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Impact of Water


Impact of water quality on TOC analyzers

Roles of water in TOC analysis
Water is used to prepare the standards necessary for the instrument calibration, to run the blanks, and in some cases to clean up or rinse the instrument. In order to optimize the performances of TOC analysis, in particular in the low concentration range, it is important to have water with very low TOC.

Water quality parameters

Organics
As mentioned, water with low TOC is necessary to run the analysis. High purity water with TOC < 5 ppb (µg/L) is delivered by water purification systems and available on-demand for TOC analysis.

Merck:/Freestyle/LW-Lab-Water/applications/TOC/LW-LC-TOC-Image1-265x194.jpg
Figure 1: Example of performances of a water purification system over time. Water purified using a pre-treatment system was fed to a polishing unit (Milli-Q®). Three different cartridges were used.

  • Ions
    As described above, some methods rely on the initial conductivity and measure the difference after oxidation. Therefore, in order to perform calibration in the best operating conditions, low conductivity solutions should be selected.
  • Particulates
    Particulates should be avoided. They can contain carbon that is not readily accessible for oxidation. They can also spoil the TOC analyzer, requiring more frequent cleaning. 
  • Bacteria
    Bacteria behave as particles and should therefore be removed. In addition, bacteria contain organic molecules that are released when bacteria are killed. 
Example illustrating the importance of water quality in TOC measurements
Experiments were performed on a TOC analyzer combining oxidation by ultraviolet (UV) light (photooxidation) and sodium peroxodisulfate at 80 °C (TOC-VWP TOC analyzer, Shimadzu Europa GmbH, Duisburg, Germany). This combination ensures a powerful oxidation of the organic molecules present in the sample. The automatic preparation of the reagents avoids the introduction of impurities and minimizes the blank values of the analyzer. Once oxidized, the organic carbon is converted to CO2 that is detected using a non-dispersive infrared (NDIR) method.

Sample Analysis.
Precautions are required when analyzing high purity water samples in order to avoid contaminating the samples. Airborne organics quickly dissolve in the water and it is therefore important to reduce the contact time of the sample with air. A major source of contamination is the container used to carry the sample. Glass containers specifically prepared for TOC analysis should be selected and rinsed with the water to be analyzed prior to sample collection. It is important as well to analyze a meaningful sample: a few liters of the water should be flushed before sampling to avoid collecting stagnant water that would inevitably be contaminated by organics.

The high purity water was prepared on a system (Direct-Q®3 system) combining several technologies that reduce the level of organics present in tap water. Reverse osmosis (RO), a membrane-based technology, is utilized to remove the bulk of the ions and organic contaminants. This membrane is protected upstream by a “pretreatment cartridge” that removes water contaminants detrimental to the RO operation (Cl2, particulates). The activated carbon present in the pretreatment cartridge also reduces the levels of organics in the water feeding the purification unit. Following reverse osmosis, the water is stored in a reservoir. Upon demand, high purity water is prepared from the water stored in the reservoir. The water is treated using a dual wavelength (185/254 nm) UV photooxidation lamp and is further purified using a mix of ion exchange resins and synthetic activated carbon. These steps result in a reduction of the TOC level below 5 ppb.

Data reported in Table 1 show the values of TOC (blank value corrected) in the tap water feeding the purification unit, in water purified using reverse osmosis and in the high purity water delivered by the system (Direct Q). The values reported are an average of three values, and the relative standard deviation (RSD) is calculated on these three values.

The data obtained in this experiment show that reverse osmosis removed a large part of the organics: above 600 µg/L down to 15 µg/L. The organic level is further reduced below 5 µg/L thanks to the photooxidation and the activated carbon purification steps.

Sample
NPOC (µg/L)
RSD (%)
Feed Water 629.4 0.9
Feed Water 637.7 1.5
Feed Water 618.8 1.3
Reservoir Water 14.8 2.1
Reservoir Water 14.8 1.5
Reservoir Water 14.9 1.3
High Purity Water 4.2 1.5
High Purity Water 4.2 5.2
High Purity Water 4.2 2.5

Table 1: Measurement of NPOC in the feed water, in the reservoir, and at the outlet (high purity water) of the Direct-Q® 3 water purification system.


Since it is known that storage of water leads to a degradation of its quality and an increase of TOC, and since contact with air contaminates high purity water, it was interesting to highlight and quantify these phenomena. TOC of commercially available analytical-grade bottled water was measured over time. TOC was measured at the opening, and then several times within three hours. After each sampling, the bottle was closed again. It is clear from the data reported in Table 2 that the water is quickly contaminated upon contact with air. This phenomenon is avoided with a purification unit, because water can be drawn upon demand at the time of the analyses.

Sample
Time
NPOC (µg/L)
RSD (%)
1 12:08 25.21 1.9
2 13:01 34.08 5.0
3 14:14 35.77 4.7
4 14:25 55.88 9.1
5 14:40 75.08 3.5
6 15:10 79.25 1.2

Table 2: Evolution of bottled water NPOC values over time.

Measuring TOC at low levels requires a suitable TOC analyzer. High purity water used in combination with the TOC analyzer optimizes the performances of the analyzer. Low blank values are obtained and results are therefore more accurate. TOC levels in water are known to increase upon exposure to air, so use of analytical-grade bottled water will result in increased baseline TOC levels after the containers are opened. Immediate use of water from an on-demand high purity water system is the most protection against contamination.


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