HPLC injection: The technique and instrumentation
HPLC injection is a technique used to load the sample into the HPLC system.
This process is done using a specially designed injection system which is a separate and prominent part of the HPLC system.
The most popular injector is the rheodyne injector.
The injection is done manually by an external syringe or autoinjector. Then another inline injection system connected to the pump and column does the actual loading of the sample.
The injection system is carefully designed to see that there are no handling errors during the HPLC analysis. Also, the amount of sample injected during repeated tests is kept constant.
Even if we load more amount of sample by external syringe, still it injects the prefixed volume excluding the excess.
HPLC injection has certain requirements:
1. The volume of injection for all the samples under an experiment should be the same.
2. The injection volume is minimal so, it should be in the range of microliters.
3. The sample injected should be free of any particulate matter.
4. The injection sample should have no air bubbles.
5. The HPLC injection is always done when the mobile phase is kept under a constant flow rate through the column.
Rheodyne injector or loop injector as HPLC injection system:
This is the injection system used widely in HPLC. The system has a load mode and an inject mode. In injecting mode, the sample from a syringe is injected into the loop. Then the injection is set to inject mode where the loaded sample is left into the flow of HPLC mobile phase.
Any excess sample is transferred to drain. The system has loops of fixed volume like 20μl, 50μl, 100μl, and 200μl. So when a sample of 25 μl is injected into a loop of 20μl, then only 20μl is passed into the column and remaining is drained out.
The software in the computer has to be set for the injection phase. Then load and inject samples are specified to record the readings of that injection.
But most modern instruments have the capability of self-injection like shown in the video below
There the system takes the exact volume of the sample from a prefixed vial and injects into the column. The software controls the whole process of injection, pump pressure and detector.