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The Importance of Z-axis Control for Accurate Pipetting

pipetting robot during z axis calibration

All automated pipetting actions on automated liquid handlers require movement to lower the pipette tip into the solution to be aspirated. The simplest implementation of this movement would be to lower the tip to a set height and aspirate from there.

However, if the liquid container is significantly full, the pipette tip may become immersed and carry excess liquid on the outside of the tip. Or if the container is low or empty, the pipette may aspirate only air from the specified height.

Achieving high accuracy and precision, the focus of this page from our Automated Liquid Handling Guide, requires full coordination between the Z-axis control, liquid-level sensing, and the pipetting drive to ensure the pipetting function is consistent regardless of the container conditions. Hamilton’s ZEUS pipetting channels incorporate all three of these aspects in an on-board intelligence that leverages algorithms to optimize movements.

Z-axis Considerations for Basic Aspiration

Common pipetting tasks like basic aspiration require multiple calculations to coordinate the pipette drive, Z-axis movement, capacitance, and pressure signals. Typical liquid handling components provide raw data and motor-move commands but leaves specific coordination to instrument designer. Hamilton ZEUS pipetting channels bring all this functionality into an integrated aspirate command that encompasses:

  1. Quick Z-axis movement down to a defined search height
  2. The Z-axis movement then slows to a search speed, stopping when the capacitive liquid level signal is found
  3. The Z-axis moves the pipette tip to a defined level below the surface of the liquid
  4. The pipetting channel begins to aspirate and the Z-axis lowers at a speed determined by the container geometry to maintain a consistent submersion below the liquid level
  5. During aspiration, pressure in the tip is monitored and analyzed for pipetting errors
  6. After aspiration is complete, the Z-axis retracts and the pipette channel aspirates a transport air gap

Automated Pipetting Error Handling Considerations

To maximize an instrument’s walk-away time, it is vital to predict possible pipetting error states and provide meaningful error codes and recovery procedures.

Examples of pipetting errors that should be predicted include:

  • No tip is installed on the pipette channel
  • The tip aspirated a clot and clogged
  • Foam is detected that is interfering with aspiration above the proper liquid surface
  • Liquid is detected, but based on container geometry there is insufficient liquid to complete the operation.
  • Liquid is detected outside of the the expected Z-axis range

ZEUS pipetting channels are programmed with these potential error states in mind and can be used to guide instrument users in troubleshooting:

Additional Resources for Automated Liquid Handling

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
Want to "own" the guide? Click for a PDF Downloadable Liquid Handling Guide
Hamilton Robotics highlights 10 Important Considerations for Accurate Automated Pipetting
Learn about Automatic XYZ Calibration
Read our advice on Addressing Challenges of Automated Pipetting