basic premise that improvement in decision reliability can be expected with use of a suitable committee TLU. Turning to the performance of individual weight vectors, i t is seen that the FID and HAD (PNP) were significantly worse than the other three. This may be the result of the absence of actual peak intensity information for about two thirds of the unknowns. It seems intuitively reasonable to expect a greater effect of discrepancies in peak :intensities on Fourier or Hadamard transformed frequency domain data, due to the fact that these transforms result in a "spreading" of peak intensity information across the entire transformed representation. This conclusion is further supported by the observation that, for the nineteen spectra where peak intensities were available, the performance of these predictors was much better (FID 4 of 19 perfect (21%) and HAD (PNP) 7 of 19 perfect (37%)) whereas for the remaining 43 spectra the performance of both were 4 perfect (9%). The performance of the two-layer 'FLU method examined in the present work further suppsorts the earlier conclusion (2) that a flexible interpretation approach, allowing the experimenter to choose the type or types of pre- and post-processing of unknown data is most likely to yield practically useful structural suggestions. For the particular set of questions asked here, the comlmittee results were best. On the other hand, if the unknowns had been obtained in the form of un-transformed ]?ID data, it is possible that the FID predictor would have proved superior. With respect to the questions initially raised, the results of this study establish that performance of the weight vectors used is satisfactory for unknowns and that results of reasonable reliability can be obtained for all of the structural questions examined. Obviously, a practicing spectroscopist would use results of this type in conjunction with whatever
other information may be available to solve actual structural problems. In that context, many more decision vectors would be required, but reliability in the range found here should be acceptable. In work under way, methods for using various types of spectral information to further enhance the confidence with which chemical structure inferences may be made by machine interpretation methods are being explored (11).
LITERATURE CITED (1) C. L. Wilkins, R. C. William, T. R. Brunner, and P. J. McCombie, J. Am. Chem. SOC.,96, 4182 (1974). (2) T. R. Brunner, R . C. Williams, C. L. Wilkins, and P. J. McCombie. Anal. Chem., 46, 1798 (1974). (3) T. R. Brunner, C. L. Wilkins, R . C. Williams, and P. J. McCombie, Anal. Chern. - - , 47.662 119751 . _, (4) P. C. Jurs, 8. R. Kowalski, T. L. Isenhour, and C. N. Reilley, Anal. Chem., 41. 690 (1969). (5) P. C.Ju&, B. R. Kowalski, T. L. Isenhour,and C. N. Reilley, Anal. Chem., 41, 1949 (1969). (6) W. Voelter, E. Breitmaier, G. Breitmaler, D. Gupta, G. Haas, and W. A. Konig, Chem. 2..97, 239 (1973). (7) L. F. Johnson and W. C. Jankowski, "Carbon-13 NMR Spectra", John
...-
~
Wiley and Sons, Inc. New York, N. Y., 1972. (8) J. F. Hlnton and B. Layton, Org. Magn. Reson., 4 (Spectral Suppi.), 448-452 (1972). (9) J. B. Stothers, "Carbon-I3 NMR Spectroscopy", Academic Press, New York, 1972. (IO) G. C. Levy and G. L. Nelson, "Carbon-13 Nuclear Magnetic Resonance for Organic Chemists", Wiley-lnterscience New York, 1972. (11) J. B. Justice, T. L. Isenhour, and C. L. Wllkins, Abstracts, Twenty-sixth PittsburghConference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March, 1975, Paper No. 198.
RECEIVEDfor review November 22, 1974. Resubmitted March 28, 1975. Accepted May 5, 1975. Support of this research through Grant CP-41515X(CLW) and Grant GP43720X(TLI) by the National Science Foundation is gratefully acknowledged.
Qualitative Analysis of Lacquer and Similiar Solvent Mixtures by Gas Chromatography Benjamin Levadle and Stephen MacAsklll State of Vermont Division of Occupational H61akh.P.O.Box 607,Barre, Vt. 05641
Evaluation of workplace air to determine whether it meets with the criteria for occupational safety and health requires qualitative and quantitative information about possible pollutants in the employees' environment. To ascertain the exposure to organic vapors and solvents, air samples are taken on activated charcoal. Absorbed vapors are eluted from the charcoal using carbon disulfide and the eluate is analyzed both qualitatively and quantitatively as required ( I , 2 ) on two or more column substrates by chromatography. Frequently, as a first step, simple qualitative information concerning materials being used is all that is needed. For the analytical laboratory such information is especially useful since it assists in the resolution of components on the chromatogram by reference to analysis of source materials, especially when paints, lacquers, varnishes, and similar systems are involved. The common procedure for the analysis of such systems, consisting of a vehicle which is a mixture of organic solvents and a nonvolatile component, is first to separate the two either by vacuum or steam distillation. The resulting distillate is subjected to a systematic solubility analysis, using mixed inorganic acids and then dimethylsulfate, for
general characterization (3). Identification of specific components may be accomplished by gas chromatography, infrared spectrometry or other suitable methods. Although the analytical methodology works quite well, problems frequently are met with during the distillation operations. They are time consuming. Systems which polymerize rapidly or change on being warmed or heated are frequently encountered. I t became necessary for our laboratory to resolve these and other difficulties with as simple and as time saving an approach as possible. We have developed an analytical procedure which takes advantage of the fact that the enclosed headspace of a container holding a paint, varnish or lacquer will be composed of a distribution of the volatile components present in the solvent vehicle. The procedure is quick, simple, and requires only readily available apparatus. It utilizes the solvent stripping technique ( 3 ) described below. Figure 1 is a photograph of the chromatographic analysis obtained for headspace of a mixture of chlorinated hydrocarbons. Bulk samples are collected in 500-ml widemouth bottles (Arthur H. Thomas 1712-E33) closed with aluminum lined
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
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t/
Flgure 3. Chromatogram of 10 pl of headspace over dimethylsulfate-washed residue from sample shown in Figures 1 and 2. All chromatographic conditions as for Figure 1
c
Figure 1. Chromatogram of a mixture of oxygenated hydrocarbons of 10 MI of headspace over aromatics and naptha
Tracor MT 150, 10% “Carbowax 600”,on 80/100 mesh Chromosorb W, 1/B-inch diameter, 20 feet long, stainless steel. Column oven temperature: isothermal at 95 ‘C; nitrogen carrier gas flow: 30 cm3/minute; flame ionizatlon detector: air flow at 600 cm3/minute, hydrogen flow at 60 cm3 per minute
Flgure 4. Chromatogram of 10 pl of headspace over mixed chlorinated hydrocarbons
Tracor MT 150; 10% Arochior 1254 on 80/100 Chromosorb W; oven temperature, 50’ isothermal; nitrogen carrier gas flow: 30 cm3/minute. FID, air flow 600 cm3/min. Hp flow at 60 cm3/min
Flgure 2. Chromatogram of 10 pl of headspace over acid-washed residue of the same sample shown in Figure 1. All chromatographic operating conditions as for Figure 1
caps (Arthur H. Thomas 2386-M90 Size 63). As a practice, 300 to 400 ml of sample is submitted. Several sample bottle caps were modified to receive a $-inch gas chromatograph septum. An appropriate sized opening was made in the plastic cap. The liner was left intact. When the analysis is to be done, the original bottle cap is replaced with the modified item and the mixture is allowed to stand in a warm room for an hour. If it is desired, it may be warmed briefly in a 50 O C water bath. A 1.75 inch No. 19 B&D stainless steel needle is used to pierce the septum and liner of the modified cap, Through this, the needle of a 10-pl “syringe pipet” which has been pre-wetted with carbon disulfide is passed and a sample of headspace vapor mixture is taken for gas chromatography. The chromatogram allows preliminary qualitative examination of the vapor in the headspace. Further qualitative analysis of the headspace vapors can be accomplished by either of two methods: 1) An all glass 10-ml hypodermic syringe (not equipped with the LuerLok modification) is fitted into the needle in the modified container cap and filled with headspace vapors. The vapor sample is transferred to a 4-ml vial fitted with a Teflonlined cap (Arthur H. Thomas 5569-E10) containing 0.5 ml of a mixture of 170 ml of 85% phosphoric acid and 100 ml of 1852
concentrated sulfuric acid by plunging the needle through the liner of the already perforated vial cap. To prevent pressure build-up in the vial, the cap is not sealed tightly. Once the vapor transfer has been completed, the vial cap is sealed and the needle and syringe are removed. The vial now may be laid on its side to allow a larger surface for the acidhapor system to interact. After five to ten minutes, the No. 19 needle is inserted through the vial cap to provide a convenient channel for the passage of the needle of a 10-pl syringe pipet which has been wetted with 1 p1 of carbon disulfide to reduce back-flush loss. A 10-pl portion of the acid washed headspace is chromatographed. The chromatogram will show only the alkyl/aryl hydrocarbons. The acid waste strips the “oxygenateds”. The acid mixture now is removed from the closed vial by means of the hypodermic needle and with an all-glass syringe, 0.5 ml of dimethyl sulfate is transferred into the vial through the same needle. The vapor system is allowed to interact with the dimethyl sulfate for five to ten minutes and 10-yl samples of the vapor remaining are chromatographed. The chromatogram will show the remaining alkyls. Aromatics have been stripped. 2) An alternative approach to this method also has been found quite useful and is quicker. As previously, a 10-p1 sample of headspace vapor is chromatographed, to obtain a chromatogram of the mixture. Then 0.5 ml of the acid mixture is aspirated into a 5-ml all-glass hypodermic syringe fitted with a No. 19 (or No. 20) needle. The needle is plunged through the septum of the sample container top and 5 ml of headspace is pulled through the acid solution in the syringe. The syringe then is laid down and the needle is replaced with another needle which has been crimped with a wire-cutter. This reduces any loss of vapor during the
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
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.
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