Envlron. Sci. Technol. 1966. 22, 561-565
Olie, K.; Vermuelen, P. L.; Hutzinger,0. Chemosphere 1977,
portant than activity, can be tailored to fit needs and conditions by changing catalytic agents and/or mixtures thereof. The low surface area, thick-walled monlithic reactor used in this study was not designed for direct commercial application. Thin-walled multicellular cores with higher surface area could pack at least 100 times more active catalytic sites into aach cubic centimeter of catalyst system. Nevertheless, estimation of scaleup potential with the present 10% CuO catalyst on low surface area A120, monolith shows that a 1000-L vapor-phase reactor operating at 500-600 “C would convert 50-70 kg of PCB/h. However, with higher surface area commerical multicell cores, reactivity could be at least 1 order of magnitude higher.
8, 455.
Lombardi, E. F.; Johnson, A. J.; Meyer, F. G.; Hunter, D. I. Incineration of PCBs Using a Fluidized Bed Incinerator; Rockwell International: Golden, CO, 1981. Chem. Eng. 1981,88(16),37-41. Akerman, L. C.; Scinta, L. L.; Bakshi, P. S.; Delumyea, R. G.; Johnson, R. J.; Richard, G.; Takata, A. M.; Sworzyn, E. M. Destruction and Disposal of PCBs by Thermal and Non-Thermal Methods; Noyes Data: Park Ridge, NJ, 1983. Akerman, D. G. “Guidelinesfor the Disposal of PCBs and PCB Items by Thermal Destruction”; U S . EPA Report 600/281-022; US. Government Printing Office, Washington, DC, 1981. Desai, F. N.; Greene, H. L.; Subbanna, P. Ind. Eng. Chem. Res. 1987, 26, 1965. Subbanna, P. M.S. Thesis, University of Akron, 1985. Prasad, R.; Kennedy, L. A,; Ruckenstein, E. Combust. Sci. Technol. 1980,22, 271-80. “State-of-the-ArtReview: PCDDs and PCDFs in Utility
Registry No. Aroclor 1254, 11097-69-1;C6H5C6H5, 92-52-4; Crz03,1308-38-9;Pt, 7440-06-4;Pd, 7440-05-3;Co304,1308-06-1; CuO, 1317-38-0;CuCr204,12018-10-9.
Literature Cited (1) Farrell, J. B.; Salotto, B. V. Effect of Incineration on Metals, Pesticides and PCBs in Sewage Sludge; National Symposium on the Ultimate Disposal of Wastewaters,and Their Residuals: Raleigh, NC, 1973. (2) Buser, H. R.; Bosshardt, H. P.; Rappe, C. Chemosphere 1978, I , 109. (3) Buser, H. R.; Bosshardt, H. P.; Rappe, C. Chemosphere 1978, 2, 165.
PCB Fluid”; final report EPRI CS-3308, Project 1263-11; Electric Power Research Institute: Palo Alto, CA, Nov 1983. Lerman, S. I.; Gordon, H.; Hendricks, J. P. A m . Lab. (Fairfield, Conn.) 1982, 14(2), 176-81. Sep-Pak Cartridges, A New Dimension in Sample Cleanup; Waters Associates: Milford, MA, 1981. A Safe, Efficient Chemical Disposal Method for PCBs;
Goodyear Tire and Rubber: Akron, OH, 1980. Received for review December 29, 1986. Revised manuscript received November 9, 1987. Accepted December 15, 1987.
A Numerical Solution for Mass Transport in Membrane-Based Diffusion Scrubbers Richard L. Corsi,” Danlei P.
Y. Chang, and Bruce E. Larock
Department of Civil Engineering, University of California, Davis, California 956 16
rn The Galerkin finite element method was used to develop a numerical model for the design of porous-wall diffusion scrubbers, devices which may be used as alternatives to diffusion denuders, and to selectively separate dissolved components from a liquid stream. It was found that the analytical Gormley-Kennedy solution for a tube with an ideally absorbing interior wall is appropriate for scrubbers characterized by a gas-liquid interface at the wall. However, the major conclusion of this work is that the Gormley-Kennedy solution is often inappropriate for application to diffusion scrubbers based upon a liquid-liquid interface at the wall. For such scrubbers, the unknown concentration boundary condition at the interior wall can be accounted for with the numerical solution, thus providing a design tool that has not been previously available. A set of penetration curves was generated for design purposes, and example applications are provided.
Introduction The diffusion denuder is a device which can be used to remove a gaseous component selectively from a gas mixture or aerosol. Denuders are typically constructed from a cylindrical tube or concentric tubes with the inner wall(s) coated with a substance to sorb a specific component. They have been used to remove water selectively from a nebulized aerosol sample while permitting the passage of sulfate particles ( I ) , to remove sulfur dioxide from airstreams containing sulfur particles (2-41,and to remove ammonia in order to prevent neutralization of sulfuric acid 0013-936X/88/0922-0561$01.50/0
aerosol collected on downstream filters (5). Additional applications for denuders have been suggested (6-8), but specific design and operational limitations have also been cited ( 4 9 ) . Limitations include a finite adsorptive capacity (3),the choice of sorbents, the sorbent concentration and form, the wall material, the denuder geometry, and the sample flow rate (9). Dasgupta (10) investigated alternative device configurations for the collection of atmospheric gases. His work led to the construction of a diffusion scrubber, a device which is similar in principle to the diffusion denuder but which employs a thin membrane tube as the collection element. In subsequent work a porous-membranetube was found to be the material of choice (11). A schematic representation of the hollow cylindrical diffusion scrubber is shown in Figure 1. It consists of a porous-wall hollow fiber. Molecules of interest diffuse through the wall and are removed by a scrubber liquid flowing counter to the transport medium. Applications of diffusion scrubbers include not only diffusion denuders for gas separation but also the separation of dissolved species from a liquid stream (12-15). Diffusion scrubbers have also been used as concentrators for continuous chemical analysis (16). In addition, Dasgupta (17)described the use of an annular helical diffusion scrubber in ion chromatography as an efficient conductance suppressor column having both low dispersion and low dead volume. Models to describe species transport in diffusion devices can be used to determine design parameters such as denuder length and the sample flow rate required to achieve
0 1988 American Chemical Society
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E X T E R I O R WILL
r
POROUS WALL
( N O T TO SC4LEl
Flgure 1. A Simplified diagram of the porous-wall diffusion scrubber illustratingthe Internal fluid stream, porous wail, and scrubber solution.
a desired species collection or removal efficiency. Analytical solutions exist to predict the penetration Pt, defined as Pt = (1) where and ceare the average concentrations at the tube inlet and outlet, respectively, of a chemical species in fully developed, laminar flow in a cylindrical tube, given either an ideally absorbing wall or known concentration profiles at the wall (18-21). These solutions are applicable to laminar flow in diffusion denuders or other devices satisfying the boundary conditions such as diffusion through silicone tubing or possibly ion-exchangemembranes. The authors are unawbe of any extant solutions which predict concentration profiles and penetration in a diffusion scrubber for which a nonuniform concentration boundary condition applies, but cannot be initially prescribed, at the inside surface of the porous wall (see Figure 1). This study was undertaken in order to investigate the effects of such a condition and to develop a numerical finite element model for use as a design tool in cases not described by existing analytical solutions such as the Gormley-Kennedy (19) solution. The latter is expressed as Pt = 0.819e-14.e272b 0.976e-89.22b 0.01896e-212b ... (2) where
e,/c,
eo
+
+
+
b = Dz/(4iiR2) (3) and D is the diffusion coefficient of the species of interest, z is the axial distance from the tube inlet, ii is the average axial velocity, and R is the tube’s inner radius. Equation 2 has been used successfully to model denuder systems with interior walls that act as perfect sinks (19-21). In order to verify model results for a porous wall, in this study a simplified case corresponding to an impermeable adsorbing inner wall which behaves as a perfect sink for the species of interest was solved by a finite element approach. The penetration of a component was then compared with values obtained from the Gormley-Kennedy solution, an analytical solution which has been widely used and tested in the field of aerosol science. A permeable wall boundary condition and the inclusion of a hydrodynamic entrance region were subsequently added to examine their effects on species transport. It should be emphasized that the numerical solution only treats the condition of laminar flow, which is the dominant flow regime for most diffusion scrubber applications. Model Methodology Governing Equations. The steady-state convectivediffusion equation governs diffusion through a homogeneous medium flowing in a cylindrical tube. It may be written as
where u is the velocity in the axial (z) direction, u is the velocity in the radial ( r ) direction, and C is the concen562
Envlron. Sci. Technol., Vol. 22, No. 5, 1988
tration of the component of interest. Axial symmetry has been assumed, along with the assumption that there are no sources or chemical transformations occurring within the tube. Equation 4 can be simplified by assuming that the radial velocity component and the axial diffusion term are negligible. I t is assumed that there is no bulk flow of fluid through the wall. Following the arguments of Tan (22), the assumption of a negligible radial velocity is valid for typical flow rates, scrubber geometries, and species diffusion coefficients for which diffusion scrubbers are applied. Ingham (20) and Tan and Hsu (21) noted the conditions for which axial diffusion is insignificant. These conditions are also easily met for typical diffusion scrubber geometries and flows (10). Use of these assumptions allows eq 4 to be simplified to
(5) Introduction of the dimensionless variables C’ = C/C, u’ = u/2ii r’ = r / R z’ = z / L leads to the nondimensional equation
where L is the effective scrubber length_. The characteristic nondimensional Fourier parameter D is
b = DL/(2iiR2) (7) For reasons which will become apparent in a later example app_lication,D is chosen for design purposes. The choice of D, ii, and R, together with a known value for D, defines a characteristic scrubber length, L. The design, or actual, scrubber length, x, is obtained as the product of L and the dimensionless axial distance z’. The selection of an appropriate value for z‘is demonstrated later in this paper. For fully developed laminar flow, the appropriate dimensionless velocity distribution is u’= 1.0 - r t 2 (8) If a significant entrance length exists, eq 8 is not an accurate representation of the velocity profile for the entire flow domain. Instead, a solution for the developing velocity field is required as input data for the numerical model. For this study, an equation based upon the linearization of momentum was used to describe the entrance region velocity field (23). Boundary Conditions. The dimensionless concentration at the inlet, z‘ = 0, was prescribed with a uniform value of 1.0. At the outlet, z’ = 1,both a zero concentration boundary condition and a second boundary condition were investigated. The alternative boundary condition was based upon a parabolic fit of the penetration which was obtained by the Gormley-Kennedy solution. Results obtained with the two boundary conditions were not significantly different. The latter condition was arbitrarily chosen for this study. Either condition imposes limitations on the minimum value which may be chosen for D , since small values of D correspond to conditions associated with high penetration that would violate the assumed boundary condition at the outlet. For permeable walls at steady state, the flux at the interior surface of the wall must necessarily be equal to the flux through the wall. It was assumed that the transport through the wall is such that a linear concentration gradient exists in the wall and that only radial diffusion is significant in the wall. This leads to C ~ ~ f ~ & N ) -
Results C' DEFINE0
BY A PARABOLIC FIT GORMLEY-KENNEDY
2"
00 I-
d C' dr' '
I
Flgure 3. A typical finlte element grid. Boundary condltions are noted.
where C:, is the dimensionless concentration in the liquid phase a t the inner surface of the wall, Cl, is the dimensionless concentration at a dimensionless radial distance Ar' from the inner surface of the wall, and t ' is the dimensionless wall thickness defined to be the ratio of the actual wall thickness t to the scrubber radius R (see Figure 2). The term W is a nondimensional wall parameter. For the case of a liquid flow medium and a gas space in the porous wall (liquid-gas interface), W is W = D&/(DiR,T)
(10)
where D, and D1 are diffusion coefficients of the species of interest in the gas and liquid phases, respectively, K is the Henry's law coefficient governing its partition between the two phases, R, is the universal gas constant, and T i s the absolute temperature. For a liquid-liquid interface of two immiscible liquids, the wall parameter is W = DlzK'/Dli
(11)
where D12is the diffusion coefficient of the species of interest in the outer acceptor liquid (which occupies the pore space in the wall or the wall itself), Dll is the diffusion coefficient in the donor liquid bearing the species of interest flowing through the tube, and K'is the equilibrium partition coefficient of the species of interest between the two immiscible liquid phases. Specifically, K' is the ratio of the species concentration in the acceptor liquid to that in the donor liquid a t equilibrium. Numerical Solution. To determine concentration and penetration values, a Galerkin finite element numerical model (24, 25) was developed. Bilinear interpolation functions were used along with four-node, isoparametric, quadrilateral elements to describe the geometry of, and concentration within, elements subdividing the diffusion scrubber volume. Figure 3 depicts a typical grid system with nodes at which concentrations were computed. The boundary conditions for this study are also denoted. Axial symmetry was assumed, and computed values of C' were numerically integrated across the scrubber cross-section to solve for the penetration at specified values of z !
The model was solved with the aid of a grid system similar to the one shown in Figure 3. A sensitivity analysis of con_centrationprofiles near the scrubber wall suggested that D should exceed a value of 1.0 to be copsistent with the outlet boundary condition. A value of D equal to 2.0 was used for this study. The algorithm of the numerical model was verified for the simplified case of an ideally adsorbing wall by comparison of model results with values obtained from the Gormley-Kennedy solution. For typical scrubber geometries and sample flow rates, the effect of a hydrodynamic entrance region was determined to be insignificant. The permeability of the scrubber wall was found to be a significant factor only for values of W/t 'less than 50.0. Given typical values of t ' less than 0.25 for porous-membrane tubes and typical values of W calculated according to eq 10, the value of W/t'is nearly always greater than 50.0 for systems composed of a liquid transport medium and a gas phase in the scrubber wall. Thus, the Gormley-Kennedy solution for laminar flow in a cylindrical tube, strictly applicable to an ideally adsorbing wall, can also be used as an accurate model for predicting penetration in porous-wall gas-liquid scrubbers. The Gormley-Kennedy solution does not, however, yield accurate results for systems with values of W/t 'significantly less than 50.0. Such systems might be based upon the interface of two immiscible liquids at the scrubber wall. Figure 4 consists of_aplot of - log Pt versus z' for various values of Wlt'and D = 2.0. Curves are shown only to z' = 0.75, which was the nondimensional distance corresponding to the last set of grid nodes at which concentrations were numerically solved. The next set of nodes corresponded to the outlet nodes where concentrations were specified as mentioned previously. The solid line indicates the Gormley-Kennedy solution. Figure 4 can be used for diffusion scrubber design in those cases where the Gormley-Kennedy solution would be inappropriate. The following examples are provided to illustrate the use of Figure 4. Example Application 1. A diffusion scrubber is to be used to remove a specific compound from a liquid stream which requires pretreatment before chemical analysis. With l-octanol as the scrubber solution and water as the transport medium, the values of D,, and Dll are 3.0 X lo4 c m 2 / s and 3.0 X cm2/s, respectively. The octanolwater partition coefficient is 30.0. A scrubber tube with an inner diameter equal to 0.625 mm and a wall thickness of 0.125 mm is available. It is desired to find the minimum scrubber length x required to remove 99% or more of the compound. The sample flow rate is expected to be less than 0.25 mL/min: Environ. Sci. Technol., Vol. 22, No. 5, 1988 563
Step 1: Solve for Dl2/Dl1= 3.0 X 104/3.0 X = 0.1. Step 2: Solve for t’ = 0.125/0.3125 = 0.4. Step 3: Solve for W/t’ = 0.10(30.0)/0.4 = 7.5. Step 4: Solve for - log Pt = - log (1.0-0.99) = 2.0. Step 5: Solve for z’at - log Pt = 2.0 for W/t’ = 7.5. From Figure 4, z’ = 0.365. Step 6: Compute the maximum average sample velocity for a flow rate of 0.25 mL/min and a radius of 0.3125 mm. The average velocity is calculated by dividing the sample flow rate by the cross-sectional area of the scrubber tube. ii = 1.36 cm/s. Step 7: Let L = x / z ‘ e d use eq 7 along with the known or calculated values of D, D, z , ii, and R to solve for x ; x = 65 cm. Use of the Gormley-Kennedy equation leads to a value of x equal to 53 cm. For W/t’ = 7.5, this corresponds to a penetration of 3%. Thus, use of the Gormley-Kennedy equation for this application may lead to an undersized (in length) scrubber and too low of a removal efficiency for the intended end use of the sample. Often, a specific length of scrubber tube is available, and the removal efficiency that can be achieved with that length is desired to be known. The following example shows how the removal efficiency can be calculated with the use of Figure 4. Example Application 2. A 30-cm segment of a scrubber tube is available to remove a compound from a liquid stream. With 1-octanol as the scrubber solution and water as the transport medium, the values of D12and Dll are 2.0 X lo+ cm2/s and 2.5 X cm2/s, respectively. The octanol-water partition coefficient is 25.0. The scrubber tube has an inner radius equal to 0.5 mm and a wall thickness of 0.1 mm. It is desired to find the compound removal efficiency given a sample flow rate of 0.2 mL/min: Step 1: Solve for Ol2/Dl1= 2.0 X 104/2.5 X = 0.08. Step 2: Solve for t’ = 0.110.5 = 0.2. Step 3: Solve for W/t’ = 0.08(25.0)/0.2 = 10.0. Step 4: Solve for the average velocity of the sample; ii = 0.42 cm/s. Step 5: Solve for L = x/z’given d, D, ii, and R. Use d = 2.0 for consistency with Figure 4. From eq 7, L = 168 cm. Step 6: Solve for z’ = x / L = 30/168 = 0.18. Step 7: Solve for the penetration (Pt) using Figure 4 with z’ = 0.18 and W/t’ = 10.0; Pt = 0.1. Thus, a 90% removal efficiency is expected with the 30-cm length of scrubber tube. Given the specific tubing, the removal efficiency could be improved by lowering the sample flow rate. This will lead to a decrease in L and a subsequent increase in z’. Figure 4 could again be used to solve for the appropriate flow rate to achieve the desired removal efficiency. Summary A numerical solution of diffusion of a single component from a fluid flowing in laminar conditions through a permeable-wall cylindrical tube has been completed. The results demonstrate that a dimensionless ratio, W/t ’, can be used as a criterion for satisfactory application of the Gormley-Kennedy solution to the mass-transfer problem. In cases where W/t’is less than 50, the numerical solution would provide more accurate results. The application of diffusion scrubbers extends to several disciplines, and the penetration curves presented in this paper should be of assistance in the design of such devices. The analysis presented here is based on the assumption of smooth walls without local mixing due to surface irregularities and pores. Future work should be directed toward verifying such an 564
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assumption for typical porous-wall diffusion scrubbers. Acknowledgments We thank Dr. Purnendu Dasgupta for his valuable comments regarding his study of diffusion scrubber design and application and Virginia Roy and Barbara Sullivan for assistance with the preparation of the manuscript. Glossary b diffusion parameter used in the Gormley-Kennedy equation; b = Dz/(4iiR2) r radial coordinate (cm) r‘ dimensionless radial coordinate; r’ = r / R t scrubber wall thickness (cm) t’ dimensionless wall thickness; t’ = t / R U axial velocity component (cm/s) U’ dimensionless axial velocity; u’ = u/%i ti average axial velocity (cm/s) V radial velocity component (cm/s) z axial coordinate (cm) 2’ dimensionless axial coordinate: z’ = z / L C concentration (g/cm3) concentration at the scrubber inlet (g/cm3) CO C’ dimensionless concentration; C’ = C / C o dimensionless concentration at a dimensionless c; distance Ar’from the interior wall of the scrubber dimensionless concentration at the interior surface C’i,, of the scrubber wall average concentration across the scrubber outlet 6, (g/cm3) average concentration across the scrubber inlet 60 (g/cm3) D diffusion coefficient of the species of interest (Cm2/s) D nondimensional Fourier parameter; b = 2b diffusion coefficient in air (cmz/s) D* diffusion coefficient in liquid (cm2/s) D1 diffusion coefficient in the liquid flowing through Dll the scrubber (cmz/s) species diffusion coefficient in the scrubber liquid D12 which permeates the porous wall (cm2/s) K Henry’s law constant of the diffusing species (atmm3/mol) K’ liquid-liquid partition coefficient L length of the diffusion scrubber (cm) Pt penetration through the scrubber; Pt = C,/c, R scrubber inner radius (cm) universal gas constant (atm.m3/mol.K) Ru T absolute temperature of the gas phase (K) nondimensional wall parameter W Literature Cited Crider, W. L.; Barkley, N. P.; Knott, M. J.; Slater, R. W. Anal. Chim. Acta 1969,47, 237. Durham, J. L.; Wilson, W. E. Atmos. Environ. 1978,12,883. Kaplan, D. J.; Himmelblau,D. M.; Chikao, K. Environ. Sei. Technol. 1981,15,558. Lewin, E.E.; Klockow, D. Proceedings of the 2nd European Symposium on the Physico-Chemical Behavior of Atmospheric Pollutants; Commission of the European Communities: Luxembourg, 1981; p 54. Appel, B. R.; Hoffer, E. M.; Tokiwa, Y.; Kothney, E. Atmos. Environ. 1982,16,589. Shaw, R.W.; Stevens, R. K.; Bowermaster, J. Atmos. Environ. 1982,16, 845. Smith, B. M.; Wagman, J.; Fish, B. R. Enuiron. Sei. Technol. 1969,3,558. Stevens, R. K.; Dzubay, T. G. Atmos. Enuiron. 1978,12, 55. Ferm, M.Atmos. Environ. 1979,13,1385. Dasgupta, P.K. Atmos. Environ. 1984, 18, 1593. Dasgupta, P.K.; McDowell, W. L.; Rhee, J. S. Analyst (London) 1986,111, 87. Aoki, T.; Munemori, M.Anal. Chem. 1983,55,209.
Environ. Sci. Technol. 1980, 22, 565-570
(21) Tan, C. W.; Hsu, C.-J. J. Aerosol Sci. 1971, 2, 117. (22) Tan, C. W. Heat Mass Transfer 1969, 12, 471. (23) Sparrow, E. M.; Lin, S. H.; Lundgren, T. S. Phys. Fluids 1964, 7, 338. (24) Finlayson, B. A. In Finite Elements in Fluids; Gallagher, R. H., et al., Eds.; Wiley: London, 1975; Vol. 1, p 1. (25) Becker, E. B.; Carey, G. F.; Oden, J. T. Finite Elements: An Introduction; Prentice-Hall:Englewood Cliffs, NJ, 1981; VOl. 1, p 10.
Aoki, T.; Uemura, S.; Munemori, M. Anal. Chem. 1983,55, 1620.
Hallowell, A.; Pacey, G. E.; Gordon, G. Anal. Chem. 1985, 57, 2851. Kiani, A.; Bhave, R. R.; Sirkar, K. K. J.Membr. Sci. 1984, 20, 125. Carlson, R. M. Anal. Chem. 1978, 50, 1528. Dasgupta, P. K. Anal. Chem. 1984, 56, 96. Davies, C. N. J. Aerosol Sci. 1973, 4, 317. Gormley, P.; Kennedy, M. Roc. R. Zr. Acad., Sect. A 1949, 52A, 163. Ingham, D. B. J. Aerosol Sci. 1975, 6, 125.
Received for review September 8,1986. Accepted November 3, 1987.
Development and Evaluation of a Procedure fo'r Determining Volatile Organics in Water Larry C. Michael and Edo D. Pelllzzari" Research Triangle Institute, Research Triangle Park, North Carolina 27709
Roger W. Wlseman National Institute of Environmental Health Sciences, Research Triangle Park, North Carolina 27709
organic compounds from various waters was developed through the use of representative volatile organic compounds as part of the EPA Master Analytical Scheme. Preliminary recoveries were determined for a broad range of analytes in distilled water, municipal wastewater effluent, and industrial (energy-related) wastewater effluent. The apparatus for isolation of the compounds from water was interfaced directly to a gas chromatograph/mass spectrometer/computer, and recoveries were determined for compounds comprising different functional groups in drinking water and municipal/industrial wastewater.
per sample and was inherently freer from interferences than closed-loop gas stripping analysis. In particular, the carbon disulfide used for desorption of the carbon trap in CLSA chromatographically obscured early-eluting analytes. Consequently, extensive method optimization was performed only on the purge and trap method. This was accomplished with 14C-labeledcompounds and is too extensive for inclusion here (2). This paper addresses method refinement, interface of the purge and trap apparatus to the GC/MS/COMP, and the method evaluation. Nonradiolabeled purgeable compounds, encompassing a broad range of compound types, were employed for method evaluation.
Introduction
Experimental Section
Analysis of organic compounds in water has traditionally been approached by selecting analytical methods on the basis of the target compounds. Examples of such an approach are the 600 series methods presented in the Federal Register (1). Alternatively, broad spectrum analytical methods provide a comprehensive assessment of all sample components. This philosophy was embraced by the U.S. EPA "Master Analytical Scheme" program (2). Analytical methods developed under this program were designed to encompassall gas chromatographable compounds with the fewest analytical methods. The overall approach to development of a comprehensive analytical scheme for purgeables in water was to employ gas chromatography/mass spectrometry/computer (GC/MS/COMP) in conjunction with an isolation technique which would include the volatile (bp < 150 "C), water-insoluble compounds and also many compounds considered to be semivolatile and semi water soluble. An additional objective was to detect and quantitate these compounds at 0.1 pg/L in drinking water, 1pg/L in surface waters, and 10 pg/L in effluent waters. On the basis of a comprehensive review of the current literature (3),only two techniques possessed adequate sensitivity to warrant experimental investigation of their range of applicability: (1)purge and trap (4) and (2) closed-loop gas stripping (Grob) analysis (CLSA) (5-7). Preliminary evaluation of both of these methods revealed that the conventional purge and trap method had a significantly shorter analysis time
Method Refinement Studies. Recovery determinations with representative compounds followed the analytical protocol outlined below: (1)Organic-free water was prepared by purging deionized water for 2 h at 90 "C with 100 mL/min helium followed by continued purging while cooling to 30 "C. (2) Aliquots (200 mL) were transferred to purge flasks containing 60 g of anhydrous Na2S04and were shaken to dissolve the salt. (3) The 30% Na2S04solution was spiked with 5 pL of the appropriate standard mixture in methanol, mixed gently, and equilibrated at 30 "C for 10 mins. (4) After the Tenax GC cartridge was attached, 500 mL of helium was passed through the solution at 25 mL/min. (5) Exposed cartridges were analyzed by thermal desorption gas chromatography with flame ionization detection (GC/FID) on either a 180 X 0.2 cm Tenax GC (80/100 mesh) packed column or a 65 m X 0.5 mm SE-30 WCOT column. (6) Recoveries were calculated by comparison to standard cartridges using electronic peak height measurements. (7) In experiments involving municipal and energy effluent samples, 20 mL of effluent was combined with 180 mL of distilled water prior to introduction into the purge flask. Evaluation of an Integrated GC/MS/COMP Purge and Trap System. A purge and trap system (Figure 1) consisting of a purge and trap module (A), an injection
A comprehensive procedure for isolation of volatile
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