Anal. Chem. 2000, 72, 1175-1178
A Multiple Channel Electrospray Source Used To Detect Highly Reactive Ketenes from a Flow Pyrolyzer Chi-Ming Hong,† Fong-Chi Tsai,‡ and Jentaie Shiea*,†
Eternal Chemical Corporation, Kaohsiung, Taiwan, and Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, Taiwan
This work detects protonated molecular ions of highly reactive pyrolytic productsscyclopentadienylideneketene, cyclohexadienylideneketenimine, and acetylketenesusing a flow pyrolyzer connected to a multiple-channel electrospray mass spectrometry. The ketene generated in the flow pyrolyzer is directly conducted into the central channel of a seven-channel electrospray ionization source by a stream of nitrogen gas. Concurrently, a methanol solution containing 0.1% trifluoroacetic acid is electrosprayed through the outside six channels. The protonated methanol ions and the charged droplets generated from the outside six electrospray channels facilitate the ionization of the neutral ketenes through ion-molecule reactions or absorption followed by protonation. Ketenes are extremely unstable compounds that can be synthesized by high-temperature pyrolysis or photolysis.1-4 Although they are highly reactive and have a very short life span, these compounds have been important molecules in the history of physical organic chemistry.1-4 Most ketenes react and disappear at temperatures above 80 K; some ketenes are more reactive and decompose or polymerize at an even lower temperature.1-6 Traditionally, the compounds are structurally characterized by absorption spectrometry using UV or IR. The analyses must be performed at an extremely low temperature using matrix isolation methods.5-8 Unfortunately, in many cases, due to rapid decarbonylation of ketene itself and interference from other reaction products such as benzyne, unambiguous determination of structures using spectroscopic methods is very difficult.7,8 Characterization of unstable ketenes by mass spectrometry and tandem mass spectrometry was developed in the early 1980s.9,10 †
National Sun Yat-Sen University. Eternal Chemical Corp. (1) Brown, R. F. C. Pyrolytic Methods in Organic Chemistry; Academic Press: New York, 1980. (2) The Chemistry of Ketenes, Allenes, and Related Compounds; Patai, S., Ed.; John Wiley & Sons, New York, 1980. (3) Chapman, O. L.; Mattes, K.; Mcintosh, C. L.; Pacansky, J.; Calder, G. V.; Orr, G. J. Am. Chem. Soc. 1973, 95, 6134. (4) Berry, R. S.; Spokes, G. N.; Stiles, M. J. Am. Chem. Soc. 1962, 84, 3570. (5) Gano, J. E.; Jacobson, R. H.; Wettach, R. H. Angew. Chem., Int. Ed. Engl. 1983, 22, 165. (6) Munzel, N.; Schweig, A. Chem. Phys. Lett. 1988, 147, 192. (7) Simon, J. G. G.; Munzel, N.; Schweig, A. Chem. Phys. Lett. 1990, 170, 187. (8) Wentrup, C.; Blanch, R.; Briehl, H.; Gross, G. J. Am. Chem. Soc. 1988, 110, 1874. ‡
10.1021/ac990462z CCC: $19.00 Published on Web 02/11/2000
© 2000 American Chemical Society
Among these techniques, electron impact ionization (EI) or chemical ionization (CI) are the two ionization techniques used. For example, Wentrup et al. modified an EI/CI source housing, coupling it to a flash vacuum pyrolyzer (FVP) for the detection of methyleneketene and other pyrolytic intermediates.11 Clemens and Witzeman characterized acetylketene by pyrolyzing the starting material placed in a GC injector port; the acetylketene was then detected by a GC/MS.12 Dass used a standard CI source (operated at a temperature below 70 °C) to detect ketene which was introduced via an all-glass inlet cooled by solid CO2 to suppress the polymerization of ketene.13 Other research groups reported on similar mass spectra.14,15 Only one reported study used field ionization (FI) to ionize the neutral ketene.16 Our previous studies have demonstrated that low-temperature atmospheric pressure ionization (LT-API) and secondary ion mass spectrometry (LT-SIMS) can be used to successfully detect intact molecules from extremely reactive compounds generated by flash vacuum pyrolysis.17-19 In these techniques, the analyte is coldtrapped in an organic solvent. Subsequently, the sample solution is injected into a homemade LT-API source or LT-SIMS interface. When the concentration of the analyte in the sample solution is low, the signal from protonated analyte ions (monomer) predominates. Unfortunately, at higher concentrations, the analyte rapidly polymerizes; even at low temperature, the signals from the analyte cluster ions predominate in the mass spectra. Our approach used to solve the above problem detects the reactive compound soon after it is synthesized. Real-time analysis of a reaction involving volatile reactant and a product containing heteroatoms is achieved by connecting the reaction vessel to a (9) Dass, C.; Gross, M. L. J. Am. Chem. Soc. 1984, 106, 5775. (10) Groenewold, G. S.; Gross, M. L. J. Am. Chem. Soc. 1984, 106, 6569. (11) Brown, J.; Flammang, R.; Govaert, Y.; Plisnier, M.; Wentrup, C.; Haverbeke, Y. V. Rapid Commun. Mass Spectrom. 1992, 6, 249. (12) Clemens, R. J.; Witzeman, J. S. J. Am. Chem. Soc. 1989. 111, 2186. (13) Dass, C. Org. Mass Spectrom. 1993, 28, 940. (14) Turecek, F.; Maquin, F.; Hill, N.; Stahl, D.; Gaumann, T. Org. Mass Spectrom. 1988, 23, 91. (15) Maquestian, A.; Flammang, R.; Pauwels, P. Org. Mass Spectrom. 1983, 18, 547. (16) Egsgaard, H.; Carlsen, L. J. Anal. Appl. Pyrolysis 1984, 7, 1. (17) Shiea, J.; Wang, W. S.; Chen, C. H.; Chou, C. H. Anal. Chem. 1996, 68, 1062. (18) Wang, C. H.; Huang, M. W.; Lee, C. Y.; Chei, H. L.; Huang. J. P.; Shiea, J. J. Am. Soc. Mass Spectrom. 1998, 9, 1168. (19) Wang, W. S.; Tseng, P. W.; Chou, C. H.; Shiea, J. Rapid Commun. Mass Spectrom. 1998, 12, 931.
Analytical Chemistry, Vol. 72, No. 6, March 15, 2000 1175
Scheme 1
Scheme 2
Scheme 3
Figure 1. Schematic diagram of flow pyrolyzer/multiple channel electrospray ionization mass spectrometry (A, Teflon tube; B, capillary column; C, PEEK fitting; D, stainless steel union).
multiple channel electrospray ionization mass spectrometry (MCES/MS).20 Results of this study demonstrate that the protonated molecular ions of gaseous organic compounds can be obtained immediately after they are synthesized. The technique may then be suitable to detect reactive compounds with short life spans, such as ketene, This investigation reports a simple set up that enables the identification of intact molecular ions of reactive ketenes by electrospray mass spectrometry. This is achieved by coupling a flow pyrolyzer (FP) to a multiple-channel electrospray ionization mass spectrometer. Three pyrolytic reactions producing different types of ketenesscyclopentadienylideneketene (CPDK), cyclohexadienylideneketenimine, and acetylketene (AK)swere performed (Schemes 1-3). These ketenes have been well studied and their structures characterized by other spectroscopic methods.21-26 EXPERIMENTAL SECTION All chemicals were obtained commercially (Lancaster, Sigma, or Aldrich). Pyrolytic reactions were performed in a flow pyrolyzer. The flow pyrolyzer consists of a quartz tube (i.d. 1.5 cm; length 45 cm) equipped with a tantalum wire heater and radiation shields. (20) Lee, C. Y.; Shiea, J. Anal. Chem. 1998, 70, 2757. (21) Brown, R. F. C.; Browne, N. R.; Coulston, K. J.; Eastwood, F. W.; Irvine, M. J.; Pullin, D. E.; Wiersum, U. E. Aust. J. Chem. 1989, 42, 1321. (22) Radziszewski, J. G.; Kaszynski, P.; Friderichsen, A.; Abildgaard, J. Collect. Czech. Chem. Commun. 1998, 63, 1694. (23) Smalley, R. K.; Suschitzky, H. Tetrahedron Lett. 1966, 29, 3456. (24) Moloney, D. W. J.; Wong, M. W.; Flammang, R.; Wentrup, C. J. Org. Chem. 1997, 62, 4240. (25) Hyatt, J. A.; Feldman, P. L.; Clemens, R. J. J. Org. Chem. 1984, 49, 5150. (26) Kappe, C. O.; Wong, M. W.; Wentrup, C. J. Org. Chem. 1995, 60, 1686.
1176 Analytical Chemistry, Vol. 72, No. 6, March 15, 2000
The tube was packed with small quartz chips. Temperature (up to 1200 °C) was data system controlled. Approximate 0.5 g (for solid sample) or 0.25 mL (for liquid sample) of the sample was introduced into the pyrolysis tube. The ketenes were pyrolyzed from 2,2,6-trimethyl-1,3-dioxin-4-one, phthalic anhydride, and isatoic anhydride, respectively, at varied temperatures (see Schemes 1-3).21-26 The ES mass spectra of these starting materials were obtained using a conventional ES/MS equipped with an ion spray source (PE Sciex API 1). Figure 1 schematically depicts a flow pyrolyzer connected to a multiple-channel electrospray ionization mass spectrometry (FP/MC-ES/MS). The details of construction of the MC-ES source has been described elsewhere.20,27 In this study, a seven-channel electrospray ionization source was constructed. The central channel in the MC-ES source is simply a Teflon tube (o.d. 0.16 cm, i.d. 0.05 cm, length 13 cm) which connected to the exit of the pyrolysis tube. After the sample was introduced into the pyrolyzer, the temperature of the pyrolyzer was increased rapidly. The valve at the right end of the pyrolytic tube was opened, and a nitrogen gas (30 mL/min) continuously swept the pyrolytic product (ketene) into the central channel of the seven-channel electrospray ionization source. The surrounding six electrospray channels were electrosprayed simultaneously with a methanol solution containing 0.1% of trifluoroacetic acid (TFA). The acidic solution (0.5 µL/min) was pumped to the six capillaries by a syringe pump. The required high voltage (∼3.8 kV, from a Classman EH20R20 power supply) for the electrospray was introduced into the tips of the six capillaries by solution conduction. The positive ions generated from FP/MC-ES were detected by a PE-Sciex API 1 mass spectrometer. The mass was scanned from m/z 10 to 400 at a rate of ∼1 s/scan. The temperature of the electrospray interface chamber was maintained at 55 ( 1 °C. The electron impact ionization mass spectrum was obtained with (27) Shiea, J.; Wang, C. H. J. Mass Spectrom. 1997, 32, 247.
Figure 2. (a) Positive ES mass spectrum of phthalic anhydride (in methanol) obtaining by electrospraying the sample solution through the commercial ES source. The inset is the El mass spectrum of phthalic anhydride. Positive ES mass spectra in (b) and (c) are the pyrolytic products from phthalic anhydride. The mass spectra were obtained by FP/MC-ES/MS. The pyrolytic temperatures were set at (b) 300 and (c) 600 °C, respectively.
a VG Quattro mass spectrometer equipped with EI/CI and FAB sources. RESULTS AND DISCUSSION Three ketenes were synthesized and detected using a flow pyrolyzer connected to a multiple channel electrospray ionization mass spectrometry. The time required for the ketene to flow from the exit of the pyrolyzer to the tip of the central channel of the MC-ES source (13 cm long) is ∼0.2 s. This is calculated from the flow rate (30 mL/min) of the purging gas in the pyrolyzer and the internal diameter (0.05 cm) of the Teflon tube connecting the pyrolyzer to the MC-ES source. If the lifetime of the gaseous ketene is longer than 0.2 s, it will reach the tip of the central channel of the electrospray source and be detected. Cyclopentadienylideneketene is a known species.21,22 The structure of CPDK has been characterized by low-temperature IR.22 However, due to interference from other products such as benzyne and carbene, an unambiguous interpretation of the IR spectrum has not yet been established.22 The mass spectroscopic evidence for the existence of CPDK is obtained by treating the pyrolysate with methanol followed by catalytical hydrogenation yielding a stable ester. The ester is then analyzed by conventional GC/MS.21 In this study, CPDK was synthesized by pyrolyzing phthalic anhydride at different high temperatures. Figure 2a depicts a conventional ES mass spectrum of phthalic anhydride dissolved
Figure 3. (a) Positive ES mass spectrum of isatoic anhydride (in methanol) obtaining by electrospraying the sample solution through the commercial ES source. (b) Positive ES mass spectrum (by FP/MC-ES/MS) of ketenimine pyrolyzed from isatoic anhydride at 260 °C.
in methanol. The ES mass spectrum was obtained by electrospraying the sample solution through the commercial ES source operated at room temperature. The protonated phthalic anhydride (MH+, m/z 149) predominates in the mass spectrum. There are traces of unknown compounds (m/z 93 and 121) that are possibly from impurities in the sample solution. The inset in Figure 2a depicts the electron impact ionization mass spectrum of phthalic anhydride. The fragments of phthalic anhydride in the EI source include ions that may be CPDK (m/z 104) and benzyne (m/z 76) ions. Figure 2b depicts the MC-ES mass spectrum of the pyrolytic product from phthalic anhydride at 300 °C. The substances from the pyrolysis tube were conducted into the central channel of the multiple channel electrospray ionization source. Because the temperature (300 °C) is not high enough to initiate pyrolysis, the protonated phthalic anhydride ion (m/z 149) still predominates in the MC-ES mass spectrum. Only a weak CPDK signal (m/z 105) is observed, suggesting that the CPDK molecules survive during transportation from the flow pyrolyzer to the MC-ES source. There is also an unknown ion at m/z 183. The signal of the ion at m/z 33 in Figure 2b is from the protonated methanol ion which is generated by electrospraying methanol solution containing 0.1% TFA from the outside six electrosprayers. Two ionization processes had been proposed in MC-ES.20 First, ionmolecule reactions (IMR) may occur when the gaseous analyte molecule collides with the protonated methanol ion (CH3OH2+) generated by the six outside electrosprayers on the MC-ES source. Analytical Chemistry, Vol. 72, No. 6, March 15, 2000
1177
Figure 4. (a) Positive ES mass spectrum of 2,2,6-trimethyl-1,3dioxin-4-one (in methanol) obtaining by electrospraying the sample solution through the commercial source. (b) Positive ES mass spectrum (by FP/MC-ES/MS) of pyrolytical products of 2,2,6-trimethyl1.3-dioxin-4-one at 200 °C.
Second, the analyte molecule can be absorbed into the fine droplet generated by the six outside electrosprayers and protonation of the analyte can occur in the droplet.20,27 Figure 2c depicts the ES mass spectrum of the pyrolytic product from phthalic anhydride at 600 °C. According to this mass spectrum, the pyrolytic reaction is complete at this high temperature and a strong signal from protonated CPDK ion (m/z 105) is detected on the MC-ES mass spectrum. The signal of phthalic anhydride (m/z 149) is hard to see. The study of the chemistry of ketenimine, a transient reactive intermediate, has been an interesting theme of organic chemistry for many years.23,24 Ketenimine can be obtained by pyrolyzing compounds such as isatoic anhydride. Such a compound was normally characterized using low-temperature IR. Mass spectroscopic evidence showing the molecular ion of ketenimine was rarely reported.24 Figure 3a depicts the ES mass spectrum of isatoic anhydride (dissolved in methanol) using a commercial ES source operated at room temperature. The mass spectrum shows sodiated and potassiated isatoic anhydride monomers (m/z 186 for MNa+ and m/z 202 for MK+) and dimers (m/z 349 for M2Na+
1178
Analytical Chemistry, Vol. 72, No. 6, March 15, 2000
and m/z 365 for M2K+). The sodium and potassium may come from the impurities present in the sample solution. The protonated isatoic anhydride ion (m/z 165) cannot be found. Figure 3b depicts the ES mass spectrum of the pyrolytic product of isatoic anhydride. The isatoic anhydride was pyrolyzed at 260 °C, and the pyrolytic product was conducted immediately into the central channel of the MC-ES source for detection. The pyrolytic reaction is complete at this high temperature, and the signal from protonated ketenimine (m/z 120) dominates the ES mass spectrum. Acetylketene and acetone are the pyrolytic products obtained by pyrolyzing 2,2,6-trimethyl-1,3-dioxin-4-one at 200 °C.25,26 Figure 4a depicts the ES mass spectrum of 2,2,6-trimethyl-1,3-dioxin-4one (dissolved in methanol) using a commercial ES source operated at room temperature. Except for the protonated molecular ion (MH+, m/z 143), the sodiated and potassiated ions (MNa+, m/z 165; MK+, m/z 181) of the 2,2,6-trimethyl-1,3-dioxin-4-one are also detected. Also, a peak appears from sodiated dimer ion (M2Na+, m/z 307). Because the ES mass spectrum was obtained at room temperature, the presence of a fragment ion at m/z 85 implies that 2,2,6-trimethyl-1,3-dioxin-4-one is not as stable as phthalic anhydride and isatoic anhydride. Figure 4b depicts the MC-ES mass spectrum of pyrolytic products of 2,2,6-trimethyl1,3-dioxin-4-one at 200 °C. The protonated acetone ion (m/z 59) now predominates in the mass spectrum. Acetone is known to be one of the pyrolytic products in this pyrolytic reaction (see Scheme 3 ). The intensity of the acetylketene ion is ∼28% of the base peak. The signal of 2,2,6-trimethyl-1,3-dioxin-4-one is less than 6%, indicating that the pyrolytic reaction was nearly complete at this temperature. CONCLUSION This work detected protonated molecules of extremely reactive ketenes generated from pyrolytic reactions by connecting the flow pyrolyzer to a multiple-channel electrospray ionization mass spectrometer. For this technique, the ionization is performed at ambient temperature and cold-trapping of volatile products is unnecessary; thermal isomerization and polymerization of the reactive compound can be avoided. The time required for the ketene molecule to move from the pyrolyzer to the exit of the central channel of the multiple channel ES source is ∼0.2 s, allowing the reactive ketene molecule to be detected before it decomposes or reacts further. ACKNOWLEDGMENT The authors thank the National Science Council, Taiwan, for financially supporting this research.
Received for review May 3, 1999. Accepted December 8, 1999. AC990462Z
