AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Semi-automated System for Measuring Oxidative Potential of Ambient Particles Collected on Filters Using Dithiothreitol (DTT) Assay
TING FANG, Vishal Verma, Rodney Weber, Georgia Institute of Technology
Abstract Number: 232 Working Group: Instrumentation and Methods
Abstract Aerosol oxidative potential has been considered as a more relevant parameter to elucidate the adverse health effects of particulate matter (PM) than the mass concentration. Consequently, many methods have been developed to measure the oxidative potential of PM. Among these, DTT assay is the most commonly used method, and which has also been found to correlate with several health markers. In this assay, DTT is catalytically oxidized by the PM and the consumption rate of DTT in units of nmol/minute is considered proportional to the concentration of redox-active species in PM. The conventional manual protocol for DTT assay is very labor-intensive and time-consuming. To address these concerns, we developed a semi-automated system using two syringe pumps (Kloehn, Inc., Las Vegas, Nevada, USA) and a liquid wave-guide capillary cell (LWCC-M-100; World Precision Instruments, Inc., Florida, USA) coupled to an online spectrophotometer (Ocean Optics, Inc., Dunedin, Florida, USA). A simplified protocol was developed by conducting the DTT oxidation in a single vial while withdrawing a small aliquot at different time intervals to calculate the rate of DTT consumption. PM samples collected via filters or impactors once extracted in a solvent (typically water) can be analyzed in batches using an auto-sampler for unattended analysis, at roughly 1 sample per hour. The performance of the automated system was evaluated by multiple field blanks (standard deviation = 26 % of mean, N=13) used as the negative control, and standards (9, 10-phenanthraquinone, standard deviation = 8%, N=35) as the positive control. The reproducibility of the system, determined by conducting the assay on seven different equal sections of the same filter extracted and analyzed separately, also yielded consistent results with a minimal standard deviation in DTT activity (<5 % of mean). The limit of detection (LOD) of the system is 0.25 nmol/minute, comparable with that of the manual protocol (0.21 nmol/minute). Additional scaled down versions, where reaction volumes were reduced by 4 to 20 times, allow similarly high analytical precision and sensitivity at PM mass requirements as low as 30 micro-gram. The scaled-down methods have been successfully used to measure filter samples collected in human-exposure CAPS (concentrated air particles) experiments and within vehicles in traffic. These systems are being used to provide an extensive data set on DTT activity as part of the Emory-Georgia Tech EPA Clean Air Research Center, SCAPE (Southeastern Center for Air Pollution & Epidemiology).