thickness of a layer that must be sputtered for an analysis to be 0.1 to 1 pm, depending on the target material. Therefore, further research and development of the glow discharge as a means for bulk analysis of layers having a thickness of the order tof 1-10 pm is appropriate. When the current strength in the Grimm glow discharge is electronically stabilized and the voltage is kept constant by controlling the pressure, spectral-line intensities vary with
Lime. The relative variation is larger, the larger the sputtering rate. These effects are attributed to “changes in the geometry” caused by the sputtering process itself. Further research is needed to explain this point more precisely.
RECEIVED for review September 13, 1971. Accepted December 27,1971.
Raman Spectrometry of Some Common Barbiturates J. N. Willis,
Jr.,1
R. B. Cook,2 and Robert Jankowa
Cary Instruments, 2724 South Peck Road, Monrovia, Gal$ 91016
Eight barbiturates and three sodium salt analogs have been examined by Raman spectrometry. The pyrimidine ring which is found in all barbituric acids and their salts can be identified using characteristic Raman group frequencies of the carbonyl stretching and the ring “breathing” vibrations. The strong Raman band at 629 & 8 A cm-’ for the free acids and at 652 + 4 A cm-l for the sodium salts is tentatively assigned to the symmetric “breathing” of the pyrimidine ring. Most barbiturates studied are readily distinguished from each other by specific band locations.
IDENTIFICATION OF COMMON BARBITURATES have been of considerable interest since barbituric acid itself was first prepared by Baeyer in 1863 ( I ) . However, widespread use of barbiturates has made their detection and characterization an ever increasing problem in clinical, forensic, and toxicological laboratories. Numerous methods, such as thin layer chromatography and ultraviolet and infrared spectrometry, are available to determine the presence of a barbiturate, but these methods usually involve separations and laborious sample preparation. In addition, these methods seldom are able to identify which particular barbiturate is present. Because of the ease of sample preparation and ability to work in aqueous solutions, Raman spectrometry was investigated as a tool to study clinically important barbiturates. Since the introduction of relatively low cost laser Raman spectrometers (less than $20,000), the use of the Raman technique has increased and reference data are now becoming available. However, very little Raman work on drugs, particularly barbiturates, has yet been reported. Three infrared studies of barbiturates have been reported ; in all cases, however, only carbonyl and N-H vibrations were discussed (2-4). The work presented here was undertaken to determine whether the Raman technique could be used to classify different barbiturates according to their skeletal structure. No detailed analysis of the Raman spectra has Present address, Jarrell-Ash, Waltham, Mass. 02154. Address correspondence to this author. a Present address, Orbisphere Corporation, Geneva, Switzerland. (1) J. F. W. A. von Baeyer, Ann., 127,199 (1863). (2) L. Levi and C. E. Hubley, ANAL.CHEM., 28,1591 (1956). (3) G. Paulig, H. Gansau, P. Knorr, and L. Erbe, Fresenius’ 2. Anal. Chem.,218,27 (1966). (4) S. Goenechea, ibid., p 416. 1228
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been attempted, but rather a general discussion of common bands and major spectral differences has been outlined. Eight of the more common barbiturates were studied. EXPERIMENTAL
All Raman spectra were measured with the Cary 82 or Cary 83 Laser Raman Spectrophotometers. The 488.0 nm line of the argon ion line laser was used as excitation wavelength with -300 mW and 60 mW power at the sample for the Cary 82 and Cary 83, respectively. Frequency accuracy of sharp bands was *2 A cm-l and was 1.4 A cm-l for broad bands. Samples were white crystalline powders and were examined using 90’ illumination. Intensities were not corrected for instrument spectral response since such correction function for 488-nm excitation is nearly flat, especially between 0 and 2000 A cm-l(5). RESULTS
All barbiturates are generally produced and used in either of two forms, the free compound, structure I, or the sodium salt of the acid, structure 11. These compounds belong to the pyrimidine class. Since either form may be present in capsules and tablets, several of the barbiturates were studied in both forms I and 11. The major differences between the various compounds studied are the two groups attached to the 5 position of the barbituric ring. Futher, five compounds studied possess a common ethyl group at the 5 position and differ only in the other substituent. The last two compounds investigated have a methyl group substituted at the 1 position. Common spectral features of the free acids and salts, structures I and 11, and distinguishing features of each individual compound will be discussed to characterize each compound. Free Acids. GENERALCOMPARISONS. All eight compounds were run as the free acid. Spectra of the barbiturates presented in Figures 1-8 have two features which are distinctly different from their sodium salts (6) : ~
~~
~
( 5 ) H. J. Sloane, R. B. Cook and H. S . Haber, “Analytical Raman
Spectroscopy 11: Intensity Considerations,” presented at the Pittsburgh Conference, Analytical Chemistry and Applied Spectroscopy, March 1970 (Available from Cary Instruments, Monrovia, Calif. 91016). (6) Listings of frequencies and intensities for the barbiturates studied in this work are available from Cary Instruments, Monrovia, Calif. 91016.
Figure 1. Raman spectrum of solid 5-ethyl-5-phenylbarbituric acid (phenobarbital)
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Figure 3. Raman spectrum of solid 5-allyl-5-(l-methylbutyl) barbituric acid (secobarbital) Carbonyl Stretching. Bands arising from the C=O stretching vibration are clearly observed in the free acids at 1692 f 6 A cm-1 and 1737 f 8 A cm-’, the former being more intense. Levi and Hubley (2) reported infrared bands of these compounds in the carbonyl region “near” 1700, 1730, and 1750 cm-l. They assigned the low frequency band to carbonyl group stretching in the 2 position and the band at -1730 cm-l to symmetric vibrations of the carbonyls in the 4 and 6 positions. The band at -1750 cm-l was assigned to
carbonyl vibrations at positions 4 and 6 moving perpendicular to the molecular axis of symmetry. This would appear to be borne out by observing the Raman spectra of these compounds. The IR band near 1700 cm-l is found in the Raman at 1692 A cm-1, and the 1730 cm-1 IR band certainly corresponds to the 1737 A cm-l Raman band and, as expected from a symmetric vibration, is very intense. Antisymmetric vibrations observed in the infrared spectra at -1750 cm-1 are found only in the spectra of phenoANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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Figure 4. Raman spectrum of solid 5-ethyl-5-isoamylbarbituricacid (amobarbital)
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Figure 5. Raman spectrum of solid 5-ethyl-5-(l-methylbutyl) barbituric acid (pentobarbital)
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Figure 6. Raman spectrum of solid 5-ethyld-sec-butyl barbituric acid (butabarbital) barbital (Figure 1) and barbital (Figure 2) at 1755 and 1762 A cm-l, respectively. This simple approach to assignments of carbonyl vibrations obviously does not take into account such factors as the coupling of these vibrations with other modes, solid state splitting of the bands, or intramolecular effects. One or more of these effects becomes significant, particularly in the cases of phenobarbital and barbital, both of which exhibit more than the three C 4 stretching bands predicted. However, the intensities of these Raman bands support the assignments of Levi and Hubley. Symmetric Breathing. The second feature is the strong band at 629 =t= 8 A cm-I which undoubtedly arises from the 1230
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
symmetric “breathing” of the pyrimidine ring. This type of ring “breathing” vibration is generally one of the most intense bands in Raman spectra of compounds containing a cyclic structure. The intensity and position of this band makes it a good group frequency for the pyrimidine ring of barbiturates. Thus, it may be used along with the carbonyl bands to characterize compounds as barbiturates. Table I lists frequencies and relative intensities of all bands found to be common to the free acids studied. SPECIFICCOMPARISONS. Individual compounds may be characterized by the unique vibrations of the substituents. Phenobarbital contains a monosubstituted phenyl group
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Figure 7. Raman spectrum of solid l-methyl-5-ethyl-5-phenylbarbituricacid (mephobarbital)
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Figure 8. Raman spectrum of solid 1,5-dimethyl-5-(l-cyclohexene) barbituric acid (hexobarbital) which has characteristic frequencies at 620, 1002, 1039, 1585, and 1597 A cm-I (7), all of which may be easily seen in the Raman spectrum, Figure 1. The only compound which possesses an alkyl group containing a double bond is secobarbital (Figure 3). This compound has characteristic vibrations at 653 and 1640 A cm-I, both of which are intense bands and well separated from other bands found in these regions. Amobarbital (Figure 4) and pentobarbital (Figure 5) are somewhat more difficult to differentiate, but differences exist in their spectra. Simpson and Sutherland (8) have found that there is a characteristic skeletal vibration for C
c-c
