The pipette, the most commonly used equipment in laboratories for extracting the samples. It is mainly used for transferring liquids or semi-solids on laboratories. It’s really interesting to know the working and internal mechanism of pipettes.
The pipette should be adjusted so that the dispensed volume will be within the required specifications. The pipettes can be mainly classified into two Air Displacement Pipettes and Positive Displacement Pipettes . Air Displacement Pipettes – These types are called as air displacement pipettes because a certain volume of air remains between the piston and the liquid. These are only used for general purpose with aqueous solutions.
• There is a piston inside the pipette which will be moved to appropriate position when the volume is set. • Piston will expel the same volume of air as indicated on the volume setting when the operating button is pressed. • The operating button will be released after immersing the tip into the liquid, which will create a partial vacuum and the specified volume of liquid is aspirated into the tip. • When the operating button is pressed to the first stop again, the air dispenses the liquid. To empty the tip completely the operating button is pressed to the second stop.
There is lot of factors like temperature, density and altitude which should be taken care otherwise it may seriously affect the accuracy of these air displacement pipettes. Positive Displacement Pipettes – It is mainly used for high viscosity and volatile liquids. The specialty of these pipettes is that the piston will be in direct contact with the liquid. The tip of it contains a disposable plunger just like a syringe, which will displace the fluid without any errors. These types are the most accurate type of pipette but the disadvantage of this one is it’s highly expensive, so this not used for general purposes.
There is lot of points to be remembered while using a pipette. First of all daily wipe the pipette with 70% ethanol so that there won’t be dirt on the pipette. Now check whether the volume is in specified range. While holding the pipette the "grippy finger rest" should rest on the index finger. To get the maximum accuracy the pipette, tip and liquid should be kept at the same temperature. Its always better to use high quality tips which is made from contamination-free polypropylene for the pipette, so that the tip will ensure maximum accuracy. The metrological characteristics of the tips will change after each time of use so it should be cleaned after use. Its better to pre –rinse the tip three to four times with the liquid to be used. It will definitely increase the accuracy especially when using positive displacement tips. Don’t turn the pipette on its side when there is liquid in the tip, liquid must go to the interior and contaminate it. Always try to store the pipettes on stands in upright position when not in use. If used daily it is recommended to check the calibration in three – month interval. Always follow the instructions of recalibration.
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