ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
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Determination of Double Bonds in Alkenes by Field Ionization Mass Spectrometry Karsten Levsen," Raymund Weber, Friedrich Borchers, Heinz Heimbach, and H. D. Beckey Institut fur Physikalische Chemie, Wegelerstr.
72,D 5300 Bonn, Federal Republic of Germany
Electron impact (El), metastable ion (MI), and field ionization mass spectra of isomeric linear alkenes (C,-C,) are reported. I n contrast to the identical M I and slmllar E1 spectra of these compounds, the F I spectra show pronounced differences in the fragmentation pattern. They are dominated by fragments due to allylic cleavage which allows an unequivocal location of the double bond. In general, allylic cleavage leads also to the most abundant fragment in branched double bond isomers although a few exceptlons are observed with these compounds.
RESULTS AND DISCUSSION Linear Double Bond Isomers. FI, EI, and metastable ion (MI) spectra have been recorded from all linear alkenes from butenes to octenes. (Metastable ion (MI) spectra were obtained by scanning the electric sector of an instrument of reversed Nier-Johnson geometry. They result from unimolecular decompositions of mass selected parent ions in the second field free region of the instrument.) The FI and MI spectra are summarized in Table I; the E1 spectra have been published previously (6). The fragments observed in the E1 spectra result from decompositions integrated over a lifetime interval of 10-'4-104 s, i.e. fragmentation from both the isomerized and the nonisomerized molecular ion contributes to the spectra. Therefore the E1 spectra are similar, but not identical. Abundance variations are especially pronounced between the 1-alkenes and the other isomers, while for instance the E1 spectra of 2- and 3-hexene, 2- and 3-heptene as well as 2-, 3-, and 4-octene resemble each other closely. However, an identification of an isomer may still be possible in favorable cases using reference samples. For the long living metastable alkene ions ( t s) complete isomerization is expected. This should lead to MI spectra which are identical within the reproducibility as indeed is observed for all isomers (Table I). I t has been shown previously for other compound classes that MI spectra are particularily useful for a direct mass spectrometric mixture analysis (7). I t is obvious from the present results that such mixture analysis is not possible for isomeric alkenes. In contrast to the MI and E1 spectra, the FI spectra of linear alkenes differ considerably in their relative abundances (see Table I). Such differences have already been observed, although not specifically commented on, in an early FI study by Beckey and Schulze (8). (In this study a platinum tip was used as emitter leading to high field strengths. Thus field dissociation processes prevailed.) The fragments observed in an FI spectrum need not necessarily arise only from unimolecular decompositions in the gas phase, but may also be formed by field dissociation or surface reactions on the emitter ( 4 ) . Although not of crucial importance for the analytical aspects discussed here, possible contributions from the latter processes will be briefly discussed. Field dissociation of linear alkene ions predominantly leads to a fragment a t m / e 29 ( 4 ) . This fragment, not included in Table I, is abundant with all linear alkenes and represents the most prominent fragment in the FI spectra of heptenes and octenes. The contribution of field dissociation to other fragments in Table I should be small or negligible. The influence of surface reactions was reduced by heating the emitter which simultaneously leads to increased fragment abundances. However, the observation of doubly charged ions which are indicative of an interaction with the emitter surface demonstrates that contributions from such processes cannot be neglected (9). Defocusing experiments suggest that the large majority of the fragments are predominantly formed by unimolecular dissociation in the gas phase ( 3 ) . The fragmentation pattern observed in the FI spectra directly reflects the position of the double bond. Ionization
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The electron impact (EI) spectra of isomeric alkenes differing in the position of the double bond are generally very similar which makes it difficult or impossible to locate the double bond by conventional mass spectrometric techniques ( I ) . This information is, however, available through characteristic ion-molecule reactions studied in a chemical ionization source as recently shown by Ferrer-Correia et al. ( 2 ) . The present paper describes that the field ionization (FI) technique offers an alternative means to differentiate between such double bond isomers. I t has been demonstrated that rapid isomerization of the molecular ion of double bond isomeric alkenes (predominantly by hydrogen migration) explains the close similarity of their E1 spectra ( 3 ) . The dependence of this isomerization process on the ion lifetime has been studied for linear octene ions using the field ionization technique ( 3 ) . In this study, it was shown that the double bond isomeric ions to a large extent retain their original structure with localized radical site up to s after ionization. Substantial isomerization is observed a t longer lifetimes leading to a mixture of interconverting structures a t several s, after ionization. These results suggest that normal FI spectra may be used to distinguish between double bond isomers. The fragments observed in such an FI spectrum, if formed by unimolecular decay in the gas phase, correspond to decompositions integrated over ion lifetimes up to about lo-" s after ionization, a time interval within which little isomerization should have occurred. T o test this assumption 19 alkenes-including several branched ones-have been studied under both E1 and FI conditions.
EXPERIMENTAL FI spectra were obtained on a double focusing instrument of reversed Nier-Johnson geometry constructed by us. This instrument was equipped with an FI source. Ten-Fm tungsten wires activated with benzonitrile and heated by a dc current of 30 mA were used as field ion emitters (total anode-cathode voltage: 10 kV) ( 4 ) . Samples were introduced via the indirect inlet system kept a t room temperature. E1 spectra were obtained on an instrument of similar geometry, but equipped with a conventional E1 source (electron energy 70 e\', source temperature -150 "C). MI spectra were measured using the MIKE technique ( 5 ) . All ion abundances are the average of at least two measurements , *lo%). All (standard deviation for FI: &a%,EI: & 5 7 ~MI: compounds were purchased from FLUKA and measured without further purification. 0003-2700/78/0350-1655$01.OO/O
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Q 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
1657
Scheme I
11 of an alkene molecule leads predominantly to a removal of an electron from the a-orbital. As the resulting radical site remains largely localized a t the original position within the lifetime interval contributing to the normal FI spectrum (vide supra), preferential cleavage of the allylic carbon-carbon bond with or without charge migration should occur as depicted in Scheme I. Inspection of Table I reveals that for all linear isomers either the allylic cleavage with charge retention or that with charge migration indeed gives rise to the most prominent peaks in the spectrum while the complementary fragment in most instances leads to the second largest peak. (Peaks due to allylic cleavage are underlined). If two alternative routes for allylic cleavage with charge retention are available, Le., in 3-heptene and 3-octene, loss of the larger alkyl radical is the more abundant process. The characteristic fragmentation of linear alkenes under FI conditions not only allows a distinction to be made between the various isomers but moreover permits an unequivocal location of the double bond even in the absence of reference samples. Branched Double Bond Isomers. The FI and MI spectra of 5 branched double bond isomers are summarized in Table I; the E1 spectra have been published previously. Not only the MI but also the E1 spectra of the three isomeric methylbutenes are almost undistinguishable while pronounced differences in the fragmentation pattern a t least between 2-methyl-1-butene and 3-methyl-1-butene are observed under FI conditions. With both compounds, allylic cleavage leads to the most prominent peak in the spectrum ( m / e55). In 2-methyl-2-butene, no C-C bond is available for an allylic cleavage. Cleavage of the allylic C-H bond ( m / e 69) is an important process (49%) and leads to the most abundant fragment if the emitter is a t room temperature, demonstrating that surface reactions contribute to this hydrogen abstraction. Finally the EI, MI, and FI spectra of 2,4,4-trimethyl-lpentene and 2,4,4-trimethyl-2-pentene have been compared. Here not only the E1 but even the MI spectra differ drastically (Table I) demonstrating that in these strongly branched isomers the threshold for decomposition is considerably lower than the threshold for isomerization. Hence there is no need to use FI spectra to distinguish between these isomers. Allylic cleavage with charge retention and charge migration again gives rise to the most abundant fragments in the FI spectrum of 2,4,4-trimethyl-l-pentene ( m / e 55 and 57) while in the spectrum of 2,4,4-trimethyl-2-pentene, the formation of the butyl ion ( m / e 57) outweighs the allylic cleavage ( m / e 97). However, defocusing of the emitter voltage from 7000 to 7030 V leads to a drastic decrease of the butyl ion suggesting that a surface reaction contributes predominantly to its formation. The results demonstrate that the fragmentation of branched isomers is again dominated by allylic cleavages, although one has to be aware of possible exceptions from this general behavior. Note Added in Proof. After submission of this paper, we became aware of a similar study by Rang et al. (12). In agreement with our conclusions, the authors demonstrated for a series of isomeric dodecenes that FI mass spectra can be used to locate the double bond in alkenes.
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
LITERATURE C I T E D (1) H. Budzikiewicz, C. Djerassi, and D. H. Williams, "Mass Spectrometry of Organic Compounds", Holden-Day San Francisco, Calif., 1967, p 55. (2) A. J. v. Ferrer-Correia, K. R. Jennings and D. K. Sen Sharma, Org. Mass Spectrom., 11, 867 (1976). (3) F. Borchers, K. Levsen, H. Schwarz, C. Wesdemiotis, and H. U. Winkier, J . Am. Chem. Soc.. 99, 6359 (1977) and references herein. (4) H. D. Beckey "Principles of Fieid Ionization and Field Desorption Mass Spectrometry", Pergamon, Oxford, 1977. (5) J. H. Beynon. R. G.Cooks, J. W. Amy, W. E. Baitinger, and T. Y. Ridley, Anal. Chem., 45, 1023A (1973). (6) E. Stenhagen, S. Abrahamsson, and F. W. Mclafferty, "Registry of Mass Spectral Data", Wiley, New York, N.Y., 1974. (7) T. L. Kruger, J. F. Litton, and R. G. Cooks, Anal. Lett., 9, 533 (1976).
(8) H. D. Beckey and P. Schulze. 2. Naturforsch. A , 20, 1355 (1965). (9) F. W. Rollgen and H. J. Heinen, Int. J . Mass Spectrom. Ion Phys., 17, 92 (1975). (10) R. P. Morgan and P. J. Derick. Org. Mass Spectrum., I O , 563 (1975). (11) A. M. Fallick, P. Tecon, and T. Gciumann, Org. Mass Spectrom., 11, 409 (1976). (12) S.A. Rang, A.-M. A. Muurisepp, M.M. Liitma, and 0. G. Eisen, Org. Mass Spectrom., 13, 181 (1978).
RECEIVED for review May 19, 1978. Accepted July 5, 1978. Financial support by the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
Analytical Application of the Room and Low Temperature (77 K) Phosphorescent Properties of Some 1,8-Naphthyridine Derivatives Clausius G. de Lima" and Ezer M. de M. Nicola Departmento de QGmica, Universidade de Braska, Bradia D. F., Brazil
Milch et al. ( 8 ) investigated the possibility of the use of ultraviolet spectrophotometric data for the characterization and determination of NA and its ethyl ester and naphthyridinic acid and its ethyl ester. As solvents HCl 0.01 M, NaOH 0.1 M, ethanol, and chloroform were evaluated. However, Milch et al. (8) concluded that these four naphthyridine derivatives cannot be determined in the presence of each other using this instrumental method. Dondi and Di Marco (5)determined NA after reaction with 2-naphthol; the compound produced has an absorption maximum a t 351 nm. Room temperature solution fluorimetry has been used by McChesney et al. (Z),Browning and Pratt ( 3 ) ,Milch et al. (81, and Staroscik and Sulkowska (9). McChesney et al. ( 2 ) demonstrated that NA became strongly fluorescent at a pH range of 0-1, using dilute sulfuric acid (ca. 2.5%) as solvent medium, showing an emission a t 375 nm after excitation a t 330 nm. Browning and Pratt (3) employed the method used by McChesney et al. (2) using, however, a sulfuric acid concentration of 60%, reading the fluorescence emission a t 408 nm and exciting at 325 nm. Using this method, Browning During an investigation of new synthetic antimicrobial and Pratt (3) were able to determine as low as 100 ppb of NA drugs, Lesher et at. (I) reported, in 1962,that some derivatives in chicken liver and in chicken muscle homogenates. Milch of l,&naphthyridine were highly effective against gram et al. (8) examined the fluorescence characteristics of some negative pathogens, in particular, one known by the trivial name of nalidixic acid (l-ethyl-1,4-dihydro-7-methyl-4-oxo- l,&naphthyridine derivatives and observed that NA, its ethyl ester, and naphthyridinic acid exhibited strong fluorescence 1,8-naphthyridine-3-carboxilic acid). Due to this fact, nalidixic in acid medium (0.01 M HCl), while the naphthyridinic ethyl acid (compound I in Table I) is at present commercially ester showed the effect in alkaline medium (0.1 M NaOH). available (2) for medical use and has also found application In the acid condition, NA showed an excitation and emission as an effective aid in the control of chicken infection (3). maximum a t 313 and 360 nm, respectively (8).Staroscik and Spectrophotometrically, nalidixic acid (hereinafter called Sulkowska (9) determined NA fluorimetrically in urine after NA) has been analyzed in tablets (4-6), in human urine, serum, extraction with chloroform, with a limit of detection of 5 pg and feces (2) and in animal tissues (2, 3) by ultraviolet abmL-'. sorption spectrophotometry and by room temperature solution In the present work we examined the application of room spectrofluorimetry. The application of ultraviolet spectrotemperature phosphorescence for the determination of NA photometry was suggested or has been used by several workers and other similar compounds (Table I) in comparison with (4-8). Salim and Shupe ( 4 ) used chloroform as a solvent, the low temperature (77 K) phosphorescence technique. Room reading the absorption either at 258 or 332 nm. Da Silva and temperature phosphorescence (RTP) is a relatively new effect Nogueira (6) employed NaOH 0.1 M as solvent, reading the which was first reported by Roth (10) and later independently absorption a t 258 nm, while Zubenko and Shcherba (7) deby Schulman and Walling (11, 12). Usually the effect is termined NA spectrophotometrically in methanol or in 0.1 observed with many salts of polynuclear organic compounds, M NaOH as solvent, a t 258 and 324 or 332 nm, respectively.
The phosphorescence characteristics of some 1,8-naphthyrldlne derivatives both at room temperature (adsorbed on paper) or at low temperature (77 K ) in dilute alkaline solution have been investigated as a method of analysis. Attention was given principally to one of the derivatives-nalidixic acid-which is a potent antimicrobial drug produced on a commercial scale. For the room and low temperature phosphorescence study, a laboratory-assembled singlemonochromator spectrophosphorimeter was used in most of the experiments and a new paper chromatography accessory was tested, to be used directly in conjunction with the room temperature phosphorescence technique. Working curves, llmits of detection, lifetimes, and the effect of different parameters (such as the NaOH concentration, effect of irradiation, and effect of temperature in the sample compartment) were also determined.
0003-2700/78/0350-1658$01.00/0 1978 American Chemical Society
