Polymerase Chain Reaction (PCR) is a laboratory technique used to amplify specific segments of DNA. Its success relies on specialized reaction vessels called PCR tubes, which are small, thin-walled containers made from high-purity polypropylene. These tubes are designed to ensure efficient and uniform transfer of temperature during the rapid heating and cooling cycles of a thermal cycler, which is fundamental to the amplification process.
Standard Capacities of Individual PCR Tubes
The maximum volume of an individual PCR tube is standardized for compatibility with thermal cycling instruments. The largest common standard size is 0.5 mL (500 microliters), often used for larger volume reactions or older thermal cyclers.
The most frequently used standard is the 0.2 mL tube (200 microliters). This size is the industry standard because it fits the well spacing and block dimensions of 96-well thermal cyclers. A low-profile variant, the 0.1 mL tube, is also widely available. These smaller tubes are favored in fast-cycling protocols because their lower thermal mass allows for quicker temperature changes, optimizing reaction kinetics.
Formats: Strips and Plates
While individual tubes are useful for low-throughput work, PCR tubes are frequently manufactured in multi-well formats to increase sample processing efficiency. The most common organized format is the 8-tube strip, consisting of eight connected 0.2 mL or 0.1 mL tubes. These strips fit into the columns or rows of a standard 96-well block, making them suitable for medium-throughput experiments.
For high-throughput applications, tubes are integrated into full PCR plates, which are large, interconnected arrays of wells. The 96-well plate is the most recognizable format, with each well typically having a 0.2 mL maximum capacity. The 384-well plate significantly increases sample density within the same footprint. To accommodate this increase, the maximum capacity of each well in a 384-well plate is reduced, often to 55 microliters or less.
Distinguishing Maximum Capacity from Working Volume
It is important to differentiate between a PCR tube’s total physical capacity and the recommended working volume, which is the amount of sample actually placed inside. The total capacity represents the absolute maximum volume the container can physically hold, but filling a tube to this maximum is rarely recommended for PCR.
The optimal working volume is significantly lower, typically ranging from 10 to 50 microliters for standard reactions. This reduced volume is necessary to ensure optimal heat transfer from the thermal cycler block to the entire reaction mixture. Larger volumes can create temperature gradients, leading to uneven amplification results. Maintaining a headspace also minimizes the risk of cross-contamination and accommodates the thermal expansion of the liquid during heating cycles.