Some of the different types of spectroscopy that will be discussed in this article include X-ray spectroscopy, flame spectroscopy, atomic emission spectroscopy (AE), atomic absorption spectroscopy (AA), spark emission spectroscopy, visible and ultraviolet (UV) spectroscopy, infared (IR) and near infared (NIR) spectroscopy, as well as nuclear magnetic resonance (NMR).
Spectroscopy Basics - Engineering Chemistry
In X-ray crystallography, X-rays of sufficient energy are used to excite the inner shell electrons in the atoms of a sample. The electrons move first to the outer orbitals and then down into the vacated inner shells, which causes the energy in this de-excitation process to be emitted as radiation.
The absorption or emission energies are characteristic of the specific atom; however, small energy variations may occur that are characteristic of particular chemical bonding. Both X-ray absorption and emission spectroscopy are therefore used to determine elemental composition and chemical bonding.
In X-ray crystallography, crystalline materials are analyzed by studying the way they scatter X-rays aimed at them. Knowing the wavelength of the incident X-rays allows the investigator to calculate the intensities of the scattered X-rays in order to determine the atomic positions and their arrangement within the crystal structure.
During flame spectroscopy, the analyte is typically in solution form or gets converted into a solution form. Once in solution, the sample is then converted to a free gaseous form in a multistage process known as atomization. Flame spectroscopy is often used for metallic element analytes present at very low concentration ranges.
AE spectroscopy excites atoms from the heat of a flame to emit light. Analysis of excited atoms can then be achieved through the use of a high-resolution polychromator to produce an emission intensity as it is released over the wavelength spectrum to detect multiple elements simultaneously.
Compared to AE spectroscopy, AA spectroscopy uses a flame of a lower temperature so as not to excite the sample atoms. Instead, the analyte atoms are actually excited using lamps that shine through the flame at wavelengths adjusted according to the type of analyte under study. The amount of analyte present in the study sample is then determined based on how much light is absorbed after passing through the flame.
Spark or arc (emission) spectroscopy
Spark spectroscopy is used for analyzing solid metallic elements or non-metallic samples that have been made conductive after being ground with graphite powder. Analysis of these samples requires the passing of an electric spark through them to produce heat that excites the atoms. The excited atoms then emit light of characteristic wavelengths, which can then be detected using a monochromator.
Analysis of metallic elements in solid samples through spark spectroscopy is qualitative, as the spark conditions are not well monitored on the whole. However, the recently introduced usage of spark sources involving controlled discharges has been shown to yield quantitative data.
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Visible and UV spectroscopy
Visible spectroscopy takes advantage of the fact that many atoms are able to emit or absorb visible light. When using this technique, the atoms of the sample must be in a gaseous phase in order to obtain a spectrum, which is similar to what is required in flame spectroscopy. It is common for visible absorption spectroscopy to be combined with UV absorption spectroscopy in UV/Vis spectroscopy.
UV spectroscopy can be used to quantify the concentration of both protein and DNA levels present within a solution. Many amino acids, including tryptophan, absorb light in the 280 nanometer (nm) range, whereas DNA absorbs light in the 260 nm range. UV spectroscopy, therefore, utilizes the ratio of 260/280 nm absorbance as a good indicator of the relative purity of a solution in terms of these entities. UV spectroscopy can also be used to analyze the fluorescence of a sample in the form of absorption spectroscopy.
IR and NIR spectroscopy
IR spectroscopy is often used to determine what types of bonds are present within a sample by measuring the different types of inter-atomic bond vibrations that arise at different frequencies. Moreover, this technique relies on the fact that molecules absorb specific frequencies that are dependent on their chemical structure, as well as the overall masses of the atoms.
NIR shows a greater penetration depth into a sample as compared to mid-IR radiation. This indicates that NIR exhibits both a low sensitivity as well as its ability to allow large samples to be measured in each scan with little (if any) sample preparation. NIR has numerous practical applications that include medical diagnosis, pharmaceuticals, biotechnology, genomic and proteomic analyses, genomics, chemical imaging of intact organisms, textiles, forensic lab application as well as various military applications.
NMR is a prominent method for analyzing organic compounds because it exploits the magnetic properties of certain atomic nuclei to determine both the chemical and physical properties of these atoms and/or the molecules containing them. NMR can also provide extensive information about the structure, dynamics, and chemical environment of atoms. Additionally, even different functional groups are distinguishable when this technique is employed; therefore, identical functional groups with differing molecular environments will still give distinguishable signals.