Improved Metrological Traceability of Particle Size Values Measured with Line-Start Incremental Centrifugal Liquid Sedimentation

Abstract

Line-start incremental centrifugal liquid sedimentation (disc-CLS) is a powerful method to determine particle size based on the principles of Stokes' law. Because several of the input quantities of the Stokes equation cannot be easily determined for this case of a rotating disc, the disc-CLS approach relies on calibrating the sedimentation time scale with reference particles. To use these calibrant particles for establishing metrological traceability, they must fulfill the same requirements as those imposed on a certified reference material, i.e., their certified Stokes diameter and density value must come with a realistic measurement uncertainty and with a traceability statement. As is the case for several other techniques, the calibrants do not always come with uncertainties for the assigned modal diameter and effective particle density. The lack of such information and the absence of a traceability statement make it difficult for the end-user to estimate the uncertainty of the measurement results and to compare them with results obtained by others. We present the results of a collaborative study that aimed at demonstrating the traceability of particle size results obtained with disc-CLS. For this purpose, the particle size and effective particle density of polyvinyl chloride calibrants were measured using different validated methods, and measurement uncertainties were estimated according to the Guide to the Expression of Uncertainty in Measurement. The results indicate that the modal Stokes diameter and effective particle density that are assigned to the calibrants are accurate within 5% and 3.5%, respectively, and that they can be used to establish traceability of particle size results obtained with disc-CLS. This conclusion has a great impact on the traceability statement of certified particle size reference materials, for which the traceability is limited to the size and density values of the calibrant particles.

Establishing SI-Traceability of Nanoparticle Size Values Measured with Line-Start Incremental Centrifugal Liquid Sedimentation

“…The PVC calibrant had an assigned particle size value of (264 ± 13) nm and particle density of (1.385 ± 0.048) g/cm 3 . The indicated uncertainties, which correspond to a confidence level of about 95%, were estimated during a previous study [6]. 50 µL of calibration material was injected before each sample measurement.…”

“…An overview of the mean and expanded uncertainty (confidence level of 95%) values calculated from the six replicate results is given in Table 2. The uncertainty values of the results obtained by the routine disc-CLS method were estimated using the bottom-up approach presented in [6]. The expanded uncertainties of the results from the reference disc-CLS method were estimated using the uncertainty budget data presented further in this article.…”

“…Materials consisting of spherical particles of polyvinyl chloride (PVC) with assigned particle size and particle density values, supplied by the main manufacturer of disc-CLS instruments (CPS Instruments, Inc., Prairieville, LA, USA) are used almost exclusively for the calibration of the sedimentation time scale. The suitability and reliability of these calibrants have been validated in a previous study [6]. In that study, the particle size and effective particle density of selected PVC calibrants were experimentally determined using different validated methods.…”

“…Thanks to the use of these PVC calibration particles, the disc-CLS method [7] has now become an important part of today's nanoparticle characterization toolbox. Although the reliability of the PVC calibration materials has thus been demonstrated [6], the actual comparability of the measured particle size results obtained for routine test samples is; however, limited because the critical properties (i.e., particle size and particle density) of the used calibration particles (PVC or other), have often not been characterized in a metrologically sound manner. For example, the "particle size" measurand is not always unambiguously defined, and uncertainties and traceability statements are often not given by the calibrant manufacturer.…”

“…The effective (buoyant) density of the silica nanoparticles was determined experimentally using isopycnic velocity interpolation and multi-velocity sedimentation approaches [6,18,19]. The measurement principle is based on the creation of a density gradient in an optically transparent cell.…”