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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

(e) which is generally accepted (2). The properties of the three columns are shown in table 1 including the peak volume, (4sv), which has more significance to the practicing chromatographer It is seen that the dead volume peak widths are very narrow (assessed as (4sv(k'=0)) and range from about 5 ml for the peak from the microbore column to about 15 ml for the 3 mm I.D. and 4.6 mm I.D. columns.   Dispersion in Contemporary GC Columns The same mathematical arguments apply to GC columns as to LC columns except that, as the mobile phase is compressible, the pressure correction factor must be applied.                              viz.         Consequently, for a GC packed column, from equation (1) and                 In addition, for a capillary column, which has no packing, the column volume will be . where (rc) is the radius of the capillary column. Furthermore, the approximate value of (Hmin.) (see Dispersion in Chromatography Columns ) will be

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 exclusively fused quartz

geometry of the fluid conduits of the detector and not its sensitivity. This provoked detector redesign, with smaller sensor volumes, different geometry and shorter connecting tubes between the column and sensor. In turn, these modifications allowed much smaller particles to be used in the column resulting in even lower column dispersion and higher efficiencies. In this way, just as in GC, detector design and column design have interacted over the years to a point where the performance of LC columns are now commensurate with those of GC columns

the detector. The logarithm of the detector output is plotted against the logarithm of the calculated solute concentration and the magnitude of the response index determined from the slope of the curve in the manner described in book 4. The response, noise and sensitivity are measured in exactly the same way as for GC detectors. Pressure sensitivity and pressure tolerance have a more important significance in LC as in multidimensional LC, the detector may be situated between two or more columns and thus must tolerate pressures up to the input pressure (e.g., several thousand p.s.i). Pressure sensitivity and flow sensitivity are also more important in LC due to the relatively high pressures involved and the sensitivity of many sensors to pressure changes (e.g., the refractive index detector and the UV detector). However, LC columns have a high impedance to flow and so pressure pulses are often smoothed out in the column and do not reach the detector. Dispersion that takes

History of Capillary Columns   There were no commercial capillary columns available in 1958 and all operational columns were 'home made'. In fact, the basic characteristics of capillary columns and their operating conditions were determined, almost exclusively, using these home made columns. The first columns were made from copper capillary 1/16 in. O.D. and 0.010 I.D. which was readily available commercially and relatively inexpensive. The standard length then was 100 ft and the stationary phase was coated on the walls of the column from a solution in a