Gas Chromatography: An Introduction
Gas chromatography is an analytical technique used to separate and analyze samples that can be vaporized without thermal decomposition. Gas chromatography (GC) refers to any chromatographic procedure in which the moving phase is a liquid, in contrast to the moving gas phase of liquid chromatography (LC). The instrument that performs gas chromatography is called a gas chromatograph. The resulting graph that shows the data is called a gas chromatogram.
Applications of Gas Chromatography
Gas chromatography is a unique and versatile technique. As an analytical tool, GC can be used for the direct separation of gaseous samples, liquid solutions,and volatile solids. It is typically coupled with mass spectrometry (MS) when information other than a comparative fingerprint is required, such as positive identification of peaks on a chromatogram.
GC is used as one test to help identify components of a mixture and determine their relative concentration. One significant application is in the area of preparation of pure substances, or narrow fractions as standards for further investigations. Additionally, gas chromatography can be used to determine vapor pressure, heat of solution, and activity coefficients.
GC is often used by industries at scale to monitor processes by testing for contamination and to ensure a manufacturing process is going as planned. Chromatography can test blood alcohol, drug purity, food purity, and essential oil quality. GC may be used on either inorganic or organic analytes, but the sample must be volatile. Ideally, the components of a sample should have different boiling points.
Example Applications of Gas Chromatography
- Drugs and Pharmaceuticals – GC is used not only in the quality control of this field but also in the analysis of new products and the monitoring of metabolites in biological systems.
- Environmental Studies – GC is often used for air pollution analyses. Many chronic respiratory diseases (asthma, lung cancer, emphysema, and bronchitis) could be results of air pollution. Air samples can be very complex mixtures, and GC is unique adaptable to the separation and analysis of such mixtures.
- Petroleum Industry – The petroleum companies were among the first to make widespread use of GC. The technique was successfully used to separate and determine the components of various petroleum products.
- Clinical Chemistry – Gas chromatography is adaptable to such samples as blood, urine, and other biological fluids. Compounds such as proteins, carbohydrates, amino acids, fatty acids, steroids, triglycerides, vitamins, and barbiturates are handled by this technique directly or after preperation of appropriate volatile derivatives.
- Pesticides and their Residues – GC in combination with selective detectors such as electron capture, phosphorus, and electrolytic conductivity selectors have made the detection of pesticides and their measurement relatively easy.
- Foods – The determination of antioxidants and preservatives in food is an extremely active part of the GC field. Adaptations and sample types are almost limitless.
How Gas Chromatography Works
- A liquid sample is prepared.
- The sample is mixed with a solvent and is injected into the gas chromatograph. Although the sample starts out as a liquid, it is vaporized into the gas phase. An inert carrier gas is also flowing through the chromatograph. This gas shouldn’t react with any components of the mixture.
- The sample and carrier gas are heated and enter a long tube, which is typically coiled to keep the size of the chromatograph manageable. Common carrier gases include argon, helium, and sometimes hydrogen. The tube may be open (called tubular or capillary) or filled with a divided inert support material (a packed column). The tube is long to allow for a better separation of components.
- At the end of the tube is the detector, which records the amount of sample hitting it. In some cases, the sample may be recovered at the end of the column, too.
- The signals from the detector are used to produce a graph, the chromatogram, which shows the amount of sample reaching the detector on the y-axis and generally how quickly it reached the detector on the x-axis (depending on what exactly the detector detects).
- The chromatogram shows a series of peaks. The size of the peaks is directly proportional to the amount of each component, although it can’t be used to quantify the number of molecules in a sample. Usually, the first peak is from the inert carrier gas and the next peak is the solvent used to make the sample. Subsequent peaks represent compounds in a mixture. In order to identify the peaks on a gas chromatogram, the graph needs to be compared a chromatogram from a standard (known) mixture, to see where the peaks occur.
At this point, you may be wondering why the components of the mixture separate while they are pushed along the tube. The inside of the tube is coated with a thin layer of liquid (the stationary phase). Gas or vapor in the interior of the tube (the vapor phase) moves along more quickly than molecules that interact with the liquid phase. Compounds that interact better with the gas phase tend to have lower boiling points (are volatile) and low molecular weights, while compounds that prefer the stationary phase tend to have higher boiling points or are heavier. Other factors that affect the rate at which a compound progresses down the column (called the elution time) include polarity and the temperature of the column. Because temperature is so important, it is usually controlled within tenths of a degree and is selected based on the boiling point of the mixture.