Development of a cascade impactor optimized for size-fractionated analysis of aerosol metal content by total reflection X-ray fluorescence spectroscopy (TXRF)

Abstract

A new cascade impactor has been developed with the arrangement of the classifying nozzles optimized for analysis of the collected particles by total reflection X-ray fluorescence (TXRF). TXRF offers detection limits in the range of a few picograms of absolute mass and therefore poses great potential for the elemental analysis of heavy metals in aerosol particles. To fully exploit this high sensitivity, particles have to be collected in the effective analysis area of the TXRF instrument, which is often smaller than typical deposition patterns of commercial impactors or filter samplers. This is achieved by a novel compact arrangement of the classifying nozzles within a circular area of a diameter of less than 5 mm. A decreasing density of the nozzle spacing from the inside to the outside of the nozzle cluster allows for constant cross-flow conditions, minimizing the mutual influence of the individual nozzles. The design of a multistage cascade impactor is presented to individually sample PM10, PM2.5 and PM1 size fractions. Considering the high sensitivity of TXRF analysis, constructive measures have been taken to prevent attrition of impactor material which might lead to methodical blank values. Experimental validation confirms that neither attrition nor cross-contamination can be observed. Furthermore, a new spin-coating method has been developed which makes it possible to apply a thin and defined adhesive layer of grease to the sample carrier with good repeatability. Application of the impactor in a case study at an urban site at Potsdamer Platz, Berlin, Germany, shows that sampling at a moderate volume flow rate of 5 L min−1, the particle mass collected in 30 min or less is sufficient for reliable TXRF analysis of heavy metal concentrations (Fe, Zn, Cu, Mn, Pb and Ni) in ambient aerosol. This high time resolution enables snapshot sampling, e.g. to quantify variations in particle source strengths. Overall, the new impactor optimized for TXRF analysis bears great potential to improve the quantification of particulate trace metals and other elements in PM10, PM2.5 and PM1 with high time resolution.