Article courtesy of Jon Evans | February 9, 2015 | Separations Now | Shared as educational material
The routine treatment and disinfection of water supplies may have led to great falls in water-borne diseases and associated increases in life expectancy, but it is not without its risks. Foremost amongst these are that commonly-used disinfectants such as chlorine and ozone can react with organic and inorganic compounds in the water to form disinfection by-products (DBPs), some of which are toxic.
Scientists have identified over 600 DBPs, but the most important in terms of their prevalence and toxicity are the haloacetic acids (HAAs), and bromate and chlorite, collectively known as oxyhalides. While these DBPs are nowhere near as dangerous as the pathogenic microbes that used to contaminate drinking water, their concentrations are still regulated by organizations such as the US Environmental Protection Agency.
This requires methods for measuring the concentration of DBPs in drinking water, but while several methods are available, many of which are based on gas chromatography, they tend either to be complicated and time-consuming or not particularly sensitive. Ion chromatography offers a simple and sensitive analytical alternative, but is hampered by the fact that drinking water contains a whole range of ionic species, which can interfere with the detection of DBPs.
One potential solution is to utilize two dimensional ion chromatography (IC), separating the DBPs from the rest of the ions in the first column and then detecting the DBPs in the second column. Up to now, however, scientists have only done this for individual DBPs, whereas it would be handy to be able to do it for all the important DBPs, comprising the five main HAAs, bromate and chlorite. This is what Hui Teh and Sam Li from the National University of Singapore have now done, by pairing a conventional IC column with a capillary IC column.
They used the conventional IC column to separate the DBPs from the rest of the ions present in drinking water, testing the process using a specially prepared mixture of DBPs with ions such as nitrate, sulphate and phosphate. This revealed that the DBPs could be separated from the other ions but rather than eluting in a big bunch they were distributed between these other ions. This meant that Teh and Li couldn’t just collect a single fraction containing all the DBPs, but had to collect multiple fractions at specific times.
To do this, they used a six-port valve to collect the eluent during four different time windows, with each window lasting between one and two minutes, allowing them to collect just the DBPs without any contamination from the other ions. All these fractions were deposited in a trap column, before being injected into the capillary IC column, which Teh and Li thought would be able to separate the DBPs faster and more effectively than a conventional column. Finally, they detected the DBPs using suppressed conductivity detection.
When they tried the whole process on a sample comprising just 100μL of tap water spiked with the DBPs, they found it worked just as they hoped, with the first column separating the DBPs from the other ions and the second column separating the seven DBPs from each other. However, in order to separate all seven DBPs they had to employ both gradient elution and temperature programming to reduce the column temperature from 35°C to 20°C after the first five DBPs had eluted.
This approach allowed them to detect the DBPs at concentrations between 0.38μg/L and 0.72μg/L, making it more sensitive than any previous method. They obtained similar results with various other types of water, including bottled water, reagent water and surface water.
Drinking water in countries that conduct routine disinfection is already much safer than it used to be, but this new IC method could help make it even safer.