Gas Chromatography

Chromatography is a technique for separating chemical substances that relies on differences in partitioning behavior between a flowing mobile phase and a stationary phase to separate the components in a mixture.

The sample is carried by a moving gas stream through a tube packed with a finely divided solid or may be coated with a film of a liquid. Because of its simplicity, sensitivity, and effectiveness in separating components of mixtures, gas chromatography is one of the most important tools in chemistry. It is widely used for quantitative and qualitative analysis of mixtures, for the purification of compounds, and for the determination of such thermochemical constants as heats of solution and vaporization, vapour pressure and activity coefficients. Gas chromatography is also used to monitor industrial processes automatically: gas streams are analyzed periodically and manual or automatic responses are made to counteract undesirable variations.

Many routine analyses are performed rapidly in environmental and other fields. For example, many countries have fixed monitor points to continuously measure the emission levels of for instance nitrogen dioxides, carbon dioxide and carbon monoxide. Gas chromatography is also useful in the analysis of pharmaceutical products, alcohol in blood, essential oils and food products.

The method consists of, first, introducing the test mixture or sample into a stream of an inert gas, commonly helium or argon, that acts as carrier. Liquid samples are vaporized before injection into the carrier stream. The gas stream is passed through the packed column, through which the components of the sample move at velocities that are influenced by the degree of interaction of each constituent with the stationary nonvolatile phase. The substances having the greater interaction with the stationary phase are retarded to a greater extent and consequently separate from those with smaller interaction. As the components elute from the column they can be quantified by a detector and/or collected for further analysis.

Two types of gas chromatography are encountered: gas-solid chromatography (GSC) and gas-liquid chromatography (GLC). Gas-solid chromatography is based upon a solid stationary phase on which retention of analytes is the consequence of physical adsorption. Gas-liquid chromatography is useful for separating ions or molecules that are dissolved in a solvent. If the sample solution is in contact with a second solid or liquid phase, the different solutes will interact with the other phase to differing degrees due to differences in adsorption, ion-exchange, partitioning or size. These differences allow the mixture components to be separated from each other by using these differences to determine the transit time of the solutes through a column.

Carrier gas

The choice of carrier gas depends on the type of detector that is used and the components that are to be determined. Carrier gases for chromatographs must be of high purity and chemically inert towards the sample e.g., helium (He), argon (Ar), nitrogen (N2), carbon dioxide (CO2) and hydrogen (H2). The carrier gas system can contain a molecular sieve to remove water or other impurities.

Sample injection system

The most common injection systems for introduction of gas samples are the gas sampling valve and injection with a syringe.

Direct injection with syringe

Both gaseous and liquid samples can be injected with a syringe. In the simplest form the sample is first injected into a heated chamber where it is vaporized before it is transferred to the column. When packed columns are used, the first part of the column often serves as injection chamber, separately heated to an appropriate temperature. For capillary columns a separate injection chamber is used from which only a small part of the vaporized/gaseous sample is transferred to the column, so called split-injection. This is necessary in order not to overload the column in regard to the sample volume.

When trace amounts can be found in the sample, so called on-column-injection can be used for capillary-GC. The liquid sample is injected directly into the column with a syringe. The solvent is thereafter allowed to evaporate and a concentration of the sample components takes place. If the sample is gaseous the concentration is achieved by so called cryo focusing. The sample components are concentrated and separated from the matrix by condensation in a cold-trap before the chromatographic separation.

Injection with valve/sample loop

Loop-injection is often used in process control, where gaseous or liquid samples continuously flow through the sample loop. The sample loop is filled in off-line position with a syringe or an automatic pump. Thereafter the loop is connected in series with the column and the sample is transferred by the mobile phase. Sometimes a concentration step is necessary.

Flame ionization detector (FID)

A Flame ionization detector (FID) consists of a hydrogen (H2)/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.

Flame ionization detectors are used for detecting hydrocarbons (HC) such as methane (CH4), ethane (C2H6), acetylene (C2H2) etc.

The sample to be analyzed is mixed with a special burner fuel, hydrogen (H2), hydrogen plus helium (He) or hydrogen plus nitrogen (N2). Ions and electrons that were formed in the flame enter the electron gap, decrease the gap resistance and thus permit a current to flow into the external circuit. The current is proportional to the rate of ion formation which depends on the hydrocarbon concentration in the gases and is detected by a suitable electrometer and displayed on an analogue output.

The FID gives a rapid, accurate and continuos reading of total HC concentration for levels as low as ppb.

Flame photometric detector

The flame photometric detector (FPD) allows sensitive and selective measurements of volatile sulphur and phosphorus compounds. The detection principle is the formation of excited Sulphur (S2*) and HPO* species in a reducing flame. A photomultiplier tube measures the characteristic chemiluminescent emission from these species. The optical filter can be changed to allow the photomultiplier to view light of 394 nm for sulphur measurement or 526 nm for phosphorus. The detector response to phosphor is linear, whereas the response to sulphur depends on the square of the concentration.

Usually, nitrogen (N2) is used as carrier gas.

Electron-Capture Detector (ECD)

Electron capture detectors (ECD) are typically used in environmental testing for detecting PCB’s, organochlorine pesticides, herbicides and various halogenated hydrocarbons.

With an electron capture detector, a beta emitter such as radioactive tritium or 63Ni is used to ionize the carrier gas. Fast beta particles generated by the radioactive source collide with the molecules of the carrier or make-up gas. By impact ionization, free slow-moving electrons are produced, which generate a measurable and steady current. If the GC effluent contains organic molecules with electronegative functional groups, such as halogens, phosphorous and nitro groups, electrons will be captured and the current will be reduced. In comparison to a signal without sample compounds, the reduction in electron flow is proportional to the quantity of electrophile sample components.

Electron Capture Detectors are up to 1000 times more sensitive than Flame Ionization Detectors and were the first detectors able to measure components at parts-per-billion (ppb) and parts-per-trillion (ppt) levels. It is this sensitivity that makes ECD the first choice for environmental measurements.