Operate a micropipette to the correct stop
The plunger has two stops and they do opposite jobs. Learn which one fills the tip, which one empties it, and why the order decides your volume.
- A plunger can have more than one resting place: Students who have only used a dropper expect one press and one release. The two-stop plunger is the whole idea behind the instrument and it has to be felt before it can be used.
- Moving air moves liquid: The pipette never touches your sample with its own parts. It moves a column of air, and the air moves the liquid, which is why leaks and fast movements cost you volume.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Two errors account for most wrong volumes, and neither of them shows up on the dial.
A protocol calls for 50 microliters. A student presses the plunger to the very bottom before putting the tip in the buffer, then releases. What is now in the tip?
Reviewed- A.Less than the dial shows, because part of the stroke was spent on air
- B.Exactly the amount on the dial, because the dial sets it
- C.More than the dial shows, because the extra stroke pulled liquid in too
- D.Nothing at all, since a plunger already at the bottom cannot lift
Show the worked solution ▾
Answer: C. More than the dial shows, because the extra stroke pulled liquid in too
- Step 1: Work out how far the plunger traveled: It went to the bottom, which is further than the dial's own stroke.
- Step 2: Turn travel into liquid: A longer stroke moves a bigger column of air, and a bigger column of air brings in a bigger sample.
Why it's right: Starting from the bottom means the plunger rises through its own stroke plus the extra travel, so the sample that follows the air is larger than the dial ever promised.
- A: Under filling comes from air escaping or from lifting the tip early. Here the stroke was longer than it should be, not shorter.
- B: The dial only sets the stroke it controls. Starting from the wrong place adds travel the dial knows nothing about.
- D: A plunger at the bottom rises perfectly well, and it is exactly that rise, longer than intended, that causes the problem.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
A tip is pushed onto the pipette loosely and it wobbles. Every volume this student delivers comes out low. What explains the pattern?
Reviewed- A.Air escapes around the loose tip instead of moving the sample
- B.A loose tip has a smaller opening, so it holds less
- C.The dial slips out of position when the tip is not seated
- D.The wobble shortens how far the plunger can travel
Show the worked solution ▾
Answer: A. Air escapes around the loose tip instead of moving the sample
- Step 1: Trace the air: The thumb pushes a column of air toward the tip, and the sample only moves because that column has nowhere else to go.
- Step 2: Give the air somewhere else to go: A gap where the tip should seal is exactly that, and the part of the column that goes out through the gap does no work.
Why it's right: A gap where the tip should seal lets part of the air column leave without pushing anything. Whatever the air does not push does not arrive, so every delivery falls short.
- B: The opening at the end of the tip is the same size however tightly it is pushed on. Nothing about the tip itself changed.
- C: The dial holds its setting independently of the tip, and a slipped dial would give scattered volumes rather than volumes that are low every single time.
- D: The plunger travels its full stroke whatever the tip is doing. The two are not connected mechanically.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
- A student whose practice weights all read low checks the tip seal before blaming the balance.
Fill these in as you work through the lesson.
- First stop (the stroke that matches the dial):
- Second stop (the extra stroke that clears the tip):
- Blow out (what the second stop does on the way down):
- Air displacement (air moves, and the air moves the liquid):
To fill, press to the stop, put the tip in, and release . To deliver, press to the first stop and then keep going to the stop, and lift the tip clear of the liquid you release.
- Describe what your thumb feels between the first stop and the second stop.
- Explain what a fast release costs you and where that liquid goes.
- Say why the dial reading can be right while the delivered volume is wrong.
You need 20 microliters. Press to the first stop with the tip in the air, lower the tip a few millimeters under the surface, release your thumb slowly, and lift straight out. Over the receiving tube, press to the first stop, keep pressing to the second stop, lift the tip clear, and only then let your thumb come up.