Analysis tool reaches a new peak in quest for safe water

Providing universal clean drinking water is one of the United Nations’ sustainable development goals, with 844 million people still lacking access to safe drinking water, as of 2015. Among the most common contaminants of water are the ‘humic’ substances produced during the natural decomposition of animals and plants. These same humic substances can cause problems in the treatment of water. Chlorine is widely used to kill disease-causing bacteria and protozoans in drinking water supplies, but it reacts with humic compounds to form the carcinogen, trihalomethane.

“In some regions with high concentrations of humic substances it is necessary to carry out a detoxification step in which these substances are removed before the water is treated,” says Hiroshi Yamamura, at the Faculty of Science and Engineering, Chuo University, Tokyo.

The quantity and composition of humic substances in water can be monitored using high- performance size exclusion chromatography (HPSEC) coupled to a detector, allowing detoxification to be precisely controlled. HPSEC, also known as molecular sieve chromatography, separates molecules according to size. Typically, the column is attached to either an ultraviolet (UV) absorption or fluorescence detector. But neither is able to detect all types of organic matter, including polysaccharides.

Since 2000, an improved detection technology, organic carbon detection (OCD), has been under development for this purpose. The OCD instruments inject a known quantity of an oxidizing agent into water samples and then irradiate them using UV light. This oxidizes and decomposes the humic substances in the water, producing carbon dioxide. “The level of organic matter is determined by quantifying the amount of carbon dioxide produced,” Yamamura explains.

HPSEC-OCD in action

The Sievers M9e OCD analyzer is compatible with Hitachi’s HPSEC systems including the high-performance liquid chromatograph Chromaster. Yamamura’s research group has demonstrated the ability of HPSEC-OCD for separating and quantifying a wide range of humic substances in the lab. They observed no overlapping peaks, enabling peak areas to be measured for the quantitative determination of compound concentrations.

HPSEC-OCD has also been tested in the field. Yamamura has the same set-up to analyze water samples taken from the Chitose River, which flows through the city of Ebetsu in Japan's Hokkaido region. The results were compared to those achieved using a UV detector. Three prominent peaks were detected using the OCD analyzer, the first of which was not detected using UV. “This example demonstrates that HPSEC-OCD allows the detection and quantitative measurement of macromolecule and polysaccharide components that cannot be detected by other detectors,” says Yamamura.

The future

HPSEC-OCD is currently only being used for research purposes, but Yamamura believes it is ready to be rolled out for routine monitoring. “In particular, it is a very useful tool for drinking water and wastewater treatment because it provides quantitative and qualitative results on which organic matter can be reduced in each treatment process meaning the water treatment process can be optimized,” he says.

While OCD detection is ready to hit the mainstream, a next-generation water analysis technique is being explored: HPSEC-excitation emission matrix (EEM) fluorescence spectroscopy.

“EEM detects the fluorescence emitted by excited organic matter when a sample is irradiated with excitation light,” Yamamura says. The output is a 3D contour plot. EEM is already used for water analysis together with a multi-component analysis algorithm called Parafac (parallel factor) analysis, which separates out overlapping peaks. However, the large number of samples needed for each Parafac analysis has limited the utility of EEM for routine use.

Yamamura’s group has recently demonstrated that an HPSEC separation before the EEM scan slashes the number of samples needed for the Parafac analysis. “We have demonstrated the possibility of greatly reducing the number of samples required for Parafac,” Yamamura says. “Currently, we are developing an instrument that connects an HPSEC system to a fluorescence spectrophotometer equipped with a flow-cell accessory.”