Accurate Total Oxide X-Ray Analysis with a XRF Spectrometer

Wavelength dispersive X-ray fluorescence (WDXRF) can test up to 84 periodic table elements in various materials, including solids and liquids, conductive and non-conductive.

The advantages of XRF over other techniques are speed of analysis, ease of sample preparation, consistent stability, precision, and a large dynamic range (from ppm to 100%).

Particle size and mineralogical effects can compromise the accuracy of powder analysis. Inhomogeneities and particle size effects can typically be reduced by grinding to 50 µm and pelletizing under high pressure. However, mineralogical effects cannot be eliminated, and some hard particles cannot be broken down to the desired size.

Fusing these oxidic materials is the most effective approach to eliminate grain size and mineralogical impacts. The technique involves heating a part of the sample with a borate flux, typically lithium tetraborate and/or lithium metaborate.

The flux melts and dissolves the material at high temperatures (1000 °C to 1100 °C). The total composition and cooling conditions must be such that the end product after cooling is a single-phase glass (Figure 1).

Figure 1. Transformation of the powder material into a glassy sample by fusion at high temperature. Image Credit: Thermo Fisher Scientific – Production Process & Analytics

The Thermo Scientific™ ARL™ X900 XRF Spectrometer can be calibrated as a complete analytical package from the factory, analyzing a wide range of minerals utilizing the general oxide calibration based on fusion sample preparation (Figure 2).

Figure 2. Many different materials can be analyzed with an ARL X900 Spectrometer calibrated with our General Oxide calibration. Image Credit: Thermo Fisher Scientific – Production Process & Analytics

Calibration Ranges and Results

Table 1 shows the types of oxides that can be treated and the concentration ranges. The Thermo Scientific^™ OXSAS^™ Analysis Software package includes multivariable regression to create a working curve for each element.

All matrix repairs are carried out using theoretical α-factors. Loss on ignition values of up to 47 % can be utilized to correct multivariable regressions—the standard error of estimate measures analysis accuracy, the average error between the certified concentrations of the standard samples, and the calibration curve for a specific oxide.

Table 2 shows the limits of detection (LoD) determined with precision tests at low concentrations for the major oxides using the universal goniometer. The analysis time per element varies from 4 to 40 seconds, depending on the element and the level of precision required.

Fixed channel monochromators for several elements/oxides can significantly reduce total counting time.

Sample Preparation

Figure 2 illustrates how standard samples are dried before being fused. Standards are 35mm diameter fused beads from ignited or non-ignited powder. When required, ignition is carried out for one hour at 1050°C.

The fusion is made from 0.7 grams of sample, 7.7 grams of flux mix (66% Li₂B₄O₇ - 34% LiBO₂), and 0.02 grams of LiBr (dilution 1:11) on a Katanax^® Inc. electrical or gas fusion machine (Vulcan or FLUXANA^®).

Samples can be prepared in two ways:

a. No Calcination of Samples

(→ Quicker preparation for clean oxides)

The software estimates the loss on ignition, so all elements must be measured for this automatic correction to operate. If other elements/oxides than the 12 measured are present, the loss on ignition should be manually entered to increase analysis accuracy.

Note: Fusion from non-ignited samples can be lethal to the Pt-Au crucible if it contains microscopic metallic particles.

b. Fusion from Ignited Samples

(→ Better accuracy and safer fusion)

Samples are ignited at 1050°C for an hour, and their loss on ignition is calculated. Samples are 35mm in diameter fused beads made from fired powder. Ignited samples are easier and safer to fuse, especially when small metallic particles are involved.

Table 1. Concentration ranges of the various oxide types with the SEE achieved. Source: Thermo Fisher Scientific – Production Process & Analytics

Table 2. Typical limits of detection in 100s obtained on various oxides using the goniometer at various power levels (fusions with 1 part sample / 11 parts flux). Source: Thermo Fisher Scientific – Production Process & Analytics

Typical Repeatability Data

The tables below illustrate typical results obtained using the universal goniometer on fusion beads of various oxidic materials. The analysis using the goniometer is sequential. Therefore, elements/oxides will be measured one after another.

The analysis duration for the first sample is 220 seconds for the nine oxides (Cr₂O₃ is not indicated because its percentage level was below the LoD).

Table 3 a. Short term repeatability on a rock fused bead - goniometer at 20 seconds per line - 40 kV/70 mA. Source: Thermo Fisher Scientific – Production Process & Analytics

Table 3 b. Short term repeatability on a cement fused bead - goniometer at 20 seconds per line - 40 kV/70 mA. Source: Thermo Fisher Scientific – Production Process & Analytics

Table 3 c. Short term repeatability on a dolomite fused bead - goniometer at 20 seconds per line - 40 kV/70 mA. Note that due to the dilution Na₂O is at the Limit of detection, hence the poor standard deviation. Source: Thermo Fisher Scientific – Production Process & Analytics

Analysis with fixed monochromator channels allows for faster evaluation while maintaining or improving precision.

Factory Pre-Calibration

The ARL X900 WDXRF Spectrometer can be pre-calibrated at the factory before it is delivered to the client. Thermo Fisher Scientific employs a series of approved standard samples created on a Katanax electrical or gas fusion machine (Vulcan VAA2 or FLUXANA VITRIOX^® Gas, depending on the customer’s preference).

This pre-calibration does not include reference samples, but it offers six stable and polished setting-up samples for long-term calibration curve maintenance.

Alternatively, a package of 24 internationally recognized oxide materials standards can be acquired, allowing the customer to calibrate the instrument on-site using their sample preparation equipment.

Conclusion

These findings demonstrate that diverse minerals, raw materials, and oxidic products may be examined accurately and precisely by combining WDXRF with sample preparation as fusion beads.

Because of smart power management, the Thermo Scientific™ ARL™ X900 XRF Spectrometer can operate at 1500 W and 2500 W without additional cooling. In these circumstances, no tap water or a water cooler is necessary.

At higher power levels (4.2 kW), sophisticated power control results in energy savings and less stress on the X-ray tube. The ARL X900 Spectrometers can be configured with a single sequential goniometer or fixed channels to speed up reaction times.

The ARL X900 series spectrometers do not require compressed air and have a low flow of P10 detection gas. The OXSAS Analytical Software, running in the newest Windows^® environment, offers powerful analytical functions while remaining easy to use.

For research use only. Not for use in diagnostic procedures. For current certifications, visit thermofisher.com/certifications © 2024 Thermo Fisher Scientific Inc. All rights reserved. All trademarks are the property of Thermo Fisher Scientific and its subsidiaries unless otherwise specified. Kantanax is a registered trademark of Kantanax, Inc. FLUXANA and VITRIOX are registered trademarks of FLUXANA GmbH & Co. KG. Windows is a registered trademark of Microsoft Corporation PPA-AN41424-EN 9/24

This information has been sourced, reviewed, and adapted from materials provided by Thermo Fisher Scientific – Production Process & Analytics.

For more information on this source, please visit Thermo Fisher Scientific – Production Process & Analytics.