There are many different types of chemical analysis, both quantitative and qualitative. Qualitative methods are used to identify unknown substances, and quantitative methods not only identify the substance but also determine the amount that is present. The type of method you choose will depend on what you are investigating, as each chemical analysis is specific for a certain type or groups of compounds.
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HPLC stands for high performance liquid chromatography. This is a type of column chromatography of which a solvent with the chemicals of interest are forced through a thin column at high pressures. The compounds are then separated out based on molecular size and polarity. There are two types of HPLC; normal-phase HPLC and reverse-phase HPLC. The most common type is reverse-phase HPLC in which the silica inside the column is actually non-polar. Normal-phase HPLC uses a polar column. HPLC can be used for both quantitative and qualitative information. Quantitative information will require internal standards.
Atomic Absorption Spectroscopy
Atomic absorption spectroscopy, or AAS, is used to determine the concentration of metals in a given sample. Metals will all absorb energy at different light wavelengths. The different wavelengths represent varying amounts of energy, a property is based on the Beer-Lambert law. Measuring this change in energy makes it possible to determine the element and concentration of a substance. An unknown sample is atomised, normally by a flame. Graphite or plasma furnaces are the most common atomisers. Once the sample is atomised, light is applied and the amount of transmitted light is captured by a detector.
FTIR, or Fourier transform spectroscopy, is a type of infrared spectroscopy. All chemicals will absorb light at different wavelengths. The amount of transmitted light for each wavelength can be compiled to create a type of fingerprint called a spectrum. To produce the spectrum the information collected from the detector needs to be transformed using a series of calculations called Fourier transformations. FTIR software will have the calculations built into the software. When compared to the spectrum of known chemicals, unknown chemicals can be determined. FTIR is used primarily to identify unknown materials, analyse the quality of a sample and determine the different components in a mixture.
There are several different methods that pair gas chromatography with other chemical analysis, such as gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectroscopy (GC-IR). Gas chromatography is used to separate out chemicals in a mixture. GC is also used to test the purity of a sample. Gas chromatography works in a similar way to HPLC except that it uses gas instead of a solvent. A carrier gas will take the sample and push it through a column. The column can be polar, nonpolar or of mixed polarity. There are many different columns, and which column you choose will depend on the type of chemicals you are analysing. The different chemicals in the sample will separate based on size and polarity. Coupling the gas chromatography with MS or IR allows the sample to be identified based on the chemicals' mass spectra.
Mass spectrometry is used to identify unknown chemicals and compounds based on their chemical properties. Mass spectrometry can be used on samples that are minute. The mass of the atoms in the compound are converted into ions. Ions are molecules that have been charged with electrons. Different molecules and molecular bonds will produce different ions. This in turn will produce an ion fingerprint for each chemical. Mass spectrometry is used to identify structures, identify the sequence of polymers, qualify and quantify compounds and much more.
Nuclear Magnetic Resonance
Nuclear magnetic resonance, or NMR, occurs when atoms in a static, or non-moving, magnetic field are put in contact with a second magnetic field that is oscillating. With the second magnetic field applied the nuclei of the atom changes from a low-energy spin to a high-energy spin. The difference between the two spins can be calculated as a frequency. Each atom has its own frequency, and by determining the frequency of the spins in a compound the molecular structure is uncovered and compounds are identified.
X-ray microscopy uses low levels of electromagnetic radiation to produce images of very small compounds. The X-ray uses films and detectors to produce the image, as X-ray radiation cannot be seen by the naked eye. Contrast imaging is used to help sharpen images. X-rays used for chemical analysis are similar to X-rays used in medical procedures.
Other Chemical Analysis
Other forms of chemical analysis include transmission electron microscopy, X-ray diffraction, X-ray fluorescence spectroscopy, neutron activation analysis, particle induced X-ray emission spectroscopy, refractive index, scanning electron microscope, pyrolysis gas chromatography mass spectrometry, resonance enhanced multiphoton ionisation, scanning X-ray microscope, Mossbauer spectroscopy, liquid chromatography-mass spectrometry, ion microprobe, ion selective electrode, laser-induced breakdown spectroscopy, gel permeation chromatography infrared spectroscopy, liquid chromatography infrared spectroscopy, field flow fractionation, differential scanning calorimetry, electron paramagnetic resonance, flow injection analysis, energy dispersive spectroscopy, cyclic voltammetry, atomic emission spectroscopy, alpha particle X-ray spectrometry, computed tomography, chromatography, capillary electrophoresis, and colorimetry.
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- "Chemical Analysis: Modern Instrumentation Methods and Techniques"; Francis Rouessac and Annick Rouessac; May 2007
- Chemguide: High Performance Liquid Chromatography --- HPLC
- Thermo Nicolet Corporation: Introduction to Fourier Transform Infrared Spectrometry
- American Society for Mass Spectrometry: What is Mass Spectrometry?
- Michigan State University Chemistry Department: Nuclear Magnetic Resonance Spectroscopy