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Introduction & History
The modern pipette has had a colorful history as a standard tool in the scientist’s arsenal. What began as the usage of straw, one’s mouth, and the scientific principles of suction is now one of the most technologically-evolved yet extremely straightforward devices in the modern laboratory.
Commonly used in genetic research, chemistry, microbiology, and pharmacological testing; pipettes and micropipettes are glass or plastic tubes used to measure, transfer, and deliver substances of accurate volumes. Most pipettes function by creating a vacuum above the space that the liquid shall fill and then control the uptake of the liquid by releasing this vacuum, suctioning the liquid upwards. Simple enough as this may sound, pipettes have come a long way–let’s look at its evolution through an exhaustive list of products that are widely used today.
The First Pipettes
The Pasteur Pipette
Aside from his more popular invention of the process of pasteurization, we also have Louis Pasteur to thank for the first pipette ever invented. As the founder of medical microbiology and the proponent of Germ Theory, it was then important for Pasteur and his experiments that things be kept clean and germ-free: hence, the invention of the first pipette. The Pasteur pipette, which are the familiar eye droppers and chemical droppers that we still encounter in everyday use, was then merely designed to prevent contamination in transferring small amounts of liquids. The addition of rubber teats at the end of long, thin, glass tubes made aspirating and dispensing of liquids a quick and easy affair. Today, these pipettes are used for the transfer of rough, uncalibrated volumes of liquids of up to 2.5 milliliters.
How is it used? Pinch the rubber teat of the Pasteur pipette with thumb and forefinger, and immerse the tip of the pipette on the surface of the liquid. Then, slowly release pressure on the rubber teat and wait for the liquid to go up. Once pressure is released, and the proper amount of liquid is aspirated, bring the full Pasteur pipette to the receiving container and pinch the rubber teat again to dispense the liquid on the side of the container’s wall.
The Transfer Pipette
Pasteur’s glass pipettes were a 19th-century hit, but problems arose from the fragility of its material. The invention of plastics in the 1940s, therefore, paved the way for a modification to the original pipette design–plastic Pasteur pipettes, also called transfer pipettes, were single pieces of molded plastic manufactured using the blow-molding process on low-density polyethylene (LDPE). Both the stem and bulb are part of the single piece of plastic, in place of the rubber attachment in glass pipettes. Today, these disposable transfer pipettes are made in a variety of sizes and shapes, and some include graduation marks for approximate volume calibrations. Note, however, that though transfer pipettes may be perfect for use with aqueous solutions, organic solvents such as acetone may dissolve the plastic material.
How is it used? Transfer pipettes use a similar procedure as with the usage of the Pasteur pipette, but instead of a rubber teat, the plastic bulb is pinched.
Volumetric and Measuring Pipettes
The high-end and complex pipette varieties that offer more accuracy and precision than Pasteur and transfer pipettes emphasize on the other aspect of pipetting, aside from the transfer and delivery of liquids–measurement. And, though considered a reliable high-precision instrument, the pipette does not achieve accuracy and precision on its own. A tool is only as good as its user, and, in the laboratory context, the wielder of the pipette must be well-prepared and knowledgeable with common pipetting techniques to make full use of its features. More importantly, scientists are expected to make informed decisions when selecting which pipette to use. Which pipette is the right pipette? How are they used? Let’s look at our list again.
The Volumetric Pipette
Volumetric pipettes resemble thin rolling pins with large bellies, blunt on one end and tapered on the tip. Typically, these pipettes are used to deliver single, specific volumes between 1 and 100 milliliters, at which they are calibrated. These glass pipettes offer more accuracy than Pasteur and transfer pipettes, emphasizing on the other aspect of pipetting, aside from the transfer and delivery of liquids–measurement. By having the middle bulge where the bulk of the liquid is stored, and the single volume marker at the thin portion, errors in accuracy are extremely lessened. The narrow diameter of the thin portion makes errors more obvious and easier to point out.
With these types of pipettes, procedures require the use of separate pipette bulbs, devices that provide suction. The common rubber bulb is fairly simple and easy to use–shaped like a fist-sized balloon, the rubber bulb is squeezed and touched to the mouth of the pipette, while the pipette tip is placed in liquid. The liquid draws up as pressure on the bulb is slowly released. Other similar tools, such as safety pipette fillers and pipetting aids, also serve the same purpose.
How is it used? First, hold the pipette at the upper portion using your thumb, pinky, ring and middle finger. Free your index finger. With your other hand, squeeze the rubber bulb and touch its opening to the top mouth of the pipette. Insert no more than a half-centimeter of the pipette into the rubber bulb. Afterwards, immerse the tip of the pipette into the liquid and slowly release your hand’s pressure on the rubber bulb. The liquid will then be drawn up into the pipette. Once the curved surface of the liquid, called the meniscus, reaches the target measurement, remove the bulb and quickly slip your free index finger over the tip of the pipette’s mouth. With your finger still firmly closed on the pipette, lift the pipette out of the liquid and into the receiving container.
It is a safe option always to collect more than, but closest to, the graduation mark so that adjustments can be easily made. Observing the pipette at eye-level, carefully raise your index finger to lessen the liquid and reach the desired amount. Depending on the physical properties of the liquid, the meniscus may either be concave or convex–for both cases, the center of the meniscus is used as the basis for measurement.
The Mohr & Serological Pipette
As opposed to volumetric pipettes, measuring pipettes are thin glass tubes that are calibrated into small divisions of volume range so that different amounts of liquid may be measured and transferred. The two types of measuring pipettes–Mohr and serological pipettes–differ according to the placement of graduations. In Mohr pipettes, graduations always end before the tapered tip, while in serological pipettes, the graduations continue to the tip.
How are they used? The procedure for the transfer of liquids in measuring pipettes is similar to t