Application of TXRF in monitoring trace metals in particulate matter and cloud water
Abstract. Ambient particulate matter and cloud trace metals are considered key elements of atmospheric processes as they affect air quality, environmental ecosystems and play an important role in cloud formation. However, they are often available at trace concentrations in these media such that their analysis requires high precision and sensitive techniques. In this study, different analytical methods were applied to quantify trace metals in particulate matter samples collected on quartz and polycarbonate filters as well as cloud water, using the Total reflection X-Ray Fluorescence (TXRF) technique. These methods considered the direct measurement of filter samples with and without chemical pretreatment. Direct measurements involved analysis of as collected polycarbonate and cloud water samples after they are brought onto TXRF carrier substrates. For the chemical treatment, different acid digestion procedures for PM on Quartz filters were assessed including reverse aqua regia, nitric acid and a combination of nitric acid and hydrogen peroxide solution. The effect of cold-plasma treatment of samples on polycarbonate filters prior to TXRF measurements was also investigated. Digestion with the reverse aqua regia solution provided lower blanks and higher recovery in comparison to other tested procedures. The detection limits of the elements ranged from 0.3 to 44 ng/cm2. Ca, K, Zn, and Fe showed the highest detection limits of 44, 35, 6 and 1 ng/cm2 while As and Se had the lowest of 0.3 and 0.8 ng/cm2, respectively. The method showed higher recovery for most trace metals when applied to commercially available reference materials and field samples. TXRF measurements showed good agreement with results obtained from ion chromatography measurements for elements such as Ca and K. Cold plasma treatment did not significantly lead to an increase in the detected concentration and the results were element-specific. Backing of the quartz filters prior to sampling showed a reduction of more than 20 % of the filter blanks for elements such as V, Sr, Mn, Zn, Sb. The methods were applied successfully on ambient particulate matter and cloud water samples collected from the Atlas Mohammed V (AM5) station in Morocco and the Cape Verde Atmospheric Observatory (CVAO). The obtained concentrations were within the range reported using different techniques from similar remote and background regions elsewhere, especially for elements of anthropogenic origins such as V, Pb, and Zn with concentrations of up to 10, 19 and 28 ng/cm2, respectively. Enrichment factor analysis indicated that crustal matter dominated the abundance of most of the elements while anthropogenic activities also contributed to the abundance of elements such as Sb, Se, and Pb. The results confirm that TXRF is a useful complementary sensitive technique for trace metal analysis of particulate matter in the microgram range as well as in cloud water droplets.