Performance check of particle size standards within and after shelf-life using differential mobility analyzer

Abstract

In aerosol research, differential mobility analyzer (DMA) is widely used for particle size distribution measurements. In general polystyrene latex (PSL) particle size standards are used to calibrate the DMAs. However, these standards are expensive, and their shelf-life is limited. Therefore in this work, we have used PSL particle standard of sizes 60 and 100nm (SRM® 1964 and SRM® 1963a, respectively) and checked their performance within and after the shelf-life period for 3 consecutive years (2013 – 2015) using a calibrated DMA (i.e., long-DMA, TSI 3081). Also the size results are compared using a reference DMA (i.e., nano-DMA, TSI 3085). Using both long- and nano-DMA, the detail results of standard particle size measurements and involved uncertainties are discussed in this paper. We observed a continuous increase in the count mean diameter (CMD) calculated from the size distribution of each standard particle size with the progress of time. Particle size measured for PSL 60 and 100nm using long-DMA are (56.01±5.56)nm and (92.86±10.81)nm in 2013, (56.37±5.66)nm and (93.71±10.44)nm in 2014, and (57.42±5.84)nm and (94.79±9.68)nm in 2015, respectively. Whereas, these results when nano-DMA was used are (56.82±4.32)nm and (94.62±9.46)nm in 2014, and (58.08±4.98)nm and (95.15±9.12)nm in 2015, respectively. Uncertainty results show that the components which contribute significantly in total uncertainty (at k=2) in CMD measurements are DMA sheath flow rate, DMA calibration, difference in certified and measured diameter, and variation in the ambient temperature and pressure. The size measurement results obtained from long-DMA and nano-DMA agreed well. Based on the measurement results, we suggest that the increasing trend of particle sizes (CMDs) with the progress of time from 2013 to 2015 is possibly because of the coating of contaminant residues (present in the standard liquid suspension) on the surface of the PSL particles which is further observed by high resolution tunnelling electron microscope (HRTEM) images of particles. Particles standards are stable and well characterized more than two years even after their shelf-life within the uncertainty limits. Size performance results for PSL 60nm using calibrated long- and reference nano-DMA show good agreement with the size value given in the certificate, however standard PSL 100nm size is found to be underestimated by ~7%.