Accurate quantification of tio₂ nanoparticles collected on air filters using a microwave-assisted acid digestion method

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.

KEYWORDS:

• Inductively coupled plasma optical emission spectroscopy
• microwave-assisted digestion
• mixed-cellulose ester filter
• polyvinyl chloride filter
• titanium dioxide
• nanoparticles

Acknowledgments

The authors greatly appreciate the preliminary efforts of Pam Kostle of the State Hygienic Laboratory (Iowa City, IA) and Brian Wells of the State Hygienic Laboratory (Ankeny, IA) to identify microwave digestion as a feasible alternative to increase recovery.

Funding

This work was funded by NIOSH (R21 OH009920; R01 OH010238).