GC Glossary

L6.13 GAS CHROMATOGRAPHY
Glossary

Below is a listing of terms commonly used when discussing Gas Chromatography. Links in the text on this page are to web pages that provide further explanations of the phenomena discussed or show pictures. Special thanks are due to the web site of Science Hypermedia, Inc., home of the on-line Encylopedia of Analytical Instrumentation, and the J&W Instruments GC Reference Site, for some of the links used below.

Analyte: a sample component that is chromatographed and measured (i.e. being analyzed). In ion chromatography, this is an ion or ionizable component.

Atomic Emission Detector: a detector that measures the emission from atomic lines of elements in the GC effluent that are excited in a microwave plasma.

Bulk Property Detector: a detector, such as a refractive index detector, that measures a property of both the solute and the mobile phase.

Calibration: a quantitative procedure performed with an integrator or data acquisition system in order to relate the known concentration of standard solutions of the components to the detector signal which is generated.

Capillary Column: open tubular fused silica columns which allow the separation of components in a mixture. For a picture, click here.

Carbon Dioxide: the most common mobile phase used in supercritical fluid chromatography.

Carrier Gas: the mobile phase in a gas chromatograph, typically hydrogen, helium or nitrogen.

Chromatogram: the output from a detector in most chromatographic techniques. A plot of detector response versus time.

Chromatography: a technique used to separate components in a sample mixture by the differential distribution of the components between a mobile phase and a stationary phase.

Column: the part of the instrument in which the separation occurs. May be capillary type of very small diameter and open tubular, with the stationary phase coated on the inner walls or may be packed type of larger diameter and packed with a solid adsorbent or solid support material coated with a liquid phase.

Component: also called solute, the compound in the sample mixture which one is interested in for the analysis.

Degassing: the process of removing dissolved gases from the mobile phase by sparging or applying a vacuum.

Detector: the part of the instrument that measures the solutes as they are eluted from the column.

Detection Limit: the minimum amount of an analyte that can be detected reliably.

Dynamic Range: sometimes known as linear dynamic range, the analyte concentration range over which response is a well defined (usually linear) function of the analyte concentration.

Efficiency: measurement of the sharpness of an eluting band or peak. High efficiencies indicate low diffusion and dispersion, which are major contributors to peak broadening. Can be calculated as theoretical plates.

Effluent: the mobile phase after it has passed through the analytical column.

Electron Capture Detector (ECD): very sensitive detector for analysis of halogenated compounds. As electronegative species pass through the detector, they capture low-energy electrons, causing a decrease in cell current.

Eluent: the mobile phase in a chromatographic separation.

Elution order: the order in which the components pass out of the column in the the process of separation.

Endcapping: the process of covering the exposed silanol groups on the stationary phase of the column.

Fixed Wavelength Detector: an absorbance detector that can be used to measure absorbance at several discreet wavelengths, but only one at a time.

Flame Ionization Detector (FID): a common type to detector used in gas chromatography and supercritical fluid chromatography.

Flame Photometric Detector (FPD): sulphur and phosphorus containing hydrocarbons burn in a flame, producing chemiluminescent species that are monitored at selective wavelengths.

Flowrate: the volumetric flowrate of the mobile phase.

Gas-Liquid Chromatography (GLC): a subclassification of gas chromatography where the mobile phase is a gas and the stationary phase is a liquid.

Gas-Solid Chromatography (GSC): a subclassification of gas chromatography where the mobile phase is a gas and the stationary phase is a solid.

HEPT (height equivalent to theorectcal plate):a calculated parameter which reflects the efficiency of a column.

Integration: a process for identifying and calculating the amount of a component.

Integrator: an electrical device commonly used to collect, perform calculations on, and display the detector output signal or chromatogram.

Internal standardization: A common method of calibration in quantitative chromatography where the response of a known substance that behaves in a manner similar to the analyte is ratioed to the analyte response in standards and unknowns to compensate for drift in instrumental parameters.

Isothermal Mode: elution of solutes using a single column or oven temperature throughout the separation.

Mass Selective Detector (MSD): molecules are bombarded with electrons, producing ion fragments that pass into the mass filter. The ions are filtered based on their mass/charge ratio. This technique yields excellent qualitative identification of components.

Mobile Phase: the liquid or gas phase that passes through the column carrying the sample through the instrument. In gas chromatography, also called carrier gas. Also called the solvent or eluent in other types of chromatography.

Nitrogen Phosphorus Detector (NPD): nitrogen and phosphorus containing hydrocarbons produce increased currents in a flame enriched by vaporized alkali salt.

Noise: the signal produced by a detector in the absence of a sample.

Packed column: a type of column made of glass or metal and is filled with particulates. For a picture, click here.

Peak: the graphical part of the chromatogram which represents the plot of detector signal versus time for a separated component (also called band or zone).

Peak Area: The area enclosed between the peak and the peak base.

PhotoIonization Detector (PID): Molecules are ionized by excitation with photons from an ultraviolet (UV) lamp. The charged particles are then collected, producing current.

Porous Layer Oper Tubular (PLOT): capillary columns with a solid stationary phase.

Qualitative Analysis: the determination of the identity of the components in the sample.

Quantitative Analysis: the determination of the amount or concentration of the components in the sample.

Resolution: a measurement of how well two chromatographic peaks are separated from each other.

Response Factor: A ratio of signal to sample size used to characterize a detector response.

Retention Time: the elasped time from the point of injection of the sample to the measurement of the maximum signal detected by the detector for a particular component.

Selectivity: for columns, the relative affinities of a pair of analytes for a stationary phase under a given set of chromatographic conditions. For detectors, the types of compounds which will cause a detector response.

Sensitivity: the relationship of analyte concentration to detector response. Mathematically, this is the slope of the linear plot of "detector response vs. analyte concentration."

Solid-Phase Extraction: a method of sample preparation that collects analytes from solution onto a solid-phase substrate in a disposable cartridge, followed by extraction of the analyte from the substrate with an appropriate solvent.

Specificity: the sensitivity that a detector has for an analyte compared to its sensitivity for another, potentially interfering, analyte.

Split Ratio: in capillary gas chromatography, the ratio of the amount of sample "split" or "thrown away" versus the amount that goes into the column for analysis.

Stationary Phase: the material inside the column which may be a solid or viscous liquid phase. Also refered to as the solvent in gas chromatography when the phase is a liquid.

Temperature Programming: elution of solutes using a temperature gradient of the column or oven temperature throughout the separation. Temperature programming can speed up an analysis without significant loss of resolution when there are both low and high volatility components in the sample.

Thermal Conductivity Detector (TCD): A filament temperature increases as analytes present in the carrier gas pass over it, causing the resistence to increase.

Wall-Coated Open Tubular (WCOT): The term used to describe capillary columns.

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