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HPLC A Praactical User'S Guide Part 7

Tham khảo tài liệu 'hplc a praactical user's guide part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 114 HARDWARE SPECIFICS I POOR GRADIENT STEP I ỊỈDEAL GRADIENT STEP I Figure 9.7 Gradient performance testing. pressures. All injectors on the market work on the principle of the loop and valve. In the load position system solvent is displaced from the injection loop with sample solution at atmospheric pressure. Sample goes into the loop at the point closest to the column and displaces loop solvent out the back end of the loop. Injection occurs by roating the injector against a Teflon seal to the inject position. This places the loop containing the sample in the solvent flow from the pump to the column. Since mobile phase enters the loop at the end opposite sample injection we achieve a last-in first-out loading of the sample onto the column head. No dilution of the sample solution occurs within the sample loop solvent volume. The loop is pressurized and the sample is washed onto the column Fig. 9.9 . Autosamplers take this same loop and valve principle and automate the filling and handle-turning sequence. The major differences between models on the market are in the way they get sample into the loop and the method of cleaning between injections. Most autoinjectors use a carousel loaded with sample valves to hold samples until their turn for injection occurs. Sample vials are usually capped with a screw cap fitted with a septum although some recent autosamplers replace the carousel with microtiter plates having 96-364 wells containing the samples for use with robotic workstations. Conical vials are available for limited samples and 1-mL injections are possible with some INJECTORS AND AUTOSAMPLERS 115 I 38b Parallel Isocratic System Figure 9.8 Gradient to parallel isocratic systems. a Gradient system b Parallel isocratic system. autosamplers. A few autosamplers are equipped with chillers to preserve sensitive samples such as proteins solutions until they are ready for injection. Some autosampler models use air pressure or a piston to push sample out of the vial into