Gas Chromatography 427 PHC

Gas Chromatograph

Gas chromatography (GC), is a common type of chromatography used in analytic chemistry for separating and analyzing compounds that can be vaporized without decomposition.

Principle: A method of analysis by which the analyte is vaporized and introduced into a stream of carrier gas. It is conducted through a chromatographic column and separated into its constituents. These fractions pass through the column at characteristic rates, and are detected as they emerge in a time sequence. The detecting cell responses are recorded on a chart, from which the components can be identified both qualitatively and quantitatively.

In GC, the moving phase (mobile phase) is a carrier gas, usually an inert gas such as helium or nitrogen. The stationary phase is a microscopic layer of liquid or polymer on an inert solid support, inside a piece of glass or metal tubing called a column. The instrument used to perform GC is called a gas chromatograph.

The gaseous compounds being analyzed interact with the walls of the column, which is coated with different stationary phases. This causes each compound to elute at a different time, known as the retention time of the compound. The comparison of retention times is what gives GC its analytical usefulness.

Gas chromatography is in principle similar to column chromatography , but has several notable differences: The process of separating the compounds in a mixture is carried out between a liquid stationary phase and a gas moving phase, whereas in column chromatography the stationary phase is a solid and the moving phase is a liquid. The column through which the gas phase passes is located in an oven where the temperature of the gas can be controlled, whereas column chromatography has no such temperature control. The concentration of a compound in the gas phase is solely a function of the vapor pressure of the gas.

Typical uses of GC include: Testing the purity of a particular substance. Separating the different components of a mixture (the relative amounts of such components can also be determined). In some situations, GC may help in identifying a compound. In preparative chromatography, GC can be used to prepare pure compounds from a mixture.

GC Instrument

Schematic Diagram of Gas Chromatograph

Carrier gas: The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependant upon the type of detector which is used. The carrier gas system also associated with pressure regulators and flow meters. In addition, it contains a molecular sieve to remove water and other impurities.

Flow control: Flow rates are controlled by a 2 stage pressure regulator: At the gas cylinder. Mounted in the chromatograph. Inlet pressure 10-50 psi → F = 25-150 ml/min with packed column F = 1-25 ml/min with capillary column The flow rate will be constant if the inlet pressure remains constant.

Sample injection port: The injector is a piece of hardware attached to the column head. It provides the means to introduce a sample into a continuous flow of carrier gas. For optimum column efficiency, the sample should not be too large, and should be introduced onto the column as vapor. Slow injection of large samples causes band broadening and loss of resolution.

Common injector types are: Microflash vaporizer direct injector: It involves the use of a microsyringe to inject the sample through a rubber septum into a flash vaporizer port located at the head of the column. The temperature of the sample port is usually about 50°C higher than the boiling point of the least volatile component of the sample. Used for packed columns, where the sample size vary from a few tenth of microliter to 20 ul.

Capillary columns require much smaller samples ( 10-3 ul), so a sample splitter system is used to deliver only a small fraction of the injected sample to the column head, with the rest going to waste.

Sample splitter (Split/Splitless) injector: The injector can be used in one of two modes; split or splitless. a sample is introduced into a heated small chamber via a syringe through a septum (the heat facilitates volatilization of the sample). The vaporized sample/carrier gas mixture then either sweeps entirely (splitless mode) or as portion (split mode) into the column. In split mode, a part of the sample/carrier gas mixture in the injection chamber is exhausted through the split vent. Split injection is preferred when working with samples with high analyte concentrations (>0.1%). Splitless injection is best suited for trace analysis with low amount of analyte (<0.01%).

For quantitative work, more reproducible sample size are required and this can be obtained by a rotary sample valve.

Rotary sample valve: gaseous samples in collection bottles are connected to what is most commonly a six- port switching valve. The carrier gas flow is not interrupted while a sample can be expanded into a previously evacuated sample loop. Upon switching, the contents of the sample loop are inserted into the carrier gas stream.

Column: There are two general types of column: Packed column Capillary column (open tubular). All the GC studies in the early 1950s were carried out on packed column. In the late 1950s capillary column were constructed that much superior in speed and column efficiency (≈ 300000 plates).

Capillary columns did not gain widespread until the late 1970s due to several reasons: Small sample capacity. Difficulties in coating the column. Tendencies of columns to clog. Short lifetimes of poorly prepared columns. Fragileness of columns. Mechanical problems in sample introduction and connection to the detector.

Capillary column: Capillary columns have an internal diameter of a few tenths of a millimeter. They were constructed of stain-less steel, aluminum, copper, plastic, or glass. They can be one of two types; wall-coated open tubular (WCOT) or support-coated open tubular (SCOT). Wall-coated columns consist of a capillary tube whose walls are coated with liquid stationary phase. In support-coated columns, the inner wall of the capillary is lined with a thin layer of support material, onto which the stationary phase has been adsorbed. SCOT columns are generally less efficient than WCOT columns, but both types are more efficient than packed columns.

In 1979, a new type of WCOT column appeared, the Fused Silica Open Tubular (FSOT) column. It was drawn from specially purified silica that contains metal oxides. These have much thinner walls than the glass capillary columns, and are given strength by an outside protective polyimide coating. These columns are flexible and can be bent into coils. They have the advantages of physical strength, flexibility and low reactivity.

Packed column: Packed columns contain a finely divided, inert, solid support material coated with a thin layer of liquid stationary phase. Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm. They are made from glass, metals, or Teflon.

Solid support materials: Hold the liquid stationary phase. Consists of small, uniform, spherical particles with good mechanical strength. It should be inert at elevated temperature. The most widely used is prepared from the naturally occurring diatomaceous earth (skeleton of thousands of single-celled plants- diatoms- lives in lakes and seas). The efficiency of a GC column increases with decreasing particle size of the solid support.

Properties and characteristics of GC columns Packed SCOT WCOT FSOT 1-6 10-100 Length, m 2-4 0.5 0.25-0.75 0.1-0.53 Inside diameter, mm 500-1000 600-1200 1000-4000 2000-4000 Efficiency, plates/m 10-106 10-1000 10-75 Sample size, ng high Low Relative back pressure slow Fast Relative speed Poorest → Best Chemical inertness No Yes Flexible

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