Custom Premix Selecting "Custom Premix" option means that all of the beads you have chosen will be premixed in manufacturing before the kit is sent to you.
Water used for surface tension measurements must not contain impurities that would reduce the surface tension of water, and give artificially low measurements.
Organics Organics, especially surfactants, decrease the surface tension of water. The polar end of the surfactant molecules interact strongly with water molecules, disrupting the strong hydrogen bonds between water molecules that cause surface tension.
Ions Salts also lower surface tension of water, though not to the extent that surfactants do.
Particles Particles will disrupt the surface tension of water.
Bacteria Since bacteria are particulates, like particles, they would disrupt surface tension. They also release by-products such as ions and organics.
Thus, water used for surface tension measurements need to of the highest purity:
Low organic contamination, measured as total oxidizable carbon (TOC)
No ionic contamination, that is, having 18.2 MΩ.cm resistivity