Gas chromatography (GC) is an analytical technique for separating compounds based primarily on their volatilities. Gas chromatography provides both qualitative and quantitative information for individual compounds present in a sample. Compounds move through a GC column as gases, either because the compounds are normally gases or they can be heated and vaporized into a gaseous state. The compounds partition between a stationary phase, which can be either solid or liquid, and a mobile phase (gas). The differential partitioning into the stationary phase allows the compounds to be separated in time and space.
The carrier gas is usually helium, hydrogen, or nitrogen. This serves as the mobile phase that moves the sample through the column. The carrier gas flow can be quantified by either linear velocity, expressed in cm/sec, or volumetric flow rate, expressed in mL/min. The linear velocity is independent of the column diameter while the flow rate is dependent on the column diameter.
The injector is a hollow, heated, glass-lined cylinder where the sample is introduced into the GC. The temperature of the injector is controlled so that all components in the sample will be vaporized. The glass liner is about 4 inches long and 4 mm internal diameter.
The GC column is the heart of the system. It is coated with a stationary phase which greatly influences the separation of the compounds. The structure of the stationary phase affects the amount of time the compounds take to move through the column. Typical stationary phases are large molecular weight polysiloxane, polyethylene glycol, or polyester polymers of 0.1 to 2.5 micrometer film thickness. Columns are available in many stationary phases sizes. A typical capillary column is 15 to 60 meters in length and 0.25 to 0.32 mm ID. A typical packed column is 6 to 12 feet long and 2.2 mm ID.
The column is placed in an oven where the temperature can be controlled very accurately over a wide range of temperatures. Typically, GC oven temperatures range from room temperature to 300°C, but cryogenic conditions can be used to operate at temperatures from about -20°C to 20°C.
As compounds come off the column, they enter a detector. The compound and detector interact to generate a signal. The size of the signal corresponds to the amount the compound present in the sample. There are several different types of detectors that can be employed, depending on the compounds to be analyzed. These detectors can measure from 10-15 to 10-6 gram of a single component.
The data recorder plots the signal from the detector over time. This plot is called a chromatogram. The retention time, which is when the component elutes from the GC system, is qualitatively indicative of the type of compound. The data recorder also has an integrator component to calculate the area under the peaks or the height of the peak. The area or height is indicative of the amount of each component.
The retention time is the total time that a compound spends in both the mobile phase and stationary phase. Retention time is generally reported in minutes.
The dead time is the time a non-retained compound spends in the mobile phase which is also the amount of time the non-retained compound spends in the column. Dead time is generally reported in minutes.
The adjusted retention time is the time a compound spends in the stationary phase. The adjusted retention time is the difference between the dead time and the retention time for a compound.
The capacity factor is the ratio of the mass of the compound in the stationary phase relative to the mass of the compound in the mobile phase. The capacity factor is a unitless measure of the column's retention of a compound.
The phase ratio relates the column diameter and film thickness of the stationary phase. The phase ratio is unitless and constant for a particular column and represent the volume ratioß.
The distribution constant is a ratio of the concentration of a compound in the stationary phase relative to the concentration of the compound in the mobile phase. The distribution constant is constant for a certain compound, stationary phase, and column temperature.
The selectivity is a ratio of the capacity factors of two peaks. The selectivity is always equal to or greater than one. If the selectivity equals one the two compounds cannot be separated. The higher the selectivity, the more separation between two compounds or peaks.
The linear velocity is the speed at which the carrier gas or mobile phase travels through the column. The linear velocity is generally expressed in centimeters per second.
The efficiency is related to the number of compounds that can separated by the column. The efficiency is expressed as the number of theoretical plates (N, unitless) or as the height equivalent to a theoretical plate (HETP, generally in millimeters). The efficiency increases as the height equivalent to a theoretical plate decreases, thus more compounds can be separated by the column. The efficiency increases as the number of theoretical plates increases, thus the column's ability to separate two closely eluting peaks increases.
Sample contains 6 aromatic hydrocarbons dissolved in a solvent (methanol). The compounds' properties are summarized in Table 2.
The compounds were separated on an nonpolar, 95% methyl, 5% phenylpolysiloxane column, 30 m long, 0.25 mm ID, and 0.25 micrometer film thickness. About 1 microliter of the hydrocarbon sample were injected. Approximately 5 nanograms (ng) of each component was injected per 1 microliter. A flame ionization detector (FID) was used.
This chromatogram shows an ideal temperature program for separation of the 6 aromatic compounds on this column. The first peak is the solvent, methanol. The compounds elute in order of increasing boiling point, that is, compounds with higher boiling points are more retained by the stationary phase. Note that para-xylene and meta-xylene cannot be separated on this column; the peak (#5) containing these compounds is broad at the baseline and shows a distinct shoulder.
This chromatogram shows the effects of an isothermal* temperature program at 60°C. The result is an increase in the retention time of all compounds. The heights of the later eluting peaks are reduced and the peak widths increased because they are more affected by the lower temperature program used. (*isothermal means a constant oven temperature was used throughout the run.)
This chromatogram shows the effects of a reduced head pressure while using the ideal temperature program. The flow rate was reduced by decreasing the head pressure. The retention time is slightly increased due to the low flow rate used. All of the peak heights were reduced and the peak widths are increased.
This chromatogram shows the effects of a higher head pressure while using the ideal temperature program. The flow rate was increased by increasing the head pressure. The retention time was reduced and all of the peak heights were increased.
This chromatogram shows the effects of a low split ratio while using the ideal temperature program. All of the peak heights were increased due to the greater amount of the sample introduced into the column.
This chromatogram shows the effects of a high split ratio while using the ideal temperature program. All of the peak heights were reduced due to the smaller amount of the sample introduced into the column.
This chromatogram shows the separation of benzene, toluene, para-xylene, meta-xylene and ortho-xylene. The first peak is the solvent, hexane. A polyalkylene glycol fused silica capillary column 30 m long, 0.25 mm ID, and 0.25 micrometer film thickness was used for separation. Para-xylene (peak #4) and meta-xylene (peak #5) can be separated on this column. This illustrates the matching of the stationary phase with the desired compounds to be separated.
Sampling & Monitoring Primer Table of Contents
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Copyright © 1997 Daniel Gallagher
Last Modified: 09-10-1997