Gas chromatography

Gas chromatography (GC) is a type of chromatography that scientists use to separate and study substances that can turn into gas without breaking apart. It is often used to check how pure a substance is or to separate the parts of a mixture to see what’s inside. It is one of the most common and useful methods in analytical chemistry because it is very fast, sensitive, and accurate. In gas chromatography, the sample is first vaporized, which means it is turned into a gas. Then, an inert gas like helium or nitrogen carries the sample through a long tube called a column. Inside the column, there is a special coating called the stationary phase. As the different parts of the sample move through the column, they stick to the coating in different ways. Some parts move faster, and some move slower. Because of this, the parts get separated by the time they come out of the column. A detector at the end then finds and records each part, allowing scientists to know what’s in the sample and how much of it there is.^[1]

Gas chromatography is used in many industries and science fields, like medicine, environment testing, food safety, fuel research, and crime investigations. It works best on organic compounds that can turn into gas and would not break down from heat. Some common uses include finding pesticides in food, checking for pollution in the air, measuring alcohol levels in blood, studying perfumes and essential oils, and testing how pure a chemical is Because gas chromatography can find even tiny amounts of substances very accurately, it is very important for quality control and following safety rules in labs and industries.^[2]

A gas chromatograph is a machine made up of several important parts. These include a sample injection port, where the sample is added; a column, which can be packed or very thin like a capillary tube; an oven that controls the temperature; and a detector that finds and measures the substances coming out of the column. Two common types of detectors are the flame ionization detector (FID) and the thermal conductivity detector (TCD). The FID is very good at finding hydrocarbons, which are found in fuels and many organic compounds. The TCD can detect most gases and is used for general purposes. Some gas chromatography systems include mass spectrometry (GC-MS). This setup not only separates the different parts of a mixture but also helps identify exactly what each part is. Because of its powerful abilities, GC-MS is used often in forensic labs and environmental testing. Gas chromatography can be done in different ways. In isothermal runs, the oven stays at the same temperature the whole time. In temperature-programmed runs, the oven slowly gets hotter during the test. This helps separate substances that have different boiling points more clearly. Sometimes, scientists use a special process called derivatization. This changes substances that do not easily turn into gas, or are too polar, so they can be tested with gas chromatography. This lets scientists study an even wider range of chemicals.^[1]

Gas chromatography (GC) has many advantages, but it also has some limits. It only works well with substances that can be turned into gas without breaking apart. If a substance is too sensitive to heat or does not evaporate easily, then GC would not work well for it. In those cases, scientists use other methods, like liquid chromatography (LC), which is better for studying these types of substances. Even with its limits, GC is still a very important tool in science. New improvements in equipment, column design, and computer software keep making it better and more powerful for analyzing all kinds of mixtures.^[3]

In selecting a carrier gas it is important to select a gas that will not react with the components of the sample. The carrier gas should also be able to withstand high temperatures due to the oven. The flow rate of the carrier gas is important in that if it is too high there is not enough time for the sample to interact with the column and no separation will be seen, if it is too slow the experiment may take a very long time.

The main type of columns used in gas chromatography is a capillary column. Packed columns are created of fused silica or stainless steel. To increase separation of a gas sample columns must be created of a large length and are coiled to fit inside of the oven. Capillary columns are separated into two categories; wall coated open tubular and support coated open tubular. WCOT are capillary tubes with a thin layer of stationary phase, and in SCOT, the tube is lined with a thin film of support material. As the sample moves through the column the individual parts of the sample become contained with the material inside of the column and are then released. This action allows for a sample to become separated.

It is important that the oven is capable of maintaining a constant temperature. Since movement of the sample through the column relies on the boiling point of the sample being analyzed, the oven should be set to a temperature that is slightly higher than its boiling point. For samples with a large boiling range, a temperature program can be used, in which the column temperature is raised. By using a high temperature, the sample moves through the column faster, while at lower temperatures the sample moves slower, but better resolution is achieved.

Many types of detectors are used in gas chromatographic separations, with the most common are flame ionization, thermal conductivity, and mass spectrometry detectors. In a flame ionization detectors, the separated sample from the column is directed into a flame. By creating a voltage near the burner tip and the detector, the ions that are produced from the flame travel towards the detector. Flame detectors are not capable of detecting H₂O, CO₂, SO₂, and CO.

With a thermal conductivity detector, the sample from the column is passed into an area that is electrically heated. The thermal conductivity of the column is reduced when the sample passes over by. When this occurs the detector heats up and measures the change in resistance. The thermal conductivity detector is capable of detecting all types of compounds.

A mass spectrometer measures the mass to charge ratio of fragmented ions of a sample.^[4] The output of a column can be feed directly into the ionization chamber of the mass spectrometer. A mass spectrometry detector is capable of being able to obtain information from incompletely separated components. Two types of mass spectrometer detectors used are quadrupole and time of flight mass analyzers. In a quadrupole detector, a voltage is produced which makes ions of a specific mass to travel to the detector. In a time of flight mass analyzer, the speed of an ion is measured allowing the mass to charge ratio to be known.

Gas chromatography is regularly used to describe what is inside a complex sample. The information obtained from gas chromatography is placed into a graph of detector response versus the time the sample leaves the column. If the separate parts of a complex sample come out at different times far apart from one another it is possible to determine what came out of the column.

If a sample is compared to a standard calibration it is possible to know how much of a separated part of the sample makes up the sample. This is useful in monitoring quality of a product such as medicine, beverage, or perfume.