Combining elemental and molecular characterization enables better and faster research for many real-world applications. The pharmaceutical, environmental, and even geochemical sectors can benefit from this complementarity of techniques. Micro-XRF provides elemental distribution over a large area without any compromise on sample preparation, while Raman microscopy can depict molecular heterogeneity under the same conditions.
In this on-demand webinar, Thibault Brulé and Jocelyne Marciano, from HORIBA Scientific, demonstrate how the combination of micro-XRF and Raman microscopy can be used to fully characterize the organic and inorganic layout of samples such as tablets, rocks or pollutant particles on filters.
JM: XRF is actually a polychromatic technique that uses the full spectrum of the X-ray tube. And we can go from zero to 50 kilowatts. In SEM/EDX, we usually work at 15 kilowatts. So, it's interesting, because we have a higher energy with XRF. Spatial resolution for scanning-electron microscopes and EDX is usually one micrometer cubed. For XRF, it depends on the size of the capillary. For smaller ones, this can be 10 microns.
Regarding the environment, XRF is able to work in vacuum too, as well as in air, partial air, or even helium. With XRF we can change the environment, and we are able to measure samples which are hydrated, which is so difficult with scanning electron microscopes.
TB: Well, in Raman, we have a probing volume which is around 1 micron cubed when you do the measurement in confocal mode. As we have a confocal Raman microscope with a motorized confocal aperture, we have the possibility to open the aperture in order to integrate a larger volume.
The interest in that case is that with such an approach on the Raman system, you can integrate more, so you have more signal, allowing you to go faster. We can also reach some volumes that are very close to those that can be obtained in XRF. In this case, we can ensure that the volume for the elemental distribution is directly related to the one obtained using Raman microscopy.
JM: We have a system in our software for colocalization. We can go from Raman to XRF by using three specific points that are registered in the environment, and then we make a triangulation, allowing us to recalculate the position when moving from Raman to XRF.
On the other hand, we can look at zero from XRF, and if we are doing mapping, we just measure the different positions of the corners of the mapped areas. We can then go to the Raman, and as Raman is able to choose its zero, we define the zero from the XRF as the zero for Raman, giving us the same position from the XRF mapping.
JM: Yes, XRF is able to measure the thickness of coating layers. It is also able to find particles inside layers. We can make colocalization between the XRF and Raman. But, of course, the minimum size would be decided by the technique with the lowest resolution, which is XRF. So, the minimum resolution for the particles would be 10 micrometers.
TB: We can do colocalization between the XRF microscope and the Raman microscope. We can do the same kind of colocalization with AFM. However, the limitation that you will have for AFM will be the size that you can measure, because we cannot cover very large areas as we can do on XRF and Raman microscopes. Moreover, you will have the nanometer spatial resolution with the AFM, but you are still limited by the lower resolution of your XRF microscopy system.
With micro-Raman, if you want to have nanometer resolution, you have to do Raman scattering analysis. In that case, it's very specific to different kinds of samples, but it cannot work with all kinds of samples. It's not so easy to work at the nanometer scale in micro-Raman.
You can do colocalization with the AFM. However, as you will probe very small features, the signal that you will have in Raman and in XRF will be very low for this specific nanometer feature. The results will not be so interesting, because you will not have a large enough signal-to-noise ratio between the substrate and your nanometer particles that you want to characterize.
TB: In Raman microscopy for tablets, when we have a concave surface, we have two possibilities. We can use a microtome to prepare the surface and flatten the sample, which is very useful because in that case, you can also remove the coating layers that can be opaque, preventing you from seeing the distribution of the components inside of the tablet.
If you have a tablet that is not flat, and you don't have any coating, or just a very small coating that is not opaque — titanium dioxide for example — in that case, you can do your Raman measurements following the surface, because we can automatically adjust the focus on the surface.
Doing an auto-adjustment of the focus, looking at the topography, we can couple that with the capabilities we have to do some fast mapping. If you have a flat surface, mapping a full tablet will take just a few minutes in micro-Raman to see the distribution of the components. After, it depends on the resolution that you want to achieve, but if you have a rough surface or a concave surface, it will take longer, maybe close to one hour.
TB: The speed is quite similar because it's highly dependent on the acquisition time that you need to produce a good spectrum in both cases. In order to do good-quality mapping in Raman or in XRF you need approximately the same acquisition time.
TB: It's completely different. In fact, EDX is very close to the XRF because with EDX you acquire elemental information. You excite electrons with an electron beam, and you then see the emission from the electrons going to their excitation levels.
In Raman, we look at molecular vibrations. We get information in Raman about the molecules, not about the electrons. Not about the elements, but the molecule itself. We'll also get information about the covalent bonds that you have in each molecule.
In this case, in Raman, the kind of information that you will have is very complementary to the elemental analysis that you can do with EDX or XRF. Once again, we couple with XRF as you have the same kind of sample preparation necessary for Raman, which can be a limitation for EDX, for example.
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