219
Anal. Chem. 1985, 57,219-223
CONCLUSIONS The thermally desorbablepassive air monitor discussed here has several advantages over commercial PSDs designed for volatile organic chemicals. Sampling rates are equivalent to those of the most commonly used commercial devices, which employ activated charcoal as the sorbent (7). However, a major advantage of this PSD over the charcoal-based commercial devices is the greatly enhanced sensitivity achievable by thermal desorption. When solvent desorption is required, less than 0.5% of the collected sample can be analyzed (e.g., 5 pL of 1mL of extraction solvent). Thermal desorption and analysis of the entire sample affords a 200-fold (or greater) increase in sensitivity, permitting the measurement of ppb concentrationswith less than 1-h exposure times. On the other hand, while 24-h exposures are possible for most volatile organic compounds for charcoal-based PSDs, the thermally desorbable PSD may be restricted to short sampling times for the more volatile chemicals. When long sampling periods are desired, it may be possible to add additional diffusion barriers to reduce the effective sampling rates. In fact, since far more sample than needed is usually collected even at ppb levels, sampling rates could be reduced without significantly affecting analytical sensitivity. The use of other thermally desorbable sorbents such as Spherocarb (Analabs, North Haven, CT) may also permit extended exposures. The anticipated effectof air velocity across the face of the PSD on the sampling rate was found to be essentially correct, and the devices should not be used under conditions where air flow is consistently below about 15 cm/s. Indeed, all passive devices are subject to boundary layer effects and should not be used under quiescent conditions. No apparent effects of high wind velocities (up to 900 cm/s) have been observed in actual field exposures (1).However, the correction factor (eq 6b) approaches unity as the air velocity increases. Therefore, at 900 cm/s, f(ve/l) = 0.93 and the concentration C, of a gas at the face of the PSD should be 93% of its true concentration C,. When Tenax GC or other hydrophobic sorbents are used, the PSD is not affected by high atmospheric humidities. Charcoal-based devices have been found to suffer from greatly
reduced sampling rates at 80% RH and above (2). Since PSDs cannot be "turned off", care must be exercised to assure hermetic sealing of the devices before and after exposure and to minimize exposure in the laboratory. For field studies conducted by these investigators, the PSDs have been sealed in small jars or cans, which are stored and transported in a larger can or jar containing activated charcoal to prevent contamination. The devices should be packaged and unpackaged in a glovebox under a clean atmosphere. Exposure to the laboratory air prior to placement in the desorption oven should be limited to a few seconds.
ACKNOWLEDGMENT We thank J. E. Strobe1 and J. V. Pustinger, formerly of Monsanto Research Corp., for their contributions to this work.
LITERATURE CITED (1) Wooten, 0. W.; Strobel, J. E.; Pustinger, J. V.; McMillin, C. R. "Passive Sampling Devlce for Ambient Air and Short-Term Personal Monitoring", Proceedings of the 3rd National-Symposium on Recent Advances in Pollutant Monitoring of Ambient Air and Stationary Sources, Raleigh, NC, May 1983; U S . Environmental Protection Agency: Research Triangle Park, NC, 1984; Report 600/4-84-050. (2) Lewis, R. G.; Coutant, R. W.; Wooten. G. W.; McMillln, C. R.; Mullk, J. D., paper presented at the 1983 Spring National Meeting of the American Institute of Chemlcal Englneers, Houston, TX, March 1983. (3) Coutant, R. W.; Lewis, R. G.; Muiik, J. D. Anal. Chem., following paper in this Issue. (4) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. "Transport Phenomena", Wlley: New York, 1960; Chapters 4 and 19. (5) Schllchting, H. "Boundary Layer Theory"; McGraw-Hill: New York, 1955. (6) Lugg, G. A. Anal. Chem. 1966, 4 0 , 1072-1077. (7) Coutant, R. W.; Scott, D. R. Environ. Sci. Techno/. 1982, 16, 4 10-4 13.
RECEIVED for review June 20,1984. Accepted September 26, 1984. Although the research described in this article was funded wholly or in part by the U.S.EnvironmentalProtection Agency through Contracts 68-02-3469 and -3487, it has not been subjected to Agency review. Therefore, it does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Passive Sampling Devices with Reversible Adsorption Robert W. Coutant* Battelle Columbus Laboratories, Columbus, Ohio 43201 Robert G.Lewis and James Mulik Environmental Monitoring Systems Laboratory, U S . Environmental Protection Agency, Research Triangle Park, North Carolina 27711 The mechanlcs of passlve sampllng utilizing thermally reverslble adsorptlon are discussed. A general model relating sampllng rates to device deslgn and spectflc sorbate/sorbent properties Is developed, and a slmpllfled thin-bed model Is presented. Results of a laboratory evaluation of a thln-bed passlve dosimeter employlng Tenax GC as the sorbent for sampllng of 17 volatile organlc compounds are used to Iiiustrate the appilcabliity of the model. The results provlde guldelines for the design and use of passlve monitors employing reverslble adsorptlon.
Most commercially available passive sampling devices 0003-2700/85/0357-0219$01.50/0
(PSD) for volatile organic compounds employ activated carbon. With this sorbent, the sorption process is thermally irreversible (in a practical sense) and solvent desorption is normally used to recover the sorbates. This process is satisfactory for a variety of industrial hygiene applications for which these devices were originally intended. However, the use of these devices for sampling of ambient air concentrations (0.1-10 ppvb) can impose severe limitations on the analytical techniques (I).Passive devices which employ thermally reversible adsorption, on the other hand, offer several advantages particularly well suited to low level sampling and analysis of volatile organics. These include (1)independent from solvent contamination, (2) increased sensitivity because of the availability of the whole sample for analysis, and (3) more 0 1984 American Chemical Society
220
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
m_ Diffusion Barrier
1
Sorbent
tA
c,
b
a
where V, is the familiar gas chromatographic (GC) retention volume for the sorbate/sorbent pair (8). The rate of change of C1is then given by
( 2 ) $)
0
Flgure 1. Coordinate representation for passive air sampling devices.
rapid sample turnaround. However, because of the reversible nature of the sorption process, the sampling behavior of these devices differs from the ideal case (normally assumed for carbon-based devices), and failure to recognize the differences can lead to biases in sampling and interpretation. In this paper, we discuss the mechanics of sampling with reversible adsorption and present a simple model for calculating sampling rates. This model provides guidelines for proper design and application of passive monitors employing reversible adsorption. The performance of a particular device, the EPA PSD (2,3),is used to illustrate the consequences of both proper and improper applications of the fundamental principles.
SAMPLING MECHANICS There are currently two passive monitor designs which depend on reversible adsorption. Both of these devices use Tenax GC as the sorbent, and their major difference is in the thickness of the sorbent bed; the EPA device is a large area system having a thin bed of sorbent, while the device developed by Brown and co-workers (4) is a small face area, thick-bed system. Recently, Moore et al. have discussed their efforts to develop a passive monitor that is similar to the EPA system (5). The fundamental mechanics of sorption are the same for these two systems, but the thin-bed system is subject to simplifications that more readily obviate the significance of the key physical parameters. We, therefore, choose to emphasize the mechanics of the thin-bed system but will comment on the relation of the thin-bed results to observations made by Brown. Fundamentals. In considering the mechanics of diffusion-limited, reversible adsorption, there are a variety of cases that can be developed depending on choice of boundary conditions, the assumed importance of the kinetics of adsorption and desorption, and the time resolution that is desired. For the purposes here, we assume that kinetics of sorption are not important, i.e., the sorbate is in equilibrium with the sorbent at each point within the bed, that the sorption isotherm is linear over the concentration range of interest, that very short-term transients are not of interest, and that synergistic effects due to the presence of multiple sorbates are negligible. These assumptions are reasonable for the case of multiple-hour sampling of ambient level concentrations of organic vapors, and the reader is referred to the comprehensive works of Crank (6) and Carslaw and Jaeger (7) for consideration of other cases. The general problem can be expressed in terms of the coordinates and parameters represented in Figure 1. In Figure 1,zone 2 (a-b) represents the diffusion barrier or entry zone for the device and zone 1 (&a) is the sorbent bed. The mass transport of material into the sorbent bed is represented by ni. The overall face area of the device is A, and the porosities of the two zones are el and e2, and D2 is the gas-phase diffusivity of the sorbate. The bed density is the sorbent weight, W ,divided by the bed volume, aA. The vapor concentration outside of the device is C,: Within the device concentrations in zones 1 and 2, C1 and C2,are functions of time and position. The weight of sorbate per gram of sorbent, W,, at any point in zone 1 can be represented as
-
= D2(
(z)
(2)
Using the value of W, given by eq 1and the definition of the bed density, p ,
(2) $) = D2(
-
or
(2) =
O2 wvb 1+-
elaA
z(2 )
(3)
(5)
(4)
Now, for any system having a practically significant sorption capacity
or
( 2 ) 2) = Dl(
where
D1 =
D2e,aA
-
(7)
wvb
That is, transport within the sorbent bed obeys Fick’s second law but with an effective diffusivity that is greatly reduced from the diffusivity in the entry region of the device. Because of this slowness of change within the sorbent bed in comparison to the rate of change possible in the entry region (for typical systems D 2> 4000D1), transport in zone 2 can be treated as a pseudo-steady-state process. The mass transport to the sorbent is thus given by Fick’s first law:
We have shown elsewhere (2, 3) that a factor is needed to correct eq 8 for the effect of air velocity on the boundary layer resistance. This factor is relatively constant for air velocities in the range of 20-200 cm/s and is omitted here for the sake of brevity. However, this correction factor is included in calculated values of sampling rates given in the Results section of this paper. Equation 8 is normally used with C, = 0 for passive devices having irreversible adsorption. With reversible adsorption, C, is a variable that is determined by the solution to eq 6. To find this solution, we recognize that there is no mass gained or lost at boundary a, and there is no mass flow across the boundary at x = 0
ezD2 b-a
= -(C, x=a
-
C,)
(9)
and at x = 0
(2)x=o =O
The Laplace transform method, or other methods illustrated
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
by Carslaw and Jaeger (7), can then be used to solve for C,, yielding
where 62D2 f f = @i(b
- a)
and
0, t a n Pna = a For practical applications, we are really interested in the effective sampling rate of the device. When C, = 0 (i.e., a t t = 0), the initial sampling rate, Ro,is dependent only on the physical parameters of the device and can be taken from eq 8
At t > 0, the instantaneous relative rate, RfRo,is proportional to (C, - Ca)/C,. However, in normal usage, it is the timeaveraged sampling rate R / R o that is relevant. This is given by
x t ( C m- C,)/C, d t
R/Ro =
(13)
Thus, we find
Equation 14 correctly describes the variation of sampling rate with time for passive samplers employing reversible adsorption, but it does not make readily apparent the relationships between device parameters such as Ro, V,, and W and the sampling rate. A much simpler expression can be gained by considering a sorbent bed that is sufficiently thin that transport within the bed is unimportant; Le., that the total bed is at equilibrium with the boundary concentration, C,. Under these conditions, most of the mass flow (eq 8) is used to increase the loading of the bed on a more or less uniform basis, or
and
where
k = Ro/ WV, (18) In a very simple and direct manner, eq 17 provides guidelines for design and application of PSDs employing reversible adsorption: the value of k for a given sorbatefsorbent pair must be small enough that the sampling rate does not change significantly with time. A small value of k also minimizes the sensitivity of the device to the adverse effects of variable exposure conditions. A changing rate per se is not objectionable as long as it is known how it is changing, but a severely changing rate coupled with a strongly variable
221
exposure condition can introduce bias in the overall interpretation of the results. EXPERIMENTAL SECTION Materials and Methods. The EPA passive sampling device was developed by the Monsanto Research Corp. for passive monitoring of ambient levels of volatile organic compounds (9). The PSD consists of a cylindrical stainless steel container with a double layer of fine-mesh screen and perforated stainless steel backing plates fitted to each end of the cylinder. The central volume between the screens is sufficient to accommodate 0.4 g of Tenax GC. The face area of the device is 7.07 cm2;the effective porosity of the diffusion barrier is 0.55; the bed depth is 0.35 cm; the effective e z / ( b - a ) is 1.37. The device can be used as a two-sided sampler, or a cap can be placed over one face to permit single-sided sampling. The EPA PSDs were exposed in triplicate to various mixtures of volatile organic compounds including chloroform, l,l,l-trichloroethane, carbon tetrachloride, trichloroethylene, tetrachloroethylene, benzene, chlorobenzene, acrylonitrile, 1,l-dichloroethylene, trichlorotrifluoroethane, 1,2-dichloroethane, trans-1,3-dichloropropene,toluene, 1,2-dibromoethane,o-xylene, a-chlorotoluene,and hexachlorobutadiene. These exposureswere conducted for various periods of time at concentrations in the range of 1-10 ppbv within the Battelle passive dosimeter test facility. The latter facility consists of a well-stirred 200-L glass chamber, with associated analytical instrumentation and device-handlingequipment. The chamber is normally operated in a dynamic mode with sufficient make-up gas being added to compensate for gas removed for analysis of component concentrations. PSDs were held under a protective clean nitrogen atmosphere at all times except during the exposures and were inserted into and withdrawn from the chamber through a special interlock system designed to guard against contamination (9). Sampling and analysis of the gas within the chamber was accomplished by at least two of three modes available, depending on the nature of a given experiment. The chamber was coupled to a capillary column gas chromatograph equipped with flame ionization and electric capture detection via a heated sampling line and gas sampling loop. The loop could be used for either direct gas injection into the GC or in a cryogenic preconcentration mode. Sampling of the chamber was also accomplished by using actively pumped Tenax GC traps that were subsequentlyanalyzed by thermal desorption into the GC. In most of the experiments, several gas samples were taken and two Tenax GC traps were simultaneously used to sample the chamber gas during the course of a device exposure. Both sets of analytical data provided information on the time-weightedaverage concentrationsthat were used to determine the apparent sampling rates of the PSDs. The Tenax GC traps and the PSDs were desorbed and analyzed by essentially the same procedures: thermal desorption, with the sample then being captured in the cryogenic sampling loop cited above, and subsequent flash injection into the GC, using conventional cryofocusing techniques to load the sample onto the column. The FID was used as the primary detector, but about 5 % of the column effluent was diverted to the ECD to provide additional qualitative identification of the sample components.
RESULTS
’
Inasmuch as sampling rates for the test chemicals had not previously been determined with the EPA PSD, all of the chemicals cited above were used in short-term (1-2 h) exposure tests to determine the rates in comparison with those calculated by using the physical parameters of the device and eq 17. In these tests, the device was used as a double-sided sampler (bed thickness per side = 0.17 cm). Subsequently, longer term exposures were conducted using the last ten of the chemicals noted above for more detailed evaluation of device performance in comparison with model predictions. Diffusion coefficients, retention volumes, and calculated 1-h time-averaged sampling rates are shown in Table I. Results of the short-term exposures are illustrated in Figure 2. It can be seen from this figure that most of the calculated results agree quite well with those measured experimentally. The
222
:pl,,,
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
E 60 -
.-c
i”’
30
l;
10
0
0
0
40
60
RIOO,*
cm2/min cm3/min
8
1 0 1 2 14 16 1 8 2 0 22 2 4
RO = 94.7 cc/min vb = 4.9 k 0.8 L/q SDEV = 2.8 cc/min
.E 60
E
t, hr Flgure 4. Decline in sampling rate for acrylonitrile as a function of time.
Table I. Calculated EPA PSD Sampling Rates
chloroform l,l,l-trichloroethane carbon tetrachloride trichloroethylene tetrachloroethylene benzene chlorobenzene acrylonitrile 1,l-dichloroethylene trichlorotrifluoroethane 1,2-dichloroethane trans-1,3-dichloropropane toluene 1,2-dibromomethane o-xylene a-chlorotoluene hexachlorobutadiene
6
4
80
Flgure 2. One-hour EPA-PSD sampling rates chemical key: (1) 1,ldichloroethyiene,(2) acrylonitrile, (3) trichiorotrifluoroethane, (4) 1,2dichloroethane, (5) chloroform, (6) l , l , 1-trichloroethane, (7) carbon tetrachloride, (8) benzene (9) 1,24ibromoethane, (10) trafls-1,3-dichloropropene, (11) toluene, (12) chlorobenzene, (13) hexachlorobutadiene, (14) o-xylene, (15) trichloroethylene, (16) tetrachioroethylene, and (17) a-chlorotoluene.
Di,O
2
t, hr
R p Predicted, cc/min
compound
vb 0.5 f 0.08 L/q SDEV = 1 . 3 cc/min
Flgure 3. Decline in sampling rate for trichlorotrifluoroethane as a function of time.
80
20
R O = 6 2 . 1 cc/min
-
20
J
20
80
R,,d
Vb,‘ L/g
cm3/min
5.33 4.76 4.97 5.25 4.97 5.59 4.48 6.35 5.51 (4.16) 5.44 4.76
79.5 71.0 74.2 78.3 74.2 83.4 66.8 94.7 82.2 62.1 81.2 71.0
18.9 11.8 16.5 39.3 154.0 42.5 372.0 0.3-7 2-6 0.23-0.47 24.4 335.0
59.0 46.8 53.9 67.7 71.5 72.2 66.0 23.0 25.0 2.0 63.9 69.9
5.09 (4.66) 4.36 4.28 (3.3)
75.9 69.5 65.1 63.9 49.2
193 183.0 2800.0 1984.0 324.0
73.7 67.6 64.9 63.7 48.7
oop
pc‘ $ 4 0 20
-
0-1
Ro = 8 1 . 2 cc/min vb = 18 k 4 L/q SDEV = 3.3 cc/min ’X Y
I l l I l l I
,
I I I I I I I I I I l i l l l T ,
t. hr
Figure 5. Decline in sampling rate for 1,2dichloroethaneas a function of time.
found similarly poor results when sampling with other passive dosimeters but have no further explanation of the observations with either this chemical or tetrachloroethylene. Chemicals used in the longer term tests were chosen to provide a mix of those for which the PSD was expected to work well and those for which the PSD was expected to display significantly variable sampling rates as a function of time. Exposures were conducted for time periods of 0.25,0.5,1,2, 4, 12, and 24 h. Results of these tests expressed as median values of the measured sampling rates are shown in Table 11. Although there is some scatter in the data, the variation is generally not significant with respect to the expected experimental error, *lo%. As predicted by eq 17, the apparent sampling rates of the very volatile compounds decline sharply as the sampling time is increased, while the rates for the less volatile compounds decline only slowly. A better picture of the correspondencebetween eq 17 and the experimental data can be gained by consideration of Figures 3-6. These figures show experimental points and curves calculated by using eq
Diffusion coefficients taken from Lugg (IO)except for estimated values given in parentheses. bCalculated sampling rates at zero time and face velocity of 100 fpm (50.8 cm/s). ‘Retention volumes of Tenax GC taken from Krost et al. (11) except where range of values is indicated. Time-weighted-average sampling rates (cm3/min) calculated for 1-h sampling times.
principal outliers in Figure 2 are tetrachloroethylene, carbon tetrachloride, and acrylonitrile. For the latter chemical, a wide range of retention volumes (0.2-7 L/g) is cited in the literature, and a mean value of 3.6 L/g was chosen for the calculated rate. Good agreement for this chemical can be obtained by choosing a value of 4.9 L/g. In the case of carbon tetrachloride, we have
Table 11. TWA Sampling Rates of EPA-PSDS (Median Values, cm3/min) chemical
00
0.25
0.5
acrylonitrile 1,l-dichloroethylene trichlorotrifluoroethane 1,2-dichloroethane trans-1,3-dichloropropene toluene 1,2-dibromomethane o-xylene a-chlorotoluene hexachlorobutadiene
94.7 82.2 62.1 81.2 71.0 75.9 69.5 65.1 83.9 (42.0)b
72.3 37.9 10.7 95.8 86.2 75.5 80.3 74.8 63.0 40.5
50.8 19.8 7.3 67.0 71.0 71.2 61.0 70.3 73.8 40.3
nEstimated (see Table I). bEstimated from current data.
averaging period, h 1 2 26.9 10.1
3.7 55.9 66.7 65.9 64.7 57.2 62.0 41.3
16.2 4.5 3.1 41.4 53.6 45.0 48.1 62.5 68.2 41.9
4
12
9.3 2.7 2.3
2.4 0.5 1.0 11.9 54.3 34.9 57.7 44.1 45.8 (27.1)
21.2
74.8 43.0 60.8 75.2 72.7 40.4
24 1.1
0.2 0.7 6.9 32.9 25.4 28.5 55.3 31.4
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
20 0
x
(XI
-
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
Flgure 6. Decline in sampling rate for hexachlorobutadiene as a function of time.
17. Conversely, retention volumes derived from the experimental data correspond well with values cited for these compounds in the literature.
DISCUSSION The results shown above emphasize the need for clear understanding of the mechanics of sorption when using Tenax GC or other sorbents displaying thermally reversible adsorption in a passive sampler. It is evident that the simple thin-bed model adequately represents performance of the EPA PSD, and the ratio of parameters given by eq 18 provides the key to proper design and usage of these types of devices. For example, the EPA PSD, in its present form, cannot be used to sample compounds having retention volumes less than about 30-40 L/g in multiple-hour exposures. To improve performance for such compounds, either the retention volume must be increased by utilizing a different sorbent or the sampling rate must be reduced by altering the diffusion barrier. We are currently pursuing both of these approaches. The alternative approach of increasing the weight of sorbent is not possible without alteration of the physical dimensions of the device. The device investigated by Brown (4) achieves both increased weight of sorbent and lower sampling rates by making the device long and thin (pencil-shaped). This approach is not satisfactory, however, as we can readily see from consideration of the form of eq 11. This equation indicates that the sorbate will tend to accumulate initially in a relatively thin zone near the leading edge of the sorbent bed; i.e., the major portion of the sorbent bed will not become available until the leading edge becomes well loaded. This means that the sampling rate will decline well before the total capacity of the bed can be utilized. This conclusion is verified by
223
Brown's measurements. Indeed, Brown suggests the use of two different sampling rates. An initial rate is to be used to represent the average rate during the initial buildup of sorbate on the leading edge of the sorbent; a secondary rate is to be used during the later, bed-diffusion-controlled sampling process. Although eq 14 could be used to more correctly represent the performance of the long, thin sampler, we conclude that the general shape of the EPA PSD is preferable because of more effective utilization of the sorbent.
ACKNOWLEDGMENT We thank G. W. Keigley and R. H. Barnes for their assistance and helpful discussions during the course of this work.
LITERATURE CITED (1) Coutant, R . W.; Scott, D. R. Environ. Sci. Techno/. 1982, 16, 410-413. (2) Lewis, R. G.; Mulik, J. D.; Coutant, R. W.; Wooten, G. W.; McMillin, C. R., paper presented at the 1983 Spring National Meeting of the American Institute of Chemical Engineers, Houston, TX, March 1983. (3) Lewis, R. G., Mullk, J. D., Coutant, R. W.; Wooten, G. W.; McMlllin, C. R. "Passive Sampling Devices for Organic Vapors in Ambient Air", Proceedings of the 3rd National Symposium on Recent Advances in Pollutant Monitoring of Ambient Air and Stationary Sources, Raleigh, NC, May 1983; U.S. Environmental Protection Agency: Research Triangle Park, NC, January 1964; Report EPA-600/9-84-001. (4) Brown, R. H.; Walkin, K. T. Proceedlngs of the Fifth International SAC Conference, Cambridge, England, May 1981, pp 205-208. (5) Moore, G.; Steinle, S.; Lefebre, H. Am. Ind. Hyg. Assoc. J . 1984, 4 5 , 145-153. (6) Crank, J. "The Mathematics of Diffusion"; Clarendon Press: Oxford, 1956. (7) Carslaw, H. S.; Jaeger, J. C. "Conduction of Heat In Solids", 2nd ed.; Clarendon Press: Oxford, 1959. (6) Wailing, J. F.; Berkley, R. E.: Swanson, D. H.: Toth, F. J. "Sampling Air for Gaseous Chemicals Using Solid Absorbents, Application to Tenax": Environmental Protection Agency: Research Triangle Park, NC, 1982; Report EPA 60015-4-82-059. (9) Lewis, R. G.; Mulik, J. D.; Coutant, R. W.; Wooten, G. W.; McMiiiin, C. R. Anal. Chem., preceding paper in this issue. (10) Lugg, G. A. Anal. Chem. 1968, 4 0 , 1072-1077. (11) Krost, K. I.; Pellizari, E. D.; Walburn, S. G.; and Hubbard, S. A. Anal. Chem. 1982, 5 4 , 810-817.
RECEIVED for review June 20,1984. Accepted September 26, 1984. Although the research described in this article was funded wholly by the US. Environmental Protection Agency under Contract 68-02-3487, it has not been subjected to Agency review. Therefore, it does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
