Gas chromatography (GC) is a powerful instrumental technique used to separate and analyze mixtures. A gas chromatograph is a standard piece of equipment in forensics, medical, and environmental testing laboratories (Figure 2.75).
To run a GC, a sample is diluted then injected into the instrument where it is vaporized. The gaseous sample is pushed through a long, thin capillary column (typically 30 meters or approximately 100 feet long) by an inert carrier gas. The column separates components of a mixture, detects them, and a spectrum is generated displaying peaks that correspond to material that has exited the column at certain times. A GC spectrum of 87 octane gasoline is shown in Figure 2.76. Every peak represents one or more compounds, and since there are at least 50 peaks, this GC spectrum demonstrates that gasoline contains at least 50 different compounds!
The quantity of time a compound spends inside a GC column before it is detected is called its "retention time," which represents the time a compound is "retained" on the column. This value is the x-axis of the GC spectrum in minutes. In many instruments, the retention time of each peak can be labeled on the spectra with the click of a button, and the value is displayed above each peak maxima. A compound's retention time is similar to \(R_f\) in thin layer chromatography, and should be reproducible between identical runs. Unlike \(R_f\), however, retention times are very precise, and retention times typically vary by no more than 0.01 minute between identical runs (when the spectra have good peak shapes).