line, the method cited involves the algebraic determina/~ tion of $2 from In (* + Y) = (CZ- 114 + C I ( ~ Z ) ~CO, where is term 1 and Y is easily evaluated from the starting temperature. C1, Cz, and CO are constants. This value of $2 was then used to calculate all terms, using Equation 11, and term 1 was again evaluated as r / F (sum of terms 2 to k ) . A new value of 4 2 was calculated from the corrected term 1, and the entire process was repeated until successive values of $2 differed by less than 0.002. Usually, about 3 to 8 iterations were necessary.
+
RESULTS The calculation procedure was first tested upon various sets of synthetic data made up to study the importance of void volume and r / F . In addition, experimental data obtained with a capillary column were studied. The results are shown in Tables I and 11. In all cases the standard measurement was taken to be the value calculated by numerical integration. The difference between the value obtained by the series method (ie., the proposed method) and that using only term 1 is the error caused by neglecting the void volume. For a packed column, void volume seldom exceeds 5 ml/g of substrate and r / F is usually of the order of 0.1. This would, for example, be a packed column operating at a flow of 100 ml/min and a temperature rise rate of lo0/ min. Except under extraordinary conditions such as a very high r / F value, the error caused by neglecting void volume is generally negligible. Starting with the most usual conditions, values successively more drastic were chosen, up to a void volume of 180 ml/g with r / F = 0.1, and 5
ml/g with r / F = 10. Under these conditions, the series calculation checked the numerical integration exceedingly well. It is shown, however, that in extremely severe cases such as when both r / F and void volume are large, the series calculation does not converge. With rare exceptions, any calculation on a packed column can be done very well by the series method. In the case of the capillary column, where the void volume was 138 ml/g, temperatures by the series method checked the numerical integration values quite well and also were usually close to the experimental values. These data were obtained with exceptional care by G. Guiochon, Ecole Polytechnique, Paris, France. Here the effect of void volume was evident but errors due to its neglect would not be large. This was a 140-m squalane column, 0.25 mm, with argon as a carrier. It is clear that the series method can be used with capilllary columns as well as packed columns. The approximate limits of applicability of this method are shown by Figure 1, where r / F is plotted us. void volume. It can be seen that successful calculations (those in which the series converges) are defined by what appears to be an hyperbola. In fact, the relationship is approximately ( r / F ) V , = 35. For success, the product should probably be less than 30 for this particular set of parameters. Received for review December 6, 1972. Accepted March 16, 1973. This work was first presented at the National Meeting of the American Chemical Society, Washington, D.C., Sept. 1971. It was supported by Grant No. G.P. 8361 from the National Science Foundation.
Adsorption as a Mechanism for Separation of Nonionic Solutes by Pellicular Ion Exchange Chromatography Joseph J. Pesek and Jack H. Frost Department
of Chemistry, Northern lllinois University, DeKalb, 111. 607 75
Ion exchange chromatography has been successfully used to separate a wide variety of organic compounds. Most separations involve charged species such as acids, bases, and amino acids or charged species produced by complexing agents ( I ) . However, a significant number of separations of nonionic compounds on ion-exchange resins have been reported. Separations of such nonionic solutes as alcohols (Z),sugars (3-11), hydantoins (12), and phenacetin and caffeine (13,14) have been achieved by ion exchange chromatography. W. Rieman and H. F. Walton, "Ion Exchange in Analytical Chernistry," Pergamon Press, Oxford, 1970, pp 162-172. C. M. Wu and R. M. McCready, J. Chromatogr., 57,424 (1971). 0. Samuelson and B. Swenson, Acta Chern. Scand., 16, 2056 (1962). 0. Sarnuelson and B.Swenson, Anal. Chim. Acta, 28,426 (1963). L. I . Larsson and 0. Sarnuelson. Acta Chern. Scand., 19, 1357 (1965). R . M. Saunders, Carbohyd. Res., 7, 76 (1968). M. E. Evans, L. Long, and F. W. Parrish, J. Chrornatogr., 32, 602 (1968). 0. Samuelson and H. Stromberg, Acta Chem. Scand., 22, 1252 (1968).
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The nature of the solute-resin interaction for the above solutes is not completely understood in all cases. There are several possible mechanisms which might contribute to the separation of nonionic compounds by ion exchange chromatography: (1) solvation of the solute by the resin acting as an organic solvent, (2) a sieving effect that would discriminate on the ability of the solute to penetrate into the pores of the resin, (3) adsorption of the solute a t the ion-exchange site, and (4) adsorption of the solute on the resin matrix. One or any combination of the above mechanisms may contribute to the retention of the solute. Mechanisms 3 and 4 may be evaluated by the use of pellicular ion-exchange resins. The pellicular resin con(9) 0. Sarnuelson and H. Stromberg, Fresenius' Z. Anal. Chem., 236, 506 (1968). (10) H. G. Walker and R . M. Saunders, Cereal Sci. Today. 15, 140 (1970). (11) P. Jonsson and 0. Sarnuelson,Ana/. Chern., 39,1156 (1967). (12) M. W. Anders and J . P. Latorre, Anal. Chern., 42, 1430 (1970). (13) R . A. Henry and J. A . Schmit. Chromatographia. 3, 116 (1970). (14) R. L. Stevenson and C.A . Burtts, J. Chromafogr., 61, 253 (1971).
A U G U S T 1973
sists of a solid impervious siliceous core surrounded by the ion exchanger ( 1 5 ) . This type of column packing allows an analysis to be carried out a t high pressures and rapid flow rates. In the pellicular resin, the internal volume of the bead associated with the ordinary ion-exchange resin is very small. The beads used in this study had a 1-micron resin thickness with an overall size of 37-54 microns. The approximate resin volume is 1-6% of the total bead volume. Therefore, mechanisms 1 and 2 which occur in the internal volume of the bead are unlikely although it has been shown that thin films of polymeric stationary phases can contribute to partitioning (16). Retention of the solute by a pellicular resin most likely depends on one of the adsorption processes. For a pellicular resin, a fifth mechanism, adsorption at the resin-support interface, may also cause retention. By varying the type of exchange site, it should be possible to distinguish between the adsorption mechanisms. If adsorption on the resin matrix or a t the resin-support interface is dominant, changing from an anion (trimethylbenzyl ammonium ion) to a cation (sulfonic acid) exchange site should have little effect on retention. Both resins are crosslinked polystyrene. Additional confirmation of the adsorption process may be obtained by studying peak shape and retention volume as a function of sample size (17). Adsorption almost always follows Langmuir-type isotherms which produce asymmetrical peaks with sharp leading boundaries and diffuse trailing boundaries. This asymmetry persists a t even the smallest sample sizes that can be detected. Peaks which are the result of adsorption also show increasing retention volume with decreasing sample size. Solution processes usually follow anti-Langmuir-type isotherms (except when hydrogen bonding or complexation occurs) a t large sample sizes. This produces asymmetrical peaks with diffuse leading boundaries and sharp trailing boundaries.
EXPERIMENTAL Apparatus. A Chromatec 3200 high-speed liquid chromatograph (Chromatec, Ashland, Mass.) equipped with pellicular cation (HF-Pellionex SCX) and anion (AF-Pellionex SAX) exchange columns (2-mm i.d. X 1-m long) was employed. The instrument was equipped with both refractive index and ultraviolet detectors. The UV detector was operated a t 254 nm. Nuclear magnetic resonance spectra of the pellicular ion-exchange resins were obtained on a Varian A 60A spectrometer. All spectra were taken a t room temperature. The Pellionex resins used for the NMR experiments were purchased from Chromatec (supplied by H. Reeve Angel). Chromatographic Procedure. All solutes were dissolved in an appropriate solvent a t concentrations of 0.1 mg/ml, 1.0 mg/ml, and 10 mg/ml. When the refractive index detector was used, the solute was dissolved in the eluant. The eluant was degassed on preparation and in the solvent reservoir a t the beginning of each day. The void volume of the system was determined by using the refractive index detector and injecting a sample of deionized water into the 0.01M KC1 eluant. For large sample sizes, the adjusted retention volume was measured a t the initial maximum point on the peak. NMR Experiments. The samples (approximately 0.5 gram) were placed in a 1.OM salt solution and allowed to settle in the NMR tube for 24 hours. The glass beads (Bio-Glas, 200-325 mesh) were also placed in a 1.OM salt solution and allowed to settle for 24 hours. The beads were silanized with Silyl-8 (Pierce Chemical Co.) by refluxing in a 10% hexane solution for 3 hours. Chemicals. Deionized water was dsed for the salt containing eluants and with reagent grade ethanol for the alcohol-water eluant. The samples were dissolved in the eluant or in reagent grade methanol. The following solutes were used in the purest form available: galactose, fructose, sucrose, 3-0-methyl-2-glucose, o-nitrophenyl-P-D-galactopyranoside,5,5-diphenylhydantoin, 5(15) J. J. Kirkland. J. Chrornatogr. Sci., 7, 361 (1969). (16) J. J. Kirk1and.J. Chrornatogr. Sci., 9, 206 (1971). (17) H. L. Laoand D. E. Martire, Anal. Chem., 44, 498 (1972).
Table I. Adjusted Retention Volumea for Various Nonionic Solutes on Pellicular Ion Exchange Resins Volume Volume cation, ml anion, ml 0 0 Methanol 0 0 Ethanol 0 0 1-Propanol 1-Butanol 0 0 0 0 Fructose 0 0 Sucrose 0 0 Galactose 3-O-meth yl-D-glucose 0 0 0.12 0.12 o-Nitrophenyl-@-D-galactopyranoside 0.75 2.85 5,5-Diphenylhydantoin 5.55 5-(p-hydroxyphenyl)-5-phenylhydantoin 1.04 0.25 0.21 Caffeine 0.48 0.33 Phenacetin Solute
a All samples are 4 pg. Column void volume: cation = 1.07 ml and anion = 1.12 ml. Exchange capacity of both resins is 10 microequivalents per gram.
(p-hydroxyphenyl)-5-phenylhydantoin, methanol, ethanol, 1-propanol, 1-butanol, phenacetin, and caffeine.
RESULTS AND DISCUSSION Table I shows the retention of 4 pg of various nonionic solutes on both pellicular anion and cation exchange columns. The eluant was 0.01M KC1 in all cases except 5,5diphenylhydantoin and 5-(p-hydroxyphenyl)-5-phenylhydantoin on the anion column which was 1.OM KC1. For elution of the hydantoins with 0.01M KCl, both of the solutes were held too strongly to be detected. Lowering the eluant concentration to 0.001M KC1 produced no change in the retention volume for the remaining solutes which showed retention. No retention for these solutes was observed when 1.OM KC1 was used. Slightly longer retention on the cation-exchange column was obtained when 0.01M KC1 was replaced by 0.01M NH4C1. Slightly shorter retention on the anion-exchange column was obtained when 0.01M KC1 was replaced by 0.01M KF. No retention was observed for the alcohols and the unsubstituted sugars when pure water was used as an eluant. Again no retention was observed for the unsubstituted sugars when a 95% ethanol-water mixture was used as an eluant. The results in Table I indicate that one of the adsorption mechanisms contributes to the separation of certain nonionic solutes, but not alcohols and sugars except the phenyl substituted saccharide. In fact, the relatively low distribution coefficient for the phenyl substituted sugar ( K D = 0.06) on the pellicular resin when compared to a value of 1.5-1.7 on a gel resin (10) indicates that surface adsorption interactions between the phenyl groups of the resin and the sugar are quite small. Therefore, mechanism 1 must be dominant in contributing to a long retention volume for ordinary ion-exchange resins because mechanism 2 would decrease the retention volume because of the relatively large size of the molecule. Using an ethanolwater mixture for the other sugars did not produce any retention by the pellicular resin indicating that the previously proposed partition mechanism for sugars in this eluant is correct. Because no retention was observed for the alcohols, the previously proposed sieving mechanism for gel resins appears to be correct ( 2 ) . All of the above results indicate that mechanism 3 is dominant where adsorption occurs. The retention volume of the solute is dependent upon both the concentration and the type of background electrolyte. Lack of retention for certain solutes using 1.OM KC1 and the excessively long retention of the hydantoins using 0.01M KC1 illustrate the effect of concentration. The general effect for all
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 9, A U G U S T 1973
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n 1
:
:
:
k
:
I
'
'
1 10 log sample size (Pg)
l.'O ' 2:O ' 3iO Volume, ml
Figure 2. Adjusted retention volume ( V ' r ) vs. log of the sample ( p g ) for phenacetin on the pellicular cation ( 0 ) and anion
Figure 1. Peak shape as a function of sample size for phenacetin
size
A , 0.4 p g ; 8,4.0 p g ; and C, 40 p g . Eluant is 0.01M KCI with instrument in UV detection mode
Eluant is 0.01M KCI
solutes is shorter retention with increasing background electrolyte concentration. Such dramatic shifts in retention indicate that adsorption of the solute is occurring a t the ion-exchange site-the principal point of interaction on the resin of the electrolyte ion. The shift in retention when changing to the ammonium form of the cation resin and the fluoride form of the anion resin also indicates that adsorption is taking place a t the ion-exchange site. Retention is also dependent upon the type of exchange site. The most dramatic example of this is the change in the retention of the hydantoins. Retention on the anion column is four to five times greater than on the cation column using a 100-fold higher eluant concentration. Because the hydantoins exhibit retention even at high electrolyte concentrations, it is possible that one of the other adsorption mechanisms or solvation by the thin polymeric film might be occurring. However, it is difficult to explain why only these compounds would exhibit such effects and why they would only occur on the anion-exchange resin. Figure 1 shows the peak shape of phenacetin on the cation-exchange column as a function of sample size. Similar behavior is observed for the solute on the anion-exchange column and for the other solutes that exhibit retention on both the anion- and cation-exchange columns. Solutes which have no retention exhibit symmetrical peaks. Langmuir-type isotherms are evident in the shape of the peaks in Figure 1 which exhibit sharp leading boundaries and diffuse trailing boundaries. Such peaks are characteristic of adsorption dominated processes. The symmetrical peaks produced by the nonsorbed solutes indicate that the asymmetrical peaks are produced by column effects rather than by the injection technique used. Figure 2 shows plots of the adjusted retention volume us. log of the sample size for phenacetin on both the anion- and cation-exchange columns. Similar straight line plots are obtained for the other solutes which are retained by the anion- and cation-exchange columns. If the initial retention volume is used, a similar straight line plot GS. log of the sample size is obtained for all retained solutes. 1764
100
( A )exchange column
This is further confirmation of the adsorption process. Increasing retention volume with decreasing sample size is characteristic of an adsorption mechanism. The nuclear magnetic resonance spectra of aqueous suspensions of both the pellicular anion- and cation-exchange resins were obtained. For the cation resin in the K + form, a single line of width 700 Hz was observed. For the anion resin in the C1- form, the line width was 600 Hz. The nuclear magnetic resonance spectrum of an aqueous suspension of 200-325 mesh glass beads in 1.OM KC1 was a single line of width 16 Hz. After the beads were silanized, the line width was 13 Hz. The nuclear magnetic resonance spectra are also indicative of surface adsorption. Ordinary ion-exchange resins usually show two rather sharp resonances ( I @ , one representing bulk water and the other representing internal water. The line width of water in the pellicular resin is probably controlled by the degree of adsorption and by exchange of water between the bulk of solution and sites in the resin, i.e., water surrounding the counter ion in the resin, that which might be adsorbed on the resin matrix, and water associated with the small internal volume of the bead. The case of nonsorbed water is illustrated by the nuclear magnetic resonance spectra of Bio-Glas. A single sharp line is the result. A slight decrease in line width is noted on silanization as the few sites for adsorption on the surface are rendered inert. Because only a single broad resonance is observed for the pellicular ion-exchange resins, the nuclear magnetic resonance spectra indicate rapid exchange of water between resin water and bulk water and/or exchange broadening. This is not the case with ordinary ion-exchange resins that have large internal volumes. In summary, it appears that adsorption is an important mechanism for separation of certain nonionic solutes by ion exchange chromatography. Peak shape and retention volume characteristics indicate that an adsorption process (18) J. E. Gordon, J. Phys. Chem., 66, 1150 (1962)
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 9, A U G U S T 1973
is dominant. No evidence of anti-Langmuir-type isotherms exists a t large sample sizes indicating that solution effects are probably quite small. Changes in retention volume when the concentration of the background electrolyte, type of background electrolyte, and type of ion-exchange site are varied indicate that the adsorption process is primarily occurring at the ion-exchange site. Adsorption on the matrix of the resin and a t the gel-support interface
appears to be of little consequence while adsorption at the ion-exchange site controls retention of these nonionic solutes on pellicular ion-exchange resins and contributes to or controls the retention on ordinary ion-exchange resins. Received for review December 7, 1972. Accepted March 22, 1973. Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
Evaluation of Substrates for Use on a Piezoelectric Detector for Sulfur Dioxide Michael W. Frechette, James L. Fasching, and Douglas M. Rosie Department of Chemistry, University of Rhode Island, Kingston, R. 1. 02887
Sulfur dioxide has long been recognized as a major pollutant from the combustion of fossil fuel. Traditional methods of SO2 analysis such as that of West and Gaeke ( I ) , although useful, suffer markedly from lack of specificity. Conductimetric methods have also been used with some success (2), but the need still exists for a portable, rugged, and yet inexpensive detector that would lend itself easily to automation. The piezoelectric detector developed by King (3-5) has been shown to have promise as a new technique to measure the concentration of various gases. The device employs an electronic oscillator and a vibrating quartz crystal. If the metal electrode of the crystal is coated with an appropriate substrate, a determination of a gaseous component can be selectively performed, since the frequency of the crystal will be lowered. The change in frequency can be expressed by the Sauerbrey equation (6) as follows:
AF = -0.38
F T
AW A
X lo6 X - X -
where I F = change in frequency due to the applied coating (Hz), F = frequency of uncoated crystal (MHz), T = thickness of quartz plate (cm), A W = weight of applied coating (grams), and A = area of electrode coated (cm)2. King (7) has predicted that, under ideal conditions, the “sorption” detector should be capable of sensing as little as gram. Utilizing the piezoelectric “sorption” device, the authors have screened a number of compounds for use as substrates for the determination of sulfur dioxide.
IT
rl CARRIER
0 RECORD
Figure 1. Block diagram of piezoelectric sorption apparatus
tal Corporation Model OT-3 9-MHz circuit. A Monsanto Model lOlA Counter Timer and a Monsanto Model 503A Digital-Analog Converter were used along with a Speedomax H Recorder (Leeds and Northrup) for visual display of the output signal. Two different sample cells were used, cell 1 having a 2 cm3 internal volume, while cell 2 had a 0.10 cm3 volume. Initially cell 1 was used in this study, but it was soon determined that a cell with a smaller internal volume would be more sensitive. A decrease in the dilution of the sample with the gas stream would allow a higher concentration of the SO2 to interact with the applied coating in a given time interval. Samples of pure SO2 were injected through a silicone rubber septum using gas tight syringes or alternatively through a gas sampling valve arrangement.
RESULTS AND DISCUSSION This study has shown that several materials would prove to be satisfactory substrates for the “sorption” detector. The substances tested are listed in Table I. Because of variations in the parameters in the system, all of our data were “normalized” using the following equation:
EXPERIMENTAL The sorption detector system was constructed as shown in Figure 1. The oscillator used in this study was an International CrysP. West and G . Gaeke, Anal. Chem., 28,1916 (1956) R. Martin and J. Grant, Anal. Chem., 37,664 (1965). W. H. King, Anal. Chem., 36, 1735 (1964). W . H. King, U.S. Patent 3,164,004, Jan. 5, 1965. (5) W. H . King. U.S. Patent 3,329,004, J u l y 4 , 1967. (6) G . 2 . Sauerbrey, Physik, 155, 206 (1959). (7) W . H. King, Res./Develop. 20 (4). 28 (1969).
(1) (2) (3) (4)
where AF*so2 = adjusted AFso2, AFso2 = frequency change due to S o n , AF subs. = frequency change due to applied substrate, sample size = cm3 of sample injected. With these considerations in mind, it would appear that tridodecyl amine and tripropyl amine would prove to be the best materials for coatings. Although they are the most sensitive toward S o n , both of these materials
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