Gas Chromatography – GC and GC-MS

Nobel Prize winner Archer John Porter Martin laid the foundation for Gas Chromatography with a gas-liquid chromatography system that he described in 1950.  Two years later a A.T. James and A.J.P. Martin published the first article describing gas chromatography system.  Gas chromatography instruments are now in almost every analytical chemistry lab. Gas chromatography can be used to quantitate compounds that can be vaporized below 300 degrees Celsius. Applications are wide spread, from environmental research to quality control testing and medical device residual analysis.

Gas chromatography works similar to other chromatography methods in that the sample flows through a chromatography column and is moved through the column using a mobile phase.  In GC, the sample is vaporized so that the the mobile phase can move it through the column.  The mobile phase in reference to GC is the carrier gas which is a gas stream that moves the sample through the column. An inert gas like Helium is the most common carrier gas for GC instruments. Molecules that make up the sample interacts with the chromatography column differently allowing different compounds to be separated.  Often, the columns are capillary tubes that are coated with a silicone based oil that absorbs the chemicals within the gas passing through. Chemicals within the sample will be absorbed by the column at different rates and are eluted at different temperatures, therefore different compounds within the sample can be separated.  Unlike other types of chromatography, GC columns are typically very long, a 100 meters or more.

There are many different types of detectors for a GC instruments.  Flame ionizing detectors (FID) are most commonly used.  FIDs use hydrogen to create a flame at the end of the chromatography column between two electrodes.  The flame breaks any up any organic compounds and hydrocarbons through a process call pyrolysis.  Carbon atoms forms cations which can be detected by the electrodes. Thermal conductivity detectors (TCD) are also common and are non-destructive so they are usually incorporated before an FID. TCDs work by determining the conductivity of gas passing electricity to a tungsten -rhenium filament. The carrier gas typically has high thermal conductivity but during elution the molecules within the sample often are less conductive which can be measured as a voltage drop at the filament.  Mass spectrometer detector are also common.

Gas Chromatography-Mass Spectrometry (GC-MS)

Gas chromatography instruments containing a mass spectrometry detector are extremely sensitive and only requires a small amount of sample.  Mass spec detectors measures the charge produced by an ion as well as the mass of the ion.  GC-MS are often used to identify unknown compounds and are used in airports as screening instruments, fire investigations, as a pharmaceutical quality control step and for medical device industry when performing extractable leachable testing.  As the molecules from the sample are eluted from the column they enter the mass spec detector which then ionizes, accelerates the fragments from each molecule separately.

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