Hi,
I am not fully understanding the description of the the touch off parameter, is there actual logic occurring for dispenses into a dry well for < 5uL or is this the suggested use? As in if I have this on and the dispense is a dry well (assume its looking up the sample tracking data base) with less than 5uL will it trigger? That seems extremely complicated, surely it can’t be doing that?
The help file describes a common use case for the function. If you are pipetting volumes less than 5μL into an empty well, the touch-off function can be used to facilitate the transfer. However, this feature is not restricted to small volumes. You can enable the function to assist with a dispense of any size. See below for an excerpt from the Liquid Handling Reference Guide:
For low volume pipetting, the function can help ensure the distance between tip and pipetting surface is minimal without blocking the tip.
I hope that helps clarify things!
Eric
Gotcha, just wanted to check there wasn’t some extra logic occuring!
I am seriously wondering if touch-off mode is truly the best approach for dispensing low volumes into a dry well. With a standard 1000 µL pipette (not a MegaPipette), once the correction curve and liquid parameters, such as flow rate and air transport volumes, are meticulously optimized, it can reliably handle volumes as small as 2 µL to 5 µL in jet dispense mode, without touching the bottom surface or the liquid surface. The precision is fully sufficient for NGS dilution and pooling, as proven by more than a year of regular operation.
The touch-off mode is highly effective at removing potential residual droplets from the bottom of the well.
What are your thoughts?
You’re right and I should further clarify that it’s not a requirement to use touch-off for any and all low volume transfers. It’s just an option and one that may make sense depending on the use case. If you have a well-defined labware and there is no concern about variability in the plasticware, then going to a fixed height with no touch off works well.
However, there may be situations where touch off on either the aspirate or the dispense is helpful depending upon the labware and properties of the liquid you’re transferring. I worked on a method where I needed to transfer 2µL from Eppendorf tubes containing only ~20µL of liquid. The tubes were placed in inserts in a 32-position sample carrier. Due to the variability in the plasticware of the tubes, how the tubes sat in the inserts, and some variability in the machined inserts, the z-bottom of the tubes varied by several tenths of mm. That was enough that a fixed height of 0.2mm would sometimes pin to the bottom of tubes or be too high and not be in liquid. Incorporating touch off on the aspirate helped. You can imagine the same scenario, but using a dispense where the height needed to be optimized so the droplet would wick off the tip and into the tube/well. You may see other variability in plasticware with PCR plates that have been thermal cycled and are slightly warped. Using touch off mode can help ensure a consistent height from the bottom for every transfer.
