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.
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.
Figure above shows schematic drawings of a six-port Rheodyne valve in which the sample fills an external loop. Compared to shorter, wider i.d. sample loops, long, narrow loops are preferred when large sample volumes are required, because of lesser band-broadening effects. Alternatively, a specially designed syringe may be used to inject a small volume (e.g., <10 ,ul) into the loop when required, although in this case the precision in the sample introduction is dependent on the precision of syringe delivery.
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.
With commercially available automatic sampling devices, large numbers of samples can be routinely analyzed by LC without operator intervention. Such equipment is popular for the analysis of routine samples (e.g., quality control of drugs), particularly when coupled with automatic data-handling systems. Automatic injectors are indispensable in unattended searching (e.g., overnight) for chromatographic parameters such as solvent selectivity, flowrate, and temperature optimization.
Most of the autosamplers have a piston metering syringe type pump to suck the preestablished sample volume into a line and than transfer it to the relatively large loop (~100 ml) in a standard six-port valve. The simplest autosamplers utilize the special vials with pressuarization caps. A special plunger with a needle, push the cap down to the vial and displace the sample through the needle into the valve loop.
Most of the autosamplers are microprocessor controlled and can serve as a master
controller for the whole instrument.