Open WL Archief (OWA)

High-Z elements such as precious metals or heavy metals are often difficult to detect with conventional EDXRF. Due to lack of excitation energy, anode material and decreasing detection capacities in the high-energy range, conventional EDXRF is usually limited to L-lines for the determination of elements with Z starting from around 40. L-line spectra are more complicated than K-lines and there is an increased risk for overlapping, e.g. the L-lines of Cd, In and Sb may overlap with the K-lines of Ca as well as with each other. In environmental soil samples it is easy to imagine that this may cause problems when e.g. dealing with Ca-rich soils. However, with the high-energy secondary-target EDXRF spectrometer, K-lines can be used for elements up to tungsten (Z = 74) and this problem is easily overcome.

The performance of the spectrometer is in some cases comparable or even better than classical reference techniques. The precious metals used in ACCs (Pt, Pd and Rh) can be determined in this matrix with equal precision and accuracy as the reference ICP-OES method. On top of this, the EDXRF procedure has the benefit of being cheaper and faster. Detection limits of around 5 ppm for this application (for a 500 s irradiation) demonstrate the improved sensitivity due to proper secondary target choice. In heavier matrices such as NiS-beads the technique is still applicable, but the performance is less spectacular. The method is faster and cheaper than an ICP-OES method, which requires an additional digestion and co-precipitation step, but the detection limits are not competitive in all materials due to large dilution (30 times or more). In very complex matrices, on the other hand, not even the secondary targets can provide an easy one-preparation-only solution for the determination of all elements.

Also in environmental applications, the improved spectrometer can provide elegant solutions for delicate analysis. Thin film analyses are very straightforward on EDXRF devices. This sample preparation can be used for e.g. air and water analysis. Aerosol filters already have low backgrounds for conventional EDXRF, but the polarizing geometry of the spectrometer provides a practically horizontal background for most secondary targets, allowing even lower detection limits than before. The higher energy and the extended applicability of the K-lines also enable better detection of harmful heavy metals and detection limits of a few ng/m³ are achieved. Determination of Cd in saline solutions is often difficult and preconcentration on a thin film sample (membrane) can offer an easy and cheap solution. The obtained detection limit, 700 ppb, is competitive with other reference methods such as ETAAS and ICP-AES. In the sediment fingerprinting application field, heavy and rare earth elements (such as e.g. Ce) can be interesting tracers, making it desirable to be able to detect them at low levels. A robust method has been optimized for the determination of a large variety of elements in Belgian soils and sediments, using a combination of SRMs and secondary standards for the calibration.