Supplier of analytical instruments
| Contact & Support | News/Archive | We represent | K-analys AB |
|
Services
| Measurements & Consultancy Services |
Products
Raman Spectroscopy is a vibrational spectroscopy technique that make it possible to study vibrations in the back-bone structure in the molecules, organic as well as inorganic. The Raman vibrations require large energies and a set of lasers from 830 nm (NIR) down to 229 nm (deep UV) are utilized. These lasers often use a very low power, from some 20mW, and this is enough to get strong Raman signals. The advantage with low power lasers are that they are cheap, safe and do not require any external cooling. Strong lasers can in fact easily burn the samples, especially biologic material, and this is of course detrimental.
Dispersive Raman Spectroscopy is well suited to study solutions in water, directly or through glass walls (beakers, reaction vessels).
Some vibration modes in molecules can be either Raman or IR active, or both Raman and IR active, or neither Raman nor IR active. A well established theory, called "the selection rules" exists and can predict
what type of spectra a certain molecule shall have.
The Spectroscopic mode
When the laser light enters the spectrometer, it is first reflected by the Notch filters into a microscope. By using a microscope, it is easy to look at the sample surface (in white light) to find the most interesting spots to analyze, focus the laser light, and also collect the shifted Raman light. The laser light that impinges on the sample is shifted in frequency (can also be expressed as energy, wavelength, wave number), but only a
small fraction is actually shifted. All the light that returns through the microscope meets the Notch filters again, before entering the spectrometer. Again all of the light with the same wave length as the laser (Reyleigh light) is totally reflected and only light shifted 50 wave numbers or more can pass through. With this, the separation is done! Now the shifted light goes through a slit, falls on a grating, where it is
dispersed according to wave length, and then it falls on the CCD-camera and the spectrum is registered.
What was described above is called the Spectroscopic mode. The result is a spectrum, intensity of peaks versus wave number shift (most commonly used unit for Raman spectroscopy). A unique feature of Renishaws Raman equipment, is that it can perform a continuous scanning. Very commonly, the width of the CCD do not cover the whole spectral region of interest. Often the coverage is 500-600 cm-1. In Renishaws
instruments the grating is moved, so that the spectrum sweeps over the CCD-area during integration and a continuous spectrum of up to 9000 cm-1 can be recorded. The advantages are many compared to stitching
static spectra together. First of all, there are no mis-matches between different sections of the spectra. Second, each and every peak has now been registered by each and every pixel and the unevenness in the CCD sensitivity has been averaged out. The intensities are comparable in the beginning and in the end of the spectrum.
By varying laser wavelength, slit opening and grating, it is possible to vary the spectral resolution. If optimized, the spectral resolution can be as good as < 0.5 cm-1. In many cases, the inherent width of peaks
occurring in real samples is much wider, and by "relaxing" the parameters, intensity can instead be increased.
Mapping
By combining the spectrometer and a microscope with an automatic xyz-stage a convenient system for mapping is easily at hand. A map can now be built by the computer by collecting spectra from different spots in an x-y raster over the sample surface. After collecting a huge amount of spectra, and analyze these for interesting features, one can for example select one peak that corresponds to the presence of a certain species and form a map that shows the occurrence of this compound over the sample surface. It is in principle possible to build an infinite number of maps of different features in the spectra. The peak intensity corresponds to concentration and the peak position corresponds to the compound. Slight shifts in peak position can be related to built-in stresses and peak width to purity of the material and so on.
Imaging
This technique is a quick way of getting spatial distribution of a species. Instead of focusing the laser spot on the sample surface, like in mapping, the laser beam is defocused to a spot of 100 µm. This spot corresponds
directly to a certain area on the CCD-detector. Instead of letting the light on to the grating, it is filtered through an angle-tuned filter and only a spectral region of about 20 cm-1 is allowed to pass. The rest of the
spectrum is wasted. This light, representing just one interesting feature, falls on to the CCD and the registered variations in intensity correspond to the distribution of sample.
Streamline
With Streamline it is possible to create Raman images of large areas very fast and still get full chemical information from each point of the area. Maps that used to take hours to create can now be created in minutes. Examples of streamline images can be seen in the this brochure.
Confocal Depth Profiling
When the laser spot is focused on to the sample, the spectral resolution is set by the slit width. But the slit width limits the area from where light is accepted. It is possible to restrict the acceptance in the z-direction, and hereby limit the contributing area to a small volume. This requires a so called pin-hole being introduced into the light path. In Renishaws Raman instruments, this is accomplished by combination of reducing the slit width and limit the number of pixel rows on the CCD that are actually used for detection (a patented technique). This gives a possibility to lower the focal point down in to the sample, and if the sample material is transparent enough, analytical information from only a limited small volume deep down in the sample will contribute to the spectrum.
Raman used today
Raman is now being used in a wide verity of applications, both in research and industry. Polymers, advanced electronics, inorganic materials, forensic investigations, pharmaceutical research, production control, and a lot of other disciplines.
One area we would like to mention particularly is the potential Raman has for in-vivo examination of skin diseases, like tumors and other superficial changes of the skin. In addition to being more "objective" and
accurate than just visual inspection by pathologists, it is also much quicker and much less painful and costly. No need for cutting and staining tissue for microscope investigations, when the skin can be studied without
being removed from the body. This has been enabled by using fiber optic probes to send the laser light out, and to collect the Raman signals back. Also a new development of a diode laser working at 830 nm has made it possible to get a very good signal to noise ratio even in a material like human skin, which is very fluorescent at any other laser wavelength.
Raman application notes
Raman and SEM geological application
note - Sandstone
Please contact us at K-analys AB for further details.
You can also visit Renishaws
website.
Do you want to know more about this product?
Please type your name, company, e-mail address and country here and we will contact you as
soon as possible: