Automated liquid handlers are precise, but they aren't mind readers. They don't inherently know if they're pipetting water, honey, or ethanol. That's where the concept of a liquid class comes in. A liquid class is a saved profile of pipetting parameters fine-tuned for a specific fluid's physical properties. Using the right liquid class is the key to moving from simply moving liquid to transferring it with accuracy and consistency.

Creating a robust liquid class involves adjusting a few core settings. The most fundamental are the speeds at which you move the liquid. Aspiration velocity controls how quickly the liquid is drawn into the pipette tip, while dispense velocity controls how fast it's pushed out.

For a standard aqueous solution, you can often use relatively high speeds. But for a viscous liquid like glycerol, a fast aspiration will create a vacuum, pulling in air bubbles and coating the outside of the tip instead of drawing a full, accurate volume. You need to slow down. Similarly, acceleration and deceleration profiles dictate how smoothly the robot starts and stops the movement. A gentle, slow ramp-up is crucial for viscous fluids to avoid shearing or splashing, while a quick, sharp profile works fine for water.

Beyond speed, we manage the air inside the tip. A leading air gap is a small pocket of air aspirated before the liquid. This creates a buffer that prevents tiny droplets of your valuable sample from getting sucked up into the pipetting head mechanism, which can cause contamination. A trailing air gap, aspirated after the sample, acts like a plug to prevent dripping, especially with low-viscosity or volatile liquids. It also helps ensure every last bit of the sample is pushed out during the dispense.

Finally, there's the 'blowout' step. This is an extra push of air after the dispense is complete to expel any residual liquid clinging to the tip wall. For viscous liquids, a 'blowout delay' might be added. This is a short pause after the main dispense but before the blowout, giving the thick liquid time to slide down the tip wall before being forced out. This simple pause can significantly improve dispense accuracy.

The real test of a liquid class is how it handles non-aqueous solutions. Viscous liquids, which have a high resistance to flow, and volatile liquids, which evaporate easily, present opposite challenges.

For a viscous fluid like a detergent-based buffer, you must use slow aspiration and dispense speeds. You might also dispense the liquid against the side of the well wall, a technique called 'touch-off' or 'tip touch,' which uses surface tension to help pull the sticky liquid out of the tip. This is often combined with a slow tip withdrawal speed to prevent a long thread of liquid from being pulled back out of the well.

For volatile liquids like ethanol or chloroform , the problem is evaporation within the tip. This increases the vapor pressure and can cause the liquid to drip out before you even reach the destination. The standard technique to combat this is pre-wetting. This involves aspirating and dispensing the liquid back into the source container one or more times. This saturates the air inside the pipette tip with vapor, equalizing the pressure and creating a more stable environment for the actual sample aspiration. A faster aspiration and dispense speed also helps by minimizing the time the liquid spends in the tip, reducing the chance for evaporation.

Adjusting these parameters is an iterative process. You start with a default liquid class for an 'aqueous' solution, then copy and modify it based on the liquid's properties. By carefully observing the robot's performance—looking for bubbles during aspiration, droplets on the outside of the tip, or inconsistent volumes in your destination plate—you can fine-tune your liquid class until it is both precise and reliable.

Now that you understand the key parameters, let's test your knowledge.