Quantification of sulphur in the presence of selenium by XPS is difficult from survey scan data due to the overlap of the S 2p peak with the Se 3p3/2 and Se 3p1/2 peaks and the S 2s peak with the Se 3s peak. It is possible, in some cases, to quantify the amounts of Se and S present using a high-resolution scan of the S 2p and Se 3p region (use a B.E. window of 174 - 154 eV). Using a Se 3p3/2 - 3p1/2 doublet separation of 5.8 eV (+/- 0.2 eV) (Se metal was found here to have a separation of 5.75 eV) and constraining the 3p3/2 - 3p1/2 peaks to a 2:1 ratio (constraint factor of A*0.5), the remaining structure may then be attributed to the S 2p peak(s). Of course this analysis will also give you information on the sulphur species present. Then, by using the correct R.S.F. values, a Se:S ratio can be obtained using a components quantification.
If the survey spectrum is collected at the same time, the survey scan data and high-resolution data can be combined using a "Regions and Comps" calculation (under the Report Spec. / Quantification Parameters window in CasaXPS) to give full quantification of all the elements present. Remember to set the R.S.F. to 0 for the Regions window of the high-resolution spectrum.
A closer look at the Se 3p spectrum shows that the Se 3p3/2-3p1/2 ratio may be closer to 7:3, or an area constraint factor of A*0.43. This may be due to some overlap of the Auger structure with the 3p peaks. A example of the final fit of the S 2p and Se 3p can be found here.
R.S.F. Values
S 2p3/2 = 0.4453
Se 3p3/2 = 0.84933