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 glass liner and a portion of the carrier gas sweeps past the column inlet to waste. As the sample passes the column opening, a small fraction is split off and flows directly into the capillary column, ipso facto this device is called a split injector. The split ratio is changed by regulating the portion of the carrier gas that flows to waste which is

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 shown in figure 6.   The sample is injected into the hot liner where it is vaporized into a gas stream flowing at (Q) ml/min. It is seen that the capillary column projects into the glass liner and the carrier gas sweeps past the column inlet and out to waste. As the vapor passes down the injector body it splits into two streams some entering the column at a flow rate of (q) ml/min. and the remainder passes out of the system to waste at a

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

nbsp; As with all small radius open tubular columns a split injection system must be used. In addition, the relatively wide boiling range of the gasoline will require a temperature program that will heat the column to 200oC or more and thus the stationary phase must be thermally stable. The components of the gasoline are present over a wide concentration range and thus, for accurate quantitative results, the linear dynamic range of the detector must also be large. These latter requisites mandates the use of an FID.   A separation of gasoline

) and these are values measured at atmospheric pressure with an inlet/outlet pressure ratio (g) of 2. Consequently, the sample volume at the inlet pressure will be proportionally smaller. The large column, however, can accept more reasonable sample volumes and at k'=5 on the 300 mm column a sample volume of nearly 8 ml can be tolerated. However, as the smaller columns provide both the fastest analysis and the greatest resolution, small sample volumes must be employed. This is achieved by using split injection systems. A diagram of a split injector is shown in figure 3. Figure 3 The Split Injector

the first part of the column. This causes the sample to be deposited at two or more positions along the tube as the solvent evaporates. When the separation is developed, each local concentration of sample can act as a separate injection and, as a result, a chromatogram containing very wide, double, or even multiple peaks may be produced. However, by removing the stationary phase from the first few centimeters of column and injecting the sample into this section of the column the sample will split and vaporize in the normal way. However, as there is no stationary phase present, the solutes will all travel at the speed of the mobile phase down the column until they reach a coated section of the column. At this point they will be absorbed into the stationary phase and all the samplewill accumulate at that point. The temperature program is then started (initially from a fairly low temperature). This helps the solutes to accumulate at one point in the column (i.e., where the