A micropipette is a specialized laboratory tool designed for the precise transfer of minute volumes of liquid, typically in the microliter range. This instrument is indispensable in fields like molecular biology, chemistry, and clinical diagnostics where accuracy in liquid handling directly affects experimental outcomes. Selecting the appropriate micropipette for a task is fundamental to maintaining the integrity and reproducibility of scientific work. This guide provides the criteria necessary for choosing the correct instrument to ensure the highest level of accuracy for any given volume transfer.
Volume Range Classifications
Micropipettes are categorized by their maximum capacity, a designation that informs the user of the instrument’s optimal working range. Common classifications include the P2 or P10 for ultra-low volumes, the P20 for up to 20 microliters, the P200 for up to 200 microliters, and the P1000 for volumes up to 1,000 microliters (1 milliliter). The numerical label often corresponds to the highest volume the pipette can accurately handle without mechanical strain or loss of precision.
A fundamental rule for achieving maximal accuracy is to select the smallest micropipette that can safely accommodate the desired volume. For example, a transfer of 150 microliters should be performed using a P200 pipette, rather than a larger P1000 instrument. Accuracy declines when a pipette is used near the absolute lower limit of its range, where the relative error of the air cushion becomes more pronounced.
Transferring a volume of 18 microliters, for instance, requires the use of a P20 pipette, which operates within its peak performance range. Attempting to use a P200 pipette for this small volume would lead to greater volumetric error compared to the smaller instrument. This selection principle ensures that the piston mechanism is utilized within its most mechanically precise window, minimizing the influence of surface tension and air cushion variability.
Defining Pipette Mechanisms and Types
The internal mechanism of a micropipette dictates its suitability for different liquid properties, extending the selection criteria beyond simple volume capacity. The most common type is the air displacement pipette, which uses a cushion of air between the internal piston and the aspirated liquid. This design is the standard laboratory workhorse, providing high accuracy for aqueous solutions and non-viscous liquids that behave similarly to water.
However, the air cushion is susceptible to changes in temperature, vapor pressure, and viscosity, which can compromise volume integrity with certain samples. For challenging samples, the positive displacement pipette is the preferred option, as its piston makes direct contact with the liquid inside a specialized capillary tip. This design eliminates the problematic air cushion, ensuring precise transfer of viscous fluids like glycerol, volatile solvents such as ethanol, or liquids with high densities.
Beyond the fundamental mechanism, pipettes are also classified by their volume adjustability and channel number:
- Fixed volume pipettes offer a high degree of calibration accuracy for a single, specific volume, making them ideal for highly repetitive tasks.
- Adjustable volume pipettes provide the necessary flexibility to measure any volume within a specified range, which is more suited for general laboratory applications involving varied experimental conditions.
- Single-channel pipettes are used for transferring liquid from one container to another.
- Multi-channel pipettes, often featuring 8 or 12 tips, are designed for the high-throughput transfer of multiple samples into microplates simultaneously.
Operational Techniques and Selection
The final choice of micropipette must account for the required operational technique, which is determined by the physical characteristics of the liquid being transferred. Forward pipetting is the standard technique, involving aspiration to the first stop and subsequent complete dispensing to the second, or blowout, stop. This method is effective and accurate for most aqueous solutions and buffers that do not exhibit high viscosity or volatility.
Certain non-aqueous liquids, however, necessitate a modification in the handling procedure to maintain volumetric accuracy. Reverse pipetting is a specialized technique employed for viscous, volatile, or foaming liquids, which tend to leave a film on the inside of the tip. This method involves aspirating a volume slightly larger than the target volume by pressing the plunger to the second stop initially.
The user then dispenses the set volume only to the first stop, leaving the excess liquid in the tip, which is then discarded. The residual liquid compensates for the film that would otherwise be left behind, ensuring the accurate delivery of the intended volume. Alternatively, a positive displacement pipette can be chosen for these challenging liquids, as its direct-contact mechanism inherently bypasses the surface tension issues.