The Basics of Elemental Analysis with XRF – Q&A

Is XRF a surface technique?

XRF is an elemental analysis technique that can quantify many elements in a sample. The characteristic X-ray photons produced in the sample have specific energy (keV) and on the way to the detector get absorbed by other atoms in the sample. The information depth depends on the energy of the element of interest and the type of sample (average atomic number). This depth ranges from one micrometer to a few centimeters. For example, the information depth of magnesium (Mg-Ka) in a brass sample is 1 micrometer and in the soil is 11 microns, whereas the information depth of tin (Sn-Ka) in a brass sample is 0.3 mm and in the soil is 14 mm. Therefore, for the low energy elements, the quality of the sample surface becomes important.

In some cases, XRF is non-destructive when the sample can be analyzed without any sample preparation and analyzed with a low power benchtop EDXRF instrument. Then the sample is still intact after the measurement. To obtain the best accurate result, sample preparation is advised and then the XRF technique can’t be considered non-destructive.

On the other hand, when a sample is measured with XRF, the same sample can be used for further analysis by any other analysis technique.  

XRF is an analytical technique that can be used to determine the chemical composition of a wide variety of sample types including solids, liquids, slurries and loose powders. XRF is also used to determine the thickness and composition of layers and coatings and can be easily used for rapid screening (semi-quantitative). It can analyze elements from beryllium (Be) to americium (Am) in concentration ranges from 100 wt% to sub-ppm levels. XRF analysis is a robust technique, combining high precision and accuracy with straightforward, fast sample preparation. No acids and chemicals are necessary since the samples don’t need to be dissolved into a liquid nor have to be diluted.

XRF also has its limitations. XRF is an elemental analysis technique and therefore quantifies the total concentration of each element in a sample. XRF can’t distinguish between different oxides. XRD would be a suitable method for that.

XRF can quantify elements between 100 wt% down to sub-ppm level. Quantifying elements in the concentration levels of lower ppb or ppt are not possible with XRF, even when longer measurements are applied.

XRF instruments can run without helium. When analyzing liquids in the floor standing WDXRF instruments, helium is required. Benchtop EDXRF instruments that can operate under the air atmosphere don’t need helium, even when analyzing liquids. Sometimes helium is used when analyzing low-energy elements (between F and Cl) in any sample to increase sensitivity.

XRF is an elemental analysis technique and therefore quantifies the total concentration of each element in a sample. XRF can’t distinguish between different oxides. XRD would be a suitable method for that.

The average lifetime of a typical XRF instrument would be around 10 years. The lifetime depends on the working conditions of the instrument, how it is treated daily, and how frequent the service was performed. Some of them are much older than 10 years, going up to 25 years for the floor standing WDXRF instruments.

Which technique is better to use for analyzing C?

In comparison with EDXRF, WDXRF is much better suited for analyzing the low-energy elements (B up to Na). When analyzing carbon with floor standing WDXRF instruments, at least 4 kW power is necessary. Using a dedicated analyzing crystal (PX4) and a course collimator (4000 µm) will further improve the sensitivity for carbon.   

Some of the EDXRF benchtop instruments like Epsilon 4 can analyze the low-energy elements C, N and F. In comparison with WDXRF, the detection limits are less optimal. Typical detection limits of a few wt% can be expected. Since the elements of interest have a very low energy, the information depth is less than a micrometer. Then the quality of the sample surface becomes important and therefore the sample preparation.

The advantage of EDXRF over WDXRF is when analyzing the higher energy elements (between Ca and Sn). Also, the resolution of the elements in the XRF spectrum (=separation of the peaks) becomes better than WDXRF in the higher energies for the elements between Mo and Sn. But the sample throughput, accuracy of the results, infrastructure (floor space) and budget are also factors to take into account.

Within Malvern Panalytical a floor standing WDXRF instrument (Zetium) has the possibility to include EDXRF technology to enjoy the benefits of both techniques. Then both EDXRF and WDXRF can be used simultaneously to improve the speed or throughput even further.

What is meant by Universal Calibrations here?

We say that calibration is “Universal” when it encompasses all (or a large number of) sample types present at a certain user. Such calibrations are normally characterized by a wide concentration range for several elements. This can be achieved by analyzing fused beads, which do not present mineralogical effects or particle size effects. One such example would be in the lab of Cement Plant, a single calibration for Clinker, Limestone, Sand, Clay, Slags and Iron ore, for example. Such wide-range calibrations, with very different matrices, is not possible with pressed pellets.

Homogeneity of a sample can be understood in many ways, depending on the parameters used for evaluation and variables of interest. In general, for XRF spectrometry (elemental chemical analysis), homogeneity can only be assessed experimentally. There are many ways to perform such test and it will depend to a great extent on variables such as particle size distribution, number of phases present, mineralogy/composition/density of each sample, among others. Also relevant are the analytical requirements such as precision, accuracy and lower limits of detection for the desired analytes. After evaluating all these parameters, a proper experiment can be designed to assess the homogeneity of the sample. Normally this will involve taking several sub-aliquots of the sample in a representative manner and analyzing a certain number of replicates for each one of them. A proper statistical analysis of the results will give insight into the degree of homogeneity of the sample.

The best approach for analysis of majors, minors, and traces in geological material is by having two different applications: One based on fused beads for the majors and minors and one based on pressed pellets for the traces. As trace analysis is less affected by mineralogical and particle size effects, results in pressed pellets are very satisfactory, whereas in fused beads the high dilution makes trace analysis normally impossible.

If you have any further questions, please don’t hesitate to contact your local Malvern Panalytical representative, or me directly here.