and Purnell (10) report obstructive coefficients of essentially unity. This suggests that there are consistent discrepancies between laboratories in the measurement of system parameters. It also suggests that end effects may be significant even though the customary precautions to minimize them are taken. In the present theory the parameters which are directly measured are utilized to fit the data. As seen from Equation 16, the magnitudes of the experimentally determined D and y values are evaluated from the slopes of best fit lines. The slopes depend on the values of m2,Ac3,and V 3 ,and, therefore, smallerrors in measuring these parameters are greatly magnified when the exponents are
applied. Regressions of this type are also sensitive to the mathematical formula, and neglect of seemingly small end effects also may have a profound influence on estimates of D and y. ACKNOWLEDGMEh’T
The authors gratefully acknowledge the assistance of Ruth Hale in performing the tests and the assistance of Denis Plane in developing the theory. The advice and encouragement of D. T. Sawyer and David A. Lowitz are also gratefully acknowledged.
RECEIVED for review January 24, 1972. Accepted May 24, 1972.
(10) J. Boheman and J. H. Purnell, J. Chem. SOC.,1961,360.
Support-Bonded Polyaromatic Copolymer Stationary Phases for Use in Gas Chromatography Edward N. Fuller Applied Automation, Inc.-Systems
Research Department, Bartlesuille, Okla. 74004
The preparation of porous polyaromatic copolymers of divinylbenzene, ethylvinylbenzene, and styrene physically bonded to a solid support is described together with initial results illustrating the utility of these materials as GC column packings. While similar in nature to the widely used porous polymer beads, the support-bonded phases provide more rapid separations and greater column efficiency. Experiments showing the effects of cross-linking and of initial dilution with inert solvent on the resulting copolymer product are also discussed.
RECENTLY Hollis (1-3) reported the development of porous polyaromatic copolymers as stationary phases for gas chromatography. Following this first disclosure, a variety of such products prepared in the form of polymer beads have become commercially available and are now widely employed as column packing materials. Their properties, as related to the separation of various classes of compounds, have been investigated by Supina and Rose ( 4 ) and more extensively by Dave (5). Other publications, particularly those devoted to specific applications, have become too numerous to mention here. The present paper describes the preparation of similar porous polyaromatic copolymers formed directly on and physically bonded to the surface of suitable solid support materials and presents initial results of experiments investigating their utility as GC stationary phases and their surface properties. The emulsion polymerization methods normally used to prepare the polymer beads are not suited to the task of pre(1) 0.L. Hollis, ANAL.CHEM., 38,309 (1966). (2) 0. L. Hollis and W. V . Hayes, J . Gas Chromatogr., 4, 235 (1966). (3) 0. L. Hollis and W. V. Hayes in “Gas Chrornatography1966”, A . B. Littlewood, Ed., The Institute of Petroleum, London, 1967, p 57. (4) Walter R. Supina and Lewis P. Rose, J. Chromatogr. Sci., 7 , 192 (1969). ( 5 ) S. B. Dave, ibid., p 389.
paring a polymer on a support surface; consequently, a different approach aimed at accomplishing the above goal was devised. Basically, the technique consists of f i s t coating a support with a solution containing the divinylbenzene (DVB), ethylvinylbenzene (EVB), and styrene (STY) monomers, inert diluent, and an initiator in predetermined proportions. The monomers are then reacted in place by gentle heating to produce the porous copolymer directly on the surface of the support. Since the extensively cross-linked polymer is formed within the surface pore structure, it becomes permanently fixed or bonded to the support. The final step, solvent removal, is accomplished by evaporation under vacuum or else during the column conditioning process. EXPERIMENTAL
Reagents. Polymerization grade styrene (STY), 99+%, and practical grade divinylbenzene (DVB), 55.5 % DVB, 39.3 % ethylvinylbenzene (EVB), and 3.4 % diethylbenzene (DEB), were obtained from stocks at Phillips Petroleum Company Research Center. The DVB composition was checked by gas chromatographic analysis using the procedure of Hannah, Cook, and Blanchette (6). The results were 55.4% DVB, 38.9% EVB, and 3.0% DEB, all three present as the meta and para isomers. A number of minor components were observed, the largest being 1.2% naphthalene. The labeled composition was assumed to be correct. In any case, minor errors in composition would not greatly alter experimental results presented here. Technical grade lauroyl peroxide (LP), used as initiator, was obtained from Wallace & Tiernan, Buffalo, N.Y. Pure grade n-heptane, 99+% normal isomer, was obtained from Phillips Petroleum Company, Bartlesville, Okla. Method of Preparation. A solid support, which may be one of the Chromosorbs, porous glass beads, etc., is placed in a weighed glass jar. The jar and contents are evacuated to remove water and other possible adsorbed contaminants (6) Ray E. Hannah, Mary L. Cook, and Joseph A. Blanchette, ANAL.CHEM., 39,358 (1967).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
1747
The plate height, H , was chosen as the measure of column efficiency and was obtained from the relation (7)
I
H = L(T/tR)’ (1) where L is the column length in centimeters, tR the retention time or first moment of the peak in seconds, and T the standard deviation or second moment about tR of the peak in seconds. The quantity chosen to compare speed of analysis between columns, was the capacity ratio, k , given by (8) 0
1
2
3
4
5
MINUTES
Figure 1. Separation of aromatic hydrocarbons on 6ft DVB-EVB-STY SUPport bonded column described in text, at 180 “C, 60 ml/min He, -0.02 kI sample size Peak 1. Benzene 2. Toluene 3. Ethylbenzene 4. +Xylene 5. Isopropylbenzene 6. n-Butylbenzene
and then weighed to determine the weight of support material. Next, a weighed solution of monomers with predetermined composition, including initiator and inert diluent, is poured onto the support. Total volume should not be so great that free liquid remains over the support. The jar, sealed with a Teflon (Du Pont) gasket and tight lid, is then placed in a tumbler-rotator under a 250-watt infrared lamp. This mild heating warms the jar to roughly 75-80 “C which is adequate to promote the reaction, normally requiring 16-48 hours for completion. The tumbling action serves to spread the monomers evenly and to maintain the support in a free flowing state by preventing sticking as the polymer forms. If the total volume of reactants, as determined by the degree of dilution and extent of loading desired, is small enough that the support is not visibly dampened, there is little danger of sticking. In this case, after tumbling for a short time to obtain a uniform coating, the reaction jar can simply be placed in an oven at approximately 80 “C until the polymerization is complete. The approximate amount of polymer material formed on the support can be determined as follows. After reaction, the jar and contents are again placed under vacuum for 2-4 hours or until a constant weight is obtained to remove diluent, low molecular weight fractions, and traces of unreacted monomers. The observed weight increase gives the approximate weight of polymer. An over-estimation is made by this method since some further weight loss occurs during column conditioning as evidenced by column bleeding. After preparation, the support-bonded packing is packed with tamping and vibration in a l/s-in. 0.d. x 0.085-in. i.d. stainless steel tube which is fitted with fine mesh stainless steel screens at each end to retain the packing (6-ft columns were used throughout this work). The column is coiled to fit the gas chromatograph and installed for conditioning at about 200 “C with helium flow of 30-50 ml/min for 8-12 hours or until a stable base line is obtained. The column is now ready for use. Column Evaluation. Column evaluation and testing were done on a Perkin-Elmer Model 900 gas chromatograph equipped for dual column operation with dual flame ionization detectors. GC output signal was sent on-line to a PDP-8 computer programmed for the calculation of peak area, retention time, plate height, etc. 1748
where tM is the retention time for an unsorbed peak. Since both methane and air are retained on porous polymer phases, their use for direct measurement of tM would give erroneous results. For this reason, tM was calculated using the procedure of Hansen and Andresen (9) in which tMis given by
(3) where tl, tz, and t3 are the retention times for a series of three adjacent normal hydrocarbons such as n-pentane, n-hexane, and n-heptane. Basic to the method is the assumption that log k is a linear function of carbon number. Extreme accuracy is not required in this case since the k’s so determined are used only for comparison. Polymer Surface Properties. Surface properties were determined by NPadsorption on bulk polymer samples prepared as previously described except that no support material was used. This approach was required, rather than direct measurements on the support-bonded polymers, since contributions from the support material tended to mask effects due to the polymer which is present in relatively smaller amounts. Another factor, perhaps even more critical, is that variations in polymer loading can cause large fluctuations in surface area and porosity of the support-bonded packings, particularly for highly porous polymer formulations. On the other hand, when considering chromatographic behavior, contributions from the suppc~tcannot be ignored; some separation or lack of it may be due to the solid support which is far from inert. NP adsorption, as a function of pressure, was measured with an automated instrument, the Adsorbtomat (American Instrument Co., Silver Spring, Md.). From the data, pore volume, pore size distribytion, and surface area for pore sizes in the range 10-300 A in radius were determined by the method of Barrett, Joyner, and Halenda (IO). Using the cylindrical pore model, the average pore radius, Jp, was estimated from the expression (11) r- = -2VP S
(4)
where V, is the pore volume per gram and S the surface area per gram. RESULTS AND DISCUSSION Performance of Support-Bonded Polymer Packings. To illustrate the performance of the support-bonded polymer packings, the separation of some aromatic hydrocarbons out (7) J. Calvin Giddings, “Dynamics of Chromatography,” Marcel Dekker, New York, N.Y., 1965, p 24. (8) ASTM Designation : E355-68, Recommended Practice for Gas Chromatography Terms and Relationships, May 1968. (9) Hans Levin Hansen and Kirsten Andresen, J . Chromatogr., 34, 246 (1968). (10) Elliott P. Barrett, Leslie G. Joyner, and Paul P. Halenda, J . Amer. Chem. SOC.,13,373 (1951). (11) Clyde Orr, Jr., and J. M. Dalavale, “Fine Particle Measurement” Macmillan, New York, N.Y., 1959, p 267.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
through butylbenzene is shown in Figure 1. Total time required was about five minutes under conditions given in the caption. The programmed temperature separation of natural gas components is shown in Figure 2; again conditions are given in the caption. The column packing used in the examples shown was prepared on Chromosorb P, 80jlOO mesh, from a formulation consisting of 38 mole % DVB, 35 mole STY, and 27 mole EVB, calculated as mole of reactant monomers (neglecting the inert diluent and DEB plus other low level impurities present in the DVB itself), with 50 wt of the total n-heptane as initial diluent and about 1 wt LP as initiator. The polymer formed on the support resulted in approximately 7 % w/w loading after a reaction time of about 18 hours. A plot of plate height, H, as a function of flow rate at 180°C for n-pentane as determined on the column just described is shown in Figure 3. For the sake of comparison, a similar plot of H for n-pentane on Porapak P, 8OjlOO mesh, is also shown. Not only is the support-bonded column more efficient, but the minimum Hoccurs at a higher flow rate and the increase in H with increasing flow is flatter, much as would be the case when comparing a lightly loaded liquid packing with one more heavily coated. The relatively flat shape of the curve shown is typical of the support-bonded polymer packings prepared from various formulations; thus, these columns can be operated at higher flow rates with only minimal loss in column efficiency. Appreciably larger values of H were observed for branched hydrocarbons as compared to normal hydrocarbons; note in Figure 2 that although elution times are close for isobutane and n-butane, the isobutane peak width is greater, and similarly for isopentane and n-pentane. It can be seen from Figure 1 that the H values for the alkyl benzenes will be greater than those for n-pentane shown in Figure 3 at the same flow rate and temperature. The reasons for these differences have not been investigated sufficiently to permit an explanation; however, column efficiencies for all the above components varied with the initial dilution and DVB content used to prepare a particular support-bonded polymer. Sample size and column temperature can also strongly influence H . Dilution and Cross-Linking Effects. Attention is next directed toward the effects of initial dilution and of crosslinking on the properties of the resulting support-bonded polymers and their effects on column performance. Dilution and cross-linking effects on DVB-STY copolymer systems have previously been investigated by Lloyd and Alfrey (12) and Millar and coworkers (13, 14). Extensive work investigating the nature of DVB-STY copolymers and their application in the entire field of chromatography has been summarized in a review by Seidl, Malinsky, Dusek, and Heitz (15). According to the model developed by Millar (14) for STYDVB cross-linking polymerization in the presence of a nonsolvating, inert diluent, highly cross-linked nuclei are formed initially. Swollen with monomers which continue to react, the nuclei become interconnected throughout the entire reaction volume by largely straight chain segments to form a separate phase. Since the nonsolvating diluent does not
z
z
I
z
z
(12) W. G. Lloyd and T. Alfrey, J . Polym. Sci., 62,301 (1962). (13) J. R. Millar, D. G. Smith, W. E. Marr, and T. R. E. Kressmann, J. Clzem. SOC.,1963, 218. (14) J. R. Millar, D. G. Smith, and T. R. E. Kressmann, ibid., 1965,
0
1
2 3 MINUTES
4
5
Figure 2. Separation of natural gas components on same column with temperature programmed from 25-160 "C at 24 "C/min, 50 ml/min He, 0 . 1 4 gas sample Peak 1. Methane 2. Ethane 3. Propane 4. Isobutane 5. n-Butane 6. 2-Methylbutane 7. n-Pentane
,4t .2 -
A 0
PORAPAK P, 80/100 MESH SUPPORT- BONDED POLYMER, 38% DVB, 5 0 % INITIAL DILUTION ON CHROMOSORB P, 80/100 MESH
swell the polymer, chain segments are highly entangled as they form and not extended by diluent; thus, upon diluent removal the polymer shrinks only slightly and permanent porosity results. Further details of the course of the reaction during phase separation have been investigated by Kun and Kunin (16). In contrast to the nonswelling case, if a swelling solvent such as toluene is used as inert diluent, the formation of a separate polymer phase occurs later in the course of the reaction and the swollen chain segments are extended with little entanglement. Consequently, upon diluent removal, the polymer shrinks to a greater extent and little permanent porosity is produced without a high density of cross-links to reduce the shrinkage. Both increases in diluent and DVB amounts speed up the onset of phase separation and result in increased porosity (14). Hence, the type and amount of inert diluent as well as DVB content are all important variables governing
304. (15) J. Seidl, J. Malinsky, K. Dusek, and W. Heitz, Adcm. Polym. Sci., 5, 1 1 3 (1967).
(16) Kenneth A. Kun and Robert Kunin, J . Polym. Sei., Part A - I ,
6,2689 (1968).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
1749
0
k FOR C i k FOR C6
A
k FOR C5
o
1
3.0
t \ 20
Olb
30
io
50
sb
io
do
I
io
Io4
WEIGHT X HEPTANE
Figure 4. Effect of initial dilution on the capacity ratio, k , for column packings prepared at 38 2 cross-linking
t
i
1
zol 01 10
20
30
40
50
60
70
80
90
I
100
WT-X HEPTANE
Figure 5. Effect of initial dilution on average pore radius of DVB-EVB-STY copolymers prepared at 38 Z cross-linking
the properties of the polymer produced. Despite the somewhat different polymerization technique used, the results described later in this section are in good qualitative agreement with the model. To investigate dilution and cross-linking effects, two series of support-bonded polymers were prepared for testing on Chromosorb P, 80jlOO mesh, at approximately 7 % w/w polymer loading. In the first series, prepared to show the effects of initial dilution, n-heptane concentration was varied from 10 to 92 wt % of the total formulation while holding DVB content constant at 38 mole% of reactants. In the second series, prepared to show the effects of cross-linking, DVB content was varied from 5 to 59 mole% of reactants, the upper limit imposed by the purity of the DVB, while holding dilution constant at 50 wt of the total n-heptane. From the first series of packings, it was found that initial dilution strongly affects column performance. The packing prepared at 25 wt Z n-heptane showed severe tailing and poor separation such that it could barely be considered a practical column. The packing at 10% n-heptane was essentially useless. On the other hand, at 40% out through 9 2 2 nheptane, the maximum value tried, symmetrical peak shapes and good separations were observed. Evidently, an initial dilution of about 30z or greater is required at the 38% crosslinked level to produce useful packings. In general, the minimum initial dilution requirement depends on the DVB content or cross-linking and on the nature of the diluent. In addition, initial dilution was found to affect retention times. The partition ratios for n-pentane, n-hexane, and n-heptane, determined at 180°C, are plotted in Figure 4 as a 1750
function of dilution. From the plot it is seen that the k's decrease at fist to a minimum at around 50Z dilution and then increase slightly at higher dilution. Since both variations in sample size and in the amount of polymer formed on a given support can affect the k's, these values should be considered only approximate; nevertheless, they are sufficiently accurate to establish the trend. It has been noted for the polymer beads that the larger the average pore radius, the more rapid elution will occur (5). Since, as will be shown later, initial dilution determines pore radius at any given level of cross-linking, what is observed here can largely be explained on the basis of average pore size. To permit comparison, the k's for the same series of hydrocarbons were determined on Porapak P at the same temperature; and the values are 2.2, 3.6, and 5.8, respectively. Although not actually determined, the very long retention times observed on Porapak Q at this temperature indicate that the k's would be much greater yet on Q. Inasmuch as the actual amount of polymer per unit column length is much less for the supportbonded polymers than for the polymer beads, this is only what would be anticipated. Thus, the above results serve to point out the more rapid analyses possible with the support-bonded materials. From the second series of packing it was determined that, in addition to a minimum level of dilution, a minimum level of cross-linking is also required. Although somewhat dependent on initial dilution, a minimum amount of cross-linking is necessary, first to produce the porous structure and then to retain it after solvent removal, as is well known from work on polymer beads ( I , 13, 14). The column with 25 m o l e x DVB showed tailing and poor separations, and going to the 10 and 5 mole DVB levels made the situation progressively worse. On the other hand, the packings prepared with 38 and 59 mole DVB were found to possess desirable characteristics as column materials. Thus, the minimum practical DVB content is evidently about 30 mole %, corresponding to 30 % cross-linking. From this second series it was further determined that DVB content affects retention time. Although partition ratios could not be accurately determined for the packing with 25 mole and less because of poor column performance, under identical conditions increased retention times were observed with increasing DVB content throughout the 5-59 mole 2 range; however, the effect is less pronounced than the increased retention observed at low dilution in Figure 4. To link the chromatographic observations just described to polymer surface properties, two series of bulk polymer samples were prepared for testing from formulations duplicating those of corresponding members in the two series of supportbonded packings. By this means, surface properties could be determined as a function of initial dilution and of crosslinking on a directly comparable basis, independent of contributions from the support itself or from variations in polymer loading on the support. However, bulk polymer properties may not correspond exactly to those of the supportbonded polymers. The support surface may exert some influence during the polymerization to alter polymer properties; furthermore, influence of the support must be taken into account when evaluating the support-bonded polymers as column materials. Nevertheless, the data provide a useful indication of the properties of the polymers on the support, help to explain trends in chromatographic behavior, and together with the chromatographic observations serve to establish limits on the ranges of initial dilution and DVB content required to produce useful columns.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
100
Table I. Effect of Initial Dilution on Surface Area and Pore Volume of 38 Cross-Linked DVB-EVB-STY Copolymers
0
z
Wt
zc7 10 25 40 50 60 92
S, m2/g 1.75 1.91 28 1 179 223 19.5
A
Vm w i g 0.0026 0.0046 0.92 0.516 0.68 0.041
80
Table 11. Surface Properties of Some Porapak Polymer Beads
Type S, m'/g 103 Porapak-Pa 634 Porapak-Qb Porapak-Rb 547 536 Porapak-Sb Porapak-Tb 306 a This work. Johnson and Barrall(17).
90
50% C7 (38 MOLE X DVB 40% C7 (38 MOLE % DVB 25% C7 ( 3 8 MOLE % DVB
VP,
cc/g
0.512 1.185 1.035 1.017 0.701
PP, A 99.4 37.4 37.8 38.0 45.7
-s -
$I*
70
60
50
4o
30
20
Before discussion of the surface properties, some mention of the readily apparent effects of initial dilution and crosslinking on the appearance and physical strength of the bulk polymer samples should prove worthwhile. As observed from the first series of bulk polymers in which initial dilution was varied while holding cross-linking constant at 38%, changes in dilution strongly alter both the appearance and strength of the polymer product. Although clear at zero dilution, at 10% n-heptane the polymer appears cloudy, indicating the onset of some internal porosity; and at 25% it takes on a milky-white appearance. As the amount of n-heptane is increased still further, the polymer becomes completely white and gradually becomes increasingly friable. At 9 2 2 n-heptane the polymer becomes fluffy with visible gross pore structure and can be easily crushed between the fingers to a fine powder. Above about 95% dilution, depending on DVB content, polymer no longer forms as a mass throughout the liquid phase but precipitates instead to the bottom of the vessel as a fine powder. The observed decrease in physical strength with increasing dilution is evidently due to the increasing probability of forming wasted cross-links, Le., chain segments linking back on themselves rather than cross-linking to another chain. Eventually, cross-linking probability drops so low that the fine powder forms, thereby fixing a maximum for initial dilution. As observed from the second series of bulk polymers in which DVB content was varied while holding dilution constant at SO%, cross-linking has little effect on appearance but does affect physical strength. At 5 mole DVB, the polymer is very tough and rubbery; as DVB content is raised, the polymer gradually becomes harder and more brittle. Turning now to the effects of initial dilution on surface properties, we will discuss next the results from the first series of bulk polymers. The surface area and pore volume are listed as a function of dilution in Table I. Obtained from Table I using Equation 4, the average pore radius for each member of the series is plotted against wt % n-heptane in Figure 5. It is not known, without more data for verification, whether the dip in V , and S at 50% C7 shown in Table I is real; if real, the smoothing shown in Figure 5 would not be justified. Although at first ?, increases with dilution as expected, the down turn at higher dilution and the
10
0
50
100
I50
200 rp t
250
300
A
Figure 6. Effect of initial dilution on pore size distribution of DVB-EVB-STY prepared at 38 cross-linking
associated drop in V , seen in Table I differ enough from anticipated results to require some explanation. The problem is easily resolved, however, once it is realized that at higher dilution much of the pore structure lies beyond the 300-A radius upper limit of the N2adsorption method and as a result makes no contribution to the measured pore volume. Consequently, the decrease in pore volume and average pore radius is only apparent. Since surface area decreases as pore size increases, the drop in S can be explained on the same basis. For comparison with the data shown in Table I and Figure 5 and for reference when considering cross-linking effects to be discussed shortly, data on the surface properties of some members of the Porapak series of polymer beads (17) are listed in Table 11. The influence of initial dilution on polymer surface properties and their relation to G C utility becomes even more apparent when the detailed pore size distribution is considered. For this purpose, pore size distribution plots, plots of dV,/dr against r , are shown in Figure 6 for the 25, 40, and 50 dilution levels. To conserve space, distribution plots for the remaining members of the series are not included; however, since the three examples shown adequately illustrate the main features of the effects of dilution, little generality is lost. At the 25% level, very little pore structure is observed, and that restricted to 25 A in radius and below. From the plot it is immediately apparent that support-bonded polymers or polymer beads prepared at low dilution levels are not likely to be practical column materials for GC because the polymer formed is essentially a solid mass. On the other hand, the
z
(17) Julian F. Johnson and Edward M. Barrall 11, J . Chrornatogr., 31,547 (1967).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
1751
Table 111. Effect of Cross-Linking on Surface and Pore Volume of DVB-EVB-STY Copolymers Prepared n-Heptane as Initial Diluent with 50 wt DVB, mol reactants S, m2/g Vm =Is 0.77 14.7 51 . O 179 377
5
10 25 38 59 40
-
20
-
I 0
10
40
30
20
50
60
% CROSSLINKING, X DVB CONTENT AS MOL % REACTANTS
Figure 7. Effect of cross-linking on average pore radius of DVB-EVB-STY prepared at 50 wt n-heptane initial dilution
z
0
59 MOLE X DVB (50% C l ) 25 MOLE X DVB(50% C 7 )
A
PORAPAK P
0
e470 -
1
20
-
listed as a function of DVB content in Table 111; and in Figure 7, average pore radius is plotted us. DVB content. Although both surface area and pore volume increase sharply with increasing DVB, the average pore radius is reduced. Evidently, the reduction results from shorter chain segments, on the average, between cross-links which would be expected to restrict pore size. The pore size distribution data for this second series provide further details on the effects of cross-linking. Pore size distribution plots for the 25 and 59% cross-linked samples, chosen as illustrative examples, are shown in Figure 8. Also included is the pore size distribution plot which was determined for Porapak P to serve as a bench mark not only for the other two curves but for the ones in Figure 6 as well. Although plotted in a slightly different manner, the pore size distributions of Porapak Q and T, determined by Johnson and Barrall(17) using N1 adsorption, are available for further reference. From an examination of the data in Figure 8, it is evident that the main effect of increasing DVB content from 25 to 59 mole of reactants is an increase in pore volume at each point over the entire range. This is a general trend observed throughout the series-ie., increasing the amount of cross-linking resulted in increased porosity everywhere in the range of pore sizes examined. Since the porosity of the samples prepared with 5 and 10 mole % DVB was even less than that at 25% DVB, the requirement for minimum crosslinking already mentioned is easily understood.
z
10 -
CONCLUSIONS I
I
0
50
100
150.
200
250
300
rp, A
Figure 8. Effect of cross-linking on pore size distribution. Pore size distribution of Porapak P shown for comparison
polymer with 40% initial dilution shows considerable pore structure in the 10-200 A radius range. As already noted, the support-bonded packing prepared at this dilution level was a useful one. The main effect of increasing dilution to 50% is to shift much of the pore structure to larger pore sizes, extending out to the 300-A limit and undoubtedly beyond. The data for 60% dilution are much like those at 50x showing more of the shift to larger pore sizes. The 92% dilution polymer is very porous; however, the pore structure in the range determinable by N2 adsorption is greatly reduced from that shown at 50% with most of the pore structure beyond the range considered here. Next, the effect of cross-linking on surface properties, as determined from the second series of bulk polymers, will be discussed. Surface area and pore volume for the series are 1752
0.0054 0.082 0.182 0.516 0.97
* ANALYTICAL CHEMISTRY,
A practical method for the preparation of porous DVBEVB-STY copolymers physically bonded to the surface structure of support materials has been described, including an evaluation of the utility of these materials as column packings for GC applications. The advantages of the support-bonded polymer phases, as compared to the polymer beads, include greater column efficiency, more rapid elution, and the capability to adjust capacity ratios over a fairly wide range by varying polymer loading. Because capacity ratios are ordinarily lower, analyses can be extended to higher molecular weight materials. Moreover, those polymer formulations which are too friable in bead form possess sufficient mechanical strength when formed within the pore structure of a support to withstand typical packing procedures. In addition, there exists the further possibility for developing similar support-bonded phases for liquid chromatography and gel permeation. Except in case of chemical attack, the cross-linked polymer will not be dissolved but only swollen by LC solvents. The most apparent disadvantage observed with these packings is tailing of some polar samples. This may be mainly due to adsorption on the support but may also be partly due to the polymer itself. In any case, the use of less active sup-
VOL. 44, NO. 11, SEPTEMBER 1972
ports and the use of an all glass chromatographic system would help to reduce this problem. Based on an investigation of dilution and cross-linking effects on surface area, pore volume, average pore size, and pore size distribution of the resulting polymer and of the relation of these surface properties to actual column characteristics, broad limits for the practical ranges of initial dilution n-heptane and and cross-linking-approximately 30-95 wt DVB 30 or greater mole of monomer reactants-have been formulated. Although no investigation was attempted on
z
other diluent systems or on the addition of other monomers such as acrylonitrile, methyl methacrylate, etc., to alter the chemical nature of the copolymers, there is no apparent reason why properties closely duplicating any of the polymer bead types could not be produced in support-bonded polymers as well as other entirely new types.
z
RECEIVED for review December 8, 1971. Accepted May 1, 1972.
Electrochemical Cell as a Gas Chromatograph-Mass Spectrometer Interface W. D . Dencker and D. R . Rushneckl Northgate Laboratories, Hamden, Conn. 06514
G . R . Shoemake2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif. 91 103 An electrochemical cell employing palladium alloy diffusion electrodes has been developed to remove the hydrogen carrier gas exiting a gas chromatograph. Electrochemical pumping removes greater than 99.9996% of the hydrogen, resulting in mass spectrometer inlet pressures less than Torr. This report gives construction details, electrode activation procedures, and performance characteristics of the cell.
THEUSE OF PALLADIUM ALLOY tubes to remove hydrogen from a carrier gas stream has been described by Lucero and Haley ( I ) and Simmonds et al. (2). In addition, Lucero (3) has given a theoretical treatment of an electrochemical cell employing palladium alloy electrodes for use as a GC/MS interface. This paper reports results of the investigation on which some of the work of Lucero (3) was based. A simplified diagram of the cell is shown in Figure 1. Hydrogen and sample from the gas chromatograph (GC) are fed into a palladium/silver (75:25) tube which serves as the cell anode. As this gas mixture flows through the tube, hydrogen selectively permeates the tube wall and is removed while the sample flows into the mass spectrometer (MS). The hydrogen is transported through the electrolyte following a potential gradient, permeates through the cathode, and is vented or recycled. When the cell is operated in electrochemical balance, the quantity of hydrogen removed at the anode is equal to the quantity of hydrogen generated at the cathode. Although the exact mechanisms of hydrogen transfer are complex and are not well defined, the migration of hydrogen through the Present address, Interface, Inc., Box 297, Fort Collins, Colo. 80521 (Author to whom requests for information should be addressed). * Present address, Texas Engineering & Science Consultants, Houston, Texas 77055. (1) D. P. Lucero and F. C. Haley, J. Gas Cliromatogr., 6 , 477
(1968). (2) P. G. Simmonds, G. R. Shoemake, and J. E. Lovelock, ANAL. CHEM., 42, 881 (1970).
( 3 ) D. P. Lucero, J . Chromntogr.Sci., 9, 105 (1971).
metal is brought about by differences in hydrogen partial pressure on each side of the metal barrier. The use of an electrolyte reduces the effective partial pressure of hydrogen to zero on the electrolyte side of the anode and increases the effective partial pressure of hydrogen to a high value on the electrolyte side of the cathode. Notice that the processes are different for the anode and cathode: on the electrolyte side of the anode there is a hydrogen vacuum whereas on the electrolyte side of the cathode there is a hydrogen excess. The hydrogen “vacuum” on the electrolyte side of the anode is brought about because hydrogen molecules cannot exist at the metal-electrolyte interface. If a hydrogen molecule existed, it would become adsorped on the metal and split into ions. These ions would then combine with hydroxyl ions in the electrolyte to form water. Since hydrogen molecules cannot exist, the effective partial pressure of hydrogen is zero, thus providing a large pressure difference across the tube wall. At the inlet end of the anode, the palladium metal is rich in hydrogen, and the transmission of hydrogen through the metal is controlled by the permeability of the metal. In this case, exchange occurs between hydrogen in the gas phase and adsorped hydrogen. Near the outlet end of the anode, the metal is starved of hydrogen, and transmission is controlled by the frequency of collision of hydrogen molecules with the metal tube walls. In designing the cell, it is important that the outlet end of the anode be kept isolated from all sources of hydrogen so that the metal remains starved of hydrogen. The hydrogen “excess” on the electrolyte side of the cathode is brought about by the decomposition of water into hydroxyl ions and hydrogen. The hydrogen thus generated is free to permeate the cathode or to evolve as gas bubbles from the electrolyte side of the cathode surface. Indeed, when a solid metal counter electrode is substituted for the tube, gas molecules evolve at the electrode surface. If the cathode is not sized and activated properly, pressure can build up within the cell, reducing cell efficiency and presenting the danger of cell rupture. An increase in the permeation rate of hydrogen through the cathode can be achieved by purging the gas phase side of the cathode with oxygen or air, or by pumping. These
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