A micropipette is a laboratory instrument designed for the precise measurement and transfer of minute liquid volumes, typically in the microliter (\(\mu\text{L}\)) range. Accuracy in scientific experiments depends on the reliable handling of these amounts, making the proper use and reading of the device a fundamental skill. Correctly interpreting the volume settings on the digital display and mechanically setting the volume ensures that the liquid transferred is exactly the amount required for the experiment.
Understanding Pipette Size Designations
Micropipettes are categorized into models designated by a “P” followed by a number indicating the maximum measurable volume in microliters. This designation dictates the instrument’s operational range and how its internal volume dial must be interpreted. Using the wrong size micropipette for a given volume is a common source of error and can damage the instrument’s internal mechanisms.
The most common P-series models are the P20 (2 to 20 \(\mu\text{L}\)), P200 (20 to 200 \(\mu\text{L}\)), and P1000 (100 to 1000 \(\mu\text{L}\)). Selecting the smallest pipette that accommodates the target volume is recommended, as it provides the highest degree of precision for that measurement.
Interpreting the Volume Dial
The volume dial is a three-digit window displaying the set volume, but the actual value is determined by the specific P-series designation of the instrument. The numbers in the window represent different place values depending on the pipette’s maximum capacity, which is where the implied decimal point shifts. Reading the dial correctly requires knowing the pipette’s size and understanding the colored digits, which often denote the decimal place.
P1000 Micropipette
For the P1000 micropipette, which measures up to 1000 \(\mu\text{L}\), the three digits correspond to the hundreds, tens, and ones places. Some models may color the top digit red to signify the thousands place. If the P1000 dial reads “085,” this translates to 850 \(\mu\text{L}\).
P200 Micropipette
The P200 micropipette is straightforward, as the three digits simply represent the hundreds, tens, and ones places in microliters. A dial reading of “150” is 150 \(\mu\text{L}\).
P20 Micropipette
Reading the P20 micropipette requires careful attention to the decimal point, as it measures very small volumes. The dial’s three digits typically represent the tens, ones, and tenths places. The bottom digit is often colored red or set apart to denote the first decimal place. A P20 dial reading of “150,” with the zero in red, indicates a volume of 15.0 \(\mu\text{L}\), while a reading of “015” (with the five in red) would be 1.5 \(\mu\text{L}\).
Mechanical Volume Adjustment
Setting the volume involves using the adjustment wheel or knob, which controls the piston stroke. To achieve the highest accuracy, always “dial down” to the final desired volume. If the user passes the target volume while increasing it, they should turn the dial past the target and then come back up to the desired setting.
This technique minimizes mechanical backlash in the internal gears, ensuring the piston is positioned correctly. Users must never adjust the volume outside the minimum and maximum range specified for the model. Forcing the dial beyond these limits can irreparably damage the instrument’s calibration and internal components. Some micropipettes include a locking mechanism to prevent accidental changes during pipetting.