Nanosampling Method Development Papers

Work with co-investigators, Tom Peters and Vicki Grassian, focus on methods to improve the sampling and analysis of nanomaterials.  One paper examines the method for extraction and analysis of TiO₂ nanoparticles to improve the sensitivity of analytical methods.  The second paper identifies a way to substitute the collection media in the NRD sampler, switching from the multi-mesh method in the original design to porous foam.

Accurate Quantification of TiO2 Nanoparticles Collected on Air Filters Using a Microwave-Assisted Acid Digestion Method

Imali A Mudunkotuwa, T. Renée Anthony, Vicki H Grassian, and Thomas M Peters

Published in the Journal of Occupational and Environmental Hygiene, available at:

http://www.tandfonline.com/doi/pdf/10.1080/15459624.2015.1072278

ABSTRACT:

Titanium dioxide (TiO₂) particles, including nanoparticles with diameters smaller than 100 nm, are used extensively in consumer products. In a 2011 current intelligence bulletin, the National Institute of Occupational Safety and Health (NIOSH) recommended methods to assess worker exposures to fine and ultrafine TiO₂ particles and associated occupational exposure limits for these particles. However, there are several challenges and problems encountered with these recommended exposure assessment methods involving the accurate quantitation of titanium dioxide collected on air filters using acid digestion followed by inductively coupled plasma optical emission spectroscopy (ICP-OES). Specifically, recommended digestion methods include the use of chemicals, such as perchloric acid, which are typically unavailable in most accredited industrial hygiene laboratories due to highly corrosive and oxidizing properties. Other alternative methods that are used typically involve the use of nitric acid or combination of nitric acid and sulfuric acid, which yield very poor recoveries for titanium dioxide. Therefore, given the current state of the science, it is clear that a new method is needed for exposure assessment. In this current study, a microwave-assisted acid digestion method has been specifically designed to improve the recovery of titanium in TiO₂ nanoparticles for quantitative analysis using ICP-OES. The optimum digestion conditions were determined by changing several variables including the acids used, digestion time, and temperature. Consequently, the optimized digestion temperature of 210°C with concentrated sulfuric and nitric acid (2:1 v/v) resulted in a recovery of >90% for TiO₂. The method is expected to provide for a more accurate quantification of airborne TiO₂ particles in the workplace environment.

Porous Polyurethane Foam for Use as a Particle Collection Substrate in a Nanoparticle Respiratory Deposition Sampler

Levi W.D. Mines, Jae Hong Park, Imali A. Mudunkotuwa, T. Renée Anthony, Vicki H. Grassian, and Thomas M. Peters

Published in Aerosol Science and Technology, available at: 

http://www.tandfonline.com/doi/pdf/10.1080/02786826.2016.1164828

ABSTRACT

Porous polyurethane foam was evaluated to replace the eight nylon meshes used as a substrate to collect nanoparticles in the Nanoparticle Respiratory Deposition (NRD) sampler. Cylindrical (25 mm diameter by 40 mm deep) foam with 100 pores per inch was housed in a 25-mm-diameter conductive polypropylene cassette cowl compatible with the NRD sampler. Pristine foam and nylon meshes were evaluated for metals content via elemental analysis. The size-selective collection efficiency of the foam was evaluated using salt (NaCl) and metal fume aerosols in independent tests. Collection efficiencies were compared to the nanoparticulate matter (NPM) criterion and a semi-empirical model for foam. Changes in collection efficiency and pressure drop of the foam and nylon meshes were measured after loading with metal fume particles as measures of substrate performance. Substantially less titanium was found in the foam (0.173 µg sampler⁻¹) compared to the nylon mesh (125 µg sampler⁻¹), improving the detection capabilities of the NRD sampler for titanium dioxide particles. The foam collection efficiency was similar to that of the nylon meshes and the NPM criterion (R² = 0.98, for NaCl), although the semi-empirical model underestimated the experimental efficiency (R² = 0.38). The pressure drop across the foam was 8% that of the nylon meshes when pristine and changed minimally with metal fume loading (∼19 mg). In contrast, the pores of the nylon meshes clogged after loading with ∼1 mg metal fume. These results indicate that foam is a suitable substrate to collect metal (except for cadmium) nanoparticles in the NRD sampler.

The following illustrates how welding fumes deposit on the two test media (mesh and foam):

Nonwoven Textile for Use in a Nanoparticle Respiratory Deposition Sampler

Donna J.H. Vosburgh, Jae Hong Park, Levi W. D. Mines, Imali A. Mudunkotuwa, T. Renée Anthony, Thomas M. Peters

Published in Journal of Occupational and Environmental Hygiene, available at:

http://www.tandfonline.com/doi/full/10.1080/15459624.2016.1263391

ABSTRACT

The nanoparticle respiratory deposition (NRD) sampler is a personal sampler that combines a cyclone, impactor, and a nylon mesh diffusion stage to measure a worker’s exposure to nanoparticles. The concentration of titanium in the nylon mesh of the diffusion stage complicates the application of the NRD sampler for assessing exposures to titanium dioxide nanoparticles. This study evaluated commercially available nonwoven textiles for use as an alternative media in the diffusion stage of the NRD sampler. Three textiles were selected as containing little titanium from an initial screening of 11 textiles by field portable x-ray fluorescence (FPXRF). Further evaluation on these three textiles was conducted to determine the concentration of titanium and other metals by inductively coupled plasma – optical emission spectroscopy (ICP-OES), the number of layers required to achieve desired collection characteristics for use as the diffusion stage in the NRD sampler (i.e., the nanoparticulate matter, NPM, criterion), and the pressure drop associated with that number of layers. Only three (two composed of cotton fibers, C1 and C2; and one of viscose bamboo and cotton fibers, BC) of 11 textiles screened had titanium concentrations below the limit of detection the XRF device (0.15 μg/cm²). Multiple metals, including small amounts of titanium, were found in each of the three nonwoven textiles using ICP-OES. The number of 25-mm-diameter layers required to achieve the collection efficiency by size required for the NRD sampler was three for C1 (R² = 0.95 with reference to the NPM criterion), two for C2 (R² = 0.79), and three for BC (R² = 0.87). All measured pressure drops were less than the theoretical and even the greatest pressure drop of 65.4 Pa indicated that a typical personal sampling pump could accommodate any of the three nonwoven textiles in the NRD sampler. The titanium concentration, collection efficiency, and measured pressure drops show there is a potential for nonwoven textiles to be used as the diffusion stage of the NRD sampler.