Micropipettors are a staple in any bio lab, especially when you're trying to accurately transfer volumes below 1 mL. They use a pretty simple mechanism to draw volume: a plunger is depressed to create a vacuum which draws up liquid into the tip. When used by hand, the distance the plunger moves is fixed with a dial, and thus a the desired volume of liquid can be picked up.
Unlike a human, the robot doesn’t need to manually adjust the dial every time it changes volume. Instead, the robot can calculate exactly how far to depress the plunger to get the desired volume. For example, if you want 5 uL from a p10, the robot pushes the plunger down 50% of the maximum distnace to transfer that exact liquid volume.
Step 1: User Calibration
Note: This calibration is extremely important, but once you do it, you can trust your volumes as long as you don’t change pipettes.
Correct volume calibration depends on the Top and Blowout positions, as these two positions set the minimum and maximum volumes. The Top position (the minimum position), in which the plunger is not pressed at all, is 0 uL, meaning no liquid will be picked up. The Blowout position (the maximum position), is the maximum volume of the pipette (for instance: 10 uL, 200 uL, or 1000 uL, depending on the pipette). Calibrating the drop tip position is important for making sure the tips are ejected properly after each transfer, although the position is not involved in the volume calibration.
Tip: The Blowout position should be as far down as the plunger will go without dropping the tip.
Step 2: Testing
Once the positions have been calibrated, the Test button picks up the volume specified in the dropdown menu (choose the maximum for the pipette you’re using). When the liquid is picked up, the user can either visually check the volume against the markings on the pipette tip, or use a scale to weigh the liquid for accuracy. If the incorrect volume has been picked up, the bottom Blowout position should be adjusted — move it up if too much has picked up, and down if too little. Once you determine the Blowout position in which the volume is correct, save it, and let the software do its thing to calculate the desired volumes.
Step 3: Software Calculates Volumes
The software uses a linear relationship between the volume and calibration positions to calculate the exact distance needed to depress the plunger for each desired volume. As long as the pipette is manually set to its maximum volume, and the calibration is done correctly, the robot will accurately dispense the correct volume throughout the experiment.
If you’re interested in more calibration data, check out our post here.