I assume the difficulty with XPS is that you don't have an ambient pressure setup available in order to work with a liquid, right?

Now, XRF is more straightforward to operate at ambient conditions, however the energy resolution is much lower so you can get an element composition, but the chemical state analysis isn't really possible. That could be done with EXAFS or XANES, but these require a synchrotron with a beamline that is equiqqed for liquid analysis.

📷 Analysis of elemental and chemical composition of the surface layers by XPS and XRF

📷 Same qualitative results for elemental analysis of the surface by XPS and XRF

X-Ray Fluorescence (XRF) product option is equipped with a unique low-energy XRF capability. This solution provides an additional channel of elemental information based on the analysis of the secondary electrons emitted from the sample since both XPS and XRF signals can be detected and analyzed simultaneously.

📷 Elemental analysis of material properties both from the surface and depth of thin films

📷 Identification of the chemical element from analysis of the secondary electron emission

📷 Simultaneous thickness and composition data without correlation

The onset of the photoelectric effect can also affect some additional interactions of light and matter. The removal of an inner shell electron resulting from an energetic incident x-ray renders instability in the electronic structure of the material being analyzed. Since each element has electronic orbitals of characteristic energy, there are a finite number of ways in which the unstable vacancy can be filled. If the unstable vacancy is filled by a higher shell electron, the difference in shell energy released as a photon (i.e. x-ray). Analysis of this characteristic secondary x-ray emission or x-ray fluorescence is known as XRF.

X-ray photoelectron spectroscopy (XPS) is mainly used to determine - oxidation state, surface chemical composition. It is a quantitative technique for measuring the elemental composition of the surface of a material, and it also determines the binding states of the elements. Notably, most changing molecules are composed of elements such as C, N, O, P, S, which can be precisely distinguished by XPS. Furthermore, the chemical shift via interactions with metal centers can also be accurately detected by XPS. XPS analysis relating to the XPS spectra can be obtained by irradiating a sample with X-rays and measuring the kinetic energy of electrons that are emitted from the top 1–10 nm of the material being investigated. Surface atoms have characteristic peaks in the XPS spectrum. The energy and intensity of these peaks enable identification and quantification of all surface elements present (except hydrogen). The surface sensitivity of XPS makes it a valuable tool in the study of graphene—since graphene is intrinsically only a few layers thick. XPS can provide quantitative information such as the number of layers, and identification of impurities present from patterning or processing. It has Successful chemical analysis (oxidation states and functional groups). Good reproducibility of XPS measurements.

ref: DOI:10.1007/s42250-022-00570-7

Influence of Ce3+ (Rare Earth Element) on the Structural, Morphological, Impedance, Binding Energy and Ferrimagnetic Properties of Spinel ZnFe2O4 Nanoparticles Fabricated by the Coprecipitation Method: Antibacterial Activity

For elemental analysis, prefer to use XRF for liquid samples.