Gas Chromatography And Mass Spectrometry GC MS Gas Chromatography And Mass Spectrometry

The gas chromatograph-mass spectrometer (GC/MS) is an instrument used to analyze the molecular and ionic composition of chemical compounds.

GC/MS technology combines two laboratory techniques: gas chromatography (GC), which separates and identifies compounds in complex mixtures and mass spectrometry (MS), which determines the molecular weight and ionic components of individual compounds.

Gas chromatography mass spectrometry (GC/MS) is an instrumental technique; a gas chromatograph (GC) coupled to a mass spectrometer (MS) complex mixtures of chemicals may be separated, identified and quantified

analysis of low molecular weight compounds found in environmental materials for a compound to be analyzed by GC/MS it must be volatile and thermally stable

Compounds may require chemical modification (derivatization), to eliminate adsorption effects that would affect the quality of the data.

Samples are usually analyzed as organic solutions materials of interest (e.g. soils, sediments, tissues etc.) extracted and the extract subjected to various 'wet chemical' techniques before GC/MS analysis is possible.

The sample solution is injected into the GC inlet where it is vaporized and swept onto a chromatographic column by the carrier gas. Usually He, lower in Atomic mass than most materials, moves out first

The sample flows through the column and the compounds comprising the mixture of interest are separated by relative interaction with the coating of the column (stationary phase) and the carrier gas, He, (mobile phase).

The latter part of the column passes through a heated transfer line and ends at the entrance to ion source where compounds ‘eluting’ from the column are converted to ions.

Two methods for ion production electron ionization (common) chemical ionization

For EI, a beam of electrons ionize the sample molecules resulting in the loss of one electron. A molecule with one electron missing is called the molecular ion and is represented by M+. (a radical cation). When the resulting peak from this ion is seen in a mass spectrum, it gives the molecular weight of the compound.

Energy imposed onto the molecular ion causes fragments producing further smaller ions with characteristic relative abundances that provide a 'fingerprint' for the molecular structure.

identify compounds of interest Characterize the structure of unknown components of mixtures Database in mass spec identifies substance qualitatively and quantitatively Daubert accepted

CI begins with the ionization of methane or helium(or another suitable gas), creating a radical which in turn will ionize the sample molecule to produce [M+H]+ molecular ions.

The next component is a mass analyzer (filter), which separates the positively charged ions according to various mass related properties depending upon the analyzer used. Several types of analyzer exist: quadrapoles (Fig. 2), ion traps, magnetic sector, time-of-flight, radio frequency, cyclotron resonance and focusing.

After the ions are separated they enter a detector the output from which is amplified to boost the signal. The detector sends information to a computer that records data produced, converts the electrical impulses into visual displays and hard copy. Computer controls the operation of the mass spectrometer.

Courtesy of Dr. Paul Gates Chemistry Dept. University of Bristol, UK http://www.bris.ac.uk/nerclsmsf/techniques/gcms.html