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LUSTLine Bulletin 42 • October 2002 ■Analytical Methods from page 5 t the MTBE would be hydrolyzed in the first 30 days of storage.If samples e.9 were acidified to pH=1 and stored at 20° C, as much as 20 percent of the s.a MTBE could be hydrolyzed in 30 days. If groundwater samples are1 0.7 - refrigerated before analysis and all 7 E 11 7 the sample preparation methods are o.a carried out at ambient temperature M os (as opposed to an elevated tempera- a — px=z al a°c ture of 80° C), there is minimal 0.4 _ pH=2 at 100c opportunity for hydrolysis of the rpH=lat{°c ether oxygenates. s 0.3 —O- pH=2 at 20-C Preventing Ether Hydrolysis 0.2 Px-1 at to°C Through Improved Sample- Preservatfon Technique U.1 -o- PH-ta120°C There are two widely used methods o of preservation: refrigeration and 0 30 60 90 120 150 chemical preservation(usually acidi- Gays of incubation fication). Often both methods are used on the same samples. If acid FIGURE 1.Predicted effect of pH and temperature on the stability of MTBE in samples of causes a problem with analysis of groundwater. MTBE and TBA, one might be tempted to not use acid and rely on warm, if they weren't preserved, if the sample,it is effective at inhibiting refrigeration alone. they were analyzed past their hold- the chemical deterioration of the It is essential, however, to use ing time,or if acid-preserved samples sample. both a chemical preservative and were analyzed using a heated Kovacs and Kampbell (1999) refrigeration for groundwater sam- preparatory method, then there is a developed an alternative procedure ples, especially if they are to be chance that some of the MTBE was for chemically preserving groundwa- analyzed for BTEX compounds. hydrolyzed to TBA. If hydrolysis is a ter samples that avoids hydrolysis of Groundwater samples from perma- possibility, then examine the quality ether oxygenates.Instead of using an nent wells typically contain micro- assurance/quality control data pro- acid to lower the pH, samples are organisms that are capable of vided with the analytical report. If preserved with a base to a pH greater degrading BTEX relatively quickly the recovery of MTBE (or other ether than 11.The elevated pH effectively when oxygen is available. Contami- oxygenate) from spiked samples is prevents the biodegradation of nants may persist in groundwater near 100 percent,then hydrolysis of organic compounds in the sample. because the plume is devoid of dis- MTBE during analysis was minimal The ethers are not subject to base-cat- solved oxygen, but groundwater and should not be of concern. alyzed hydrolysis,and a basic pH has samples from wells invariably con- We must reiterate that both a no adverse effect on BTEX or the tain dissolved oxygen,particularly if chemical preservative and refrigera- alcohol oxygenates (O'Reilly et al. samples were collected with a bailer. tion should be used to preserve sam- 2001).The pH is elevated by adding a In samples that have not been pre- ple integrity. Refrigeration by itself salt of a weak acid (trisodium served, BTEX compounds may be may slow the rate of biological degra- phosphate dodecahydrate, or TSP), completely biodegraded in less than dation,but not to a useful extent.A instead of a solution of a strong base two weeks (Wilson et al. 1994) and conventional refrigerator is often such as potassium hydroxide.Table 1 MTBE and TBA may be completely near 10° C and refrigerated storage compares MTBE hydrolysis in sam- degraded within two weeks of stor- for samples is usually near V C. ples that were preserved with acid to age(Kane et al.2001). The temperature of groundwater samples preserved with TSP. There As good practice,samples should in the northern half of the United was no evidence of MTBE hydrolysis be packed in ice for shipment and States ranges from 10°C to 150 C.As to TBA in the samples that were pre- refrigerated during storage. The tem- a consequence, the microorganisms served with TSP. perature and general condition of the collected along with a groundwater The Kovacs and Kampbell(1999) samples upon receipt by the labora- sample are already adapted to cold procedure is safe and convenient.In tory should be indicated on the conditions.Storage of samples with- the laboratory,between 0.40 and 0.44 chain-of-custody. Samples should be out a chemical preservative at 10°C gram of TSP is added to each 40 mL cold (preferably close to 4° C upon to 4°C will only slow the rate of bio- sample vial.Because it is more conve- arrival at the lab),they should be pre- logical degradation of BTEX by a fac- nient to measure the required served,and they should be analyzed for of two to four at most.Although amount of TSP on a volume basis within prescribed holding times. refrigeration is only minimally effec- rather than by weight,staff of the R.S. If samples arrived at the lab tive in retarding biodegradation of Kerr Center use a precalibrated 6