One thing we are really proud of at Rigaku Automation is the accuracy with which we can dispense all types of stocks relevant to making protein screens. An important aspect is of course that we can “tap off” drops from our syringe (with the so called “Tapper” technology) but when it comes to measuring the accuracy of our dispense mechanism, we are really interest how difficult to dispense stocks behave.
Water, DMSO and other stocks with low viscosities are easy to dispense. That’s why we wanted to take a moment and discuss how we can dispense stocks with higher viscosities such as 100% polyethylene glycol 400 (PEG 400) or 50% PEG 8000 with high precision.
First we dispensed 100% PEG 400 using an unused small syringe without any further preparation into 20 consecutive wells. What happens is that for the first 6 wells no PEG is dispensed at all and after that, a drop of around 6 ul is dispensed (the target dispense volume was 1 ul). Figure 1 shows this behavior over the 20 wells.
Figure 1: Dispensing results for PEG400 with a previously unused small syringe
What happens is that a previously unused syringe aspirates an air bubble when working with stocks with higher viscosities. This air bubble acts as a cushion that prevents the Tapper of doing its work correctly, avoiding tapping off the drops from the syringe tips. The drop increases in size during several dispensing steps until it finally grows to a size where gravity causes the drop to be dispensed.
Luckily the solution to this problem is fairly easy: Wetting the syringe walls by manually aspirating the stocks solution we want to dispense before placing the BirdFeeder into the Alchemist deck. If the syringe walls are wetted, viscous materials can be more easily aspirated which suppresses the formation of air bubbles. Repeating our experiment with a wetted syringe generates quite different results:
Figure 2: The same dispensing experiment using PEG400, but with a manually primed small syringe.
The improvement is dramatic for both medium and small syringes (and should be used for large ones as well). With primed syringes you can achieve coefficients of variation or CV’s of 2% or less, as shown in Table 1.
Table 1: Dispensing accuracies for primed and unprimed syringes for 100% PEG 400 and 50% PEG 800.
So, for which viscosities is priming required?
As it turns out, if you are dispensing stocks with with viscosities of 5 or higher, you should definitely plan on adding a manual priming step before automatic dispensing. Otherwise you risk drops not being correctly tapped of from the syringe tips, with negative effects on dispensing accuracy. Table 2 shows our test results:
Table 2: Drop removal behavior by tapping for primed and unprimed syringes.
Finally, I want to acknowledge my colleague Matt Lundy for carrying out the experiments and interpreting the results.
