One of the most frequently used tools in a typical laboratory is the air-displacement or air-cushion pipette. Convenient and easy to use, single-use pipette tips minimize the chance of cross contamination between samples.
Do you know how to select the correctly sized pipette for maximum accuracy? Let’s first look at the mechanics.
Air displacement pipettes are based on a simple concept:
- A piston of consistent diameter is pushed through a seal to move a column of air; the air movement, in turn, moves liquids in and out of a disposable pipette tip
- An air cushion exists inside the pipette tip between the liquid and the end of the pipette; the liquid never touches the pipette itself
- After a pipette tip is affixed, the piston is moved from its resting position and through the piston seal to push air out of the pipette tip
- The tip is then immersed just below the surface of the liquid to be aspirated; the spring-action piston is returned to its resting position, creating a vacuum that fills the pipette tip with liquid
- Adjustable volume pipettes use a screw mechanism to change the stroke length to set the volume at 50% of the nominal capacity
For pipettes with nominal volumes of 100 µL or greater, the inaccuracy of an air displacement pipette is treated as a constant volume throughout the range of the pipette. ISO 8655 Part 2 lists this value at ≤± 0.8% of the nominal capacity (i.e. ≤± 0.8 µL for a 100 µL pipette) and depends mostly on the mechanics of the pipette. For smaller volumes, the inaccuracy increases due to the compressibility of air and other factors.
Typically, pipette data sheets or catalogs present the accuracy as a percentage of the nominal volume, often with no additional information, so the user must determine the implications. When estimating the accuracy percentage for partial volumes, the numerator (the amount of inaccuracy) remains the same; the denominator (the set volume) changes.
Accuracy of Partial Volumes for a 100 µL Pipette
| Volume Setting | Percentage of Nominal Capacity | Volume of Inaccuracy | Pipetted Volume Accuracy |
|---|---|---|---|
| 100 µL | 100% | ≤± 0.8 µL | ≤± 0.8% |
| 75 µL | 75% | ≤± 0.8 µL | ≤± 1.1% |
| 50 µL | 50% | ≤± 0.8 µL | ≤± 1.6% |
| 20 µL | 20% | ≤± 0.8 µL | ≤± 4% |
| 10 µL | 10% | ≤± 0.8 µL | ≤± 8% |
Clearly, an air displacement pipette is most accurate at or close to its nominal capacity. Furthermore, accuracy decreases significantly as the set volume goes below 50% of the nominal capacity. Most pipettes therefore have a stated volume range with both upper and lower volume limits.
The amount of liquid to be pipetted often falls within the range of several different pipettes. For example, 20 µL could be measured with 2 to 20 µL, 5 to 50 µL, or 10 to 100 µL pipettes. The volume would be most accurately pipetted using the 2 to 20 µL pipette. If that size is not available, choose the pipette with the lowest capacity relative to the volume you need. In this case, the 5 to 50 µL pipette is a better choice than the one that measures from 10 to 100 µL.
If your lab has pipettes with only marginally overlapping ranges, the addition of one or two more pipettes with more overlap may increase the accuracy of your measurements.
The BRAND Transferpette Sis a manual single-channel pipette available in ten different adjustable volume sizes (0.1 to 1µL up to 1 to 10 mL); these sizes cover nearly the entire range from 0.5 µL to 10 mL, overlapping the upper halves of the pipette ranges. Similarly, manual Transferpette 8- and 12-channel pipettes are available in five adjustable volume ranges (0.5 to 10 µL to 30 to 300 µL) for 1 to 300 µL sample volumes.
The BRAND Transferpette is also available in an electronic version with similar options. Five different single-channel Transferpette electronic pipettes cover volume ranges from 0.5 to 10 µL to 250 µL to 5 mL; five multichannel models cover volumes from 0.5 to 10 µL to 15 to 300 µL.
NOTE: In most literature, “accuracy” is used to denote the amount of inaccuracy. To stay consistent with other literature, we use “accuracy” in this article when referring to percentages.
