Wikipedia
Milli-Q is a trademark created by Millipore Corporation to describe 'ultrapure' water of " Type 1", as defined by various authorities (e.g. ISO 3696), as well as their devices for producing such water. The purification processes involve successive steps of filtration and deionization to achieve a purity expediently characterised in terms of resistivity (typically 18.2 MΩ·cm at 25 °C). The term is also commonly used as a genericised trademark to refer to other purified waters and purification equipment.
Milli-Q water purifiers use resin filters and deionization to purify the water. The system monitors the ion concentration by measuring the electrical resistivity of the water. Higher resistivity means fewer charge-carrying ions. Most Milli-Q systems dispense the water through a 0.22 μm membrane filter.
Milli-Q purifiers are capable of producing water pure enough to get reproducibility and accuracies within parts per million when using ion mass spectrometers in analytical geochemistry. Other users of this system include molecular biology and atmospheric science.