Anal. Chem. 1986, 58,969-972
Registry No. Thioglycolicacid, 68-11-1; dithiodiglycolic acid, 505-73-7; sphalerite, 12169-28-7.
LITERATURE CITED sect. C , in (1) ~ ~ D.; Ragh~ S, Trans. ~ Inst. i Mln.~ ~ ~ press. (2) Raghavan, S.; Unger, K. Trans. Insf. Min, Mete//. Sect. C 1983, 9 2 , 95. (3) Walker, G. T. Manuf. Chem. 1953, 2 4 , 376. (4) Walker, 0. T.; Freeman, F. M. Manuf. Chem. 1955, 26, 13. (5) Tseng, P. K. C.; Gutkencht, W. F. Anal. Chem. 1975, 47, 2316. (6) Oganesyan. L. 6.; Kon’kova, N. F. J . Anal. Chem. USSR (fngl. Trans/.) 1960, 12, 1296. (7) Beicher. R.; Bogdanski, S.L.; Townshend, A. Anal. Chlm. Acta 1973, 6 7 , 1. (8) Belcher, R.; Bogdanski, S. L.; Knowies, D. J.; Townshend, A. Anal. Chim. Acta 1975, 79. 292.
969
(9) Leussing, D. L.; Kolthoff, 1. M. J . Nectrochem. SOC. 1953, 100, 334.
(IO) O’Dom, G.; Fernando, Q. Anal. Chem. 1965, 3 7 , 893. (11) O’Dom, G.; Fernando, Q. Anal. Chem. 1986, 38, 844. (12) Schubert, S. A.; Clayton, J. W., Jr.; Fernando, Q. Anal. Chem. 1979, 51, 1297. (13) Schubert, S. A.; Clayton, J. W., Jr.; Fernando, Q . Anal. Chem. 1980, , 52. 963. (14) Tzeng, J.-H.; Fernando, Q. Anal. Chem. 1982, 5 4 , 971. (15) Oae, S. “Organic Chemistry of Sulfur”; Plenum: New York, 1977. (16) Burguera, M.; Bogdanskl, 8. L.; Townshend, A. CRC Crit. Rev. Anal. Chem. 1960, 185. (17) Gregg, S. J.; Sing, K. S. W. “Adsorption, Surface Area and Porosity”; Academic Press: New York, 1982.
Received for review October 3,1985. Accepted December 6, lgg5* We are grateful to the Arizona Mining and Resources Research Institute for financial assistance.
CORRESPONDENCE Consecutive Laser-Induced Photodissociation as a Probe of Ion Structure Sir: Photodissociation hae become an important means for studying the spectroscopy, structure, and thermochemistry of ions in the gas phase (1-8). A widely used method involves storing ions in a static magnetic ion trap, such as an ion cyclotron resonance cell, and detecting the fragment ions that are produced by photon absorption. Dunbar and co-workers have shown that ion photodissociation spectra are highly characteristic of the structure of an ion (4). Most ion photodissociation experiments have utilized visible and infrared lasers, but we are interested in investigating ion photodissociation induced by ultraviolet (UV) light. Our first experiments showed that oligopeptide ions are efficiently fragmented with 193-nm light from an excimer laser (9). The photofragment ions were detected by Fourier transform mass spectrometry (FTMS). An advantage of the excimer laber for these experiments is that UV radiation is strongly absorbed by the carbonyl and aryl groups in polypeptides. At 193 nm each photon deposits 6.42 eV specifically into electronic excitation of the ion, and absorption of even one W photon may be sufficient to cause fragmentation. In this paper we describe a new technique in which parent ions stored in an FTMS analyzer cell are photodissociated by a first laser pulse to produce daughter ions, and a particular daughter ion of interest is then isolated in the analyzer cell and subsequently photodissociated by a second laser pulse. The general scheme is parent
hu
daughter
hv
granddaughter
(1)
This scheme is similar to MS/MS/MS experiments except that UV radiation instead of collisional dissociation is used to fragment the ions (10,ll).We are interested in investigating whether photodissociation with UV radiation is more efficient than collisional dissociation for fragmenting high-mass ions. High-mass ions are not efficiently fragmented by collisional dissociation because of the increasing amount of energy that a large molecule can accommodate before dissociating (the number of vibrational modes increases) and the decreasing amount of energy that can be transferred during a collision (center of mass effect) (12-14).For the two examples presented, bromobenzene and Gly-Phe-Ala, the photodisso0003-2700/86/0358-0969$01.50/0
ciation yields for daughter ions are as high as 25%, and it appears that consecutive laser-induced photodissociation may become a practical and powerful method for elucidating ion structures.
EXPERIMENTAL SECTION A block diagram of the apparatus used in this work is shown in Figure 1. The excimer laser is a Lambda Physik Model EMG-103MSC operated on the line of ArF* at 193 nm. When the laser is triggered, a 20-11s pulse of UV radiation enters the vacuum chamber through a magnesium fluoride window. The beam passes through a 2.5-cm-diameter hole in the front plate of a cubic trapped ion analyzer cell (5.5 X 5.5 X 5.7 cm) and intersects ions that are trapped inside the analyzer by a 1.2-T magnetic field. The beam exits the cell through a hole of the same size in the back plate. This hole is covered with electroformed copper mesh (31 liies/cm). We found that the mesh was nedessary because without it the mass resolution is severely degraded. The laser beam is not strongly focused and has a cross section of approximately 1 X 2 cm as it passes through the cell. Ions formed by electron ionization or laser photodissociation are stored inside the analyzer cell and detected by Fourier transform mass spectrometry (15-18).FTMS is a pulsed detection method that provides high sensitivity and high mass resolution. In addition, a complete mass spectrum can be obtained after each laser pulse. Basically, FTMS operates by accelerating ions at their cyclotron frequenciesand detecting the coherent ion image current signals that are induced in the plates of the analyzer cell. Fourier analysis of the ion image current signals yields a mass spectrum. An IonSpec Model 2000 FTMS data system, shown in Figure 1, was used to control the analyzer cell, detect the ions, plot the data, and trigger the laser. The data system is based on an IBM Instruments CS-9002 computer with 1M byte of system memory. The FTMS signal is digitized and stored in a 64K byte static random access memory. A floating point array processor performs the Fourier transform calculation, and a new mass spectrum is displayed every 4 s. The pulse sequence utilized for our consecutive laser-induced photodissociationstudies is shown in Figure 2. Initially, a quench pulse is applied to one of the side plates to remove all ions from the analyzer cell. The electron beam is then pulsed for 50 ms to ionize the sample. All ions formed by the electron beam are trapped in the analyzer cell by the combined effects of the magnetic and electrostatic fields. The parent ions of interest are 0 1986 American Chemical Society
e70
ANALYTICAL CHEMISTRY, VOL. 58,NO. 4, APRIL 1986
M/AM
= 16,000
77
~.
1
I‘
BROMOBENZENEGRANDDAUGHTER PHOTOFRAGMENT cqii;
..]
n l n n * 5,500
l i Flgure 1. Block diagram of the Fourier transform mass spectrometer used for photodlssociation of ions stored in the analyzer cell.
PULSE SEQUENCE
I
QUENCH PULSE
ELECTRON B E A M PULSE
FUNCTION
I
REMOVES A L L I O N S F R O M ANALYZER C E L L
ELECTRON I O N I Z A T I O N OF MOLECULE
I S O L A T E PARENT IONS P I I N A N A L Y Z E R CELL
LASER PULSE
PULSE
GENERATES DAUGHTER IONS
I S O L A T E DAUGHTER I O N S D2 I N ANALYZER C E L L
I LASER PULSE
GENERATES GRANDDAUGHTER IONS
O B T A I N COMPLETE MASS SPECTRUM O F ALLGRANDDAUGHTER I O N S GENERATED
Figure 2. FTMS pulse sequence for consecutive laser-induced photodissociatlon of ions. The first laser pulse fragments the parent Ions and produces daughter ions. The daughter ions of Interest are isolated in the analyzer cell, and a second laser pulse fragments them to produce granddaughter Ions.
isolated in the analyzer cel by applying a sweepout pulse (20 V p-p rf) that ejects all other ions. After a delay time of 10 ms, the first laser pulse is triggered to fragment the parent ions and produce daughter ions. A mass spectrum of the daughter ions may be obtained at this point, or the daughter ions can be manipulated further as follows. As shown in Figure 2, a particular daughter ion of interest can be isolated by a second sweepout pulse. These daughter ions can then be irradiated by a second laser pulse to produce granddaughter ions. The granddaughter ions are stored in the analyzer cell, and a mass spectrum can be obtained at this point. In principle, this process can be continued as long as there is a detectable ion signal. Bromobenzene, degassed by several freezepumpthaw cycles, was admitted to the vacuum chamber through a variable leak valve to a pressure of 6 X torr. Glycylphenylalanylalanine (Gly-
Figure 3. FTMS mass spectra of fragment ions produced by consecutive photodissociation of bromobenzene molecular ions: (a) m l z 77 produced by photodissociation of C6H,Br+- and (b) m / z 5 1 granddaughter ions produced by photodissociation of m l z 77 daughter ions.
Phe-Ala) was obtained from Sigma, and its 0-methyl ester derivative was prepared as described previously (9). The derivatized sample was placed on the tip of a heated direct insertion probe mounted directly below the analyzer cell. The temperature of the vacuum chamber was maintained at 100 “C.
RESULTS AND DISCUSSION Bromobenzene was selected to demonstrate the feasibility of consecutive laser-induced photodissociation because the kinetics, cross sections, and energetics for the bromobenzene photodissociation reaction C,H6Br+.
-k C6H5++ Br.
(2)
have been carefully investigated (19-22). Dunbar and coworkers have reported a threshold of 2.81 f 0.07 eV (440 f 10 nm) for this reaction (22). Using the FTMS pulse sequence shown in Figure 2, we have developed a double laser pulse experiment to investigate the consecutive photodissociation of bromobenzene ions. The reaction sequence is
(3) The C6H5Br+parent ions, formed by electron ionization, are stored in the analyzer cell, and a sweepout pulse ejects all other ions. The first laser pulse (17 mJ at 193 nm) produces C6H5+ and C4H3+daughter ions. The conversion efficiency for parent ions to daughter ions is about 15%. Figure 3a shows the FTMS mass spectrum for the m / z 77 daughter ion. After the second sweepout pulse has ejected all ions from the analyzer cell except m / z 77, the laser is triggered again and m / z 77 daughter ions are photofragmented to yield granddaughter ions. Figure 3b shows the FTMS mass spectrum of the m / z 51 granddaughter ions produced by photodissociation. The photodissociation yield for granddaughter ions is about 1570, so the relative intensity of the m / z 51 ion in Figure 3b is approximately 2.5% of the original parent ion signal. The mass resolution shown in Figure 3 (16000 for m / z 77 and 5500 for m / z 51) is far lower than the theoretical resolution of our FTMS instrument under these conditions. This may be due to excess kinetic energy released to the fragment ions during photofragmentation. The thresholds for both of these fragmentations are well below the photon energy of 193 nm (6.42 eV) used in our experiments, and the fragment ions may have several electron volts of excess kinetic energy.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
CH2
971
CH3
I C6H5
PARENT I O N S :
DAUGHTER I O N S :
A l l i o n s e x c e p t m/z 120 a r e e j e c t e d froin t h e a n a l y z e r c e l l . The s e c o n d low p r e s s u r e l a s e r pulse p r o d u c e s c I1ell11 c a 1 1 o 11 i L d t i o n III/Z 91 a n d 7 7 . Figure 4. Breakdown scheme for the consecutive photodissociation of protonated Giy-Phe-Ala-OCH,/CD,. The flrst laser pulse produces four daughter ions. When m / z 120 is isolated in the analyzer cell and irradiated by a second laser pulse, m / r 77 and m / z 91 are produced.
Protonated p e p t i d e i o n s a r e formed b y
The f i r s t laser pulse, 193nm a n d 1 7 m J , produces daughter i o n s .
The ability to cleave a high-mass ion consecutively into smaller fragments would be a valuable technique for elucidating the structure of the ion. This technique is demonstrated in Figure 4 for the 0-methyl ester derivative of the tripeptide Gly-Phe-Ala. Protonated parent ions, m/z 308 and 311, were generated by low-pressure chemical ionization, and all other ions were ejected from the analyzer cell with a sweepout pulse. The first laser pulse (16 m J at 193 nm) fragments the protonated parent ions and produces the four daughter ions shown in Figure 4. We have found that the total fragment ion signal intensity is directly proportional to the laser pulse energy up to 25 mJ, which is the maximum energy that can presently be delivered to the analyzer cell. This shows that the photodissociation is a one-photon process. The total yield for the daughter photofragment ions is 24.8% with a 15 mJ pulse, and of this amount 75% of the daughter ions are m/z 120 (NHz=CHCHzC6H6),which results from the internal phenylalanine. The next most abundant daughter ions are m/z 104 and 107, which result from cleavage of the C-terminus alanine. Two other ions are formed a t a relative abundance of approximately 6%. The m / z 91 peak is a fragment of phenylalanine, and m / z 30 indicates that the N-terminus amino acid is glycine. In order to confirm the structure of the m/z 120 daughter ion, a sweepout pulse is used to isolate it in the analyzer cell and a second laser pulse (16 mJ at 193 nm) is triggered to generate granddaughter ions. Figure 4 shows that two granddaughter fragments are produced, m/z 77 and m / z 91,
both of which are clearly indicative of the benzyl R group of phenylalanine. In conclusion, we have demonstrated that 193-nm radiation efficiently photofragments oligopeptide ions and that consecutive laser pulses can be used to elucidate the structure of the daughter ions produced. The success of this method depends upon having a high efficiency for photodissociating the ions. Using FTMS we have found conversion yields up to 25% per laser pulse, and we expect further improvement is possible with more efficient coupling of the laser light into the analyzer cell. Experiments using our present FTMS instrument are limited by the volatility of the sample, but a new FTMS instrument with an external ion source is currently being constructed which will provide for fast atom bombardment ionization of high mass biomolecules (23-25). With this instrument we plan to investigate the consecutive laser-induced photodissociation of high-mass ions. Registry No. Gly-Phe-Ala-OCH3, 100207-61-2; Gly-PheAla-OCH3.H+,100207-62-3;C6HsBr,108-86-1;C6HsBr+,5545033-4; CBHS', 17333-73-2;H3N+-CH(CH3)-C(O)-OCH3,10020763-4; HZN+=CHCHz(C6H,), 100207-64-5;HzN+=CH2,28963-72-6.
LITERATURE CITED (1) Dunbar, R. C. I n "Gas-Phase Ion Chemistry, Volume 3"; Bowers, M. T.. Ed.; Academic Press: New York and London, 1984; Chapter 20. (2) Frelser, B. S.; Beauchamp, J. L. Chem. Phys. Lett. 1975, 35, 35-40. (3) Freiser, B. S.; Beauchamp, J. L. J. Am. Chem. SOC. 1977, 99. 3214-3225. (4) Dunbar, R. C. Anal. Chem. 1976, 48, 723-726.
972
(12) (13) (14) (15) (16) (17) (18)
(19) (20)
Anal. Chem. 1986, 58,972-974 Morgenthaler, L. N.; Eyler, J. R. Int. J . Mass Spectrom. Ion Phys. 1981. 97. 153. Jaslnskl, J. M.; Rosenfeid, R. N.; Meyer, F. K.; Brauman, J. I . J . Am. Chem. SOC. 1982, 104, 652. Rosenfeld, R. N.; Jasinski, J. M.; Brauman, J. I . J . Am. Chem. SOC. 1982, 104, 658. Thorne, L. R.; Beauchamp, J. L. I n "Gas-Phase Ion Chemistry, Volume 3"; Bowers, M. T., Ed.; Academic Press: New York and London, 1984; Chapter 18. Bowers, W. D.; Delbert, S. S.; McIver, R. T., Jr. J . Am. Chem. SOC. 1984, 106, 7288-7289. McIver, R. T., Jr.; Bowers, W. D. In "Tandem Mass Spectrometry"; McLafferty, F. W., Ed.; Wlley: New York, 1983; Chapter 14. Cody, R. 6.; Burnler, R. C.; Cassady, C. J.; Freiser, B. S. Anal. Chem. 1982. 54. 2225-2228. Amster, I:J.; Baldwin, M. A.; Cheng, M. T.; Procter, C. J.; McLafferty, F. W. J . Am. Chem. SOC. 1983, 105, 1654. Neuman, 0. M.; Derrick, P. J. Org. Mass Spectrom. 1984, 19, 165. Shell, M. M.; Derrick, P. J. Org. Mass Spectrom. 1985, 2 0 , 430. Commlsarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 2 5 , 282. McIver, R. T. Jr. Am. Lab. (Falrfleld, Conn.) 1880, 12(11), 18. Wilkins, C. L.; Qross, M. L. Anal. Chem. 1981, 53, 1661A. Bowers, W. D.; Hunter, R. L.; McIver, R. T.. Jr. Ind. Res. D e v . 1983. 25(11), 124-128. Dunbar, R. T.; Fu, E. W. J . Phys. Chem. 1977, 81, 1531-1536. Van Veizen, P. N. T.; Van Der Hart, W. J. Chem. Phys. 1881, 61, 325-334.
(21) Honovich, J. P.; Dunbar, R. C. J . Phys. Chem. 1984, 87, 3755-3758. (22) Dunbar, R. C.; Honovich, J. P. Int. J . Mass Spectrom. Ion Processes 1884, 58, 25-41. (23) McIver, R. T., Jr.; Hunter, R. L.; Bowers, W. D. Int. J . Mass Spectrom. Ion Processes 1985, 6 4 , 67. (24) Hunt, D. F.; Shabanowltz, J.; McIver, R. T., Jr.; Hunter, R. L.; Syka, J. E. Anal. Chem. 1985, 5 7 , 765. (25) Hunt, D. F.; Shabanowitz, J.; Yates, J. R., 111; McIver, R. T., Jr.; Hunter, R. L.; Syka. J. E.; Amy, J. Anal. Chem. 1985, 57, 2733.
William D. Bowers Sherri-Sue Delbert Robert T. McIver, Jr.* Department of Chemistry University of California Irvine, California 92717
RECEIVED for review October 7,1985. Accepted December 23, 1985. This work was presented at the 33rd Annual Conference on Mass Spectrometry and Allied Topics, May 26-31, 1985, San Diego, CA. We acknowledge support by Grant CHE8024269 from the National Science Foundation and Grant GM34327 from the National Institutes of Health.
Hollow Cathode Plume as an Atomization/ Ionization Source for Solids Mass Spectrometry Sir: The development of glow discharge atomization/ionization sources for solids analysis by mass spectrometry has been of interest in this laboratory for some years (1-4). The qualities of relatively uniform atomization and ionization, low ion energies, and source stability make glow discharge mass spectrometry (GDMS) an attractive alternative to the thermal (5) and high voltage-spark ionization techniques (6). The acceptance of the technique as an analytical tool has become evidenced by the commercial availability of a glow discharge mass spectrometer system (7). Many types and configurations of glow discharges have been employed as ion sources dating back to the Aston discharge source (8)used in early isotope ratio experiments. The most common electrode configurations have been the planar diode (9) and hollow cathode geometries (10). While most glow discharges are operated in a dc potential mode, Coburn et al. have demonstrated the utility of ratio frequency (rf) powered discharges to sputter atomize and ionize nonconducting samples such as lathanide oxides (11). We have recently described (12,13)the use of a unique type of glow discharge, the hollow cathode plume, as an atomic emission source for direct solids analysis. The plume phenomenon arises from a hollow cathode discharge confined in the orifice of a s m d sample disk. Adjustment of the discharge pressure and current causes extrusion from the orifice of a plasma plume, which contains a high density of sputtered atoms. These atoms are collisionally excited in the intense plasma allowing for elemental analysis by atomic emission. Emission from ionic species is also observed, indicating that a significant ionic population exists and that the hollow cathode plume might be advantageously utilized as a mass spectrometric source. We report here the use of the hollow cathode plume (HCP) as an atomization/ionization source for solids analysis. The source appears to offer certain advantages over other glow discharge devices, notably in the ability to generate a large atomic population and to energy-discriminate against backgrdund gas ions.
EXPERIMENTAL SECTION Figure 1 illustrates the components employed in the mass spectrometric sampling of the HCP. The quadrupole system used in these studies has been described previously (14). A Bessel box energy analyzer, a three-element lens with a central stop for photons and neutrals, is incorporated in the system as a means of allowing only ions of a narrow kinetic energy spread to enter the quadrupole, improving its resolution (15). The value of this energy window may be varied in order to maximize analytical signal or to study the kinetic energy of ionic species that exit the plasma. Mass spectral data may be taken in either an analog or digital format. A DEC MINC-11 microcomputer is employed to control the maw spectrometer, accumulate data, and store spectra. The HCP source is housed in a stainless steel six-way cross (Nor-Cal Products, Yreka, CA), which allows mounting of optical windows and gas inlets. After an initial evacuation and an argon flushing to reduce residual water vapors and other background gases, the source is operated in an argon atmosphere at pressures of 1-10 torr. The discharge is maintained by a Kepco Model BHK power supply operating in a constant current mode up to 200 mA. The HCP samples take the form of disks, which are 1.5 mm thick and 4.5 mm in diameter with a 1.5 mm orifice drilled through the center. The sample is held in a graphite hollow cathode holder (25.4 mm long, 4.5 mm i.d., and 6 mm 0.d.). R E S U L T S AND DISCUSSION There are inherent properties of the hollow cathode plume source that distinguish it from other glow discharge ion sources: these are a high sputter atomization rate and a large energy disparity between the sputtered and gaseous ion species. As a glow discharge device, the HCP utilizes sputtering as its means of sample atomization. The discharge occurring in the orifice of the disk sample, where the majority of the discharge current is being delivered, exhibits the characteristics of hollow cathode discharges (16). Among these is the ability to operate at relatively high currents with low discharge voltages, typically 100-200 mA and less than 350 V. These values are in contrast to those a t which the coaxial pin cathode source ( 1 7 ) employed in this laboratory is operated, 1-5 mA and 1kV. Glow discharges operating in the 1-10
0003-2700/66/0358-0972$01.50/00 1986 American Chemical Society
