CLINICAL LAB SCIENEC

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ESSENTIALS OF CLINICAL LABORATORY SCIENCE
To fill a buret, close the stopcock at the bottom and use a funnel. You may
need to lift up on the funnel slightly, to allow the solution to flow in freely. You
can also fill a buret using a disposable transfer pipette. This works better than
a funnel for the small, 10-mL burets. Check the tip of the buret for any air
bubbles. To remove an air bubble, whack the side of the buret tip while solution
is flowing. If an air bubble is present during a titration, volume readings may be
in error. Rinse the tip of the buret with water from a wash bottle and dry it carefully.
After a minute or so, check for solution on the tip to see if the buret is leaking.
The tip should be clean and dry before you take an initial volume reading.
When your buret is conditioned and filled and with no air bubbles or leaks, take
an initial volume reading. A buret reading card with a black rectangle can help
you to take a more accurate reading. Read the bottom of the meniscus. Be sure
your eye is at the level of meniscus, not above or below. Reading from an angle,
rather than straight on, results in an error of measurement called a parallax
error. The endpoint should be approached slowly, a drop at a time. Use a wash
bottle to rinse the tip of the buret and the sides of the flask. Deliver solution
to the titration flask by turning the stopcock. The solution should be delivered
quickly until a couple of milliliters from the endpoint, when it is advantageous to
slow the flow for better control. The clinical laboratory instructor can show you
how to deliver a partial drop of solution when near the endpoint.
Micropipetters and Micropipettes
Micropipetters (to include pipette tips) are designed to deliver extremely small
volumes such as a few microliters in most cases and are engineered for precision
and accuracy in transferring maximum volumes of up to 1 mL (Figure 8-13).
One type of micropipette, the Eppendorf, dispenses varying volumes of liquid
from a disposable tip. The pipette body contains a plunger that provides the suction
to pull liquid into the tip when a button is pressed. The maximum displacement
of the plunger is set by a dial on the pipette body, enabling the delivery
volume to be changed. A number of pipettes and pipette tips for single-use that
require a reusable pipette are available. The pipettes or pipette tips are normally
disposed of following their use.
Mechanics of Micropipetters
Sterile technique prevents liquid from coming into contact with the pipette when
disposable tips are used. The liquid is drawn into and dispensed from a disposable
pipette tip, which is changed between transfers. Depressing the tip ejector
button removes the tip, which is cast off without being handled by the operator
and disposed of safely in an appropriate container. The plunger is depressed to
both draw up and dispense the liquid. Normal operation consists of depressing
the plunger button to the first stop while the pipette is held in the air. The tip is
then submerged in the liquid to be transported and the plunger is released in a
slow and even manner. This draws the liquid up into the tip. The instrument is
then moved to the desired dispensing location. The plunger is again depressed
to the first stop, and then to the second stop, or “blowout,” position. This action
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