Application of TXRF in monitoring trace metals in particulate matter and cloud water
Authors: Fomba Khanneh Wadinga , Deabji Nabil , Barcha Sayf El Islam , Ouchen Ibrahim , Elbaramoussi El Mehdi , El Moursli Rajaa Cherkaoui , Harnafi Mimoun , El Hajjaji Souad , Mellouki Abdelwahid , Herrmann Hartmut 
Published: September 2020

Abstract. Trace metals in ambient particulate matter and cloud are
considered key elements of atmospheric processes as they affect air quality,
environmental ecosystems, and 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 (PM) 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 measurement of filter samples directly without
and with chemical pretreatment. Direct measurements involved the analysis of
PM samples collected on polycarbonate filters and cloud water samples after
they are brought onto TXRF carrier substrates. The chemical treatment method
involved the assessment of different acid digestion procedures on PM sampled
on quartz filters. The solutions applied were reverse aqua regia, nitric
acid, and a combination of nitric acid and hydrogen peroxide. The effect of
cold-plasma treatment of samples on polycarbonate filters before 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 cm−2. Ca, K,
Zn, and Fe showed the highest detection limits of 44, 35, 6, and 1 ng cm−2, while As and Se had the lowest of 0.3 and 0.8 ng cm−2, 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. Baking 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, and Sb. The methods were applied successfully on ambient particulate matter and
cloud water samples collected from the Atlas Mohammed V station in Morocco
and the Cape Verde Atmospheric Observatory. 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 m−3, 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.