274
Anal. Chem. 1984, 56, 274-278
(7) Noden, F. G. I n “Lead In the Marine Environment”; Branica, M., Konrad, Z.,Eds.; Pergamon Press: Oxford, 1980; pp 83-91. (8) Birnie, S. E.; Hodges, D.J. Environ. Techno/. Left. 1981, 2, 433-442. (9) Potter, H. R.; Jarvie, A. W. P.; Markall, R. N. Water follut. Control. (Maidstone, Engl.) 1977, 123-128. (10) Gross, S . 6.; Parkinson. E. S . A t . Absomr. News/. 1974, 13. 107-108. (11) Murthy, L.; Menden, E. E.; Eller, P. M.; Pertering, H. G. Anal. Blochem. 1973, 53, 365-372.
(12) Jarvie, A. W. P.; Markall, R. N.; Potter, H. R. Environ. Res. 1981, 25, 241-249. (13) Chau, Y. K.; Wong, P. T. S.; Bengert, G. A,; Kramar, 0. Anal. Chem. 1979, 51, 188-188.
for review August 8, 1983* Accepted October 5, 1983.
Nonreactive Coadsorption of Solutes on a Sampling Adsorbent Karen J. Hyver and Jon F. Parcher* Chemistry Department, The University of Mississippi, University, Mississippi 38677
The effect of a domlnant component In an alr sample on the sampling selectivity and capaclty of an adsorbent was Investlgated. Gas-solid equlllbrlum Isotherms were determlned for benzene on a common sampling adsorbent (Carbopack C) at 10, 30, and 50 ‘C. The effect of adsorbed benzene on the adsorptlon of other sample components was studled by measuring the retentlon volumes of lnflnltely dilute samples of n-pentane, acetone, nltromethane, propanol, and tetrahydrofuran. The results showed that adsorbed benzene caused both enhanced and dlmlnlshed retentlon of the other solutes depending upon the surface coverage. At low coverage the retentlon volumes of the lnflnltely dllute samples decreased with Increased benzene adsorptlon. At fractional surface coverage from 0.3 to 0.5 the retentlon volumes Increased slgnlficantly with the amount of benzene adsorbed due to lateral lnteractlons between benzene and the solute In the condensed phase. Monolayer formatlon was accompanled by a drarnatlc decrease In the retentlon volumes of all of the solutes.
Several different types of solid sorbents, such as charcoals, polymers, and graphitized carbon blacks (GCBs), are commonly used for analytical sampling (preconcentration) of large volume samples of gases and liquids. Many standard protocols call for this type of sampling, and the method has proven successful for a wide range of sample types. However, many analysts have noted that this preconcentration step is often the limiting factor which determines the validity and accuracy of the complete analysis. The composition of the concentrated sample must accurately reflect the composition of the original (large volume) sample, or else the analysis is of questionable utility no matter how precise the subsequent analytical procedures might be. For this and other reasons, it is desirable to investigate and, if possible, quantitatively measure the physicochemical interactions of sample components in the condensed phase on the surface of a sampling sorbent. These interactions along with the finite monolayer capacity of the sorbents allow one component of a sample mixture to significantly influence the collection efficiency and capacity of the sorbent for a different component. In many cases samples may contain one or more dominant, i.e., high concentration, components, such as water, carbon dioxide, or hydrocarbons. The effect of these dominant components on the adsorption of other components, especially at trace levels, is of particular interest to the analytical chemist. Graphitized carbon black adsorbents, such as Carbopack and Carbotrap, are commonly used for such sampling, and they are of particular significance because of the homogeneity
of the surface of the solids. Bertoni et al. (1)studied the effect of vapor concentration of one component on the breakthrough volumes of other components in a synthetic sample of polluted air on GCB. The general observation was that the breakthrough volumes of any component decreased exponentially with increasing vapor concentration. The study covered a concentration range of 1-300 ppm in the gas phase, and no attempt was made to determine the amounts of any component actually adsorbed on the adsorbent surface. Other studies (2-5) have shown that one component adsorbed on a surface may actually enhance the adsorption or retention of a second component. It has also been shown that very complex, sometimes bizarre, behavior is observed for the adsorption and retention of multicomponent vapors on these adsorbents. For example, preadsorbed propane diminished the retention of butane on GCBs at very low surface coverage with propane (4). At high coverages (20--80% of a monolayer) the adsorbed propane enhanced the retention of small samples of butane. At full monolayer coverage with propane the retention of butane diminished significantly to a relatively small value. Similar effects have been observed for other systems, including GCBs with a nonvolatile liquid deposited on the surface (6, 7). Enhanced or “cooperative”adsorption is usually rationalized as the result of lateral interactions (specific and nonspecific) between solutes in the condensed phase. However, this simple explanation is not sufficient to explain many of the observed effects, especially at low surface coverage. The magnitude of the enhancement is often very large even for solutes, such as hydrocarbons, for which the lateral interactions should be relatively small, and the enhancement effects have never been observed for breakthrough volume experiments, even a t low pressures. In the present study, multicomponent adsorption systems were studied with benzene as a dominant vapor component. Benzene was chosen because (i) the intermolecular interactions in the adsorbed phase should be nonspecific and weak and (ii) a good mathemptical isotherm model was available. Many authors (8)have mesured the gassolid equilibrium isotherms of benzene on GCBs with different surface areas. It was generally observed that adsorption was uniform (at given temperature and pressure) when normalized to a square meter base. The BET equation (9) was shown to accurately describe the adsorption isotherm and to demonstrate that the BET parameters, V , and C, were independent of temperature over a reasonable range.
EXPERIMENTAL SECTION The equilibrium isotherms were determined by mass spectrometric tracer pulse chromatography (10). This procedure is a variation of the classical tracer pulse methods in which non-
0 1984 American Chemical Society 0003-2700/84/0356-0274$01.50/0
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
1' -
Table I. BET Parameters for Benzene on the Graphitized Carbon Blacks
sorbent Carbopack B Carbopack C,
teomp, C
pmol/g
C
surface area, m'/g
50 30, 50
301 23.6
52 72
79 6.2
10, 30, 50 10, 30, 50
19.3
68
5.1
33.8
58
8.9
V,,
275
lot 1
Carbopack C, lot 2 Carbopack C, lot 3
radioactive tracers are employed. In this case, [2H8]benzene (Columbia Organic Chemicals) was used as the probe for the isotherm measurements. Two GC/MS systems were used for the study, Hewlett-Packard Models 5985 and 5995. The 5985 was used with no separator between the GC and the MS. The 5995 contained a membrane separator. No influence on the experimental results was observed for either the direct inlet valve or the membrane separator. Both mass spectrometers were operated in the selected ion monitor (SIM) mode to detect the deuterated benzene without responding to the natural benzene in the carrier gas. The carrier gases used were mixtures of helium and benzene of different compositions in which helium acted only as a gas phase diluent. The carrier gas composition was determined by comparison with standard gas mixtures on a separate gas chromatograph (Hewlett-Packard
0
1
0
2
0
3
0
4
0
5
0
bnzene Prerrun(torr)
Figure 1. Adsorption isotherm of benzene on Carbopack C at 30 OC: A, lit. value (8); 0, this work.
5840).
The elution solutes (n-pentane, acetone, nitromethane, tetrahydrofuran, and 1-propanol)were injected by syringe at a sample size of less than 1pmol per component. These solutes were also detected by the MS in the SIM mode. Neon was injected with the solutes, and the retention time of this inert gas was used as the dead-time of the system. The columns were made of l/g in. 0.d. stainless steel of various lengths up to 1m. Several different types and batches of GCBs were used. The two types were Carbopack B with an advertised area of ca. 100 m2/g and Carbopack C with an area of ca.10 m2/g (Supelco, Inc.). Two different lots of the Carbopack C were used. In one case a partial deactivation or decrease in surface area was observed in the course of the experiments. The change was irreversible and observed only for one lot. The altered batch was treated simply as a different lot with a slightly smaller surface area.
RESULTS AND DISCUSSION Accurate determination of the surface area of each of the different GCBs was necessary in order to normalize all of the retention and adsorption data. The surface areas were determined from the BET equation fit to each set of isotherm data for benzene on each lot of GCB at several temperatures. These parameters are given in Table I. The area occupied by an adsorbed benzene molecule has been reported as 40 A2 a t 20 "C by Avgul and Kiselev (8). However, McClellen and Harnsberger (11)recommended a value of 43.6 A2 for a wide variety of adsorbents and temperatures. The surface areas given in Table I were calculated from the V , values with an area of 43.6 A2 for benzene. These calculated surface areas were used to normalize all of the experimental data. Figure 1 is a plot of this normalized data for three of the adsorbents at 30 "C along with the data of Avgul and Kiselev (8). The line is the best fit to the BET equation for the entire data set. The present data are scattered compared to the averaged literature data; however, the agreement with the literature data is very good. The same type of agreement was observed for the data at 50 OC. Table I1 is a compilation of the adsorption and retention data for the combined Carbopack C adsorbents. Carbopack B was not investigated further because the large surface area caused inconveniently long retention times for most of the solutes. The isotherm data are shown in Figure 2 plotted with relative benzene pressure, P / P , where PO is the vapor
0.0
0.1
0.2
0.3
Re1 a t i ve
0.4
0.5
OS
Pressure
Figure 2. Adsorption isotherm of benzene on Carbopack C: 0,10 O C ; 0, 30 OC; A, 50 OC.
pressure of pure benzene. The plots coincide for the three temperatures investigated. Thus, a single BET isotherm equation with V , = 3.8 pmol/m2 and C = 70 is an accurate mathematical model for the entire data set. The effect of preadsorbed benzene on the adsorption of other solutes was investigated by measuring the retention volume of several polar and nonpolar solutes at infinite dilution as a function of the partial pressure of benzene in the carrier gas and the amount of benzene adsorbed on GCB. These data are also given in Table 11. The retention volume of these infinitely dilute samples is a measure of the limiting (P 0) slope of the isotherm of the solute on the benzenemodified surface. For systems with linear isotherms, these values can also be directly related to the breakthrough volumes commonly used to describe the sampling capacity of a sorbent for a given component.
-
276
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
0
2
Benzene
4
6
8
Adsor b e d (pmol/m2
)
Figure 3. Retention volume of acetone as a function of the amount of benzene adsorbed. Legend is the same as Figure 2.
The results for acetone as a solute are shown graphically in Figure 3. These results are typical for a weakly interacting system of solute and modifier. The retention volume of acetone generally decreased with increased temperature and increased amounts of adsorbed modifier (benzene). However, the changes with surface coverage were far from simple, and the excursions were greatest at the lowest temperature. At each temperature there was a distinct maximum in the retention volume at a fractional surface coverage, 0, of 0.3-0.5. These maxima are more pronounced and occurred over a narrower range of 6' at the lowest temperatures. Another obvious feature of the plot is the sharp drop in the retention volumes at the point of completion of a monolayer (6' = 1 at 3.8 bmol/m2). In the cases where enough benzene was adsorbed to form more than two layers, the retention volumes subsequently increased with the amount of modifier adsorbed, and the system behaved more like a gas-liquid system. The same general phenomenon was observed for an alkane solute (n-pentane) as shown in Figure 4. The measured retention volumes were of much greater magnitude than those of acetone; however, the variation of the retention volumes with surface coverage by benzene was very similar for acetone and pentane. The same was true for the other solutes investigated, i.e., 1-propanol,nitromethane, and tetrahydrofuran. The magnitude of the bulk liquid contribution can be estimated from the measured activity coefficient of the solute in liquid benzene. The activity coefficients at 10 ' C available from the literature were 2.2 for n-pentane (12) and 1.7 for acetone (13-15). The bulk liquid contribution to the measured retention volumes was calculated from the equation V , (liq) = 273Rnade/Poy,where nads and y represent the moles of benzene adsorbed per square meter and the activity coefficient of the solute in benzene, respectively. The results are shown as the dotted line in Figure 3. At 0 > 3 the total retention volume of acetone is due to gas-liquid partition effects. On the other hand, the solubility of pentane in benzene is so low compared to the adsorption of pentane that no significant increase in the retention volume was observed. Benzene and GCB form an almost ideal adsorbate-adsorbent system. They are chemically similar; intermolecular interactions for the adsorbate are small; and the solid surface is very homogeneous. Thus, it is reasonable to assume uniform
(
Benzene Adsorbed pmol/m2
)
Figure 4. Retention volume of n-pentane vs. the amount of benzene adsorbed. Legend is the same as Figure 2.
0
I
c\ \
-
m 0
'
0
2
Benzene
4
6
8
I
A d s o r b e d (pmol/rnZ)
Figure 5. Retention volume of benzene vs. the amount of benzene adsorbed. Legend is the same as Flgure 2.
distribution of the adsorbate on the adsorbent at any surface coverage. This distribution isolates and maximizes the effects of lateral interactions between adsorbed benzene and adsorbed solute. In other words, the adsorption energy of a site adjacent to an occupied site is greater than for an isolated site. This is the classical explanation for the maximum observed in the retention (adsorption) of the solutes on a surface already 30-50% covered by adsorbed modifier. This interpretation is reasonable for these systems; however, the magnitude of the enhancement observed at the lowest temperature is surprisingly large. One additional factor to be considered at the lower temperature is the freezing point of benzene. The freezing point of the bulk liquid is 5.5 'C, and the freezing point of the liquid adsorbed on GCB may be considerably higher than for the bulk liquid (16). However, this enhancement phenomenon has been observed for many other systems far removed from
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
Table 11. Retention and Adsorption Data for Benzene and Several Solutes on Carbopack C retention volume, mL/m2 benzene tetranitroadsorbed, pentane hydrofuran methane propanol acetone pmol/m* re1 pressure
277
benzene
50 "C 0.000 0.002 0.004 0.011 0.029 0.071 0.084 0.110 0.147
0.00 0.56 0.96 1.93 2.65 3.21 3.39 4.01 4.15
0.95 1.02 1.04 1.02 0.75 0.48 0.46 0.41 0.32
0.57 0.60 0.65 0.71 0.63 0.56 0.53 0.45 0.41
2.21 2.81 3.09 2.05 1.43 1.04 0.99 0.77 0.74
2.84 3.37 3.60 3.21 2.95 1.73 1.55 1.32 1.08
7.66 6.90 6.09 6.34 3.77 1.57 1.22 1.02 0.72
25.5 15.9 13.5 11.0 6.24 3.02 2.51 2.15 1.78
4.81 4.26 4.25 4.43 3.58 2.61 2.45 2.40 1.64 1.59 1.97 1.16
5.54 7.02 6.82 6.61 5.49 4.32 3.09 2.65 2.59 2.73 1.79
16.1 12.8 14.6. 13.5 6.83 4.79 4.02 2.28 1.98 1.59 2.08 0.68
48.9 30.4 24.3 20.6 11.5 8.72 7.12 4.98 4.44 4.12 4.33 2.62
15.6 16.6 15.0 20.5 18.3 22.0 24.0 20.1 7.70 5.05 4.39 3.95 3.78 3.99 4.80
38.6 34.7 37.4 40.0 40.0 46.3 43.5 34.4 5.37 5.10 1.69 1.45 1.13 1.09 1.15
124 107 107 94.4 88.3 82.2 74.8 58.4 12.1 7.56 6.24 5.49 5.07 5.98 6.31
30 " C
0.126 0.143 0.151 0.265
0.00 1.01 1.40 1.59 2.85 3.24 3.71 3.82 3.93 4.13 4.60 4.86
1.82 2.14 2.13 2.03 1.09 0.91 0.88 0.70 0.72 0.64 0.67 0.52
1.09 1.43 1.31 1.42 1.10 0.94 0.99 0.86 0.94 1.00 0.83 0.87
0.000 0.0031 0.0033 0.0040 0.0044 0.006 0.008 0.012 0.125 0.231 0.356 0.416 0.480 0.512 0.538
0.00 0.96 1.01 1.06 1.13 1.56 1.59 1.89 4.05 4.68 5.94 6.11 6.51 7.45 9.08
4.18 4.20 3.45 5.51 4.75 5.63 5.58 4.55 1.43 1.25 1.16 1.08 0.97 1.09 1.50
6.18 2.68 2.76 2.98 2.90 3.37 4.98 3.53 2.79 2.22 2.05 2.17 2.44 2.54 3.44
0.000 0.004 0.008 0.011 0.036 0.055 0.071
0.110
10 "C
the melting point of the adsorbate. For example, significant cooperative adsorption was observed for several solutes with ethanol adsorbed on GCB at 30 "C,which is almost 150 "C above the freezing point of liquid ethanol. On the other hand, no enhancement at all is observed for deuterated benzene as a solute as shown in Figure 5. This is a clear illustration of the distinction between infiiite dilution retention volumes and breakthrough volumes. The retention volumes are determined by the ratio niadS/Piof the solute at the limit P 0, whereas the breakthrough volumes are determined by the same ratio at the pressure of the solute in the vapor. The retention volume of the deuterated benzene is determined by the ratio at the pressure of natural benzene in the vapor. Thus, it is actually a breakthrough volume rather than a retention volume. This is the operational definition for tracer pulse chromatography. In order to compare the conditions in these experiments with typical sampling conditions, the retention data for acetone were also plotted as a function of the concentration of benzene in the vapor sample in Figure 6. The results show that the observed cooperative adsorption effects occur in the range 200-1000 ppm, and that a monolayer is formed at ca. 2000-3000 ppm. These conditions are unique to this system at 10 "C; however, they are not atypical. Fortunately, these concentrations are considerably higher than those expected in an analytical sample (1-100 ppm).
20.0 12.7 13.0 14.5 17.9 19.0 11.3 6.78 3.83 2.82 2.75 3.34 2.89
-
c
i
0 1 10'
1
10'
10'
~ e n z a n e C o n c e n t r a t i o n (ppm)
Flgure 6. Retention volume of acetone as a function of the concentration of benzene in the vapor phase at 10 O C .
CONCLUSIONS One or more dominant components in a vapor mixture may significantly alter the sampling selectivity and capacity of the adsorbent for all of the other components in the sample. The selectivity may be enhanced or diminished for any particular component. However, enhanced adsorption and selectivity have only been observed for infinitely dilute vapor samples with a dominant component present at higher pressures than
270
Anal. Chem. 7984, 56,278-283
normally encountered in analytical samples. Conditions for such sample interference will, however, vary significantly with sample composition, type of adsorbent, and temperature. Because of these factors, the analyst must be aware of the possibility of distortion of the analysis caused by sample interference and the significance of the formation of a monolayer. A monolayer may be formed of one or several components; however, once a monolayer is formed, the adsorbent no longer functions as an efficient trapping material. The system will act as a vapor-liquid system with the sampling capacity determined by the liquid solubility properties of the sample components in the adsorbed component or mixture. Under normal sampling conditions the sample volume is limited to less than the breakthrough volumes of any of the components, and sample interference cannot affect the results. However, if these conditions are not met or if the desorption process is not 100% efficient, the possibility exists for severe distortion of the analytical results.
Registry No. Carbon, 7440-44-0;benzene, 71-43-2;pentane, 109-66-0; acetone, 67-64-1; nitromethane, 75-52-5; propanol, 7123-8; tetrahydrofuran, 109-99-9. LITERATURE CITED (1) Bertonl, G.; Bruner, F.; Libertl, A.; Perrino, C. J. Chromatogr. 1981, 203, 263-270. (2) von Ryblnskl, W.; Findenegg, G. H. Ber. Bunsenges Phys. Chem. 1979, 83, 1127-1130.
(3) Parcher, Jon F.; Lin, Plng J. Anal. Chem. 1981, 53, 1889-1894. (4) Lin, Plng J.; Parcher, Jon F. J. Colloid Interface Sci. 1983, 91, 76-86. (5) Parcher, Jon F.; Hyver-LoCoco, Karen J . Chromatogr. Sci. 1983, 21, 304-309. (6) Di Corcia, A.; Liberti, A. "Advances in Chromatography"; Giddings, J. C., et al., Eds.; Marcel Dekker: New York, 1976; Vol. 14, pp 305-366. (7) Bruner, Fabrizio; Ciccioll, Paolo; Crescentini, Giancarlo; Pistolesl, Maria T. Anal. Chem. 1973, 45, 1851-1859. (8) Avgul, N. N.; Klselev, A. V. "Chemistry and Physics of Carbon"; Walker, Phillip L.. Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6. (9) Brunauer, S.;Emmett P. H.; Teller, E. J. Am. Chem. SOC.1938, 60, 309-319. (IO) Parcher, Jon F.; Selim, Mustafa I. Anal. Chem. 1979, 51, 2154-2156. (11) McClellan, A. L.; Harnsberger, H. F. J. Colloid Interface Sci. 1967, 23, 577-599. (12) Wang, Jack L. H.; Lu, Benjamin C.-Y. J. Appl. Chem. Biotechnol. 1971, 21, 297-299. (13) Schrelber, Loren B.; Eckert, Charles A. Ind. Eng. Chem. Process Des. Dev. 197f9 10, 572-576. (14) Thomas, Eugene R.; Newman, Bruce A.; Nicolaides, George L.; Eckert, Charles A. J. Chem. Eng. Data 1982, 27, 233-240. (15) Thomas, Eugene R.; Newman, Bruce A.; Long, Thomas C.; Wood, Douglas A.; Eckert, Charles A. J. Chem. Eng. Data 1982, 27, 399-405. (16) Parcher, Jon F.; Lin, Ping J. J. Chromatogr. 1982, 250, 21-34.
RECEIVED for review July 11,1983. Accepted October 25,1983. Acknowledgment is made to the National Science Foundation (Grant No. CHE-8207756) and to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
Solvatochromic Investigation of Polarizable Polymeric Liquids' James E. Brady, Dale Bjorkman, Christian D. Herter, and Peter W. Carr* Department of Chemistry] Smith and Kolthoff Halls, University of Minnesota] Minneapolis, Minnesota 55455
The polarlty characterlstlcs of methyVphenyl OV type GLC stationary phases (OV-101, -3, -7, -11, -17, -22, -25) and low molecular weight analogues (hexamethyldlslloxane, octamethyltrlslloxane, decamethyltetraslloxane, and bls(tr1methyls1lyl)methane) are examined via the solvatochromlc comparison method. As expected, the differentlal polarity of the OV llqulds Is due In large measure lo differential polarizablllty effects and Is well correlated wlth a reactlon fleld based model. I n addltlon, a nearly constant low level of hydrogen bondlng acceptor strength Is observed. The presence of measurable hydrogen bondlng acceptance In these llqulds has Important lmpllcatlons to the Interpretation of retention of strong hydrogen bond donors on these stationary phases In GLC. Llkewlse, the lnteractlon of strong hydrogen bondlng solutes wlth the slloxane backbone of slllca gel In llquld chromatography may be a slgnlflcant, and generally neglected, effect. I t appears the detalled analysis of the orlgln of retention of strong hydrogen bond donors on these materlals may warrant reexamlnatlon.
represent solvent dipolarity-polarizability, hydrogen bond donicity, and hydrogen bond accepting strength, respectively. The relationship between the a* scale and the shift in frequency of maximum absorbance of a solvatochromic indicator is given by the equation v = vo sa* (1) where vo is nominally the indicator's frequency of maximum absorbance in cyclohexane and s is a measure of the sensitivity of a particular indicator to changes in solvent dipolar-polarizability. T o date the a* values for over 100 aprotic solvents, using a number of judiciously selected solutes from a set of 45 indicators (2), have been determined. No investigations of high molecular weight liquids have been undertaken to this point. In this work the Kamlet-Taft solvatochromic approach is applied, for the first time, to the study of a series of polymeric liquids (see Table I) of systematically varying composition. The liquids are the siloxane polymers shown below:
+
% PHENYL
Over the past 5 years Kamlet, Taft, and their co-workers have systematized the use of solvatochromism as a basis for exploring the nature and strength of interactions between a solute and a solvent (I). This had led to the development of the a*,a,and /3 linear solvation free energy scales which 'This work is dedicated to Piet Kolthoff on the anniversary of his 90th birthday. 0003-2700/84/0356-0278$01.50/0
0 1984 American Chemical Society
ov-101 OV-3 OV-7 ov-11 OV-17
0 10 20 35 50
OV-22 OV-25
65 75
