Pipettes are classified into five grades based on their use, testing, maintenance, and measurement.
Transfer pipettes, single-channel pipettes, multichannel pipettes, graduated/serological pipettes, and repeat pipettes are the five categories of laboratory pipettes. The manner in which the equipment is handled, from the most simple transfer pipette dropper to the most complex repeat dispensing pipette, will affect the accuracy of the test results.
It’s the most basic pipette and can only be used for rough measurements. When using a disposable pipette, however, a standard pipetting technique must be followed.
Always use a new pipette and discard it after testing. Aspirate liquid at a 90-degree angle, dispense at a 45-degree angle, and touch off to ensure that all liquid is dispensed.
It is a non-disposable pipette, typically of the air-displacement variety, that produces accurate measurement results with the use of a single disposable tip. Single-channel pipetting is commonly associated with two techniques:
- Forward Technique. This is the intended function and the most commonly used pipette measurement technique. The first step is pressing the plunger to partially submerge the pipette tip in the liquid, then aspirate the measured volume by gradually releasing the plunger to avoid bubbles. To dispense the liquid, press the plunger slowly through the first stop to the final blow out position while ‘touching off’ the last drop from the tip.
- Reverse Technique. When working with viscous or bubble-prone solutions, we can use a reverse pipetting technique to reduce interference from air bubbles. To use this technique, push the plunger all the way down to the third stop position (all the way down), slightly submerge into the liquid, and slowly return the plunger to the top to aspirate the liquid into the tip.
The technique and technology used in multi channel pipettes are similar to those used in single channel pipettes, with the exception that it accepts more than one tip at a time.
Because the liquid is being aspirated from the same well into multiple channels at the same time, you must ensure that the aspirated liquid levels are equal before dispensing the liquid in the tubes.
The method is similar to the single channel technique, but the result is vastly different:
- Set the desired volume and install the appropriate tips to each channel.
- Depress the plunger to the first stop while holding the pipette vertically.
- Immerse the plunger tip in the liquid, then return it to its resting position.
- Set the tip at a 45-degree angle against the wall of the vessel that will receive the liquid.
- Depress the plunger to the first stop, wait a second, then press the plunger to the second stop and expel all of the liquid while ‘touching off’ the last drop.
- Remove the tip of the plunger from the liquid and return it to its resting position.
The final volume of the subject is determined when using this type of pipette by calculating the difference in liquid level before and after the liquid is dispensed, similar to a burette.
This is the most common way to use a graduated pipette:
- Do not touch the bottom of the pipette while holding it in solution.
- Squeeze the bulb and secure it to the pipette’s tip.
- To control the volume of aspiration, place your forefinger on top of the pipette.
- To ensure proper measurement, subtract the amount required into a separate beaker while remaining eye level.
- Measure the solution from the bottom of the meniscuses, the crescent-shaped surface of the liquid visible in the pipette.
- Subtract the required volume from the initial volume to find the volume required to release it in order to obtain the desired amount.
Repeat Pipette Dispenser
This pipette allows for dispensing and setting a specific volume into multiple receptacles without the need to aspirate. This ability to multi dispense saves time and effort.
The design of the repeat pipette dispenser differs from that of a standard pipette. A dispensing and filling lever, as opposed to a plunger, distinguishes the two.
Follow this method to successfully use a repeater pipette:
- Pull the filling lever all the way down.
- Raise the locking clamp to its highest position.
- Close the lever after inserting the syringe-style tip into the barrel and clicking it into place.
- At a 90-degree angle, immerse the tip in the liquid.
- Fill the tip completely by slowly sliding the filling lever upward.
- The liquid from the first dispense should be discarded to prime the tip.
- The pipette is now ready to use.
Prior to pipetting technique and liquid measurement, it’s critical to assess the type of test you’re performing and determine which pipette type is best suited to it. Using the incorrect instrument, or even the correct instrument, can have a significant impact on your results.
Accuracy and repeatability are two of the most important aspects of pipette testing; however, technicians cannot expect precise results unless they are well-versed in the operation of all lab equipment.