Gas Chromatography- Definition, Principle, Parts, Steps, Uses
Gas chromatography (GC) is a powerful and widely used analytical technique for separating and identifying different components of a mixture based on their volatility and affinity for a stationary phase. GC can be used to analyze organic and inorganic compounds that can be vaporized without decomposition, such as gases, liquids, and low-molecular-weight solids. GC can also be used to prepare pure compounds from a complex mixture by collecting the fractions of interest as they elute from the column. GC involves injecting a small amount of sample, either in gaseous or liquid form, into a stream of carrier gas (usually an inert gas such as helium, nitrogen, or hydrogen) that flows through a column containing a stationary phase. The column is typically a narrow tube made of metal or glass that is coated or filled with a liquid or solid material that acts as the stationary phase. The column is usually enclosed in an oven where the temperature can be controlled.
The principle of gas chromatography is based on the separation of the components of a sample between a mobile gas phase and a stationary phase. The mobile gas phase, also called the carrier gas, is usually an inert or unreactive gas such as helium, nitrogen, hydrogen or argon. The stationary phase can be either a solid or a liquid that is coated on a solid support. The separation of the components depends on their relative affinity for the stationary phase and their volatility. The more volatile a component is, the less time it spends in the column and the faster it elutes. The less volatile a component is, the more time it spends in the column and the slower it elutes. The affinity for the stationary phase is determined by the intermolecular interactions and the polarity of the component and the stationary phase. The more polar a component is, the more it tends to interact with a polar stationary phase and the longer it stays in the column. The less polar a component is, the more it tends to interact with a non-polar stationary phase and the shorter it stays in the column. The separation process can be described by an equilibrium constant called the distribution constant (Kc) or the partition coefficient, which is defined as the ratio of the concentration of a component in the stationary phase to its concentration in the mobile phase:
Kc = s/m
where s is the concentration of component A in the stationary phase and m is the concentration of component A in the mobile phase. The distribution constant controls the movement of the components through the column and determines their retention time (Rt), which is the time taken for a component to elute from the column. The higher the distribution constant, the longer the retention time and vice versa. The retention time can be used to identify and quantify the components by comparing them with known standards. The separation efficiency of gas chromatography depends on several factors, such as:
- The type and quality of the stationary phase
- The type and flow rate of the carrier gas
- The temperature and length of the column
- The injection technique and sample size
- The type and sensitivity of the detector
Gas chromatography is mainly composed of the following parts:
- Carrier gas: This is the mobile phase that carries the sample through the column. The carrier gas should be inert, chemically stable, and compatible with the detector. The most commonly used carrier gases are helium, nitrogen, hydrogen, and argon. The carrier gas is supplied from a high-pressure cylinder with pressure regulators and flow meters to control the flow rate and pressure of the gas. The flow rate and pressure of the carrier gas affect the separation efficiency and retention time of the sample components.
- Sample injection system: This is where the sample is introduced into the carrier gas stream. The sample can be a gas or a liquid that is vaporized in the injection port. The injection port is a heated metal block that has a septum through which a syringe needle can pierce and inject the sample. The injection port should be hot enough to vaporize the sample quickly and completely, but not so hot that it causes thermal degradation of the sample components. The injection port also has a split or splitless valve that controls how much of the sample enters the column. In split mode, only a fraction of the sample goes into the column and the rest is vented out. This mode is used for samples with high concentrations or low boiling points. In splitless mode, all of the sample goes into the column. This mode is used for samples with low concentrations or high boiling points.
- Column: This is where the separation of the sample components takes place. The column is a long and narrow tube that contains the stationary phase. The stationary phase can be either a solid adsorbent (gas-solid chromatography) or a liquid coated on a solid support (gas-liquid chromatography). The column can be either packed or capillary. A packed column is filled with small spherical inert particles that are coated with the stationary phase. A capillary column is a thin-walled tube that has the stationary phase bonded to its inner surface. The column is placed inside an oven that controls its temperature during the separation. The temperature of the column affects the partitioning of the sample components between the stationary and mobile phases, and thus their retention times and separation efficiency.
- Detector: This is where the eluted sample components are detected and measured as they exit the column. The detector generates an electrical signal that is proportional to the concentration of the sample component in the carrier gas stream. The detector should be sensitive, selective, linear, and fast enough to respond to the eluted peaks. There are many types of detectors for gas chromatography, such as flame ionization detector (FID), thermal conductivity detector (TCD), electron capture detector (ECD), mass spectrometer (MS), etc. Each detector has its own advantages and limitations depending on the nature of the sample components and their detection limits.
- Recorder: This is where the output signal from the detector is recorded and displayed as a chromatogram. A chromatogram is a plot of detector signal versus time or distance along the column. The chromatogram shows peaks that correspond to different sample components based on their retention times and peak areas or heights. The recorder can be either an analog device that uses a pen and paper, or a digital device that uses a computer and software. The recorder allows for qualitative and quantitative analysis of the sample components by comparing their retention times and peak areas or heights with those of known standards or reference materials.
The procedure of gas chromatography involves four main steps: sample injection, vaporization, separation, and detection.
- Sample injection: A small amount of liquid or gaseous sample is injected into a heated injection port using a syringe or a valve. The injection port is connected to a carrier gas stream, which is an inert or unreactive gas such as helium, nitrogen, hydrogen, or argon. The carrier gas serves as the mobile phase that transports the sample into the column.
- Vaporization: The sample is rapidly vaporized in the injection port by heating it above its boiling point. The vaporized sample then mixes with the carrier gas and enters the column.
- Separation: The column is a long and narrow tube that contains a stationary phase, which can be a solid or a liquid coated on a solid support. The column is placed inside an oven that controls its temperature during the separation. The sample components are separated based on their different affinities for the stationary and mobile phases. Components that have a higher affinity for the stationary phase will spend more time in the column and elute later than components that have a higher affinity for the mobile phase. The separation efficiency depends on several factors, such as the column type, length, diameter, packing material, temperature, carrier gas flow rate, and pressure.
- Detection: The separated sample components exit the column and enter a detector that senses their presence and generates a signal. The detector can be based on different principles, such as thermal conductivity, flame ionization, mass spectrometry, or others. The detector signal is recorded as a function of time and produces a chromatogram that shows the peaks corresponding to each component. The peak area or height can be used to quantify the amount of each component in the sample.
The following code block shows an example of a chromatogram obtained by gas chromatography.
Time (min) | Detector Signal (mV) 0 | 0 1 | 0 2 | 0 3 | 5 4 | 10 5 | 15 6 | 10 7 | 5 8 | 0 9 | 0 10 | 0 11 | 20 12 | 40 13 | 60 14 | 40 15 | 20 16 | 0 17 | 0
The chromatogram shows two peaks, indicating that the sample contains two components. The first peak has a retention time of about 5 minutes and a peak height of 15 mV. The second peak has a retention time of about 13 minutes and a peak height of 60 mV. This suggests that the first component has a lower affinity for the stationary phase than the second component. It also suggests that the second component has a higher concentration in the sample than the first component.
Gas chromatography (GC) is a versatile technique that can be used to analyze a wide range of samples in various fields, such as environmental, clinical, pharmaceutical, biochemical, forensic, food science, and petrochemical. Some of the common applications of GC are:
|Manufacturing Quality Control
Gas chromatography is a powerful and widely used analytical technique for the separation and identification of volatile organic compounds. However, like any other method, it also has some advantages and limitations that need to be considered.
Advantages of Gas Chromatography
Some of the main advantages of gas chromatography are:
- High resolution: Gas chromatography can separate closely related compounds with high efficiency and precision, due to the large number of theoretical plates and the small diffusion coefficients of the analytes in the gas phase.
- High sensitivity: Gas chromatography can detect very low concentrations of analytes, in the range of picomoles or parts-per-billion, depending on the detector used. Some detectors, such as mass spectrometry or flame ionization, can provide selective and specific detection of certain compounds or functional groups.
- Fast analysis: Gas chromatography can perform rapid separations in a matter of minutes, by using high carrier gas flow rates, short columns, and high temperatures. This reduces the analysis time and the consumption of reagents and solvents.
- Wide applicability: Gas chromatography can analyze almost any type of organic compound that is volatile or can be made volatile by derivatization. It can also handle complex mixtures from various sources, such as environmental, biological, pharmaceutical, or industrial samples.
- Reliability: Gas chromatography is a mature and well-established technique that has been extensively validated and standardized. It can provide reproducible and accurate results with good precision and accuracy.
Limitations of Gas Chromatography
Some of the main limitations of gas chromatography are:
- Limited to volatile compounds: Gas chromatography is restricted to the analysis of compounds that can be vaporized without decomposition at the injection port. Non-volatile or thermally unstable compounds cannot be analyzed by gas chromatography, unless they are derivatized to increase their volatility or stability. This may introduce additional steps and errors in the analysis.
- Limited to low to medium molecular weight: Gas chromatography is not suitable for analyzing high molecular weight compounds, such as proteins, peptides, or polymers, because they have low vapor pressures and high boiling points. They also tend to adsorb strongly on the stationary phase or cause column degradation.
- Incompatible with aqueous samples: Gas chromatography cannot handle samples that contain water or other polar solvents, because they interfere with the carrier gas flow and the detector response. Aqueous samples need to be dried or extracted with organic solvents before analysis by gas chromatography.
- Thermal stability required: Gas chromatography requires high temperatures for vaporization and separation of the analytes. This may cause some compounds to decompose or react with the stationary phase or the carrier gas. The choice of the stationary phase and the carrier gas should be carefully considered to avoid unwanted reactions or losses of analytes.
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