appears in the equation. Also, the presence of any species capable of changing k2 in a sample will introduce serious error in this technique. Double-Point Method. By making a second determination of the amount of al and u2 reacted a t a longer time, t ' , after al has reacted to completion b u t prior to complete reaction of UZ, Equation 1 can be rewritten as:
where x' is the amount of al and uz reacted a t t'. Equations 1 and 3 can now be solved simultaneously for a! to give the expression:
the factors to be considered in choosing t and t' is presented in an earlier work (2,s). (4)
Thus, by measuring x and x' a t t and t' and substituting into Equation 4, a: can be calculated (a and b being known). Note that Equation 4 does not contain the rate constant k2, which makes this method in a sense temperature independent. The temperature must remain constant during a single determination of a!, but can vary from determination to determination without introducing error into a series of determinations. A detailed discussion of
LITERATURE CITED
(1) Hanna, J. G., Siggia, S., ANAL. CHEM.34. 547 (1962). (2) Lee, T. S.; Koithoff,' I. M., Ann. N . Y. Acad. Sn'. 53, 1093 (1951).
(3) Reilley, C. N., Papa, L. J., ANAL. CHEM.34, 801 (1962). (4)Sieda. S.. Hanna, J. G., Ibid.,. 33,. . 896-0961).'
LOUISJ. PAPA HARRYB. MARK,JR CHARLES N. REILLEY Department of Chemistry The University of North Carolina Chapel Hill, N. C.
A Simple Device for Qualitative Functional Group Analysis of Gas Chromatographic Effluents Benito Casu and Luigi Cavallotti, lstituto Scientific0 di Chimica e Biochimica "Giuliana Ronzoni" and Stazione Sperimentale Combustibili, Milano, Italy
are encountered S in the qualitative interpretation of gas chromatograms of complex heteroERIOUS DIFFICULTIES
functional mixtures, for example in samples of natural origin. Because several compounds may have the same retention characteristics for the chosen experimental conditions, even the use of a complete list of reference retention data, if available from the literature, can lead only to the indication of several possibilities. Knowledge of the chemical class to which each component belongs may help in discriminating among the above possibilities. The general approach for relating chemical functional groups to gas chromatographic peaks involves either reactions on the original mixtures or further analysis of effluents. In the first case certain classes of chemical compounds may be removed selectively from the original sample. The corresponding disappearance of some peaks from a newly recorded chromatogram indicates the presence in the disappeared compounds of a functional group reactive toward the particular preliminary reaction involved [Bassette, R., Whitnah, C. H., ANAL. CHEY. 32, 1098 (1960)l. The second case involves mainly the collection of fractions for further analysis by other techniques, for instance infrared spectrometry. An easy functional group analysis of column effluents nevertheless can be accomplished without use of additional expensive instrumentation. Walsh and Merritt [ANAL.CHEM.32, 1378 (1960)l described a simple device for qualitative functional analysis of gas chromato15 14
ANALYTICAL CHEMISTRY
graphic effluents. Their system uses a stream splitter on the exit line of a gas chromatograph, allowing the split gas effluents to bubble through a series of solutions of suitable group reagents. A three-way stopcock was also provided on the exit tube, to make poyible a rapid change of reagent vials while a first series of vials was in use. We have developed another simple technique for the qualitative analysis of some functional groups of gas chromatographic effluents. Our system
does not involve any interruption of the gas stream and does not require the attention of the operator during the run. Basically it reveals the single effluents by spots which develop with specific qualitative reagents on a horizontally advancing strip just beneath the exit t,ube. The rate of advancement of the strip is the same as that of the recorder chart. The strip can be compared with the gas chromatogram to provide a rapid group-type discrimination of the separated compounds.
\
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Figure 1.
Drawing of assembly
The device, shown in Figure 1, consists of a gear system, A , connected to the recorder for transmitting its motion, by way of a shaft, B, to a slide, 8. This slide moves on a guide, C, supported on the fractometer side. It is adjusted so the exit tube of the chromatograph is aligned with the slide. The motion is transmitted from the shaft to the slide, through a chain and sprocket, to a pinion, p , which drives a rack on the slide support. The gears are selected in such a way that the rate of advancement of the slide is the same as that of the recorder chart. The movement of the train, a, is started by pushing on the shaft, sh, which has two stop positions, engaged and disengaged. The transmission shaft, B, from the recorder to the slide is connected by spherical joints and can be engaged and disengaged easily, as it is fitted with a telescopic part held in place by a small spring. The distance from the slide support to the exit tube can be modified by moving the whole system along the slot, d. Our device has been installed on a Perkin Elmer Vapor Fractometer Model 154 D, with a Leeds and Northrup Speedomax recorder. For the gear system, A , installed on a Lucite support, we use the accessory gears from the recorder. The couple, a-a', has 54 straight teeth; the couple, b-b', has 10 and 20 helical teeth respectively. The pinion that drives the rack has 8 teeth.
This gear system drives the strip a t the same speed as the recorder chart. The difference is less than 1%. For fixing the support, C, of the slide we use a hole on the left side of the instrument, which is meant for the back-flush system. The total length of the slide support is 60 cm., providing a slide excursion of 30 cm. The strip is held in place on a Lucite chassis, e, by a retaining slide. A little lever, f, allows the easy extraction of the strip from the support. The strips are of the type used in thin-layer chromatography-in glass, covered with a silica gel-chalk layer and wet with the suitable reagent. The performance of the device was checked for system of alcohols and aromaticolefinic unsaturation, employing the classification reagents used by Walsh and Merritt, nitrochromic acid and sulfuric acid-formaldehyde. The device is very simple and inexpensive. When in use it does not prevent the normal operation of the gas chromatograph; if necessary, it can be removed in 2 minutes. As the strip is always positioned a t a certain distance (ca. 1 mm.) from the exit tube, the gas flow or other operating parameters are not affected during the analysis. The gas chromatogram is not, therefore, altered in any way and no shift of base line occurs.
Our system does not allow the analysis of several functional groups in one single operation, as opposed to the system of Walsh and Merritt which splits the effluents from the gas chromatograph into five different reagents. We have performed on some occasions the simultaneous analysis of two functional groups using two strips cut in half longitudinally. The strips were impregrated with two different reagents, and the partition line was positioned exactly under the outlet of the gas chromatograph. When necessary to check for several functional groups, more than one run is necessary. This does not imply that a greater quantity of sample than that used by Walsh and Merritt is required. I n fact, although sensitivity in spot testing is also dependent upon the concentration of the reagent and the type of support, our system gives for the samples actually run a somewhat better sensitivity than that reported using the method of Walsh and Merritt. Chromatograms illustrating two typical analyses are presented in Figure 2. The bottom of the figure s h o m the corresponding strips obtained a t the same time the chromatogram Fas run. Although in the case of these analyses an easy identification of individual components may be possible by con-
z
W
W
z 22 iLi W LL
i
0
I 0 _-I
r-
FW
4 0 F0 0
il
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I
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Figure 2. Left: Gas chromatogram of typical mixture of alcohols with ethyl ether, water, and methyl-ethyl-ketone IMEK), compared with corresponding strip wet with nitrochromic acid Column: 2 meters dibenzylether on Celite 545; temperature: 75' C.; carrier gas flow: 180 cc. of He/min.; total sample quantity: 2 pl.; chart speed: 2 minutes per inch
Right: Gas chromatogram of typical mixture of aliphatic and aromatic hydrocarbons compared with corresponding strip wet with HzSOlHCHO Column: 2 meters di-2-ethyl-hexyl-sebacate on Celite 545; temperature: 100' C.; carrier gas Row: 180 CC. of He/min.; total sample quantity: 2 pl, chart speed: 2 minutes per inch
VOL. 34, NO. 1 1 , OCTOBER 1962
0
1515
sidering the retention volume for the specific chemical class, our technique is obviously more useful for more complex mixtures. This is particularly so for high boiling compounds, for which little information regarding retention values is reported. Walsh and Merritt have shown that, in those cases in which it is possible to make use of the correlation plots like log retention volume 21s. carbon number for homologous series, the knowledge of the component
functionality can lead to its exact identification. As an additional feature, the device described here allows in some cases the control of the purity of a gas chromatographic peak. When a light-colored spot corresponds to a large peak, this may be due to the peak being the result of more than one component. I n this case one should also try other reagents. However, it may be that the light color of the spot is due to a single compound
in which the functional group is diluted in the molecule. In making this evaluation one must consider the effect of the nonlinear response of a gas chromatographic detector for compounds of different classes and for different molecular weight members of a n homologous series. ACKNOWLEDGMENT
The authors thank Mario Psallidi for technical help.
An Improved Digital Readout Method Ralph Eno, Applied Physics Corp., Monrovia, Calif.
people, such as R. E. Biggers and G. P. Smith of Oak Ridge National Laboratory. J. E. Mapes of Brookhaven National Laboratories, and R. C. Molter of Libby-Owens Ford, are using digital readout equipment with spectrophotometers, gas chromatographs, and other types of instrumentation. Invariably, they use the same basic system of readout for the data (3) lvhich involves separating the wavelength or time axis into equal intervals. At each interval, a reading of the recorder pen deflection and wavelength is recorded. When using this method with a spectrophotometer, it is necessary that readout be made a t intervals not greater than the instrument resolution to avoid loss of spectral information. Therefore, the recording of an ultraviolet-visible spectra (1850A. to 655OA.) on a high resolution instrument, would require the recording of about 4700 points tb characterize the curve. With this method of recording. the sections of the curve without spectral information are read out as often as those of high spectral information. An extreme example of this is shown in Figure 1, which is the visible transmission curve of an interference filter. Using the above method of readout, 3500 poirits would have been recorded, but only 950 points T o d d have fallen in areas of spectral information. Reducing the number of points would reduce the time needed by a computer to identify a chemical compound by a like factor. An alternative method of recording data is proposed. This method produces data points in greatest number in those sections of the curve having the highest spectral information. Rather than using equal intervals of wavelength, the absorbance (or transmittance) scale is divided into equal intervals a t which the absorbance value and wavelength are recorded. I n the curve shown in the figure, had each interval been 0.25y0transmittance, ANY
15 16
ANALYTICAL CHEMISTRY
only 745 points n-ould have h e n recorded and all but four would have fallen in the section of spectral information. A further refinement of this method can be made. Nothing is gained by recording the points falling on the steep sides of peaks so long as the recorder pen motor is running a t mayimum speed. Under this condition, the pen merely draws a straight line. To eliminate these points, a derivative sensing device is placed on the pen system that prevents it from recording points when the rate-of-change of absorbance (or transmittance) with respect to wavelength is greater than some given value. One of many such devices is an A-C tachometer ( 2 ) attached to the recorder pen drive motor. The rectified output from the tachometer is then used to adjust the readout rate of the system. Details about the system can best be obtained from the manufacturer of digital systems ( I , 4 ) . KOloss of spectral information occurs with this equipment, becausp the pen is moving a t less than full speed on the skirts, shoulders, and a t points of inflection of the curve. By adjusting the derivative sensing equipment so that it will allow readout, except in the shaded area shown in the figure, the number of data points is cut to about 220. The high initial cost of derivatiLe sensing equipment may discourage many prospective users, but fortunately there is an inexpensive compromise-a device
which limits the readout rate to some maximum value and permits recording data points a t even intervals of pen deflection. One of the slower IBM card punches has a maximum punching rate of 30 data points per second, thus allowing a maximum of 30 points during a full chart maximum velocity excursion of the recorder pen. This is an adequate punch rate because a t normal scanning speeds for spectrophotometers, it vr-ould allow readout a t intervals of one third of instrument resolution. Points of inflection on the steep slopes of curves would be characterized by a number of points having small differences in absorbance due t o the lower velocity of the pen in this area. This device xould note all the points recorded using the derivative sensing device, yet would allow only 30 extra point. to be recorded in the area of low spectral information. X o loss of precision or spectral information occurs with the compromise> method. At the peaks of curves, the pen will be moving s l o ~ l yand allonthe punch to read out a t each equal change in absorbance. Weak peaks will be seen if readout is made for equal intervals of absorbance, not larger than about two to three times the s!*stem noise. Precise records of absorbance or resolution can be made because data points in the sections of low spectral information are eliminated. The application of either of the improved methods of digital readout TT ill significantly reduce the amount of computer memory required to stow spectral data and will reduce computation time to a more realistic figure. LITERATURE CITED i l ’i Datex Corp., Monrovia, Calif.
Figure 1. Transmittance curve for Spectrolab Type 2347 Interference Filter
itzgerald, A. E., Kingsley, C., w t r i c Machinery,” p. 468, McGrawr k , 1952. D., R.C.A. Engineer, 7 , No. 1, p. 49, June-July 1961. (4) Radio Corp. of America, Moorestown,
N. J.
