Gas Chromatography(GC) |
All chromatographic systems (paper, column, thin layer, gas, etc.) have two phases: a stationary phase and a moving phase. As the terms imply, the stationary phase is some kind of surface on which the sample is placed. The moving phase, true to its name, moves or flows over the stationary phase carrying some compounds with it and leaving some compounds behind. If the components of a mixture have varying tendencies to stick to the stationary phase or flow with the moving phase, they can be separated by chromatography, analyzed for identity and quantity, and even collected for further use since the process of separation has purified the original mixture into its components.
In the type of chromatography which we will use, gas chromatography (GC), the stationary phase is inside of a long piece of hollow metal tubing. The moving phase is helium gas.
A schematic diagram of the GC is shown below. Metal or glass tubing is filled with small, solid, insoluble particles coated with a high boiling liquid. This is the chromatographic column and is the stationary phase. The column is coiled and placed in an oven so that it can be maintained at temperatures above those of ambient conditions.
An inert gas such as helium is passed through the column as a carrier gas and is the moving phase. A sample is injected into a port which is much hotter than the column and is vaporized. The gaseous sample mixes with the helium gas and begins to travel with the carrier gas through the column. As the different compounds in the sample have varying solubility in the column liquid
and as these compounds cool a bit, they are deposited on the column support. However, the column is still hot enough to vaporize the compounds and they will do so but at different rates since they have different boiling points. The process is repeated many, many times along the column. Eventually the components of the injected sample are separated and come off of the column at different times (called "retention times").
There is a detector at the end of the column which signals the change in the nature of the gas flowing out of the column. Recall that helium is the carrier gas and will have a specific thermal conductivity, for example. Other compounds have their own thermal conductivities. The elution of a compound other than helium will cause a change in conductivity and that change is converted to an electrical signal. The detector, in turn, sends a signal to a strip chart recorder or to a computer. Detectors come in several varieties, for example, thermal detectors, flame-ionization and electron capture detectors.
The readout shows the order of elution (order of components coming off the column), the time of elution (retention time), and the relative amounts of the components in the mixture. The order of elution is related to the boiling points and polarities of the substances in the mixture. In general, they elute in order of increasing boiling point but occasionally the relative polarity of a compound will cause it to elute "out of order". This is why it is important to run a standard before using your sample.
| Compound | Boiling Point (0C) |
| pentane | 36 |
| hexane | 69 |
| cyclohexane | 80 |
| isooctane (2,2,4-trimethylpentane) | 99 |
| toluene | 110 |
| 4-methyl-2-pentanone | 117 |
| octane | 126 |
Gas Chromatography (GC)
Introduction
Gas chromatography is a chromatographic technique that can be used to separate volatile organic compounds. A gas chromatograph consists of a flowing mobile phase, an injection port, a separation column containing the stationary phase, and a detector. The organic compounds are separated due to differences in their partitioning behavior between the mobile gas phase and the stationary phase in the column.Instrumentation
Mobile phases are generally inert gases such as helium, argon, or nitrogen.
The injection port consists of a rubber septum through which a syringe needle is inserted to inject the sample. The injection port is maintained at a higher temperature than the boiling point of the least volatile component in the sample mixture. Since the partitioning behavior is dependant on temperature, the separation column is usually contained in a thermostat-controlled oven. Separating components with a wide range of boiling points is accomplished by starting at a low oven temperature and increasing the temperature over time to elute the high-boiling point components. Most columns contain a liquid stationary phase on a solid support. Separation of low-molecular weight gases is accomplished with solid adsorbents. Separate documents describe some specific GC Columns and GC Detectors.
Schematic of a gas chromatograph
Type of GC Detectors:
Thermal Conductivity Detector (TCD)
Introduction
A TCD detector consists of an electrically-heated wire or thermistor. The temperature of the sensing element depends on the thermal conductivity of the gas flowing around it. Changes in thermal conductivity, such as when organic molecules displace some of the carrier gas, cause a temperature rise in the element which is sensed as a change in resistance. The TCD is not as sensitive as other detectors but it is non-specific and non-destructive.Instrumentation
Two pairs of TCDs are used in gas chromatographs. One pair is placed in the column effluent to detect the separated components as they leave the column, and another pair is placed before the injector or in a separate reference column. The resistances of the two sets of pairs are then arranged in a bridge circuit.Schematic of a bridge circuit for TCD detection
The bridge circuit allows amplification of resistance changes due to analytes passing over the sample thermo conductors and does not amplify changes in resistance that both sets of detectors produce due to flow rate fluctuations, etc.
Flame-Ionization Detector (FID)
Introduction
An FID consists of a hydrogen/air flame and a collector plate. The effluent from the GC column passes through the flame, which breaks down organic molecules and produces ions. The ions are collected on a biased electrode and produce an electrical signal. The FID is extremely sensitive with a large dynamic range; its only disadvantage is that it destroys the sample.Schematic of FID
Electron Capture Detector (ECD)
Introduction
The ECD uses a radioactive Beta emitter (electrons) to ionize some of the carrier gas and produce a current between a biased pair of electrodes. When organic molecules that contain electronegative functional groups, such as halogens, phosphorous, and nitro groups pass by the detector, they capture some of the electrons and reduce the current measured between the electrodes. The ECD is as sensitive as the FID but has a limited dynamic range and finds its greatest application in analysis of halogenated compounds.Schematic of an ECD
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