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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?

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    $\begingroup$ FTIR and Raman microscopy exists, you can buy such instruments off the shelve. Other than that, I think you should keep on studying, because IR/Raman and XPS have basically nothing in common. Completely different techniques that answer completely different questions about your sample. $\endgroup$ – Karl Aug 5 '20 at 12:39
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    $\begingroup$ I think my question isn't very clear. 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? $\endgroup$ – Paperreader Aug 5 '20 at 14:00
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    $\begingroup$ @Paperreader Keep in mind that, energetically speaking, comparing the three methods with each other, XPS is a high energy / short wavelength method. Energy is used to excite (core / inner) electrons of atoms to a higher, previously unoccupied level and their radiant relaxation is observed. By comparison, IR and Raman are soft energy/long wavelength; recording the change of dipole moment of bond vibrations (IR), or the change of polarizability of bonds (Raman). $\endgroup$ – Buttonwood Aug 5 '20 at 17:04
  • $\begingroup$ @Paperreader If you have studied those methods, then you should know what results they give. Do you? $\endgroup$ – Karl Aug 5 '20 at 19:38
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    $\begingroup$ Buttonwood, thanks for the comment, I'll keep that in mind. Karl, I do know the different results that they give. But I am talking about identification of unknown elements. All four can do it, but I guess it just depends on the application which one is best suited. $\endgroup$ – Paperreader Aug 5 '20 at 20:25
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I think your main question is related to the difference? Have you heard a famous poem, The Blind Men and the Elephant? It is a story, I'd rather copy from Wikipedia

It is a story of a group of blind men who have never come across an elephant before and who learn and conceptualize what the elephant is like by touching it. Each blind man feels a different part of the elephant's body, but only one part, such as the side or the tusk. They then describe the elephant based on their limited experience and their descriptions of the elephant are different from each other. The moral of the parable is that humans have a tendency to claim absolute truth based on their limited, subjective experience as they ignore other people's limited, subjective experiences which may be equally true.

Elephant

So replace these men with these analytical techniques and the sample with the elephant. XPS /EDS can see what Raman/FTIR cannot and vice versa.

XPS/EDS are elemental techniques and they will tell you what elements are present. Sometimes XPS spectrum, for an expert, can also reveal what is the oxidation state of that element, if you do high resolution scans. Each sample may take hours and hours.

Raman and FTIR can used to probe surfaces (they are fancy surface vibrational spectroscopy techniques) and it will tell you the molecular information, and even the angle at which the molecule is standing on the surface.

From the Raman spectra of an unknown it will be very hard to guess what elements are present.

In short, these are complementary techniques. Someone who is trying to study surfaces would use all of them - if needed.

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    $\begingroup$ Thanks for your answer. My question was a bit too broad I think, but this helps me answer it. $\endgroup$ – Paperreader Aug 5 '20 at 20:25
  • $\begingroup$ Indeed, the one you like is the one you have in your lab. You will use it even if it is suboptimal for what you are looking for. $\endgroup$ – Jon Custer Aug 5 '20 at 22:16

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