By Afsaneh Khetrapal BSc (Hons)
X-rays of sufficient energy are used to excite the inner shell electrons in the atoms of a sample. The electrons move to outer orbitals then down into the vacated inner shells and the energy in this de-excitation process is emitted as radiation.
The absorption or emission energies are characteristic of the specific atom and small energy variations may occur that are characteristic of particular chemical bonding. The X-ray frequencies can be measured and X-ray absorption and emission spectroscopy is 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 calculation and eventually the intensities of the scattered X-rays give information about the atomic positions and their arrangement within the crystal structure.
Usually the analyte is in solution form (or converted into one) that is then converted to a free gaseous form in a multistage process (atomization). This method is often used for metallic element analytes present at very low concentration ranges.
Atomic Emission Spectroscopy (AE)
This method uses atoms excited from the heat of a flame to emit light. The analysis can be done with a high resolution polychromator to produce an emission intensity vs. wavelength spectrum to detect multiple elements simultaneously.
Atomic absorption spectroscopy (AA)
Compared to AE spectroscopy, a flame of lower temperature is used so as not to excite the sample atoms. Instead, the analyte atoms are actually excited using lamps which 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 determined based on how much light is absorbed after passing through the flame.
Spark or arc (emission) spectroscopy
This is used for analyzing solid metallic elements or non-metallic samples made conductive by being ground with graphite powder. Analysis requires passing an electric spark through it to produce a heat that excites the atoms. The excited atoms emit light of characteristic wavelengths which can be detected using a monochromator.
Analysis of these metallic elements in solid samples is qualitative as the spark conditions are not well monitored on the whole however the recently introduced usage of spark sources involving controlled discharges yields quantitative data.
This uses the fact that many atoms are able to emit or absorb visible light. The atoms must be in a gaseous phase in order to obtain a spectrum just as those obtained 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 protein and DNA in a solution. Many amino acids (including tryptophan) absorb light in the 280 nm range whilst DNA absorbs light in the 260 nm range. Using this knowledge indicates 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 fluorescence from a sample in a form of absorption spectroscopy.
Infrared (IR) and Near Infrared (NIR)
IR spectroscopy is used to show what types of bonds are present in a sample by measuring different types of inter-atomic bond vibrations at different frequencies. It relies on the fact that molecules absorb specific frequencies which is dependent on their chemical structure. This is determined by factors such as the masses of the atoms.
NIR shows a greater penetration depth into a sample than mid-infrared radiation. This indicates a low sensitivity but also that it allows large samples to be measured in each scan by NIR spectroscopy with little (if any) sample preparation. It has numerous practical applications that include: medical diagnosis pharmaceuticals, biotechnology, various analyses (genomics, proteomic) and chemical imaging of intact organisms, textiles, forensic lab application and various military applications.
Nuclear magnetic resonance
This is a prominent method for analyzing organic compounds because it exploits the magnetic properties of certain atomic nuclei to determine the properties (both chemical and physical) of these atoms or the molecules containing them. It can provide extensive information about the structure, dynamics, and chemical environment of atoms. Additionally, even different functional groups are distinguishable, and identical functional groups in differing molecular environments still give distinguishable signals.
Last Updated: Dec 20, 2016