Mass Spectrometry is a very useful technique that is used for identification of compounds by their mass-to-charge ratios.
An advantage of Mass Spectrometry is its ability to identify the unknown compounds using only picogram (10-12) amounts.
The sample, which may be a solid, liquid or vapor is introduced into a high vacuum system through an inlet; which is then ionized by an ionization source. The ions are separated according to their mass-to-charge ratios in the mass analyzer. The ions are then collected by a detector which is connected to a data system for data processing that produces a Mass Spectrum.
Mass Spectrum is a plot of the m/z ratios of the ionic species present versus abundance. The molecular weight and the structure of the compounds can be obtained from the information of the ion fragmentations and their abundances.
In daily life Mass Spectrometry is used...
Electron Ionization (EI) and Chemical Ionization (CI) are complementary techniques and are usually combined into one ion source so that the rapid switching between the two can take place.
EI is the bombardment of the sample by very fast moving electrons. The sample is chemically influenced by the fast moving electrons, causing an ejection of its own electron, forming a radical cation. This process takes about 10E-14 seconds. These unstable parent ions fragment to form stable daughter and granddaughter ions.
CI is a relatively gentle process of proton transfer from pre-ionized reagent gas, such as methane. Ionization is made by collision of sample molecules with the reagent ions.
Chemical ionization produces quasimolecular ions which do not fragment as readily as the molecular ions formed by electron ionization. Therefore, CI spectra are normally simpler than EI spectra in containing abundant quasimolecular ions and few fragment ions. It is advantageous to run both CI and EI spectra on the same compound to obtain complementary information.
In Fast Atom Bombardment (FAB), an atom gun is used to project "heavy" fast atoms (often argon and xenon) onto the surface of a target solution (matrix).
Usually FAB results in molecular or quasimolecular ions which have little excess of internal energy and therefore do not fragment. This ionization method is mild and good for obtaining molecular mass (molecular weight) information.
A quadrupole analyzer is a mass filter that creates a quadrupole field with a DC component and an RF component in such a manner as to allow transmission only of ions having a selected mass-to-charge ratio.
Quadrupole analyzer consists of four parallel rods arranged symmetrically. The opposite two are connected electrically to RF generator and DC potential. By varying the ratio of RF/DC, the ions of selected m/z ratio can avoid collision to the rods and goes through the central space between the rods to reach the detector.
Other mass analyzers, such as double-focusing electrostatic/magnetic sector, ion trap and time-of-flight, are also commonly used.
Gas Chromatography-Mass Spectrometry (GC-MS) is an analytical technique combining the advantages of a GC instrument with those of a mass spectrometer.
GC means of separating components of mixtures by passing them through a chromatographic column so that they emerge sequentially.
MS is a means of examining a compound, also in the gas phase, so that its structure or identity can be deduced from its mass spectrum.
The combined GC-MS allows more information to be gained than is obvious from the simple sum of the two separate instruments.
Liquid Chromatography-Mass Spectrometry (LC-MS) is an analytical technique combining the advantages of a LC instrument with those of a mass spectrometer.
LC, or sometimes HPLC (High Pressure Liquid Chromatography), is a means of separating components of mixtures by passing them through a chromatographic column so that they emerge sequentially.
MS is a means of examining a compound, also in the gas phase, so that its structure or identity can be deduced from its mass spectrum.
Because LC operates in the liquid phase but MS is a gas phase method, it is not a simple matter to connect the two. In order to allow separated components of a mixture to be passed sequentially from the LC into the MS an interface is needed. Having interfaced the two, LC-MS is a very powerful technique.
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We would like to thank Mr. Kim Harich , Mr. Levent Yilmaz, Mr. Haiqing Yuan and Analytical Services in theDepartment of Chemistry at Virginia Tech for their help.
Also, Spectrometric Identification of Organic Compounds, by R.M Silverstein, G. Clayton Bassler and Terence C. Morril and Back to Basics, by Fisons Instruments were very helpful and useful references.
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