1490
Anal. Chem. 1987, 59, 1490-1494
Thermodynamic Studies into a Sorption Mechanism within the Cross-Linked Polysiloxane Stationary Phases Michal Roth,' Josef Novlk,',2 P a u l David: and Milos N o ~ o t n y * ~ Institute of Analytical Chemistry, Czechoslovak Academy of Sciences, Brno, Czechoslovakia 61142, and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
The retention mechanism has been studled for n-alkane analytes withln a cross-Inked commercial poly(dlmethylslioxane) statlonary phase and laboratory-synthesizedpoly(methyl-n-aikylsHoxane) substrates with dmerent alkyl chaln lengths. Model capillary columns were prepared with these phases and the specHlc retention volumes of the anaiytes measured wlthln the range of 80-160 'C. The foiiowlng thermodynarnlc calculations Indicate that bulk dissolution appears to be the dominant mechanlsm for retentlon of alkanes within these stationary phases.
Cross-linked (immobilized) polysiloxane stationary phases have now been widely used in both gas and supercritical fluid chromatography (1). The advantages offered by in situ cross-linked stationary phases include insolubility in common solvents and an increased film stability at elevated temperatures. These features of cross-linked stationary phases contribute to improved column lifetimes as compared to conventional stationary phases. Recently, there has been considerable activity in both theoretical ( 2 , 3 )and experimental (2,4-6) investigations of the sorption properties of chemically bonded stationary phases for high-performance liquid chromatography. In comparison, little is known about the mechanism of sorption within the cross-linked stationary phases (7) used in capillary gas and/or supercritical fluid chromatography. The goal of the experiments presented here is to determine whether partition (i.e., bulk dissolution) or adsorption is operative as the retention mechanism for alkanes in cross-linked nonpolar stationary phases. Two simple, methodical approaches have been devised to reach this goal. METHODICAL APPROACHES The first approach makes use of a series of capillary columns with stationary-phase film structures shown schematically in Figure 1. Each column has in it a film of in situ cross-linked stationary phase. The immobilized film is then overcoated with a conventional, i.e., non-cross-linked layer of the same stationary phase. The thickness of the cross-linked film is kept constant within the series while the thickness of the overcoating layer is allowed to vary from one column to another. Assuming that the retention of a model analyte in the non-cross-linked layer is due to partition only, the plots may be investigated showing the overall net retention volume of a model analyte as a function of the amount of non-crosslinked (=liquid) stationary phase in the column. One may argue that there should be two classes of such plots distinguished just by the mechanism of sorption being operative within the cross-linked film. If the analyte is retained in the cross-linked film by bulkdissolution, the overall net retention volume is expected to Institute of Analytical Chemistry, Czechoslovak Academy of Sciences. *Deceased April 6, 1986. Department of Chemistry, Indiana University. 0003-2700/87/035Q-1490$01.50/0
increase in proportion to the amount of non-cross-linked overcoat as shown in Figure 2. The contribution to the overall retention from the cross-linked film (line a) is supposed to be independent of the amount of non-cross-linked stationary phase in the column. The contribution to the overall retention from the non-cross-linked layer (line b) is directly proportional to the amount of non-cross-linked stationary phase in the column. The resulting overall retention volume is made up additively from the two contributions as shown by the heavy line in Figure 2. If the analyte is retained by adsorption on the surface of the cross-linked film, the dependence of the overall net retention volume on the amount of non-cross-linked stationary phase is expected to be hyperbolical as shown by the heavy line in Figure 3. In this case, the contribution to the analyte retention from the cross-linked film (line a) is supposed to decrease gradually as the surface becomes deactivated through coverage with increasing amounts of non-cross-linked stationary phase. The contribution to the overall retention from the non-cross-linkedlayer (line b) is again directly proportional to the amount of non-cross-linked stationary phase in the column. The second methodical approach involves the use of model poly(methylalkylsi1oxane) stationary phases of the general formula
with R being n-octyl, n-dodecyl, n-hexadecyl, or n-octadecyl, and R1 being trimethylsilyl or dimethylvinylsilyl (see the Experimental Section). In these stationary phases, the n-alkyl chains are bonded to the siloxane backbone, so that they are packed more closely than in corresponding liquid n-alkanes. Therefore, the question arises whether all methylene groups in the alkyl chains actually contribute to the analyte retention. Is the analyte molecule actually capable of penetrating through the alkyl chains up to the siloxane backbone? If so, the retention volume of the analyte per one siloxane chain unit would be expected to increase in proportion to increasing alkyl chain length. If not, the retention volume of the analyte per one siloxane chain unit would be expected to be independent of the alkyl chain length. EXPERIMENTAL SECTION Apparatus. A Fractavap 4160 series gas chromatograph (Carlo Erba, Milan, Italy) equipped with a flame ionization detector was used throughout this study. The instrument was operated in the split injection mode. Helium was employed as the carrier gas, while methane injections were used to mark the dead retention time. Columns. Pyrex glass capillary columns of 250-pm i.d. were used for all retention measurements. Glass capillaries were drawn from 2.5-mm4.d. Pyrex glass tubing by using a glass drawing machine (Hupe & Busch, Karlsruhe, West Germany), leached dynamically with 18% hydrochloric acid at 110 O C (€9, and deactivated with octamethylcyclotetrasiloxane at 425 "C (9). 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
1 1
II
II
II
I
Figure 1. Stationary-phase film proportions in the columns used for the first set of experiments (schematic representation): the shaded area is the crosslinked portion of the film; the nonshaded area is the non-cross-linked portion of the film.
columns were tested by measuring the capacity ratio of n-decane at 100 "C. The crcws-linked films were then separately overcoated with 0.05, 0.1, 0.2, 0.3, and 0.4-pm films of conventional SE-30 elastomer, without further cross-linking. One column was left without any overcoat (cf. Figure 1). For the second set of experiments, the poly(methy1-n-alkylsiloxane) stationary phases were synthesized from commercial dichlorosilanea (Petrarch Systems, Bristol, PA) according to the procedure described by Kuei et al. (12) for the n-odyl phage. The siloxane chains were terminated by trimethylsilyl groups for n-octyl and n-dodecyl phases and by dimethylvinylsilyl groups for n-hexadecyl and n-octadecylphases. The Pyrex glass columns of 250-pm i.d. were coated with 0.2-pm films of poly(methy1-nalkylsiloxane) phases. Procedure. For all 10 columns, the specific retention volumes of C8-C12normal alkanes were measured within the temperature range of 80-160 "C. The mean linear flow velocity of the carrier gas in the column did not exceed 25 cm/s. Very small volumes (CO.01 p L ) of the properly composed mixture of five n-alkanes were injected by using the "wet needle" technique. The detection system was operated at 1/2-1/16 of the full sensitivity. D a t a Reduction. The net retention volume, VN, and the specific retention volume, V,", are related to the experimental parameters by VN = ( t -~tm)%(Ta, PatmIJ'T/Ta (1)
vc = ( v~/ W 2 )(273.15/ T )
'"E 0
e
I 8
Amount of liquid stationary phaselarbitrary units
w e 2. The overaH net retention volume of the anatyte as a functlon of the amount of noncross-linked (i.e., liquid) stationary phase in the column in case of bulk dissolution of the analyte in the cross-linked film (heavy line): (a) contribution to the net retention volume from the cross-linked film; (b) contribution to the net retention volume from the noncross-linked overcoat.
Amount of liquid stationary phase/srbitrary units
Flg~re3. The overall net retentbn volume of the analyte as a function of the amount of non-cross-linked ( l a , liquid) stationary phase in the column In case of adsorption of the analyte on the surface of the cross-linked film (heavy line): (a) contribution to the net retention volume from the cross-linkd film; (b) contribution to the net retention volume from the non-cross-linked overcoat.
For the first set of experiments, a series of six columns was prepared, each column containing a statically coated 0.2-pm film of SE-30 elastomer cross-linked in situ with azo-tert-butane (10, 11). The columns were subsequently rinsed with n-pentane and methylene chloride solvents to remove the portion of the film that failed to cross-link. After overnight conditioning at 220 " C the
1401
(2)
where tR is the retention time of the analyte, t~ is the retention time of a nonsorbed compound, F3(Ta,Patm) is the volumetric flow-rate of the carrier gas at the ambient absolute temperature Taand barometric pressure Pat,. T i s the absolute column temperature, j is the James-Martin compressibility correction, and w 2is the weight of the stationary phase in the column. Neglecting the end-group effect, the retention volume per one siloxane chain unit is given by Vmo = VgoMu (3) where Mu is the molecular weight of one siloxane chain unit (-SiCH&O-). The infinite-dilution Raoult law activity coefficient of the analyte in the stationary phase is related to the specific retention volume of the analyte (13) by -ylw = 2 ~ 3 . 1 5 ~ / ( v g D ~ 1 exp[(VILo 0~2,) ~11)P1O/(Rr)lexp[(2B13 - ~ I L " ) P , J ~ ~ / ( R T (4) )I where the symbol J34is given ( 1 4 ) by Here, R is the molar gas constant, M2" is the number-average molecular weight of the polymeric stationary phase, Poand Piare the column outlet and inlet pressures, respectively, Pl0 is the saturated vapor pressure of the pure analyte, V,LO is the saturated liquid molar volume of the pure analyte, Ell is the second virial coefficient of the pure analyte vapor, E13 is the s_econdcross-virial coefficient of the analyte-carrier gas pair, and VIL" is the partial molar volume of the analyte, diluted infinitely in the stationary phase. When helium is used as the carrier gas, the second exponential term can usually be neglected. The auxiliary quantities Plo,V1~O,and Ell can be calculated from suitable correlated equations (15-17). From the activity coefficients and their temperature dependencies, partial molar excess thermodynamic functions of the analyte infinitely diluted by the stationary phase may be obtained, such as Gibbs energy, enthalpy, and entropy. For calculation of the excess Gibbs energy or entropy, the absolute values of the activity coefficients must be known,while to calculate the excess enthalpy, only the slope of the temperature dependence of the activity coefficient is needed: a l E m = R 8 In rl"/W/T) (6) Thus, an estimate of the mean value of MlE" within the temperature range investigated can be obtained from the slope of the linear least-squares fit of the plot In -ylw - 1/T. RESULTS A N D DISCUSSION SE-30 Columns. Specific retention volumes at 80 "C were
plotted against the weight of non-cross-linked SE-30 elastomer
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
I
2m 10
-4 0
12
5
4
22
1
2.4
Weight of nan-crosslinkedSE-30/mg
Figure 4. Specific retention volumes of n-alkanes as functions of the amount of non-cross-linked SE-30 in the column: 0, n-octane; 0, n-nonane; 0 ,ndecane; A , n-undecane; 0, ndodecane.
22
24
26
2.6
28
1OW K/T
Figure 6. Temperature dependencies of the -ylW&f2,, products for n-alkanes on one of the SE-30 columns (0.2-pm film of cross-linked SE-30): 0, n-octane, 0, n-nonane; 0 ,n-decane; A, n-undecane; 0,n dodecane.
28
1OOO KIT
Figwe 5. Temperature dependencies of the qecifk retentlon volumes of n-akanes on one of the SE-30 columns (0.2-pm film of cross-linked SE-30 0.05-km film of non-cross-linked SE-30): 0,n-octane; 0 n-nonane; 0 ,ndecane; A v n-undecane; 0, n-dodecane.
+
in the column, as shown in Figure 4. While calculating specific retention volumes, the total amount of stationary phase in a column has been used, i.e., the combined weights of both cross-linked and non-cross-linked portions of the stationary phase. As seen from the graph, the specific retention volume of any alkane is independent of the amount of non-cross-linked stationary phase in the column. In principle, Figure 4 is equivalent to Figure 2, indicating that the retention of C8 to C12n-alkanes in the cross-linked film of SE-30is due to bulk dissolution. Moreover, Figure 4 suggests that the sorption properties of SE-30 elastomer are not significantly affected by cross-linking. The temperature dependencies of the specific retention volumes of the alkanes on one of the used columns are shown in Figure 5. The appearance of these plots with the other columns was similar. The slight nonlinearity of the plots is primarily due to the temperature dependence of the standard enthalpy of condensation of 1mol of the pure analyte from an ideal-gas state at the column temperature to the liquid state a t the same temperature and the mean column pressure (18). T o calculate the activity coefficients from eq 4, the number-average molecular weight of the stationary phase must be known. As this quantity was unknown, we were only able to calculate the values of the -y1-MZn products. For one of the SE-30 columns used, these values are plotted against the reciprocal column temperature in Figure 6. From the slopes of the lines (Figure 6), infinite-dilution partial molar excess enthalpies of the analytes in SE-30 can be estimated. The values obtained from all six SE-30 columns are average
mIE"
Stationary phase methylene number
Figure 7. The retention volumes at 80 "C of n-alkanes per one siloxane chain unit as functions of the methylene number of the alkyl chain of the stationary phase: 0,n-octane; 0, n-nonane; 0 ,n decane; A, n-undecane; 0, ndodecane.
Table I. Average Values of ARIE"Obtained from All Six SE-30Columns analyte
H I E " / Jmol-'
analyte
MIE"/J mol-'
n-octane n-nonane n-decane
-80 180 550
n-undecane n-dodecane
930 1310
listed in Table I. These values measure the heat of solution of 1 mol of the analyte in a very large quantity of SE-30 elastomer at 120 "C. For n-octane, heat is released in such a process, while increasing amounts of heat are absorbed for the longer-chain n-alkanes. Poly(methy1-n -alkylsiloxane) Columns. Retention volumes per one siloxane chain unit of CBto CI2n-alkanes at 80 "C were plotted against the methylene number of the alkyl chain in the siloxane stationary phase, as shown in Figure 7. The V," values of n-alkanes increase linearly on increasing the stationary-phase alkyl chain length. This suggests that all methylene groups in the alkyl chains contribute to analyte retention, or, in other words, that the analyte molecules are actually able to penetrate through the alkyl chains of the stationary phase all the way to the siloxane backbone. Table I1 summarizes the parameters of the lines from Figure 7. The a values measure the contributions to the analyte retention from a single methylene group in the alkyl chain of the stationary phase, while b values give the contribution from the rest of the molecule of the stationary phase (Le., the siloxane
ANALYTICAL CHEMISTRY, VOL. 59, NO. 11, JUNE 1, 1987
Table 11. Constants of the Equation Vm0= Na
+ b , 80
analyte
a / L mol-'
b/L mol-'
n-octane n-nonane n-decane n-undecane n-dodecane
2.71 6.11 13.76 30.77 68.55
13.71 28.34 57.85 117.50 237.00
1493
Oca
' N is the number of methylene groups in the stationary phase alkyl-chain. Table 111. Average Values of ARIE" Obtained from All Four Poly(methy1-n -alkylsiloxane) Columns
analyte n-octane n-nonane n-decane
mlE"/J mol-'
analyte
-770 -780 -700
n-undecane n-dodecane
U I E " / J mol-'
-.
I
22
24
26
28
1OOO K/T
-590 -490
Flgure 0. Tempeature dependencies of the -y,-M,, products for n alkanes on the poly(methyCn-octylsiloxane)column: 0, noctane: 0. n-nonane; 0 ,n-decane; A, n-undecane; 0, n-dodecane.
Table IV. Retention of n -Dodecane Relative to n -Undecane on Poly(methy1-n -alkylsiloxanes) at 80 OC.
stationary phase /
71
SE-30 (C,) C8
C'Z c 1 6 C18
2.2
2.4
2.6
2.8
1OOO K/T
Figure 8. Temperature dependencies of the speclflc retenth volumes of n-alkanes on the poly(methy1-n-octadecylslloxane) column: 0, n-octane; 0, n-nonane, 0 , ndecane; A, n-undecane; 0, ndodecane.
chain plus the end groups and methyl groups attached to Si atoms, plus methyl groups terminating the alkyl chains). The plot of temperature dependence of the specific retention volumes, with the five n-alkanes, on n-octadecyl stationary phase is shown in Figure 8. As can be expected, the retention volumes are generally larger than those observed with SE-30. products for The temperature dependencies of the ylmM2n n-octyl phase shown in Figure 9, form a pattern different from that noted with the SE-30 elastomer (cf. Figure 6). In Table 111, the average values are listed of the partial molar excess enthalpies of the analytes in methyl-n-alkylsiloxane phases. These values are generally more negative than those observed with S E 3 0 (cf.Table I); this indicates that the alkane analytes interact more strongly with a more favorable hydrophobic environment provided by the methyl-n-alkylpolysiloxane phases. Finally, from the analytical point of view, there does not seem to be too much point in increasing the alkyl chain length above n-octyl. As illustrated by the values of relative retention, a,of n-dodecane to n-undecane a t 80 "C (Table IV), there is some increase in "selectivity" of these phases when going from SE-30 to the phases with longer alkyl chains. However, a major part of this increase is already attained by the n-octyl phase, and there appears to be a kind of asymptotic behavior of a with longer alkyl chains.
CONCLUSION Bulk dissolution (or partition) appears to be the retention mechanism for alkanes in both the cross-linked SE-30 films
ff
2.03 2.15 2.18 2.19 2.19
and in poly(methy1-n-alkylsiloxane)films. Apparently, the density of cross-links in our columns was not high enough to prevent the alkane analytes from penetrating into the bulk of the stationary phase. In the hypothetical limit of an extremely high degree of cross-linking, a dense, tridimensional network could be formed within the stationary film, preventing alkane molecules from penetrating into the bulk of the stationary phase. In such a case, the retention (if any) would be due to surface adsorption only. Therefore, the relative significance of surface adsorption can be expected to increase with both the increasing size of the analyte molecule and/or the increasing degree of cross-linking of the stationary phase. Extension of the current studies to more polar solutes and stationary phases would be worthwhile. However, such investigations are greatly complicated by the column wall adsorption, which will occur whether or not the stationary phase is cross-linked.
LITERATURE CITED Lee, M. L.; Yang, F. J.; Bartle, K. D. Open Tubukr Column Gas Chromatography; Wiley: New York. 1984; pp 75-81. Horvath, C.; Melander, W.; Molnar, L. J. Chromatogr. 1979, 125, 129-156. Martire, D. E.; Boehm, R. E. J. Phys. Chem. 1983, 8 7 , 1045-1062. Lochmuller, C. H.; Wilder, D. R. J. Chromatogr. Sci. 1979, 17, 574-579. Tanaka, N.; Sakagaml, K.; Araki, M. J. Chromatogr. 1901, 199, 327-337. Lochmuller, C. H.; Hunnicutt, M. L.; Muilaney, J. F. J. Phys. Chem. 1905, 8 9 , 5770-5772. Bererkin. V. G.; Korolev. A. A. Chromatcgaphia 1985, 2 0 , 462-486. Lee. M. L.; Vassilaros, D. L.; Phillips, L. V.; Hercules, D. M.; Azumaya, H.; Jorgenson, J. W.; Maskarinec, M. P.; Novotny, M. Anal. Lett. 1979, 72, 191-192. Stark, T. J.; Dandeneau, R. D.; Mering, L. Presented at the 1980 Pttsburgh Conference, Atlantic City, NJ, 1980; Abstract 002. Wright, B. W.; Peaden, P. A.; Lee. M. L.; Stark, T. J. Chromatogr. 1982, 248. 17-34. Richter, B. E.; Kuei. J. C.; Park, N. J.; Crowley, S. J.; Bradshaw, J. S.; Lee, M. L. W C CC,J. High Resoiut. Chromatogr. Chromatogr. Commun. 1903, 6 , 371-374. Kuei, J. C.; Tarbet, 8. J.; Jackson, W. P.; Bradshaw J. S.; Markiies, K. E.; Lee, M. L. Chromatographia 1985, 2 0 , 25-30. Crulckshank, A. J. B.; Wlndsor. M. L.; Young, C. L. Roc. R. SOC. London, A 1988, 295, 259-270. Everett, D. H. Trans. Faraday Soc. 1985, 6 1 , 1637-1645. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids, 3rd ed.; McGraw-Hilt New York, 1977.
Anal. Chem. 1987,5 9 , 1494-1498
1494
(16) Spencer, C. F.; Adler. S. 8. J . Chem. Eng. Data 1978, 23,82-89. (17) Hayden, J. 0.;OConnell, J. P. Ind. Eng. Chem. Process Des. Dev. 1975, 74, 209-216. (18) ROth, M.; NOVHk, J. MaCfOmOleCUleS 1986, 79, 364-369.
RECEIVED for review June 23, 1986. Resubmitted March 5,
1987. Accepted March 5, 1987. This study was supported by the grants from the ~ ~ t ihience ~ ~ Foundation a l ( N ~cm . 82-00034) and the Office of Naval Research. Presented, in part, at the 191st National Meeting of the American Chemical Society, New York, April 13-18, 1986.
Continuous Aerodispersive Enrichment Unit for Trace Determination of Pollutants in Air Zbynek Ve5ei.a a n d Jaroslav J a n i k * Institute of Analytical Chemistry, Czechoslovak Academy of Sciences, Leninova 82, 611 42 Bmo, Czechoslovakia
A method and a device operatlng on the prhclpk of balanced accumulation of pollutants from a gas by means of a polydispersive aerosol of a liquid are descrlbed. The devke enables preconcentratlon of analytes of different chemlcal propertleshom Wfer quantities d the gas to microliter volumes of the Ilquld. Its dlmendons are such as to enable up to a 5 X I O 4 Increase of anatyte concentratlon In the llquld In comparlson wlth Its orlglnal concentratlon In the gas phase without b d n atlected ~ by the relathre hmkllty of the air. The device Is appllcah for conthwowr monitoring. The reoponse tlme Is dependent on the means of detection of the analyte In the conoentrate; Le. lt usually Is tens of seconds maximally, but the lower thne lknlt for response can be comparable to the rate of supplying the analyte to the enrlchment unlt. Repoduclbillty and rellablllty of the results were vermed for concentrations of phenol and o-cresol of I O a to I O d g, 2naphthol of IO-' g, and toluene of IO-' g In a llter of the alr In alr-water and alr-n-decane models. The results correspond wlth the theory very well.
Growing civilization and human interferences in the environment result in increasing pollution of the atmosphere by means of various pollutants of both organic and inorganic origin (1,2). As some of them are directly or i n p c t l y biologically active even in their trace concentrations, when their direct detection and quantification are often difficult, enrichment techniques have become an integral part of trace analysis of pollutants (3). Enrichment of traces of different substances from a gas medium is usually carried out either statically (extraction) or dynamically (chromatography) by means of sorption on solid materials, in suitable liquids, or by condensation of an analyte (4,5). A number of recent surveys (6) and approaches to this problem (7-12) prove that this is still the case. From the point of view of enrichment, the methods based on adsorption by solid materials are generally more efficient than those based on absorption in liquids, especially with respect to a higher value of the distribution constant of the analyte at a given temperature, a more favorable dimension of the interphase boundary, and a higher rate of adsorption than that of dissolution. However, the higher value of the distribution constant of the analyte a t adsorption allows the analyte to be released from the sorbent only with the use of thermal desorption or extraction by means of a suitable liquid. Then the enrichment process is discontinuous by nature and 0003-2700/87/0359-1494$01.50/0
the enrichment efficiency cannot easily be estimated. The general problem of preconcentration of analytes from the atmosphere seems to be a permanent and variable content of water vapor in air, which nearly always exceeds the amount of trace components by orders of magnitude and complicates the enrichment process. Therefore it is necessary to use special solid sorbents (6, 7,IO),adjustment of liquid absorbers (II), or special instrumentation (8, 12). An interesting method for enrichment of trace components from air in water was described by Dawson and Farmer (13, 14). The essence of this approach is the absorption of soluble gases in water vapor condensed from air on a cooled cuprous plate. The method is suitable for enrichment of highly soluble gases, the sorption being controlled by their rate of dissolution. The advantages of the adsorption process have been made use of together with the advantages of the use of a liquid medium in such a way that the analyte is accumulated by means of polydispersive aerosol of a liquid (15). This has been done with respect to the work of Freed (16) and Kat0 et al. (17). Freed determined organic substances liable to thermal decomposition in the air by means of their cocondensation with saturated vapors of n-pentane with an aim to reach their quick and quantitative absorption in an organic solvent. Kato et al. used the method of spraying a water solution of flavine mononucleotide and direct chemiluminescence to trace sulfur dioxide in the atmosphere. This work presents theoretical analysis and practical solution of the problem of preconcentration: a liquid is dispersed in a gas in such a way as to provide sufficient amounts of aerosol aggregates with a surface enabling analyte adsorption to become the controlling process of distribution of the analyte between gas and liquid phases. Adsorption shows a substantially faster rate of reaching equilibrium than is enabled by dissolution. Under such circumstances it is also possible to use a water mist for considerable preconcentration of analytes of various chemical properties from the atmosphere (generally from gas media) with different and variable water vapor content. The enrichment unit can be miniaturized and is applicable for continuous monitoring of gaseous environments.
THEORY When a gas contaminated by the analyte i gets into contact with a liquid phase able to sorbe the analyte, the sorption reaches equilibrium at a certain definite rate. Its instantaneous value can, at usual ways of accumulation of pollutants from a gas into a liquid, be described (Dic/DiL= 104-105) by means of the following diffusively convective equation (18) 0 1987 Amerlcan Chemical Soclety
