Anal. Chem. 1996, 68, 93-99
Design and Evaluation of a New Thermospray Liquid/Liquid Extractor for the Extraction of Semivolatile and Nonvolatile Organic Compounds from Water E. Sebastian Farrell and Gilbert E. Pacey*
Department of Chemistry, Miami University, Oxford, Ohio 45056
The recovery of several semivolatile organic compounds (SVOCs) using a new thermospray liquid/liquid extractor (TSLLE) was investigated. The base system includes a 300 mL multiport extraction vessel, jacketed in a 500 mL cooling flask, a dual-stage condenser for progressive cooling, several thermospray probes, and solvent/sample delivery systems. Aqueous mixtures of SVOCs were used to evaluate the TSLLE. For most compounds, recovery values of 80-100% were obtained during a single cycle in 24 h), this gap will slowly decrease and may lead to plugging and overpressure in the extractor. For operation exceeding 24 h, a larger gap size is recommended, and periodic defrosting should be done if necessary. A vapor bypass assembly at the lower end of the condenser reduced contact between the condensate and the extractor walls.
Table 1. Summary of Experimental Parameters for Selected Thermospray Liquid/Liquid Extractions sample
flow rate (mL/min)
probe temp (°C)
condenser status
sample type
row
vol (mL)
concn (µg/mL)
sample
solvent
sample
solvent
prestage
poststage
chiller temp (°C)
HC mix 2 SV mix 2 SV mix 7 HC mix 2 HC mix 2
A B C D E
100 200 50 50 50
2.2 2.0 8.0 0.400 0.400
5.0 4.0 4.0 4.0 5.0
2.0 3.0 3.0 3.0 3.0
163 160 160 160 162.5
145 130 130 67, 141 159.4
on on on on on
off off off on on
0 5 5 0 0
Instrumentation. Extracts were analyzed on a Perkin-Elmer 8500 gas chromatograph (GC) equipped with a split/splitless injector, an Omega analytical workstation, and a flame ionization detector (FID). The FID was operated with H2 set at 20 psi, air set at 22 psi, and N2 carrier set at 70 mL/min. Separations were performed on a J&W Scientific capillary column (DB-5, 30.0 m × 0.32 mm i.d., with a film thickness of 1.0 µm). The column temperature was initially set at 40 °C for 4 min and then increased to 270 °C at 10 °C/min for 10 min, while the injector and detector temperatures were set at 250 and 300 °C, respectively. Concentrator. Extract concentration was accomplished using a Kuderna-Danish (K-D) concentrator heated by a Tekmar Model 1052 constant temperature water bath (Cincinnati, OH). K-D concentration proceeded initially with a three-ball Snyder column and subsequently with a micro-Snyder column to a final volume of 1.0 mL. Procedure. Aqueous solutions containing 0.400, 2.0, 2.2, or 8.0 µg/mL of the standard SVOCs mixture were prepared respectively by adding 50, 250, 275, or 1000 µL of the 2000 µg/ mL stock in 10, 50, 60, or 100 mL of methanol and then slowly diluting to mark in a 250 mL volumetric flask with triply distilled water. The 2.2 µg/mL solution of HC mix 2 was prepared by adding 550 µL of the stock solution to 100 mL of methanol, followed by slow addition of 20 mL of acetone and enough water to give a final volume of 500 mL. All solutions were prepared and used on a daily basis. Standard solutions of each compound were prepared in methylene chloride to give the same concentrations as the aqueous solutions. Sample Extraction. Specific extraction parameters for each sample are shown in Table 1, rows A-C. Following the implementation of these parameters, triply distilled water was extracted to obtain a system blank and ascertain probe temperatures equilibration. The sample pump was then turned off momentarily for replacement of the water with the sample. When the extraction was completed, 5 mL of deionized water was circulated through the transfer line to remove residual SVOCs. The sample pump and heater were then turned off, while the solvent delivery system continued for an additional minute to ensure collection of all extractable materials. After a 5 min cooling-off period, the lower solvent phase was collected and dried over anhydrous Na2SO4. Extracts containing >2.0 µg/mL SVOCs were diluted to 25 or 50 mL with CH2Cl2 prior to GC analysis. Those containing 0.400 µg/mL SVOCs were concentrated to 1 mL using a K-D concentrator with Snyder columns. Calibration standards were prepared in CH2Cl2 at the concentration levels of the aqueous sample. K-D concentration or dilution was performed as for the sample. Aliquots of 1 mL each of the extract and the standard were spiked with 1 µL of 1,3-dichloropropane internal standard. Only 1 µL of this mixture was injected on column for GC/FID
analysis. The system blank was concentrated and analyzed as for the sample. RESULTS AND DISCUSSION Design Considerations. Thermospray nebulizers appear to have found greater utility in applications which involve thermally sensitive metabolites, SVOCs, and nonvolatile organic compounds.13-15 Based on this capability, a continuous liquid/ liquid extraction system equipped with thermospray nozzles was designed for the extraction and preconcentration of SVOCs and nonvolatiles from water. Figure 1 sketches the TSLLE used in this work. A critical factor which must be considered during lowlevel extractions is the surface area of the extraction chamber. To minimize contact of the analytes with the surface, the TSLLE design incorporated a vapor bypass arm, a funnel assembly, and a small extraction chamber (300 mL). During the design phases of the extractor, Teflon stoppers were used to fix the thermospray nozzles within the movable arm of a ball-and-socket joint. Figure 2 shows the extraction profile of a 0.400 µg/mL solution of HC mix 2 in a movable arm TSLLE as a function of boiling point at two solvent probe temperatures. The other experimental conditions are shown in Table 1, row D. Note that in both cases, the order of recovery for the lower boiling compounds is reversed, indicating loss of the more volatile components of the mixture. Using conditions in Table 1, row E, a mass balance experiment was performed to determine where the loss was occurring. During this process, the exit end of the condenser was connected to a liquid nitrogen/CH2Cl2 trap. Following extraction, a nitrogen carrier was used to purge the headspace onto a trap. The aqueous phase was reextracted with three 20 mL portions of methylene chloride in a 250 mL separatory funnel. The combined organic phases from the batch process, along with the other extracted fractions, were analyzed separately. As shown in Table 2, residual amounts of some analytes were found in the reextracted sample at levels ranging from 2 to 4%, but losses resulting from condenser breakthrough were not observed. Additional experiments were performed without a dry ice/acetone mixture in the poststage condenser. Again, vapor breakthrough was not observed, indicating the effectiveness of the cooling jacket during the extraction process. Therefore, the integrity of the Teflon seals in the movable arm joints appeared to have deteriorated with prolong heating over time and led to vapor loss. This prompted a design change in which fixed joints with stainless steel compression fittings were used. (13) Mutlib, A.; Chui, Y. C.; Young, L.; Abbott, F. S. Proceedings of the 39th ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN, May 1924, 1991; pp 611-612. (14) Slatter, J. G.; Rashed, M. S.; Pearson, P. G.; Han, D. H.; Baillie, T. A. Chem. Res. Toxicol. 1991, 4, 157-161. (15) Chang, S. Y.; Moore, T. A.; DeVaud, L. L.; Taylor, L. C. E.; Hollingsworth, E. B. J. Chromatogr. 1991, 562, 111-118.
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Figure 2. Profile of the percent recovery as a function of boiling point for the extraction of HC mix 2 in a movable arm TSLLE at different temperatures. Table 2. Mass Balance Extraction Data from an Experiment Performed during the Design Phase of the Movable ARM TSLLE recovery (%) compound
spray-extracted fraction
batch-reextracted fraction
1,3-dichlorobenzene 1,4-dichlorobenzene 1,2-dichlorobenzene hexachloroethane nitrobenzene isophorone 1,2,4-trichlorobenzene hexachlorobutadiene hexachlorocyclopentadiene 2-chloronaphthalene 2,6-dinitrotoluene 2,4-dinitrotoluene azobenzene
7 8 9 0 73 74 9 5 13 37 82 87 76
0 0 0 0 3 4 0 0 0 0 3 3 2
One other design consideration worth mentioning is the relative orientation of the probes. If the probe tips lagged behind the wall of the extractor, of if they were placed at a relative angle of 180°, the collection of intermittent droplets at the tips impeded free flow and nebulization process. Except where otherwise noted, the probes were oriented at right angles to each other and about 3 mm from the surface of the extraction vessel. The vapor bypass assembly described above also helped to eliminate this problem by redirecting the condensate away from the walls of the extractor. Solvent Reduction Consideration. The effectiveness of the concentration step depends largely on the physical properties of the analytes in the mixture and the nature of the sample matrix. Since chemical equilibrium condition between phases in the concentrator is governed by Henry’s law,16 in K-D concentration, the choice of temperature significantly affects the distribution of the analytes between liquid and vapor phases. Based on U.S. EPA method protocol,1 the allowable temperature range for methylene (16) Namiesnik, J.; Gorecki, T.; Torres, L. Anal. Chim. Acta 1990, 237, 4-21.
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Analytical Chemistry, Vol. 68, No. 1, January 1, 1996
chloride reduction in a K-D concentrator is 60-80 °C on a water bath or 35-40 °C for nitrogen evaporation. As a quality assurance measure, 50 mL aliquots of a 0.400 µg/mL methylene chloride solution of HC mix 2 were placed in a K-D apparatus and reduced to 1 mL on a water bath at various temperatures. Table 3 lists the recoveries and relative standard deviations of these compounds. Presumably, entrainment of the sample in the solvent vapor during the concentration step, and the temperature or rate at which the concentration step is performed, can lead to loss in sample recovery.2,16 Losses of up to 40% were sustained by many compounds, with more loss and less variability occurring at higher temperatures. Similar losses by K-D concentration have been reported, even for more polar analytes.17,18 Thus, TSLLE was evaluated using high concentrations (low milligrams per liter range) of SVOCs so that direct analysis of the extract was possible without solvent reduction. Temperature and Flow Rate Considerations. Nebulization of a liquid sample into droplets requires energy.19 The total power, W, which must be coupled into the liquid to vaporize a fraction, f, at a flow rate F (g/s) is given by the relationship
W ) fFHv + F(1 - f)CL(T2 - T0)
where Hv is the specific enthalpy (J/g) to convert liquid with an entrance temperature T0 to an exit vapor with temperature T2; T0 is the entrance temperature; T2 is the exit temperature; and CL is the specific heat capacity of the liquid. Therefore, by controlling the power input, the fraction vaporized can be controlled. But since the temperature is a function of both the liquid composition and the flow rate,11 fluctuations of the latter will have a significant (17) Baker, R. J.; Gibs, J.; Meng, A. K.; Suffet, I. H. Water Res. 1987, 21, 179190. (18) McNally, M. E.; Snyder, J. L.; Grob, R. L.; Oostdyk, T. S. P. Anal. Chem. 1993, 65, 596-600. (19) Todoli, J. L.; Canals, A.; Hernandis, V. Spectrochim. Acta 1993, 48B, 373386.
Table 3. Recovery and Reproducibility Data (%) Resulting from Variable Temperature Concentration of HC Mix 2 in a K-D Apparatus 45 °C
56 °C
65 °C
compound
recovery (N ) 5)
RSD
recovery (N ) 5)
RSD
recovery (N ) 5)
RSD
1,3-dichlorobenzene 1,4-dichlorobenzene 1,2-dichlorobenzene hexachloroethane nitrobenzene isophorone 1,2,4-trichlorobenzene hexachlorobutadiene hexachlorocyclopentadiene 2-chloronaphthalene 2,6-dinitrotoluene 2,4-dinitrotoluene azobenzene hexachlorobenzene
72 69 66 72 69 76 69 70 75 86 96 110 103 110
17 25 20 22 16 14 18 16 17 12 10 15 15 13
81 69 70 73 70 73 71 80 71 79 87 92 88 93
22 10 9 9 8 4 10 26 7 10 6 12 12 12
64 66 64 64 60 62 62 61 63 68 78 84 83 83
3 10 7 2 10 13 8 10 12 11 13 16 13 17
Figure 3. (A) Probe temperature as a function of extraction time, demonstrating stability over the course of a typical extraction run. (B) Solvent probe temperature profile after disabling the sample probe heater to illustrate the magnitude of probe-to-probe heat transfer effect.
impact on extraction efficiency and reproducibility. In the case of TSLLE, system stability was established over the course of a typical extraction run. Figure 3A demonstrates the temperature stability profile in which reagent water and methylene chloride were nebulized respectively at flow rates of 5 and 3 mL/min and temperatures of 156 and 91 °C. At time zero, the flow rate in each probe is also zero, and a temperature elevation is observed. However, when the pumps are activated and flow equilibrium is attained, the temperature increase quickly subsided. In Figure 3B, it can be observed that heat transfer from the sample probe (T ) 156 °C) to the solvent probe (T ) 91 °C) has a significant effect on the tip temperature of the latter. With the experimental conditions set as defined for Figure 3A, the sample heater was turned off (this corresponds to time zero on the graph), while the solvent probe temperature was monitored for an additional 15 min. The observed temperature for the solvent probe changed by as much as 50 °C. On the other hand, the latter probe had no
Figure 4. Thermospray liquid/liquid extraction of SV mix 7 from water, demonstrating the effect of solvent flow rate on extraction recovery.
impact on the observed temperature of the sample probe. Such a heat transfer effect can be minimized when the temperature difference between probes is small. Figure 4 shows the recovery of 50 mL of 0.400 µg/mL SV mix 7 as a function of the solvent probe flow rate. Temperatures in the sample and the solvent probes were fixed at 140 and 75 °C, respectively, while the sample flow rate was held constant at 4 mL/min. The chiller temperature was set at 0 °C. Under these conditions, higher sample/solvent ratio led to higher extraction recovery down to a ratio of 4:2 mL/min, where nebulization ceased and the advantages of surface area enhancements were no longer realized. Below this point on the curve, the process is essentially a batch extraction process, where modest increase in recoveries Analytical Chemistry, Vol. 68, No. 1, January 1, 1996
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Table 4. Thermospray Liquid/Liquid Extractor Recovery and Reproducibility Data (%) compound (HC mix 2)
recovery (N ) 5)
RSD
1,3-dichlorobenzene 1,4-dichlorobenzene 1,2-dichlorobenzene hexachloroethane nitrobenzene isophorone 1,2,4-trichlorobenzene hexachlorobutadiene hexachlorocyclopentadiene 2-chloronaphthalene 2,6-dinitrotoluene 2,4-dinitrotoluene azobenzene hexachlorobenzene
91 93 97 76 112 115 98 64 72 108 98 93 96 98
8 6 9 18 5 4 2 12 11 6 7 1 2 7
Table 5. Percent Recovery Data for the Extraction of SV Mixes 2 and 7 from Water by Thermospray Liquid/ Liquid Extraction Figure 5. Thermospray liquid/liquid extraction of SV mix 7 from water, demonstrating the effect of solvent probe temperature on extraction recovery.
can be realized with increasing volume of solvent. To test the effect of solvent probe temperature on extraction efficiency, a 50 mL aliquot of 0.400 µg/mL SV mix 7 was extracted, with the sample and solvent flow rates set at of 4.0 and 3.0 mL/min, respectively. The chiller and sample probe temperatures were set at 0 and 152 °C, respectively. From Figure 5, it can be seen that extraction efficiency increased with increasing temperature over the range tested. Reproducibility and Recovery Studies. When the dualphase condenser and the extraction flask circulator are operational, the TSLLE can be used at elevated temperatures to extract modified aqueous samples containing organics at a wide range of volatility in a single extraction step without substantial evaporative loss. To evaluate the extraction efficiency and precision of the TSLLE system, five independent 100 mL aqueous samples containing 2.2 µg/mL HC mix 2 and modified with 20% methanol and 4% acetone were extracted with methylene chloride (see Table 1, row A). These extractions were performed with both the coil condenser and the extraction flask cooling jacket operational, the Neslab chiller set to 0 °C, and the solvent and sample probe temperatures set at 145 and 163 °C, respectively. A sample-tosolvent ratio of 5:2 mL/min has been employed for the work reported in Table 4. The values in this table indicate very good acceptable recovery and precision for many of the analytes tested. Hexachlorocyclopentadiene (and possibly hexachlorobutadiene) is known to undergo chemical reactions in acetone solution, along with thermal and photochemical decomposition.20 Consequently, their extraction efficiency appears to be reduced significantly. To further test the effectiveness of TSLLE for the isolation of polar, acidic, or chlorinated SVOCs, phenols and various compounds found in SV mixes 2 and 7 were extracted as stipulated in Table 1, rows B and C. The extraction results are summarized in Table 5. Again, the data show good recoveries for all compounds except pentachlorophenol and nitrophenols. (20) U.S. EPA. SW-846, 3rd ed.; Test Methods For Evaluating Solid Waste, Method 8270; U.S. GPO: Washington, DC, 1987.
98 Analytical Chemistry, Vol. 68, No. 1, January 1, 1996
a
compound
SV mix 7
SV mix 2
phenol 2-chlorophenol 1,3-dichlorobenzene 1,4-dichlorobenzene 1,2-dichlorobenzene 2-methylphenol 4-methylphenol 2-nitrophenol 2,4-dimethylphenol benzoic acid 4-chloro-3-methylphenol 2,4,6-trichlorophenol 2,4,5-trichlorophenol 2,4-dinitrophenol 4-nitrophenol 2-methyl-4,6-dinitrophenol pentachlorophenol
npa np 85 98 97 np np np np np np np np np np np np
87 98 95 95 96 94 87 96 92 82 96 99 89 111 95 115 76
volume extracted (mL) concentration (µg/mL)
50 8
200 2
Not present in mixture.
Safety Considerations. To prevent accidental breakage of the extractor and related injuries, great care must be taken to avoid undue strain on the fixed probes and extraction chamber. To reduce the risk of injury and inhalation of solvent vapor, the extractor should be operated in a partially closed fumehood. Additional safety precaution can be taken by enclosing the extraction vessel in a plexiglass shield. CONCLUSION Thermospray liquid/liquid extraction is a viable alternative to EPA SW 846 Method 3520 for continuous liquid/liquid extraction of semivolatile or nonvolatile organic compounds from water. Until it is commercially available, TSLLE construction may require access to a scientific glassblower and supporting instruments. For the work reported in this paper, pH or salt concentration adjustments were not required. The technique gave acceptable recoveries of SVOCs in significantly less time, partly because of (a) the ability to optimize temperature, flow rate, and probe diameter of all probes independently, (b) the ability to minimize solvent consumption while adjusting system parameters to in-
crease extraction efficiency, and (c) the rapid breakup of emulsion with minimal heat application. Further investigation and optimization of this system are continuing. ACKNOWLEDGMENT We thank Joseph A. Partlow, Scientific Glassblower, Department of Chemistry, Miami University, for the construction of the
thermospray liquid/liquid extractor during the design phase of this work. Received for review July 5, 1995. Accepted October 16, 1995.X AC9506544 X
Abstract published in Advance ACS Abstracts, December 1, 1995.
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