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 being separated such as essential oils and flavor components. However, glass has pressure limitations and for long packed columns, stainless steel columns are used as they can easily tolerate the necessary elevated pressures. The sample must, of course, be amenable to contact with hot metal surfaces. Short columns can be straight, and installed vertically in the chromatograph. Longer columns can be U-shaped but columns more than a meter long are usually coiled. Such columns can be constructed of any practical length and relatively easily installed. Pyrex glass columns are formed to the desired shape by coiling at about 700˚C and metal columns by bending at room temperature. Glass columns are sometimes treated with an appropriate silanizing reagent to eliminate the surface hydroxyl

-nickle but, although satisfactory for dispersive stationary phases, also produced unstable films with polar materials. Stainless steel proved little better and the surface of the stainless steel exhibited catalytic activity resulting in some substances being oxidized and others to suffer molecular rearrangement. At that time only dispersive and some semie-polar substances could be used as stationary phases. In 1957 Scott (2) demonstrated the effective use of Nylon columns for use as capillary columns. He found that they coated well with squalane and dinonyl phthalate and could be used in extremely long lengths (e.g. 1000 ft) to produce very high efficiencies. A column 10000 ft long 0.020 in. I.D. was shown to give efficiencies of up to one million theoretical pates. Nylon, although an interesting material and due to its ready availability was a simple way of producing long columns and high efficiencies, is not a suitable material for general use due to its operating temperature being

Open Tubular Columns   The majority of contemporary open tubular columns are made from fused quartz although, even today, in some applications (mainly hydrocarbon mixtures) stainless steel columns are still used providing equivalent performance. Nevertheless, fused quartz columns are perceived as "state of the art" columns. The internal surface of an open tubular column normally requires some deactivation and/or cleaning before it can be coated with stationary phase. Deactivation procedures (although usually very simple) are usually considered as highly proprietary. Under certain circumstances samples may need the column to be specially treated including particular column deactivation procedures, but most samples, can be successfully analyzed on columns

the sample loop should not be left in-line during development. The valve should be returned to the 'flush' position immediately after injection, so that the 'tail' of the sample left in the loop does not cause dispersion and tailing on the column. This procedure may entail sample recovery from the residue contained in sample valve. Preparative Columns Preparative columns (GC or LC) are usually made of glass or stainless steel the latter being used for high pressure systems. Preparative columns must be designed to accommodate the inlet pressure necessary to obtain the required flow rate through the packed bed which is determined by the size of the particles selected for the packing. The larger the column diameter, the stronger must be the column and the thicker the walls. Large column operating at high pressures with relatively small particles can become extremely bulky and heavy. In addition, the construction of wide columns (3 in. O.D. and greater), irrespective of the

Components present at a level of 0.001% can be easily separated and determined quantitatively without any preliminary fractionation or concentration. The Capillary or Open Tubular Column Capillary columns are fabricated from stainless steel or quartz. Metal capillary columns must be carefully cleaned to remove traces of extrusion lubricants before they can be coated, usually by washing with methylene dichloride, methanol and then water. After removing oil and grease, the columns are washed with dilute acid to remove metal oxides or other corrosion products that may remain adhering to the walls, washed with water and the again washed with methanol and methylene dichloride. Finally the column is dried in a stream of hot nitrogen. Metal columns provide the high efficiencies expected from open tubular columns and were used for the analysis of petroleum and fuel oils, etc. Metal columns, however, have some disadvantages as although easily coated with

chromatograph are made from stainless steel, however for biologically sensitive, or labile substances, biocompatible materials may be necessary for certain parts of the apparatus. To fabricate the bulky preparative components (e.g. the column) from titanium that have sufficient strength can become inordinately expensive and plated base metal (e.g. using gold or some other suitable biocompatible metal) may be a more economic alternative. Pumps The pump must have adequate capacity, and for columns up to 1 in. diameter, pumping rates of at least 150 ml/min. should be available. For larger columns, pumping rates of 500 ml to 1 liter per min. may be necessary. Owing to the limited strength of large diameter columns pressures much above 6000 p.s.i. are rarely required. Any glands involved in the pump should be made from appropriately inert material (e.g. 'Teflon' or 'loaded' 'Teflon', polytetrafluoroethylene,). In preparative chromatography, the sample is often placed on the column