Gas Analysis Glossary

The following are a list of terms that could of assistance to you in understanding Gas Chromatography better.

Adsorbent
Solid, granular material used to pack columns and on the surface of which sample components are held by adsorptive forces.

Abort
Cancels run. Increases temperature to highest programmed temperature then resets program.

Air Peak
The peak of a substance which is not retarded by the column material, thus indicating the time necessary for the carrier gas to travel from the injection point to the detector. A "marker".

Ambient Temperature
The temperature outside the chromatographic system.

Attenuator
An electrical element containing a series of resistances which may be selected by a switch to produce the reduction of input voltage to a recorder by a fixed factor.

Band
see Peak.

Baseline
The portion of the chromatogram recording the detector response when only the mobile phase emerges from the column.

Bonded Phase
A stationary phase which is covalently bonded to the support particles or to the inside wall of the chromatographic column.

Bypass Injector
A device in which the sample is first introduced into a chamber (loop) temporarily isolated from the mobile phase system by valves which can be changed so as to make an instantaneous switch of the mobile phase stream through the chamber to carry the sample to the column.

Capillary Column (Open Tubular Column)
A column made of fused silica, glass, or stainless steel with an internal diameter equal to or less than 0.53 mm. The tube wall is coated with the stationary phase and there is an open unrestricted path for the mobile phase.

Carrier Gas
The gas, usually helium, nitrogen, argon, or hydrogen which consistantly flows from a pressurized source through the column, transporting components of the sample with it.

Chromatogram
A graphical or other presentation of detector response, concentration of analyte in the effluent or other quantity used as a measure of effluent concentration vs. effluent volume or time.

Chromatograph (verb)
To separate by chromatography.

Chromatograph (noun)
The assembly of apparatus for carrying out chromatographic separation.

Chromatography
A physical method of separation in which the components to be separated are distributed between two phases one of which is stationary (stationary phase) while the other (the mobile phase) moves in a definite direction.

Column
A metal or glass tube packed or internally coated with the column material through which the sample and carrier gas flow and in which the chromatographic separation is performed.

Column Head Pressure
Pressure at the inlet (head) of the column.

Column Material
The material contained in the column. In adsorption chromatography, the column material is the adsorbent itself; in partition chromatography, the column material consists of a stationary phase distributed on inert solid support or coated on the inside wall of the column.

Column Oven
A precisely heated and controlled area containing the column(s), the temperature of which can be varied over a wide range.

Column Temperature
The temperature of the column under isothermal operation. Also known as Separation Temperature.

Component
Pure compound contained in the sample mixture.

Concentration Sensitive Detector
A detector whose response is proportional to the concentration of materials in the gas stream passing through it (as is the TCD or ECD), rather than the mass. Sensitivity is calculated per unit concentration in the mobile phase.

CONT
Continue with the temperature program after a HOLD.

Dead Volume
Volumes within the chromatographic system which are not swept by the mobile phase. Extra column volume.

Detector
A measurement device, usually electrical, which emits a signal in the presence of a component which is eluted from a chromatographic column.

Detector Sensitivity
The detector sensitivity is equal to the signal output per unit concentration or unit mass of a substance in the mobile phase entering the detector.

Detector Temperature
The temperature of the detector. In the case of a detector incorporating a flame, it refers to the temperature of the detector base.

Direct Injector
A device which directly introduces the sample into the mobile phase stream.

Drift
The average slope of the noise envelope expressed in microvolts, amperes or absorbance units per hour.

Dynamic Range
The dynamic range of a detector is that range of concentration or mass flow rates of a substance over which an incremental change is concentration or mass flow rate produces an incremental change in detector signal.

Effluent
The separated sample after having passed through the column. Also called eluant.

Elution
The transport of a component through and out of the column by the carrier gas.

External Standard
A compound present in a standard sample of known concentration and volume which is analyzed separately to the unknown sample under identical conditions. Alternatively, the volume of the external standard (standard sample) need not to be known if identical to that of the unknown sample.

FIA
Flame Ionization Amplifier board.

Filament
A type of thermal conductivity device in which a fine tungsten or similar wire is used as the variable resistance element in the cell chamber. Also known as Hotwire or Element.

Final Temperature
The highest temperature to which the column is programmed.

Flash Vaporization
A device in which a liquid sample is introduced into the carrier gas stream with simultaneous evaporation and mixing with the carrier gas prior to entering the column.

Flow Rates
The volume of mobile phase passing through the column in unit time.

Fronting
Asymmetry of a peak such that, relative to the baseline, the front is less steep than the rear.

Gas Chromatography (GC)
Collective noun for all chromatographic methods in which the mobile phase is a gas. Gas chromatography is always carried out in a column.

Gas-Liquid Chromatography (GLC)
Chromatographic method in which the stationary phase is a liquid distributed on a solid support. The separation is achieved by partition of the components of a sample between the phases.

Gas-Solid Chromatography (GSC)
Chromatographic method in which the stationary phase is an active solid (adsorbent). The separation is achieved by adsorption of the components of a sample.

Gas Sample Valve
A bypass injector permitting the introduction of a gaseous sample of a given volume into a gas chromatograph.

HOLD
Stop the timers, ramp increase and other parameters of the temperature program. Typically used when insufficient time has been allowed in the isothermal or programmed section, manually to give a bit more. Note that the separation process does not stop, and so the RUN TIME timer continues running

Injection Temperature
The temperature within the injection device.

Initial Temperature
The temperature of the chromatographic bed (column) at the start of the analysis. Temperature programming might start immediately upon sample introduction or be preceded by a short isothermal period.

Inlet Pressure
The absolute inlet pressure to a chromatographic column.

Integrator
A mechanical or electromechanical device for producing a continuous summation of detector output with respect to time, yielding a measurement of included area of a chromatographic peak.

Internal Standard
A compound added to a sample in known concentration to facilitate the qualitative identification and/or quantitative determination of the sample components.

Ionization Detectors
Chromatographic detectors in which the measurement of a sample is derived from the current produced by the ionization of sample molecules induced by thermal, radioactive, or other excitation sources. Examples: FID, DID, ECD, PID, NPD.

Isothermal Chromatography
A procedure in which the temperature of the column is kept constant during the separation.

Linear Range
The linear range of a chromatographic detector represents the range of concentrations or mass flows of a substance in the mobile phase at the detector over which the sensitivity of the detector is constant within a specific variation, usually ± 5%.

Liquid Phase
A liquid which is relatively nonvolatile at the column temperature and is sorbed on the solid support where it acts as a solvent for the sample. The separation achieved differs with differences in solubility of the various components of the sample in the liquid phase. This is also called the stationary phase or substrate.

Make-up gas
A gas supply, usually the carrier gas, which is added to the gas flow after the column to increase the volume flow through the detector, normally to reduce the effects of dead volumes.

Mass Sensitive Detector
A detector whose response is proportional to the mass of material passing through it. This means independent of the dilution. Sensitivity is calculated per unit mass of the test substance in the mobile phase entering the detector.

Marker
A reference substance chromatographed with the sample to assist in identifying the components.

Mobile Phase
A fluid which percolates through or along the stationary bed, in a definite direction. In gas chromatography, this fluid is a gas. The expression Carrier Gas may be used for the mobile phase.

Noise
The amplitude expressed in millivolts, amperes or absorbance units of the envelope of the baseline which includes all random variations of the detector signal of a frequency on the order of one or more cycles per minutes. In the case of photometric detectors, the amplitude may be expressed in absorbance units per unit cell length.

On-Column Injector
A device in which the sample is directly introduced into the column. In GC, the on-column injector permits the introduction of the liquid sample into the column without prior evaporation.

Open-Tubular Column
see Capillary Column.

Outlet Pressure
The pressure at the exit of the chromatographic column. Oven 1 Board Controls the first three heated zones as well as the Run Control controller and connector.

Oven Board 2
Controls the second three heated zones as well as the
External Timed Events controller and connector.

Overload
When the amount of material in the column exceeds its capacity. Overload normally results in flat topped or other poorly shaped peaks.

Packing
The active solid, stationary liquid plus solid support, or swollen gel contained in a column.

Packed Column
A column of internal diameter generally between 2 and 9 mm, which is packed with granules of an inactive support, coated with stationary phase.

Peak
The portion of a differential chromatogram recording the detector response when a single component elutes from the column. If separation is incomplete, two or more components may elute as one unresolved peak.

Porous-Wall
Column in which the inside wall is porous. This porosity Open-Tubular can be achieved by either chemical means (e.g. etching) or Column (PLOT) by the deposition of porous particles on the wall from a suspension. The porous layer may serve as a support for a liquid stationary phase or as the stationary phase itself.

Potentiometric Recorder
A readout device in which a pen, whose deflection is proportional to the voltage output of the chromatographic detector, writes on paper chart which is moving at a constant speed.

Programmed Chromatography
A procedure in which the temperature of the column is changed systematically during a part or the whole of the separation. Commonly used in the analysis mixtures with a high range of boiling points. Also referred to as"Temperature Ramping".

Program Rate
The rate of increase in column temperature. The rate of temperature increase is usually linear (°C/min.) but it may also be nonlinear (e.g. ballistic temperature program). During one analysis the temperature rate may be changed and/or the temperature programming may be interrupted by an isothermal period (Multiple Programming). In multiple programming each program must be specified by its initial and final temperatures and program rate.

Reset
Stop the temperature program where it is and immediately return as quickly as possible to the initial temperature and valve/timed events positions. Typically used when an injection has been made and failed to get material into the column. Also see Abort.

Resolution
A measure of the quality of separation between peaks.

Retention Time
The elapsed time from injection of the sample to recording of the peak maximum of a component band.

Run time
The length of time between injection and the emergence of the last peak in the analysis.

Sample
A mixture consisting of a number of components the separation of which is attempted on the chromatographic bed (column) as they are carried or eluted by the mobile phase. One of the sample components may be a solvent. The sample may also consist of a single component.

Sample Components
The chemically pure constituents of the sample. They may be unretained (that is not delayed by the stationary phase, partially retained (that is eluted at different times) or retained permanently.

Sample Injector
A device by which a liquid, solid or gaseous sample is introduced into the mobile phase stream. The sample can be introduced directly into the carrier gas stream, or into a chamber (loop) temporarily isolated from the system by valves which can be changed so as to instantaneously switch the gas stream through the chamber.

Segment
One of ten possible parts of a temperature program, each consisting of a ramp rate, an upper temperature and an upper time, for which the temperature will be held.

Selective Detector
Responds to a related group of sample components in the column effluent.

Separation
The elapsed time between elution of two successive components, measured on the chromatogram as distance between the recorded peaks.

Shutdown
Cools the oven down in a controlled way, i.e. the fans remain running, the vent door is open, and the heaters and detectors are turned off.

Solid Support
A solid that holds the stationary phase but, ideally, does not contribute to the separation process.

Solute
see Sample Components.

Span
The signal level (# of millivolts) required to produce a change in the deflection of the recorder pen from 0 to 100% on the chart scale.

Specific Detector
Responds to a single sample component or to a limited number of components having similar chemical characteristics.

Split Injection
A way to inject small samples onto a capillary column without overloading the column. The samples for this technique are usually quite concentrated. As the sample is injected, a small part of it goes onto the column. The remaining majority of the sample is vented through the split vent.

Splitless
Capillary injection technique for the introduction of trace level samples onto a capillary column. To make and injection, the split valve is closed and held closed for a period of 0.3 to 1.0 minutes after the injection is made. The sample is concentrated on the head of the analytical column. The split valve is then opened for the rest of the analysis to flush the remaining sample from the instrument.

Splitter
A T-fitting attached to the column to divert a portion of the carrier gas flow. An inlet splitter allows introduction of very small samples to a capillary column. An effluent splitter may be used to permit parallel operation of two detectors.

Stationary Phase
The stationary phase is one of the two phases forming a chromatographic system. It may be a solid, a gel, or a liquid. If a liquid, it is distributed on a solid. This solid may or may not contribute to the separation process. The liquid may also be chemically bonded to the solid (see Bonded Phase).

Support-Coated Open-Tubular Column
A version of PLOT columns in which the porous layer serves as a support and was deposited from a suspension.

Tailing
Asymmetry of a peak such that, relative to the baseline, the front is steeper than the rear.

Temperature Programming
see Programmed Analysis.

TCA
Thermal Conductivity Amplifier board.

Thermal Conductivity
A physical property of materials which is their ability to conduct heat from a warmer to a cooler object.

Triac Board
Heater supply board.

Wall-Coated Open-Tubular Column (WCOT)
Column in which the liquid stationary phase is coated on the essentially non-modified smooth inside wall of the tube.