The Unseen Foundation: Accurate Pipetting

Introduction

  • In the medical laboratory, the measurement of volumes is often done in microliters (µL) and nanoliters (nL).
  • Pipettes are essential tools in controlling small volumes of liquid, making accurate pipetting a crucial skill in biotechnology.

Definition of Pipette

  • A pipette is a laboratory device used to measure out or dispense small amounts of liquid, typically in volumes of milliliters (mL) or microliters (μL).
  • The term "pipette" originates from a French word that translates to "small pipe."

Principle of Micropipettes

  • Air Displacement Pipette: The volume of air that is displaced within the pipette correlates to the selected aspiration volume.
  • Positive Displacement Pipette: This type utilizes a plastic tip that integrates an internal piston, ensuring precise dispensing of samples.

Types of Pipettes

  • Single-channel Pipettes: Designed to dispense one sample volume at a time.
  • Multichannel Pipettes: Capable of dispensing multiple samples simultaneously, which streamlines parallel processing tasks.
  • Pasteur Pipette: Used primarily for transfer of liquids without precise volume measurement.
  • Serological Pipettes: Used for transferring larger volumes of liquid, providing increased precision and measurement.

Classifications of Pipettes

  • Single-channel vs. Multichannel:
    • Single-channel allows for only one liquid transfer at a time.
    • Multichannel enables simultaneous transfer, usually in blocks of eight, twelve, or more.
  • Manual vs. Electronic Pipettes:
    • Manual pipettes require hand operation, while electronic pipettes automate the dispensing process for higher accuracy and convenience.
  • Fixed vs. Adjustable Volume Pipettes:
    • Fixed pipettes are set to a specific volume, while adjustable pipettes can be set to various desired volumes within their range.

Parts of a Micropipette

  • Plunger: Controls the movement of the piston within the pipette.
  • Volume Adjusting Knob: Allows for setting the desired volume to dispense.
  • Tip Ejector Button and Ejector Arm: Facilitates the removal of the pipette tip post-dispensing.
  • Digital Volume Display Window: Displays the set volume for accurate dispensing.
  • Plastic Shaft and Tip Cone: Serves as the entry point for the liquid to be aspirated.

Pipetting Techniques

  • Forward Filling: A technique where the sample is drawn into the pipette from the stock solution and dispensed into the receiving vessel.
  • Reverse Filling: Used to re-aspirate or refill the pipette with new liquid after partial dispensing.
  • First Stop and Second Stop: Refers to the two levels of plunger movement: the first is for aspirating, and the second for expelling contents.

Accuracy and Precision

  • Definitions:
    • Accurate: Measurement that is close to the true value.
    • Precise: Measurements that yield consistent results when repeated.
  • Scenarios depicted demonstrating various degrees of accuracy and precision, indicating:
    • Accurate and precise.
    • Accurate but not precise.
    • Precise but not accurate.
    • Neither accurate nor precise.

Calibration of Pipette

  • Gravimetric Method: The weight of a known volume can be measured to determine accuracy.
    • For instance, 100μL=1mg100μL = 1mg, 1000μL=1g1000μL = 1g.
  • Colorimetric Method: Utilizes color change or intensity to quantify a sample's volume based on a calibration curve.

Common Sources of Pipetting Error and Mitigation Strategies

  • Poor Technique: Leads to inconsistent volumes and aerosol generation.
    • Mitigation: Use smooth, consistent motions; pre-wet tips; immerse tips to the correct depth; select the appropriate pipette for the volume.
  • Thermal Effects: Air displacement pipettes are sensitive to temperature changes, leading to over-delivery.
    • Mitigation: Equilibrate all reagents and the pipette to room temperature before use.
  • Wrong Tip Usage: Using incorrect tips can create a poor seal, leading to volume inaccuracies.
    • Mitigation: Always use high-quality, manufacturer-recommended tips.
  • Not Calibrating Regularly: Pipettes can drift out of specification.
    • Mitigation: Implement regular professional calibration and frequent self-checks using gravimetric analysis.
  • Human Fatigue: Over time, fatigue can diminish precision and accuracy of pipetting.
    • Mitigation: Utilize electronic pipettes for high-throughput tasks and ensure regular breaks are taken to maintain performance.

Immersion Depth and Angle

  • Describes how the immersion depth affects accuracy:
    • 1 cm immersion: 0.2-0.4% inaccuracy.
    • 3 cm immersion: 0.6-0.8% inaccuracy.
    • 4 cm immersion: 1-1.2% inaccuracy.
  • Immersion angle should ideally remain vertical; deviations of over 20 degrees can lead to inaccuracies due to excess liquid entering the tip.

Pipetting Improvement Techniques

  • Dispensing Techniques: Contacting the wall of the tube while dispensing ensures no sample remains clinging to the orifice; this technique can enhance accuracy by up to 1%.
  • Aspiration Techniques: Maintaining a consistent rhythm during liquid aspiration prevents splashing and loss of sample volume, improving accuracy by up to 5%.
  • Pre-rinsing/Pre-wetting: Releasing the plunger preemptively a few times helps to neutralize capillary effects and equalizes temperature, improving accuracy by up to 0.2%.
  • Angle of Immersion: Should remain as vertical as possible, ideally avoiding deviations greater than 20 degrees, to decrease inaccuracies caused by excess liquid in the tip.

Conclusion

  • As laboratory demands for accuracy and precision increase, mastering effective pipetting techniques becomes essential for reliable results in experimental and clinical settings.