There are two types of columns in common use in GC and they are the conventional packed column and the open tubular column. The former are usually 2 to 4 mm I.D. and 1 to 4 meters long and, packed with a suitable adsorbent, are mostly used for gas analysis. As a result of the simpler injection procedure and the more precise sampling method, the packed column tends to give greater quantitative accuracy and precision. However, despite its problems with sample injection, the open tubular column is seen as the 'state of the art' column and is by far the most popular column system in general use. The length of open tubular columns range from about 10 m to 100 m and can have internal diameters from 100 mm to 500 mm. The stationary phase is coated on the internal wall of the column as a film 0.2 mm to 1 mm thick. The Packed GC Column Packed columns are usually constructed from stainless steel or Pyrex glass. Pyrex glass is favored when thermally labile materials

are other problems that need addressing, once packed, the practical lifetime of a column is also uncertain. The changes in performance of a preparative HPLC column that occurs with time depends upon the stability of the packed bed. Frequently, the bed settles after operation for even a short time and the top of the column needs to be repacked. Sometimes channels are formed in the bed, in which case the entire column has to be repacked. The rate of settling again depends upon the diameter of the column. This bed instability arises because there is a significant change in wall support as the column diameter increases. In analytical columns the walls are relatively close to the center of the column and 'bridges' of packing particles can be formed across the bed, as shown in Figure 16. These bridges allow the longitudinal forces acting on the packing within the column to be dissipated to the walls. When a column is packed, it is never in its optimal configuration and there are always areas

nbsp; Packed Column Injectors In general, the sample injected onto a packed GC column ranges in volume from 0.5ml to 5ml and usually contains the materials of interest at concentrations ranging from 5%v/v to 10%w/v. The sample is injected by a hypodermic syringe, through a silicone rubber septum directly into the column packing or into a flash heater. Although the latter tends to produce broader peaks it also disperses the sample radially across the column. Direct injection into the packing constrains the sample into a small volume, but can cool the front of the packing. An example of a septum injection system used for packed columns is shown in figure 7. The silicone septum is compressed between metal surfaces in such a manner that a hypodermic needle can pierce it, but when it is withdrawn

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 accuracy and better precision and repeatability. Packed GC columns are usually made of stainless steel or glass and open tubular column almost

Effect of Mobile Phase Compressibility On the HETP Equation for a Packed GC Column  As the pressure falls along the column length, the velocity changes and, as the solute diffusivity depends on the pressure, the diffusivity of the solute will also change. The multi-path term, which contains no velocity or gas pressure dependent parameters, will be unaffected and the expression that describes it the same. The other terms in the  HETP equation, however, all contain parameters that are affected by gas pressure (solute diffusivity and mobile phase velocity) and, therefore, need to be modified to accommodate

may need to be determined by experiment. During the initial pressure adjustment some of the packing passes into the column and forms a lightly packed bed at the bottom of the column. The exit valve is hen rapidly opened and the sudden flow of gas packs and compacts the bed at the same time. After packing, the reservoir is carefully removed so as not to loosen the top of the packing and connected to the sampling system. LC Columns If particle sizes in excess of 20 mm are used, then the column can often be dry packed, with appropriate tapping, or, even better, with longitudinal and radial sonic vibration. The variance per unit length obtainable from a preparative LC column should be less than 2 particle diameters (determined using analytical scale samples). It is worth remembering that (as already discussed) when designing preparative columns, it is better to obtain the necessary efficiency using a longer column packed with larger particles, than the converse. The long column