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Gas chromatography is a type of chromatography used in chemistry for analyzing and separating compounds that can be convert to gas without decomposition. Uses of this technique include testing the purity of a Gaseous substance and separating the different components of a mixture. Sometimes, it may help in identifying the compound. It can also be used to prepare pure compounds from a mixture. Gas chromatography is similar to fractional distillation, as both processes separate the mixture components primarily based on their vapor pressure differences.
Principle of GC Gas chromatography is based on the principle of partition(differential distribution) of an volatile compound(gas) in two phases - a liquid phase covering the adsorbent surface and a gaseous phase of the eluting gas. With a fixed set of parameters (length and diameter of column, temp., nature and flow rate of the eluting gas etc.)
Compounds A and B interact with the stationary phase through intermolecular forces. A interacts more strongly with the stationary liquid phase and is retained relative to B, which interacts weakly with the stationary phase. Thus B spends more time in the gas phase and advances more rapidly through the column and has a shorter retention time than A. Typically, components with similar polarity elute in order of volatility. Thus alkanes elute in order of increasing boiling points; lower boiling alkanes will have shorter retention times than higher boiling alkanes.
Sample is injected into the injection port. Sample vaporizes and is forced into the column by the carrier gas ( = mobile phase which in GC is usually helium) Components of the sample mixture interact with the stationary phase so that different substances take different amounts of time to elute from the column.The separated components pass through a detector. Electronic signals, collected over time, are sent to the GC software, and a chromatogram is generated.
Components of a Gas Chromatograph Gas Supply: E.g. N2 or He Sample Injector: syringe or septum Column: Tubing packed with small uniform size, inert support coated with thin film of nonvolatile liquid Detectors: - Thermal conductivity (TC) - Flame ionization detector (FID) - Electron Capture (ECD) - Photo Ionization (PID)
Gas-Supply Carrier gases, which should be chemically inert and non reactive, include Helium, Nitrogen, and Hydrogen. Along with the gas supply there are pressure regulators, gauges, and flow meters. In addition, the gas carrier system often contains a molecular sieve to remove water and other impurities.
Sample Injection System Column efficiency depends upon that Sample should be of suitable size and introduced as a “plug” of vapor. Slow injection of oversized samples causes poor resolution and band spreading . The most common method of sample injection involves the use of micro syringe to inject a liquid or gaseous sample through a self-sealing, silicone-rubber diaphragm or septum into a flash vaporizer port located at the head of the column.
Column Configurations Two types of columns are commonly used in gas chromatography Packed and open tubular Capillary type Chromatographic columns vary in length from <2 to 50 m or more. They are made up of stainless steel, glass, Teflon or fused silica. In order to fit into an oven for thermo stating, they are usually formed as coils having diameters of 10 to 30 cm.
Column Ovens Column temperature is a very important variable that must be controlled for precise result. Thus, the column is housed in a thermo stated oven. The optimum column temperature depends upon the evaporation point of the sample and the degree of separation required. Usually a temperature more than or equal to average boiling point of a sample results in a reasonable elution time (2 to 30 min). For samples with a broad boiling range, it is desirable to employ temperature program, where the temperature is increased either continuously or in steps as the separation proceeds.
Detection Systems Properties of the Ideal Detector: The ideal detector for Gas Chromatography should have following characteristics: Good stability and reproducibility. A linear response to solutes that extends over several orders of magnitude. A temperature range from 25oC room temperature to at least 400oC. A short response time that is independent of flow rate. High reliability and ease of use.
Similarity in response toward all solutes or a highly selective response toward one classes of solutes. Should be Nondestructive toward sample.
Flame Ionization Detectors (FID) The flame ionization detector is the widely used and most applicable detector for Gas Chromatography. The effluent from the column is mixed with hydrogen and air and then ignited electrically. Most organic compounds, when pyrolyzed at the temperature of a hydrogen/air flame, produce ions and electrons that can conduct electricity through the flame.
Thermal Conductivity Detectors(TCD) An old age detector for Gas Chromatography, and one that still have wide application, is based upon changes in the thermal conductivity of the gas brought about by the presence of analyte molecules. The sensing element of this detector is an electrically heated element whose temp. at constant electrical power depends upon the thermal conductivity of the surrounding gas. Heated element - Fine platinum, gold, or tungsten wire or a semiconducting thermistor.
The advantage of the thermal conductivity detector is its simplicity, its large linear dynamic range, its response toward both organic and inorganic species, and its nondestructive nature, which permits collection of solutes after detection. A limitation is its relatively low sensitivity (~10-8 g solute/mL carrier gas). *Other detectors exceed this sensitivity by factors as large as 104 to 107.
Electron-Capture Detectors(ECD) The electron-capture detector has become one of the most widely used detectors for environmental samples because this detector selectivity detects halogen containing compounds, such as pesticides and polychlorinated biphenyls. The effluent from the column is passed over a emitter, usually nickel-63. An electron from the emitter causes ionization of the carrier gas and the production of a burst of electrons. In the absence of organic species, a constant standing current between a pair of electrodes results from this ionization process. The current decreases markedly, however, in the presence of those organic molecules that tend to capture electrons.
Advantage: The electron-capture detector is selective in its response. It is highly sensitive to molecules containing electronegative functional groups such as halogens, peroxides, quinones, and nitro groups. Limitation: It is insensitive toward functional groups such as amines, alcohols, and hydrocarbons. *An important application of the electron-capture detector has been for the detection and determination of chlorinated insecticides.
Atomic Emission Detectors (AED) The atomic emission detector is used commercially. In this device the eluent is introduced into a microwave-energized helium plasma that is coupled to a diode array optical emission spectrophotometer. The plasma is sufficiently energetic to atomize all of the elements in a sample and to excite their characteristic atomic emission spectra.
Thermionic Detectors (TID) The thermionic detector is selective toward organic compounds containing phosphorus and nitrogen. Its response to a phosphorus atom is greater than to a nitrogen atom. Compared with the flame ionization detector, the thermionic detector is approximately 500 times more sensitive to phosphorus-containing compounds and 50 times more sensitive to nitrogen bearing species. These properties make thermionic detection particularly useful for detecting and determining the many phosphorus-containing pesticides.
GAS CHROMATOGRAPHIC COLUMNS The choice of column depends on the sample and the active measured. The main chemical attribute regarded when choosing a column is the polarity of the mixture, but functional groups can play a large part in column selection. The polarity of the sample must closely match the polarity of the column stationary phase to increase resolution and separation while reducing run time. The separation and run time also depends on the film thickness (of the stationary phase), the column diameter and the column length.
Open tubular Columns Open tubular, or capillary, columns are of two basic types,namely, wall—coated open tubular (WCOT) and support-coated open tubular (SCOT). Wall-coated columns are simply capillary tubes coated with a thin layer of the stationary phase. In support-coated open tubular columns, the inner surface of the capillary is lined with a thin film (~30 m) of a support material, such as diatomaceous earth. This type of column holds several times as much stationary phase as does a wall-coated column and thus has a greater sample capacity.
Packed Columns Packed columns are fabricated from glass, metal (stainless steel, copper or aluminum), or Teflon tubes that typically have lengths of 2 to 3 m and inside diameters of 2 to 4 mm. These tubes are densely packed with a uniform, finely divided packing material, or solid support, that is coated with a thin layer (0.05 to m) of the stationary liquid phase. In order to fit in a thermo stating oven, the tubes are formed as coils having diameters of roughly 15 cm.
The Stationary Phase Desirable properties for the liquid phase in a gas-liquid chromatographic column include: Low volatility (ideally, the boiling point of the liquid should be at 100oC higher than the maximum operating temperature for the column) Thermal stability chemical inertness solvent characteristics such that k` and values for the solutes to be resolved fall within a suitable range. The retention time for a solute on a column depends upon its distribution constant which in turn is related to the chemical nature of the stationary phase
Film Thickness Commercial columns having stationary phases that vary in thickness from 0.1 to 5m. Film thickness primarily affects the retentive character and the capacity of a column. Thick films are used with highly volatile analytes because such films retain solutes for a longer time, thus providing a greater time for separation to take place. Thin films are useful for separating species of low volatility in a reasonable length of time. For most applications with 0.26- or 0.32-mm columns, a film thickness of 0.26 m is used. With mega bore columns, 1- to 1.5 m films are often used.
Qualitative Analysis Gas chromatogram is used as criteria of purity for organic compounds. Contaminants present are revealed by the appearance of additional peaks. The areas under these peaks provide rough estimates of the extent of contamination. The technique is also useful for evaluating the effectiveness of purification procedures. Retention times should be useful for the identification of components in mixtures. Gas chromatography provides an excellent means of confirming the presence or absence of a suspected compound in a mixture.
Quantitative Analysis The detector signal from a gas-liquid chromatographic column has wide use for quantitative and semi quantitative analyses. An accuracy of 1% relative is attainable under carefully controlled conditions. Reliability is directly related to the control of variables.The nature of the sample also plays a part in determining the potential accuracy
Gas Chromatography/Mass Spectrometry (GC/MS) The flow rate from capillary columns is generally low enough that the column output can be fed directly into the ionization chamber of the mass spectrometer. For packed columns and mega bore capillary columns however, a jet separator must be employed to remove most of the carrier gas from the analyte.
Advantages of Gas Chromatography Requires only very small samples with little preparation Good at separating complex mixtures into components Results are rapidly obtained (1 to 100 minutes) Very high precision Only instrument with the sensitivity to detect volatile organic mixtures of low concentrations Equipment is not very complex (sophisticated oven)
