Pipetting is the laboratory technique of measuring and transferring precise volumes of liquid using a handheld instrument called a pipette. It’s one of the most fundamental skills in any biology, chemistry, or medical lab, used millions of times a day worldwide to move tiny amounts of liquid from one container to another with high accuracy. Whether a technician is running a diagnostic blood test or a researcher is setting up a DNA experiment, pipetting is almost certainly involved.
How a Pipette Actually Works
The most common type, an air displacement pipette, operates on a surprisingly simple principle. Inside the instrument, a spring-loaded piston moves up and down within a cylinder. When you press the plunger, the piston descends and pushes air out of the bottom of the pipette. When you release the plunger, the piston rises back up and creates a partial vacuum inside the disposable plastic tip attached to the end. Atmospheric pressure then forces liquid up into the tip, filling the space left by the displaced air.
The volume of air the piston displaces is exactly equal to the volume of liquid drawn in. You dial in your desired volume on the pipette, which adjusts how far the piston travels. To dispense the liquid, you press the plunger again, and the compressed air pushes the liquid back out of the tip. There is always a small cushion of air between the piston and the liquid itself, so the mechanical parts never touch the sample.
Volume Ranges and Pipette Sizes
Different pipettes handle different volumes. Micropipettes, the ones you’ll see most often in molecular biology and diagnostic labs, typically cover ranges from less than 1 microliter up to about 1,000 microliters (1 milliliter). A microliter is one-millionth of a liter, roughly the size of a tiny dewdrop. For context, a single drop of water from an eyedropper is about 50 microliters.
For larger volumes, labs use serological pipettes, which are long, graduated tubes that can transfer anywhere from 0.1 milliliters up to 25 milliliters, with some sizes reaching 100 milliliters. These are common in microbiology and cell culture work, where technicians need to move milliliter-scale volumes of growth media, bacterial cultures, or chemical solutions between flasks and test tubes. Serological pipettes are powered by a separate suction device (a pipette controller or bulb) rather than a built-in piston.
Air Displacement vs. Positive Displacement
Most labs use air displacement pipettes because they’re less expensive and work well with the vast majority of water-based liquids. But that air cushion between the piston and the sample can cause problems with certain liquids. Volatile chemicals can evaporate into the air gap, and very thick or viscous solutions resist being drawn in smoothly. Temperature extremes also affect the air cushion’s behavior, throwing off accuracy.
Positive displacement pipettes solve this by eliminating the air gap entirely. Instead of a fixed piston inside the pipette body, these use a disposable tip with a tiny built-in piston that makes direct contact with the liquid. Viscous samples get pushed out more completely, volatile liquids can’t evaporate inside the tip, and hot or cold samples don’t affect accuracy. The tradeoff is cost: positive displacement tips are significantly more expensive because each one contains its own piston mechanism.
Single Channel and Multichannel Models
A single-channel pipette has one tip and transfers one volume of liquid at a time. It’s the default tool for most general lab work. When experiments involve microplates (plastic trays with dozens or hundreds of tiny wells), single-channel pipetting becomes painfully slow. That’s where multichannel pipettes come in.
Multichannel pipettes have 8 or 12 tips arranged in a row, letting you fill an entire row of a 96-well plate in one motion. Sixteen-channel versions exist for 384-well plates, which have even smaller, more densely packed wells. These are standard equipment for high-throughput applications like ELISA immunoassays, PCR genetic testing, and cell culture work. Electronic multichannel models automate the plunger action with a motor, reducing hand fatigue during long sessions.
Forward and Reverse Pipetting Techniques
Forward pipetting is the standard technique. You press the plunger to the first “stop” (a point of resistance), immerse the tip in the liquid, release the plunger to draw liquid in, then press the plunger past the first stop to a second stop to blow out every last bit of liquid. This works well for most water-like solutions.
Reverse pipetting flips the approach. You press the plunger past the first stop before aspirating, which draws in more liquid than needed. When dispensing, you only press to the first stop, leaving a small residual volume in the tip. This technique is useful for quickly dispensing equal small volumes into multiple wells from the same source, such as distributing antibody solutions or coated microparticles across a plate. The extra liquid in the tip acts as a buffer, ensuring each dispensed volume is consistent.
Why Technique Matters for Accuracy
Small errors in pipetting technique can have outsized effects on results. One of the most common mistakes is immersing the tip too deeply into the liquid. Doing so roughly doubles the inaccuracy of the measurement. If you also hold the pipette at a 30 to 40 degree angle while immersed too deeply, inaccuracy increases three to five times. The ideal approach is to hold the pipette vertically and immerse the tip just a few millimeters below the liquid surface.
Other common sources of error include pipetting too quickly (which can introduce air bubbles), failing to “pre-wet” the tip by aspirating and dispensing the liquid once before taking the actual measurement, and using the wrong size tip for the pipette. Each of these can introduce enough variation to compromise an experiment or diagnostic test, especially when working with volumes under 10 microliters where even tiny discrepancies represent a large percentage of the total.
Calibration and Maintenance
Pipettes drift out of calibration over time as internal seals wear and springs lose tension. The standard practice for accredited laboratories, based on ISO 17025 requirements referenced by the FDA, is annual calibration and preventive maintenance. During calibration, a technician tests the pipette’s actual output against its set volume using gravimetric methods (weighing the dispensed water on a precision balance). A calibration label is then affixed to the pipette showing the service date and the date when the next calibration is due.
Between calibrations, users can do basic checks by weighing dispensed water themselves. Pipettes that are used heavily, dropped, or used with corrosive chemicals may need more frequent service. Keeping pipettes stored upright on a stand when not in use helps prevent liquid from creeping back into the piston mechanism.
Ergonomics and Repetitive Strain
Pipetting is one of the most physically repetitive tasks in laboratory work. A researcher running a large experiment might press a pipette plunger thousands of times in a single day, and the resulting strain on the thumb, hand, wrist, and shoulder is a well-recognized occupational hazard. Repetitive strain injuries from pipetting are common enough that major universities publish specific prevention guidelines.
The University of Pittsburgh’s ergonomics program recommends taking a two-minute break for every 20 minutes of continuous pipetting. Keeping your elbows close to your body, sitting with back support, and maintaining a neutral neck position (avoiding the instinct to hunch forward and peer into the tip) all reduce strain. Alternating which fingers operate the plunger, switching between thumb and index finger, helps distribute the workload across different muscle groups.
Equipment choices matter too. Electronic pipettes eliminate the need for manual plunger force. Light-touch models require less thumb pressure. Latch-mode features on some pipettes hold the plunger in position without continuous finger pressure during repetitive dispensing. Rotating pipetting duties among coworkers, when practical, is one of the most effective strategies for preventing chronic injury.