Sniffing out the causes of odorous drinking water

Nov. 6, 2003
While woody or musty aromas often lead consumers to believe their water is unsafe, the problem is usually caused by harmless compounds.

Nov. 5, 2003 -- Rivers, lakes and reservoirs usually reach their warmest temperatures in late summer and early fall.

As the water gets warmer, the resident algae and microbes grow more rapidly -- and consumers start calling their local water companies with concerns about unpleasant flavors and odors.

While woody or musty aromas often lead consumers to believe their water is unsafe, the problem is usually caused by harmless compounds.

For years, it was commonly accepted that unpleasant aromas or flavors in drinking water were associated with three compounds: geosmin, MIB (2-methylisoborneol) and haloanisoles.

However, confirming their presence in water has been a significant analytical challenge because these compounds are odorous at very low concentrations.

Detecting the culprit compounds

The human nose is an especially sensitive detector, alerting us to the presence of these compounds at remarkably low thresholds:

* Geosmin's earthy odor is noticeable at 1 to 10 ng/L
* MIB's woody or camphor-like odor is noticeable at 5 to 10 ng/L
* Haloanisoles' musty odor is noticeable at 0.5 to 4 ng/L

Two analytical methods are currently used to detect these compounds, but both have drawbacks. Closed-loop stripping analysis (CLSA) is used most often but the process is slow and cumbersome, involving heating, transfer to charcoal, elution from charcoal and analysis via gas chromatography (GC).

The other common method, "purge and trap," is based on the same principles as CLSA but lacks sufficient sensitivity to detect the odor-causing compounds at levels below 100 ng/L. Additionally, neither stripping method is efficient enough to quantify compounds that are less volatile or more polar.

Surpassing conventional methods

Researchers at Anjou Recherche in Saint-Maurice, France, have developed a novel extraction technique that is simple and fast--and offers an alternative to conventional stripping methods. This method is described in detail in the Agilent application note, "Stir Bar Sorptive Extraction: A New Way to Extract Off-Flavor Compounds in the Aquatic Environment."

The stir bar sorptive extraction (SBSE) technique uses Agilent GC and mass spectrometry (MS) equipment along with an olfactometric detector, thermal desorption unit and PTV inlet from Gerstel GmbH & Co. (an Agilent partner).

From a chromatography perspective, GC linked with MS is the only method that provides sufficient separation, identification and quantification capability to detect the offending compounds at the required levels.

SBSE uses a magnetic stirring bar incorporated into a glass jacket coated with a 0.5-mm layer of polydimethylsiloxane (PDMS). Extraction is performed by placing a suitable sample amount in a vial, adding a stir bar, and stirring for 30 to 120 minutes.

After extraction, the stir bar is introduced into a glass desorption tube and placed in a thermal desorption unit where it is desorbed at 200 to 300°C. GC/MS is then used to detect the compounds. This technique makes it possible to extract and analyze more than 20 samples per day.

Solving real cases

The Anjou Recherche team has used this method to identify off-flavor problems in drinking water systems near Paris, and the application note describes three such cases. All involved the analysis of water samples related to complaints about flavor and odor problems. In one case, two different samples were collected at the home of a consumer. One sample gave a very strong musty odor and the other gave off soft musty and strong metallic odors.

Using SBSE, the samples were analyzed for geosmin, MIB and the haloanisoles. The concentration levels found in both samples explained the musty odor, caused by geosmin in the first sample and a haloanisole in the other. Another pass of SBSE on the second sample revealed the presence of tetrachlorobenzene and dibromoiodomethane (a chlorination byproduct).

Cases like this will likely occur every year when the weather is warm, causing concerned consumers to call their local water companies. Sniffing out the cause quickly and accurately can do more than help solve the problem: it can also provide reassurance to consumers that their water is safe.