Piston Pumps for Sample Aspiration and Transfer

01Define the sample and its contact path first

A sample channel must obtain a defined aliquot from a limited specimen, preserve its composition and deliver it to the reaction position. First decide where the sample will remain. With system-fluid displacement, the sample can stay mainly in the probe and front-end tubing while the pump contains system fluid. If sample passes through the pump, the pump, valves and tubing all become sample-contact surfaces that require cleaning. This decision comes before selecting the pump-head material or wash volume.

For example, the Roche GLUC3 method accepts serum, specified plasma types and urine under its respective method settings. FOREACH configures the metering channel around these defined conditions. When system fluid drives the sample, list the pump-contact liquid separately from the specimen contacting the probe.^[1]

Available specimen volume is not the same as the delivered dose. A 5 μL reaction dose does not mean that a tube containing only 5 μL can be sampled reliably. Probe immersion, inaccessible liquid at the container bottom, aspiration reserve and repeat tests all consume the specimen budget. For limited specimens, reduce unusable front-end volume and unnecessary aspiration loss first. This page concerns serum, plasma and urine prepared as required by the assay; cellular whole blood and disposable-tip pipetting require their own fluid-path design.

02Compare 5 μL and 20 μL doses with the complete stroke budget

The assay determines the working dose. Roche GLUC3 on cobas c 303, c 503 and c 703 uses a normal sample volume of 1.5 μL. That method point illustrates an even shorter stroke; it is not a FOREACH small-volume performance claim. The 5 μL and 20 μL points below are an independent capacity-selection example.^[1]

The nominal full-step volumes are 0.025 μL for the 50 μL configuration and 0.05 μL for the 100 μL configuration. More full steps for the same dose permit finer command increments, but transmission behavior, reversal, valve switching and tip retention still affect delivery. Accuracy does not improve in direct proportion to the number of steps.^[2]

If a project sets a mean-volume error target of ±1% at 5 μL, the absolute allowance is ±0.05 μL—one nominal full step of the 100 μL / 2,000-step configuration. This comparison calls for both mechanical and liquid-delivery evaluation; the availability of motor microstepping alone does not establish compliance. The overview explains why full-stroke specifications and the datasheet's 2%-stroke displacement tests cannot replace validation of this delivered dose.

Include auxiliary movements in the same uninterrupted stroke. For example, 20 μL of useful sample, a nominal 5 μL air gap and 5 μL of auxiliary displacement total 30 μL, leaving 20 μL of nominal capacity in a 50 μL pump. Adding another 30 μL segment in that stroke raises the budget to 60 μL; compare a 100 μL configuration or change the sequence. These auxiliary volumes are calculation assumptions, not standard sample-program settings. Check gas compression and segment position under the actual pressure and sequence.

Limited specimen volume calls for reduced front-end waste. Large auxiliary segments call for a stroke-budget check. Restricted installation space calls for comparison with SM or TM. These are different constraints and do not all lead to a larger pump.

03Keep the sample at the front end and deliver the aliquot to the vessel

For patient samples that change from tube to tube, a FOREACH piston pump can move system fluid while the sample remains mainly in the probe and front-end tubing. This limits the extent of sample contact with the pump, but does not remove interface mixing, tip retention or wash residue. Direct aspiration into the pump is also possible in a suitable dedicated fluid path, provided every wetted surface can be cleaned and sample composition is preserved.

Research on liquid-filled automated handling identifies liquid segments, air gaps and motion speed as variables affecting sample integrity. In a simple 5 μL delivery example, replacing 0.1 μL of sample with analyte-free system fluid lowers concentration by 2%, assuming additive volumes and no analyte loss. Measuring a total volume of 5 μL therefore does not, by itself, confirm the effective sample fraction.^[3]

Fix the tip-delivery arrangement early. Contact dispensing uses the vessel wall or existing liquid to assist droplet transfer, so contact position and subsequent cleaning matter. Non-contact dispensing requires the drop to detach completely; dispensing speed, outlet geometry and liquid surface properties work together. If the final drop remains on the tip, extra motor steps may affect the next delivery rather than repair the current one. Tune retraction as a separate action after the effective dose has transferred.

A deliberate air gap is different from an uncontrolled bubble in the pump. The former is a defined part of the segment program; the latter can consume displacement as pressure changes. When composition is wrong, inspect interfaces and delivery order. When volume fluctuates under an unchanged program, investigate air aspiration, leaks and trapped gas. Metering motion, valves and probe handling must be tuned against the same receiving position; motor completion and valve confirmation are not substitutes for the final liquid result.

04Separate slow aspiration, clots, analytical interference and bubbles

Distinguish liquid that has not reached its intended position in time from a specimen that is unsuitable for the assay. Fluid-path and motion changes may improve the former; the latter requires the method's specimen-handling rules. Treating every abnormal sample as a viscosity problem leads to the wrong compensation.

For confirmed slow aspiration without a blockage, first check inlet probe and tubing length and bore. Reduce aspiration speed, soften acceleration and allow the liquid column to settle while the probe remains in its specified position. Tune dispensing separately instead of slowing the entire cycle proportionally. These changes consume time: if volume recovers but throughput fails, improve the inlet path or redistribute the cycle time. Tecan's pump-selection guidance likewise relates liquid behavior to aspiration, dispensing, settling and throughput.^[6]

A stable low mean under repeatable conditions may justify working-point calibration after residue and dilution have been excluded. An abnormal first dose followed by normal doses points to idle recovery or priming. Occasional large deficits point first to clots, air aspiration or bubbles. A fixed volume offset is not an effective correction for a changing fault. EAS supports chamber deaeration; it does not provide specimen-clot or low-level detection.

Carryover can be translated into an assay error budget. If a 3 mmol/L glucose segment has 0.1% of its volume replaced by a preceding 30 mmol/L sample, an ideal mixing calculation gives an increase of (30 − 3) × 0.001 = 0.027 mmol/L, or 0.9% of the low value. This is a worked error-budget example, not a universal carryover limit; the assay defines the acceptable effect.

05Verify volume, sample integrity and recovery at the receiving position

EA offers an optional ISC1000 driver with RS-232, RS-485 or CAN interface configurations. Initial-position sensing and optional motor closed-loop optical encoder feedback are distinct options. FOREACH can match the metering drive, interfaces and valve components to the application; probe motion and assay coordination remain instrument-level functions.^[2]

For an illustrative acceptance target of ±1% mean-volume error and CV ≤1% at 5 μL, the acceptable mean is 4.95–5.05 μL and standard deviation is no more than 1% of the measured mean. A ±1% mean-volume target at 20 μL corresponds to 19.8–20.2 μL. These are project acceptance examples, not new small-dose product specifications. Define accuracy, repeatability and CV consistently with the selected test procedure.

Assess continuous sampling, the first dose after the longest planned idle period, the first dose after sample-change washing and sampling at low liquid level separately. Volume measurement establishes how much arrived; an appropriate tracer or analytical method checks unintended system-fluid dilution; a high-to-low sequence measures carryover. Verification of a liquid handler must cover the actual task and sample integrity as well as volume.^[7][3]

At 5 μL, a ±1% allowance is only ±0.05 μL. Record the specimen or surrogate, temperature, density, replicate count and measurement uncertainty, and control evaporation. Balance display resolution alone does not establish that the measurement method is adequate.

For configuration discussions, provide a concrete brief: fixed-probe serum channel; 5 μL routine and 20 μL maximum dose; identified system fluid and cleaners; auxiliary aspiration segments; complete cycle time; and separate first-dose and steady-state bias/CV targets. This narrows the choice to a useful capacity, wetted assembly and drive configuration.

References

Related guides

• Piston-pump capacity and configuration
• Predilution and resampling
• Reagent dispensing and first-dose recovery