So I have been studying the four methods the past couple of weeks. To my understanding, XPS is a very surface-sensitive technique. It can give information about the composition of the surface but also of the bulk material via depth profiling. I think it is mainly used for qualitative analysis, i.e. find out the elements of the sample. The same goes for EDS/EDX (in SEM), it is used to identify samples. Strictly speaking, EDS is not a surface sensitive technique I think, because the spectral resolution is not as good as XPS. XPS looks at the first few layers of atoms, while EDS looks more at the bulk of the sample.
Now my question is: can't we do the same with FTIR or Raman? Except of course that with FTIR or Raman it would be difficult to study metals, whereas with EDS/XPS it is easy. What are the advantages of FTIR or Raman w.r.t EDS/XPS or the other way around? I know that there are handheld devices for FTIR and Raman, so they would be easier to use to identify unknown samples and require less sample preparation than XPS or EDS.
To me it seems that all four can be used interchangeably, except for maybe XPS, because it is so surface-sensitive. I know it depends on the applications, but in general, can either of these be used to identify unknown materials?
Summary of the question by the OP from their comments: I mean to say that either of the methods can be used to identify unknown samples. But of course, it depends on the sample and what you want to know from the sample to know which method works best. While I agree that the methods are different and answer different questions regarding the sample, I don't agree that they are "completely" different. To some extent they do answer the question of what the unknown sample is. Right?