Process Gas Chromatography Glossary


Accuracy: The difference between the maximum and minimum measurements of 5 analyses of a known standard. Max Deviation (%) – Min Deviation (%)


BTEX: Benzene, Toluene, Ethylbenzene and Xylene. Aromatic hydrocarbons. Common carcinogenic VOC’s frequently found in combustion exhaust.


Carrier gas: A pure gas used to drive sample through the GC process. Typically Nitrogen, Hydrogen or Helium

Chromatogram: The graphical results from a GC analysis.

Column: The portion of a gas chromatograph which allows various gasses present in a sample to separate into their constituent parts. See our FAQ titled: How does a GC work / what’s the difference between total and GC analysis

Concentration (%, ppm, ppb, ppt): Per Cent, Parts per Million, Parts per Billion and Parts per Trillion. 10% = 100,000 ppm. 10 ppm = 10,000 ppb. 10 ppb = 10,000 ppt


The difference in response to a known standard measured 24 hours apart. [( Initial Cal – 24 Hour Cal ) / ( Cal Value )] * 100

Dwell: The period of time that a multi-point sampling system spends monitoring a given point.


eV: Electron Volt. A measure of the energy of a photon. Closely related to the frequency and wavelength of light.


Flame-Ionization Detector. A gas detector which uses a hydrogen flame to displace and collect electrons from a sample. Typically used for hydrocarbons.


GC: Gas Chromatograph. A device used to measure both composition and concentration of a gas sample


Ionization Potential: A measure of energy required to free an electron from a given atom or molecule. See eV.


MDQ: Minimum Detectable Limit. The lowest concentration a given detector can quantify.

The Analytical Method. The method defines all the fixed parameters within a given analysis. Column temperature, carrier gas flow rate, injection timing, concentration alarm levels and calibration information are all examples of Method parameters.


Permanent Gas: A class of gasses which will condense except at extremely low temperatures. E.G.: Hydrogen, Oxygen, Nitrogen and Carbon Monoxide.

Photo-Ionization Detector. A gas detector which uses high energy ultra-violet light to displace and collect electrons from a sample. Typically used for VOCs.

Programmable Logic Controller / Data Collection System. Computerized devices which collect information from various locations in a process and make control decisions based on this information

Pounds per Square Inch. A measure of gas pressure commonly used in the US


Repeatability: The % Relative Standard Deviation of the measurements of 5 consecutive measurements. [ Std Dev ( Conc1 … Conc5 ) / Average ( Conc1 … Conc5 ) * 100


Sample loop: A length of tubing used to collect and inject a fixed volume of sample into a column. See our FAQ titled: How does a GC work / what’s the difference between total and GC analysis

Multi-port Sequence. The sequence defines which sample locations will be monitored and in what order. This gives the user flexibility to monitor higher risk areas more frequently.


TCD: Thermal Conductivity Detector. A universal gas detector which measures the difference in thermal conductivity between a sample and reference gas stream to determine sample content.

Total Hydrocarbon Analyzer: A device used to measure the total hydrocarbon content of a gas sample. This device does not measure composition of the sample.


VOC: Volatile Organic Compound. A broad class of chemicals which evaporate easily at room temperature and typically have ionization potentials below 10.6 eV. See BTEX for some examples.


WITS: Wellsite Information Transfer Standard. A data protocol standard used to provide real time data to oil exploration drilling rigs.