Trace the sample path component by component
| Arrangement | Fluid movement | Selection checks |
|---|---|---|
| Pull loading | A downstream metering device draws sample through the needle and valve into the storage path | Inlet resistance, continuity of the liquid column, designed air segments and stopping position |
| Push loading | The metering drive sends sample toward the loading port and loop | Sample-contact materials, outlet resistance, residue and validated overfill |
| Injection state | Mobile phase transfers the loaded sample to the column | Whether the metering device remains pressure-connected; valve isolation and switching sequence |
Pump displacement is not automatically loaded sample volume
Displacement can be used to fill the needle, connecting tubing and valve passages before the intended sample reaches the loop. Incomplete priming, trapped gas or early switching can therefore separate theoretical pump movement from actual loaded volume.
Define the injection strategy and validate path-fill and overfill volumes. Count the volume that this pump must move per cycle, including required path displacement, while keeping the routine metering segment within a useful stroke range.
The guide to accuracy, repeatability and resolution explains why commanded displacement is not sufficient evidence of loaded sample volume. Check the loop, connecting volumes and loading mode together.
Compare full-loop and partial-loop requirements
| Mode | Pump requirement | Customer implication |
|---|---|---|
| Full-loop | Move the loop volume plus path filling and experimentally established overfill | Sample consumption may exceed nominal injection volume; check the usable volume in small vials |
| Partial-loop | Meter the target sample segment within the limits of the injection procedure | Volume control and path condition become particularly important; validate each working point |
There is no universal overfill multiple for every loop. Geometry, dispersion, loading strategy and sample properties change the required excess. Use the instrument procedure and loading experiments rather than a fixed multiplier.
Coordinate aspiration, settling, switching and recovery
| Control point | Question | Acceptance observation |
|---|---|---|
| Aspiration speed | Does filling cause bubbles, volatilization or vial-surface disturbance? | Consistent loaded sample across representative liquids |
| Post-aspiration dwell | Does the liquid column settle before switching? | Stable loaded volume as dwell is adjusted |
| Valve movement | Is metering complete and valve position confirmed? | Correct transfer with defined interlocks for failed switching |
| Wash and reset | Is the path clean and the starting position verified? | Acceptable carryover and drift over a sequence |
If loop loading becomes inconsistent after switching or priming, use the guide to air bubbles and dispensing error to investigate gas in the transfer path before increasing the loading volume.
Choose the capacity around transfer volume and useful stroke
EA offers a broad capacity range for transfer tasks; SM is a compact candidate within its capacity range. TM should enter the comparison when the 50–500 µL range and a severely constrained installation are both relevant. None is selected before confirming pressure exposure and required isolation.
For direct sample contact, examine adsorption, particles, salt precipitation and wash recovery. For a system-liquid drive, verify the system liquid against the complete head, piston, seal and valve assembly. Request configuration-specific evidence at your working volume.
Review pump-head material selection with the actual sample or system liquid and wash solvent. The selected head is only one part of the complete wetted-path assessment.
Validate loaded volume and sample use together
- Test loading bias, repeatability and linearity across the intended volumes.
- Measure sample consumed per cycle, including discarded overfill and priming.
- Check loop, seat and valve carryover after repeated high-concentration injections.
- Repeat at representative viscosity, volatility and vial level.
- Test interlocks for a needle out of position, valve failure and inlet air entry.
Common questions about loop size
Does a 20 µL loop require only 20 µL of sample aspiration?
Not necessarily. Full-loop loading may also fill the needle and valve path and discard excess sample. Partial-loop programs control a defined segment. Establish the mode first, then calculate total sample use.
Can a larger pump handle every loop without a trade-off?
A larger capacity may reduce refills, but a routine small dose may occupy very little travel. Compare total transfer volume, stroke fraction, sample consumption and cycle time together.