Procedure for calibration of micropipettors


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Good Laboratory Practice dictates that all equipment used for the generation, measurement or assessment of data shall be adequately tested, calibrated and/or standardized. A procedure describing a pipettor calibration method, records of test performance and repairs should be available for laboratory personnel. This procedure describes a method for evaluating the accuracy and precision of micropipettors used for immunoassay.

Fixed volume, adjustable or variable volume, or repeating volume pipettors with dispensing volumes in the range of 100 to 1000 mL; analytical balance; weighing boat; distilled or deionized water.

Assign a unique identifier code to each pipettor and label each with an indelible marker. Establish data logs for routine recording of testing data and historical performance assessment. Record the identifier code, the pipettor manufacturer's model number and the serial number in the logbook. Record any maintenance and repair activity in the logbook. Accuracy and precision specifications given by the manufacturer can be used to evaluate pipettor performance until the laboratory can establish its own (see 'Expected Results'). A testing frequency for this protocol should be established as part of the laboratory standard operating procedures. Monthly testing is recommended for micropipettors in use on a daily basis.

1. Fill a beaker with distilled water.
2. Tare a weigh boat on the analytical balance.
3. Select the pipet volume to be tested.
4. Attach the appropriate pipet tip or syringe (if repeat volume pipet) to the pipettor and wet the tip by withdrawing and dispensing several volumes of distilled water in the beaker.
5. Deliver one aliquot of water to the weigh boat and record the weight.
6. Leave this weigh boat in place and tare again. Repeat steps #5 and 6 nine times.
7. Calculate the mean, standard deviation and coefficient of variation ( or RSD) for the ten weights and record on the worksheets.
8. Compare results with established specifications (see 'Expected Results') for mean weight (accuracy) and % CV (precision).

Pipettors that do not meet specifications should be disassembled and cleaned thoroughly. If necessary, the seals should be replaced according to the manufacturer's instructions. After this is done repeat the test procedure. Pipettors which remain out of specification should be removed from service and submitted to the manufacturer for repair.

Pipettor manufacturers often will provide technical data showing the degree of inaccuracy and imprecision for their products but each laboratory should collect sufficient data to establish its own specifications. The following ranges are given as guidelines for specifications based on a data set of 100 replicates of the test protocol described above for each micropipettor volume.

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