Precautions Observed in Manual Pipetting

A list of pointers for pipetting manually in the clinical laboratory.

Automation is the trend in clinical laboratories today because it saves time, avoids human errors and could perform several procedures simultaneously.   Even in the pipetting of liquids, reagents or samples, manual pipetting is rarely utilized.

It is not an excuse though not to learn the proper precautions that should be observed in manual pipetting.  Here are important precautions involved in this procedure:

1.     Before attempting to pipet anything, Personal Protective Equipment (PPE) should be worn at all times.  This consists of a hair cap, a pair of gloves, a laboratory gown, a mask and goggles.

2.     Be aware of the solution’s specific properties.  Is it explosive, flammable, or corrosive?  Knowing about these would allow you to be extra careful in these areas.

3.     Make sure that the pipette you will be using is dry and clean, because any liquid left inside the apparatus would be adding to the total volume of   liquid pipetted.

4.     Use the pipette with the nearest calibration to the volume that you are pipetting. e.g.  2 mL (milliliter)-pipettes for 1.8 mL- volumes, or 5 mL for 4.5 mL- volumes.

5.     When you commence pipetting, the pipette should be immersed all the way down into the beaker containing the reagent to be pipetted.

6.     The pipette should be held upright in an upright or vertical position and not held horizontally.  This would allow you to take note of the correct reading.

7.     Liquid to be pipetted should be aspirated beyond the 0 ml mark.

8.     Your index finger is the best finger to control the flow of the liquid because it is more flexible. The index finger must be dry. Do not use the thumb as it would be more difficult to manipulate.

9.     Do not forget to wipe the sides of the pipette with a tissue or filter paper. This would remove excess liquid clinging to the external sides of the pipette, which may inaccurately add to the total volume of the solution.  Use your other hand to do this.

10.            The liquid should be exactly at the mark that you want to measure and this is done by gently releasing the liquid with the use of your index finger.

11.            Viscous liquids are read at the upper meniscus while clear liquids are read from the lower meniscus.

12.              If there are two “blasted rings” at the top, upper portion of the pipette.  This would indicate that the pipette is a “blow-out” type and therefore the content should be “blown” into the receiving vessel by the use of the aspirating bulb or aspirator. 

13.            If it is not a “blow-out” pipette, then the liquid should only be allowed to drain into the receiving vessel by touching the tip of the pipette to the sides of the receiving vessel.

14.            Do not immerse the pipette into the liquid already in the receiving vessel, unless the pipette is a “blow-out” in which you can aspirate and blow out the liquid at the tip of the pipette to assure proper volume has been delivered.

Being aware of these precautions would avoid laboratory accidents and inaccurate measurement of solutions.

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