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a considerable amount of basic physical chemical data, such as the adsorption isotherms of each solute measured over a wide range of concentration, before it can be applied to a practical problem. Only if the separation problem demands an extremely high through-put, and the process must be as economic as possible, will it be worthwhile to gather the necessary basic data. The problem of mass overload is more conveniently and economically solved by a simple experimental approach. Depending on the ultimate concentration of solute at the point of injection, a number of effects can take place when a large sample mass is placed on the column. In the first instance, there will be the effect resulting from the limited capacity of the stationary phase. On injection, the sample will spread along the column, carried by the mobile phase, until it contacts sufficient stationary phase surface to allow it to be held on the surface under equilibrium conditions. This will result in a spreading

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

" 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 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

analyses carried out using the high efficiency small diameter capillary columns may have limited accuracy and precision, depending on the nature 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

valves are set so that solvent from the reservoir is pumped through the injection valve to the column and the column outlet valve is set to the fraction collector. After the sample has been placed on the column, the column outlet valve is set to pass the column eluent back to the pump and the solvent selection valve set to the column eluent. Thus the mobile phase is continuously circulated through the column, through the detector, back to the pump and then back to the column. As a result the column is used over and over again many times, and each time the sample passes through the column, the resolution is improved. Unfortunately, the resolution is not necessarily proportional to the number of cycles, as significant peak dispersion can occur each time it passes through the pump. Nevertheless, there is a substantial net gain in resolution on each cycle. This procedure can be very time consuming if long retention times are involved, but has the great advantage of solvent economy.

..24).   Two types of polymer coating were used on the fibre, polydimethylsiloxane (100 mm thick) and polyacrylate (95 mm thick). The fibers were prepared by heating the fibers in the injection port at about 250�C until no peaks could be detected on a blank run. A Varian Model 3400 gas chromatograph coupled to a Varian Saturn ion-trap mass spectrometer was used for the analysis. The capillary column was 30 m long, 250 mm I.D. and carried a film of SPB-5 stationary phase, 0.5 mm thick. The column was held at 40�C for 5 minutes, heated to 100�C at 50�/min., then to 275�C at 5�/min. and finally heated to 300�C at 30�/min. where it was held for 2 min. The injector and transfer lines were held at 250�C. the mass range scanned was 45-400 mu. A chromatogram showing the separation of 60 pesticides, using the polyacrylate fiber for extracting the sample, is shown in figure 64. The concentration of each pesticide in the water was 100 ng/liter indicating a very high sensitivity and a very