In regards to detecting the liquid height in the center of the well then moving to the side of the well, this is possible. The tips will retract to traverse height between the detection and the actual aspiration step, but it will still give the desired results. The aspiration can be setup with zero volume, then followed by the steps found in these posts (Capacitive liquid level detection (LLD) without aspiration/dispense and Finding the liquid height after aspiration and passing it on - #2 by Pascal) to retrieve a fixed height to be used in the following aspiration.
Pipetting remaining volume with a second tip could work, but I am in agreement that this would add unnecessary time to the overall run. Based on the volumes you’re working with, this should not be required and could run into similar issues with inconsistent volumes.
When I’ve worked on supernatant removals like these, I used a different technique that focused on using fixed heights instead of cLLD so that I could precisely control my volumes. This requires determining the height of the liquid in a well at the desired volume, then aspirating in one of the following ways:
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Set the aspirate to a fixed height (no liquid following) to the calculated height and remove the supernatant. In some instances, the fixed height will need to be adjusted slightly higher or lower than the calculated height depending on the properties of the liquid. There may also be some additional liquid class property adjustments to ensure the pellet is not disturbed, but this technique removes a lot of variability. This technique can still be used it your target volumes have well-to-well variability, as pipetting steps can use an array of fixed heights similar to an array of volumes.
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If the tips can not be plunged into the liquid to a fixed height like in option 1, then the calculated height can be used to determine a “false bottom” of the well that sits higher than the actual well bottom. When using liquid following, the tip will only travel as far as the inside bottom of a well, or to a container definition’s Max Pipetting Depth. This allows the tip to travel from the surface of the liquid to only as far as the calculated height. To implement this, either adjust the Max Pipetting Depth of the plate’s container or use a virtual shift to adjust the plate’s Z position at runtime. Note that this technique would require using one channel at a time if your target volumes have well-to-well variability.
Let me know if you have any questions.
Thank you,
Dan