Micropipetting and aseptic technique
Wed, Dec 16, 2026 · Week 17 · Principles of Biomedical Technology (Principles of Biomedical Science)
Today's goal: Practise setting and using a micropipette, and explain why aseptic technique changes the number you get.
This is a model of the work you should turn in today. Use it to check your own: match the structure and the level of detail, do not copy it. Your data and wording should be your own.
The pipette I was issued: a variable P100, range 10 to 100 microliters. I wrote the range down first, because setting 5 or 150 on this barrel is outside what it can deliver, and forcing the dial past its stop damages it.
Setting the volume: I turned the dial down to 50 and read the display as 0 5 0, which on this model is 50 microliters. My partner read it back to me before I drew anything.
Seating a tip: press the shaft straight down into the tip in the box, one firm push. A tip on crooked leaks air, and a leak means the volume in the tip is not the volume on the dial.
The two-stop plunger, in order:
- 1. Press down to the FIRST stop before the tip goes anywhere near the liquid.
- 2. Put the tip just under the surface and release slowly to draw the set volume.
- 3. Move to the receiving tube and touch the tip to the tube wall.
- 4. Press to the first stop to deliver, then to the SECOND stop to push out what is left in the tip.
- 5. Eject the tip. Do not let the plunger back up while the tip is still in liquid, or the sample gets sucked back.
What a wrong first stop does: if I press past the first stop before drawing, I blow out extra air and pull up more than 50 microliters. Nothing about that looks wrong. The tube fills, the run finishes, and the number at the end is simply too high.
Labeling rule I wrote for myself: every tube gets labeled before any liquid moves, on the side and on the cap, because caps get swapped while a rack sits open. A tube I label afterwards is a tube I am labeling from memory.
Why contamination is worse than an obvious failure: if I reuse a tip between samples, a little of sample 1 rides into sample 2. Sample 2 still develops, the reading still lands in a believable range, and there is nothing in the number itself that tells me it is wrong. A dropped tube announces that it failed. A shared tip does not. That is why the tip changes between every sample and the bench gets wiped before I start.
| Step | Plunger position | What is happening |
|---|---|---|
| 1 | First stop | Air pushed out before the tip enters the liquid |
| 2 | Released slowly | The set volume is drawn into the tip |
| 3 | First stop | The set volume is delivered into the receiving tube |
| 4 | Second stop | Residual liquid is pushed out of the tip |
| 5 | Eject | Tip discarded before the next sample is touched |
This model shows the level of evidence and organization needed to complete: Completes the technique note-taking task: the issued pipette's volume range, the two-stop plunger written out in order, what a wrong first stop does to a transfer, the labeling rule, and why contamination produces a believable wrong number.
- Date and label the entry.
- Record the procedure, observation, or design decision clearly.
- End with what the evidence means and the next step.
Keep the structure. Replace the question, facts, measurements, and evidence. Then recheck units, vocabulary, and whether the conclusion goes beyond the evidence.
Also due today: Complete the assigned PLTW online activity on micropipetting and aseptic technique, then keep your notes for Wednesday's screen.
Claim ceiling: Today's evidence supports a classroom claim about today's lesson. It cannot prove causation, diagnose a real patient, or justify action outside this room.
One exam-style question that uses exactly what you practiced today. Try it before you reveal the answer, then read why each choice is right or wrong.
Tap an answer to see the full explanation. Nothing is recorded or graded.
It builds this reusable test skill: Explaining why a reused pipette tip is a hidden error rather than an obvious failure.
- Name the concept or data pattern being tested.
- Cross out choices that violate that rule or the evidence.
- Justify the best remaining choice before checking the answer.