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.
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.
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.
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.
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
- Degassing: the process
of removing dissolved gases from the mobile phase by sparging or applying
- 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.
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
- Endcapping: the
process of covering the exposed silanol groups on the stationary phase of
- Fixed Wavelength Detector: an
absorbance detector that can be used to measure absorbance at several discreet
wavelengths, but only one at a time.
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.
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
- 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
- Response Factor: A
ratio of signal to sample size used to characterize a detector response.
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
- 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
- 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.
- Wall-Coated Open Tubular
(WCOT): The term used to describe capillary columns.
to Skill Development Module L6.13 Gas Chromatography