Page 8: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Trihalomethanes
6.0 Analytical methods
The THMs can be determined by a number of different analytical techniques. The U.S. Environmental Protection Agency (EPA) has approved three methods (EPA Method 502.2; EPA Method 524.2 and EPA Method 551.1) for the analysis of THMs in drinking water. Method 502.2 uses purge and trap capillary column gas chromatography with photoionization and electrolytic conductivity detectors in series (P&T/GC-ECD); Method 524.2 determines THMs using capillary column gas chromatography/mass spectrometry (GC-MS) and Method 551.1 uses liquid-liquid extraction and gas chromatography with electron-capture detection (LLE/ECD) (U.S. EPA, 2005). Method detection limits (MDLs) for these methods are not listed.
Health Canada uses a purge and trap (P&T), liquid-liquid extraction (LLE), and direct aqueous injection in combination with a chromatographic system to analyze THMs. The chromatographic system will permit concurrent determination of all four THMs. The MDL by the P&T and LLE methods is approximately 0.1-0.2 µg/L (Health Canada, 1995).
Some of the techniques are known to give different values; for example, chloroform levels in water analysed by direct aqueous injection are usually higher than levels determined by the purge-and-trap technique. The variation is attributed to the formation of chloroform from the breakdown of CDBP precursors in the hot injection port of the gas chromatograph used in the direct aqueous injection technique.
Health Canada studies on DBPs in drinking water used the LLE method from the EPA (Method 551.1), and adapted it to incorporate analysis by gas chromatography-electron capture detector (GC-ECD). Samples were also determined using the P&T technique followed by gas chromatography-mass spectrometry (ion trap) detector (GC-ITD).
The LLE approach also allows for the concurrent determination of other DBPs including chloral hydrate, di- and trichloropropanones, haloacetonitriles, and chloropicrin, which are not explicitly covered in this guideline document. The method was later modified to include the concurrent determination of cyanogen chloride (LeBel and Williams, 1996, 1997; LeBel and Benoit, 2000) and other halogenated acetaldehydes (Koudjonou and LeBel, 2003). An essential requirement of the method was the pH adjustment (pH 4.5) of the water samples at the time of field sampling to prevent further production of chloroform during storage of the sample between collection and analysis; the effect due to pH diminished with time (distance) in the distribution system (LeBel and Williams, 1995).
In order to ensure that the sample is representative of THM exposure, all reactions should be stopped at the time of sample collection. This is achieved by the addition of a preservative to quench the chlorine and by pH adjustment in the field to prevent the transformation of intermediate products. Data from recent Health Canada studies indicate that 1,1,1-trichloro-2-propanone (LeBel et al., 2002) and trihalogenated aldehydes (Koudjonou and LeBel, 2003) will degrade in water to their corresponding THMs at increased pH and temperature. However, they are stable in water at sampling/storage conditions (pH 4.5, 4°C).
Both the P&T/GC-ITD and LLE/GC-ECD techniques can be used for the determination of THMs in drinking water samples. For similarly treated samples (same pH and preservative), the results using both techniques are comparable, but the P&T technique gives slightly higher values of chloroform due to breakdown of some chlorinated intermediates (LeBel and Williams, 1995). As well, the P&T technique is not generally amenable to the analysis of the more hydrophilic DBP analytes targeted by the LLE approach. Therefore, the LLE approach is preferred for its versatility and reliability.
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