All reagents used in laboratories today are delivered with labels indicating the degree of purity and the maximum concentration levels for likely contaminants. Yet, the purityof the major reagent used in all laboratories is often not fully monitored and recorded, says J F Pilette
Laboratory grade water often constitutes 99 per cent of the mass of solutions used in laboratory experimentation. Ionic contamination of ultrapure water is generally monitored through resistivity, but the degree of water contamination by organic substances is often left undetermined.
This contamination can be reported as total oxidisable carbon (TOC). This single parameter takes into account the various organic substances that can be present in pure water and is related to the concentration and nature of the organic substances.
For instance, acetone (CH3COCH3) has a molecular weight equal to 58 Dalton and contains three carbon atoms with an atomic weight of 12 Dalton each. The carbon ratio in acetone is therefore equal to 36/58 = 0.62 and a 1ppb acetone solution will display a TOC level equal to 0.62ppb. A similar calculation shows that the methanol carbon ratio is equal to 0.375 and that a 2.66ppb methanol concentration corresponds to 1ppb TOC.
Examples of the concentration required to reach 1ppb TOC values are given in Table 1 for various organic substances. The TOC parameter expresses the organic contamination of water. For instance, a solution containing 1.33ppb methanol and 1.25ppb formaldehyde will generate 1ppb TOC. TOC may reach levels of 1,000 to 2,000ppb in tap water and up to 5000ppb in deionised water produced by ion-exchange resins that are regularly regenerated.
The highly variable organic contamination of water purified by regenerated ion-exchange resins can be attributed to the release of organic substances from the polystyrene/divinylbenzene resin bead matrix after regeneration by strong acid and bases, and the growth of micro-organisms on old resins in use for over a year.
Organic contaminants in water can cause interference in many laboratory applications. In high performance liquid chromatography (HPLC), LC and ILC for instance, organics may coat the outside of resin beads and prevent access of the molecules or ions to the exchange sites, reducing the separation resolution; they may also generate high baselines.
A sensitive HPLC method for measuring ascorbic acid by electrochemical detection was developed by medical researchers at a national research laboratory. However, even low TOC levels (50ppb) in the high purity water used to make the mobile phase, interfered significantly with the analysis, generating a high background and decreasing the analysis sensitivity. Reducing the water TOC level to <5ppb increased sensitivity and improved the baseline and quantification by decreasing background levels.
Millipore recognised this need early on and developed a compact TOC monitor in collaboration with Anatel, the world leader in TOC measurement. This A10 TOC monitor works as follows: it measures the conductivity compensated at 25oC of a water sample entering its analytical cell. The sample is then irradiated by a 185 nm UV lamp. This promotes the generation of hydroxyl radicals from the oxygen dissolved in water. These radicals oxidise organic substances in the sample, producing carbon dioxide that dissolves in water, yielding bicarbonate ions and protons that cause conductivity increase.
A complex set of algorithms monitors the conductivity variation, adapts the oxidation time to ensure complete oxidation all organic substances and calculates the TOC value from a calibration curve when the final conductivity value is stable.
The A10 TOC monitor is available as a stand alone unit that can be linked to a Millipore water purification system delivering water with resistivity above 5Ma.cm. All Millipore systems now have a built-in TOC monitor, enabling them to detect organic contamination down to 1 ppb TOC.
Enquiry No 52
J F Pilette is product manager worldwide for Millipore Lab Water Division. Millipore's headquarters are in Bedford, Massachusetts, USA.
