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Characterization of porous polymers by polar strength and selectivity
Journal of Chromalography,
404 (1987) 145-154
Elsevier Science Publishers B.V., Amsterdam -
Printed in The Netherlands
CHROM. 19 720
CHARACTERIZATION AND SELECTIVITY
OF POROUS POLYMERS
BY POLAR STRENGTH
MARK A. HEPP* and MATTHEW S. KLEE* Smith Kline & French Laboratories,
Swedeland,
PA (U.S.A.)
(First received February 20th, 1987; revised manuscript received May 14th, 1987)
SUMMARY
Chromosorb Century Series, Porapak, and Gas-Chrom porous polymer adsorbents for gas chromatography were characterized with respect to relative polar strengths and selectivities. The phases fell into two general classes of selectivity based on polar interaction with hydrogen bond donor, acceptor and dipole polarity probes. Suggestions for the most useful phases are given based on widest differences in selectivity and chromatographic characteristics. The results form a basis for straightforward selection of phases for mixed-phase optimizations.
INTRODUCTION
Improving resolution of co-eluting or closely eluting components is the goal of development and optimization of chromatographic methods. In gas chromatography, resolution can be increased by increasing column length or by improving the separation factor, ~1.Increasing the column length necessarily increases analysis time and, in the case of packed columns, can require impractically high carrier pressures, so it is advantageous to change a. The separation factor for two closely eluting components is most dramatically changed by choosing a stationary phase with different selectivity from the one originally tried (a much less dramatic means of changing selectivity might be effected by changing temperature). Mixing stationary phases with different selectivities or coupling different columns in series is often necessary to achieve optimum separation for a mixture of many solutes. To make efficient choices in stationary phases, it is important to know relative selectivities and chromatographic characteristics. This has not been straight forward for the porous polymer phases which are commonly used to separate low molecular weight solutes (gas analysis) and in process environments where durability and stability of the chromatographic system is critical. One has typically had to rely on published examples of column packings which have separated compounds similar to
* Present address: Chemistry Department, Georgetown University, Georgetown, Washington, DC, U.S.A. 0021-9673/87/$03.50
f?
1987 Elsevier Science Publishers B.V.
146
M. A. HEPP.
M. S. KLEE
those of interest or on tables which suggest classes of compounds for which a given stationary phase has been used. Stationary phases for gas chromatography have been characterized in a number of ways. Rohrschneider’ classified stationary phses by their ability to retard polar solute probes. McReynolds2 subtracted the dispersive contribution to the retention index of these probes to yield dl and used the sum of AZ values to describe the total polarity of the phase. 0thers3-5 have used the Rohrschneider-McReynolds probes to classify liquid stationary phases into different groups. Dave6 listed the retention indices of many classes of compounds for the Porapak series and a few of the Chromosorb series porous polymers. The list served as a guide for choosing a phase for a given class of compounds. Caste110 and D’Amato’ used the least polar porous polymer, Chromosorb 106, to estimate retention indices which correspond to the nonpolar contribution to the retention of probe solutes. These values were subtracted from I to yield AZ in the manner of McReynolds, and thereby related the polarity of the other phases. In a subsequent papers they compared the polarity of the polar phases relative to the “Kovats Phase”, a synthetic liquid hydrocarbon phase, C87H176. Using the liquid phase as the non-polar reference resulted in negative AZ values for some of the probes on many of the polar phases. For this reason, it is not appropriate to reference a gas-solid adsorbent to which solutes adsorb to a non-polar gas-liquid phase in which solutes partition. For this study we used graphitized carbon black (GCB) as the non-polar phase because it is based on the same retention mechanism as the porous polymers and gives very low retention indices for the probe solutes (retention is due primarily to non-specific dispersive forces). Graphitized carbon blacks from two different suppliers were evaluated and found to be statistically indistinguishable in their chromatographic behavior towards the polarity probes used. Results from the two GCBs were averaged and used as the values for the reference phase. The selectivity triangle, developed by Snyder l Ofor use in selecting liquid phases for liquid chromatography, was extended for gas-liquid stationary phase selection by Klee et al. l*, and is further extended here to the porous polymer adsorbents. The use of selectivity triangles and prisms for graphic representation of selectivity parameters quickly allows one to see differences in selectivities and strengths of the polar interactions for given phases. EXPERIMENTAL
Chromatograms for all phases except Porapak N and T were run at 200°C on a Perkin-Elmer Sigma 2000 gas chromatograph equipped with flame ionization detector (N and T were done at their maximum operating temperature of 190°C). The data were collected and analyzed with an HP 6000 computer and Nelson Analytical TECON software. A statistical dead time was calculated by the method of Grobler and Baliz12 through linear regression of the retention times of a homologous series of alkanes used for the Kovats plots for at least three separate runs. The calculated dead times were found to be statistically identical to the dead time determined for nitrogen using a thermal conductivity detector. Carbopack C HT (GCB) and Porapak series porous polymers were obtained
CHARACTERIZATION
OF POROUS
POLYMERS
147
from Supelco (Bellefonte, PA, U.S.A.). Graphpak GC (GCB) and Chromosorb Series and Gas-Chrom 220, 254 porous polymers were obtained from Alltech/Applied Science (Deerfield, IL, U.S.A.). These gas-solid adsorbents were packed in 2 m x 3.2 mm stainless-steel columns and were conditioned per manufacturers’ instructions. All columns were operated with helium carrier gas at approximately 30 ml/min. Reagent-grade probe solutes (ethanol, n-butanol, nitromethane, l-nitropropane, and 1,Cdioxane) were kept in septum-cap vials and approximately l-20 ~1 of headspace was injected to determine the retention indices for each probe. At least three injections were made for each probe near the limit of determination in order to minimize potential problems of mixed retention mechanisms and sample overload. Alkane mixtures from a McReynolds’ kit (PolyScience, Niles, IL, U.S.A.) were used to establish Kovats plots for phases with moderate retention. For highly retentive phases, a Scott 1% Cr-C6 normal alkane mix in nitrogen (Alltech Assoc.) was used. Reagent-grade polar test solutes (acetonitrile, 2-propanol, triethylamine, 1,2dichloroethane, and octane) were mixed in a septum-capped vial and sampled in a manner similar to the polarity probes. McReynolds constants (AZ) were calculated by subtracting the average retention indices of the probe solute on GCB from the indices on each of the porous polymer phases. The selectivity parameters (xi) were calculated from the following equation: AIi xi = AI,, + AI,
+ AZ,
where AZb, AZ,, and AZd are the McReynolds constants for the probes n-butanol, nitropropane, and l,Cdioxane, respectively. The denominator (CAZ) describes the relative polar contribution to retention for that stationary phase. The larger Cdl, the more significant the polar interactions are to retention of polar probes relative to dispersive interactions. RESULTS
AND
DISCUSSION
n-Butanol (hydrogen-bond donor) was used to measure H-bond acceptor characteristics, 1,6dioxane (H-bond acceptor) was used to measure H-bond donor characteristics, and nitropropane was used to measure dipole characteristics of the porous polymer phases. It must be stressed that every polar molecule is capable of several polar interactions, and that retention on any particular stationary phase, whether it is liquid or solid, will depend on the total interaction. However, one can gain much insight and utility from the relative interactions of polar probes providing that they have identifiable polar characteristics which predominate. Ethanol and nitromethane were used by Klee et al.’ l originally to allow direct comparison with the solvent groupings on the Snyder selectivity triangle lo. Nitropropane and n-butanol were selected for the present study because their boiling points and retention times are more similar to the third probe, 1,4-dioxane. The longer retention times of the behavior probes resulted in greater accuracy and reproducibility in the determined in-
148
M. A. HEPP,
M. S. KLEE
dices (nitromethane and ethanol are often eluted very close to the dead time under the conditions required for characterization of the phases). The selectivity parameters and CAZ for 16 porous polymer phases are given in Table I. Fig. 1 is a plot of the Chromosorb Series and Gas-Chrom porous polymers on the selectivity triangle. Chromosorb 103 reacted with nitropropane and therefore was not characterized. Likewise, Fig. 2 shows the location of Porapak porous polymers on a similar triangle. The scales of the triangles are magnified (i.e., ranges not O-l) in order to help visualize differences in the phases. Since polar interactions are never “pure” it is impossible ever to have a selectivity parameter of 1 for any particular interaction. The relative position of the phases on the triangle is the important parameter. With the exception of Chromosorb 105, all of the phases fall into two broad groups of similar selectivities outlined with circles in Figs. 1 and 2. The phases in Group I are expected to be more selective for molecules with large dipole moments and H-bond donation than those in Group IT. Again, since retention is governed by the total energy of interaction, the extent to which any selectivity is exhibited depends on the amount of polar interaction relative to non-polar interaction. This is given by the CAZ values and, as was the case for liquid phases’ l, only phases with CAZ values close to or exceeding 1000 exhibit any inherent selectivity. The ambiguity in the minimum required value for the LYAZ required for selectivity to be exhibited exists because phases with selectivities more toward a corner of the selectivity triangle require a lower CAZ in order to exhibit the same influence on retention that phases which have larger CAZ values but which are less pure in selectivity (in the same direction but more toward the center of the selectivity triangle).
TABLE
I
POROUS
POLYMER
SELECTIVITY
Chromosorb 101 Chromosorb 102 Chromosorb 103* Chromosorb 104 ( 190°C) Chromosorb 105 Chromosorb 106 Chromosorb 107 Chromosorb 108 Porapak N Porapak P Porapak PS Porapak Q Porapak QS Porapak R Porapak S Porapak T (190°C) Gas-Chrom 220 Gas-Chrom 254 l
Chromosorb
103 reacts
PARAMETERS
(2Oo’C)
0.258 0.258 _
0.368 0.367
0.374 0.375 _
751.9 487.5 -
0.289 0.315 0.270 0.299 0.297 0.305 0.259 0.265 0.276 0.283 0.290 0.300 0.301 0.263 0.260
0.387 0.356 0.365 0.394 0.384 0.375 0.37 I 0.367 0.366 0.359 0.404 0.374 0.383 0.366 0.368
0.325 0.329 0.366 0.307 0.319 0.320 0.371 0.368 0.358 0.358 0.306 0.326 0.317 0.371 0.373
1624.0 743.6 405.7 802.1 1029.0 667.2 778.3 8SO.S 458.8 441.1 559.0 579.7 925.3 484.2 690.2
with nitropropane,
precluding
determination
of selectivity
parameters.
CHARACTERIZATION
OF POROUS POLYMERS
149
‘;TfiT I ONHKY PtiRSE
acceptor
0 - Chromosort~ llil n - Chromosorb 102 + - Chrumosarb
104
X - Chramosort,
IUS
0 _ Chromasort>
IIJb
V - ChromosorL
ILv
q - Chromosort
donor
Gas
Chrom
220
$-
Gas
Chrom
254
dipole
0.45
0.35
Fig, I. Selectivity triangle showing relative selectivities polymer phases. Chromosorb 103 was not characterized
108
X-
of Chromosorb Series and Gas-Chrom porous due to reactivity with nitropropane probe.
To facilitate comparison, selectivity parameters have been plotted in Fig. 3 as a prism with Cdl being the third dimension. Those phases toward the back (high Znr) have the potential to exhibit the most selectivity. A test mixture was prepared and separated on each of the phases to test the validity of this classification scheme. The test mixture was composed of a proton donor (Zpropanol), a proton acceptor (triethylamine), a weak dipole (1,2-dichloroethane), a strong dipole (acetonitrile) and octane which has no polar functionality. Some of the physical characteristics of the probes are given in Table II. The retention of these solutes on GCB, shown in Fig. 4, is due almost solely to dispersive interactions. The solutes elute in order of their boiling points and molar volumes.
acceptor STATIONARY PHfiSE 0 - Paropak
N
A - Pnropak
P
+ - Poropok
PS
X - Porapak
0
0 - Porapak
OS
V - Porapak
R
fZ - Porapak
S
X - Poropok
T
0
do nor Fig. 2. Selectivity
0.45
triangle
showing
relative
selectivities
of Porapak
dipole porous
polymer
phases.
M. A. HEPP,
Fig. 3. Selectivity prism showing selectivities of the more polar porous polymers.
and Cdl, the relative polar contribution
M. S. KLEE
to retention
for four
Group I phases
Of the phases in Group I, those with significant CA1 values are Porapak T (925) Chromosorb 108 (1020) and Chromosorb 104 (1624). Fig. 5 illustrates selectivities of these phases and Chromosorb 107 with the polar test mix. The Cdl of 802 for Chromosorb 107 indicates that the potential for polar interactions of Chromosorb 107 with polar solutes is not significant compared with its potential for non-polar interactions and is illustrated by the fact that it retains the polar components much less relative to octane than do the two phases with the highest CA1 in Fig. 5. The two test solutes capable of dipole interactions (1,2-dichloroethane and acetonitrile) are retained longer relative to the other polar probes as ZAZ increases. Chromosorb 104, with the highest ZAl, exhibits this selectivity most dramatically for the dipole solutes which are retained longer than the octane peak, and 2-propanol is retained more than triethylamine (the H-acceptor). Chromosorb 108 has an intermediate
TABLE
II
PHYSICAL
PROPERTIES
OF TEST MIXTURE Bding
Acetonitrile 2-Propdnol I ,2-Dichloroethane Triethylamine Octane
point
PROBES
Molar volume (ml/mole)
Dipole moment
(“C) 81.6 82.4 83.5 89.3 125.7
52.5 16.5 79.0 139.1 162.6
3.44 1.66 1.86 0.66 0.0
(Debye)
CHARACTERIZATION
OF POROUS
POLYMERS
275633
12
3
L
5 6 7 Time (m11-1)
Fig. 4. Polar test mixture on graphitized to non-polar, dispersive interactions.
carbon
8
9
10
black (Carbopak
C HT). Retention
is almost entirely due
Cdl, and exhibits the same selectivity as the other two to an extent intermediate between the other two; just as its CAZ would predict. The other phases in Group I (Porapaks S, N, and R) have ,XAZ values which are much less than 1000 and the elution order of test probes is very similar to that 98076
,
3136,
isI’ , 1 -l--T’--T-123~56789
zx
T- -,_7_
‘.r _,‘.. ,--. 10
11
12
Time (min)
Fig. 5. Chromatograms for polar test mix on four of the more polar porous polymers. tonitrile, 2 = isopropanol, 3 = 1,2-dichloroethane, 4 = triethylamine, 5 = octane.
Peaks:
1 = ace-
M. A. HEPP,
152
)
1
‘x
101
,(hrmosorb
3675.
I
M. S. KLEE
1
I
0.5
1.0
1.5
2.0 2.5 Time (min)
3.0
3.5
Fig. 6. Chromatograms for polar test mix on three non-polar 2 = acetonitrile, 3 = 1,2-dichloroethane, 4 = triethylamine,
4.0
porous polymers. 5 = octane.
Peaks:
1 = isopropanol,
on GCB. This shows that Cdl must be near or exceed 1000 for a phase to exhibit its inherent polar selectivity. The base triethylamine is retained longer on Porapaks N and S and with poor peak shape relative to the other phases in Group I and a higher amount must be injected in order to be seen. This indicates at the least very poor adsorption characteristics and at the most reaction with the phases. Retention characteristics of the other probes are as predicted. Group ZZ phases
The Group II phases are those which are shifted more toward the H-donor corner relative to the other phases. These phases can be expected to more selectively retain compounds with H-bond acceptor characteristics. All of these phases have Cdlvalues that are less than 1000 and exhibit their selectivities only to a small degree. In Group II there are only three phases with CAZ values greater than 500; Porapaks P and PS and Chromosorb 101. Porapak PS is the silanized version of P and one would expect it to be less polar than P, however the larger CAZ indicates that a larger portion of its total interaction with polar molecules is due to polar interactions compared with Porapak P. This could mean either an increase in the polar contribution or the same absolute amount of polar contribution with concomitant decrease in nonpolar contribution. The net effect of the difference in CAZ values for Porapak P and PS is the inversion in elution order of 1,2-dichloroethane and triethylamine on Porapak PS, and poorer peak shape for triethylamine (possible acidity in Porapak PS due to byproducts from the silanization). Since the phases of Group II are farther away from the H-acceptor corner of the triangle, it would be expected that they show little affinity for H-donor compounds. This is indeed the case for each of the three phases with ,EAZ over 500
CHARACTERIZATION
OF POROUS
15.3
POLYMERS
(Porapak P and Gas-Chrom 254) 2-propanol elutes before acetonitrile. The remaining phases in Group II show retention characteristics similar to GCB. In some cases, most notably for Gas-Chrom 220 and Chromosorb 102, triethylamine tails greatly indicating that these are not appropriate phases for the analysis of basic compounds. In fact, Chromosorb 103, which could not be characterized due to irreversible adsorption of nitropropane, was found by diffuse reflectance Fourier transform infrared spectrometry to have free carboxylic acid groups13. A sample of Chromosorb 103 from another supplier was found not to have carboxyl groups to the extent of the original, but the apparent batch to batch variation indicates that Chromosorb 103 is also not a good choice from the available phases. Gas-Chrom 254, Porapak P, and Chromosorb 101 have virtually identical selectivity parameters and Cdl values (suggesting that they are the same polymer) and showed excellent efficiencies and chromatographic characteristics, even for triethylamine. There was some indication that these three phases may change selectivity over time (possible oxidation) in that triethylamine and 1,Zdichloroethane are resolved on new columns of Porapak P and Gas-Chrom 254, but co-eluted on a column of Chromosorb 101 and a column of Prapak P which had been used for approximately three months. Chromatograms for these columns are compared in Fig. 6. The outlier Chromosorb 105 (a polyaromatic phase), although potentially useful because of its apparently unique selectivity, had very poor chromatographic efficiency (80 theoretical plates/m) which overwhelmed any advantage in selectivity. CONCLUSIONS
The porous polymer phases with the highest CAZ values all have similar polar selectivity favoring interaction with dipoles. These are Porapak T and Chromosorbs 108 and 104. Of these, Chromosorb 104 is most apt to yield a change in a for closely eluting polar molecules, especially if they do not have the same polar functional group(s). Porous polymer phases in Group II exhibit virtually no selective polar interactions and the best choices for routine use are probably those with the highest efficiencies and best peak shape for polar solutes. The phases in this Group generally have the higher efficiencies than those in Group I. Suggestions for general purpose phases to have available for methods development and possible optimizations (using methods such as window diagramming) are Porapak P, Gas-Chrom 254, or Chromosorb 101 as moderately non-selective phases, and Porapak T and Chromosorb 104 as polar phases. REFERENCES I L. Rohrschneider, J. Chromutogr., 22 (1966) 6. 2 W. 0. McReynolds, J. Chromatogr. Sci., 8 (1970) 685. 3 R. V. Golovnya and T. A. Misharina, J. High Resolut. Chromatogr. Chromatogr. 4. 4 F. Saura-Calixto, A. Garcia-Raso, J. Canellas and J. Garcia-Raso, J. Chromatogr. 5 L. V. Semenchenko and M. S. Vigdergauz, J. Chromatogr., 245 (1982) 177. 6 S. B. Dave, J. Chromatogr. Sci., 7 (1969) 389. 7 G. Castello and G. D’Amato, J. Chromatokr., 254 (1983) 69.
Commun.,
3 (1980)
Sci., 21 (1983) 267.
154 8 9 IO II I2 I3
M. A. HEPP,
G. Castello and G. D’Amato, J. Chromatogr., 269 (1983) 153. L. R. Snyder, J. L. Glajch and J. J. Kirkland, J. Chromatogr., 218 (1981) 299. L. R. Snyder, J. Chromatogr., 92 (1974) 223. M. S. Klee, M. A. Kaiser and K. B. Laughlin, J. Chromatogr., 279 (1983) 681. A. Grobler and G. Balizs, J. Chromatogr. Sci., I2 (1974) 57. M. A. Kaiser, E. I. DuPont de Nemours & Co., Inc., Experimental Station, Wilmington, communication.
M. S. KLEE
DE, personal
404 (1987) 145-154
Elsevier Science Publishers B.V., Amsterdam -
Printed in The Netherlands
CHROM. 19 720
CHARACTERIZATION AND SELECTIVITY
OF POROUS POLYMERS
BY POLAR STRENGTH
MARK A. HEPP* and MATTHEW S. KLEE* Smith Kline & French Laboratories,
Swedeland,
PA (U.S.A.)
(First received February 20th, 1987; revised manuscript received May 14th, 1987)
SUMMARY
Chromosorb Century Series, Porapak, and Gas-Chrom porous polymer adsorbents for gas chromatography were characterized with respect to relative polar strengths and selectivities. The phases fell into two general classes of selectivity based on polar interaction with hydrogen bond donor, acceptor and dipole polarity probes. Suggestions for the most useful phases are given based on widest differences in selectivity and chromatographic characteristics. The results form a basis for straightforward selection of phases for mixed-phase optimizations.
INTRODUCTION
Improving resolution of co-eluting or closely eluting components is the goal of development and optimization of chromatographic methods. In gas chromatography, resolution can be increased by increasing column length or by improving the separation factor, ~1.Increasing the column length necessarily increases analysis time and, in the case of packed columns, can require impractically high carrier pressures, so it is advantageous to change a. The separation factor for two closely eluting components is most dramatically changed by choosing a stationary phase with different selectivity from the one originally tried (a much less dramatic means of changing selectivity might be effected by changing temperature). Mixing stationary phases with different selectivities or coupling different columns in series is often necessary to achieve optimum separation for a mixture of many solutes. To make efficient choices in stationary phases, it is important to know relative selectivities and chromatographic characteristics. This has not been straight forward for the porous polymer phases which are commonly used to separate low molecular weight solutes (gas analysis) and in process environments where durability and stability of the chromatographic system is critical. One has typically had to rely on published examples of column packings which have separated compounds similar to
* Present address: Chemistry Department, Georgetown University, Georgetown, Washington, DC, U.S.A. 0021-9673/87/$03.50
f?
1987 Elsevier Science Publishers B.V.
146
M. A. HEPP.
M. S. KLEE
those of interest or on tables which suggest classes of compounds for which a given stationary phase has been used. Stationary phases for gas chromatography have been characterized in a number of ways. Rohrschneider’ classified stationary phses by their ability to retard polar solute probes. McReynolds2 subtracted the dispersive contribution to the retention index of these probes to yield dl and used the sum of AZ values to describe the total polarity of the phase. 0thers3-5 have used the Rohrschneider-McReynolds probes to classify liquid stationary phases into different groups. Dave6 listed the retention indices of many classes of compounds for the Porapak series and a few of the Chromosorb series porous polymers. The list served as a guide for choosing a phase for a given class of compounds. Caste110 and D’Amato’ used the least polar porous polymer, Chromosorb 106, to estimate retention indices which correspond to the nonpolar contribution to the retention of probe solutes. These values were subtracted from I to yield AZ in the manner of McReynolds, and thereby related the polarity of the other phases. In a subsequent papers they compared the polarity of the polar phases relative to the “Kovats Phase”, a synthetic liquid hydrocarbon phase, C87H176. Using the liquid phase as the non-polar reference resulted in negative AZ values for some of the probes on many of the polar phases. For this reason, it is not appropriate to reference a gas-solid adsorbent to which solutes adsorb to a non-polar gas-liquid phase in which solutes partition. For this study we used graphitized carbon black (GCB) as the non-polar phase because it is based on the same retention mechanism as the porous polymers and gives very low retention indices for the probe solutes (retention is due primarily to non-specific dispersive forces). Graphitized carbon blacks from two different suppliers were evaluated and found to be statistically indistinguishable in their chromatographic behavior towards the polarity probes used. Results from the two GCBs were averaged and used as the values for the reference phase. The selectivity triangle, developed by Snyder l Ofor use in selecting liquid phases for liquid chromatography, was extended for gas-liquid stationary phase selection by Klee et al. l*, and is further extended here to the porous polymer adsorbents. The use of selectivity triangles and prisms for graphic representation of selectivity parameters quickly allows one to see differences in selectivities and strengths of the polar interactions for given phases. EXPERIMENTAL
Chromatograms for all phases except Porapak N and T were run at 200°C on a Perkin-Elmer Sigma 2000 gas chromatograph equipped with flame ionization detector (N and T were done at their maximum operating temperature of 190°C). The data were collected and analyzed with an HP 6000 computer and Nelson Analytical TECON software. A statistical dead time was calculated by the method of Grobler and Baliz12 through linear regression of the retention times of a homologous series of alkanes used for the Kovats plots for at least three separate runs. The calculated dead times were found to be statistically identical to the dead time determined for nitrogen using a thermal conductivity detector. Carbopack C HT (GCB) and Porapak series porous polymers were obtained
CHARACTERIZATION
OF POROUS
POLYMERS
147
from Supelco (Bellefonte, PA, U.S.A.). Graphpak GC (GCB) and Chromosorb Series and Gas-Chrom 220, 254 porous polymers were obtained from Alltech/Applied Science (Deerfield, IL, U.S.A.). These gas-solid adsorbents were packed in 2 m x 3.2 mm stainless-steel columns and were conditioned per manufacturers’ instructions. All columns were operated with helium carrier gas at approximately 30 ml/min. Reagent-grade probe solutes (ethanol, n-butanol, nitromethane, l-nitropropane, and 1,Cdioxane) were kept in septum-cap vials and approximately l-20 ~1 of headspace was injected to determine the retention indices for each probe. At least three injections were made for each probe near the limit of determination in order to minimize potential problems of mixed retention mechanisms and sample overload. Alkane mixtures from a McReynolds’ kit (PolyScience, Niles, IL, U.S.A.) were used to establish Kovats plots for phases with moderate retention. For highly retentive phases, a Scott 1% Cr-C6 normal alkane mix in nitrogen (Alltech Assoc.) was used. Reagent-grade polar test solutes (acetonitrile, 2-propanol, triethylamine, 1,2dichloroethane, and octane) were mixed in a septum-capped vial and sampled in a manner similar to the polarity probes. McReynolds constants (AZ) were calculated by subtracting the average retention indices of the probe solute on GCB from the indices on each of the porous polymer phases. The selectivity parameters (xi) were calculated from the following equation: AIi xi = AI,, + AI,
+ AZ,
where AZb, AZ,, and AZd are the McReynolds constants for the probes n-butanol, nitropropane, and l,Cdioxane, respectively. The denominator (CAZ) describes the relative polar contribution to retention for that stationary phase. The larger Cdl, the more significant the polar interactions are to retention of polar probes relative to dispersive interactions. RESULTS
AND
DISCUSSION
n-Butanol (hydrogen-bond donor) was used to measure H-bond acceptor characteristics, 1,6dioxane (H-bond acceptor) was used to measure H-bond donor characteristics, and nitropropane was used to measure dipole characteristics of the porous polymer phases. It must be stressed that every polar molecule is capable of several polar interactions, and that retention on any particular stationary phase, whether it is liquid or solid, will depend on the total interaction. However, one can gain much insight and utility from the relative interactions of polar probes providing that they have identifiable polar characteristics which predominate. Ethanol and nitromethane were used by Klee et al.’ l originally to allow direct comparison with the solvent groupings on the Snyder selectivity triangle lo. Nitropropane and n-butanol were selected for the present study because their boiling points and retention times are more similar to the third probe, 1,4-dioxane. The longer retention times of the behavior probes resulted in greater accuracy and reproducibility in the determined in-
148
M. A. HEPP,
M. S. KLEE
dices (nitromethane and ethanol are often eluted very close to the dead time under the conditions required for characterization of the phases). The selectivity parameters and CAZ for 16 porous polymer phases are given in Table I. Fig. 1 is a plot of the Chromosorb Series and Gas-Chrom porous polymers on the selectivity triangle. Chromosorb 103 reacted with nitropropane and therefore was not characterized. Likewise, Fig. 2 shows the location of Porapak porous polymers on a similar triangle. The scales of the triangles are magnified (i.e., ranges not O-l) in order to help visualize differences in the phases. Since polar interactions are never “pure” it is impossible ever to have a selectivity parameter of 1 for any particular interaction. The relative position of the phases on the triangle is the important parameter. With the exception of Chromosorb 105, all of the phases fall into two broad groups of similar selectivities outlined with circles in Figs. 1 and 2. The phases in Group I are expected to be more selective for molecules with large dipole moments and H-bond donation than those in Group IT. Again, since retention is governed by the total energy of interaction, the extent to which any selectivity is exhibited depends on the amount of polar interaction relative to non-polar interaction. This is given by the CAZ values and, as was the case for liquid phases’ l, only phases with CAZ values close to or exceeding 1000 exhibit any inherent selectivity. The ambiguity in the minimum required value for the LYAZ required for selectivity to be exhibited exists because phases with selectivities more toward a corner of the selectivity triangle require a lower CAZ in order to exhibit the same influence on retention that phases which have larger CAZ values but which are less pure in selectivity (in the same direction but more toward the center of the selectivity triangle).
TABLE
I
POROUS
POLYMER
SELECTIVITY
Chromosorb 101 Chromosorb 102 Chromosorb 103* Chromosorb 104 ( 190°C) Chromosorb 105 Chromosorb 106 Chromosorb 107 Chromosorb 108 Porapak N Porapak P Porapak PS Porapak Q Porapak QS Porapak R Porapak S Porapak T (190°C) Gas-Chrom 220 Gas-Chrom 254 l
Chromosorb
103 reacts
PARAMETERS
(2Oo’C)
0.258 0.258 _
0.368 0.367
0.374 0.375 _
751.9 487.5 -
0.289 0.315 0.270 0.299 0.297 0.305 0.259 0.265 0.276 0.283 0.290 0.300 0.301 0.263 0.260
0.387 0.356 0.365 0.394 0.384 0.375 0.37 I 0.367 0.366 0.359 0.404 0.374 0.383 0.366 0.368
0.325 0.329 0.366 0.307 0.319 0.320 0.371 0.368 0.358 0.358 0.306 0.326 0.317 0.371 0.373
1624.0 743.6 405.7 802.1 1029.0 667.2 778.3 8SO.S 458.8 441.1 559.0 579.7 925.3 484.2 690.2
with nitropropane,
precluding
determination
of selectivity
parameters.
CHARACTERIZATION
OF POROUS POLYMERS
149
‘;TfiT I ONHKY PtiRSE
acceptor
0 - Chromosort~ llil n - Chromosorb 102 + - Chrumosarb
104
X - Chramosort,
IUS
0 _ Chromasort>
IIJb
V - ChromosorL
ILv
q - Chromosort
donor
Gas
Chrom
220
$-
Gas
Chrom
254
dipole
0.45
0.35
Fig, I. Selectivity triangle showing relative selectivities polymer phases. Chromosorb 103 was not characterized
108
X-
of Chromosorb Series and Gas-Chrom porous due to reactivity with nitropropane probe.
To facilitate comparison, selectivity parameters have been plotted in Fig. 3 as a prism with Cdl being the third dimension. Those phases toward the back (high Znr) have the potential to exhibit the most selectivity. A test mixture was prepared and separated on each of the phases to test the validity of this classification scheme. The test mixture was composed of a proton donor (Zpropanol), a proton acceptor (triethylamine), a weak dipole (1,2-dichloroethane), a strong dipole (acetonitrile) and octane which has no polar functionality. Some of the physical characteristics of the probes are given in Table II. The retention of these solutes on GCB, shown in Fig. 4, is due almost solely to dispersive interactions. The solutes elute in order of their boiling points and molar volumes.
acceptor STATIONARY PHfiSE 0 - Paropak
N
A - Pnropak
P
+ - Poropok
PS
X - Porapak
0
0 - Porapak
OS
V - Porapak
R
fZ - Porapak
S
X - Poropok
T
0
do nor Fig. 2. Selectivity
0.45
triangle
showing
relative
selectivities
of Porapak
dipole porous
polymer
phases.
M. A. HEPP,
Fig. 3. Selectivity prism showing selectivities of the more polar porous polymers.
and Cdl, the relative polar contribution
M. S. KLEE
to retention
for four
Group I phases
Of the phases in Group I, those with significant CA1 values are Porapak T (925) Chromosorb 108 (1020) and Chromosorb 104 (1624). Fig. 5 illustrates selectivities of these phases and Chromosorb 107 with the polar test mix. The Cdl of 802 for Chromosorb 107 indicates that the potential for polar interactions of Chromosorb 107 with polar solutes is not significant compared with its potential for non-polar interactions and is illustrated by the fact that it retains the polar components much less relative to octane than do the two phases with the highest CA1 in Fig. 5. The two test solutes capable of dipole interactions (1,2-dichloroethane and acetonitrile) are retained longer relative to the other polar probes as ZAZ increases. Chromosorb 104, with the highest ZAl, exhibits this selectivity most dramatically for the dipole solutes which are retained longer than the octane peak, and 2-propanol is retained more than triethylamine (the H-acceptor). Chromosorb 108 has an intermediate
TABLE
II
PHYSICAL
PROPERTIES
OF TEST MIXTURE Bding
Acetonitrile 2-Propdnol I ,2-Dichloroethane Triethylamine Octane
point
PROBES
Molar volume (ml/mole)
Dipole moment
(“C) 81.6 82.4 83.5 89.3 125.7
52.5 16.5 79.0 139.1 162.6
3.44 1.66 1.86 0.66 0.0
(Debye)
CHARACTERIZATION
OF POROUS
POLYMERS
275633
12
3
L
5 6 7 Time (m11-1)
Fig. 4. Polar test mixture on graphitized to non-polar, dispersive interactions.
carbon
8
9
10
black (Carbopak
C HT). Retention
is almost entirely due
Cdl, and exhibits the same selectivity as the other two to an extent intermediate between the other two; just as its CAZ would predict. The other phases in Group I (Porapaks S, N, and R) have ,XAZ values which are much less than 1000 and the elution order of test probes is very similar to that 98076
,
3136,
isI’ , 1 -l--T’--T-123~56789
zx
T- -,_7_
‘.r _,‘.. ,--. 10
11
12
Time (min)
Fig. 5. Chromatograms for polar test mix on four of the more polar porous polymers. tonitrile, 2 = isopropanol, 3 = 1,2-dichloroethane, 4 = triethylamine, 5 = octane.
Peaks:
1 = ace-
M. A. HEPP,
152
)
1
‘x
101
,(hrmosorb
3675.
I
M. S. KLEE
1
I
0.5
1.0
1.5
2.0 2.5 Time (min)
3.0
3.5
Fig. 6. Chromatograms for polar test mix on three non-polar 2 = acetonitrile, 3 = 1,2-dichloroethane, 4 = triethylamine,
4.0
porous polymers. 5 = octane.
Peaks:
1 = isopropanol,
on GCB. This shows that Cdl must be near or exceed 1000 for a phase to exhibit its inherent polar selectivity. The base triethylamine is retained longer on Porapaks N and S and with poor peak shape relative to the other phases in Group I and a higher amount must be injected in order to be seen. This indicates at the least very poor adsorption characteristics and at the most reaction with the phases. Retention characteristics of the other probes are as predicted. Group ZZ phases
The Group II phases are those which are shifted more toward the H-donor corner relative to the other phases. These phases can be expected to more selectively retain compounds with H-bond acceptor characteristics. All of these phases have Cdlvalues that are less than 1000 and exhibit their selectivities only to a small degree. In Group II there are only three phases with CAZ values greater than 500; Porapaks P and PS and Chromosorb 101. Porapak PS is the silanized version of P and one would expect it to be less polar than P, however the larger CAZ indicates that a larger portion of its total interaction with polar molecules is due to polar interactions compared with Porapak P. This could mean either an increase in the polar contribution or the same absolute amount of polar contribution with concomitant decrease in nonpolar contribution. The net effect of the difference in CAZ values for Porapak P and PS is the inversion in elution order of 1,2-dichloroethane and triethylamine on Porapak PS, and poorer peak shape for triethylamine (possible acidity in Porapak PS due to byproducts from the silanization). Since the phases of Group II are farther away from the H-acceptor corner of the triangle, it would be expected that they show little affinity for H-donor compounds. This is indeed the case for each of the three phases with ,EAZ over 500
CHARACTERIZATION
OF POROUS
15.3
POLYMERS
(Porapak P and Gas-Chrom 254) 2-propanol elutes before acetonitrile. The remaining phases in Group II show retention characteristics similar to GCB. In some cases, most notably for Gas-Chrom 220 and Chromosorb 102, triethylamine tails greatly indicating that these are not appropriate phases for the analysis of basic compounds. In fact, Chromosorb 103, which could not be characterized due to irreversible adsorption of nitropropane, was found by diffuse reflectance Fourier transform infrared spectrometry to have free carboxylic acid groups13. A sample of Chromosorb 103 from another supplier was found not to have carboxyl groups to the extent of the original, but the apparent batch to batch variation indicates that Chromosorb 103 is also not a good choice from the available phases. Gas-Chrom 254, Porapak P, and Chromosorb 101 have virtually identical selectivity parameters and Cdl values (suggesting that they are the same polymer) and showed excellent efficiencies and chromatographic characteristics, even for triethylamine. There was some indication that these three phases may change selectivity over time (possible oxidation) in that triethylamine and 1,Zdichloroethane are resolved on new columns of Porapak P and Gas-Chrom 254, but co-eluted on a column of Chromosorb 101 and a column of Prapak P which had been used for approximately three months. Chromatograms for these columns are compared in Fig. 6. The outlier Chromosorb 105 (a polyaromatic phase), although potentially useful because of its apparently unique selectivity, had very poor chromatographic efficiency (80 theoretical plates/m) which overwhelmed any advantage in selectivity. CONCLUSIONS
The porous polymer phases with the highest CAZ values all have similar polar selectivity favoring interaction with dipoles. These are Porapak T and Chromosorbs 108 and 104. Of these, Chromosorb 104 is most apt to yield a change in a for closely eluting polar molecules, especially if they do not have the same polar functional group(s). Porous polymer phases in Group II exhibit virtually no selective polar interactions and the best choices for routine use are probably those with the highest efficiencies and best peak shape for polar solutes. The phases in this Group generally have the higher efficiencies than those in Group I. Suggestions for general purpose phases to have available for methods development and possible optimizations (using methods such as window diagramming) are Porapak P, Gas-Chrom 254, or Chromosorb 101 as moderately non-selective phases, and Porapak T and Chromosorb 104 as polar phases. REFERENCES I L. Rohrschneider, J. Chromutogr., 22 (1966) 6. 2 W. 0. McReynolds, J. Chromatogr. Sci., 8 (1970) 685. 3 R. V. Golovnya and T. A. Misharina, J. High Resolut. Chromatogr. Chromatogr. 4. 4 F. Saura-Calixto, A. Garcia-Raso, J. Canellas and J. Garcia-Raso, J. Chromatogr. 5 L. V. Semenchenko and M. S. Vigdergauz, J. Chromatogr., 245 (1982) 177. 6 S. B. Dave, J. Chromatogr. Sci., 7 (1969) 389. 7 G. Castello and G. D’Amato, J. Chromatokr., 254 (1983) 69.
Commun.,
3 (1980)
Sci., 21 (1983) 267.
154 8 9 IO II I2 I3
M. A. HEPP,
G. Castello and G. D’Amato, J. Chromatogr., 269 (1983) 153. L. R. Snyder, J. L. Glajch and J. J. Kirkland, J. Chromatogr., 218 (1981) 299. L. R. Snyder, J. Chromatogr., 92 (1974) 223. M. S. Klee, M. A. Kaiser and K. B. Laughlin, J. Chromatogr., 279 (1983) 681. A. Grobler and G. Balizs, J. Chromatogr. Sci., I2 (1974) 57. M. A. Kaiser, E. I. DuPont de Nemours & Co., Inc., Experimental Station, Wilmington, communication.
M. S. KLEE
DE, personal