When dispensing liquid from an autopipette, to which stop do you depress the plunger?

Question

BlackTom AI · Accepted Answer

Autopipette dispensing uses a two-stop plunger design: the first stop dispenses the calibrated volume and the second stop (the “blow-out”) expels residual liquid from the tip.

1. Relevant knowledge points and concepts
- Two-stop plunger function: first stop = dispense set volume; second stop = blow-out residual liquid.  
- Proper technique preserves accuracy (delivering the intended volume) and precision (consistent delivery) and prevents aerosols or splashing.  
- A short pause after the first stop allows the liquid to evacuate the tip by gravity and surface tension before using the blow-out.

2. Step-by-step reasoning process
- After aspirating the liquid and placing the tip against the inside wall of the receiving vessel, press the plunger to the first stop. This action dispenses the calibrated volume of liquid into the container.  
- Pause briefly (about 1 second) to allow remaining liquid in the tip to flow out due to capillary action and avoid dripping.  
- Press to the second stop (blow-out) to expel the small residual volume that remains after the first stop. This completes the dispense and ensures the full intended volume has been transferred.

3. Intermediate steps (practical technique)
- Hold the pipette vertically for aspiration; angle slightly for dispensing.  
- Contacting the vessel wall helps form a meniscus release point.  
- Pause between stops so liquid decants before blow-out.

4. Why the correct answer is correct
- Using first stop, pause, then second stop ensures the calibrated volume is delivered accurately and any residual in the tip is expelled, matching manufacturer-recommended technique for most pipettes.

5. Why the other options are incorrect
- “First stop only.” Omits blow-out, leaving residual liquid in the tip and under-delivering the true volume.  
- “Straight to second stop.” Skipping the pause causes turbulent ejection, possible splashing, loss of accuracy, and formation of aerosols.

6. Conclusion
The correct technique is: First stop, pause, then second stop. This matches the provided answer.

When dispensing liquid from an autopipette, to which stop do you depress the plunger?单项选择题

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