ever, because of the many simplifying assumptions required by the calculations, a series of experiments was conducted to verify the results. Seutron shadowing was negligible in most instances, being even less than predicted. I n the gamma attenuation experiments, considerable difference in count rate was observed between the calculated and the observed values for the light metals. This behavior, shown in Figure 1 , presumably resulted from the high energy beta particles not being absorbed in the lighter matrices (1). A National Bureau of Standards steel sample having a known oxygen content of O.lOS%, was selected for use as a standard. Table I shows comparative oxygen results for a group of metals analyzed by neutron activation and by other commonly used analytical techniques.
DISCUSSION
The optimum method for analysis would conceivably be to irradiate metal samples without the use of a container. However, this would require sufficient metal to make a specific size rod and would require machining time. Also, the sample could possibly pick up oxygen-containing materials from the transfer tube and from handling. Encapsulating the sample in a lowoxygen metal container has the advantages that the analysis sequence can be completely automated, a powder sample can be handled easily, and the sample is not exposed to the atmosphere during the analysis. This latter aspect is particularly important in analyzing reactive metals. The disadvantages are that the container contributes to the background and unless the container
material always has a constant amount of oxygen, the background effect will vary; and exclusion of oxygen from the container is essential, otherwise a correction must be applied. LITERATURE CITED
( 1 ) Anders, 0. I?.,Briden, D. W., A N ~ L .
CHEM.36, 287 (1964).
(2) Coleman, R. F., Iron Steel
Inst.
(London) Spec. Rept. 68 (1960). ( 3 ) Stallwood. R. A,. Mott. W. E.. Fanale, D.' T., A N ~ L .CHEW 35, 6 (1963). (4) Steele, E. L., General Atomic, San Diego, Calif., private communication, 1964.
K. G. BROADHEAD H. H. HEADY
Reno Metallurgy Research Center Bureau of Mines U. S. Department of the Interior Reno, Nev.
Mass Spectral Analysis of Carbonyls Regenerated from Their 2,4-Dinitrop henylhyd razones An Extension of the Procedure of Ralls SIR: Successful regeneration of carbonyl compounds from 2,Pdinitrophenylhydrazone derivatives for qualitative analysis by gas chromatography was introduced by Ralls (7') and has been modified recently in attempts to improve the quantitative aspects of the procedure (6). Often it is difficult to absolutely identify compounds only from retention time data obtained from gas chromatography columns. Therefore, separation and purification of 2,4-dinitrophenylhydrazones, using published procedures (3,8), followed by regeneration, for mass spectrometry, of the original carbonyl compound from the purified derivative offered the possibility of a rapid simple procedure for gaining positive identifications based on mass spectral analysis. The components of some mixtures might be completely elucidated if the gas chromatography data were used to supplement the mass spectrometric data. This report deals with the successful adoption of such a procedure in our laboratories. EXPERIMENTAL
Apparatus. An attachment (Figure 1) for the regeneration and introduction of samples into the CEC-21103C mass spectrometer was constructed similar to that introduced 760
ANALYTICAL CHEMISTRY
by Bazinet and Merritt ( 2 ) . I t was modified to accommodate a sample regeneration tube shown in Figure 1B and an attachment so that both Ascarite and Molecular Sieve 4X (Molecular Sieve Products Company, Tonawanda, N. y.) could be included in the system simultaneously. Alternatively, for small samples (2 to 3 mg.), the apparatus shown in Figure 1 B may be used alone by simply including the molecular sieve and ascarite in the regeneration tube and separating it from the sample with a plug of glass wool In this case the amounts of ascarite and molecular sieve one can use are decreased accordingly and the regeneration tube is attached directly to the mass spectrometer inlet. Mass spectra were obtained on a CEC Model 21-103C mass spectrometer equipped with a gas inlet system at ambient temperature. All samples passed through a 3-liter expansion volume at 100" C. before entering the ionization chamber. Ascarite received no special treatment except that when a fresh bottle was opened it was stored in a desiccator charged with calcium chloride when not in use. Molecular sieve was flushed with nitrogen for 3 to 4 hours while it was heated a t 350" C. in a furnace. I t was cooled in a desiccator and desiccation was maintained until the sieve was parceled into the regeneration apparatuc. Molecular sieve prepared in thiq manner will trap water to the extent of about 107' of it. weight. Procedure. Four mg. of each of
several 2,4 - dinitrophenylhydrazone derivatives, prepared from high purity carbonyl compounds and recrystallized at least once, were accurately weighed and triturated with 12 mg. of a-keto glutaric acid. After the triturated samples were placed in the regeneration tubes, shown in Figure 1B. they were attached to the regeneration apparatus (Figure 1'4). The entire apparatus was then attached to the mass spectrometer inlet. Approximately 5 gm. each of ascarite and molecular sieve 4A were placed in the separate traps before the apparatus was assembled. The stopcocks leading to the mass spectrometer inlet and the molecular sieve and ascarite bulbs were closed and the carbonyl compound was regenerated a t atmospheric pressure by subjecting the sample to a temperature of 250' C. for 30 seconds. This was accomplished with a silicone oil bath; the top of the sample in the regeneration tube was well below the surface of the oil. Next, the regenerated carbonyl compound was frozen out in liquid nitrogen and the system, including the ascarite and molecular sieve traps, was opened to the roughing pump on the mass spectrometer to remove the air. The stopcock leading to the inlet of the mass spectrometer was again closed and the system was warmed to slightly above room temperature. Nest, the vapors were cryogenically pumped first into the molecular sieve and then into the ascarite allowing equilibration of about 5 minutes with each. Finally,
, ,12/30
I
,
d. Tubinq
s
10 mm
0.
d x I30 mm
4 mm 0 . d . x 90 mm
(
e)
k;ompie
Figure 1. (A) Apparatus for regeneration of carbonyl compounds from 2,4-dinitrophenylhydrazones their showing bulbs for ascarite and molecular sieve and 12/30 joint for attachment to mass spectrometer inlet (B) Regeneration tube to hold sample when used with apparatus in Figure 1A or to hold ascarite, molecular sieve, and sample when used alone
the sample was warmed gently with a heat gun and introduced into the mass spectrometer by opening the stopcock to the inlet system. For the higher molecular weight carbonyls it was necessary to warm the inlet leading into the expansion chamber to increase the vapor pressure. Mass spectra were obtained with the CEC Mascot mass spectrum digitizer and the relative fragmentation patterns were compared with published spectra. When smaller samples were used, manipulations were the same except no cryogenic pumping was required since the ascarite and molecular sieve were included in the regeneration tube with the sample. The sieve was placed first in the path of the vapors so that the bulk of the water would be removed before contacting the ascarite. If the ascarite became moist, it yielded water to the lower pressure as soon as the mass spectrometer inlet system was opened. After regeneration and removal of the air as in the previous case, the sample was equilibrated with the ascarite and molecular sieve for 15 minutes before introducing the vapors into the inlet of the mass spectrometer.
RESULTS AND DISCUSSION
Mass spectra were obtained, in the manner described, for acetone, 2-pentanone, 3-methyl-2-butanone, 2-heptanone, butanal, 2-methyl propanal, pentanal, heptanal, nonanal, and benzaldehyde. A satisfactory spectrum for ethanal was not obtained unless larger samples were used. The upper limit of molecular size, because of lower volatility, is about 8 carbon atoms. For example, when decanal was regenerated, fragment peaks were large but insufficient sample was present in the source to give a detectable parent mass peak. The presence of a detectable parent mass peak was considered essential for identifying unknown carbonyl compounds. Attempts to obtain good spectra for ethanal failed presumably because this molecule is small enough to be trapped by the molecular sieve. Figure 2 contains some representative spectra obtained in the manner described. The aldehydes agreed fairly well with published spectra ( I , 4, 5) as did the ketones (9) and positive identifications could readily be made in all cases. However, some slight discrepancies did occur. Notably, small peaks appeared consistently at m/e 133, 96, and 95. These were probably due to breakdown products of 2,Minitrophenylhydrazine. Also, considerable CO, was produced during the heating process, presumably from decarboxylation of a-ketoglutaric acid, and, even though most of it was removed on ascarite, enough entered the inlet ionization chamber to cause some error in the intensity of the peak at m/e 44. For this reason, the m/e 44 peaks in these spectra were of little value for identification purposes. With the alternative procedure, smaller samples could be analyzed but only a t the expense of diluting the sample introduced into the inlet with large amounts of nitrogen and oxygen. Apparently, when the sample was regenerated in presence of the molecular sieve, some of these gases from the air were retained. Probably smaller amounts of carbonyls having higher vapor pressures could be analyzed successfully although we have not thoroughly tested this tenet. Mass spectra of unknown mixtures of 2,4-dinitrophenylhydrazones regenerated in this manner were valuable in showing parent masses of carbonyl compounds in the mixture. The use of this procedure in conjunction with published procedures for classifying and separating 2,4-dinitrophenylhydrazones extends the procedure of Ralls t o provide additional qualitative information of considerable value for identification of carbonyl compounds.
HEPTAWAL
I
I
BEN2AtOEHYOE
I
ACETONE
rn/e
Figure 2. Mass spectra of some aldehydes an ketones regenerated from their 2,4-dinitrophenylhydrazone derivatives in the manner described in the text
LITERATURE CITED
(1) American Petroleum Institute, “Catalog of Mass Spectral Data,” API Research Project 44, Serial No. 645. (2) Bazinet, M. L., Merritt, Charles, Jr., ANAL.CHEM.34, 1143 (1962). (3) Corbin, E. A., Ibid., p. 1244. ( 4 ) Gilpin, J. A., McLafferty, F. W., Ibid., 29, 990 (1957).
( 5 ) Manufacturing Chemists’ Association Research Project, “Catalog of Mass
Spectral Data,” Serial No. 83. (6) Ralls, J. W., ANAL. CHEM.36, 946 (1964). ( 7 ) Ibid., 32, 332 (1960). (8) Schwartz, D. P., Parks, 0. W., Keeney, M., Ibid., 34, 669 (1962). (9) Sharkey, A. G., Jr., Shultz, J. L., Friedel, R. A., Zbid., 28, 934 (1956). MICHAEL E. MASON BOBBYJOHNSON Department of Biochemistry Agricultural Experiment Station Oklahoma State University Stillwater, Okla. MYNARD C. HAMMINQ Continental Oil Co. Ponce City, Okla. WORKsupported in part by a grant from the Corn Products Institute of Nutrition. VOL. 37, NO. 6, MAY 1965
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