Check whether the metering device sees system pressure
The term autosampler metering pump does not mean low-pressure operation. Some flow-through designs expose the metering path to mobile phase after injection; other arrangements isolate the aspiration side. The instrument's valve-state diagram, rather than the component name, defines the pressure requirement.[1]
Consider EA, SM or TM only within the selected configuration's permitted pressure, media and drive limits. If the device is connected to the chromatographic high-pressure side, verify the complete-cycle pressure and isolation arrangement first. A general liquid-handling piston pump must not be substituted on the basis of dose volume alone.
| Provide | Establish |
|---|---|
| Valve diagrams during aspiration, switching, injection and washing | Whether the pump chamber communicates with the high-pressure path at any point |
| Steady pressure and switching transients | Required limits for the head, seals, fittings and drive |
| Original metering and control arrangement | Flow-through operation, pressure balancing, homing and interlock requirements |
Distinguish direct sample contact from system-liquid displacement
Sample enters the metering chamber
The head, piston, seals and associated valves become sample-contact components. Compatibility, adsorption, particles, residual liquid and cleaning recovery must be evaluated throughout this path.
System liquid moves a separate sample segment
The pump displaces system liquid while the sample remains primarily in the needle and front-end tubing. Pump displacement still matters, but sample-contact materials and carryover control concentrate around the needle, seat, valve and loop. Keeping sample out of the chamber does not eliminate carryover elsewhere.
For direct sample contact and system-liquid operation, the pump-head material selection guide helps frame the compatibility discussion. Include seals, valves and cleaning exposure, not only the visible pump head.
Use capacity and installation needs to shortlist EA, SM or TM
| Series | Nominal capacity | Reason to evaluate | Performance boundary |
|---|---|---|---|
| EA | 50 µL–20 mL | Broader capacity and configuration choice | Full-stroke accuracy and repeatability are specified at ≤0.5% under stated test conditions; verify the actual working dose separately. |
| SM | 50 µL–1 mL | Compact microvolume or system-liquid metering | Base configuration: 2,000 full-stroke steps and specified full-stroke repeatability ≤0.5%. Confirm accuracy for the configuration and working dose. |
| TM | 50–500 µL | Severely constrained installation space | Displayed configuration: 2,540 full-stroke steps. Pressure, accuracy, repeatability and service life require configuration-specific validation. |
Capacity, nominal volume per step and the smallest reliable delivered volume are different quantities. A smaller housing does not establish better small-dose accuracy. Compare a TM configuration only when its installation advantage is relevant.
Use the piston-pump selection guide to compare the capacity, drive and installation inputs after the pressure boundary is established. TM remains a space-driven option, not a default choice for every autosampler.
Evaluate the complete metering channel
- Check inlet seals, priming and valve timing when bubbles change the effective liquid column.
- Include fittings, valve cavities, needle seats and loops when calculating hold-up volume and sample consumption.
- Locate residue on the needle exterior, valve passages and seals before increasing wash-pump output.
- Test volume bias, repeatability and carryover at the target injection volumes rather than extrapolating full-stroke specifications.
Review the distinction between accuracy, repeatability and resolution before converting motor steps into an injection-volume specification. Acceptance must be measured at the working volume with the complete channel.
What the pump provides, and what the instrument must provide
Controlled piston travel creates liquid displacement. The drive defines aspiration and discharge motion, while the head and piston configuration determine available capacity and material options. These capabilities support a repeatable fluid-transfer sequence.
Source selection, needle positioning, liquid-level recognition, high-pressure isolation and washing depend on the surrounding valves, sensors, tubing and control program. The pump does not independently detect an empty aspiration or guarantee chromatographic peak-area precision.
Compare 100 µL and 500 µL capacities at the working dose
List minimum, routine and maximum injection volumes and the path displacement genuinely needed in each cycle. Once capacity is sufficient, compare the useful stroke fraction, refill count and cycle time.
For two illustrative configurations with 2,000 steps per full stroke, 100 µL capacity gives 0.05 µL per nominal step and 500 µL gives 0.25 µL per nominal step. A nominal 10 µL movement uses 200 versus 40 steps. This compares control headroom; it does not demonstrate 10 µL delivery accuracy.
A smaller capacity may require extra refills and valve changes for larger transfers. Select the capacity that balances the routine dose with the complete cycle, not simply the smallest or largest available pump.
Resolve material and small-dose questions before ordering
Does a full-stroke specification apply to every injection volume?
No. Short travel, a different liquid or another valve arrangement may produce different errors. Measure mean volume bias and repeatability at the minimum, routine and maximum working volumes.
Which head should be used with organic solvents?
Supply solvent identities, proportions, temperature, exposure time and cleaning agents. Check the head, piston, seals and valves together. A ceramic piston or a general chemical-resistance description cannot establish compatibility of the complete assembly.