MS analysis

Abstract

The factors influencing the signal noise and its contribution to the uncertainty of a single AgNP size in spICP-MS were investigated. The possibility of the reproducible delivery of Ag masses (0.045–5.4 fg) corresponding to nanoparticles with pre-selected diameters (20–100 nm) by time segmentation when introducing homogeneous ionic standards is experimentally proven. The calculation of ionic standard concentration ensuring the delivery of desired Ag mass within a certain dwell time (t_d) and the dilution factor of nanocolloid samples was done by the developed theoretical model. The behavior of the time-segmented signals was studied for five different Ag masses measured within four t_d in the interval of 3–10 ms. The results show that both the sensitivity and the reproducibility of the signals are independent of the selected t_d. The hypothesis of dispersion homogeneity was verified by Bartlett's test and in all cases, the calculated values of the Bartlett criterion (B_exp) were much lower than B_crit, which prove that signal noise depends only on the Ag mass and can be used as a reliable estimate of the contribution of ionization in plasma and ion flux transport to uncertainty. The correspondence of both calibrations by ionic standards and AgNPs RM confirm that the signals are comparable, regardless of whether Ag is introduced via ionic solutions or directly as nanoparticles. The combined uncertainty (RSD_c%) was calculated for Ag masses consistent to the size of nanoparticles, and the contribution of different quantities was evaluated. The AgNPs' diameter confidence intervals (1σ, P = 68%) were calculated. Two main trends were observed: (i) with decreasing size, U_c increased and (ii) due to the cubic relationship between the mass and diameter of NPs, the two half-intervals became asymmetric. The approach was used for the calculation of the size U_c of AgCl, AgI, Ag₂O, and Ag₂S nanoparticles, which are of ecological interest, but no reference materials are available. The average uncertainty interval depends on the physical properties of the nanoparticles and varies between 3.4 nm and 5.5 nm for Ag and AgI, respectively. This allows the experimental evaluation of the spICP-MS size resolution and to propose a criterion for the cluster dimension when size distribution is obtained. The method was applied for the analysis of AgNPs in a commercial pharmaceutical product. The suggested approach for calculating the uncertainty in the diameters is flexible and can be applied to different types of nanoparticles.