Water for IC

Impact of Water

Various contaminants in the water can have an effect on the ion chromatography systems and on the results of the analyses. These effects are reviewed, together with specifications for the quality parameters to consider for high purity water.

Ions
Ions are obvious contaminants to remove, because they can:

• Interfere with samples
• Interfere with standards
• Contaminate the mobile phase, leading to negative peaks in the chromatograms when the sample contains less of a specific ion than the mobile phase. 

In addition, the eluent generator and the suppressor also perform better when the water used contains very few ions: higher stability and lower baseline conductivity. Water quality parameter: To ensure “ion-free water”, water should have an 18.2 MΩ•cm resistivity.

Organics
Some organics have affinity for the polymer-based columns, which has some effects over time on the column performances:

• Reduce the column lifetime
• Seemingly modify elution time over time 

Some charged organics, such as ammoniums and carboxylates can also interfere with the analyses: they can be detected on the chromatogram, but they can also complex inorganic ions that are being analyzed. Water quality parameter: A TOC < 10 ppb is a safe target for the level of organic contaminants in the high purity water.

Particulates
Particulates will increase the back-pressure of the columns, and may have an effect on the injection pump. Water quality parameter: A 0.22 µm screen filter set up at the outlet of the water purification unit avoids the release of particulates.

Bacteria
Bacteria can release inorganic ions and small organic acids (acetate, formate, oxalate, pyruvate) that can show up on the anion chromatograms. Bacteria also behave as particulates and, therefore, have an effect on the column back-pressure. Water quality parameter: A 0.22 µm screen filter set up at the outlet of the water purification unit ensures a bacteria level remains < 1 CFU/mL

Figure 1: Comparison of chromatography profile of various sources of water.

High purity water with a resistivity of 18.2 MΩ•cm, a TOC below 10 ppb, and filtered through a 0.22 µm filter optimizes the performances of the ion chromatography systems and avoids interferences of the water with the analyses.

Sources of Contamination

Being able to use freshly produced high purity water is certainly one of the major benefits of a water purification system. When water or mobile phase is stored, CO2 dissolves in the water to generate significant amounts of bicarbonate. This leads to variation of the pH of the mobile phase and, consequently, can lead to variations of the elution time.

Figure 2: Hydrogen carbonate contamiated water changes the eluent strength. The anions, especially phosphate and sulfate, are eluting at later retention times.

Air-borne Contamination
Major contaminating ions are Na+, Ca2+, and Cl-. These contaminate quickly the standard solutions. Since an autozero is done before acquiring the data, the contamination goes undetected. However, a higher concentration of ions (e.g. Na+, Ca2+, or Cl-) than believed is actually present in the standard, leading to an off-set calibration curve. The concentration of ions in the sample is then under-estimated.

Contamination by the Container
The plastic from the container can release ions. While a careful selection of the brand of containers, as well as thorough cleaning and rinsing steps are highly advisable, avoiding contamination from the container is always very difficult.

Figure 3: Example of contamination of a standard solution upon storage (18h) in an analytical vial.

Resources

• Application Note: High Purity Water for Percholorate Analysis
• R&D Notebook 007: Trace Ion Chromatography

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