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(7) Hofmann, K.; Halasz, I. J . Chromatogr. 1979, 173, 211. (8) Hofmann, K.; Halasz, I. J . Chromatogr. lg80, 199, 3. (9) Synder, L. R.; Adler, H. J. Anal. Chem. 1976,48, 1017. (10) Snyder, L. R.; Adler, H. J. Anal. Chem. 1976,48, 1022. (11) Snyder, L. R. J. Chromatogr. 1976, 125, 287. (12) Huber, J. F. K.; Mulsman, J. A. R. J.; Meljers, C. A. M. J. Chromatogr. 1971, 62, 79. (13) Coq, B.; Cretler, G.; Rocca, J. L.; Porthault, M. J . Chromatogr. Sci. 1981, 19, 1. (14) Scholten, A. H. M. T.; Brlnkman, U. A. Th.; Frei, R. W. J. Chromatogr. 1981,205, 229. (15) Karlberg, B.; Thelander, S. Anal. Chim. Acta 1978, 9 8 , 1. (16) Bergamln, H.; Medeiros, I . X.; Reis, B. F.; Zagatto, E. A. G. Anal. Chlm. Acta 1978, 101, 9. (17) Nord, L.; Karlberg, B. Anal. Chim. Acta 1980, 718, 285.
(18) Karlberg, B.; Thelander, S. Anal. Chim. Acta 1980, 714, 129. Introduction to Modern Liquid Chromatography"; Wlley: New York, 1979. (20) Brlnkman, U. A. Th.; Welllng, P. L. M.; de Vries, G.; Scholten, A. H. M. T.; Frei, R. W. J. Chromatogr. 1981,217, 463. (21) Lawrence, J. F.; Brlnkman, U. A. Th.; Frei, R. W. J. Chromatogr. 1979, 185, 473. (22) Schlabach, T. D.; Regnler, F. J. Chromatogr. 1978, 158, 349. (23) Studebaker, J. F. J . Chromatogr. 1979, 185, 497. (24) Cockshott, I. D.; Payne, R.; Copsey, P. 8.research disclosure, Nov 1979,p 639.
(19) Snyder, L. R.; Kirkland, J. J.
Received for review May 6,1981.Resubmitted May 18,1982. Accepted June 7, 1982.
Molten Organic Salt Phase for Gas-Liquid Chromatography Frank Pacholec, Hal T. Butler, and Colln
F. Poole"
Department of Chemistty, Wayne State University, Detroit, Michlgan 48202
The molten organlc salt ethylammonlum nltrate Is shown to be sultable for use as a statlonary phase In gas-liquid chromatography. I t has a usable temperature range of ~ 4 0 - 1 2 0 OC. I t Is shown to behave as a polar llquld wlth a stronger lnteractlon than Carbowax 20M for test compounds having large dlpole or hydrogen-bondlng functlonal groups. The chromatographicproperties of the molten salt are illustrated for the separatlon of alcohols and monofunctional benzene derlvatlves. Amlnes are not eluted from the column withln the accessible operatlng temperature range for the stationary phase.
Separations by gas-liquid chromatography occur due to differences in the residence time of the solutes in the stationary phase. All solutes have the same residence time in the mobile phase. Consequently, retention, selectivity, and resolution are adjusted by changing the stationary phase or the experimental operating conditions, as appropriate, in gas-liquid chromatography. This realization has led to the description of numerous liquid phases for general use in gas-liquid chromatography (I). Indeed, the expression "stationary phase pollution" has been coined to describe the substantial redundancy that exists in the market place for the sale of liquid phases having essentially identical properties as well as the tendency of some vendors to repackage common liquid phases under their own brand name (2-4). In recent years, this situation has eased somewhat due to the general acceptance of the Rohrschneider/McReynolds schemes for stationary phase characterization and their widespread publication in the scientific and trade literature (1). In light of the large number of stationary phases already available, there would seem to be only two reasons for the introduction of further phases. Firstly, a need exists for thermally stable polar reference stationary phases. As most common liquid phases are polymeric materials defined in terms of an average molecular weight, they have properties which may vary from batch to batch. Further composition changes may occur due to the selective loss of low-molecular-weight oligomers during column conditioning. This problem has been addressed by Kovats, who prepared a synthetic hydrocarbon (24,24-diethyl-19,29-dioctadecyl-
hepatetracontane, CS7Hl7,Jfor use as a nonpolar reference stationary phase (5, 6). We have made similar attempts to prepare polar reference stationary phases having a mphenylene oxide backbone substituted with polar functional groups (7). These phases are characterized by a clearly defiied chemical structure having physical and chromatographic properties independent of the method of synthesis. The second reason for introducing a new stationary phase would be to provide some unique selectivity advantage over available materials. For example, the groups of Bayer (8)and Verzele (9) have described the preparation of stationary phases with chiral centers for the separation of optical isomers. In general, selectivity is mainly a property of the polar interactions of the stationary phase with the solute and is governed by the type and concentration of functional groups present in the phase. The majority of polar stationary phases contain cyano, nitro, amine, or amide functional groups which influence retention largely through dipole and hydrogen bonding interactions. In this paper we wish to describe our work using a molten organic salt as a novel stationary phase containing ionic groups. This material was investigated as its ionic character is likely to promote selective interactions with polar solutes which differ in both magnitude and type from those obtained with conventional phases. The use of salt systems as stationary phases in gas chromatography is not a new concept. Saturated solutions of silver nitrate in ethylene glycol, glycerol, and benzyl cyanide have been used as selective phases for the separation of unsaturated compounds by charge transfer interactions (10-12). Fused inorganic salt systems have been used to separate mixtures of various inorganic compounds (1,13,14). In these systems, separations were achieved by selective complexation of the solutes within the molten salt stationary phase. These inorganic melts are generally unsuitable for the separation of organic compounds due to the poor solubility of the organic solutes in the inorganic salt systems. It is very likely that organic solutes would be much more soluble in a molten organic salt so that partitioning between the stationary and mobile phases would be possible. This partitioning, in conjunction with any complexation/ionic interactions which may occur between the organic solute and the molten salt stationary phase should lead to some intriguing separation possibilities. To test this hypothesis, ethylammonium nitrate
0003-2700/82/0354-1938$01.25/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
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Table I. A Comparison of Stationary Phase Selectivity for the Ten McReynolds Test Probes on Ethylammonium Nitrate and Carbowax 20Ma retention time, min capacity factor ethylethylammonium Carbowax ammonium Carbowax ratio test probe bp, "C nitrate 20M nitrate 20M KWIKCW 0.79 1.5 1.9 benzene 0.40 80.1 0.50 4.16 21.2 5.1 5.33 1.30 117.3 1-butanol 1.94 8.6 4.4 2.17 1.13 101.8 1,4-dioxane 1.48 13.6 9.2 3.43 2.33 130.5 1-nitropropane 4.42 36.7 8.3 9.20 2.10 115.5 pyridine 3.14 11.4 3.6 2.87 120.5 0.93 2-methyl-2-pientanol 0.72 2.0 2.8 0.53 0.73 130.5 1-iodobutane 0.58 1.5 2.6 0.67 0.40 138.8 2-octyne 1.59 4.1 2.6 0.67 2-pentanone 1.05 102 0.65 :1.9 2.9 0.77 0.50 161.1 cis-hydrindance a
Column 6 ft X 0.1215 in. i.d., 10% (arlw), nitrogen linear velocity 12.2 cm s-', and column temperature 80 "C.
was selected as a candidate phase.
EXPERIMENTAL SECTION Unless otherwise statad, all chemicals and solvents were general laboratory or analytical grade in the highest purity available. Column packings were prepared by using the rotovapor technique and Gas-Chrom Q (1001-120 mesh) as support. Methanol or acetone were used as sollvent for ethylammonium nitrate. The packings containing stationary phase loadings of 2-10% (w/w) were air-dried at 40 "C and packed with the aid of vacuum in columns of 3 to 9 ft lengths. A Varian 3700 gas chromatograph with heated on-column injector, temperature programmer, and flame-ionization detectors was used for column evaluation. Synthesis of Ethylaimmonium Nitrate. A solution of 70% (v/v) aqueous ethylamine was neutralized by the slow addition of 20% (v/v) nitric acid over a period of about 1h (15). Excess water was removed on a rotovapor at 90 "C. Final traces of water were removed by double lyophilization. The anhydrous ethylammonium nitrate was further purified by extraction with methylene chloride. Thle final product was a stable, viscous, odorless, clear liquid.
RESULTS AND DISCUSSION To date, there have been no reports of studies using organic molten salts as stationary phases in gas chromatography. There are logical reasons for this. In general, most organic ionic melts have poor thermal stability with decomposition temperatures below or close to their melting points. Their thermal stability is very much influenced by the presence of trace impurities in the gtarting materials used for their synthesis. Many salts are air sensitive. Im recent times the properties of organic molten salts have become better understood and several examples of organic ionic melts thermally stable above their clearing temperatures are known (16-19). As a candidate stationary phase ethylammonium nitrate was selected for study. Its choice was influenced by reports of good chemical and thermal stability, existence as a liquid over a wide temperature range, and some physical evidence indicating solution properties similar to water (19). The column operating temperature range for ethylammonium nitrate was established with o-dichlorobenzene as a test probe. The capacity factor, plate count, and peak skew value were determined at a column temperature of 40 "C. The column oven teimperature was then increased by 10 OC increments with a half hour hold a t each temperature increment. The oven temperature was then returned to 40 OC and the column performance parametem were remeasured. No change in retention was noticed until the column was conditioned at a temperature of 130 "C. A second column maintained at a temperature of 120 "C for 1week showed no change in retention for the test probe, and it is assumed to be the maximum safe operating temperature for ethyl-
ammonium nitrate. This has been confirmed by temperature programming the column and observing the color of the detector flame. The stationary phase imparts a green hue to the flame which can be seen to occur when the temperature of the column reaches 130 "C. Also, for the extended conditioning period the peak shape and column efficiency remained constant indicating that no thermally induced changes occurred. The upper temperature limit for the phase is established by its vapor pressure and not by thermal decomposition. Temperatures lower than 40 "C were not investigated. The clearing temperature for ethylammonium nitrate is 12 OC, so the lowest allowable operating temperature is somewhere between 12 and 40 "C. To establish the polarity and selectivity of ethylammonium nitrate, and to compare these properties to nonionic stationary phases, we made an attempt to measure its McReynolds' constants. This proved to be unsuccessful due to the fact that the Kovats index values could not be determined for the test probes. The n-hydrocarbons used as the fixed points on the Kovats scale were not retained by the stationary phase. In fact, a mixture of C5-Cl0 n-hydrocarbons eluted together a t the column void volume a t all practical operating temperatures. This is consistent with the notion that the organic molten salt is extremely polar with little hydrophobic character. Likewise, the stationary phase shows little ability to separate saturated/unsaturated hydrocarbon mixtures due to a lack of retention. Since the McReynolds constants of ethylammonium nitrate could not be determined due to a lack of retention of the n-hydrocarbons but not the test probes themselves, the relative polarity of the phase was established by comparing the retention of the test probes on ethylammonium nitrate to that on a standard polar stationary phase. Carbowax 20M was used as the standard polar phase as its chromatographic characteristics are well-known. All measurements were made with the same stationary phase loading, support, column length, temperature, and mobile phase velocity. The results of the comparison for the 10 McReynolds test probes are summarized in Table I. In each instance where the ratio of the capacity factor values (Kom/Kcw) is >1, this implies a stronger interaction for that test probe with ethylammonium nitrate than with Carbowax 20 M. The largest values for this ratio were found for butanol, 2-methyl-2-pentanol, and pyridine. This implies greater selectivity than Carbowax 20M for the separation of alcohols, nitriles, acids, bases, heterocyclic compounds, and branched chain compounds containing a polar functional group (20). A value for the capacity factor ratio
