SRIF Background Paper
Gas Chromatography


Hewlett Packard's 5890 Gas Chromatograph

Introduction

Gas chromatography (GC) separates a mixture of chemicals (a sample) into individual components. These components (or analytes) can then be analyzed for their identity and their concentration in the sample. In general, analytes must be volatile or semivolatile (able to be vaporized) in order to be anlyzed on the GC. Analytes must also be dissolved in an organic solvent at very low concentrations.

The uses for GC are numerous. They are used extensively in the medical, pharmacological, environmental, and law enforcement fields. SRIF has two GC systems. One has an electron capture detector (GC-ECD) and the other has a mass spectrometer (GC-MS). They very popular instrument and are used for many types of academic projects and research. This paper discusses the GC-ECD and another SRIF background paper discusses the GC-MS.

General Chromatography

In all chromatography, separation occurs when the sample mixture is introduced (injected) into a mobile phase. In liquid chromatography (LC), the mobile phase is a solvent. In gas chromatography (GC), the mobile phase is an inert gas such as helium.

The mobile phase carries the sample mixture through what is referred to as a stationary phase. The stationary phase is a usually chemical that can selectively attract components in a sample mixture. The stationary phase is usually contained in a tube of some sort. This tube is referred to as a column. Columns can be glass or stainless steel of various dimensions.

The mixture of compounds in the mobile phase interacts with the stationary phase. Each compound in the mixture interacts at a different rate. Those that interact the fastest will exit (elute from) the column first. Those that interact slowest will exit the column last. By changing characteristics of the mobile phase and the stationary phase, different mixtures of chemicals can be separated. Further refinements to this separation process can be made by changing the temperature of the stationary phase or the pressure of the mobile phase.

Gas Chromatography

SRIF's GC has a long, thin column containing a thin interior coating of a solid stationary phase (5% phenyl-, 95% dimethylsiloxane polymer). This 0.25 mm diameter column is referred to as a capillary column. This particular column is used for semivolatile, non-polar organic compounds such as the PAHs we will look at. The compounds must me in an organic solvent.

The capillary column is held in an oven that can be programmed to increase the temperature gradually (or in GC terms, ramped). this helps our separation. As the temperature increases, those compounds that have low boiling points elute from the column sooner than those that have higher boiling points. Therefore, there are actually two distinct separating forces, temperature and stationary phase interactions mentioned previously.

As the compounds are separated, they elute from the column and enter a detector. The detector is capable of creating an electronic signal whenever the presence of a compound is detected. The greater the concentration in the sample, the bigger the signal. The signal is then processed by a computer. The time from when the injection is made (time zero) to when elution occurs is referred to as the retention time (RT).

While the instrument runs, the computer generates a graph from the signal. (See figure 1). This graph is called a chromatogram. Each of the peaks in the chromatogram represents the signal created when a compound elutes from the GC column into the detector. The x-axis shows the RT, and the y-axis shows the intensity (abundence) of the signal. In Figure 1, there are several peaks labeled with their RTs. Each peak represents an individual compound that was separated from a sample mixture. The peak at 4.97 minutes is from dodecane, the peak at 6.36 minutes is from biphenyl, the peak at 7.64 minutes is from chlorobiphenyl, and the peak at 9.41 minutes is from hexadecanoic acid methyl ester.


Figure 1: Chromatogram generated by a GC.

If the GC conditions (oven temperature ramp, column type, etc.) are the same, a given compound will always exit (elute) from the column at nearly the same RT. By knowing the RT for a given compound, we can make some assumptions about the identity of the compound. However, compounds that have similar properties often have the same retention times. Therefore, more information is usually required before an analytical chemist can make an identification of a compound in a sample containing unknown components.


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