Gas chromatography (GC) is an analytical technique used to separate and analyze complex chemical mixtures. The method is designed for compounds that can be easily vaporized without breaking down, making it suitable for analyzing volatile organic compounds, gases, and semi-volatile substances. Separation occurs as the sample is carried by an inert gas through a long tube, where components are physically and chemically separated based on their properties. GC allows scientists to determine the identity and relative amounts of substances present.
Essential Components of a Gas Chromatograph
The process begins when a liquid sample is introduced into the instrument through the injector port. This port is a heated block that immediately vaporizes the sample into a gaseous state. The vaporized sample is then swept into the separation unit by a continuous flow of an inert mobile phase, known as the carrier gas, typically helium or nitrogen.
The heart of the instrument is the column, a long, coiled tube housing the stationary phase—a thin layer of non-volatile liquid or polymer coated on the inside wall. The column is contained within a temperature-controlled oven that maintains the thermal conditions necessary for separation. As separated compounds exit the column, they flow into a detector, which registers their presence and sends an electrical signal to a computer.
The Principle of Differential Partitioning
The separation of compounds relies on a continuous process known as differential partitioning, which occurs inside the column. This principle describes how each chemical component constantly distributes itself between the two phases: the moving carrier gas (mobile phase) and the stationary liquid coating. The time a compound spends in each phase governs how quickly it travels through the column.
Volatility
A compound’s volatility, or how easily it remains in the gas phase, influences its movement. More volatile components spend more time dissolved in the mobile gas stream, allowing them to be carried quickly toward the detector. Conversely, less volatile components tend to condense onto the stationary phase more often, slowing their progress.
Chemical Affinity
The chemical affinity of a compound for the stationary phase is another factor in this partitioning equilibrium. Components with a strong chemical attraction to the stationary phase spend a greater proportion of time adsorbed to the coating, moving slowly. For instance, a polar compound will have a stronger interaction with a polar stationary phase, increasing its retention time compared to a nonpolar compound.
These differences in volatility and chemical affinity cause the components of the initial mixture to travel at distinct speeds. Components that interact most weakly with the stationary phase or are highly volatile exit the column first. This achieves complete separation of the mixture.
Reading the Results
The final output of a gas chromatography analysis is a graph called a chromatogram, which plots the detector’s response on the vertical axis against time on the horizontal axis. Every separated compound that exits the column and passes through the detector generates a distinct signal, which appears on the graph as a peak. The number of peaks in the chromatogram directly indicates how many different compounds were successfully separated from the original mixture.
The identification of each separated compound relies on its specific retention time, which is the time elapsed from the moment the sample was injected until the peak for that compound reaches its maximum height on the chromatogram. Under identical operating conditions, such as temperature and carrier gas flow rate, a given compound will always exhibit the same retention time. Analysts compare the retention times of unknown peaks to those of known reference standards to confirm the identity of the substances in the sample.
Beyond identification, the size of the peak provides information about the quantity of the compound present. The area under a peak is directly proportional to the amount of that specific substance that passed through the detector. By measuring this area and comparing it to a calibration curve prepared from samples of known concentration, scientists can accurately determine the concentration of each component in the original mixture.