1 NOTES
I
Gas-Liquid Chromatography Separations of Hydrocarbons Using Columns with Aqueous Solutions of Complexing Metal Ions as Stationary Phases S. P. Wasik and W. Tsang National Bureau of Standards, Washington, D. C. 20234
WE WISH to demonstrate that interesting solute selectivity may be achieved in gas-liquid chromatography (GLC) separations by using aqueous solutions of complexing additives as the stationary phase. Purnell(1) and Conder, Locke, and Purnell(2) have shown that in the absence of solute adsorption on the solid support the experimentally observed partition coefficient, Kobs, for aqueous solutions of complexing electrolytes at infinite dilution is given by Koba = KL $L Z
KJ
$: K L K ~ C ~ V
(1) 24
where K L = the partition coefficient for the bulk solution Ks = the surface partition coefficient A = the active surface area V = the total volume of the liquid phase K c = the stability constants for the reaction of the solute, X i , with the additive, &, to form the complex Sixj Cc = the molar concentration of the additive.
The above equation is based on the assumption that there is no coupling effect between the different complexing additives. For a majority of cases in GLC, solute selectivity is obtained by a proper choice of the stationary phase where all but the first term of the r.h.s. of Equation 1 is essentially zero. For columns containing aqueous solutions of complexing additive for the stationary phase any term of the r.h.s. of Equation can be made large relative to the other terms by the proper choice of additive and solvent coverage. We have chosen aqueous solutions of silver and mercuric salts as our stationary phases to illustrate how solute selectivity may be obtained because of the large surface and complexing effects of this system. Water as the stationary phase presents no experimental difficulties (3). In Figure 1A is shown a chromatogram of the elution of a mixture of alkanes, olefins, and aromatics from a column containing only water as the stationary phase. In the absence of additives, the solvent selectivity is poor. In Figure 1B is shown a chromatogram of the elution of the same mixture from a column containing AgN03 and Hg(NO& as additives. Mercuric ions complex very strongly with olefins (Kc-N 10,000 (1) J. H. Purnell, “Gas Chromatography 1966,” Butterworth, London, 1966, p 3. (2) J. R. Conder, D. C. Locke, and J. H. Purnell, J. Phys. Chem., 78, 701 (1969). ( 3 ) S . P. Wasik and W. Tsang, J. Phys. Chem., 74, 2970 (1970). 1648
6
12 C
0
3
T!Y (U!fLTESl
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Figure 1. Chromatograms of hydrocarbon mixture: 2methyl-cyclohexene, nonane, decane, undecane, dodecane, benzene, toluene, p-xylene, o-xylene, and vinyltoluene at 20 “C A . 15% w/w water; B. 25% wjw of an aqueous solution of 3.5M AgN03 and 0.05M Hg(N03)~;C. 50 w/w of an aqueous solution of 5.0MAgN03and O.OSMHg(NO& The elution peaks of the other aromatic compounds are not shown in this chromatogram. Solid supports, 60-80 mesh chromosorb P
acid wash
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
liters/mole) and essentially acts as a barrier for the elution of any olefins. Mercuric ions do not complex with the alkanes or aromatics while the silver ions complex weakly with aromatics ( K A 1-2 I./mole). The net result is that the aromatics are eluted at longer times relative to the alkanes while the olefins for practical consideration, are retained completely. The chromatogram presented in Figure 1B is from a column of high solvent coverage (25%). For most GLC systems, this high coverage would make the K,A/V term essentially zero. For aqueous solutions as the stationary phase this is not the case. At 25 coverage there is still an appreciable contribution from the KsA/V to Kobs. The alkanes are affected by the surface effect more than the aromatics (higher Ka’s). The aromatic retention may be increased relative to the alkanes by increasing V and CAgNOs.In Figure 1C is shown a chromatogram of the elution of some normal alkanes and benzene from a column of 50% coverage of 5M aqueous AgN03 solution. Benzene is retained longer than dodecane even though there is a difference of 140 “C in their boiling points. The retention times of benzene relation to decane for the chromatographs in Figure lA, B , and C are 1.20,0.248, and 0.082, respectively. This present note is restricted to the separation of hydrocarbon using aqueous solution of Ag+ and HgZ+salts as sta-
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tionary phases for GLC columns. It demonstrates the feasibility of separating compounds by groups such as olefins, alkanes, and aromatics. This technique, however, does not have to be restricted to hydrocarbon analyses, there are other salts that can be used to complex a large variety of compounds (4). In addition to complexing additives there are other types of additives that may be used to gain solute selectivity such as the sodium salt of 2,4-dimethylbenzenesulfonic acid (saltingin effect), other noncomplexing inorganic salts (salting-out effect), and soaps (solubilization effect). By a combination of additives, it should be possible to design columns for specific classes of compounds. The practical utility of this type of separation is self-evident. This separation have important use in air and water pollution analyses where there are usually many unidentified compounds. The use of such highly specific columns will ease considerably the problem of positively identifying a particular component. Work along these lines is now in progress. RECEIVED for review May 13,1970. Accepted August 7,1970. (4) L. G. Sillen and A. E. Martell, “Stability Constants,” Metacalfe
and Copper Limited, London, 1964.
Determination of Trace Amounts of Contaminants in Water by Isotope Dilution Gas Chromatography S. P. Wasik and W. Tsang National Bureau of Standards, Washington, D . C. 20234
AN IMPORTANT ASPECT of water pollution analyses is the maintenance of sample integrity. In any sample processing, such as storage, transfer, extraction, handling, or concentrating, there is inevitably some loss of contaminants. This has been emphasized in a recent review by Brown ( I ) . The problem is particularly serious in water pollution analysis where the contaminants must be concentrated a great deal before analysis can be performed. In this report we point out and demonstrate the suitability of the isotope dilution method using gasliquid chromatography, GLC, for solving this problem. Isotope dilution methods are routinely used in quantitative mass spectrometric analyses. This involves adding to the sample a known quantity of an isotope (in the same chemical and physical form) as the contaminant to be determined and measuring the appropriate isotope ratio. The concentration of the contaminant is then determined by multiplying the known concentration of the isotope by the isotope ratio. The isotope dilution technique is not restricted to mass spectrometric measurements. Any physical or chemical method may be used that can measure isotope ratios. In recent years, various workers have demonstrated that GLC is a good method for separating fully deuterated compounds from their light isomers. Wasik and Tsang (2) have separated aromatics and olefins from their deuterated isotopes using aqueous silver nitrate solutions as the column’s stationary phases. (1) R. A. Brown, Mater. Res. Stand. 2 (12), 983-988 (1962). (2) S.Wasik and W. Tsang, J . Phys. Chem., 74, 2970 (1970).
Other investigators ( 3 ) have used silver nitrate dissolved in ethylene glycol as the stationary phase for separating deuterated olefins from their lighter isomers. Gaumann and HoignC ( 4 ) have separated deuterated alkanes using capillary columns. Cartoni, Liberti, and Pela (5) have separated polar compounds such as methyl alcohol, ethyl alcohol, and chloroform from their respective deuterated isomers. Van Hook and Phillips (6) have separated acetylene from deuterated acetylene. The above brief survey of isotope separations using GLC is far from complete. It does demonstrate that a large variety of compounds can be separated from their isotopic isomer by GLC. In those cases where complete or even partial separations have been reported, the isotope dilution method could be applied using conventional detectors such as the hydrogen flame detector. In principle any “tagged” molecule may be used as a tracer in isotope dilution analysis although the separation would be more difficult. An attempt was made in this laboratory to separate ethylene from 13C-labeled ethylene using a 12-foot X lJa-inch0.d. column containing an aqueous solution 5M in AgNO3 as the stationary phase. Within experimental error, the two compounds had the same retention time. (3) R. J. Cvetanovic, F. J. Duncan, and W. E. Falconer, Can. J . Chem., 41, 2095 (1963).
(4) T. Gaurnann and J. HoignC, “Aspects of Hydrocarbon Radi-
olysis,’’ Academic Press, London, 1968. ( 5 ) G. P. Cartoni, A. Liberti, and A. Pela, ANAL.CHEM., 39, 1618 (1967). (6) A. Van Hook and J. T . Phillips, J. Chromatogr.,30, 21 1 (1967).
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