Why is it important to know the viscosity of the samples that will be aspirated / dispensed?
The viscosity of the sample will have a direct influence on which pipette is required. For aqueous samples which are low viscosity, an air displacement pipette is ideal. These pipettes are driven by a piston in an airtight sleeve which generates a vacuum. For more viscous or heavy liquids, a positive displacement pipette should be used. These pipettes are driven by a disposable piston which comes into direct contact with the sample.
How will samples required for PCR, ELISA, or other immunoassay applications affect which pipette should be used?
PCR, ELISA, and many other immunoassay methods utilize microwell plates. Microplates come in many configurations: 6, 24, 96, 384, and 1536 well plates arranged in a 2x3 matrix are typically used. In order to accommodate faster throughput with such methods, many pipette manufacturers offer multichannel pipettes. These allow for faster pipetting of multiple samples: instead of having to fill 96 wells individually, an 8 channel pipette can be used, reducing the number of aspirations and dispenses to 12.
How does the volume of samples being worked with influence which pipette is the best fit?
Pipettes come in a variety of different sizes to suit whichever volume needs are required. If you know you will always be pipetting the same volume of liquid, then a fixed volume pipette will be best. If the amount to be pipetted is changing from sample to sample, then a variable pipette will be ideal.