Letters: April 2020

We are concerned by the sensationalist tone of the article entitled ‘Are the last half century’s worth of results from widely-used spectroscopy tool wrong?’ (Chemistry World, March 2020, p10).

The article is associated with a picture of an XPS instrument and an operator, which is sourced from the Science Photo Library with appropriate permissions. Both the instrument and operator are easily identifiable and those with a moderate interest in XPS will recognise that the picture was taken at the National Physical Laboratory (NPL) in Teddington. The XPS instrument was manufactured by Kratos Analytical, which made the world’s first commercial XPS instrument in 1969. The association of the NPL and Kratos with the work of the authors is, we assume, accidental. However this accident invites a response.

NPL, along with other metrological institutes such as Nist in the US, have provided reference data and procedures to calibrate XPS instruments.These have been available in ISO standards for 20 years or so, and have been developed by experts in electron spectroscopies within ISO Technical Committee 201 on Surface Chemical Analysis, Sub-Committee 7 for XPS, to help analysts perform meaningful and consistent measurements. Instrument manufacturers and users generally pay careful attention to these standards and also contribute to their development. Experts will not rely on absolute peak energies to identify chemical states in insulators but on peak shapes, the presence of other elements, energy differences, stoichiometry and other information.

The title of the article poses a question that has a simple answer: no. The counter-argument is presented in the article itself, in the growth of academic papers using XPS as an analytical method and the fact that XPS instrument sales are at an all-time high to both industry and academia. No-one would invest in such expensive equipment unless it provided useful information and appropriate value, which it clearly does. To conflate the charge referencing issue with the validity of all XPS results is misleading.

The real issue is one of education, because charge referencing and assignment errors, among many other types of error, are usually made by multi-disciplinary non-experts who need simple, automated methods. Don Baer, who was featured in the article, is taking positive action in this regard and has commissioned a series of ‘How to perform XPS’ guides in the Journal of Vacuum Science and Technology.

Given the current concerns about the reproducibility of academic science it would be nice to see RSC journals take similar action for all forms of chemical analysis. As Don says: ‘the problems are not unique to XPS’