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The Stationary Phase

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p132-134

2026-07-06

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The Stationary Phase

The column containing the stationary phase was described above. As the mobile phase passes through the stationary phase, the various components of the analyte mixture are exposed to the surface of the stationary phase. There are two principles that determine how long it will take a substance to traverse the entire column: its molecular weight and its polarity. All other factors being equal, a lower molecular weight substance will traverse the column faster than a heavier one. The degree of influence that polarity has on the time depends upon the polarity of the stationary phase and the polarities of the analytes. Because of their differences in structure, cocaine and caffeine have different weights and polarities and are easily separated by GC. Figure 1 is a diagram that shows how an analyte mixture is separated by a GC stationary phase. The operator has quite a bit of control over the conditions of the GC experiment. The time it takes for analytes to traverse the stationary phase (the retention time) and the degree of separation of the analyte components can be controlled by varying the polarity of the stationary phase, the speed of the mobile phase as it passes through the stationary phase and, most importantly, the temperature of the stationary and mobile phases. The rule of thumb for GC is that every 10 °C rise in temperature of the experi ment halves the retention time of a material. This is a powerful factor in designing an experiment to separate a complex mixture. If the mixture contains substances that are low molecular weight and high molecular weight and of varying polarity, one can use temperature programming to vary the temperature of the experiment during its progress. For example, gasoline contains more than 300 different compounds. Their polarities are generally similar but there is a large variation in the weights of the com pounds. If the separation is done at a low temperature, the lower weight compounds will separate from each other but the process will take a long time. If the temperature is raised so that the process takes a short time, the low weight compounds will all elute together and won’t separate. This is where temperature programming comes in. The oven can be set at a low temperature (around 50 °C) at the beginning and then it is allowed to gradually rise during the run. This provides good separation of low weight compounds and completes the run in a reasonable time. Figure 2 shows a chromatogram of a sample of gasoline that was partially burned in a fire. The chart proceeds from left to right, with the lowest weight components on the left side.

FIGURE 1 The partitioning process of analyte molecules between a stationary phase and a mobile phase. The partitioning depends upon the polarities of the analyte components and the relative polarities of the stationary and mobile phases.

FIGURE 2 A gas chromatogram of partially burned gasoline. Each peak represents a component of the gasoline. Whole gasoline contains more than 300 substances. The most volatile substances evaporate when the gasoline is subjected to heat.