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Gas Chromatography
Technique:
Gas Chromatography (GC) is an analytical technique used for the separation and quantification of components of complex mixtures of volatile compounds. It is suitable for most compounds with boiling points of less than 250°C (500°F) and uses a gas as the mobile phase. The GC is similar to HPLC, but is traditionally used for simpler compounds.
Application:
GC's are used at Armstrong in many different areas.
• Detection and characterization of Ignitable Liquids.
• Quantification of compounds in Industrial Hygiene.
• Quantification of compounds in Environmental Testing.
• Forensic Identification & Quantification of Drugs of Abuse.
The Science Behind:
Gas chromatography uses a gas as the mobile phase, carrying the test sample through a packed or capillary column. Packed columns are useful for low molecular weight species such as gases and alcohols. Sensitivity and separation of the components is limited however. Columns are lined or packed with a stationary phase designed to separate the individual components of the test sample.
In order to get the separations needed, Gas Chromatographs have programmable temperature and pressure controls. The reproducibility of these complex programs is the heart of Gas Chromatography. The art of the GC is knowing what temperature and pressure program to use to achieve desired separation results. A standard GC program will start near room temperature and a constant pressure, ramping thru various stages, up to 250°C, allowing heavier compounds elute completely. Then doing it over and over again, in the exact same manner.
After the sample components are separated they pass through one or more detectors. These detectors measure the amount of each component of the sample as it exits the column and the data is captured via a computer. The analyst processes the data to determine the identification and the amount of each component under study.
Currently FID, ECD & PID detectors are used at Armstrong:
- FID (Flame Ionization Detector) is the most common and easiest to use. FID, a workhorse of most labs, uses a flame to ionize the sample. The resulting charged gases are measured. The more of a compound present in the sample, the greater the ionization and a larger response.
- ECD (Electron Capture Detector) uses a radioactive source, and measures the reduction in alpha radiation by the sample components. The ECD is a selective detector, providing sensitivity and selectivity for compounds containing halogens and electronegative elements.
- PID (Photo Ionization Detector) uses an ultraviolet (UV) source to separate molecules into ions which are then measured. PID is a selective detector providing excellent sensitivity for aromatic compounds and does not respond to most diphatic compounds.
Wikipedia Definition of Gas Chromatography
Capillary - Refers to the size of the column, up to 100 meters long tube with an interior diameter smaller than a human hair.
Column - Thin tube, lined/filled with the stationary phase. Capillary columns are typically made from glass and purchased for uniformity in quality. Packed columns can be purchased or packed by hand.
Detector - A device used to measure or record changes in a system.
Gas (or Carrier gas) - Typically Helium or Hydrogen used to move the sample through the system.
Halogenated - A compound containing one or more of the halogen elements (fluorine, chlorine, bromine, iodine). Mobile Phase - The moving part of all chromatography, in this case a gas.
Packed - Columns have the stationary phase in a glass or metal tube around ¼ of an inch in diameter.
Stationary Phase - The portion of chromatography that interacts with the sample, separating components of the sample from each other, while not moving.
Volatile - Is to evaporate easily at normal temperatures and pressures.
Column - Thin tube, lined/filled with the stationary phase. Capillary columns are typically made from glass and purchased for uniformity in quality. Packed columns can be purchased or packed by hand.
Detector - A device used to measure or record changes in a system.
Gas (or Carrier gas) - Typically Helium or Hydrogen used to move the sample through the system.
Halogenated - A compound containing one or more of the halogen elements (fluorine, chlorine, bromine, iodine). Mobile Phase - The moving part of all chromatography, in this case a gas.
Packed - Columns have the stationary phase in a glass or metal tube around ¼ of an inch in diameter.
Stationary Phase - The portion of chromatography that interacts with the sample, separating components of the sample from each other, while not moving.
Volatile - Is to evaporate easily at normal temperatures and pressures.