Selecting the Proper Air Displacement Pipette For Maximum Accuracy
One of the most frequently used tools in a laboratory is the air displacement pipette, also known as an air-cushion pipette. These pipettes are widely used due to their convenience, ease of operation, and ability to minimize cross-contamination when paired with disposable pipette tips.
But do you know how to select the right pipette for maximum accuracy? The accuracy of an air displacement pipette depends on multiple factors, including volume settings, temperature, and pipette mechanics. Choosing the correct pipette for your volume range is essential to ensure precision, reproducibility, and reliable lab results.
In this blog, we’ll explore how air displacement pipettes work, how accuracy is affected by volume settings, and how to select the right pipette for your specific needs.
For quick reference, be sure to check out our downloadable PDF guide—perfect for printing and keeping in your lab as a handy accuracy reference.
At its core, an air displacement pipette is a simple yet precise instrument. It operates using a piston and an air cushion to aspirate and dispense liquids. Here’s a step-by-step breakdown:
- When a pipette tip is attached, the piston moves forward, pushing air out.
- The tip is then immersed in liquid, and the piston retracts, creating a vacuum.
- Atmospheric pressure pushes the liquid into the tip, filling it with the measured volume.
- To dispense, the piston moves forward again, displacing the liquid from the tip.
Importantly, the liquid never comes into direct contact with the pipette itself—only the tip. This prevents contamination and ensures consistent measurement accuracy.
For adjustable-volume pipettes, the stroke length of the piston is varied. A 50% volume setting means the stroke length is halved, and accuracy depends on how precisely this adjustment is made.
Temperature fluctuations can significantly impact pipetting accuracy, particularly when using air displacement pipettes. This is due to the thermal expansion and contraction of air inside the pipette, as well as changes in the density and viscosity of the liquid being pipetted.
- Cold liquids (below room temperature) shrink, leading to under-pipetting (less liquid aspirated than expected).
- Warm liquids (above room temperature) expand, leading to over-pipetting (more liquid aspirated than expected).
- The pipette mechanism itself can be affected by environmental temperature changes, which may alter precision.
Additionally, liquid viscosity changes at different temperatures:
- Warmer liquids are less dense and more fluid, making them easier to aspirate but more prone to evaporation.
- Cooler liquids are denser and more viscous, requiring more force and time to pipette accurately.
- Allow liquids and pipettes to equilibrate to room temperature before pipetting.
- Use reverse pipetting for viscous liquids to improve accuracy.
- Pre-rinse the pipette tip with the liquid to normalize temperature differences.
- Use positive displacement pipettes for highly volatile or viscous liquids to minimize errors.
For pipettes with nominal volumes of 100 µL or greater, inaccuracy remains relatively constant across different volume settings. According to ISO 8655 Part 2, this value is set at ≤± 0.8% of the nominal capacity (equivalent to ≤± 0.8 µL for a 100 µL pipette).
However, as volume decreases, inaccuracy becomes more pronounced. This occurs due to:
- Mechanical limitations of the piston movement.
- The compressibility of air, which impacts smaller volumes more significantly.
Laboratory pipette datasheets often list accuracy as a percentage of the nominal volume, but users must interpret what this means at different settings.
Accuracy of a 100 µL Pipette at Partial Volumes
To illustrate how accuracy declines at lower volumes, consider the following accuracy chart for a 100 µL pipette:
| Volume Setting, µL | Percentage of nominal capacity | Volume of inaccuracy,≤± µL | Accuracy percentage of pipetted volume, ≤± |
|---|---|---|---|
| 100 | 100% | 0.8 | 0.8% |
| 75 | 75% | 0.8 | 1.1% |
| 50 | 50% | 0.8 | 1.6% |
| 20 | 20% | 0.8 | 4% |
| 10 | 10% | 0.8 | 8% |
Key Takeaways:
- Pipette accuracy is highest at or near its nominal capacity.
- Below 50% of nominal capacity, accuracy drops significantly.
How to Choose the Right Pipette for Maximum Accuracy
To ensure the most precise measurement, always use a pipette where your desired volume falls within the upper range of its capacity.
Example Pipette Selection: Measuring 20 µL
If you need to pipette 20 µL, you have several options:
- 2–20 µL pipette – Best choice for highest accuracy
- 5–50 µL pipette – Acceptable but slightly less accurate
- 10–100 µL pipette – Less accurate, avoid if possible
If your lab lacks a wide range of pipette sizes, adding one or two additional instruments can greatly improve accuracy for smaller or variable-volume samples.
Discover BRAND Pipettes for Precision Pipetting
BRANDTECH offers a range of air displacement pipettes designed to maintain high accuracy across different volume settings.
Manual Pipettes
BRAND® Transferpette® S Pipettes
- Available in 10 adjustable volume sizes (0.1-1 µL up to 1-10 mL).
- Designed to cover the full volume range without sacrificing accuracy.
- Multichannel models (Transferpette -8/-12) allow simultaneous pipetting of multiple samples.
Electronic Pipettes
BRAND® Transferpette® Electronic Pipettes
- Available in single and multichannel models.
- Five adjustable volume ranges (0.5-10 µL to 250-5000 µL).
- Designed for high-throughput labs needing greater precision.
- Choosing the right pipette model depends on your lab’s needs and whether manual precision or automated efficiency is required.
Ensuring Precision: Choosing the Right Pipette for Maximum Accuracy
Choosing the right air displacement pipette is essential for accurate, reliable laboratory measurements. By understanding how volume settings impact accuracy and selecting pipettes accordingly, you can achieve highly precise results.
Key Takeaways:
- Pipette accuracy is highest at full capacity and declines as volume decreases.
- Always use a pipette that keeps your target volume within 50-100% of its nominal range.
- For measuring small volumes, opt for a pipette with a narrower volume range to maintain precision.