neither be verified nor contradicted. However, the configuration about the D-ring of gitoxigenin derivatives eluted from a GLC column is apparently different from presently accepted assignments. The separations achieved for trisaccharide glycoside TMSi ethers demonstrated clearly that the affinity of gitoxin TMSi ether for OV-17 is significantly greater than that of digoxin TMSi ether and is less than that of digitoxin TMSi ether. If we assume (a) that dipole-dipole interactions between the aromatic liquid phase and the 14P-OH (as well as the lactone ring) were significant during gas chromatography of the aglycone TMSi ethers and that they became much less significant during GLC of the trisaccharide TMSi ethers and (b) that vapor pressure differences provide an immeasurably small contribution to retention time differences in the cases of the trisaccharide TMSi ethers, the relative retention time relationships observed for the trisaccharide TMSi ethers correlate with results expected from gas chromatography of the aglycone
TMSi ethers. The importance of dipole-induced dipole interactions in determining relative affinity of the various glycoside TMSi ether derivatives for OV-17 is more clearly demonstrated than was the case for the aglycone TMSi ethers at the lower temperatures. ACKNOWLEDGMENT
The authors acknowledge the generous assistance of E. C. Horning in providing accessibility to the tandem gas chromatography-mass spectrometer facilities at the Institute for Lipid Research, Baylor University School of Medicine, Houston, Texas. The authors are indebted to J. H. Doherty for a sample of dihydrodigoxigenin. The authors acknowledge the generosity of F. C. Chang in permitting utilization of the nuclear magnetic resonance spectrometer. RECEIVED for review November 15,1968. Accepted February 6, 1969.
Continual Analysis of Gas Chromatographic Effluents by Rapid Repetitive Infrared Scanning Burton Krakow The Warner & Swasey Company, Control Instrument Dioision, 32-16 Downing St., Flushing, N . Y . 11354 Rapid, on-the-fly, infrared analysis of gas chromatographic effluents was investigated using a grating spectrometer that continually produced infrared spectra from 2.7 to 9 at the rate of 1.6 spectra per second. The gas eluting from the gas chromatograph was passed continuously through a flow-through infrared cell where its spectrum was scanned repetitively by the spectrometer. The quality of the spectra obtained was adequate for functional group analyses of a number of organic compounds studied. Potential ability to detect unresolved GC peaks appeared to be a particularly valuable feature of this analytical method.
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Two TECHNIQUES have been used for obtaining infrared spectra of gas chromatograph effluents: (1) trapping isolated fractions of the effluent containing G C peaks (hopefully single pure peaks) and study of each fraction at leisure with an ordinary infrared spectrometer, (2) on-the-fly rapid-scanning of the infrared spectrum of each fraction as it elutes from the G C column. Trapping is tedious and time consuming. Moreover, ordinary spectrometers take several minutes to scan, while the gas chromatograph elutes peaks at intervals of a few seconds. Backlogs of trapped samples can develop very quickly this way. Rapid, on-the-fly scanning would be desirable. A number of attempts have been made to meet this need by accelerating the scanning speed of standard types of infrared spectrometers ( I , 2, 3). A filter wheel spectrometer ( 4 ) and an interferometer ( 5 ) have been applied to the problem. These instruments took from one second (5) to 45 seconds ( 2 ) to scan a spectrum. Up to 800 scans per second, with good spectral quality, are produced by the Warner & Swasey Model 501 rapid scanning spectrometer (6). A modification, Model 503, which has been made for analysis of gas chromatograph effluents, produces 1.6 scans per second. Such a rate represents sufficient speed to follow
variations in column effluents without producing excess spectra that would make the job of data processing unnecessarily gross. This paper will describe experiments to evaluate the performance of the modified spectrometer and the utility of on-the-fly rapid scanning. EXPERIMENTAL
A schematic diagram of the apparatus used for studying infrared spectra of gas chromatograph effluents is shown in Figure 1. As gas elutes from the gas chromatograph, it is passed continuously through a light pipe that serves as an infrared absorption cell. Radiation from the infrared source passes through the light pipe and is focused on the entrance slit of a rapid-scanning grating spectrometer. The rapid-scanning spectrometer uses a unique method of scanning with corner mirrors through the intermediate focal plane of a double-pass monochromator, keeping the grating and all other components of the dispersing train immobile (6). The spectrometer repetitively scans the infrared absorption spectrum, from 2.7 to 9 p, of the sample in the light pipe. One complete spectral scan takes '/s second, there is a pause of second during which time no spectra are observed, then the scan is repeated, and the sequence is continued indefinitely. A single case, which can be flushed with dry nitrogen, contains the entire optical system, including the source, (1) A. M. Bartz and H. D. Ruhl, ANAL.CHEM., 36,1892 (1964). (2) P. A. Wilks, Jr. and R.A. Brown, ibid., 36,1896 (1964). (3) C. W. Warren, J. J. Heigl, R. A. Brown, and J. M. Kelliher, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 8, 1968, Paper 217. (4) G. T. Keahl, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February 25, 1966, Paper 209. ( 5 ) M. J. D. Low and S . K. Freeman, ANAL.CHEM., 39, 194 (1967). (6) S. A. D o h , H. A. Kruegle, and G. J. Penzias, Appl. Opt., 6, 267 (1967). VOL. 41, NO. 6, MAY 1969
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Figure 1. Block diagram of Model 503 rapid-scanning spectrometer chopper, sample cell, transfer optics, scanning monochromator, and detectors. A light pipe with a 10-cm optical path and a volume of l-cm3 is used. It has a metal body and salt windows. The inside surface of the body is polished and gold plated for good reflectivity. The thermally insulated cell, inlet line, and outlet line can be separately temperature controlled from ambient to 250 O C . The inlet line extends out of the spectrometer case and into the furnace of a gas chromatograph to provide an uninterrupted heated path from the column to the infrared cell. The outlet line, which conducts the sample out of the spectrometer case after it has passed through the light pipe, is also heated in order to prevent plugging due to condensation. Because infrared analysis is nondestructive, the gas issuing from this outlet line can still be used for further analysis, by mass spectroscopy or other techniques. RESULTS AND DISCUSSION
Identifiability of Spectra. The performance of the Model 503 spectrometer was tested by studying spectra of a n assortment of organic compounds. Vapor of each compound was introduced into the light-pipe, one compound a t a time. The absorption spectrum of the compound’s vapor was scanned in 0.5 seconds, displayed on a n oscilloscope, and recorded on a Polaroid picture. A background spectrum was superimposed on each picture for reference. These pictures are shown in Figures 2-7. The absorption spectrum of methyl ethyl ketone is shown in Figure 2. The strongest band in the methyl ethyl ketone spectrum, at 5.8 p, stems from the stretching vibration of the carbonyl band. Another prominent band is the characteristic skeletal vibration of ketones a t 8.5 p. Also characteristic of ketones is the strong absorption in the C H deformation region around 7 p , which in this molecule is comparable in strength to the absorption due to C H stretching at 3.4 p. Figure 3 shows the spectrum of butyraldehyde, an isomer of methyl ether ketone. The CO stretch at 5.8 p still gives the most prominent feature. However, the C H vibrational
Figure 2. Spectrum of methyl-ethyl ketone, scanned in 0.5 sec 816
ANALYTICAL CHEMISTRY
bands are much different from those of the ketone. Quantitatively, the bands due to CH stretching are much stronger than those produced by CH deformation. A qualitative difference is the presence in this spectrum of the doublet, a t 3.6 and 3.8 p , which is characteristic of aldehydes. The size of the carbon chain per aldehyde group is indicated by the relative intensities of the 3.4 to 3.8 p peaks. Figure 4 shows a 503 spectrum obtained with toluene vapor. Two CH stretching frequencies are clearly resolved. The one at 3.3 p represents the motion of hydrogen atoms attached to the aromatic ring. The peak a t 3.5 p is produced by vibrations in the methyl side chain. Absorption peaks due to aromatic skeletal vibrations and C H band deformations occur between 6 and 7 p. Note that the peak at 3.3 p is a little stronger than the one at 3.5 p. This conforms to the fact that the molecule has five CH bonds involving ring carbon atoms and only three CH bonds on the side chain. On this basis we might anticipate a reversal of the relative intensity of these two peaks in o-xylene where there are four C H bonds associated with the aromatic ring and six C H bonds on the side chains. A 503 spectrum of o-xylene shown in Figure 5 bears out this reasoning. Of course, different vibrations d o not always have such similar transition probabilities. In this case, however, the vibrations are so akin in nature that the similarity of their transition probabilities is not surprising. These illustrations show that spectra produced by rapidscanning are useful for detecting variations in functional groups, even in some fairly similar compounds. Sensitivity. The infrared cell’s high path length to volume ratio, 10 cm-2, is needed in order to study small samples of gas chromatograph effluents with high time-resolution. The sensitivity and the detail with which a gas chromatograph peak can be analyzed vary widely with the compound involved, the GC column, and the column operating conditions. OBSERVED SPECTRAOF SMALLSAMPLES.The spectra shown in Figures 2-4 were obtained with relatively large samples of over 100 pg in the infrared cell of the spectrometer. Useful spectra can be obtained with smaller samples as indicated in Figures 5-7. A 503 spectrum of 40 pg of a typical aromatic hydrocarbon, o-xylene, is shown in Figure 5 . This spectrum has already been discussed. Identifiable spectra of carbonyl compounds can generally be produced with smaller samples than those required for hydrocarbons. A case in point is Figure 6 which shows a spectrum obtained with 25 pg of acetone in the sample cell. In addition to the very strong absorption peak from the carbonyl stretching vibration at 5.8 p, we can also see the characteristic peaks at 3.5,6.9, 7.3, and 8.3 p. Figure 7 shows a spectrum of 45 pg of n-pentane, a n example of an aliphatic hydrocarbon. The only absorption expected with such a compound is the C H stretching vibration
Figure 3. Spectrum of butraldehyde, scanned in 0.5 sec
Figure 4. Spectrum of toluene, scanned in 0.5 sec
Figure 5. Spectrum of40 pg of o-xylene, scanned in 0.5 sec
Figure 6. Spectrum of 25 pg of acetone, scanned in 0.5 sec
at 3.5 p and the C H deformations at 6.9 and 7.3 p. These peaks can all be seen in Figure 7. EFFECTOF G C PEAKWIDTH ON SENSITIVITY. The 503 spectrometer continually scans the spectra of G C effluents as they pass through its infrared cell, producing one spectrum every 0.625 seconds. The largest sized sample of a component that will be observed in any one of these spectral scans will depend on the carrier gas flow rate and the GC peak width as well as on the amount of the component originally injected. Suppose we are using a carrier gas flow rate of 30 cm3 per minute and make the simplifying assumptions that the G C column and the IR cell are at the same temperature and we are dealing with a triangular G C peak. At this flow rate, 2 seconds will be required for the gas chromatograph to evolve the 1 cm3 of gas necessary to fill the infrared cell. This is just a little more than the time necessary for the 503 spectrometer to make three spectral scans. Consequently, material from a G C peak will be in the infrared cell for at least the period necessary for the 503 to observe three spectra. The actual number of scans in which the compound in question is observed and the fraction of the original sample that is in the cell during any given scan vary radically with the G C peak width. If the G C peak is very sharp, less than 0.1 sec in width, and it enters the infrared cell during the 0.1 sec just prior
Figure 7. Spectrum of 45 pg of n-pentane, scanned in 0.5 sec
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to the beginning of a spectral scan, then 100 of the original sample will be observed in each of the three succeeding spectra, which will be obtained during the period of approximately 2 seconds that it will take the peak to traverse the cell. The entire sample will then flow out of the cell before a fourth scan has progressed substantially. In the case of a peak that is one second wide, or 0.5 ,ma in volume, the sample will be observed in five spectral scans. The entire sample will be present throughout only one of the five, the third one. The first scan would observe only the leading edge of the peak and the fifth scan would observe only the trailing edge. Qualitative agreement among the five spectra of the peak is a measure of the purity of the peak. The last case we will consider in detail is a gas chromatography peak that is five seconds wide. In this case, a specimen from the G C peak will be present in the infrared cell during 11 scans. The infrared cell will not contain the entire original sample at any time. The most it will ever contain will be 6 4 x of the original sample. The 1 cm3 infrared cell can only contain 4 0 z of the 2.5 cm3 of carrier gas that conveys the sample in the five second peak. However, when we account for the concentration variation in a triangular peak, we find that the center 4 0 x of the effluent contains 6 4 z of the sample by weight. If the G C peak contains a total of 100 pg of the compound under study, the largest amount used at any one time in a spectral observation will be about 64 pg. This quantity would be present in the infrared cell during the sixth of the 11 spectral scans of this peak. All other spectra would be weaker with the first and last scans involving less than 2 of the original quantity of component injected. If the sample is large enough to provide reasonable strength in many of these weaker
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Figure 8. Concentration of sample as a function of position at four different times during passage of a 1-sec GC peak through the 503 gas handling system
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Figure 9. Concentration of sample as a function of position at three different times during passage of a 5-sec GC peak through the 503 gas handling system Per cent by weight Time, sec of sample in IR cell
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Figure 10. Gas chromatogram of synthetic mixture
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Figure 11. Infrared spectra of different GC fractions of synthetic mixture Acetone B. +Butyraldehyde C. Isovaleraldehyde D. Diethyl ketone +heptane E. /?-Heptane diethyl ketone F. Toluene G. Butyric acid A.
spectra, they would provide a very good analysis of the purity of the GC peak. Examples of the variation in the contents of the infrared cell as gas chromatograph peaks pass through it are described in Figures 8 and 9. They show the distribution by weight of samples in triangular GC peaks, with widths of one and five seconds, respectively, at several distinct times during their passage through the 503 gas handling system. The dotted curve in Figure 8 shows the sample in the inlet tube just before it starts to enter the IR cell. The dashed distribution curves in Figures 8 and 9 represent the condition 0.5 seconds later. Half of the sample of the one second peak has entered the IR cell and the other half (by volume as well as by weight) is still in the inlet tube. For the 5-second peak, the quantity of sample in the cell is 10 % by volume (because 10 of the base of the triangle is inside the region representing the IR cell) and 2 by weight (because 2 of the triangle's area is above the region representing the IR cell). When the center of the GC peak is at the center of the IR cell, infrared absorption will be at its greatest. The sample distribution for this case is shown by solid lines in Figures 8 and 9. At this time, the entire 1-second peak is in the IR cell and would be observed in a n infrared measurement. For the 5-second peak, the cell contains 4 0 z of the sample by volume and 6 4 z of the sample by weight. The dot-dash curves show the peak when it has just entirely emerged from the cell. This condition occurs 3 seconds after the first trace of the one second peak starts to enter the cell. For the 5-second peak, this interval between initial entry
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and final exit is 7 seconds, time for 11 spectral scans. In 3 seconds, Model 503 completes about 5 spectral scans. Gas chromatograph peaks are not true triangles, and rounding of the peak top would tend to equalize the spectra obtained from scans five, six, and seven of the 11 obtained for the 5-second peak. As the GC peak becomes broader, spectral scans are made on a larger number of isolated segments of it. On the other hand, a decrease occurs in the fraction of the sample present in the largest of these segments. A 10-second GC peak would appear in 19 spectra and the largest sample observed spectrally would contain 36% of the injected component. A 20-second GC peak would appear in 35 spectra and the largest sample observed spectrally would contain 19 of the injected component. This means that the 503 system studies broader GC peaks in greater detail to better establish their purity or identify their impurities but larger samples are requircd tc d o so. Operation of 503 Spectrometer with a Gas Chromatograph. The Model 503 spectrometer was used to obtain infrared analyses of the effluents of a Beckman GC-4 chromatograph using a 6-foot long l/a-inch diameter silicone packed column,
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a 30-cc/min helium carrier gas flow rate, and flame ionization detectors. A 1O:l splitter divided the effluents between the flame ionization detector and the infrared cell. Ten parts went to the infrared cell and one part to the flame ionization detector. ?'he spectra were recorded on a Honeywell Visicorder recording oscillograph. Figures 10 and 11 show data obtained when this system was used to analyze 2 p1 of a synthetic mixture containing 10 acetone, 10 n-butyraldehyde, 10 % isovaleraldehyde, 20 diethyl ketone, 10 n-heptane, 20% toluene, and 20% butyric acid. Figure 10 displays the gas chromatogram. Figure 11 shows seven selected scans obtained by the Model 563 spectrometer on samples corresponding tu those observed in Figure 10 at points A , B , C , D , E, F, and G, respectively. Each of these spectra is labeled with a letter from A to G in accordance with the point in Figure 10 to which it corresponds. Spectra A , B, and C indicate that the first three peaks in the gas chromatogram are acetone, n-butyraldehyde, and isovaleraldehyde. During the rise and fall of each of these three G C peaks, several more IR spectra were recorded and were characterized by coincident rise and fall of all observed infrared peaks. This behavior indicates that these first three G C peaks are homogeneous and probably pure. The fourth peak in Figure 10 is impure, containing both diethyl ketone and n-heptane. The chromatogram shows no evidence of this impurity but the infrared spectra provide clear proof of it. The infrared spectra of the effluents of the leading edge of this peak are qualitatively different from those of the trailing edge. The differences are illustrated by
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spectra D and E of Figure 11. Each of these tracen represents a spectrum of a mixture of diethyl ketone and n-heptane. However, the concentration of the ketone can be seen to be much greater in D than E and the hydrocarbon is more concentrated in E. Fifteen other spectra were recorded between D and E. In these 17 spectra (along with a few before and after them), all infrared peaks can be seen to rise and fall with the ketone peaks rising and falling before the hydrocarbon peaks. The fifth and sixth G C peaks in Figure 10 are produced by toluene and butyric acid, respectively. This is indicated by spectra F and G in Figure 11. Qualitative similarity within the group of spectra produced by the fifth G C peak and within the group of spectra produced by the sixth G C peak indicates that each of these two G C peaks is homogeneous. CONCLUSIONS
This preliminary investigation indicates that rapid on-thefly infrared scanning is potentially capable of yielding useful information to help identify G C effluents. Ability to detect unresolved G C peaks appears to be a particularly valuable feature of this analytical method. Chemical sensitivity varies widely with the G C peak width and flow rate, and with the molecular weight and the infrared transition probabilities of the sample. Samples used in this study were of a size that may be employed in larger packed columns.
RECEJVED for review December 31, 1968. Accepted March 10, 1969.
Activation Analysis of Halogens in Photographic Emulsions Using a Neutron Generator E. P. Przybylowicz,' Gilbert W. Smith, J. E. Suddueth, and S. S. Nargolwalla Institute f o r Materials Research, National Bureau of Standards, Washington, D. C. 20234 A nondestructive neutron activation technique for the analysis of chloride and iodide in a silver bromide matrix is described. Chlorine was measured after activation with 14.7-MeV neutrons. The 3.1-MeV gamma rays from were measured without interference. Calibrations were carried out using photographic emulsions containing 10 to 200 mg of chlorine. The relative standard deviation of a single determination at the 10 mg level is 5%: at the 200 mg level it approaches 1%. Iodine was measured via lZ8l produced by (n,?) activation with 2.8-MeV neutrons. A straight line curve was established for 2 to 420 mg of iodine. The relative standard deviation of a single determination at these two levels was 20% and 1%, respectively. The method offers an attractive alternate to existing chemical and instrumental methods for the determination of iodide and chloride in silver halide mixtures because it has the potential for providing rapid analyses with reasonably good precision.
MANYANALYTICAL PROCEDURES have been reported for the determination of chloride, bromide, and iodide in mixtures. The abundance of methods testifies to the importance of the problem and is tacit proof that no accepted methods exist Research Associate from the Eastman Kodak Company, Rochester, N. Y . , at the National Bureau of Standards, 1968.
for the halides which can be applied directly to a variety of matrices. Because the primary light-sensitive properties of a photographic emulsion are determined by the composition of the silver halide component, a need exists to have accurate and rapid methods of analysis for the halides in this matrix. Several methods have been applied to this problem, however, none of these provide the desired speed, coupled with acceptable accuracy. One of the earliest analytical methods used for halides in photographic emulsions was the potentiometric titration with silver nitrate ( I ) . This procedure, while capable of high accuracy and precision, is very lengthy. An alternate titrimetric method which has been used is based upon the automatic coulometric titration of the halides with silver after separation by differential oxidation (2). The method is somewhat faster than the potentiometric procedure, however, because iodide, which is often a minor component, is determined by difference, the relative standard deviation for its measurement is large. Russell (3) reported a specific method for iodide in emul~
(1) W. Clark, J. Chem. SOC.,1926, 749. (2) E. P. Przybylowicz, A. J. Moyse, and T. N. Tischer, unpub-
lished work, 1964.
(3) G . Russell, Sci. Ind. Photogr., 28, 297 (1957). VOL. 41, NO. 6, MAY 1969
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