Define the complete injection cycle
Metering is only one part of the sequence. Needle position, valve state, aspiration speed, dwell time and switching must be coordinated. Check pressure isolation in every state before evaluating a FOREACH configuration; this sequence is not a universal plumbing diagram.[1]
Translate the injection volume into a useful working stroke
Routine dose
Use the normal dose range to choose a repeatable, calibratable working stroke, rather than sizing only from the largest transfer. Reserve capacity for path filling only where the cycle actually requires it.
Priming and compensation
Needles, valve passages and tubing can consume displacement before a sample reaches its destination. Establish this volume from the actual path and experiments; do not count it automatically as injected sample.
Nominal step volume
Capacity divided by commanded full-stroke steps estimates command resolution. Mechanical lost motion, compressibility, bubbles and valve timing still affect the liquid result. Microstepping alone does not establish a minimum reliable injection volume.
Before specifying a short working stroke, compare accuracy, repeatability and resolution. A nominal volume per step does not establish the smallest reliable sample dose.
Separate volume errors, carryover and short-stroke variation
| Observed symptom | Possible contributors | First checks |
|---|---|---|
| Low or variable delivered volume | Air entry, incomplete needle immersion, incomplete filling or valve leakage | Inlet seals, vial level, priming, valve position and dwell time |
| A high sample affects the next result | Residue at the needle, seat, valve or poorly flushed regions | Wash chemistry, internal versus external cleaning, wash volume and blank recovery |
| Poor repeatability at small doses | Too little useful stroke, compressible gas or lost motion | Capacity choice, aspiration speed, reversal compensation and settling time |
When delivery varies after priming or idle periods, review air bubbles and dispensing error to plan checks for trapped gas and compressible volume. Confirm the cause before changing capacity or calibration.
Give the supplier the operating window, not only a target dose
| Input | Include |
|---|---|
| Volume and tolerance | Minimum, routine and maximum injection volumes; acceptable bias and repeatability at each point |
| Liquid and contact path | Sample or system liquid in the pump; solvent composition, viscosity, temperature, particles and gas-release risk |
| Program | Aspiration speed, dwell, reversal and valve timing; any deliberately introduced air segment |
| Hydraulics | Needle/tubing volume, inlet height and pressure in every valve state |
| Throughput | Injections per hour, uninterrupted running time and first-injection requirements after idle |
| Recovery | Carryover limit, wash procedure, bottle-change recovery and maintenance interval |
Verify the volume at the point where the sample is received
- Measure bias and repeatability at actual working volumes, using an appropriate validated measurement method.
- Include representative low- and high-viscosity samples and liquids prone to gas release.
- Follow a representative high-concentration sample with blanks to evaluate carryover and wash recovery.
- Compare initial startup, restart after idle, low reservoir level and continuous operation.
Keep pump delivery measurements separate from chromatographic peak-area repeatability. Sample stability, separation conditions and detector behaviour can change peak area even when displacement is repeatable.
Investigate when the error occurs before replacing the pump
An abnormal first injection points toward priming, the liquid column after idle or retained liquid. An error only at low vial level calls for immersion and inlet checks. Continuous variation requires checking bubbles, seals, drive commands and valve states together.
Establish a baseline with a fixed vial, liquid, needle position and aspiration program. Change one variable at a time. This distinguishes an undersized or unsuitable metering configuration from a correct pump operating in an unstable fluid path.
Use the dispensing-drift diagnosis guide to separate gradual drift from first-cycle and intermittent errors, then repeat the checks under the intended injection sequence.
Questions about speed and low carryover
Will faster aspiration always improve throughput?
No. Excessive speed can introduce air or prevent a restrictive path from filling promptly. Find a stable aspiration window for the representative liquids, then optimize needle travel, dwell and valve timing around it.
Is carryover eliminated when the sample stays outside the pump?
No. Sample can remain in the needle, seat, valve grooves and loop. A system-liquid drive reduces sample contact with some components but does not clean the entire injection path.