Enhancement of Perchlorate Analysis using Solid Phase Extraction (SPE) Cartridges Victor F. Medina*, Steven L. Larson, Barbara Extine United States Army Corp of Engineers Engineer Research and Development Center Vickburg MS. * victor.f.medina@erdc.usace.army.mil, (601) 634 4283 Introduction Perchlorate is a contaminant of increasing concern at U.S. Army facilities - primarily from weapons production and firing of solid fuel rockets. Perchlorate has a high solubility (about 200 g/l) and is resistant to degradation (Espenson 2000). The Engineer Research & Development Center (ERDC) is conducting studies on biodegradation of perchlorate. Although methods using technologies such as electron spray mass spectroscopy have been presented in the literature (Magnuson et al. 2000), analysis of perchlorate is typically performed using ion chromatography. Perchlorate treatment levels are in the low ppb range (the CADHS action level is 4 ug/l). Large injection loop injection (with injection loops of 0.5 to 1 ml) has been demonstrated to be effective for perchlorate analysis down to these low concentration levels (Dionex, undated; CADHS 1997; Hautman et al. 1999; Ellington and Evans 2000; Jackson et al 2000; Tsui et al. 2000; Collette et al. 2001). However, this method is not convenient for our needs. To begin with the initial concentrations of our samples are in the range of 20 to 40 mg/l. We would have to change the configuration of the instrument for initial and final concentration measurements, or use extensive dilution. In addition, our instrument is not available for dedicated use for the project. It is extensively used for analysis of chloride, phosphate, nitrate, and other anions, supporting 4 other projects. Development of another, reliable method to reach low concentration levels without requiring a change in our current operating configuration would be of a great value to us. Solid phase extraction (SPE) uses columns of sorbent material in which samples are filtered. The material removes the contaminant of interest by physico-chemical means. The contaminant is then extracted in a smaller volume using an appropriate solution, thereby concentrating the contaminant. SPE is commonly used for the analysis of low levels of pesticides (Manuel 2000) and munitions (Jenkins et al. 1995; Larson et al. 2002). Perchlorate exists as an anion. Anionic SPE filters exist and are commonly used for compounds such as arsenate. The removal process is ion exchange. Ion exchange has been extensively studied as a treatment technology, and these provide key background for SPE application (Batista et al. 2000, 2002; Gingas and Batista 2002; Viera 2000). Generally, these studies indicate that perchlorate is readily exchanged. There are some resins that perform very poorly, but with the right ion exchange materials, it is possible to achieve about 90% removal or higher. However, a challenge in ion exchange treatment of perchlorate is that, for some materials, the sorbed pechlorate is not recoverable by 1
regeneration. Strong base anion exchange resins generally remove high levels of perchlorate, but tend to be resistant to regeneration. Weak base anion exchange resins may not remove perchlorate effectively. However, some resins, both strong and weak base, remove high levels of perchlorate and are allow for high recovery of the perchlorate. These findings indicate that resin selection was likely to be an important parameter SPE filtration. This paper investigates the use of anionic solid phase extraction cartridges to concentrate perchlorate, improving its detection in our existing IC configuration. Preliminary results detailing its effectiveness are presented. Methods SPE Filtration & Extraction Figure 1 summarizes our basic approach. Large volumes of water can be filtered through the SPE cartridge, concentrating perchlorate solution on the solid media. Then, the perchlorate can be recovered using a regenerant. Figure 1. Basic Method for SPE Filtration of Perchlorate Contaminated Water Pretreat Sample as needed (centrifugation, preliminary filtration) Filter the 1-L sample through the NH2 SPE Cartridge. If concerned about breakthrough, take a sample of the last 2 ml passed through the filter. Carefully measure 1-liter of the sample solution Extract & recover sorbed perchlorate with 2 ml of 1% NaOH (12% NaCl used in some experiments) Set up NH2 SPE filter(s) (1.0 ml filter volume) on filtration manifold Analyze extract by Ion Chromatography Pretreat SPE filter: 2 ml methanol followed by 2 ml DI water Calculate solution concentration (mass balance) We chose 1-L as our standard filtration volume because this has been commonly used in other SPE applications (Larson et al. 2002). Filtration of 1-L of relatively clean water can take 100 or more minutes, so although larger volumes could improve detection, it might not be practical from a time sense for large numbers of samples. The filtration velocity for clean water averaged 10 ml/min, and filtration pressure was about 20 psi. The results presented in this paper are for perchlorate solutions in deionized (DI) water. Table 1. Ion Chromatograph Specifications Parameter Name/Setting/Value Instrument Dionex DX 500 Flow Rate 1.5 ml/min Mobile Phase Isocratic: 33.5 mm NaOH Run Time 12 minutes Injection Loop 10 µl Separation Column AS-11 Detector Dionex CD 20 Conductivity Detector Detector Amperage 100 ma Approx. Perchlorate 6.80 minutes Peak Time Filtration times for DI water samples range took 100 to 150 minutes. However, since many samples could be processed simultaneously. Care must be taken to avoid excess spills, pouring samples into the wrong cartridge, identifying leaks, etc. Ion Chromatography Table 1 summarizes key operating parameters for the IC used in this study. Notice that the sample injection loop was 10 ul, which is 100 times smaller than the loop used in USEPA Method 314.0, the large sample injection method for low-level perchlorate analysis. Method Detection Limit (MDL) Method detection limit (MDL) was analyzed and calculated using the approach given in Standard Methods (Greenberg et al. 1992), which is consistent with the approach specified by the Federal Register (1984). This involves collection and analysis of 7 2
replicate samples. The standard Table 2. Filter Material and Regenerant Study deviation is calculated from these samples. The MDL is defined as Filtered Filtered Mass Extracted Extracted Filter Concent. Volume removed Concen. Volume Material Extractant (mg/l) (ml) (mg) (mg/l) (ml) the product of the standard deviation and the t-statistic value, which for 7 samples is equal to 3.14. The Federal Register stipulates that the concentration used for the MDL test must be within a range of 1 to 5 times the calculated MDL. Figure 2. Comparison of Chromatograms of Perchlorate Extracted from an NH2 SPE with 12% NaCl and 1% NaOH. Note Scale Difference. 3,500 PER C H LO R ATE1ppm _N ac l_n H 2_5M M _070203 N ac l_n H 2filter-125:7 ECD 2,000 1,000 1-1.180 2 - perchlorate - 6.640-5 0 0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 1 0.0 1 1.0 1 2.0 12% Sodium Chloride Extraction (perchlorate peak = 146 mg/l) 200 PERC H LO R ATE1PPM _N AO H_N H2_5M M _070203 N ao H _N H 2filter-125:7 ECD 2-1.303 150 100 50 1-1.200 3 - perchlorate - 6.753-2 0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 1 0.0 1 1.0 1 2.0 1% Sodium Hydroxide Extraction (perchlorate peak = 126 mg/l) Results Filter & Regenerant Selection Our study initially focused on selecting a commercially available SPE type, and a regenerant. We tested three commercially available media, NH2, WCX, and PSA. Each media type is available from a variety of suppliers, including Supelco and Varian. 3 ml cartridges were used in these experiments. Each media type proved to be effective at removing perchlorate from solution. Trial and error studies indicated that each filter type could remove about 0.270 mg of perchlorate before breakthrough, this corresponding to about 200 ml of a 1.34 mg/l solution Two regenerants, 1% NaOH and 12% NaCl, were tested. These choices were based on ion exchange research published by Batista et al. (2002). No perchlorate was recovered off the WCX material (Table 2). 84% of the applied perchlorate was recovered from the PSA material using 2% NaCl, but only 9% with NaOH. >90% recovery was achieved from the NH2 columns with both NaCl and NaOH. Therefore, NH2 was chosed as the SPE material for subsequent work. We chose to focus on NaOH as the regenerant because NaCl creates a large Cl - peak (Figure 2). Although this does not adversely effect perchlorate measurement, it is more likely to interfere with perchlorate transformation products than the 1% NaOH solution. Mass Recovered (mg) Percent recoverey NH2 12% NaCl 1.34 200 0.268 146.26 2 0.293 109% NH2 1% NaOH 1.34 200 0.268 126.04 2 0.252 94% WCX 12% NaCl 1.34 200 0.268 0 2 0.000 0% WCX 1% NaOH 1.34 200 0.268 0 2 0.000 0% PSA 12% NaCl 1.34 200 0.268 112.41 2 0.225 84% PSA 1% NaOH 1.34 200 0.268 11.71 2 0.023 9% Figure 3. Comparison of analysis of 0.135 mg/l perchlorate solution, with and without SPE concentration 45.0 PERCHLO RATE_0.1_0.001ppm _NAO H_NH2_070903 #8 Final0.1ppm _1L_7-07-0325:7 ECD 30.0 20.0 10.0 1-1.273 2-1.490-0.5 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 Chromatogram for 0.135 mg/l perchlorate. Perchlorate is below instrument detection level. 45.0 PERCHLO RATE_0.1_0.001ppm_NAO H_NH2_070903 #9 NaO H-1_NH2_1L0.1ppmPerchl25 ECD_ 1-1.287 30.0 20.0 10.0 2-1.487 3 - perchlorate - 6.790-0.5 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 Chromatogram for 0.135 mg/l perchlorate concentrated to 168 mg/l using SPE (1-Liter filtered, extracted by 2 ml NaOH) 3
Comparison of Analysis, With & Without SPE Preconcentration Figure 3 compares two chromatograms. The first was a direct injection of a 0.090 mg/l solution using the configuration given in Table 1. The chromatogram reveals that the concentration was below the instrument detection limit. Below is a chromatogram of the same solution, which was concentrated using an NH2 SPE cartridge. A perchlorate peak was easily identified, as it had a concentration of 168 mg/l. The actual concentration of the solution was calculated by mass balance, in this case it equals 0.084 mg/l. This represents a recovery of about 93% of perchlorate in the sample. Table 4. MDL Analysis on SPE Filtration Method Specified in Methods Section (NH2 Cartridge, 60 ug/l, 1-Liter Volume, Extraction with 2-ml of 1% NaOH) Recovery Versus Table 3 summarizes perchlorate recovery data for various initial solution concentrations. For the concentration range from 1340 ppb down to 60 ppb, perchlorate recoveries were 87% or higher. Based on this trend, we expected recovered concentration of about 15 mg/l for 30 ppb initial perchlorate concentration and about 3 mg/l for 6 ppb. However, the percent recovery declined at these lower levels, which ultimately affected the effectiveness of the SPE method. We do not yet have an explanation for this decline in recovery. Method Detection Limits Table 4 summarizes the calculation of the method detection limit (MDL) for the SPE analysis method, using the IC configuration in Table 1 and the method specified above. 60 ug/l was chosen as the concentration to perform the MDL analysis. The analysis yielded an MDL of about 37 ug/l. This is about 15 times better than the level calculated for direct injection (Table 5). However, this level is about a factor of 10 higher than the CADHS action level of 4 ug/l. Extracted (mg/l) Calc. Soln Conc. (mg/l) Calc. Soln Conc. (µg/l) Recovery (%) Replicate 1 36.97 0.0739 73.94 123.24% Replicate 2 28.06 0.0561 56.12 93.54% Replicate 3 36.43 0.0729 72.86 121.44% Replicate 4 28.46 0.0569 56.92 94.86% Replicate 5 35.79 0.0716 71.58 119.31% Replicate 6 23.59 0.0472 47.18 78.64% Replicate 7 23.54 0.0471 47.08 78.47% Average 30.41 0.0608 60.81 101.36% Std. Dev. 5.93 0.0119 11.87 Rel. Dev. 20% 20% 20% Deviation from Initial Solution (60 µg/l) 0.81 µg/l or 1.35% MDL 0.0373 37.27 Table 3. Perchlorate Recovery Versus Initial Initial (ppb) Recovered (mg/l) Percent Recovery 1* 0 0% 6 0 0% 10 0 0% 12* 0 0% 15 1.35 18% 30* 8.7 58% 60* 30.4 101% 90 42 93% 450** 78 87% 1340** 126 94% * = average of 7 replicates ** = 200 ml filtered instead of 1-L Table 5. Comparison of MDLs for SPE Versus Direct Injection for IC Configuration in Table 1. Configuration Approx. IDL Conc. for MDL analysis 1 2 3 4 5 6 7 Ave. Std. Dev. MDL Direct Injection 500 2,750 2,534 2,657 2,722 2,830 3,075 2,864 2,964 2,806 184.5 579.5 SPE 25 to 30 60 73.94 56.13 72.86 56.92 71.58 47.18 47.08 60.81 11.87 37.27 IDL = Instrument Detection Limit. Defined as lowest concentration in which 90% of analyzed samples will have a detectable signal (Standard Methods1992) 4
Conclusions Overall, the method improved the MDL for our current configuration by a factor of about 15. However, the overall level is still above regulatory levels. We believe that we can lower the MDL, perhaps to about 10 ug/l, by refining our methodology. Studies by Tsui et al (2000) suggest that peaks can be made sharper and more distinct by increasing the NaOH (they actually used KOH as their solvent) concentration of the IC solvent. However, to reach regulatory levels, it may be ultimately necessary to increase the injection loop. Competing anions may be the greatest problem with the method. We plan on conducting studies using solutions prepared with ground-, surface-, and sea-water to quantify changes in the method s effectiveness with these solutions. Eventually, the method may be coupled with large column injection to achieve extremely low levels of detection and quantification. References Batista, J.R.; McGarvey, F.X.; Viera, A.R. 2000. Removal of Perchlorate from Waters using Ion Exchange Resins. In: Urbansky, E.T. ed. 2000. Perchlorate in the Environment. Kluwar Academic/Plenum Publishers. New York, NY. Chapter 13: p. 135-145. Batista, J.R.; Gingas, T.; Viera, A.R. 2002. Combining Ion-Exchange (IX) Technology and Biological Reduction for Perchlorate Removal. Remediation. 13(1):21-38. California Department of Health Services (CADHS). 1997. Determination of Perchlorate by Ion Chromatography. Collette, T.W.; Robage, W.P.; Urbansky, E.T. 2001. Ion Chromatographic Determination of Perchlorate: Analysis of Fertilizers and Related Materials. EPA/600/R-01/026. Dionex. Undated. Analysis of Low s of Perchlorate in Drinking Water and Ground Water by Ion Chromatography. Application Note 12. Ellington, J.J.; Evans, J.J. 2000. Determination of Perchlorate at a Parts-Per-Billion Levels in Plants by Ion Chromatography. Journal of Chromatography A. 898:193-199. Espenson, J.H. 2000. The Problem and Perversity of Perchlorate. In Urbansky, E.T. ed. Perchlorate in the Environment. Kluwar Academic/Plenum Publishers. New York, NY. Chapter 1: p. 1-7. Federal Register. 1984. Appendix B Definition and Procedure for the Determination of the Method Detection Limit. 49(209):198-199. Gingas, T.M.; Batista, J.R. 2002. Biological Reduction of Perchlorate in Ion Exchange Regenerant Solutions Containing High Salinity and Ammonium Levels. Journal of Environmental Monitoring. 4:96-101. 5
Greenberg, A.E.; Clesceri, L.S.; Eaton, A.D. 1992. Standard Methods for the Examination of Water and Wastewater. 18 th Edition. American Public Heath Association. Washingon, D.C. Hautman, D.P.; Munch, D.J.; Eaton, A.D; Haghani, A.W. 1999. Method 314.0. Determination of Perchlorate in Drinking Water Using Ion Chromatography. USEPA National Exposure Research Laboratory. Cincinnati, OH. Jackson, P.E.; Gokhale, S.; Rohrer, J.S. 2000. Recent Development in the Analysis of Perchlorate using Ion Chromatography. In: Urbansky, E.T. ed. 2000. Perchlorate in the Environment. Kluwar Academic/Plenum Publishers. New York, NY. Chapter 5. p. 37-44. Jenkins, T., and L. Escalon. 1995. Evaluation of Clean Solid Phases for Extraction of Nitroaromatics and Nitramines from Water. U.S. Army Corp of Engineers. Cold Regions Research & Engineering Laboratory. CRREL Special Report 95-22. Larson, S.L., Felt, D.R.; Davis, J.L. and Escalon, L. 2002. Analysis of CL-20 in Environmental Matrices: Water and Soil. Journal of Chromatographic Science. 40:201-206. Magnuson, M.L.; Urbansky, E.T.; Kelty, C.A. 2000. Microscale Extraction of Perchlorate in Drinking Water with Low Level Detection by Electrospray-Mass Spectroscopy. Talanta 52:285-291. Manuel, C. 2000. Evaluation of a Passive Sampling Devise (PSD) to Sample Pesticides in Irrigation Water. M.S. Thesis. Washington State University. Pullman, WA. Tsui, D.T.; Clewell, R.A.; Eldridge, E.; Mattie, D.R. Perchlorate Analysis with the AS16 Column. In: Urbansky, E.T. ed. 2000. Perchlorate in the Environment. Kluwar Academic/Plenum Publishers. New York, NY. Chapter 7. p. 59-80. Viera, A.R. 2000. The Removal of Perchlorate from Waters Using Ion-Exchange Resins. M.S. Thesis. Department of Civil & Environmental Engineering. University of Nevada, Las Vegas. 6