Using XRF analysis in footwear screening

X-ray fluorescence spectrometry can be used to determine the presence of metals but has practical limitations.

by John Hubbard

The introduction of requirements into national legislation on the presence of lead (CPSIA and California Proposition 65), cadmium (EU) and other metal-based compounds (REACH SVHC list) has resulted in the increased use of

X-ray fluorescence spectrometers (XRF guns) to determine whether any of the metals are present in materials such as plastics, leathers and textiles. As XRF guns are handheld and portable, as well as offering non-destructive testing of materials, this makes them very useful for analysing large numbers of products in a short time scale which would meet the needs of quality screening or market surveillance.

The principal of XRF is the use of a high energy X-ray that dislodges an electron from the inner shell of the atom. This results in a gap which is filled by an electron falling from an outer shell (higher energy) to the inner shell (lower energy). The resulting difference in energy is given off as secondary X-rays. By detecting distinctive X-rays, the element can be identified. The X-rays’ intensity provides a measurement of the amount of the element present.

To test a material, the XRF spectrometer is placed directly onto the surface of the sample and a measurement is taken. The results can then be displayed on the screen and stored for transfer to a computer.

As there is no requirement for sample preparation, staff with little or no training can use these instruments. However, when it comes to understanding the results, there are a number of issues which may be misleading or cause confusion when compared to laboratory-based tests using Atomic Absorption Spectroscopy (AAS) or Inductively Coupled Plasma Spectroscopy (ICP).

Making XRF measurements

The substrate’s matrix material can influence the determination. Therefore, a suitable calibration material should be used, depending on the materials to be analysed. These calibration materials may be polymers, leathers, textiles or painted surfaces – as similar as possible to the material to be measured. If the calibration material is of a different type to the matrix to be measured, errors may be introduced.

XRF guns only penetrate a short way into the material and only at the point where they are in contact with the sample. Therefore, if the sample is not homogenous or has a coating, the results will be unreliable and probably incorrect. It is also important to consider the result in the context of the whole product (which may consist of a number of individual materials) to determine if the whole product is compliant with any limitations for the metal compounds.

Interferences can exist between some metals, which may mask some results. Hence, the gun should be used in conjunction with laboratory testing. This is to confirm that the results obtained by the XRF technique show at least a proportional relationship with those obtained using ICP or AAS for any given material.

The XRF gun measures the total amount of the metal present in the material, which may not be the same as the amount available for extraction. Therefore, it will not give a complete assessment for the metals covered by the Toy Safety Standard EN 71-3 for extractable metals. It may, though, provide a screening test to confirm if any of the elements are present. If the metal is not present, it cannot leach out.

The use of XRF guns is growing, but care should be taken on interpretating results. To develop confidence in a monitoring programme, a support series of tests using alternative, more reliable, techniques should be put in place.

Publishing Data

This article was originally published on page 6 of the April 2014 issue of SATRA Bulletin.

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