Basic liquid handling concepts - Visualized
The basic function of a pipettor is a simple 2-step process: aspirate and dispense. These 2 steps take place entirely within the tip of the pipettor. Aspirate means bringing liquid into the tip, and dispense mean expelling that liquid from the tip. This process can be seen by clicking several times on the Play button in this animation.
It is possible to make a single aspiration and then use that liquid to make multiple dispenses. To see this in operation, press the Play button on the animation below.
There are many parameters that are important when performing liquid handling. These include:
- Speed. The speed of aspiration and dispensing actions is usually programmable. It becomes important in relation to the viscosity of the liquid being pipetted.
- Dispense Contact. The dispense operation can take place into empty air (dry) or the tip can be lowered into liquid that is already present in the target location (wet).
- Tip Touch. The tip can be "touched" against the side wall of the target container to wipe off any drop that might be hanging on.
- Blowout. A final burst of pressure can be used to blow out the last drop of liquid that tends to stay behind in the tip.
- Mixing. Liquid can be repeatedly aspirated and dispenses while the tip remains in the liquid, this causing a mixing action.
An automatic liquid handler can be equipped with different numbers of pipetting tips, from 1 to 384 or more. The pipetting tips can be one of 2 types:
- Fixed Tips. A thin stainless steel tube that is used repeatedly and is washed between pipetting steps. Usually Teflon-coated to improve washing.
- Disposable Tips. These are made of various types of plastic and are designed to be replaced after each pipetting action. Automated pipettors are most frequently used for the following liquid handling applications:
This is a basic operation for a multichannel pipettor. The goal is to replicate the contents of the source plate to the destination plate(s). The pipettor aspirated from all wells of the source plate, then moves to the destination plate and dispenses the same pattern of liquid. It is common to produce multiple destination plate replicates. If a single replicate is produced, this is called a 1:1 plate replication. If two replicates are produced, this is called a 1:2 plate replication. Four replicates is 1:4, and so forth. Automated pipettors can be programmed to easily accomplish plate replications.
This is also a common operation for multichannel pipettors. Plate reformatting involves moving samples between the 96 and 384 well microplate formats. Plate expansion is a reformatting operation that spreads the wells of a 384-well plate to four 96-well plates. Plate consolidation (or compression) is the reverse, moving the contents of four 96-well plates to a single 384-well plate.
Reagent refers to any liquid that needs to be added to the wells of a destination plate. Typically, the reagent is stored in a reservoir as shown below. The wells of the destination plate are usually filled with some liquid that will be mixed with the reagent.
The transfer of a set of compounds, or samples, from the source plate to the destination plate. The wells in the destination plate usually already contain some liquid. The compound is being added to the existing liquid as part of an experiment.
A serial dilution is used to set up a series of different sample concentrations for an experiment. To accomplish this, a high concentration of sample is used as a starting point. A measured amount of sample is transferred to another well and mixed with dilution solvent, creating a lower concentration. For example, if 10 microliters of sample is mixed with 90 microliters of solvent, the resulting liquid will be 10% as concentrated. This is called a 1:10 dilution. If the same process is repeated on the 1:10 dilution, the next well will have a 1:100 dilution. Dilutions are repeated "serially" in this fashion the desired number of times.
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