X-ray fluorescence spectrometry is used extensively for geochemical analysis. Simple sample preparation, high accuracy and precision, and good to excellent detection limits across large parts of the periodic table are the principal reasons for this choice.
Accurate trace element analysis is becoming increasingly important in geology for many reasons including superior process control with associated cost savings, minimizing and controlling contamination and environmental pollution during mineral processing, geochemical exploration, quality control of products and pure research. The demand for lower and lower LLDs is ever increasing. For a typical high-power wavelength dispersive spectrometer configured for geological applications, average LLDs for most elements of geological and environmental importance are below 1 ppm.
Accurate quantification of trace elements close to their limits of detection requires accurate, spectrally clean net peak intensities, accurate corrections for inter-element matrix effects and good standards. Zetium with Pro-Trace software and its standards make up a unique package that consistently produces very high-quality trace element analyses in a wide variety of geological materials, including rocks, soils, sediments, ores, minerals and mineral sands.
To demonstrate the performance of this system, 9 elements were selected as representative of the suite of elements that can be calibrated with the
Pro-Trace standards. These elements range across the periodic table and illustrate a variety of analytical situations. For example, the use of K- and L-lines, the position of peaks on curved or linear backgrounds, elements suffering strong line overlap, and for some elements the necessity to ‘jump’ major element absorption edges to make mass attenuation coefficient (MAC) corrections.
Preparation of standards and samples
For Pro-Trace to operate correctly, high-purity blanks, single or multi-element interference standards, MAC and concentration standards are essential. A special set of standards consisting of 25 pressed powder multi-element standards, blanks and two monitors for the analysis of 40 trace elements is available from PANalytical for this purpose. The Pro-Trace standard set has two calibration standards for each of the 40 trace elements, and each one is validated against more than 200 internationally certified reference materials. Alternatively, a laboratory can use its existing standards and/or prepare additional standards to match its specific requirements.
The preparation of routine samples is easily mastered, safe and relatively inexpensive. Oven-dried samples are pulverized for ~20 minutes in a planetary ball mill together with 20 % wax/styrene additive. The additive acts as a binder and a grinding agent, resulting in a uniform grain size distribution ( ≤ 40 µm). Approximately 12 g of the mixture is then pressed into 36 mm diameter pellets using a hydraulic press operated at 20 tonnes pressure.
Zetium and Pro-Trace: the essential XRF trace- element hardware and software
Zetium is a fully integrated wavelength dispersive XRF analyzer, consisting of a spectrometer, X-Y sample handler and analytical software. Designed to provide accuracy, precision and sub-ppm detection limits for many elements, the Zetium, coupled with the unique Pro-Trace software and standards, provides the highest quality trace-element analysis of up to 44 elements and targets a wide range of materials of geological and environmental significance.
Pro-Trace standards in special storage cups and case
• 25 standards, 2 monitors
• High-purity constituents
• Robust pressed powders in aluminium cups
• Storage cups prevent contamination of standards
Precision and instrument stability
The precision, repeatability and reproducibility of the Zetium in combination with Pro-Trace are impressive, not only for short- term measurements (consecutive measurements, Table 1), but also for longer-term measurements (over a period of ten days). For comparison, the counting statistical error (CSE) is also shown in Table 1. In this case 20 consecutive measurements of a sample deliver standard deviations better than 2 % relative at the 25 ppm level, e.g. 25.6 ± 0.4 ppm Ga. More importantly, this level of precision is maintained for measurements carried out over a period of 10 days, e.g. Nb, Ga, Co and V, illustrating the long-term stability of the system.
Table 1. Analytical precision
Analyses are based on two-point calibration lines (Pro-Trace standards) forced through the origin (Figures 1-2). Pro-Trace calibration standards are shown as (+), and the crosses (x) are international CRMs analyzed as unknowns. The quality of the data (Table 2) results from the combination of the Pro-Trace software that produces accurate net peak intensities, together with the Pro-Trace setup standards. Calibration plots for vanadium and hafnium (Figures 1-2) give a graphic illustration of the accuracy of the system, and the CRM data (x) show no systematic error or bias.
Table 2. Calibration accuracy based on 200 s counting time oer element (100 s peak, 100 s background)
Figure 1. Calibration plot for V
Figure 2. Calibration plot for Hf showing good fit and no bias, even below 20 ppm Hf
Detection limits for the elements studied are given in Table 3.
Table 3. Range of detection limits in typical geological samples
Figure 3 shows detection limits lower than 2 ppm for the majority (31 out of 39) elements between scandium and uranium using the Zetium/ Pro-Trace system. For many elements the LLDs are below 1 ppm. Arsenic was determined uFsing the Kβ line, hence the higher LLD compared to the neighboring elements. The other 8 elements are measured on their L emission lines and thus suffer from less than optimal sensitivity.
Figure 3. typical detection limits in geological samples measured with the Zetium and Pro-Trace combination
Pro-Trace is designed to give the best performance by using proper and proven procedures for:
• Background corrections
• Spectral line overlap corrections
• Corrections for tube spectral impurities
• Matrix corrections, with MACs with jump edge corrections, influence coefficients and FP
• Accurate background correction for goniometer and fixed-channel measurements
• Fast, accurate trace element analysis with LLDs below 1 ppm for many elements
• Best quality trace element data from LLDs up to ~2000-3000 ppm in concentration
• Reliable trace element analysis of a wide range of sample types and compositions
• Reliable LLDs and counting error for every element in every sample
• Good quality trace element data is achievable, even with only two calibration standards
Components typeset in bold were present in the spectrometer used to obtain the data in this note