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nbsp; Examination of figure 6 shows that the descending steps of the curves, which correspond to the tails of the normal elution curves, are very diffuse compared with the ascending steps, which correspond to the fronts of the normal elution curves. If the sample is placed on the column by allowing the mobile phase to flow through the loop for a given time (determined from the flow rate and the volume of sample selected for injection) and the valve then rotated to allow the mobile phase to pass directly to the column, the dispersion effect of the sample tube is virtually eliminated. The improvement resulting from this technique (often called back-cutting injection) is shown in the upper curve in figure 6. It is seen that the descending steps of the curve are very similar to the ascending steps showing that the tube dispersion has been significantly reduced. It should be pointed out that this injection procedure

The glass liner can be fitted with a separate heater and the volatalization temperature can, thus, be controlled. This "flash heater" system is available in most chromatographs. By using a syringe with a long needle, the tip can be made to penetrate past the liner and discharge its contents directly into the column packing. This procedure is called 'on-column injection' and, as it reduces peak dispersion on injection and thus, provides higher column efficiencies, is often the preferred procedure.   Open Tubular Column Injection Systems Due to the very small sample size that must be placed on narrow bore capillary columns, a split injection system is necessary, a diagram of which is shown in figure 8. Figure 8 The Split Injection System The basic difference between the two types of injection systems is that the capillary column now projects into the

diameter open tubular columns have been employed that would permit on-column injection. The columns have an I.D. of about 0.056 in., which is slightly greater than the diameter of a specific hypodermic needle. The injection system is shown in figure 7.     Figure 7. Device for On-Column Injection in Large Bore Capillary Columns   Unhappily, this type of injector also is far from ideal, not so much from poor accuracy and precision but from its effect on column resolution. On injection, the sample breaks up into discrete segments, due to bubble formation in the first part of the column. As the solvent evaporates the sample is deposited at two or more locations along the column. When development commences, each local concentration of sample acts as a unique injection and a chromatogram containing very wide or multiple peaks is produced. There have been a number of procedures introduced in an attempt to eliminate the sample splitting on the column. The first solution

of the sample. This problem was partially solved by using larger diameter columns that would permit on-column injection. The columns are constructed to have an I.D. of about 0.056 in; which is slightly greater than the diameter of a certain hypodermic needles. This injection system is depicted in figure 9. Figure 9 On-Column Injector for Large Bore Open Tubular Columns   However, there are also difficulties associated with this type of injector. On injection, the sample breaks up into separate portions, and bubbles form at the beginning of the column causing the sample to be deposited at different positions along the open tube as the solvent evaporates. On starting to develop the separation, each local concentration of sample acts as a separate injection. As a consequence, a chromatogram containing very wide or multiple peaks is produced. Procedures have been introduced in an attempt to eliminate sample splitting in this manner

has evolved over the years but it was not until the late 1950s that the various terms used for the characteristics of a chromatogram were rationalized. A summary of the nomenclature is shown diagramatically in figure 4. The various terms are defined as follows. Figure 4 The Nomenclature of a Chromatogram. The baseline is any part of the chromatogram where only mobile phase is emerging from the column. The peak maximum is the highest point of the peak. The injection point is that point in time/position time when/where the sample is placed on the column. The dead point is the position of the peak-maximum of an unretained solute. The dead time (to) is the time elapsed between the injection point and the dead point. The dead volume (Vo) is the volume of mobile phase passed through the column between the injection point and the dead point

nbsp;   Injection Systems   Due to the small dimensions of the column, a very small mass must be injected and this must take the form of a very sharp band of solute entering the column. There are a number of different injection devices that can be used and the appropriate form will depend on the dimensions of the capillary column, in particular its internal diameter. For columns that have diameters that preclude the entry of a syringe needle, a split injector must be used, a diagram of which is