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graphic showing liquid vapor and the effect of pressure on pipetting

The Physics of Pipetting a Liquid

This page is part of the comprehensive Hamilton Automated Liquid Handling Guide.

When pipetting volatile liquids like isopropanol, acetone, ethanol, and chloroform, it is common to experience dripping of liquid from the pipette tip. Air Displacement Pipetting (link) requires a vacuum pressure in the tip above the liquid column to hold the liquid in the tip. When a vacuum is applied to volatile liquids they vaporize which increases the pressure in the pipette tip. The increased pressure pushes fluid out of the tip.

Common techniques for avoiding drips and improving pipetting accuracy include:

  • Reverse Pipetting
  • Pre-wetting the tip
  • Anti-Droplet Control

Reverse Pipetting Volatile Liquids

Typical pipetting involves aspirating an air gap and then aspirating the solution. The solution is then dispensed and the air gap is used to force (or blow out) the last droplets of liquid from the pipette tip. However, for volatile solutions this procedure can be reversed by overaspirating the liquid and then dispensing the precise volume with no blow out. This technique leaves some excess liquid in the pipette tip so the dispense volume is not impacted by minimal dripping during transfer. The negative aspect of reverse pipetting is that the liquid can still drip onto the work surface.

Pre-wet the Tip to Improve Pipetting Accuracy with Volatile Liquids

Prewetting the pipette tip involves aspirating and dispensing the liquid several times prior to attempting to transfer a precise volume. Prewetting saturates the air inside the pipette tip with vapors from the solution. Then when the sample is aspirated, the dripping is minimized because no additional vapors are forming so the pressure above the liquid column stays constant. One consideration with pre-wetting is that it does increase the time that it takes to complete each liquid transfer.

Anti-Droplet Control for Automated Transfers of Volatile Liquids

Pipette channels equipped with Anti-Droplet Control (ADC) monitor the pressure in the pipette tip above the liquid column. After aspiration, ADC maintains a constant pressure by retracting the plunger as the pressure builds in the pipette tip. This prevents the formation of a droplet and ensures accurate pipetting with volatile liquids.

Additional Resources for Automated Liquid Handling

See Hamilton Automated Plate Sealers and Small Devices
Check out the Hamilton Carriers and Pedestals
See the line of Hamilton Liquid Handler Reservoirs, Tips and Consumables
Learn about all of Hamilton Company's Automated Liquid Handling solutions
Check out the homepage of our Automated Liquid Handling Guide
Read our recommendations for Step by Step Automated Liquid Handler Setup
Get our input on the process used to Measure Transferred Liquid Volume
The team at Hamilton gives recommendations for Best Practices for Liquid Handling Activities
Want to "own" the guide? Click for a PDF Downloadable Liquid Handling Guide
Read our comparison of Manual Pipetting vs Semi Automation vs Automation
Hamilton Robotics highlights 10 Important Considerations for Accurate Automated Pipetting
Learn about Automatic XYZ Calibration