Standard test data for estimating peak parameter errors in x-ray photoelectron spectroscopy III. Errors with different curve-fitting approaches

Abstract

We present results in the final part of a three-part study employing standard test data (STD) to estimate errors in peak parameters derived from data analysis procedures used in x-ray photoelectron spectroscopy (XPS). The XPS-STD are simulated doublet and singlet XPS spectra based on spline polynomial models of measured C 1s spectra. The XPS-STD contained 10 sets of spectra, each of which consisted of 18 doublets and 4 singlets. The doublet spectra in each set were created from a factorial design with three factors: peak separation; relative intensity of the component peaks; and fractional Poisson noise. Each of the XPS-STD sets contained the same complement of doublet and singlet spectra and Poisson noise at the same levels; however, the noise distributions were unique in each spectral set. Seven curve-fitting approaches were used by a group of 20 analysts to represent single peaks: a single Gaussian (G), a Gaussian–Lorentzian (G–L), a Voigt function, a G function with an asymmetry term, a G–L function with an asymmetry term and dual G or G–L functions. We statistically analyzed errors in binding energies and intensities by the type of curve-fitting approach using the Kruskal–Wallis test and the Mann-Whitney U-test. Bias was used to measure the accuracy of a curve-fitting approach, whereas random error was used to measure the precision of the approach. Despite the fact that individual peaks were nearly symmetrical, curve-fitting approaches that accounted for peak asymmetry proved to be the most accurate for determining both peak intensities and binding energies. For the doublets exhibiting the larger peak at the higher binding energy, the use of dual G–L functions to fit individual peaks, as a way to account for peak asymmetry, produced the highest accuracy in determining peak binding energy. This dual peak-shape approach for this particular spectral condition is also preferable for determining peak intensities and produces better precision. The XPS-STD and the statistical analysis methods presented here provide a means to distinguish differences in the accuracy of various curve-fitting approaches for spectral conditions that resemble those of the XPS-STD. To obtain the XPS-STD and to have an online evaluation of curve-fitting accuracy and precision, consult the following web site: http://www.acg.nist.gov/std. Copyright © 2000 John Wiley & Sons, Ltd.