Free Books and Brochures

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

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

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

Consider the column depicted in Figure 3. Figure 3. Pressure and Velocity Distribution Along a Packed Column The column may be packed or an open tube but in this example, a packed column will be specifically considered. Let the column have a length (L) and inlet and outlet pressures and inlet and outlet velocities of (Pi), (Po), (ui) and (uo), respectively. The pressure and velocity at a distance (x) from the front of the column will be defined as (Px) and (ux), respectively. According to D'Arcy's equation for fluid flow through a packed bed, at any point in the column,    &

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

Types of Small Bore Columns   The first small bore column to be explored was the packed column. The mass sensitivity of a detector-column combination is a function of the I.D. of the column and so the reduction of the columns diameter is highly desirable for trace analysis (mass sensitivity is the minimum detectable mass, as opposed to the minimum detectable concentration and depends not only on the detector sensitivity but also on the dimensions of the column). The I.D. of small bore packed columns is considered to be 1 mm or less but GC columns having an I.D of less than 0.5 mm are extremely difficult to pack. Small bore packed columns, however, are popular and extensively used in LC. The wall coated open tubular column (WCOTT), the impressive term given to capillary columns, have I.D.s lying between 100 mm and 550 mm and carry a film of stationary phase on the wall surface that ranges between 5 mm to 50 mm thick.   The third type of small bore column is the porous