A10 TOC Monitor

While resistivity is a sensitive and accurate way to measure and display the level of ionic concentration in ultrapure water, it gives no indication of the level of dissolved organic compounds that may be present.

Ultrapure water with a resistivity of 18.2 MΩ.cm @25 °C has a low inorganic salt* level, but concentrations of organic substances still may remain high.

Many laboratory experiments will be affected by even relatively low levels (50 ppb TOC) of organic contaminants. As a result, it is necessary to monitor these contaminants in order to give scientists a complete and accurate picture of the quality of high purity water used in the laboratory.

*The theoretical resistivity of ultrapure water containing no other ions aside from the protons and hydroxyl resulting from the water molecule dissociation is equal to 18.2 MΩ.cm @25 °C. The addition of only 1 μg/l of NaCl makes the resistivity value drop to 17.5 MΩ.cm @ 25 °C.

TOC Reduction Improves Laboratory Results

For instance, a moderate TOC level in high purity water used for chromatography applications will degrade the baseline quality and reduce the sensitivity of an analysis. Additionally, the presence of TOC will shorten the lifetime of chromatography columns by fouling the chromatography column beads and subsequently preventing a good mass transfer between the beads and the mobile phase.

The variable presence of undetermined organic substances in ultrapure water used for the preparation of culture media or buffers will also affect experiments including cell culture or electrophoresis separations.

For all these reasons, it is important for laboratories to be able to regularly measure the TOC level of high purity water.

Millipore TOC Monitors

To meet this need, Millipore’s Milli-Q water purification systems are designed with a built-in TOC monitor to ensure regular, on-line measurement of the level of organic substances in ultrapure product water at the time of delivery.

Millipore also offers an alternative solution to scientists who wish to measure the TOC level in high purity water delivered by systems without a built-in TOC monitor. The independent Millipore A10 TOC monitor is a small analytical instrument that can be connected to any high purity laboratory water system in order to provide a rapid and accurate measurement of the TOC level in the water produced.

The Millipore A10 TOC monitor will measure accurately Total Oxidizable Carbon between 1 and 999 ppb TOC values, and can be submitted to a TOC suitability test as described in the US and European Pharmacopoeia.

The operation principle of the Millipore A10 TOC monitor is described in Figure 1. The monitor is made of a small volume quartz cell with 2 built-in titanium electrodes to measure the water conductivity and a thermistor to measure the temperature.

Figure 1: Instrument Design

During the first phase of a test, water flows for 3 minutes through the 0.8 ml quartz TOC analysis cell that contains 2 titanium electrodes. Then the solenoid valve closes, capturing a water sample and the conductivity of the high purity is measured, compensated at 25 °C . This is the baseline conductivity.

The second phase then starts: the UV lamp located aside of the quartz cell is then powered “ON”, emitting light at 254 and 185 nm.

The energy of the 185 nm light generates ozone that produces hydroxyl (•OH) radicals from the oxygen dissolved in water. This process is catalyzed by the titanium dioxide coating formed on the electrodes. The •OH radicals are powerful oxidizing agents that will react with the organic substances dissolved in water. The oxidation process will generate charged molecules, and the conductivity of the water sample will start to increase.

The end product of organic molecule oxidation is carbon dioxide, which dissolves and reacts
with water to produce charged ions that increase the conductivity. The A10 monitor will measure conductivity compensated at 25 °C and perform the oxidation until all organic molecules are oxidized and the conductivity value becomes stable. The test can therefore last 3 or 6 minutes depending on the nature and concentration of organic molecules in water.

At the end of this process, a complex set of algorithms confirms complete oxidation, calculates TOC value from the difference between baseline and final conductivity, and displays the result.

Then the solenoid valve closing the oxidation cell outlet opens, allowing water to flush the cell, and a new measure cycle can start.

Quality Assurance

Millipore A10 TOC monitors are calibrated in accordance with Millipore’s Quality Assurance System standard procedures and specifications. Traceability of measurement equipment is performed according to the Millipore Calibration System.

Millipore A10 TOC monitors are delivered with a Certificate of Calibration. They can be recalibrated at user request and have been designed to take into account the suitability test requirements as specified in USP <643>.