For viscous samples, which pipetting mode is recommended?

• A. Forward mode
• B. Reverse mode
• C. Both forward and reverse
• D. None of the above

Answer: (B)

Handling viscous samples can mess with accuracy because sticky liquids cling to the tip and don’t all get expelled during dispensing. In reverse pipetting, you aspirate more than the target amount and then dispense only the exact volume you need. The extra liquid in the tip offsets the resistance and adhesion of the viscous fluid, so what actually leaves the tip matches the intended volume more closely. This reduces under-delivery and variability that are common with thick liquids. That’s why reverse mode is recommended for viscous samples. Forward mode is more prone to under-delivery in this case due to the liquid’s tendency to stick to the tip.