Validation of a Simple Microwave-Assisted Acid Digestion Method Using Microvessels for Analysis of Trace Elements in Atmospheric PM2.5 in Monitoring and Fingerprinting Studies

Abstract

Two microwave-assisted digestion procedures, followed by inductively coupled plasma mass spectrometry (ICP-MS) analysis, were evaluated for the determination of trace elements in fine atmospheric aerosols (PM2.5) for air monitoring purposes. The first procedure used 40%(v/v) HNO3 and a digestion program with a maximum temperature of 175 o C. The second digestion procedure provided the dissolution of silicate matrix through the use of a mixture of HNO3/HF/H3BO3 at 200 o C. Both digestion procedures employed microvessels (MicroVessel TM ), which significantly reduced the amount of reagents needed, and increased the sample throughput by two-fold. The effectiveness of both digestion procedures was studied through the analysis of NIST standard reference materials (SRM), namely SRM 2783 (PM2.5 Air Particulate Matter deposited on a polycarbonate filter membrane), and SRM 1648 (Urban Particulate Matter). The accuracy was also checked through the analysis of co-located ambient PM2.5 samples collected within the Canadian National Air Pollution Surveillance (NAPS) network. Comparison of analytical results for PM2.5 samples showed that 40% (v/v) HNO3 could dissolve most of the metals of interest to the same degree as the HNO3/HF/H3BO3 mixture. Good agreement was also obtained by comparison with Energy Dispersive X-ray Fluorescence Spectrometry (ED-XRF). The efficiency of the simple microwave digestion procedure (no HF) was not dependent on the origin, mass loading, or elemental composition of the analyzed samples, thus it is quite suitable for monitoring purposes. However, the total dissolution of several trace elements such as Ti, Cr and U in PM2.5, and Ti, Cr, Al, lanthanides, and Sb in coarse fractions of PM (PM10-2.5) would require digestion in the presence of HF.