For the choice of the right peaks, the Thermo Fisher learning center is always a good address: https://www.thermofisher.com/de/de/home/materials-science/learning-center/periodic-table.html

Nitin T Shelke, To determine the elemental ratio in a compound using XPS, first obtain high-resolution spectra for each element of interest and measure the peak areas after proper background subtraction. Then, correct these peak areas by dividing them by the element-specific Relative Sensitivity Factors (RSFs) to account for differences in photoionisation efficiency. The atomic ratio between elements is calculated by comparing their corrected intensities. Keep in mind that XPS analyses only the top few nanometres of the surface, so the ratios reflect surface composition, which may differ from the bulk due to contamination, sample charging, or sputtering effects. Accurate quantification requires careful peak fitting, background correction, and consideration of these surface-sensitive factors.

Hi! you can find a detailed answer in the attached file