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March2003 • LLISTLine Bulletin43 "Under normal environmental The authors state that their TBE at pH 1.5 at temperatures results show that"acid hydrolysis of ranging from 20 to 80°C.Inspecting conditions ethers do not undergo MTBE,in properly handled ground- Figure 1 at the typical purge time of hydrolysis at significant rates water samples,does not compromise 11 minutes, reportable TBA (> 5 without enzyme cafahvsis;even in the integrity of dissolved MTBE and pg/L) can occur at temperatures acidified(pH<2)groundwater TBA analyses."This is very good evi- above 65°C.In practice,samples with samples, ethers are generally dence that hydrolysis should not be a concentrations above 500 to 1,000 stable(Church, et al., 1999)." "Therefore, if water samples are Solution of MTBE at pH 1.5 Production H m a t t preserved with acid, there i's an understandable concern as to 30 01 whether or not any of these data 80°C are valid. . . . If[acidified] 25 groundwater samples are _j 71 °C refrigerated before analysis and-- m20 all the sample preparative o methods are carried out at a; 15 ambient temperature(as 010 opposed to an elevated c temperature of 800C), there is m 5 minimal opportunity for hydrolysis 45°C of the ether oxygenates." 6 �/� LUSTLine#42 0 5 10 15 26 25 30 "Analytical Metads for Fuel Oxygenates" Heating Time (min) has been well documented (O'Reilly et al., 2001). At very low pH (e.g., problem when Figure 2 Changes in Hydrolysis Rate below 1.5)and at high temperatures samples are han- Constant (e.g., 80°C) hydrolysis rates will be dled and ana- relatively fast, but those conditions lyzed according 12 are unusual except perhaps during to commonly the heated P&T or heated headspace used standard 10 extractions. protocols. ? 8- -As shown in LL#42, at 4°C, Using the c MTBE hydrolysis is negligible under method outlined v 6 almost any condition. Even at ambi- in O'Reilly et al., g? 4 ent temperature there is insignificant it is useful to cal- hydrolysis over the standard holding culate some pos- 2 times of VOC samples(14 days)if the sible effects of 0 1 1 Ili j sample pH is 2. (Of course, samples hydrolysis under 0 20 40 60 60 166 should never be stored this way!) a variety of typi- For example,in a recent paper by cal analytical Temperature, degrees C Douthit et al. (2002), the calculated conditions. pseudo-first-order-rate constant for Using an upper MTBE hydrolysis for a solution con- range of heated purge conditions pg/L are diluted prior to analysis. taining 900,000 ppb of MTBE at 26°C (450C), a sample containing 1,000 This practice both dilutes the acid, and pH 2 is 0.0022 per day—that is, pg/L of MTBE theoretically will yield raising the pH, and reduces the 02 percent of the MTBE will be trans- 0.7 pg/L of TBA at pH 1.Under the MTBE concentration to a point where formed into TBA each day. At same conditions,a sample containing any resulting TBA concentration is 100,000 pg/L MTBE and the same 10,000 pg/L MTBE will yield 7 pg/L below reporting limits. temperature and pH, the calculated of TBA, and a sample containing 100 To see that temperature effects rate constant is 50 percent slower pg/L of MTBE will yield 0.1 pg/L of are of greatest concern above 40°C, (0.0011/day). That same study TBA.So even when pH is extreme,if look at Figure 2. As temperature showed no measurable hydrolysis of moderate heating is used,hydrolysis increases, the rate constant for MTBE to TBA above detection limits should be insignificant. hydrolysis increases by nearly an as low as 5 jig/L in samples stored at To illustrate this point more rig- order of magnitude between 45°C 4°C for 7 to 31 days and analyzed by orously, Figure 1 shows the calcu- and 80°C. Keeping purge tempera- P&T/GC-MS. Unpreserved control lated amount of TBA that can tures below this range should greatly samples were also analyzed.No TBA theoretically result from the hydroly- reduce potential rates of hydrolysis. was detected at 5 to 20 ppb. sis of a sample containing 500 gg/L ■continued on page 20 19