these methods. ^ When using both techniques, a more complete picture of type and overall quantity of organic acids formed during ozonation may be obtained.

As a prerequisite for GC, organic acids have to be concentrated and derivatized. Extraction is usually accomplished by liquid/liquid extraction or SPE. In all drinking water methods listed in Table 13.3, the extraction step was preceded by aqueous derivatization of a ketofunction, thus


limiting these GC methods to ketoacids.39'40'88'89 Simple, aliphatic mono- and dicarboxylic acids cannot be determined with these methods. However, approaches taken for GC analysis of organic acids in rain could be equally as well applied to drinking water, thus covering additional mono- and dicarboxylic acid.34'36'74-76'78-80

AEC methods have also been used for organic acid analysis in drinking water although inorganic anions such as chloride, sulfate, and carbonate are usually present in much higher concentrations (low to medium mg/l) than organic acids (low /Ag/l range). This can lead to masking or incomplete separation of organic acid peaks and therefore to identification and quantification problems. Hence, it is surprising that IEC, where inorganic anions elute up-front with the system peak, has not been applied to organic acid analysis in drinking water. This may be due to problems anticipated when using IEC, for example interferences from weak inorganic acids such as carbonate, and separation problems between the early eluting system peak and organic acids with low pK& values such as oxalate or pyruvate.29,155

AEC has been predominantly employed for formic, acetic, and oxalic acid,42-45,86,87 although


other acids have also been measured. , , Slightly different approaches have been taken, usually keeping sample pretreatment to a minimum. Direct, large volume injections have been combined with the separation on a high capacity column. In this case, sample pretreatment, i.e., filtration, is


only necessary if samples are turbid. , , However, high sulfate concentrations may interfere with oxalate quantification and therefore this method was further developed utilizing a "heart-cut" technique.43,87 In another direct injection method, removal of mercuric cations, which were added to the sample for preservation purposes, was necessary to avoid column poisoning. This is accomplished in a time-saving manner by using a cation exchanger in-line between the autosampler and the injection loop.42,44 A third approach requires the removal of chloride, sulfate, and phosphate prior to injection, which is achieved by pushing the samples through cartridges filled with silver or barium salts, a technique which is often used in bromate analysis.184-186 All of these relatively fast methods achieve detection limits at very low /Ag/l concentrations, thus being sensitive enough to detect organic acids in drinking water in the presence of inorganic anions.

0 0

Post a comment