Evaluation of a TEM based Approach for Size Measurement of Particulate (Nano)materials

Abstract

An approach for the size measurement of particulate (nano)materials by transmission electron microscopy was evaluated. The approach combines standard operating procedures for specimen preparation, imaging, and image analysis, and it was evaluated on a series of certified reference materials and representative test materials with varying physical properties, including particle size, shape, and agglomeration state. The measurement of the median value of the minimal external particle diameter distribution was intra-laboratory validated. The validation study included an assessment of the limit of detection, working range, selectivity, precision, trueness, robustness, and ruggedness. An uncertainty that was associated to intermediate precision in the range of 1–7% and an expanded measurement uncertainty in the range of 7–20% were obtained, depending on the material and image analysis mode. No bias was observed when assessing the trueness of the approach on the certified reference materials ERM-FD100 and ERM-FD304. The image analysis method was validated in an inter-laboratory study by 19 laboratories, which resulted in a within-laboratory precision in the range of 2–8% and a between-laboratory precision of between 2% and 14%. The automation and standardization of the proposed approach significantly improves labour and cost efficiency for the accurate and precise size measurement of the particulate materials. The approach is shown to be implementable in many other electron microscopy laboratories.

Physicochemical Characterization of the Pristine E171 Food Additive by Standardized and Validated Methods

“…The magnification selected for the quantitative TEM analysis was suitable because a Fmin smaller than the LLOQ was measured only for 0.3% of constituent particles, and no particles with a Fmin larger than the ULOQ were found (Table 1). In all samples, more than 300 (and for optimized protocols about 1000) particles were measured, which is sufficient to obtain an expanded measurement uncertainty (Ucx, k = 2) below 10%, as determined for representative test materials NM-100 and NM-103 by Verleysen et al [59]. Table 6 and Figure 5 summarize the quantitative TEM measurements for all 15 E171 materials using optimized sample preparation protocols (P1 for pearlescent pigments and P6 for the anatase E171 materials and rutile material E171-F) and image analysis settings.…”

“…Measurement uncertainties on the medians of the Fmin distributions of the E171 materials were estimated based on the validation study of Verleysen et al [59] for representative test materials NM-100 (TiO 2 materials with particles in the order of 100 nm) and NM-103 (TiO 2 materials with particles in the order of 20 nm). The expanded measurement uncertainties (Ucx, k = 2) for median Fmin measurements are reported to be 8.5% and 9.2% for NM-100 and NM-103, respectively [59].…”

“…Precision parameters were determined by analyzing three independent replicates of the representative test material NM-100 (JRC, Ispra, Italy) on each of five different days and assessed via one-way analysis of variance. As a proxy for trueness estimation ("apparent trueness"), the median ESD determined by spICP-MS was compared to the median Fmin determined by TEM for NM-100 (i.e., 100 nm; [59]). The recovery of particle mass was determined as a percentage of the theoretical concentration, which is 1 g/g.…”

“…Evaluation of sample preparation protocols based on the medians of the Fmin, Fmax, and aspect ratio (AR) distributions for (a) constituent particles and (b) agglomerates of anatase E171 material E171-06. For the constituent particles, expanded measurement uncertainties (Ucx, k = 2) are estimated based on the validation study of Verleysen et al [59] for representative test material NM-100.…”

“…The TEM specimen (grid) preparation, imaging, and image analysis procedures were previously validated intra laboratory [59], while validation performance characteristics for a standardized spICP-MS methodology are given in this publication.…”