Improving the sensitivity for ionic solutes analysis


Credit: Dr. Shin-Ichi Ohira

Researchers from Kumamoto University in Japan have developed a new method to improve the sensitivity of analytical systems for various ionic solutes, such as water from rivers, lakes, or even the faucet. Certainly, clean running water is important for community health, and impurity analysis can easily be performed by highly sensitive equipment, such as a mass spectrometer. However, these devices can be prohibitively expensive for low income areas or if only a few samples are to be analyzed. In these cases, an enrichment process is used to improve the sensitivity of more easily accessible analytical systems.

The limits of detection of trace ionic solutes, such as those left over after purification, in drinking water should be at least a few micrograms per liter and even less for ultrapure water used in various industrial processes. There are several different enrichment methods that can be used to increase ionic solute levels in a sample, such as heating with/without a vacuum, nitrogen-flow evaporation, co/precipitation for heavy metal ions, liquid-liquid extraction, solid phase extraction, or electrodialysis, to improve analysis sensitivity.

Researchers used the electrodialytic ion transfer enrichment method since quantitative ion transfer can be achieved and the transferred ions become enriched if the sample solution flow rate is higher than the acceptor solution, i.e., the solution traveling to the analyzer, flow rate. They found that if the flow rate for the sample solution (Fs) was higher than that of the acceptor solution (Fa), the enrichment effect became equivalent to the ratio of the two flow rates (when Fa is not 0), and ion enrichment could be performed in just a few seconds.

Upon testing this enrichment method, researchers found that ion chromatography system detection limits were improved by a factor of ~10 for inorganic cations and ~50 for heavy metal ions, which follows the flow rate ratios of Fs/Fa = 10 and 50 respectively. Similar detection limit results were received in a flow injection analysis (FIA) system. With the present method, the sensitivity of any analytical system can be improved.

Finally, the byproducts of drinking water chlorination, chloroacetic acid (MCAA), dichloroacetic acid (DCAA), and trichloroacetic acid (TCAA) were assessed in Japanese tap water with a HPLC-UV detector using the researcher's electrodialytic enrichment process. The analysis required a relatively higher voltage than that for strong acids to obtain a flow rate ratio of 10, and enrichment factors for the tap water ions averaged an acceptable 9.8.

"Our in-line, real-time, enrichment of ionic solutes improves the sensitivity of testing systems," said Professor Shin-Ichi Ohira, leader of the research project. "What's more, it can be done in just a few seconds. We envision this pretreatment to be implemented in future, fully automated systems."


This research was published online in the Journal Talanta on 18 Dec. 2017.


Ohira, Shin-Ichi, Takayuki Yamasaki, Takumi Koda, Yuko Kodama, and Kei Toda. "Electrodialytic in-Line Preconcentration for Ionic Solute Analysis." Talanta 180 (April 2018): 176-181. doi:10.1016/j.talanta.2017.12.054.

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