Anal. Chem. 1993, 65, 2812-2818
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Molecular Cooling and Supersonic Jet Formation in Laser Desorption Jian-Yun Zhang, Davinder S. Nagra, and Liang Li' Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
A method has been developed to study molecular cooling during the laser desorption (LD) process. It involves the use of resonant two-photon ionization (R2PI) spectroscopy to examine the molecular population distribution among internal states of the molecules generated from LD. A continuous-flow probe is developed to introduce sample and matrix through a capillary tube and onto a stainless steel frit, upon which laser desorption is carried out with a COZlaser. The sample molecules expand into the acceleration region of a reflectron time-of-flight mass spectrometer,whereR2PI is performed with a tunable dye laser. It is demonstrated that small sample molecules are internally cooled during the gas expansion process in LD. The molecular cooling is believed to be the result of a supersonic jet expansion. It is further shown that the jet expansion process in LD is similar to that observed in a molecular beam experiment with a pulsed nozzle source. In addition,it is found that velocity distributionsdepend on the extent of the molecular cooling. Finally, the relation between the internal temperature and the translational temperature is examined. INTRODUCTION Laser desorption (LD) is a powerful method for the generation of ions and neutrals.'-Q In particular, matrixassisted laser desorption (MALD)has become an increasingly important technique for ion generation in mass spectrometry.1-8 The technique involves mixing a proper matrix with samples such as peptides and proteins on a substrate, followed by laser desorption. In a time-of-flight mass spectrometer, molecular ions can be observed with little fragmentation even for proteins as large as 300 000 Da.I4 Apparently, although a large amount of energy is implanted to the sample and the sample substrate, these fragile biopolymers can still survive the desorption process. It is also known that the addition of a proper matrix to a sample (1)Karas, M.; Bachmann, D.; Bahr, U.; Hillenkamp, F. Int. J. Mass Spectrom. Ion Processes 1987,78,53. (2)Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshida, T. Rapid Commun. Mass Spectrom. 1988,2, 151. (3)Beavis, R. C.; Chait, B. T. Rapid Commun. Mass Spectrom. 1989, 3,233;1989,3,432; Anal. Chem. 1990,62,1836. (4)Nelson, R. W.; Rainbow, M. J.; Lohr, D. E.; Williams, P. Science 1989,246,1585. (5)Spengler, B.; Cotter, R. J. Anal. Chem. 1990,62,793. (6)Hillenkamp, F.; Karas, M.; Beavis, R. C.; Chait, B. T. Anal. Chem. 1991,63,1193A,and references cited therein. (7)Lubman,D. M.;Li, L. InLasers and Mass Spectrometry; Lubman, D. M., Ed.; Oxford New York, 1990; Chapter 16,pp 353-382. (8)Engelke, F.; Hahn, J. H.; Henke, W.; Zare, R. N. Anal. Chem. 1987, 59,909. (9)Grotemeyer, J.; Boesl, U.; Walter, K.; Schlag, E. W. Org. Mass Spectrom. 1986,21,595. 0003-2700/S3/0365-2812$04.00/0
facilitates the generation of intact neutral molecules by LD.'&'* At present, the mechanism for LD is not well understood though.'"U In studying LD, it is perhaps advantageous to separate the LD process into two temporal events. One is the interaction between the laser beam and the sample and/or the sample substrate. The other one is the gas expansion after the interaction. Because most experimental measurementa are performed after the molecules have expanded a finite distance, it is important to characterize the expansion process well so that any theoretical treatment or experimental measurement of the initial interaction can account for ita contribution. This work is directed toward the study of the gas expansion process in laser desorption. Recently, several groups have argued that molecular cooling during the expansion after the laser impact reduces the probability of thermal degradationof samplemolecules.4fiJ~~ In this work, we have designed an experiment to provide information concerning the internal energies of the desorbed molecules in LD. The experimentinvolves the use of resonant two-photon ionization (RPPI) spectroscopy%to examine the molecular population distribution among internal states of the molecules generated from the desorption process. The key to success in studying the organic molecules spectroscopically is to develop a sample introduction system so that a stable, repetitive desorption event can take place over an extended period to allow a spectrum to be taken. We note that Lustig and LubmanM have developed a flow probe to deliver samples from a capillary tube to the probe face for repetitive laser desorption. In their technique, neutrals generated by LD are entrained into a supersonicjet and carried into the ionization region of a time-of-flight maas spectrometer (TOFMS) where multiphoton ionization mass spectra are obtained. In this work, a continuous-flow probe is developed (10)Li, L.; Lubman, D. M. Reu. Sci. Instrum.1988,59,557. (11)Li, L.; Lubman, D. M. Appl. Spectrosc. 1989,43,543. (12)Beavis, R. C.; Lindner, J.; Grotemeyer, J.; Schlag, E. W. Chem. Phys. Lett. 1988,146, 310. (13)Kinsel, G.R.; Lindner, J.; Grotemeyer, J. Org. Mass Spectrom. 1991.26. 1052. (14)Becker, C. H.; Jusinski, L. E.; Moro, L. Int. J. Mass Spectrom. Ion Processes 1990,95,R1. (15)Ehring, H.; Karas, M.; Hillenkamp, F. Org. Mass Spectrom. 1992,
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(16) Gimon, M. E.; Preston, L. M.; Solouki, T.; White, M. A.; Russell, D. H. Org. Mass Spectrom. 1992,27,827. (17)Ens, W.; Mao, Y.; Mayer, F.; Standing, K. G.Rapid Commun. Mass Spectrom. 1991,5, 117. (18)Lincoln, K. A,; Covington, M. A. Int. J. Mass Spectrom. Ion Processes 1975,16,191. (19)Williams, P.; Schieltz, D.; Luo, C. W.; Thomas, R. M.; Nelson, R. W. in Laser Ablation, Mechanism and Applications; Miller, J. C., Haglund, R. F., Jr., Eds.; Springle-Verlag: New York, 1991;pp 154-159. (20)Vertes, A,; Levine, R. D. Chem. Phys. Lett. 1990,171,284. (21)Kimbrell, S.M.; Yeung, E. S. Appl. Spectrosc. 1991,45,442. (22)Beaws, R. C.; Chait, B. T. Chem. Phys. Lett. 1991,181,479. (23)Fehre, T. H.; Becker, C. H.RaprdCommun. Mass Spectrom. 1991, 5,378. (24)Pan, Y.; Cotter, R. J. Org. Mass Spectrom. 1992,27,3. (25)Forareview,seeforexample: (a)Lin,S.H.;Fujimura,Y.;Neueeer, H. J.; Schlag, E. W. Multiphoton Spectroscopy ofMolecules; Academic Press: New York, 1984. (b) Hayes, J. M. Chem. Rev. 1987,87,745. (26)Lustig, D. A.; Lubman, D. M. Reu. Sci. Instrum. 1991,62,957. 0 l9S3 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 65, NO. 20, OCTOBER 15, 1993
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Flguro 1. Schematlc of the contlnuous-flow probe and the ionization region of the reflectron tlme-of-flight mass spectrometer used for resonant
two-photon lonlratlon spectroscopic studies of the laser desorption process.
to deliver sample and matrix through a capillary tube and onto a stainless steel frit, upon which laser desorption is carried out. The sample molecules expand directly, without the carrier jet, into the acceleration region of a TOFMS, where RBPI is performed with a tunable dye laser and wavelength spectra are recorded. Note that there are some previous reports of using spectroscopicmeans to study the state distributions for atomic or molecular speciessuch as NO and aniline.3- In the studies of molecules, they are either desorbed from cryogenic sample films or generated from the dissociation of a solid sample such as a polymer upon desorption. Thus, the experimental conditions are different from those used in this work where a matrix is used and the sample probe temperature remains close to the room temperature. In the reported experiments, although the rotational and vibrational temperatures of the desorbed molecules can be lower than the translational temperatures, they are usually found to be higher than the initial substrate temperatures.In some studies, the internal temperatures of the desorbed species are found to be as high as several thousands of degrees.2Q-31 In this report, we demonstrate that sample molecules, at least for the model compounds studied herein, can be internally cooled during the laser desorption process. It is found that vibrational temperatures of the small molecules can be significantly lower than the room temperature or the temperature of the sample surface. Furthermore, it is shown that the cooling is matrix dependent. This work therefore provides strong evidence supporting the supersonic jet model. It is believed that the matrix species form a jet during LD and provide cooling for the sample molecules. In this work, (27)King, D. S.;Cavanagh, R. R. In Advances in Chemical Physics; Lawley, K. P., Ed.; Wiley: New York, 1989; Vol. 76, pp 45-89, and references cited therein. (28)Nogar, N. S.;Estler, R. C.; Miller, C. M. Anal. Chem. 1985,57, 2441. (29)Dreyfus, R. W.; Kelly, R.; Walkup, R. E. Nucl. Instrum. Methods 1985,B23, 557. (30)Srinivasan, R.;Braren, B.;Dreyfus, R. W. J.AppLPhys. 1987,61, nrrn
OIL.
(31)Koren, G.Appl. Phys. 1988,B46,147. (32)Mase, K.; Mizuno, S.; Achiba, Y.; Murata, Y. Rev. Solid State Sci. 1990,4,721. (33)Cousins, L. M.; Levis, R. J.; Leone, S. R. J.Chem. Phys. 1989,91, 5731. (34)Schwarzwald,R.; Modl, A.; Chuang, T. J. Surf.Sci. 1991,242,437. (35)Elokhin, V. A,; Krutchin8ky, A. N.; Ryabov, S. E. Chem. Phys. Lett. 1990,170,193. (36)Elokhin, V. A.; Krutchinsky, A. N.; Ryabov, S.E. Rapid Commun. Mass Spectrom. 1991,5,251.
studies of the parameters affecting the supersonic jet expansion are also reported.
EXPERIMENTAL SECTION Figure 1shows the schematic of the experimental setup used for RBPI studies of MALD. The reflectron time-of-flight mass spectrometer has been described previously.a'@ In brief, the system consists of an angular reflectron TOF mass spectrometer (R. M. Jordan Co., Grass Valley, CA) mounted vertically in a six-port cross pumped by a 6-in. diffusion pump (Varian Associates, Inc., Lexington, MA). The 1-m-long flight tube is differentially pumped by a 4-in. diffusion pump (Varian). The pressure in the detection region is usually below 1 X 1od Torr during the operation. The design concept of the flow probe is, in some aspects,similar to the frit-type probe used in continuous-flowfast atom bombardment mass spectrometry (CF-FABMS).SBA silica capillary tube (75-pm i.d., 363-pmo.d., 1m long) (PolymicroTechnologies, Phoenix, AZ) is inserted into a 1.27-cm-0.d. and 0.635cm4.d. stainless steel tube and extends to the probe tip (see Figure 1). For electric insulation, the top section of the probe (-7.62 cm long) is made of Vespel. The probe tip (5-mm o.d., 1-mmi.d., and -5 mm long), also constructed of Vespel, is then screwed onto this insulator. A septum is placed in between the probe tip and the insulator for vacuum sealing. The capillary tube punctures through this septum and is placed -1 mm away from the surface of the tip. A 1.55-mm hole is drilled in the center of the tip face to a depth of 1mm for housing the stainless steel frit (1.59-mm 0.d. and 0.794 mm thick) (Chromatographic Specialties Ltd., Brockville, ON, Canada). The frit is pushed into the hole to make direct contact with the end of the capillary tube. The flow probe is inserted into the TOFMS via a custommade solid probe lock. In this experiment,a microsyringe pump (Orion Research Inc., Boston, MA) continuously introducesthe sample and the matrix onto the frit probe surface. The flow rate is in the range of 2.05.0 pL/min. The samples are prepared by dissolving the andyte in a water/glycerol mixture to produce a solution in the concentration range of 1-5 mM. The glycerol content is varied from 0% to 50%. The probe temperature is between 5 and 10 "C, dependingon the percentage of glycerol used in the solution. It is found that this continuous-flow sample introduction technique can provide excellent long-term signal stability (less than * 5 % signalvariation),which is the key for performingR2PI studies. Moreover, relativelyvolatile matrices such as water (see Results and Discussion)can be used. This is because a dynamic
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(37)Li,L.;Hogg,A.M.;Wang,A.P.L.;Zhang,J.Y.;Nagra,D.S.Anol. Chem. 1991,63,974. (38)Nagra, D. S.;Zhang, J. Y.; Li, L. Anal. Chem. 1991,63,2188. (39)Ito, Y.; Takeuchi, T.; Ishi, D.; Goto, M. J. Chromatogr. 1985,346, 161.
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equilibrium between the liquid flow and the vacuum pumping is established in the flow probe. This equilibrium is affected by the flow rate, the matrix volatility, and the local pumping speed. Higher flow rate and lower pumping speed favor the retardation of the volatile matrix on the probe. For laser desorption, a pulsed COzlaser (Allmark Model 852, A-B Lasers Inc., Acton, MA) which generates a 10.6-pm IR radiation with a 75-11s initial pulse and
