ABSTRACT
In gas chromatography (GC) (originally called vapor phase chromatography), the mobile phase is a gas, usually nitrogen, helium, or hydrogen, introduced to the column through a pressure regulator from a cylinder of compressed gas. These mobile phase gases do not have signicant solvation interactions with analyte molecules. Initially, the stationary phases were high boiling liquids dispersed on a particulate solid support, which was then “packed” into a column of metal tubing 1/16 to 1/4 in. in diameter, generally made of copper or stainless steel, hence the term packed column GC. The coated particles were secured by plugs of glass wool or porous metal frits at each end. The column (often wound in a coil to t) was maintained in a thermostatted oven set to a controlled constant temperature. Higher molecular weight (MW) analytes, with higher boiling points, would require correspondingly higher temperatures to partition sufciently into the gaseous mobile phase to be eluted in a reasonable time. True liquid stationary phases would eventually begin to vaporize slowly at such elevated temperatures and be carried off the column as well, a phenomenon referred to as column bleeding. In time, such liquid stationary phases were replaced with very high MW polymers, especially long-chain, substituted silicones (cf. end of Section 11.5), which would behave like liquids to the analytes but which were stable at higher temperatures. At high enough temperatures, these would eventually bleed as well, but instead of volatilization of intact molecules of the stationary phase, this was now due to stepwise loss of small fragments from the ends of the polymer chains. Mixtures of volatile liquids (e.g., the components of a petroleum oil fraction, mixed products of an organic synthesis procedure, lipid mixtures) would be sampled with a syringe and injected on the GC column for separation and detection of separated components. The mobile phase gas, called the carrier gas, would be introduced to the head of the column into an unpacked open space above the restraining frit or plug. This empty volume lay beneath a plastic or rubber septum, which sealed the top of the column and through which the syringe needle could be inserted. The liquid mixture was injected, vaporized at the elevated oven temperature, and swept along by the carrier gas to initially pile up as a narrow band on the beginning of the packing. Its components subsequently progressed more slowly, at different rates along the packed portion of the column, and the chromatographic separation ensued. If the carrier gas were helium or hydrogen, these very low MW gases had much higher thermal conductivity than almost any analyte they might carry off the column. A simple detector at the efuent end of the column produced a signal proportional to the amount of analyte in each eluting peak. This resulted from the increase in electrical resistance of a wire placed in the efuent stream and heated by passage of an electrical current. This was initially thought to be a consequence of its increase in temperature due to the decrease in thermal conductivity and thus cooling power of the carrier gas stream, which was caused by the presence of the heavier analyte vapors. So GC had a nearly universal detector that readily produced an electrical signal that could
be plotted on a chart recorder to produce a chromatogram. A very simplied block diagram of a generic GC instrument was depicted in Figure 11.1.
