In Situ Generation of Hydrofluoric Acid during Microwave Digestion of Atmospheric Particulate Matter Prior to Trace Element Analysis Using Inductively Coupled Plasma Mass Spectrometry

Abstract

Results from a systematic evaluation of numerous microwave-assisted digestion techniques to solubilize trace elements from ambient airborne particulate matter prior to analysis using inductively coupled-plasma mass spectroscopy (ICP-MS) are reported. Direct handling of HF was avoided by generating it in situ in closed Teflon-lined vessels during digestion by heating a mixture of NaF, HNO₃, and the sample and later complexing any remaining HF using stoichiometric excess of boric acid. The effects of microwave digestion variables including dwell time (20–120 min), temperature (150–200°C), pressure (38–200 psig), acid type (HNO₃ and HF), and absence or presence of peroxide on extraction efficiency were determined. Principal component analysis was conducted on the recoveries of 20 elements (Mg, Al, K, Ti, V, Cr, Mn, Fe, Ni, Co, Cu, Zn, As, Rb, Cd, Cs, Ba, La, Pb, and U) from urban particulate matter Standard Reference Material 1648, which revealed that the digestion matrix was the most important variable of all those experimentally investigated. Using H₂O₂ improved the clarity of the digested sample by ensuring complete oxidation of organic matter thereby facilitating subsequent ICP-MS analysis. Quantitative recoveries of Al, K, Ti, Rb, and La were possible only when using HNO₃, NaF (HF), H₃BO₃, and H₂O₂, suggesting that these elements were associated with the siliceous matrix of urban airborne particulate matter even though this matrix increased detection limits compared to using HNO₃ alone. Even though this method was not suitable for Cr recovery, it was found to extract significantly higher concentrations of Al, Mg, Ni, Co, and Ti from airborne line particulate matter samples obtained in Houston, TX, compared to X-ray fluorescence.