WHAT IT IS
Injection type in liquid chromatography (LC) describes how a sample is introduced into the mobile phase and onto the column. Injections must be precise and reproducible, since errors affect peak shape, retention, and quantification. Injection can be manual or automated, and depends on how the sample loop or flow path is used.
HOW IT WORKS
Injection is carried out through a high-pressure valve. In manual systems, the operator loads the loop with a syringe and rotates the valve to start the run. In automated systems, an autosampler or sample loader aspirates the liquid, washes the needle, and injects the sample by switching the valve electronically. The type of injection defines how the loop is filled or bypassed.
TYPES
Full-Loop Injection
The loop is overfilled (≈3× its volume) to ensure complete filling. Provides excellent precision but consumes more sample.
Partial-Loop Injection
Only part of the loop is filled under autosampler control. More economical in sample use, but less precise than full-loop.
µL-Pickup / Direct Injection
The autosampler aspirates only the required volume and injects it directly. Minimal waste, suitable for scarce samples, but more prone to carryover.
Manual Injection
The sample is loaded with a syringe and introduced by rotating the valve. Low-cost and robust but operator-dependent.
KEY FEATURES
Injection Valves: Six-port rotary valves are standard; multi-port designs allow multiple loops or column switching.
Autosamplers: Provide unattended operation, programmable sequences, and precise loop filling.
Sample Loaders: Support higher-throughput formats (e.g., well plates, racks of vials).
Injection Volumes: Sub-µL in nano-LC, 1–100 µL in analytical LC, and up to mL in preparative LC.
Carryover Control: Needle wash stations minimize contamination between injections.
IMPACT ON PERFORMANCE
Accuracy and Precision: Determined by injection type and valve quality.
Peak Shape: Overfilling or underfilling loops can distort chromatographic peaks.
Sample Economy: Direct injection minimizes waste, valuable for limited material.
Throughput: Automated loaders enable long sequences without operator input.
CHALLENGES AND LIMITATIONS
Sample Use: Full-loop wastes material; direct modes risk carryover.
Reproducibility: Partial-loop and direct injections depend on autosampler calibration.
Hardware Wear: Valves, syringes, and needles degrade and must be maintained.
Scaling: Preparative LC requires robust valves and loops for large volumes.
Manual Variability: Human operation reduces reproducibility compared with autosamplers.