Definitions of Terms

Gas Chromatography (GC) 
A technique used to resolve a mixture of compounds for the purpose of identification and quantitation. Suitable for compounds that capable of generating a vapour pressure when heated up to a temperature of 400°C.  Separation affected by interaction between a solid or liquid stationary phase and a gaseous mobile phase.

Multi-dimensional Gas Chromatography (GCxGC)
The total output of one capillary column is transferred in short pulses onto a second much shorter column, usually of a radically different stationary phase polarity.  This provides a huge increase in column efficiency/resolving power, equivalent to the product of the two columns in use. A fast-scanning detector is required for this technique as very narrow peaks are produced.

Multi-dimensional Gas Chromatography using Time-of Flight Mass Spectrometry as the detector (GCxGC ToF MS). (see GCxGC and GC-ToF MS)

Gas Chromatography – Mass Spectrometry (GC-MS)
An example of a “hyphenated” technique combining two analytical methods.  A Gas chromatograph (GC) used to separate mixtures of compounds is linked to a mass spectrometer (MS) which acts as a detector for the resolved compounds.

Gas Chromatography-Time of Flight Mass Spectrometry (GC-ToF-MS)
An example of GCMS using a specific design of mass spectrometer known as Time-of-Flight. This type of mass spectrometer is capable of either very high scan speeds, or very high accurate mass measurements, but not both simultaneously.

Multi-dimensional Gas Chromatography (Heart-Cutting)
In this technique only a small fraction of eluent (ca 5 secs worth) is transferred (“cut”) from the primary analytical column onto the second analytical column. Data is acquired from two detectors one each for the primary and secondary columns eluent, which are usually of greatly different polarity to enhance resolution of the “cut” fraction. (See also GCxGC)

Large Volume Injection (LVI)
Technique used to inject large amounts of sample onto a gas chromatography column, where the volume may be typically >4 – 5000µl.  Theoretically there is no upper limit to the injection volume but logistics and mechanics impose actual limits.

Programmed Temperature Vaporising (PTV) Injector is a temperature programmed sample introduction inlet allowing greater flexibility for sample introduction in GC. Hot or Cold injection, Large Volume injection and positive discrimination of sample injections can all be achieved by control of the inlet temperature, gas flows and split valve positions throughout the sample transfer process. Some PTV inlets also enable in-liner derivatisation, thermal desorption and Pyrolysis.

Selective Discrimination
Using a combination of temperature, gas flow and vent valve position to control the transfer of sample components either onto the capillary column or discharge through the vent valve.  

Thermal Desorption      
Process of releasing compounds from a sample matrix by the application of heat in an inert atmosphere.  Temperatures do not exceed 350°C so no chemical bonds are broken, only vaporisation occurs.

Pyrolysis     
Sample is subjected to a heating profile at temperatures above 350°C in a stream of inert gas.  These temperatures cause chemical bonds to break, producing smaller molecular fragments which may then be analysed. Applied to polymers and macromolecules.

Static Head Space (SHS)
Solids and liquids exist in equilibrium with a vapor phase at the solid/air, or liquid/air interface.  The position of the equilibrium is determined by several variables (temperature, pressure, etc.).  If the material is in an enclosed container, the remaining void not occupied by the material is termed the “headspace” and will reach an equilibrium vapor concentration that is representative of and proportional to the source material. It is then possible to sample and analyse this headspace in place of the parent material. This technique is most appropriate for volatile compounds that produce an appreciable vapor pressure. In SHS analysis a fixed aliquot of the headspace is removed for analysis.

Dynamic Head Space (DHS) 
Solids and liquids exist in equilibrium with a vapor phase at the solid/air, or liquid/air interface.  The position of the equilibrium is determined by several variables (temperature, pressure etc).  If the material is in an enclosed container, the remaining void not occupied by the material is termed the “headspace” and will reach an equilibrium vapor concentration that is representative of and proportional to the source material. It is then possible to sample and analyse this headspace sample in place of the parent material. This technique is most appropriate for volatile compounds that produce an appreciable vapor pressure. In DHS the headspace volume is continually flushed onto a trap where analytes are concentrated and then released for analysis and is therefore more sensitive than SHS.
 
Purge and Trap (P&T) is a concentration technique used for volatile solutes.  An inert gas is bubbled through a sample to purge all volatile components onto an adsorptive trap which is then heated to desorb the trapped components onto a GC column for analysis.  Removes all the volatile component present, rather than an equilibrium portion, and so the most sensitive volatile techniques

Solid Phase Extraction (SPE) is a sample preparation method that concentrates analytes out of solution onto a solid-phase substrate. This is then followed by extraction or desorption of the analyte from the substrate using a much smaller portion of an appropriate solvent. Can also separate analytes from matrix components as a “clean-up” technique.  Concentration factors up to 2000-5000x. (1 litre sample to 200ul extract)

Solid Phase Micro-Extraction (SPME)
Single fiber samplers are used to collect volatile and semi-volatile analyses.  Fibers are coated with a range of stationary phases similar to those used in capillary columns, in a range of thicknesses (capacities).  Samples are taken by exposing the fibre to atmosphere or immersing in a liquid and then desorbed in the injection inlet of a GC.

Stir Bar Sorptive Extraction (SBSE)
Similar to the SPME technique but applied to a stir bar in place of a single fiber.  The stir bar has a much greater surface area and the phase is much thick allowing a greater adsorptive capacity and thus greater sensitivity.  The SBSE bar is added to a sample and allowed to “stir”. After sampling it is removed, rinsed, dried and placed in a TD tube for analysis by TD-GC.