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Micropipettes are routinely used by molecular biologists, organic chemists, and other bench scientists to deliver precise volumes of liquid on the microlitre to millilitre scale. On occasion, they must be calibrated to verify their accuracy. This is typically done by pipetting a volume of water on
to an analytical balance, measuring the weight of the water, and then using the density of the water at room temperature to calculate the volume delivered. This works fine and dandy for volumes greater than fifty microlitres or so, but below that, the weights of the microdroplets become difficult to measure.
My solution to this problem is the development of a specialized micropipette tip that measures the volume of liquid taken up by the pipette. On the inside of this tip are printed extraordinarily fine circuit traces that measure the level of the special, conductive testing fluid. The traces that are exposed to the fluid are arrange radially so that a "slice" of fluid will close the circuit at the next measurement level. One may need to use several different tips to calibrate over the range of the pipette. For example, a 10 uL pipette may have three different tips for calibration at 1 uL, 5 uL, and 10 uL.
The base of the tip "plugs in" to a battery operated, wristwatch-sized computer with an LCD screen that displays the volume delivered and calculates the error. The tip is then flushed, dried, and stored for reuse at a later date.
I don't know how you would print circuit traces on the inside of the tip, but I suppose there's someone out there who could figure it out.
[link]
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Why do micropipettes need to be recalibrated ? Does their volume change somehow ? |
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If they're adjustable (as most are), the adjuster
can slip out of calibration. However, 99% of
pipette faults are due to leaky pistons, which can
be checked much more easily (fill pipette; place
finger over end; depress plunger and wait 5
seconds; remove finger; if liquid isn't expelled,
there's a leak). |
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This is a nice idea, though. My only quibble is
that, especially if the inside of the tip isn't
uniform or is slightly dirty, the liquid/air interface
may not be perfectly level, leading to an over- or
under-reading. But using a special tip which is
very long and narrow would solve this problem. |
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my assumption would be that an accurate TD measure is not achieved until the tip has been filled with fluid. |
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how does one address the strange behavior of fluid at that volume? The meniscus is a dynamic thing and tiny droplets of fluid are capricious and very responsive to surface conditions, even static. I'm afraid that I would have very little trust that my investment in such a device would be worthwhile. After all, I needn't measure the mass of a single delivery, I can measure more accurately the mass of 10, 20, 50, 100 deliveries in bulk and do some math which should give adequate significance given the vagaries affecting my accuracy (thermometer and scale accuracy and resolution). |
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TD accuracy is always a little hit and miss. I assume that the values are different if the tip is submerged, and don't even get started with multiwell tips. |
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To be honest, I can't think of a lot of situations
where even a 5% pipetting error is a big deal. And
there are very few where a 1% error is worth worrying
about. |
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You are more likely to get larger errors through bad
pipetting technique; failing to mix defrosted
solutions before sampling; failing to correctly touch
off the tip after dispensing; and a half a dozen other
factors. |
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//a 10 uL pipette may have three different tips for calibration at 1 uL, 5 uL, and 10 uL// |
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/the strange behavior of fluid at that volume/
Yes. Probably there is a better way to move tiny amounts of fluid than a pipette. |
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Should be µl - microlitres. |
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What does meniscus formation do to your
measurement? Are you still planning to have a
specific calibration fluid? |
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The meniscus problem is why you would want a
long, thin tip rather than a regular conical tip. If
the tip is polypropylene (as most are), water will
form only a very slight positive meniscus. |
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Moreovermore, before these new-fangled Gilsons,
a micropipette was a small glass capillary with a
line indicating a defined fill volume. These were
used in conjunction with a "spit tube" (capillary
holder with tube and mouthpiece) or, since health
and safety was invented, a little eyedropper-style
bulb. You can still get these, in volumes from 1µl
to a few hundred µl. |
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//Should be µl - microlitres// |
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Thanks. I guess mcl was already taken... |
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MB, spent more time than I care to think about with a tiny pipette between my fingers, like some sort of horrible cramping drug addiction. very reliable so long as you take care about where your tears fall. |
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The best micropipette calibration tip is "±10% is fine
for biology". |
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