Since both H202 and organic peroxides carry out this reaction, this method measures total peroxides. However, the contribution of H202 can be separated by adding catalase, which decomposes H202 but not the organic peroxides. The instruments therefore usually have two channels, one in which catalase is added to give the organic peroxides, and one in which it is not, giving total peroxides. The difference between the two channels gives H202. Another approach involves using the different solubilities of H202 and organic peroxides (Staffelbach et al., 1996).

The organic peroxides can be separated by HPLC prior to detection (Kok et al., 1995). Sauer et al. (1996, 1997), for example, used HPLC with a POPHA detector to measure peroxides in air and in rainwater in Germany and at a marine coastal site in France. Although no organic peroxides were found in air, several were identified in some rainwater samples, including hydroxymethyl hydroperoxide (HOCH2OOH) and 1-hydroxyethyl hydroperoxide (CH3CH(OH)OOH). Hydroxymethyl hydroperoxide is expected to be formed in the atmosphere from the reaction of the one-carbon Criegee biradical (-CH200-) with water, i.e., from the reaction of ethene and terminal alkenes with 03 in air (see Chapter 6.E.2) and perhaps from the reaction of H0CH200- radical with HOz (see Chapter 6.E.2).

Fels and Junkermann (1994) reported both hydroxymethyl hydroperoxide and CH3OOH in air in a rural area in Germany, with these two compounds comprising more than 90% of the total organic hydroperoxides. Ethyl hydroperoxide and peroxyacetic acid were also detected in some samples. Methyl hydroperoxide is expected in low-NOx environments from the reaction of CH302 with H02 (see Chapter 6). The organic hydroperoxides were about 10-40% of the H202 concentrations measured simultaneously. This is similar to the observations of Tremmel et al. (1994), who found that organic hydroperoxides in air over the northeastern United States were typically about half that of

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