Compare the results for testing the P20 pipet at the 2 ul se

Compare the results for testing the P-20 pipet at the 2 ul setting from Test Group 3 with those of Test Group 4. Is the difference significant? If so, any ideas why?

Group 3 2.6 5.6 Group 4 1.4 1.6 1.7 1.4 1.9

Solution

In general, micropipettes are designed to operate with accuracies within a few percent (generally <3%) of the intended value. The accuracy of a micropipette decreases when micropipettes are set to deliver volumes close to the lowest values in their range, like in the present case. Precision of measuring by the experimenter provides information about reproducibility, without any reference to a standard. Thus precesion reflects random errors. Assuming random error, all the measurements will fall within one standard deviation of the mean, and 95% of measurements will fall within two standard deviations of the mean.

Test group 3: P-20 pipette at 2 ul setting: Results range from a minimum value of 2.6 to maximum of 5.6. Thus the volume measured by this pipette for 2 ul is higher by 0.6 to 3.6 ul. This accuracy will further decrease if larger volume is taken from this pipette. Keeping in mind the random precision errors, the errors presented in this case are much beyond maximum permissible limits (<=0.03ul).

Test group 4: P-20 pipette at 2 ul setting: Results range from a minimum value of 1.4 to 1.9  ul. Thus the volume measured by this pipette for 2 ul is lower by 0.1 to 0.6 ul. In this case also, the errors are exceeding maximum permissible limits, although results are comparitively better than test group 3.

The Difference between the volume desired and the actual olume pipetted in both cases are significant.

This could be due to following reasons;

1.The nose of the barrel of pipette damaged/not stored properly/or tips are not well fitted.

2. Pipette not calibrated.

3. Positioning of pipette while use is not accurate.

4. Accuracy of pipette decreases naturally when operated at close to lowest volumes.

5. Room temperature and viscosity of the liquid to be pipetted plays a role. Sample should be at room temperature and reverse pipetting is required while pipetting viscous solutions.