PP is the optimum material for PCR tubes. It provides the most efficient heat transfer, as well as an inert surface with low binding capabilities for nucleic acids, proteins and other molecules. However, the material is not thermally stable in a plate format, and expands and contracts during each PCR cycle (Figure 1, click on image to expand).
Such thermal expansion will weaken the plate seal and leads to sample evaporation mainly from corner wells and outer rows. Thermal cycler blocks do not prevent thermal expansion of PCR plates. PCR blocks do not support PCR plates from the sides, and the high temperatures from the thermal block and heated lid accelerate expansion of the plates (Figure 2, click on image to expand).
Evaporation from one piece PP plates is highest in the outer wells. Since thermal expansion and movement of wells in one piece PP plates is enhanced around the edges of the plates (see Figure 1), evaporation is the highest from the two outer rows of wells.
Figure 3 (click on image to expand) illustrates the levels of sample evaporation from different areas of PP plates. only the inner 32 wells of a one piece 96 well plate show low levels of evaporation, but sample loss is high from the two outer rows which contain more than 65% of the wells.