Validation of the Analytical Method for Different Types of Carbon Nanotubes and Carbon Nanofiber and Exposure Assessment of Workplaces

Abstract

The major purposes of this study were to establish analytical method of elemental carbon (EC) in carbon nanotubes (CNTs) and carbon nanofiber (CNF) and to assess the airborne CNT at the workplaces Specific aims of this study were 1) to set up the analytical methods of EC according to the types of CNTs and CNF, 2) to evaluate the correlation between EC and the metrics measured by real-time monitors, and 3) to measure the CNT at the workplaces in Korea using EC sampling and real time monitors. The study was consisted of three parts

First, to evaluate existing methods and proposing new method for EC in CNT and CNF using bulk CNTs/CNFs, second, to evaluate airborne CNTs in exposure chambers. Third, to evaluate airborne CNTs at the workplace. Domestic CNTs/CNFs were used to evaluate the EC analytical method and airborne CNT in the exposure chamber. Based on guidelines of National Institute for Occupation and Safety (NIOSH) for air sampling and analytical method development and evaluation, recovery rate experiment for EC in CNT and CNF were performed to establish analytical methods. The existing methods such as NIOSH Method (NIOSH Manual of Analytical Method) 5040, Modified NIOSH Method 5040 and Modified IMPROVE (Interagency Monitoring of PROtected Visual Environments) A method were tested and a new method to increase recovery rate with better precision was also proposed. Both filter-based sampling for EC and real-time monitoring for aerosol monitoring was performed in exposure chambers equipped with HEPA filter to evaluate the correlation between EC measurements and real time nanoparticle sampling devices. Exposure assessment of workplaces was targeted to one manufacturer and one laboratory. For single-walled carbon nanotubes (SWCNTs) and CNF, NIOSH Method 5040 proved to be appropriate with the high recovery rate (97.7±5.4%) and good precision (pooled relative standard deviation

0.0012). For multi-walled carbon nano tubes (MWCNTs), existing analytical methods such as NIOSH Method 5040, Modified NIOSH Method 5040 showed a wide range of recovery rates by MWCNT types with relatively large standard deviation. The new proposed method which increased temperature to 930 ℃ and time to twofold of the thermal carbon analyzer showed a good recovery rate of 99.4±5.2% and a high precision with pooled relative standard deviation of 0.0028. In the exposure chamber, a relatively good correlation was found between EC level and Black carbon (BC) measured by Aethalometer (the Pearson correlation coefficient was 0.73) and surface area concentration by SAM (the Pearson correlation coefficient was 0.16) though the former was an integrated sampling and the latter was a real time monitoring instrument. At the workplace, airborne EC concentration was 6.03+1.70 µg/m3 in A workplace and 2.70+0.88 µg/m3 in B laboratory. Twenty-eight samples among thirty-four samples (82%) of the total EC samples exceeded the recommended exposure limit (REL) of one µg/m3 suggested by NIOSH. NIOSH Method 5040 was proven to be appropriate to analyze SWCNT/ CNF. A new method was proposed for MWCNT. EC for CNT integrated sampling and Aethalometer for real time monitoring may be the useful to CNT exposure assessment (the Pearson coefficient was 0.79). However, other nanoparticle sampling devices and electron microscopy analysis in parallel would be necessary to understand to CNT exposure assessment. During the monitoring of workplaces, most of EC samples exceed the REL of NIOSH. Exposure control strategy and hierarchy of controls is recommended to mitigate exposure to CNTs.