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Lus?Line 6nlletin 43 • hrardh 2003 - -— ■MTBE Acid Hydrolysis ation study(U.S.EPA, 2002)showed from page 17 good recoveries(>85%)of MTBE and What's Purge other ethers at ambient temperatures. and Trap? analyte for UST sites—this despite That same study did show poorer numerous recommendations by a TBA recoveries at ambient tempera- Think of the purge and trap tech wide variety of authorities over the ture and also showed slightly better pique as a very small-scale air- last five years. (See page 21 for a list recoveries of ethers using the 80°C sparging application in a closed of reviews andg uidance.)Analyzing heated purge. It did not evaluate system in which you are captur- tem Eor all alkyl ethers (e.g., MTBE, purge g recoveries at temperatures ing the volatilized gases in acar- TAME,DIPE,ETBE)and alcohols has between ambient and 80°C. bon canister. In practice, a small been widely recommended ever However, P&T can be an effec- volume of water(usually 5 ml, since the first comprehensive evalua- tive TBA extraction method even at but sometimes larger volumes tion of oxygenate impacts to water ambient temperatures as shown by a Yg P are used to improve method per- quality was published in the National Lawrence Livermore National Labo- formance) is purged with a gas Science and Technology Council's ratory study (Halden et al., 2001). helium to remove the 1997 report Interagency Assessment of These same authors provided similar (usually ) Oxygenated Fuels.On an annual basis information several years earlier in a soluble VOCS from that sample. this recommendation has been reiter- report for California EPA(Happel et The sample may be heated (e.g., ated by a series of other EPA reports al., 1998).Their results showed that in the 20°to 45°C range)to or pronouncements. gasoline ether oxygenates (i.e., enhance the removal process, So, obviously, every site should MTBE,TAME,ETBE, and DIPE)and and the purging time may vary, be assessed for the potential presence TBA could be effectively measured although 11 minutes is very of all oxygenates at some point, using P&T/GC-MS at ambient tem- common.The volatilized chemi- preferably early on, as LL#42 sug- peratures. They observed that a cals are "trapped" on a sorbent gests. Does this mean you have to method detection limit of 35 µg/L column that is then heated analyze every site, for every oxy- could be obtained for TBA but noted genate,every time using GC-MS?Of that at 40°C, and using a larger sam- slowly to facilitate the sequential course not, and LL#42 makes that ple volume(10 ml versus 5 ml),they separation of chemicals with same point,indicating that there will were able to reach a detection limit of varying boiling points in the gas be situations where simple 4.6 µg/L. So if a heated purge is chromatograph (GC).The con- approaches like Method 8015 will desirable, a temperature of 45°C stituents are then identified in work just fine. should probably be high enough. It the mass spectrometer(MS). Certainly it makes sense to evalu- should provide acceptable recoveries, ate every site for all oxygenates at and minimize some of the potential some time to make sure you know problems noted later in this article. desirable,but may not always be nec- what's there.Your data needs may be In a very recent paper discussed essary to document good method different for any given sampling in more detail later in this article, performance if recoveries are consis- event during the life cycle of any site, Evans and Colsman (2003) demon- tent among all samples. and a careful decision on the appro- strate another example of effective Finally, there is some anecdotal priate analytical method to use MTBE and TBA analysis using ambi- evidence from lab technicians that should made for each round of data ent temperature P&T. In that study, when using an 80°C P&T extraction, collection. three California labs using Methods complicating issues with water man- 5030/8260 were able to get good agement may result in worse recov- Do You Need to Heat MTBE and TBA recoveries,as well as ery of TBA than would be the case at Samples to 80°C to accurate results at low detection lev- 45°C or lower.The potential problem Effectively Extract TBA and els (-12 yg/L). While their results here is that at this high temperature, Ethers from Water Samples don't mean that every lab performs some water may also be evaporated, as well, they do document that those ending up in the instrument's water During P&T? methods are capable of delivering trap.Some TBA may be in that water LL#42 states that"if purge and trap satisfactory performance. and not find its way into the GC, is used...it must be modified to Purging effectiveness is clearly leading to poor recovery.So perhaps increase method sensitivity. One more important for TBA than for the 45°C is a good compromise tempera- straightforward approach is to heat ethers, but keep in mind that if ture when TBA is a target analyte. the sample to 80°C." While it is true proper calibration procedures are that P&T techniques are most effec- performed, good results can be Is Hydrolysis of MTBE to TBA tive for nonpolar chemicals like obtained,even with lower recoveries. a Concern under Normal BTEX,there is also good documenta- The relative recovery of TBA from a Conditions? tion that more polar compounds, lab's calibration sample should be such as MTBE and other ethers, can similar to that from field samples, The rate of hydrolysis of MTBE to be purged successfully at either and so a correction factor can be TBA is affected primarily by pH, ambient room temperature (-25'C) applied to determine the "correct" temperature, concentration, and or slightly heated(e.g.,<45°C). concentration.Higher recoveries(i.e., time.The impact of pH and tempera- EPA's oxygenates method evalu- 75 percent and more) are certainly ture on the rate of MTBE hydrolysis 18