reaction period. Our experience has shown that, even with the uncrimped needle, vapor loss is minimal. Once the acid has been in contact with the headspace vapor sample for five to ten minutes, a 1O-pl portion of the acid extracted vapor can be obtained directly from the open end of the syringe. If the sample syringe is handled carefully, there is little danger of acid reaching the needle of the microliter pipet. The acid extracted portion is chromatographed. The acid mixture now is discharged carefully through the original hypodermic needle. Now 0.5 ml of dimethyl sulfate is aspirated into the syringe containing the acid washed vapor and permitted to be in contact with it for five to ten minutes after which the vapor is again chromatographed as above. Qualitative analysis is by relative retention volume (benzene = 1).Solvent mixtures frequently contain components which produce overlapping peaks. By using the above selective stripping technique, we find it possible to make practical and useful analyses even when peak overlapping occurs. For, once oxygenated compounds and aromatics have been removed stepwise from a solvent system containing mixed petroleum alkyls also, only the latter will be present. Figures 1, 2, and 3 show progressively the effect of this technique on a solvent mixture containing oxygenated compounds, aromatics, and petroleum hydrocarbons. Figure 1 is a chromatogram of the whole mixture. Figure 2 is the result of stripping with the acid mixture. Figure 3 represents the petroleum hydrocarbon portion. Detector re-
sponse has been attenuated in these runs to keep the peaks on the chart. Dimethyl sulfate tends to remove some of the lighter hydrocarbons and this is demonstrated clearly in Figure 3. We use 20-foot long stainless steel columns, %-inch o.d., packed with 10% substrate on 80 to 100 mesh washed firebrick. The substrates are: for general analysis: free fatty acid phase (FFAP); for good preliminary separation of light hydrocarbons from polar compounds: polyethylene glycol 600; for partition of chlorinated hydrocarbons (including clean and effective distinction between carbon tetrachloride and 1:l:ltrichloromethane) Arochlor 1254 (Figure 4). We have had excellent qualitative results using this system. The application of the law of partial pressures might present an opportunity for insight into such vapor systems. We have not attempted to do this.
ACKNOWLEDGMENT The authors thank Mrs. Joann Wood for her help in preparing the manuscript. LITERATURE CITED (1) C. L. Fraust and E. R. Hermann, Am. Ind. Hyg. Assoc. J.. 27, 68-74 (1966). (2) L. D. White, D. G. Taylor, P. A. Mauer, and R . E. Kupel, Am. hd. Hyg. Assoc. J., 31, 225-232 (1970). (3) M. B. Jacobs and L. Scheflin, "Chemical Analysis of Industrial Solvents," Interscience Publishers, New York, N.Y., 1953.
RECEIVEDfor review March 24, 1975. Accepted June 6, 1975.
Rapid Separation of C4 Hydrocarbons at 50 OC by Modified GasSolid Chromatography Antonio Di Corcia and Roberto Samperi lstituto di Chimica Analitica, Universita di Roma, 00 185 Roma, ltalia
Separation of the Cq hydrocarbons commonly encountered in refinery operations is of great interest to the petroleum industry. A considerable number of investigators have suggested the use of many types of liquid phases and adsorbing media. Yet, in the routine use of such column packing materials, two main difficulties are commonly encountered. One arises from the fact that these columns must be generally operated a t such low temperatures which cannot be controlled by common gas chromatographic apparatus. This is particularly true when operating with gas-liquid columns. The second arises from the fact that the commonly suggested absorbing media, that is silica gel and alumina, are degraded by trace amounts of moisture in the carrier gas. In a very recent paper ( I ) , we reported a first evaluation on the feasibility of graphitized carbon black (GCB), such as Vulcan-G (-110 m2/g), modified with either picric acid or PEG (Carbowax) 1500 for the separation of hydrocarbons in the C1 to Cq range. The object of this work is to show that Carbopack C, which is another example of GCB with a relatively low surface area, modified with 0.19% w/w picric acid is capable of separating all Cq hydrocarbons as well as propene from
propane at 50 OC. Pentane is eluted within 16 minutes. By coating Carbopack C with 0.7% picric acid, pentane is eluted after only 2.5 minutes a t 47 O C . Under these conditions, however, butene-24s and butene-2-trans are eluted as one peak. EXPERIMENTAL The GCB used in this work was Carbopack C (Supelco Inc., Bellefonte Pa.), which has a surface area of about 13 m2/g. This type of GCB has chromatographic characteristics similar to Sterling FT-G (s = 15 m2/g) (Supelco Inc., Bellefonte, Pa.). Column packings were prepared by dissolving weighed samples of picric acid in methylene chloride and adding the solution to a known weight of GCB (100-120 mesh) in a flat dish. Packings were slowly dried a t room temperature. While drying, stirring of the material must be avoided, as it causes crushing of graphitized carbon particles. In any case, dried materials have to be resieved to maintain the proper mesh range. With the materials prepared in this fashion, columns made from stainless steel tubing (2-mm i.d.1 were packed by moderately vibrating with the aid of a vibrator. T o maintain uniformity of the packed material, it is necessary that the void tubing be already coiled and ready for connection to the gas chromatograph. Columns were then conditioned for 12 hr a t 100 OC. The apparatus used was a Carlo Erba gas chromatograph model GI (Milano, Italy). Nitrogen was used as carrier gas.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
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.-
0
I
L
Figure 1. C4 hydrocarbons
12
8
r t m e / m r n / 16
at 50 "C on a 2.2-m X 2-mm column
0
+
0.19% picric acid: containing Carbopack C (100-120 mesh) pressure drop: 4.8 Kg/cm2: linear carrier gas velocity, 10.9 cm/sec
RESULTS AND DISCUSSION In previous paper ( I ) we showed that adding picric acid to Vulcan-G resulted in considerable modifications of the elution order of hydrocarbons as compared to that on a bare GCB surface. Moreover, just by varying the relative amount of picric acid added to the carbon surface, a large range of selectivity could be made available. When separating a typical C4 hydrocarbon mixture of interest in the petroleum industry, methane, ethylene, acetylene, and ethane are generally not present. There may be some propane and propylene, but also some iso- and n-pentane. In making this separation, 0.19% wlw was found to be the optimum percentage of picric acid to be added to Carbopack C. As shown in Figure l, base-line separation of all C4 hydrocarbons is accomplished within 16 minutes. From a practical point of view, it is noteworthy that this column packing can be operated at 50 "C. This is a realistic column temperature which can be easily controlled by common gas chromatographic apparatus. To obtain high efficiency columns, the GCB particle size range used was 100-120 mesh. As a matter of fact, a t the
1
2 3 time lmin)
-
Figure 2. C4 hydrocarbons at 47 "C on a 2.2-m X 2-mm column containing Carbopack C (100-120 mesh) 0.7% picric acid: pressure drop 2 Kg/cm*; linear carrier gas velocity, 5.75 cm/sec
+
optimum carrier gas velocity, the calculated column efficiency was seen to be more than 3300 plates per meter. Figure 2 shows a chromatogram of the same hydrocarbon mixture eluted on Carbopack C coated with a monolayer of picric acid. As can be seen, in this instance, the elution time is greatly reduced. However, butene-2-cis and butene-2trans are eluted as one peak. Picric acid-coated Carbopack C columns are thermally stable up to 110-120 "C. We have used the columns continuously at 50 " C for some weeks without observing any alteration. A flame ionization detector was used. At the maximum sensitivity of the amplifier system, we had no problem of base-line stability.
LITERATURE CITED (1) A. Di Corcia and R. Samperi, J. Chromatogr., 107, 99 (1975).
RECEIVEDfor review December 9, 1974. Accepted May 19, 1975.
Separation of Oligosaccharides by Partition Chromatography on Ion Exchange Resins Jaroslav Havlicek and Olof Samuelson Department of EngineeringChemistry, Chalmers University of Technology, 5-40220 Mteborg 5, Sweden
Oligosaccharides are important constituents of beverages, food, and waste water from industries which produce products containing polysaccharides, e.g., the cellulose industry. Adsorption chromatography on charcoal-Celite ( I ) and paper chromatography (2) have been used to separate these compounds for preparative and analytical purposes. Efficient separations which depeGd on differences in the degree of polymerization (DP) of the oligosaccharides are achieved by gel permeation chromatography (3-6) and partition chromatography in aqueous ethanol on ion exchange resins (6, 7). In a recent investigation of oligomeric sugar alcohols by this method it was shown that the distribution 1854
coefficients were dependent not only on the D P and the sugar moieties but also on the type of glycosidic linkages (8). The present paper deals with the separation of different types of oligosaccharides containing glucose units linked by various types of glycosidic linkages.
EXPERIMENTAL The B-(1+3)-linked D-glucose oligomers were prepared by acid hydrolysis of laminarin with 0.1M hydrochloric acid a t 100 "C for 1 hr (50 cm3 of acid per gram of laminarin). Oligosaccharides of p(1-6)-linked D-glUCOSe series were kindly supplied by J.-C. Janson, Uppsala, Sweden, The other saccharides were the same as used in previous work (6, 7 ) .
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
