Page 109 - The Veterinary Laboratory and Field Manual 3rd Edition

78 Willy Schauwers

• Put the tip firmly on the pipettor. is leakage. The leakages arise from damage
• Press the plunger of the pipettor to the sec- either to the seals, pistons or tip cones. Often
ond stop. not detectable by the naked eye, leaks lead to
• With the pipettor vertical and the tip 1–2 mm significant volume errors. Air-displacement
below the surface of the liquid, release the pipettes must be checked at regular intervals
plunger smoothly and slowly to aspirate the and the results must be compared with the
fluid. Wipe the outside of the tip, do not error limits. However, a calibration certificate
touch the open end of the tip. only reflects the results at the time of testing.
• To dispense, touch the tip at an angle against The time between these calibrations is cru-
the side of the receiving vessel, press to the cial, since leaks can occur at any time. While
first stop and wait a few seconds. Do not the PLT unit cannot replace regular gravimet-
press to the second stop (the liquid in the tip ric testing, daily pipette checks can provide a
should be discarded). safeguard during the periods between calibra-
tions. Even the smallest leaks are detected!
Micropipettor tips can be packaged in bulk or in Process reliability for the pipettes is thus
tip-boxes (racks, no hand contact with the tip significantly improved.
needed). Tips can be sterile, non-sterile and with • Every 6 months the micropipettors should
or without filter. be checked for leakage, accuracy and preci-
sion. If leakage, accuracy and precision are
carE, uSE and MaIntEnancE within limits, the micropipettors can be used
• First organize your work station for maxi- for another 6 months, if the micropipettor
mum efficiency and minimum fatigue. fails some parts have to be replaced and it
• Make sure the tip is properly mounted and has to be recalibrated. Some micropipettors
fits well before you set the volume. can be re-calibrated; others have to be sent
• Adjust the volume. back to the manufacturer. Testing micropipet-
• Choose the mode of pipetting (reverse or for- tors requires the use of an analytical balance
ward mode) adapted to your sample. (sensitivity 0.0001 g, usually not available in
• Eject the used tip and store the pipette in an a district laboratory) and some specialized
upright position to avoid damage and cross- training. See:
contamination.
• Do not leave your pipette lying on the work- • ‘SOP-LABORATORY-EQUIP-07 Quality
bench where it can come into contact with assurance of mechanical pipettes’
chemicals or fall off and break. Always store • ‘FORM-SOP-LABORATORY-EQUIP-07–01
your pipette vertically to prevent liquids from Pipette Calibration Worksheet’
running inside the shaft of the pipette. • ‘FORM-SOP-LABORATORY-EQUIP-07–02
• Use the right tip for the pipette, there is a Gravimetric pipette test worksheet for
very wide range of tips. Pipetman P-1000’.
• Do not re-use tips.
Dispensers
quaLIty aSSurancE
• Air-displacement pipette leakages can be Bottle top dispensers (Figure 2.43) are widely
tested using the BRAND PLT unit (Pipette available for rapid repetitive dispensing of
Leak Testing unit). The most frequent cause reagents. Some dispensers can be autoclaved.
7
of inaccuracy in air displacement pipettes Dispensers are available for different volume

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