The technique of dispensing a sample from a pipette into the target receptacle is an important consideration in the liquid handling process, and is just as critical as the proper aspiration technique.
When using forward mode pipetting, dispense the sample while lightly touching the pipette tip against the side of the receiving container, so that the entire sample can be dispensed out of the pipette tip and no droplets remain sticking to the tip. Dispense the sample in a moderately slow, even speed, similar to the aspiration speed. It is good practice to wait a second after dispensing, or to streak the tip upwards along the container wall to remove the last little bit of sample. This assumes that the receiving container will be subsequently mixed to “wash down” the sample from the container side.
Should the receptacle already contain other reagents/solutions, do not dispense sample directly into this solution and “rinse” the tip by successive aspiration/dispense cycles. The danger of retaining and removing sample mixture in the tip or on its outside is relatively high, as is the danger of cross-contamination.
If very small volumes are dispensed while touching an existing solution (a.k.a. wet-dispense), they should be dispensed by forward-mode pipetting at or just under the surface of the solution, and the pipette tip should be removed from the solution before releasing the plunger. Use caution to not immerse the tip too deeply into the solution to reduce the possibility of retaining droplets on its outside. It is prudent to inspect the tip carefully for remaining droplets after sample delivery.
Some operators, or even entire labs, are trained to dispense “into air” — not touching either the container or the liquid. Some automated liquid handling manufacturers call this “jet dispensing.” This method is not recommended for handheld pipetting, since it is difficult to generate the velocity and sharp cut off needed to achieve a complete dispense. In handheld “jet dispensing,” it is very common for a small droplet to remain either inside the tip, or to wrap around and remain clinging on the outside of the tip. There may be valid assay reasons to dispense this way, but this technique is less precise and less accurate than touching while dispensing.
In the case of reverse mode pipetting, caution needs to be exercised when dispensing against the side of the container. Once the first stop is reached, promptly remove the pipette tip from the container wall to avoid the danger of additional sample being wicked out of the tip due to surface tension effects.
When using reverse mode pipetting, you should never dispense into an existing solution, as this will result in a dramatic over-delivery of sample.
A. Bjoern Carle, PhD, is the PCS Product Manager at Artel, where he supports product development, scientific marketing, and technical training efforts. Dr. Carle earned his Vordiplom (B.S.) in Chemistry at the University of Stuttgart (Stuttgart, Germany), his M.S. and PhD in Organic Chemistry at the University of Cincinnati (Cincinnati, OH), and performed post-doctoral research at Harvard Medical School / Massachusetts General Hospital (Boston, MA).