to use for hydration spheres contribute to molecular dimensions. Grubisic, Rempp and Benoit (25) report that viscometric hydrodynamic volume determines retention in gel permeation columns. Figures 7 and 8 show that the Donnan salt exclusion effect can be eliminated in gel filtration by the presence of sufficient electrolyte in the eluent. The same has been done in osmometry (26) to eliminate osmotic pressure contributions from small membrane diffusable ions. In gel filtration, the presence of sufficient inert electrolyte enables ionic solutes to elute as sharp, symmetrical peaks having elution volume independent of sample amount. The beneficial effect of electrolyte upon separations is illustrated in Figure 9 for equimolar mixtures of Na2S04and NaCl. In comparison, identical elutions with water did not give as nearly complete separations. The behavior of salt mixtures eluted with water from Sephadex G-10 (Figure 9B) is in good agreement with the data for the individual salts presented in Figures 4, 5, and 6. That the fractionation improved with sample loading, although opposite to normal chromatographic experience, was to be expected from the divergence of the Na2S04 and NaCl curves in Figure 6. (Similar behivior would be expected for mixtures of Na5P3010or Na2HP04with any of the alkali metal chlorides shown in Figure 5.) Because the concentration gradients on the lower front side of the NaCl peaks were reproducible and independent of sample amount (Figure 4)) it was possible to predict that a small amount of Na2S04and a large excess of (25) Z. Grubisic, P. Rempp, and H. Benoit, J. Polymer. Sci.,B5, 753 (1967). (26) C. Tanford, “Physical Chemistry of Macromolecules,” Wiley, New York, 1961 p 221.
NaCl would fractionate as shown in Figure 9B. In addition, the NaCl impurity level under any given NazSOl peak would be independent of the total amount of NaCl in the mixture. The need of inert electrolyte for the separations of ionic solutes on Sephadex gels has been demonstrated. In choosing the proper electrolyte for a particular separation, a number of factors need to be considered. The electrolyte should be highly ionized to give a level of ionic strength sufficient to overcome the Donnan salt exclusion effect. For completely ionized salts, the concentration need not be greater than 10-2M. The electrolyte should have its cation or anion in common with that of one of the salts in the sample in order to reduce the total number of cation-anion combinations that might result from exchanges. The hydrodynamic volume of the electrolyte should be small enough to allow penetration into essentially all of the gel interior. For instance, a salt such as Na5P3010would not penetrate Sephadex G-10 sufficiently (see Figure 5) to neutralize the effect of all ionic charges within the gel interior. For the separation of certain chemical systems, background electrolyte concentrations as high as 10-2M might not be desired or tolerated. In those cases, compromises between background impurity level and peak resolution would have to be made. Optimum conditions would need to be determined empirically. RECEIVED for review November 1, 1967. Accepted February 2, 1968. This work was supported in part by the U. S. Atomic Energy Commission under contract AT (11-1)-1222. Presented at the Division of Analytical Chemistry, 154th National Meeting of the American Chemical Society, Chicago, September 1967.
Quantitative Infrared Analysis of Mixtures of Isotopically Labelled Gases Mark M. Rochkind Bell Telephone Laboratories, Inc., Murray Hill, N.J. A new spectrophotometric method based on an unconventional technique of low-temperature sample preparation provides a general method of qualitative and quantitative analysis for isotopically labelled gases. The method, infrared pseudo matrix isolation, involves condensing specially diluted gas samples onto a cold infrared transmitting substrate by controlled-pulse deposition. The simple vibrational spectra which result are recorded using commercial instrumentation. Data for deuterated methanes and ethylenes as well as methyl ether, propionaldehyde, and neopentane are presented. The latter, which represent a range of molecular polarity and bulk, serve to support the general quantitation of the method. As little as 0.2 pmole of certain gases may be detected; acute selectivity is shown by the successful spectrometric separation of the 3 ethylene-d2 isomers. Pseudo matrix isolation proves far superior to gas chromatography for molecular isotope analysis. As such, the most promising application appears to be in the area of photochemistry.
A NEW SPECTROPHOTOMETRIC technique (infrared pseudo matrix isolation) was recently proposed (1, 2 ) for qualitative analysis of multicomponent gas mixtures. The technique requires conventional spectrometric instrumentation but uses cryogenic temperatures to effect an unusual method of sample 762
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preparation. The latter results in condensed-phase spectra characterized by relatively sharp absorptions, few in number, occurring at highly reproducible frequencies. Such spectra contrast markedly with normal gas-phase spectra and encourage use of low temperature sample preparation for gas analysis. In work previously reported ( I ) , pseudo matrix isolation spectra of thirteen C1-CI hydrocarbons were recorded and multicomponent mixtures containing random collections of these were analyzed. That work constituted a feasibility study, for satisfactory gas chromatographic methods already existed for the analysis of light hydrocarbon mixtures which matched the sensitivity and qualitative selectivity of the experimental infrared technique. Nonetheless, low-temperature sampling proved a practicable means of spectrochemical analysis and we engaged in further studies. A large number of gases and volatile liquids representing widely differing classes of chemical compounds have been investigated and synthetic multicomponent mixtures containing isomeric molecules as well as molecules of distinct structural classes have (1) M. M. Rochkind, ANAL.CHEM., 39,567 (1967). (2) M. M. Rochkind, Enairoti. Sci. Techtiol., 1, 434 (1967).
been successfully analyzed. Recently, a modification of the apparatus used in the initial studies has resulted in enhanced spectral sensitivity (by almost a factor of 3) and essentially strict adherence to Beer's law, using peak absorbance measurements, in every instance of quantitative investigation. Thus, recent work indicates that infrared pseudo matrix isolation is a very general method of qualitative and quantitative gas analysis. In a paper currently in preparation, data on more than 60 distinct chemical compounds will be presented and methods of data handling, i.e., mixture analysis, will be discussed. A detailed account of the essential apparatus and experimental methods is being prepared. Despite the widespread use of infrared spectroscopy in chemical analysis, there has not been demonstrated any satisfactory infrared method for direct qualitative and quantitative analysis of the isotopic molecular composition of gas mixtures. The insuperable band overlap problem and the resolution demands made on conventional instrumentation by the very sharp absorptions characteristic of gas-phase spectra rule out direct gas-phase analysis as a practicable approach. Nonetheless, molecules differing in isotopic composition often display quite distinct infrared spectra. Mass spectrometry and, more recently, gas chromatography (3-7) have had to provide the major methods of analysis in this area. While the former is definitive, instrumentation necessitates a major commitment and mass spectral data often require complex analyses. The latter, on several grounds, is not a satisfactory approach. It offers retention times of hours to effect only marginal separations and it requires specially packed columns commonly hundreds of feet long which are ipso facto systemspecific. In this paper we report the application of infrared pseudo matrix isolation spectroscopy to the qualitative and quantitative analysis of isotopically labelled gases. Like mass spectrometry it is a one-step spectrometric method which is identical for all gases. On the other hand, as in gas chromatography, analysis of the data is straightforward. Thus, infrared pseudo matrix isolation spectroscopy borrows salient features from each of the major analytical methods in use. Data are presented for deuterium-labelled methanes and ethylenes. Pseudo matrix isolation spectra of gas mixtures composed solely of molecular isotopes are reported and some data which bear on the general quantitative properties of pseudo matrix isolation as an analytical tool are discussed. Successful quantitative determinations for mixture components differing by only one deuterium atom are indicated and the capability to analyze for isotopic isomers is demonstrated. EXPERIMENTAL Deuterated methanes and ethylenes (each at least 98% atom pure) were purchased from and analyzed by Merck, Sharp and Dohme of Canada, and used without purification. Methane (U.H.P. 99.95 %) and ethylene (C.P. 99.5 % min.), obtained from the Matheson Co., were used likewise without purification. Air Products and Chemicals prepurified nitro.; gen (99.998%) was used as a diluent after flowing the gas at -100 mm liter/min through a 12-foot coil cooled by liquid nitrogen. All per cent composition indications of purity (3) K. E. Wilzbach and P. Riesz, Science, 126,748 (1957). (4) R. J. Cvetanovic, F. J. Duncan, and W. E. Falconer, Can. J. Cliern., 41, 2095 (1963). (5) E. K. C. Lee and F. S. Rowland, ANAL.CHEM.,36, 2181 (1964). (6) . . J. W. Root, E. K. C. Lee, and F. S. Rowland. Science. 143. 676 (1964). (7) G. P. Cartoni, A. Liberti, and A. Pela, ANAL.CHEM.,39, 1618 (1967). ,
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Figure 1. Schematic diagram of deposition and analysis apparatus
High vacuum stopcocks, (b) measured volume -16 ml, (c) CsI windows, (d) cooled, rotatable CsI substrate (a)
listed are those claimed by the suppliers. Spectra were recorded on a N2-purged Beckman IR-12 spectrophotometer: spectral slitwidth usually
