Avoiding pitfalls in the determination of halocarboxylic acids: the photochemistry of methylation

Haloethanoic (haloacetic) acids are formed during chlorination of drinking water and are regulated by the Environmental Protection Agency (EPA). These compounds are normally quantified by gas chromatography with electron capt ure detection (GC-ECD) as the methyl esters. EPA Method 552 uses diazometha ne (CH2N2) for this purpose, but has only been validated by EPA for HAA6: c hloro-, dichloro-, bromo-, dibromo-, bromochloro- and trichloroacetic acids . EPA Method 552.2 was developed and validated for all nine analytes (HAA9 = HAA6 + dibromochloro-, bromodichloro- and tribromoethanoic acids). Since the promulgation of Method 552.2, which uses acidic methanol, a debate has ensued over discrepancies observed by various laboratories when using diazo methane instead. In an effort to identify and eliminate potential sources f or these discrepancies, a comparative study was undertaken for HAA9. Better accuracy and precision were observed for all HAA9 species by Method 552.2; recoveries were satisfactory in de-ionized and tap water. Method 552 remai ns satisfactory for HAA6. Systematic differences in instrumental response a re observed for the two methods, but these are precise and may be accounted for using similarly treated standards and analyte-fortified (spiked) sampl es. That notwithstanding, Method 552 (CH2N2) was shown to be unsuitable for dibromochloro-, bromodichloro- and tribromoethanoic acids (HAA9-6). The pr imary problem appears to be a photoactivated reaction between diazomethane and the HAA9-6 analytes; however, side reactions were found to occur even i n the dark. Analyte loss is most pronounced under typical laboratory lighti ng (white F40 fluorescent lamps + sunlight), but it is also observed under Philips gold F40 lamps (lambda greater than or equal to 520 nm), and in the dark.