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Importance of Pure Water in Modern Ion Chromatography

Ion chromatography (IC) is evolving to meet the analytical demands for more rapid analyses using significantly smaller sample volumes. It is essential that sensitivity and reproducibility are maintained.

by Paul Whitehead, Ph.D.

Ion chromatography (IC) is evolving to meet the analytical demands for more rapid analyses using significantly smaller sample volumes. It is essential that sensitivity and reproducibility are maintained. The purity of the water used has a key role to play.

Introduction

IC a form of liquid chromatography, it uses ion-exchange based stationary phase materials to separate atomic or molecular ions for qualitative or quantitative analysis.

Use of pure water in Ion Chromatography

When analyzing samples at such trace levels, the only way to ensure confidence in the data is to have a reliable IC instrument, high-quality reagents and, especially, pure water. Water is used in all aspects of IC, including the dilution of samples, sample preparation or pre-treatment, preparing blanks and standards, the rinsing of equipment and as an eluent. In reagent-free ion chromatography (RFIC) systems the only flow stream is water, therefore, any impurities present in the water can interfere with the analysis in a number of ways and a reliable source of water free of contamination is essential for reproducible results.

So what type of water impurities is a chromatographer at risk of encountering and what effects could they have?

Types of water impurities

Ultrapure (type I) water is usually produced by the multi-stage treatment of potable mains water but the unique ability of water to dissolve to some extent or other virtually every chemical compound and support practically every form of life means that natural waters inevitably contain a wide range of impurities and contaminants such as ions, dissolved organics, dissolved inorganic compounds, suspended particles, microorganisms and dissolved gases. As the water is rendered potable, many of the contaminants are removed, but some others may be introduced, such as plasticizers from pipe-work systems or bituminous coatings from tanks.

Effects of contamination

With a sensitive technique such as IC, the effects of contamination of the water could have serious consequences, with the potential to negate experimental results.

But what impact can impurities really have on the reliability and reproducibility of an IC analysis?

As summarized in Figure 1, the effects of contamination from ions, organics, colloids, bacteria and gases can all impact sensitivity and reproducibility to some degree. Contaminating ions tend to have a significant but short-term effect, producing high blanks, high background and chemical interferences that directly degrade results and reduce sensitivity. Varying levels of contaminating ions would result in higher variances in the observed results. While organics, colloids and bacteria will also affect background/blanks, they also tend to have a long-term impact through media fouling and surface coating that can affect parts of the instrumentation, such as the chromatography column, the detector or inner surfaces of the system itself. The net effect of this type of fouling is anomalous baseline shifts, unknown peaks on the baseline, high noise etc.

Figure 1

Figure 2 demonstrates interference from poor water quality in the eluent encountered during ion chromatography. Data in Figure 2 shows the effects on trace cation analysis.

Experimental conditions
Dionex DX500 with GP40 pump, CS12 column, CD20 conductivity detector
Eluent 20mM methanesulphonic acid at 1ml/min

Figure 2

Typical water purity required

For basic isocratic IC applications, good quality general laboratory grade water (Type II pure water) may be adequate with a typical specification. However, even a “basic measurement” will only benefit from using water purified to Type I ultrapure standards. Extremely low limits of detection (down to ppt levels) can be achieved using IC by pre-concentrating the ions to be measured on a short ion exchange column and then eluting into the eluent stream for separation and analysis. In this case, extremely pure water is essential, requiring a water purification system to deliver 18.2 M?-cm resistivity and low TOC. The requirements for water purity are more stringent for ultratrace IC, with ng/L levels of ions and low organics

Many water purification systems are required to conform to industry standards set by Pharmacopoeia, ASTM, ISO and CLSI.

Conclusion

For IC applications, especially at trace levels, contaminants of all types (organic, particulate, bacterial, and gaseous as well as ionic) can seriously affect the quality of the data produced. A Type I ultrapure water system is required that can guarantee the ionic purity of the water and ensure consistent organic purity. Water suitable for such applications can only be produced by a combination of technologies removing all contaminants. When choosing a laboratory water purification system for analytical applications such as IC, it is essential to consider systems that combine such technologies and incorporate real-time monitoring of water purity in order to have confidence in the water and confidence in the experimental results.

References

1) Ion Chromatography: an Overview and Recent Developments”, Swartz M. Chromatography Online, 1st July 2010
2) Dionex (UK) private communication.
3) Clinical and Laboratory Standards Institute. Preparation and Testing of Reagent Water in the Clinical laboratory; Approved Guideline – 4th Edition. CLSI document C3-A4 (2006)
4) “Ultrapure Water for Ion Chromatography” Whitehead P. Journal Of Chromatography A 770(1997) 115-118

Source: ELGA LabWater