It's easy to measure your pipetting accuracy – just pipette the same volume of water ten times onto a tared balance, note the weight each time. Then measure the relative standard deviation in the dispensed masses by calculating the standard deviation and expressing it as a percentage of the average. Calculate the accuracy of the pipette. To calculate the accuracy you can use the equation A = 100 x Vavg/V0, where A is the accuracy of the pipette, Vavg is the average calculated volume and V0 is the value you set the pipette to dispense. Accuracy should be between 99-101%.
Aside from sending your pipette off for calibration every year, your pipettes should be checked for accuracy on a regular basis. How often you do it depends on the frequency you use them. The faithful pipette you use every day? Check it every three months. The beloved but not used so often pipette? An annual check should suffice. The most common way to check your pipette accuracy is by weighing water. One caveat before beginning, be sure to use proper pipetting technique when measuring the accuracy of your pipette. Proper technique involves drawing up (aspirating) and dispensing (ejecting) the liquid in a smooth motion. Whatever you use your pipette for, take care of it, and it will take care of your experiments in the long run.
Take distilled water in a beaker and record its temperature. Gather your pipette and the correct tips based on both the small and large volumes that the pipette can dispense. Place a weigh boat on a balance that can accurately weigh in the microgram range, and set it to zero after closing the balance door. Pre-rinse the tip by aspirating and dispensing the set volume three times and push fully to remove any remaining liquid. Aspirate the calibration volume without bubble formation and dispense the liquid slowly into the weigh boat. Then, record the weight on the balance and repeat the process ten times.
The error of a pipette is an abstract concept that cannot be determined in real world pipetting environments. It is defined as the difference between the measured value and the true value. If a pipette user measures 1002 µL with their automatic pipette when they intend to measure 1000 µL, the error would be 2 µL. A pipette is calibrated with a series of graduation lines to allow the measurement of more than one volume. Measure using the bottom of the concave surface of the liquid in pipette. Reading the value at the bottom of the meniscus shows that 3.19 mL of liquid was delivered.
Precision is usually expressed in terms of the deviation of a set of results from the arithmetic mean of the set (mean and standard deviation to be discussed later in this section). The student of analytical chemistry is taught - correctly - that good precision does not mean good accuracy. However, It sounds reasonable to assume otherwise. There are certain basic concepts in analytical chemistry that are helpful to the analyst when treating analytical data. The accuracy is a measure of the degree of closeness of a measured or calculated value to its actual value. The percent error is the ratio of the error to the actual value multiplied by 100. The precision of a measurement is a measure of the reproducibility of a set of measurements.
Reverse pipetting is a technique to dispense a measured quantity of liquid by means of air displacement, which reduces the risk of splashing, foam or bubble formation. Reverse pipetting is more precise in dispensing small volumes of liquids containing proteins and biological solutions compared to forward pipetting.
For example, if you have to measure out 4.5 mL of liquid, a 5 mL pipet would be more accurate than a 10 mL pipet. This is because smaller sized pipets have smaller diameters and more precise graduations. Therefore, there will be less error when judging the meniscus of the sample when using a smaller sized pipet.
A disposable piston moves within the tip, making direct contact with the liquid. The piston then moves upwards, drawing the sample into the tip (No air cushion is present as with air displacement pipettes, so a physical void of the selected volume is created).
At 1 000 m above sea level, the volume error of a 100 μL pipette is about –0.3%. When using special tips, that is, tips that significantly differ from standard tips in their geometry, changing the adjustment can improve the dispensing accuracy (systematic error).