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the modern liquid chromatograph. In the early years of the LC renaissance, there were two types of pump in common use; they were the pneumatic pump, where the necessary high pressures were achieved by pneumatic amplification, and the syringe pump, which was simply a large, strongly constructed syringe with a plunger that was driven by a motor. Today the majority of modern chromatographs are fitted with reciprocating pumps fitted with either pistons or diaphragms. For more information on HPLC pump requirements see the pump section in the HPLC supplement. The Pneumatic Pump The pneumatic pump has a much larger flow capacity than the piston type pumps but, nowadays, is largely used for column packing and not for general analysis. The pneumatic pump can provide extremely high pressures and is relatively inexpensive, but the high pressure models are a little cumbersome and, at high flow rates, can consume considerable quantities of compressed air. A diagram of

Unfortunately, even today, there is no LC detector that has an equivalent performance to the flame ionization detector (FID) used in GC. In general, LC detectors have sensitivities of two to three orders of magnitude less than their GC counterparts and linear dynamic ranges one to two orders of magnitude lower. Only highly specific LC detectors have sensitivities that can approach those of GC detectors. See also the section on detectors in the HPLC supplement. Detector Specifications Detector specifications are like those for GC detectors and are listed as follows, 1. Dynamic Range 2. Response Index or Linearity 3. Linear Dynamic range 4. Detector Response 5. Detector Noise Level 6. Detector Sensitivity or Minimum Detectable Concentration 7. Total System Dispersion 8. Sensor

preparative purposes and it was not until gas chromatography (GC) was introduced by Martin and Synge (1) was the technique used for analytical purposes. Even after the introduction of GC, liquid chromatography (then called column chromatography) was still used largely for preparative work. Liquid column chromatography evolved from a preparative procedure into an analytical technique during the late nineteen sixties, largely provoked by the development of high performance liquid chromatography (HPLC), which, in turn,� was largely sparked off by the successful development of GC. Initially, column loads were increased for preparative purposes by increasing the dimensions of the column both in GC and in HPLC. However, this approach has distinct limitations. If the column radius is increased, unless special packing techniques are employed, the packing procedure becomes inefficient and the packing itself unstable. In addition to maintain the optimum mobile phase velocity, the flow rate

in 1941 (3) and in their paper recommended the replacement of the liquid mobile phase with a suitable gas which would accelerate the transfer between the two phases and provide more efficient separations. Thus, the concept of gas chromatography was born. In the same paper in 1941, Martin and Synge suggested the use of small particles and high pressures in LC to improve the separation which proved to the critical factors that initiated the development of high performance liquid chromatography(HPLC). "Thus, the smallest H.E.T.P. (the highest efficiency) should be obtainable by using very small particles and a high pressure difference across the column

in the reservoir which ensures that the top of the column is tightly packed. The column is disconnected, the packing secured with a suitable frit, and then connected to the chromatograph. The major difference in equipment used for larger scale chromatography lies in the technology required to pack and maintain high-performance LC columns of large diameter. Another difference is that industrial-scale equipment is often dedicated to just one specific separation. Once the diameter of an HPLC column approaches 5 cm, additional difficulties arise in its preparation and operation. It is possible to pack a column 5 cm in diameter by conventional high-pressure slurry packing techniques but this may not be easy to do, particularly in a laboratory environment, and it becomes even more difficult and expensive for columns of wider diameter. There are other problems that need addressing, once packed, the practical lifetime of a column is also uncertain. The changes in performance of a

nbsp; Because any density gradients caused by the compression are longitudinal rather than radial they will not affect the flow profile along the bed. Thus, the bands remain undistorted and very high efficiencies can be obtained with this technology. In addition, there is no restriction on column diameter, and HPLC columns of this type with diameters as large as 80 cm have been made. A diagram of the axial compression packing system is shown in figure 18.   The apparatus consists of a column (capped at the top with a stainless steel frit) having a precisely controlled internal diameter and finish. The column contains a close fitting piston. The piston, which is mounted on a constant-pressure hydraulic jack, can be moved throughout the entire length of the column. It contains a porous frit at