Gas chromatography

Gas chromatography
Name	Gas chromatography
Caption	A simplified diagram of a gas chromatograph
Acronym	GC
Classification	Chromatography
Analytes	Organic compounds, Inorganic compounds, Gases
Manufacturers	Agilent Technologies, Shimadzu, Thermo Fisher Scientific, PerkinElmer
Related	High-performance liquid chromatography, Mass spectrometry, Gas chromatography–mass spectrometry

Gas chromatography. It is an analytical technique used to separate and analyze compounds that can be vaporized without decomposition. The method involves a sample being carried by a mobile phase of inert gas through a column containing a stationary phase, leading to separation based on differential partitioning. The separated components are then detected and quantified, providing data used in fields from forensic science to environmental monitoring.

Principles and theory

Separation in this technique is governed by the differential distribution of analytes between the mobile gas phase and the stationary phase within the column. The theoretical foundation is described by the van Deemter equation, which relates column efficiency to the linear velocity of the carrier gas, considering factors like eddy diffusion and mass transfer. Key thermodynamic parameters, such as the partition coefficient, determine how long a compound is retained, a value known as its retention time. The process is influenced by temperature, often controlled via a programmed oven, and the chemical nature of the stationary phase, which can range from polydimethylsiloxane to polyethylene glycol.

Instrumentation

A standard system consists of several key components. The injector, such as a split/splitless injector, introduces the vaporized sample into the carrier gas stream, often helium or hydrogen. The column, housed in a temperature-controlled oven, is where separation occurs; common types include packed columns and capillary columns coated with a stationary phase. Following separation, the detector generates a signal; widely used detectors are the flame ionization detector, the thermal conductivity detector, and the electron capture detector. The resulting chromatogram is processed by a data system from manufacturers like Agilent Technologies or Waters Corporation.

Methodology and operation

A typical analysis begins with sample preparation, which may involve derivatization to increase volatility or extraction techniques like solid-phase microextraction. The operator sets parameters on the instrument, including injector temperature, oven temperature program, and carrier gas flow rate. After injection, the separated components elute from the column at characteristic times and pass to the detector. The identification of compounds is often achieved by comparing retention times to those of known standards analyzed under identical conditions, a principle central to analytical chemistry.

Applications

This technique has broad utility across numerous scientific and industrial fields. In petroleum analysis, it is used for characterizing hydrocarbons and in simulated distillation. Environmental laboratories employ it to detect pesticides, polychlorinated biphenyls, and volatile organic compounds in air and water. Within pharmaceutical development, it aids in drug purity testing and residual solvent analysis. It is also indispensable in forensic toxicology for screening biological samples, in food safety for analyzing flavor compounds and contaminants, and in chemical synthesis for monitoring reactions.

Types and variations

Several specialized forms of the technique exist. Gas chromatography–mass spectrometry couples the separation power with the identification capability of mass spectrometry, a standard tool in laboratories like those at the United States Environmental Protection Agency. Multidimensional gas chromatography, such as heart-cutting or comprehensive two-dimensional gas chromatography, uses two columns with different phases for complex mixtures. Other variants include pyrolysis gas chromatography for analyzing non-volatile materials like polymers, and inverse gas chromatography used for studying surface properties of solids. The choice of detector also defines specific methods, such as using a nitrogen–phosphorus detector for nitrogen-containing compounds.

History and development

The origins trace back to the early 20th century with the work of Mikhail Tsvet on chromatography. The foundational work for the gas-based method was performed in the 1940s and 1950s; key pioneers include Archer John Porter Martin and Richard Laurence Millington Synge, who were awarded the Nobel Prize in Chemistry in 1952 for their invention of partition chromatography. The development of the capillary column by Marcel Golay in the late 1950s, promoted by companies like PerkinElmer, dramatically improved resolution. Subsequent decades saw the commercialization of robust instruments by Hewlett-Packard and the critical hyphenation with mass spectrometry, advancing fields from metabolomics to space exploration, such as the Viking program's analysis of the Martian soil.

Category:Chromatography Category:Analytical chemistry