The Column and Column Oven

GC ovens usually require an operating range from about 5°C to about 400°C although the majority of GC analyses are carried out between temperatures of 75°C and 200°C. In contrast, LC column ovens cover a more limited range of temperatures viz., 0°C to 120°C. Temperature programming is an essential feature for all GC column ovens and is necessary to handle a sufficiently wide molecular and polarity range of samples. Linear programming is the most common although other functions of time are often available. LC column ovens are rarely provided with temperature programming facilities as the technique appears to be far less effective compared with GC, gradient elution being a far more useful alternative. The thermostatting medium used in GC ovens is almost exclusively 'forced air' as the heat capacity of the GC mobile phase (i.e., a gas) is relative small. Consequently, air has sufficient heat capacity to change the column temperature rapidly without significant cooling from the carrier gas. Air ovens are also employed in LC column ovens but are far less effective as the mobile phase, a liquid, has a much higher heat capacity and, thus, a stronger cooling effect. This problem is partly alleviated by using mobile phase pre-heaters but these introduce a significant volume between the solvent supply system and the column which will distort the profile of any solvent gradient that is employed. Nevertheless, a liquid thermostatting medium introduces difficulties when changing columns and with column detector connections and is, thus, not commonly used. The temperature program can be controlled by a microprocessor incorporated in the programmer or can be controlled from a central computer that governs the operation of the whole instrument.

The GC column can be a packed or open tubular and, thus, the oven must be capable of taking both. The open tubular column is by far the most popular partly because they are considered state of the art and not because they necessarily provide an improved performance. Open tubular columns will always provide the highest efficiencies but, if correct operating procedures are adopted, in general, analyses carried out on packed columns, are likely to provide greater quantitative accuracy and better precision and repeatability. Packed GC columns are usually made of stainless steel or glass and open tubular column almost exclusively fused quartz.

Almost all LC columns are packed, although they can vary widely in length and diameter depending of the nature of the sample and the resolution required. They are usually manufactured of stainless steel or titanium (reputed to provide greater stability for labile materials of biological origin) and the connection to the sample valve and detector should be as short as possible and have a very small diameter to reduce extra column dispersion.