Analysing Elemental Composition Through a Plastic Barrier

In most elemental analysis techniques, a sample needs to be exposed to the excitation source and/or placed under vacuum. For instance, atomic absorption (AA) and ICP-MS need ablation or sample digestion and then exposure to the excitation source, while SEM-EDS requires sample exposure to the electron beam under a low vacuum.

There are certain situations where it ishighly desirable to leave the sample in a plastic barrier bag or analyze the sampleat atmospheric pressure. For example, the analyst could be concerned aboutcontamination from toxic, corrosive or radioactive samples or may need to preventenvironmental contamination of the sample.

For micro-spot elemental analysis, the Orbis micro-XRF spectrometer (Figure 1) is unique as an elemental analysis tool in thatthe analysis can be done through a plastic bag and, if necessary, at atmospheric pressure.

The Orbis PC Micro-XRF system

Figure 1. The Orbis PC Micro-XRF system

Comparison with Existing Solutions

While using SEM/EDS for analysis, the sample needs to be removed from the barrier bag as the exciting electron beam cannot penetrate the wall of the plastic bag and the analysis is carried out under vacuum conditions. Particle debris and wet sample outgassing will put the SEM column at risk of contamination. While using ICP-MS for analysis, the sample should be removed from the barrier bag for analysis. The equipment may be contaminated with radioactively decaying materials.

In comparison, Micro-XRF offers the following:

  • It is possible to perform analysis at atmospheric pressure and through a plastic barrier bag enabling analysis of slurries and powders
  • Analysis can be done on tiny spots to measure defects and inclusions
  • Both quantitative and qualitative analysis can be done and additional features such as primary beam filters extend the analytical detection limits

Testing and Analysis

X-rays are used as the exciting source for Micro-XRF, with fluoresced X-rays typical of the elements, as the signal. Lighter elements (Sodium – Argon) fluoresce lower energy X-rays which are scattered or absorbed more easily while being transmitted through air at atmospheric pressure or through a plastic barrier.

Higher energy X-rays are fluoresced by heavier elements, and whilst travelling through plastic barrier materials or the atmospheric path to the detector, they experience minimal attenuation. Therefore in situations where keeping a sample in a plastic containment bag and/or analyzing the sample at atmospheric pressure is preferred, valuable elemental analysis data can be obtained with micro-XRF even if there is a compromise on the detection limits of lighter elements.

Results for uranium rich glass, mining standard powder, and moon rock are included in this article, all of which were tested under varying chamber pressures and contained within plastic bags. In all samples, signal attenuation is present at the lighter emissions below 4keV whereas there is minimal attenuation at the higher emissions.

Uranium Glass Overlay

Three spectra with variation in the plastic bagging is shown in Figure 2. While using the exposed sample in air, it was found that the determined uranium concentrations were 1.68 wt%.

The exposed uranium glass without any barrier is shown in red, that kept in a single 50µm bag is shown in blue, and that enclosed in two 50µm bag layers is shown in green. There is very little signal attenuation more than 4keV. Conversely, there is a significant attenuation below 4keV.

Three spectra with variation in the plastic bagging

Figure 2. Three spectra with variation in the plastic bagging

Mining Standard Powder

Figure 3 shows a mining sample powder of Canmet DH-1 with uranium concentration at 0.26 wt% when compared to a measured concentration in the previous glass sample as 1.68 wt%.

The red line shows the powder in a thinner Mylar envelope, the blue line represents the powder in a thicker 50µm plastic bag and the green line indicates the powder in the same 50µm sample bag with a rhodium primary beam filter.

This filter enhances the lower detection limits for the uranium X-ray lines. Without using a filter, the blue and Mylar plastic bag samples were studied. It can be seen from the blue and green spectra that filtering enhances detection limits.

Results for mining sample powder of Canmet DH-1

Figure 3. Results for mining sample powder of Canmet DH-1

Lunar Rock

Figure 4 shows a lunar rock having transition elements instead of higher energy actinides. The data shows elemental plots instead of spectral plots. Beginning from the left these are results from an uncontained sample, followed by samples having 50.8µm nylon, 50.8µm Teflon, and 127µm Teflon containment bagging with the analysis performed in vacuum.

Stronger signal intensity with minimal signal absorption by the barrier material is shown by the higher intensity color. Lower energy elements could also be effectively analyzed with the barrier in place while using the high-density Teflon barrier when compared to nylon.

The aim was to study the effect of using high-strength, gas-tight containment bags on the elemental analysis using micro-XRF.

Results for lunar rock with transitional elements

Figure 4. Results for lunar rock with transitional elements

Conclusion

In order to offer valuable elemental analysis on samples in the barrier bag under vacuum and atmospheric pressure, the Orbis micro-XRF spectrometer, has been used. There are two capabilities shown here; elemental mapping and single point analysis.

The ultra-high intensity 30µm polycapillary optic is especially useful where analyzing defects and inclusions or higher resolution mapping is desired. Hence the Orbis PC configuration with this optic type is suggested for this kind of work.

The standard hardware suite includes the primary beam energy filters, which are used here for enhanced detection limits over limited spectral ranges as well as the automated stage for sample mapping and positioning. Two large spot collimators can also be used to configure the Orbis along with the polycapillary optic for sample measurements more suited to a larger spot analysis.

The key features of the Orbis PC Analyzer are:

  • Vision Software suite including Microsoft operating system and Microsoft Office
  • Precision automated stage
  • Chamber vacuum pump
  • Dual color CCD cameras
  • 6 primary beam energy filters
  • X-ray optics: 30µm poly capillary, optional 1 mm and 2mm collimators
  • Silicon Drift Detector with LN-free cooling
  • Optional Mapping Software

About EDAX Inc.

EDAX is the global leader in Energy Dispersive X-ray Microanalysis, Electron Backscatter Diffraction and Micro X-ray Fluorescence systems. EDAX manufactures, markets and services high-quality products and systems for leading companies in semiconductors, metals, and geological, biological, material and ceramics markets.

Since its founding in 1962, EDAX has utilized its knowledge and expertise to develop ultra-sensitive silicon radiation sensors, digital electronics and specialized application software that facilitate solutions to research, development and industrial requirements.

EDAX is a unit of AMETEK Materials Analysis Division. AMETEK, Inc. is a leading global manufacturer of electronic instruments and electric motors with annualized sales of more than $1.8 billion.

This information has been sourced, reviewed and adapted from materials provided by EDAX Inc.

For more information on this source, please visit EDAX Inc.

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