Anal. Chem. 1989, 6 1 , 777-779
Table I. Calculated and Measured’ Relative Intensities for the n = 21 Ammonium Adduct Isotope Multiplet mlz
3457 3458 3459 3460 3461 3462 3463 3464 3465 3466
relative intensity calcd, % measd; % 28.4 63.0 92.1 100.0 89.5 68.4 46.1 27.9 15.4 7.8
35.5 68.6 92.4 100.0 89.5 70.3 47.1 30.2 15.7 7.6
re1 deviation, % 22.2 8.5 0.0 0.0 2.7 2.1 7.9 1.9 2.6
From peak heights. of the calculated and observed relative intensities deviate from one another by less than 3 % . The median deviation is 2.6 % . The peaks tail to the low mass side, with peak height measurements becoming less accurate in this direction. We obtained these reasonable isotope ratios after a minimum amount of tuning. In the work described here a simple sample cylinder was used to contain the polysiloxane solution instead of a pump. The pressure was fixed at roughly 5.9 MF’a (58 atm) (measured with the uncalibrated gauge described in the Experimental Section), the vapor pressure of the C 0 2 with 5% 2-propanol at room temperature. It might be argued that the method described is in reality subcritical fluid injection since the critical pressure of the mix is not exceeded. However, the critical pressure is defined (and only has physical significance) at the critical temperature (11). At a temperature only slightly above the critical temperature, the pressure vs density isotherm shows a dramatic change in density (and solvating strength) about the critical point. This discontinuity in the pressuredensity isotherm becomes less dramatic as the temperature of the system increases. The isotherm is nearly linear in this region at a reduced temperature of three for pure COz, for example. At the point of injection into the mass spectrometer ion source, the polysiloxane solution is clearly far above its critical temperature. Whether the pressure is slightly above or slightly below the “critical” pressure at this temperature has little effect on the solvating power of the carrier, on the flow rate of modified COz through the restrictor, and on the pressure within the ion source region. This is borne out by our observations of the ion source pressure during SFI/MS with our sample cylinder and during normal pressure-programmed SFC/MS (1). Thus we feel justified in our
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use of this simple method to approximate conditions encountered in SFC/MS and in our use of the term “supercritical” in SFI/MS. During these early trials, SFI/MS of polysiloxanes dirtied the ion source and analyzer over a 1-day period. In more recent experiments, less concentrated solutions (0.5%) produced strong, stable ion currents over a 5-day period without rapid deterioration in ion source and analyzer conditions. These results indicate that SFI/MS of polysiloxane solutions is a reasonable tuning and calibration tool for SFC/MS. It is also useful in studying instrument performance over a wide mass range. Figure 1provides a glimpse at the potential power of SFI/MS for quickly characterizing oligomeric mixtures, especially those that are difficult to analyze by desorption/ionization techniques such as fast atom bombardment MS or plasma desorption MS.
LITERATURE CITED (1) Pinkston, J. D.; Owens, G. D.; Burkes, L. J.; Delaney, T. E.; Mllllngton, D. S.; Maltby, D. A. Anal. Chem. 1988, 60,962-966. (2) Sin, C. H.; Pang, H. M.; Lubman, D. M.; Zorn, J. Anal. Chem. 1986, 58, 487-490. (3) Smith, R. D.; Udseth, H. R. Anal. Chem. 1983, 55, 2266-2272. (4) Smith, R. D.; Udseth, H. R.; Hazlett, R. N. Fuel 1985, 64, 810-815. (5) Bertrand. M. J.; MaRals. L.; Evans, M. J. Anal. Chem. 1987, 59, 194-197. (6) Chester, T. L.; Innis, D. P.; Owens, G. D. Anal. Chem. 1985, 5 7 , 2243-2247. (7) Wright, B. W.; Smith, R. D. J . Chromatogr. 1986, 355, 367-373. (8) Hacker, D. S. in Supercr/f/ca/Flu/& - Chemical and €ng/nesr/ng Principles and Applications; Squires, T. G., Paulaitis, M. E., Eds.; ACS Symposium Series 329; American Chemical Society: Washington, DC, 1987; pp 213-228. (9) Brady, J. E., University of Pittsburgh, personal communication, 1988. (IO) Yonker, C. R.; Frye, S. L.; Kalkwarf, D. R.; Smith, R. D. J . Phys. Chem. 1988, 9 0 , 3022-3026. (11) CRC Handbook of Chemistry and Physics, 57th ed.; CRC Press: Cleveland, OH 1976; p F-98. To whom correspondence should be addressed.
J. David Pinkston* Grover D. Owens The Procter & Gamble Company Miami Valley Laboratories P.O. Box 398707 Cincinnati, Ohio 45239-8707
Ernest J. Petit The Finnigan-MAT Institute 4450 Carver Woods Drive Cincinnati, Ohio 45242
RECEIVED for review October 4,1988. Accepted December 27, 1988.
Supersonic Jet Spectroscopy of Nonvolatiles f rorn Pulsed High Pressure Ammonia Expansions Sir: The sharp spectral features available by using supersonic jet expansions in conjunction with laser spectroscopic methods have been demonstrated as a means of obtaining enhanced selectivity in chemical analysis ( I ) . In the past, this technique has generally been limited to molecules which can be easily volatilized into the gas phase for expansion with a light carrier gas. More recently there have been a number of methods developed for volatilizing nonvolatile and thermally labile molecules into supersonic jets (2-18). These
methods have the potential for extending the selectivity of jet spectroscopy to a broad class of molecules of biological and pharmaceutical importance. Direct heating methods have been used for some time for volatilization of relatively stable large polynuclear aromatic hydrocarbons (PNAHs) with high melting points (>200 OC) into supersonic jet expansions (2-4). In order to study more labile species several groups have developed specialized heating methods in order to minimize thermal decomposition (5,6).
0003-2700/89/036 1-0777801.50/0 0 1989 American Chemical Society
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However, these methods are generally applicable to only a limited number of chemical systems which are still relatively stable. More recently other methods have been developed to volatilize labile samples into the gas phase that avoid the use of continuous heating. One such method utilizes pulsed laser desorption, where a high-powered pulsed infrared laser is used to induce rapid heating that desorbs neutral molecules from a surface before they have time to decompose (7-9). Following desorption, the molecules are entrained into a pulsed supersonic jet and swept into a mass spectrometer for study by resonant two-photon ionization (R2PI) spectroscopy. This method has been used by Li and Lubman (10) for the study of the jet spectroscopy of tyrosine derivatives and dipeptides and by Levy and co-workers (11) for the study of tryptophan-containing di- and tripeptides. This technique avoids the decomposition that can result from heating methods; however, it requires the use of an expensive pulsed COz laser for performing the volatilization. An alternate method that has been explored for volatilizing labile molecules into jet expansions involves the use of supercritical fluids (12-18). This method is based upon the property of these fluids whereby they can dissolve certain molecules like a liquid, but then expand as a gas upon introduction into vacuum, i.e. a supersonic jet. This method though has been limited to relatively nonpolar fluids such as COz and N20 in which thermally labile polar biological molecules generally have very low solubility. Thus, in this work we have utilized high pressure pulsed ammonia expansions as a means of volatilizing nonvolatile molecules into supersonic jets. In this method, the sample is dissolved in liquid ammonia under high prssure (- 150 atm) but at room temperature. The liquid ammonia and sample are injected into vacuum using a pulsed injection source where the liquid is rapidly heated to supercritical conditions. Due to the short residence time in the pulsed valve, thermal decomposition is minimized. It is demonstrated herein that this method can be used for introducing both nonvolatiles and thermally labile polar molecules into jet expansions a t relatively low temperature and with minimal decomposition.
EXPERIMENTAL SECTION The experimental setup has been described in detail elsewhere (19)and consists of a differentially pumped vacuum system with a time-of-flight(TOF) mass spectrometer sitting vertically on top of the chamber. The laser beam ionization source enters the chamber through quartz windows and R2PI is produced in the acceleration region of the TOFMS. The key component of this experiment is the pulsed valve which can inject the carrier fluid at up to 200 atm and 180 "C. The design of this valve is described elsewhere (20) and is based upon a solenoid operator. The unique feature of the valve is that it has been designed so that the solenoid coil is not in contact with the fluid as in earlier high pressure pulsed valve designs (21). Thus, the valve can be used for injection of corrosive gases such as ammonia into supersonic jets. The high pressure pulsed valve injects the supersonic jet into an expansion chamber which is pumped by a 4-in. diffusion pump and several liquid N2 traps which rapidly pump the condensible NH3. The jet is then skimmed by a liquid N2 cooled skimmer which has been designed with a sharp edge to slice the beam and then is shaped to deflect away molecules not transmitted through the orifice in order to minimize the shock waves that would otherwise occur against a flat surface. The skimmer uses a large orifice (0.95 cm) to allow maximum transmission of the supersonic beam. The beam then expands into a second chamber pumped by a 6-in. diffusion pump and several liquid N2traps. The pressure in this chamber remains below 2 X 10" Torr. The TOFMS acceleration region is located in this chamber and the laser ionization beam is synchronized to intercept the jet as it enters the TOFMS. The resulting ions are mass separated in the time-offlight drift tube, and the mass spectrum is recorded. Sample introduction is performed with high pressure liquid NH3 (- 20 "C, 120 - 180 atrn), delivered by a Varian 8500 syringe
I 1 I/
325
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323
1
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WAVELtNGTH
1 320
I
319
I 318
Inm)
Figure 1. RPPI mass selected wavelength spectrum of carbazole obtained with the use of supercritical NH, injection at 180 atm and 160
"C. pump. The liquid NH3passes through a high pressure inline filter (NUPRO SS-4F10-7) in which the sample has been placed beforehand. The sample is extracted by the liquid NH3 (Scott Specialty Gases) and carried into the pulsed valve for injection. The nozzle is heated up to 140 "C by a thermocoax heater in order to convert liquid NH3into supercriticalNH3. The residence time is relatively short (- 5 s) in order to minimize the decomposition during the injection process. Under the typical jet conditions used in this experiment, Le. 150 atm back pressure expansion through a 330-pm orifice at 10-Hz repetition rate, the pressure in the source chamber does not rise above 1 X lo4 Torr Torr. and the pressure in the ionization chamber is -1.5 X The laser system used is a Quanta-Ray DCR-SA Nd:YAG pumped dye laser, which is frequency doubled in KD*P to produce tunable near-UV light. The typical doubled dye laser energy used in this work to obtain wavelength spectra is -2 mJ. The laser beam is collimated with a positive-negative lens combination to a 2-3 mm beam. The delay between the pulsed valve and the laser is optimized by using a delay generator at -700 ps. The best spectra are obtained near the beginning of the molecular beam pulse, since further into the pulse either shock waves or clustering may lower the degree of cooling achieved. In these spectra, the molecular ion only is monitored in the TOFMS as a function of wavelength using an SRS 250 gated integrator. The mass spectra were obtained with a frequency-quadrupled (266 nm) Nd:YAG laser at 4 mJ and were signal averaged with the LeCroy 9400 waveform recorder over 100 laser pulses.
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RESULTS AND DISCUSSION
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In Figure 1 is shown a mass-selected R2PI wavelength ) of carbazole. This spectrum of the So SI ( ~ - n *transition spectrum was obtained by dissolving carbazole in liquid NH3 at 180 atm and 20 "C and then heating to 150-160 "C upon injection from the pulsed valve to obtain a gaseous jet of NH3 molecules. The melting point of carbazole is 245 "C and in an expansion formed from 1 atm back pressure of Ar at a temperature of 20 "C no signal is observed. The cooling obtained from the NH, expansion is comparable to that obtained from either a supercritical COz (13) or NzO (14) expansion of carbazole obtained in earlier work. The degree of translational cooling obtained from NH, expansions can be estimated from the work of McClelland et al. (22) as M = A(-y)(X/D)?-lwhere A ( 7 ) = 3.90 for NH3 and X = 22 cm in our experiment so that M = 29 in the laser ionization region. This corresponds to a translational temperature of 3 K. In Figure 2 is shown a mass-selected R2PI wavelength spectrum of tryptamine. In the corresponding mass spectrum the molecular ion is the dominant peak with only minor fragments at m / z 130 and 30. A small peak is observed at m / z 18 due to charge exchange with ammonia. The wavelength spectrum exhibits relatively sharp, cold spectral fea-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
150atm NH3 l5OoC
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of clustering can be minimized when the laser pulse is timed to intersect the front edge of the molecular beam pulse. As the laser pulse probes further into the molecular beam pulse, the degree of clustering becomes much more extensive and clusters of carbazole-", of up to M + 9NH3 are observed. This phenomenon may be due to several effects including an increasing number density in the molecular beam later in the pulse which results in more three body collisions. Thus by timing the laser delay accordingly, the clustering effect has been minimized in these experiments. High pressure liquid NH3 thus serves as a means of volatilizing polar nonvolatiles for injection into supersonic jets. The NH3 is heated to supercritical conditions before expansion and since the residence time is relatively short decomposition is minimized. By use of this method, cold wavelength spectra of these molecules can be obtained in the NH3 jet expansion as well as molecular ions for identification in the R2PI mass spectrum. In addition, minimal solvent-solute cluster formation is observed due to the pulsed injection method in which the laser probes the gas pulse before there are sufficient collisions to induce clustering.
I
Tryptamine
I
779
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I
286.50 286.00 285. WAVELENGTH (nm) Figure 2. RPPI mass selected wavelength spectrum of tryptamine obtained with the use of supercritical NH3 injection at 150 atm and 150 OC.
tures with the dominant peak being the main origin band at 286.3 nm (5). The spectrum is similar to that obtained by using pulsed laser desorption/volatilization with entrainment into an-expansion from 1 atm of COz carrier (23). It is also similar to that obtained by Levy and co-workers (5) using an expansion from several atmospheres of He with detection by RPPI or laser-induced fluorescence (LIF). However, there are differences in the relative intensities of the various origin conformer bands which are probably due to the ability of the different carrier gases to collisionally relax each of the conformers. In addition, the degree of cooling from high-pressure NH,expansions is not as extensive as that obtained from pure He. The typical bandwidth of the transitions observed here is -12 cm-' which corresponds to an estimated rotational temperature of -60 K. The rotational temperature has been estimated by comparing an experimentallyobtained spectrum of aniline (for which the A, B, C rotational constants are known), under similar supercritical NH, expansion conditions to those obtained for carbazole and tryptamine, to spectral plots generated as a function of rotational temperature using an asymmetric rotational contour simulation program (24). In addition, our spectrum has also been compared to calculated rotational contour spectra obtained in the work of Amirav et al. (25). Although the cooling is not as extensive as for argon carrier, the relatively sharp bands obtained by using NHS jet expansions provides a means of uniquely identifying these compounds. One drawback of high-pressure NH3 injection is that some solvent-solute clusters are observed in almost all mass spectra. Strong cluster formation is expected due to the tendency of NH3 to hydrogen bond to the highly polar solute molecules. Nevertheless, the degree of cluster formation is still less than 5% of the total ion yield. This may be partially due to the formation of a transient shock wave induced in the high pressure gas pulse emitted by the pulsed valve which destroys the weakly bonded clusters. A second effect is that the degree
LITERATURE CITED (1) Lubman, D. M. Anal. Chem. 1988, 5 9 , 31A, and references cited therein. (2) PepGh, B. V.; Caliis, J. B.; Danielson, J. D. Sheldon; Gouterman. M. Rev. Sci. Instrum. 1988. 57 878. (3) Amirav, A.; Even, U.; Johner, J. J . Chem. Phys. 1979, 71, 2319. (4) Chan. I.Y.; Dantus, M. J. J . Chem. Phys. 1985, 82. 4771. (5) Park, Y. D.; Rizzo, T. R.; Peteanu. L. A.; Lew, D. H. J . Chem. Phvs. 1986, 84, 6539. (6) Sipior, J.; Sulkes, M. J . Chem. f h y s . 1988, 88, 6146. (7) Li, L.; Lubman, D. M. Rev. Sci. Instrum. 1988, 59, 557. (8) Engeike, F.; Hahn, J. H.; Henke, W.; Zare, R. N. Anal. Chem. 1987, 59. 909. (9) Grotemeyer, J.; Boesi, U.; Walter, K.; Schlag. E. W. Org. Mess Spectrom. 1988. 21. 595. IO) Li, L.; Lubman, D. M. Appl. Spectrosc. 1988. 4 2 , 418. 11) Cable, J. R.; Tubergen, M. J.; Levy, D. H. J . Am. Chem. SOC. 1987, 109, 6198. 12) Randall, L. G.; Wahrhaftlg, A. L. Anal. Chem. 1978, 50, 1703-1705. 13) Sin, C. H.; Pang, H. M.; Lubman, D. M.; Zorn, J. C. Anal. Chem. 1988, 58, 487. 14) Fukuoka, H.; Imasaka. T.; Ishibashi, N. Anal. Chem. 1988, 58. 375. 15) Imasaka, T.; Yamaga, N.; Ishibashi. N. Anal. Chem. 1987, 59, 419. 16) Anderson, B. D.; Johnston, M. V. Appl. Spectrosc. 1987, 4 1 , 1358. 17) Goates, S. R.; Barker, A. J.; Zakharla. H. S.; Khoobehi, B.; Sheen, C. W. Appl. Spectrosc. 1987, 41. 1392. (18) Sin. C. H.; Pang, H. M.; Lubman. D. M. Anal. Instrum. 1988. 17. 87. (19) Tembreull. R.; Lubman, D. M. Anal. Chem. 1987, 59, 1082. (20) Li, L.; Lubman, D. M. Rev. Sci. Instrum. 1988, 59. 2460. (21) Pang, H. M.; Sin, C. H.; Lubman, D. M.; Zwn, J. C. Anal. Chem. W86, 58, 1581. (22) McClelland, G. M.; Saenger, K. L.; Valentini, J. J.; Herschbach, D. R. J . Phys. Chem. 1979, 83, 947. (23) Li, L.; Lubman, D. M. Anal. Chem. 1988. 60, 2591. (24) Pierce, L. Asymmetric Rotational Contour Program, Notre Dame Unlversity. (25) Amirav, A.; Even, U.; Jortner, J.; Blrss, F. W.; Ramsay, D. A. Can. J . Phys. 1983, 61, 278.
Ho Ming Pang David M. Lubman* Department of Chemistry The University of Michigan Ann Arbor, Michigan 48109 RECEIVED for review October 4, 1988. Accepted January 5, 1989. We gratefully acknowledge support of this work by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and by the National Science Foundation under Grant CHE 8720401 and NSF Grant DMR 8418095 for acquisition of the Chemistry and Materials Science Laser Spectroscopy Laboratory. David M. Lubman is an Alfred P. Sloan Research Foundation Fellow.
