Columns for Gas Chromatography: Performance and Selection

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We cannot take responsibility for items which are lost or damaged in transit. In gas-liquid chromatography GLC , we coat the packing material with a liquid mobile phase. To prevent any uncoated packing material from adsorbing solutes, which degrades the quality of the separation, the surface silanols are deactivated by reacting them with dimethyldichlorosilane and rinsing with an alcohol—typically methanol—before coating the particles with stationary phase.

Other types of solid supports include glass beads and fluorocarbon polymers, which have the advantage of being more inert than diatomaceous earth. To minimize the effect on plate height from multiple path and mass transfer, the diameter of the packing material should be as small as possible see equation Compared to capillary columns, which are discussed below, a packed column can handle larger sample volumes, typically 0.

The column in Figure You can use equation A capillary, or open tubular column is constructed from fused silica and is coated with a protective polymer coating. The interior surface of the capillary has a 0.

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Capillary columns are of three principle types. In a wall-coated open tubular column WCOT a thin layer of stationary phase, typically 0. A capillary column provides a significant improvement in separation efficiency because it has more theoretical plates per meter and is longer than a packed column. For example, the capillary column in Figure On the other hand, a packed column can handle a larger sample. Elution order in gas—liquid chromatography depends on two factors: the boiling point of the solutes, and the interaction between the solutes and the stationary phase.

If two solutes have similar boiling points, however, then a separation is possible only if the stationary phase selectively interacts with one of the solutes. As a general rule, nonpolar solutes are more easily separated with a nonpolar stationary phase, and polar solutes are easier to separate when using a polar stationary phase. Table Many stationary phases have the general structure shown in Figure A stationary phase of polydimethyl siloxane, in which all the —R groups are methyl groups, —CH 3 , is nonpolar and often makes a good first choice for a new separation.

The order of elution when using polydimethyl siloxane usually follows the boiling points of the solutes, with lower boiling solutes eluting first. Increasing polarity is provided by substituting trifluoropropyl, —C 3 H 6 CF, and cyanopropyl, —C 3 H 6 CN, functional groups, or by using a stationary phase of polyethylene glycol Figure An important problem with all liquid stationary phases is their tendency to elute, or bleed from the column when it is heated.

The temperature limits in Table Capillary columns with bonded or cross-linked stationary phases provide superior stability. Cross-linking, which is done after the stationary phase is in the capillary column, links together separate polymer chains, providing greater stability.

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Another important consideration is the thickness of the stationary phase. From equation The most common thickness is 0. Thinner films are used when separating low volatility solutes, such as steroids.

12.4: Gas Chromatography

A few stationary phases take advantage of chemical selectivity. The most notable are stationary phases containing chiral functional groups, which can be used for separating enantiomers. Three considerations determine how we introduce a sample to the gas chromatograph. Second, the analytes must be present at an appropriate concentration. Finally, the physical process of injecting the sample must not degrade the separation.

Not every sample can be injected directly into a gas chromatograph. A solute of low volatility may be retained by the column and continue to elute during the analysis of subsequent samples.

A liquid—liquid extraction of analytes from an aqueous matrix into methylene chloride or another organic solvent is a common choice. An attractive approach to isolating analytes is a solid-phase microextraction SPME. In one approach, which is illustrated in Figure The fiber, which is coated with a thin film of an adsorbent, such as polydimethyl siloxane, is lowered into the sample by depressing a plunger and is exposed to the sample for a predetermined time. After withdrawing the fiber into the needle, it is transferred to the gas chromatograph for analysis.

Two additional methods for isolating volatile analytes are a purge-and-trap and headspace sampling. In a purge-and-trap see Figure 7. These compounds are carried by the purge gas through a trap containing an absorbent material, such as Tenax, where they are retained. Heating the trap and back-flushing with carrier gas transfers the volatile compounds to the gas chromatograph. In headspace sampling we place the sample in a closed vial with an overlying air space. After allowing time for the volatile analytes to equilibrate between the sample and the overlying air, we use a syringe to extract a portion of the vapor phase and inject it into the gas chromatograph.

Alternatively, we can sample the headspace with an SPME. Thermal desorption is a useful method for releasing volatile analytes from solids. We place a portion of the solid in a glass-lined, stainless steel tube. After purging with carrier gas to remove any O 2 that might be present, we heat the sample. Volatile analytes are swept from the tube by an inert gas and carried to the GC. Because volatilization is not a rapid process, the volatile analytes are often concentrated at the top of the column by cooling the column inlet below room temperature, a process known as cryogenic focusing.

Once the volatilization is complete, the column inlet is rapidly heated, releasing the analytes to travel through the column. The reason for removing O 2 is to prevent the sample from undergoing an oxidation reaction when it is heated. To analyze a nonvolatile analyte we must chemically convert it to a volatile form. For example, amino acids are not sufficiently volatile to analyze directly by gas chromatography. Reacting an amino acid with 1-butanol and acetyl chloride produces an esterfied amino acid.

A side benefit of many extraction methods is that they often concentrate the analytes. If an analyte is too concentrated it is easy to overload the column, resulting in peak fronting see Figure Injecting less sample or diluting the sample with a volatile solvent, such as methylene chloride, are two possible solutions to this problem. In Section We also introduce an additional source of band broadening if we fail to inject the sample into the minimum possible volume of mobile phase.

There are two principal sources of this precolumn band broadening: injecting the sample into a moving stream of mobile phase and injecting a liquid sample instead of a gaseous sample. An example of a simple injection port for a packed column is shown in Figure The top of the column fits within a heated injector block, with carrier gas entering from the bottom.

The sample is injected through a rubber septum using a microliter syringe such as the one shown in Figure Injecting the sample directly into the column minimizes band broadening by mixing the sample with the smallest possible amount of carrier gas. The needle pierces a rubber septum and enters into the top of the column, which is located within a heater block.

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The needle pierces a rubber septum and enters into a glass liner, which is located within a heater block. In a split injection the split vent is open; the split vent is closed for a splitless injection. In a split injection we inject the sample through a rubber septum using a microliter syringe. Instead of injecting the sample directly into the column, it is injected into a glass liner where it mixes with the carrier gas. At the split point, a small fraction of the carrier gas and sample enters the capillary column with the remainder exiting through the split vent.

By controlling the flow rate of the carrier gas entering the injector, and the flow rates through the septum purge and the split vent, we can control what fraction of the sample enters the capillary column, typically 0. In a splitless injection , which is useful for trace analysis, we close the split vent and allow all the carrier gas passing through the glass liner to enter the column—this allows virtually all the sample to enters the column.

Gas Chromatography Column Selection Simplified Chromatography Today

Because the flow rate through the injector is low, significant precolumn band broadening is a problem. For samples that decompose easily, an on-column injection may be necessary. In this method the sample is injected directly into the column without heating. The column temperature is then increased, volatilizing the sample with as low a temperature as is practical. For this reason the column is placed inside a thermostated oven see Figure In an isothermal separation we maintain the column at a constant temperature.

To increase the interaction between the solutes and the stationary phase, the temperature usually is set slightly below that of the lowest-boiling solute. One difficulty with an isothermal separation is that a temperature favoring the separation of a low-boiling solute may lead to an unacceptably long retention time for a higher-boiling solute.

Temperature programming provide a solution to this problem. As the separation progresses, we slowly increase the temperature at either a uniform rate or in a series of steps. You may recall that we called this the general elution problem see Figure The final part of a gas chromatograph is the detector. The ideal detector has several desirable features, including: a low detection limit, a linear response over a wide range of solute concentrations which makes quantitative work easier , sensitivity for all solutes or selectivity for a specific class of solutes, and an insensitivity to a change in flow rate or temperature.

As the mobile phase exits the column it passes over a tungsten-rhenium wire filament see Figure Because of its high thermal conductivity, helium is the mobile phase of choice when using a thermal conductivity detector TCD. Read more Read less. Review "This book provides the necessary guidance for column selection and type of column chosen with the injection system and detectors in mind. This resource provides essential guidance for scientists and technicians, including: Methods of choosing both capillary and packed columns Selection of dimensions column length, I.

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