Rheodyne Injector

Injectors for liquid chromatographic systems should provide the possibility of injecting the liquid sample within the range of 0.1 to 100 ml of volume with high reproducibility and under high pressure (up to the 4000 psi). They should also produce minimum band broadening and minimize possible flow disturbances.

Generally, the most useful and widely used sampling device for modern LC is the microsampling injector valve.

Rheodyne injector

Because of their superior characteristics, valves are now used almost to the exclusion of syringe injection. With these sampling valves, samples can be introduced reproducibly into pressurized columns without significant interruption of flow, even at elevated temperatures.

A clockwise rotation of the valve rotor places the sample-filled loop into the mobile-phase stream, with subsequent injection of the sample onto the top of the column through a low-volume, cleanly swept channel. Other valve types (e.g., Siemans and Valco) use an internal sample cavity consisting of an annular groove on a sliding rod that is thrust into the flowing stream. The minimum injection volume which can be made with the valve-type injectors is 60 nl.

Valve injection allows the rapid, reproducible, and essentially operator-independent delivery of a wide range of sample volumes (e.g., from 60 nl up to several milliliters), at pressures up to 7000 psi with less than 0.2% error. High-performance valves provide extracolumn band-broadening characteristics comparable or superior to that of syringe injection. Manually operated valves are only moderately expensive, and automated versions can be obtained at somewhat higher cost. A minor disadvantage of most sample valves is that the sample loop must be changed to obtain various sample volumes, but this can often be achieved in a few minutes. Another advantage of sampling valves is that they can be located within a temperature-controlled oven for use with samples that require handling at elevated temperatures (0′ – 150’C).

Low-volume switching valves are also available (e.g., Valco, Rheodyne, Siemans) for use in special techniques such as recycle chromatography and column switching. Some of these valves can be operated at pressures up to 7000 psi, and often they can be used at elevated temperatures. The more common valves can be obtained in 3-, 4-, 6-, 8-, or 10-port configurations, for use in either the manual or automated mode.

Most of the autosamplers are microprocessor controlled and can serve as a master controller for the whole instrument.