Fourier Transform Infrared Spectroscopy (FTIR)
In FTIR spectroscopy, the sample is exposed to IR radiation. Some of the radiation is absorbed by the sample and some of the radiation is passed through (transmitted). The instrument then detects these signals that are produced by the various methods (i.e. transmission, ATR, diffuse reflectance) to generate interferograms. An FTIR spectrum is derived from the decoding of interferograms into recognizable spectra. The patterns in these spectra help identify the sample, since molecules exhibit specific IR fingerprints
Raman Spectroscopy
In Raman microscopy and Raman spectroscopy the sample is illuminated using monochromatic light (from a laser) which causes a change in the polarizability of the molecule due to the interactions between the light and the molecules. When these energies of transition are plotted to produce a spectrum, a ‘molecular fingerprint’ of a given molecule is produced. As Raman spectroscopy detects fundamental vibrations, the Raman bands have a good signal to noise ratio and are non-overlapping, which makes the technique highly specific. In techniques such as FTIR, presence of water in a given sample can lead to heavy interferences by the water bands that are produced. But as the Raman spectrum for water is weak and unobtrusive, good spectra can be acquired even for species in aqueous solutions. As the measurement intensity of a Raman species is directly proportional to the concentration, a Raman analysis could be performed to measure species concentration as well. Raman spectroscopy is non-destructive and spectra can be produced in a short time making it a versatile technique for chemical analysis.
Ultraviolet/ Visible Spectroscopy (UV-Vis)
UV-Vis Spectroscopy is used for the quantitative determination of different analytes at one given time or over a desired period of time. Analytes include transition metal ions, highly conjugated organic compounds, and biological macromolecules which are usually carried out solutions. The samples are dispensed into a cuvette which is placed in-between the ultraviolet and visible light (UV/Vis) source and a detector. The absorption of light across the UV/Vis wavelengths through the liquid sample is detected and the concentration of the compound in the solution can be determined using Beer-Lambert’s law.
Fluoroscence Spectroscopy
In fluorescence spectroscopy, a sample that is to be analysed is at first excited by using a light beam. The absorption of photons from a light beam causes chemical species present in a sample to be excited from its ground state to an excited electronic state. These excited molecules collide with other molecules to lose vibrational energy until the lowest vibrational state of the excited electronic stage is reached. The molecule then returns to the ground electronic state, emitting a photon in the process. As the molecules may drop down to any of the several vibrational levels in the ground state, the emitted photons will have different energies and thus, different frequencies. Therefore, by analysing the different frequencies of light emitted in fluorescence spectroscopy, along with their relative intensities, the structure of the different vibrational levels can be determined.
Nuclear Magnetic Resonance Spectroscopy (NMR)
In nuclear magnetic spectroscopy, the local magnetic fields around a sample are observed. The sample to be analysed is placed in a strong magnetic field and is excited by using radio waves. The nuclei exhibit nuclear magnetic resonance which is used to produce the NMR signals. As the intramolecular magnetic field around an atom changes based on the electronic structure of the molecule and its individual functional groups, this information can be used in the characterization of compounds. Given the NMR peaks produced for individual compounds are highly characteristic NMR has become the definitive method for the identification of organic compounds. Apart from identification purposes, NMR spectroscopy can be used to obtain detailed information regarding the structure, dynamics, reaction state and chemical environment of molecules.
X-Ray Diffraction Spectroscopy (XRD)
The X-ray diffractometers consist of three basic elements: an X-ray tube, a sample holder, and an X-ray detector. The X-rays are produced by a cathode ray tube by heating a filament to produce electrons which are filtered to produce monochromatic radiation, collimated and directed towards the sample. This X-ray beam interacts with the planes of atoms in the sample where parts of the beam are transmitted through, absorbed by, refracted, scattered and diffracted by the sample. As the X-rays diffract differently by each mineral depending on the composition and arrangement of atoms in the crystal lattice, the distance between the planes of atoms can be measured by applying Bragg’s Law. This law relates to the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample. Due to the unique diffraction pattern exhibited by samples, the XRD analysis provides a ‘fingerprint’ of the crystals present in the sample. This can be compared with standard reference patterns and measurements to identify the crystal form.
X-Ray Photoelectron Spectroscopy (XPS)
XPS is a quantitative spectroscopic technique which analyses the average surface chemistry of a sample up to a depth of approximately 5nm. This technique quantitatively measures the elemental composition, atomic concentrations and chemical states of elements present at a samples surface. XPS can detect all elements with an atomic number greater than 3, therefore Hydrogen and Helium are not possible to detect. For analysis beyond the top 1-5nm, an inert gas ion gun (normally Argon) can be used to sputter off the surface layers before analysis. Alternating sputtering and XPS spectral acquisition permits chemical depth profiles to be obtained.
