Chromatography Forum

Hi all, would appreciate some advices/inputs on the following:

I noticed that a small drop almost always remains at the micropipette tip when dispensing protein sample into the diluent (water or buffer). I use forward pipetting - aspirate to first stop and dispense by pushing all the way to second stop. The volume dispensed is in the range of 50uL to 400 uL. May i know if this is indication of poor pipetting technique? If so, any tips on how to improve this? Also, will this small residual drop significantly affect the accuracy of volume dispensed?

Many thanks (in advance) for your kind inputs!

I have always (unless forbidden somehow by the method) rinsed my pipette tip when aliquoting any sample into matrix by pipetting up and down. Dispense the sample to the second stop, remove the tip from the matrix so you don't accidentally aspirate matrix into your tip and possibly contaminate the filter at the top of the pipette, and then push down to the first stop and dip the pipette tip as if you were going to draw up sample from the matrix and pipette up and down a few times before dispensing.

As long as there is no sample adhering to the outside of the pipette tip, you have the actual volume inside the tip. If you dispense and some remains then it is due to the surface tension of the liquid keeping it attached to the tip. I have seen times when pipetting an oil that after dispensing if you wait several seconds the bottom of the tip will fill up with several ul that slowly drain down the inside of the tip.

The rinsing method listed above works well to get that last bit out. Another technique if you are disposing of the tips each time is to touch the tip to the side of the vessel you are pipetting into and let the surface tension draw the last bit out of the tip. If you plan to use the tip for another aliquot, that could introduce contamination so be careful with that.

If there is some adhering to the outside of the tip then I normally touch the tip to the side of the vessel that I remove the initial aliquot from and let it wick away any extra that might be on the outside of the tip.

I agree with the last two posters above for disposable pipette tips. I touch the tip to the side of the volumetric or to the top of the liquid after dispensing. If there is still a drop inside... For 1mL tips I flip the drop to the end and dispense it. MLA pipettes tend to do that. This seems to be correct because I check the calibration with a balance and DI water every day that I use it. Note: perhaps self correcting since I calibrate the 1 mL tip that way. However, for 100uL tips, the instructions say you must rinse the tip with solvent to get full delivery.

If its at all goey or sticks to the outside of the tip, you are better off weighing it.

For fixed volumetric glass pipettes its noted on the pipette whether its TD or TC. If its TD you can touch it to the side or to the surface but should not rinse or blow that last bit out.