Overview
This lecture covers the correct use of pipettes in the lab, focusing on steps for accurate liquid transfer and minimizing errors or contamination.
Choosing and Adjusting the Pipette
- Select a pipette that matches the volume you need to dispense.
- For 100 microliters, use a pipette labeled for 20 to 200 microliters.
- Adjust the pipette so the display shows the correct volume before use.
Preparing the Pipette and Tip
- Attach the appropriate tip to the pipette, ensuring a snug fit.
- Color coding may help select tips, but always confirm proper fit.
Drawing Up the Liquid
- Before inserting into liquid, press the plunger until you feel resistance (first stop).
- Immerse the tip just below the surface of the liquid, not too deep.
- Slowly release the plunger to draw liquid into the tip.
- Keep the tip submerged to avoid air bubbles, which cause inaccurate volumes.
Dispensing the Liquid
- Hold the receiving container at a 45-degree angle and keep the pipette vertical.
- Dispense the liquid against the side of the container to ensure complete transfer.
- When you feel resistance, continue pressing to expel all liquid.
- Avoid creating bubbles during dispensing.
- Withdraw the tip before releasing the plunger fully.
Common Sources of Error
- Poorly sealed or leaky tips reduce measurement accuracy.
- Reusing pipette tips decreases precision.
- Not dispensing against the container wall can leave drops in the tip.
Technique Validation
- Test accuracy by dispensing several 100 microliter aliquots onto a scale; each should weigh 100 milligrams.
Key Terms & Definitions
- Pipette — A tool used to transfer precise liquid volumes.
- Plunger — Button on the pipette used to draw and dispense liquids.
- Aliquot — A measured sub-volume of a liquid sample.
Action Items / Next Steps
- Practice pipetting and test your accuracy using water and a scale.
- Make sure to always use a fresh, properly fitting pipette tip for each transfer.