X-Ray Fluorescence Analysis for Elemental Analysis

Several techniques are available when it comes to elemental analysis. However, it must be noted that, of the available options, none have the same degree of flexibility in terms of material type and analytical range as X-Ray fluorescence.

The measurement of up to 83 elements of the period table is enabled by wavelength dispersive X-Ray fluorescence (WDXRF) in samples of various forms and nature: whether liquids or solids, conductive or nonconductive.

Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

There are a number of significant benefits of XRF over other techniques, which include:

• Analysis speed
• Ease of sample preparation
• Extremely stable and precise
• Wide dynamic range (ranging from ppm levels to 100 %)

Accuracy of standards and sample preparation are the two largest factors affecting the precision of the analysis in WDXRF. Typically, multiple analyses performed on the material by several different laboratories produce these standard accuracies. Round robin testing is how the most common form of this type of analysis is performed.

To achieve a certified reference value for the measured elements, a statistical computation of all results is performed. Through several government agencies and private companies, these materials are commercially available.

Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

As relates to the second issue, considering both non-homogenous solids and powder, the most accurate method of sample preparation for XRF is fusion. An optimal way to completely remove both grain size and mineralogical effects is by fusing oxide materials.

The method essentially comprises heating a mixture of a borate flux and the sample – the borate flux is either lithium tetraborate and/or lithium metaborate at high temperature (1000 °C- 1200 °C), which then melts and dissolves the sample.

The composition and cooling conditions must be ideal in order to combine and produce a one-phase glass after cooling. The Thermo Scientific Total Oxide X-Ray Analyzer (TOXA) enables analysis by WDXRF of a broad range of minerals and is a full-service analytical package that uses the General Oxide calibration based on a sample preparation by fusion.

Instrument

An elemental analysis was undertaken using a 4200 W system from the Thermo Scientific ARL PERFORM’X series spectrometer for demonstrative purposes (Fig. 1). The system’s features are as follows:

• The Thermo Scientific ARL PERFORM’X series spectrometer has six primary beam filters, four collimators, two detectors, up to 9 crystals, helium purge and 5GN+ Rh X-Ray tube for superior performance from heavy to ultra-light elements as it also boasts a 50 µm Be window.
• Month after month, this new X-Ray tube fitted with a low current filament ensures an unmatched analytical stability.
• High-quality performance and sample analysis safety are provided by The ARL PERFORM’X.
• The series of features in the unique LoadSafe design prevent any issues during sample loading and pumping.
• Even in cases of human error, the liquid cassette recognition guarantees that the liquid sample is not exposed to vacuum. Should the X-Ray exposure time be too long, the liquid sample will be automatically ejected.
• A specially-designed bespoke container in the Secutainer system collects loose powder to protect the primary chamber. This container can then be easily removed and cleaned.
• The ARL PERFORM’X uses a helium shutter for spectral chamber protection, designed for absolute protection of the goniometer during liquid analysis under helium operation.
• A special X-Ray tube shield in the “LoadSafe Ultra” configuration offers total protection against sample breakage or liquid cell rupture. The ARL PERFORM’X analyzer also features small spot and elemental mapping analysis permitting 1.5 mm and 0.5 mm areas. By offering contamination identification, additional screening, inclusion analysis and segregation/non-homogeneity mapping, these features markedly improve XRF system capabilities.

Figure 1. Thermo Scientific ARL PERFORM’X series spectrometer. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Calibration Ranges and Results

Table 1 shows the types of oxides that can be addressed along with their concentration ranges.

Table 1. Concentration ranges of the various oxide types with the Standard Errors of Estimate achieved. Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

For each element, a working curve is drawn using the multivariable regression incorporated in the sophisticated Thermo Scientific OXSAS software package. For all matrix corrections, theoretical alpha factors are used. Loss on ignition values, as can be noted, spread up to 47 %, and these can be used in the multivariable regression for correction purposes.

A measure of the accuracy of analysis, the Standard Error of Estimate (SEE) is the average error between the certified concentrations of the standard samples and the calibration curve for a given oxide.

Table 2. Typical limits of detection in 40 s obtained on various oxides (fusions with 1:12 dilution). Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Table 2 lists the limits of detection (LOD) determined with precision tests at low concentrations for the range of oxides.

Depending on the element and the precision required, the analysis time per element can range from 4 to 40 seconds.

Finally, the team performed a 12-day stability test. Over twelve days, the sample was run sixty times and did not generate any drift correction during this period. The ARL PERFORM’X spectrometer has high stability, ensuring precise analytical results, as is shown by these results (summarized in Table 3). 

Table 3. 12-day stability test without any drift correction. Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Conclusion

The ARL PERFORM’X sequential XRF spectrometer can perform an analysis of fused beads. In both the short-term and long-term analyses, high precision and accuracy are obtained. With the simple addition of more certified reference standards, all calibration ranges can be extended.

Moreover, the operation is simplified through the state-of-the-art Thermo Scientific OXSAS software, which is operated using the latest Microsoft Windows 7 packages.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers.

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