This page is part of Hamilton's Automated Liquid Handling Guide.
- Blow-out volume – Prior to aspiration of liquid, a small volume of air should be aspirated to be used later as a blow-out volume. This blow-out volume is important when trying to completely empty the pipette tip. Some liquid tends to linger in the tip and the blow-out volume provides an extra dispense to ensure the tip is fully emptied.
- Reverse pipetting – For some viscous liquids, the blow-out volume is not sufficient to completely empty the tip. In these cases reverse pipetting may be a preferable option. For reverse pipetting, there is no blow-out volume, instead excess liquid is picked up during aspiration. Then during the dispense step, the desired volume can be expelled precisely and the excess waste volume remains in the tip.
- Transport air volume – After liquid is aspirated into the pipette tip, it is common to move the tip to a new location prior to dispense. This movement results in forces that affect the equilibrium of the liquid in the tip. One possible effect is a small drip forming on the pipette tip during transit. To mitigate this issue, pipettes can aspirate a “transport air volume” after the liquid is in the tip. This additional air prevents the droplet formation during transit.
- Pre-wetting the tip – A wet tip behaves differently than a fresh, dry tip. This has to do with the surface tension between the liquid and the tip material as well as the saturation of the air in the tip. Pre-wetting the tip by repetitive aspiration and dispensing before drawing the desired aspiration volume can improve the accuracy for many liquids but is especially useful for viscous and volatile liquids.
- Over aspiration volume – Pre-wetting the pipette tip can improve accuracy but it also increases the duration of the pipetting task. Over-aspirating a liquid and then immediately dispensing the additional liquid can have a similar effect to pre-wetting without considerably increasing pipetting time.
- Optimize swap speed – After aspiration, it is possible that some liquid remains on the outside of the tip. The amount of that liquid can be influenced by the speed that the tip is removed from the liquid (swap speed). A slower speed can ensure that the liquid has time to drop off the tip into the reservoir. Minimizing liquid on the outside of the tip improves the accuracy of the subsequent dispense.
- Settling time – After aspirating a liquid, it is critical to wait for the liquid and the air in the tip to reach equilibrium prior to dispensing the liquid. An appropriate settling time is dependent on the volume and the properties of the liquid being aspirated.
- Stop-back volume – When jet dispensing aliquots of liquid, it is critical to achieve a clean cut between the dispensed volume and the liquid retained in the tip. This is partially achieved with a high dispense velocity, but it can be further enhanced with a stop-back volume. After the plunger moves the desired distance to dispense the liquid, the motor is immediately reversed and the plunger aspirates to create a stop-back volume of air, resulting in a greater velocity change and a clean droplet.
- QPM on/off - Real-time monitoring of the pressure curve while pipetting can prevent aspiration of foam, prevent clots, and eliminate aspirating air from an empty tube. Hamilton’s pipetting channels feature a QPM (qualitative pressure monitoring) function that can be tuned to match application requirements.
- Jet vs. surface dispense – Jet dispense is useful to keep the pipette tip above the liquid in the tube while dispensing, but is not recommended for viscous liquids. Surface dispense is used to dispense with the tip at or below the liquid level. Surface dispense is best for viscous liquids and to deliver the liquid to the bottom of a tube that already contains other liquid.
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
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 tips to accurately Pipette Volatile Liquids
Learn how Liquid Properties affect automated liquid handling
Read our comparison of Manual Pipetting vs Semi Automation vs Automation