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XRF Analyzer Sample Preparation

by Ray Franklin

XRF analyzers are powerful tools for screening toys and consumer goods for heavy metals and some toxic chemicals. With proper sample preparation, the results will be sound and will resist challenges from others who might question the sampling process.

Correct Preparation Ensures Good Toy Screening Results

Lead, cadmium, chromium and mercury are all heavy metals, toxic to people in small quantities, and banned in electronics by the European Union. Even though the toxicity of these metals has been known for decades, they were banned from electronics only in 2006, and they continue to be found in toys, dinnerware and other consumer products.

This situation has driven a number of consumer watchdog organizations to use a high-tech method to detect the presence of these metals: X-Ray Fluorescence, or XRF. Modern, handheld, XRF analyzers can quickly identify all of these metals and others. However, without a good understanding of the capabilities and limits of this technology, well-meaning inspectors can totally invalidate the results by mishandling the tool.

XRF Analyzer X-MET5000 by Oxford Instruments

XRF Analyzer X-MET5000 photo courtesy of Oxford Instruments

The key to obtaining good results with an XRF analyzer is sample preparation. To understand why sample preparation is so important, requires a bit of knowledge of how XRF works.

Without getting too technical, an XRF analyzer directs X-Rays at an object to be analyzed. A detector in the analyzer measures reflected X-Rays and uses well-established rules of physics and chemistry to display the elements present in the sample. A good analyzer can also determine how much of each element is present, with moderate accuracy.

If the sample is non-metallic, say plastic, wood or glass, the X-Rays will penetrate up to about one centimeter into the object. The results will identify the elements in that zone. The story is very different if the material is metallic.

Unlike plastic, metal is very dense. It is also conductive. X-Rays barely penetrate metals. A metallic coating of only a few micro-inches will completely block the X-Rays from a typical XRF analyzer. So if a toy has a non-toxic metallic layer over a toxic substrate, the XRF analyzer will not detect the toxic material.

When the product has a complex structure with multiple materials, good results are possible only by destructive analysis. To prepare a good sample for XRF, the sample must be ground up. Grinding, or chopping, will expose all the materials. A tablespoon of finely ground toy will deliver excellent results with an XRF analyzer.


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