Anal. Chem. 1902, 64, 2079-2083
2070
Axial Introduction of Laser-Desorbed Ions into a Quadrupole Ion Trap Mass Spectrometer Aubrey McIntosh, T r a c y Donovan, and J e n n i f e r Brodbelt' Department of Chemistry and Biochemistry, University of Texas, Austin, Texas 78712-1167
spectrometer may offer some advantages for development of an ultrasensitive analytical technique with greater flexibility for ion-molecule reactions of laser-desorbed species because quadrupole ion traps's operate at much higher pressures than FTICR instruments, and thus can accommodatethe increased gas loads associated with admission of reagent gases without degradation of instrumental performance or need for pulsed valves. Additionally, the standard use of helium buffer gas in quadrupole ion traps may assist in the collisional deactivation and trapping of kinetically hot laser-desorbed ions. Furthermore, with the recent introduction of the mass range extension mode of operation,'6 the quadrupole ion trap is useful for analysis of ions greater than 20 OOO Da. With respect to laser desorption in a quadrupole ion trap, several designs have been reported. The first involved a pulsed IR laser technique in which desorbed metal ions were injected into a quadrupole ion trap mass ~pectrometer.'~ The injection of gold and tantalum ions was demonstrated with this arrangement, which used a three-element einzel lens to transmit ions into the trap, but the technique was not utilized for desorption of biomolecules. In another report, infrared desorptionlionization of organics was performed inside the cavity of a quadrupole ion trap.'* This method involved INTRODUCTION directing the beam of a C02 laser through a hole drilled in the ring electrode to a sample probe. Cation attachment The developments in thermal'-6 and matrix-assisted6-12 products for several small biomolecules including sucrose and laser desorptionlionization mass spectrometry over the past leucine-enkephalinwere presented;however, the applicability decade have demonstrated its importance for the sensitive of this technique was limited by space charge effects and the determination of increasingly high molecular weight biomollow mass range of the quadrupole ion trap detector. In another ecules and other involatile materials. Most of the advances report, it was shown that collisionally activated dissociation in laser desorption methods and applications have involved could be performed on laser-desorbed salt ions in an ion trap the use of time-of-flight6-BJ1-13 or Fourier transform ion cyclotron resonance (FTICR) mass s p e c t r o m e t e r ~ . ~ -In ~ ~ ~ 0 , ~mass ~ spectrometer19 by using an infrared laser desorption method similar to the one described in ref 18. More recently, particular, trapping mass spectrometers have capabilities for it was shown that ultraviolet matrix-assisted laser desorpmultistage experiments, such as collisional-activated dissotion could be performed directly in the cavity of the quadciation and selectiveion-molecule reactions, techniqueswhich rupole ion trap20 by using a design similar to the one described afford additional ion structural information. The combinain ref 19, but in conjunction with the frequency-quadrupled tion of laser desorption with a quadrupole ion trap mass output of a Nd:YAG laser. These reports demonstrated the viability of laser desorptionlionization in a quadrupole ion (1)Hillenkamp, F. In Proceedings of the Second International Workshop on Ion Formation form Organic Solids; Benninghoven, A., trap, but a system in which laser desorption occurs outside Ed.; Springer-Verlag: New York, 1983. of the trap offers one potential advantage. A recent study of (2) Coates, M. L.; Wilkins, C. L. Anal. Chem. 1987,59, 197. the dramatic effects of collisional cooling in a quadrupole ion (3) Land, D. P.; Pettiette-Hall, C. L.; Sander, D.; McIver, R. T., Jr. Rev. Sci. Zmtrum. 1990,61, 1674. trap highlights the significance of ion kinetic energy on ion (4) Fung, E. T.;Wilkins, C. L. Biomed. Enuiron. Mass Spectrom. 1988, storage and detection.21 On the basis of these results, we 15,609. believed that the external generationof laser-desorbed species (5) McCreary, D. A,; Ledford, E. B., Jr.; Gross, M. L. Anal. Chem. Pulsedinfraredlaser desorption is accomplished with a quadrupole Ion trap by means of a fiber optic interface. The fiber optic probe lo located external to the ion trap device, and desorbed ionspatsthroughthe holm in one end-cap electrode and are trapped. This new method allows desorption and ionization by gabphase catlon attachment of biologically relevant molecules, including Gramicidin S which has a molecular weight of 1141. Trapping lo mort effective by uslng a hlgh helium buffer gas pressure (11 mlorr), a long storage Way (150ms) prlor to detection, and a low rltrapping potential (350-650 V ), during the desorption pulse. Ion-moiecuie reactions of neutrals with trapped laserdosorbed ions are also demonstrated. Aikaiknetai catlonsor halideanlons may be formed by desorption of aikalknetaihalide salts, and these ions may be selectively stored for subsequent attachment to volatile organics, such as polyethers. Additlonally, transitionmetal ions desorbed at higher laser power densities undergo selectlve attachment to aromatic substrates such as naphthalene.
1982,54, 1435. (6) Strobel, F. H.; Solouki, T.; White, M. A.; Russell, D. H. J.Am. SOC. Mass Spectrom. 1991,2,91. (7) Karas, M.; Barh, U.; Hillenkamp, F. Znt. J.Mass. Spectrom. Ion Processes 1989, 92, 231. (8) Karas, M.; Hillenkamp, F. Anal. Chem. 1988,60, 2299. (9) Beavis, R. C.; Chait, B. T. Anal. Chem. 1990,62,1836. (IO) Hettich, R. L.; Buchanan, M. V. J. Am. SOC. Mass Spectrom. 1991, 2, 22. (11) Hillenkamp,F.; Karas, M.; Beavis, R. C.; Chait, B. T. Anal. Chem. 1991,63, 1193A. (12)Levie, R. J.; Romano, R. J. J. Am. Chem. SOC. 1991,113, 7802. (13) Spengler, B.; Cotter, R. J. Anal. Chem. 1990, 62, 793. (14) Chiarelli, M. P.; Gross, M. L. In Lasers and Mass Spectrometry; Lubman, D. M., Ed.; Oxford University Press: New York, 1990; Chapter 12. Nuwaysir, L. M.; Wilkins, C. L.; Zbid., Chapter 13. 0003-2700/92/0364-2079$03.00/0
(15) March, R.; Hughes, R. Quadrupole Storage Mass Spectrometry; Wiley-Interscience: New York, 1989. (16) Kaiser, R. E.; Cooks, R. G.; Stafford, G. C.; Syka, J. E. P.; Hemberger, P. H. Int. J. Mass Spectrom. Ion Processes 1991, 106, 79. (17) Louris, J. N.; A m y , J. W.; Ridley, T. Y.; Cooks, R. G . Int. J.Mass Spectrom. Zon Processes 1989,88, 97. (18)Heller, D. N.; Lys, I.; Cotter, R. J.; Muy, 0. Anal. Chem. 1989,61, 1083. (19) Glish, G. L.; Goeringer, D. E.; Asano, K.G.; McLuckey, 5.A. Znt. J. Mass Spectrom. Ion Processes 1989, 94,15. (20) Goeringer, D. E.; Chambers, D. M.; McLuckey, S. A.; Glish, G. L. Presented at the 4th Annual ASMS Sanibel Conference on Mass Spectrometry, January 1992, Sanibel, FL. (21) Wu, H.-F.; Brodbelt, J. Int. J. Mass Spectrom. Ion Processes 1992, 115, 67.
0 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER 15, 1992
would allow greater opportunity for collisional cooling of ions that might otherwise be too kinetically hot to trap (compared to laser-desorbed species formed inside the electrode assembly). We report herein the design of a pulsed infrared desorption technique in conjunction with a quadrupole ion trap maas spectrometer (QITMS)that is distinguished by extemal ion formation and axial introduction of ions without auxiliary injectionoptics. The method employs a fiber optic laser probe interface that alleviates the need for strategically placed optical windows in a mass spectrometer and often can provide superior versatility and portability to a conventional optical assembly. Laser desorption by means of a fiber optic interface was previously shown in a sector mass spectrometer22and in a triple quadrupole mass spectrometer,23and photodissociation in a quadrupole ion trap24 was accomplished with a fiber optic probe. More recently, a fiber optic probe demonstrated for laser desorption/ionization with a FTICR mass ~pectrometer~~ afforded many of the features most important for an interface to a QITMS. In this report, the laser desorption method is demonstrated to be useful for several types of experiments, including desorption of organic ions and ionmolecule reactions of selected desorbed cations and anions. The effects of several parameters, including buffer gas pressure, ion storage time, and the trapping voltage, on the effectiveness of ion storage and detection are also described.
EXPERIMENTAL SECTION The laser desorption probe consists of a rotatable stainless steel sample tip which is mounted via a screw mechanism on a 3/8-in.stainless steel probe shaft. The 3/8-in. probe is sealed to a '/Tin. probe shaft by a Swagelok fitting. The l/rin. probe fits the standard solids probe port of a Finnigan ion trap mass spectrometer (QITMS) (see Figure l), and the probe is admitted through a bellows valve vacuum interlock. Along the side of the sample tip is mounted a fiber optic guide tube which directs the fiber optic to point at the sample tip. The fiber optic is also admitted through the probe shaft and externally sealed by a '/*-in. Swagelok fitting. The sample probe and fiber optic are positioned orthogonally to the cylindrical symmetry axis of the ion trap assembly. This end-cap is actually a second detectortypeend-capelectrode,and thus it has multiple holes for improved transmission of laser-desorbed species. In the laser desorption configuration, there is no filament assembly, and the fiber optic probe is about 1cm from the end-cap electrode. The plume of desorbed material emitted from the surface is aligned directly on-axis with the holes in the end-cap electrode. A Nd:YAG laser operated in the Q-switchmode at both 532 and 1064 nm provides the desorption pulse, and power densities are adjusted from 2 X lo6 to 2 X 1Oe W/cm2depending upon the desired type of laser desorption. This results in typical pulse energies of 7-20 mJ as measured at the outlet end of the fiber optic. The laser beam is focused into the fiber optic by a single 80-mm focal length lens. The fiber optic is a 600-pm-core, Teflon-clad fiber. With each laser shot, ions and neutrals are desorbed from the sample tip, drift through the end-cap electrode,and some of the ions are trapped. The laser spot size on the surface is about 0.25 mm2. The sample tip is rotated to expose fresh sample surface. Typically,a helium buffer gas pressure of 2 2 mTorr is used, and a storage time of 2200 ms is used to assist in cooling of ions. The sample is dissolved in a suitable solvent and is sprayed on the probe tip to which an alkali-metalhalide salt solution has typically been applied. The rotatable sample tip allows >lo00 laser shots on fresh surface. All samples were obtained from Aldrich or Sigma and used without further purification. ~~~~
(22) Cechetti, W.; Polloni, R.; Maccioni, A. N.; Traldi, P. Org. Mass Spectrom. 1986,21,517-518. (23) Emary,W. B.; Wood,K. V.;Cooks, R. G.Anal. Chem. 1987,59, 1069-107 1. (24) Lourie, J. N.; Brodbelt,J. S.;Cooks,R.G. Int. J. Mass Spectrom. Ion Processes 1987, 75,345-352. (25) Hogan, J. D.; Beu, S. C.; Laude, D. A,; Majidi, V. Anal. Chem. 1991,63,1452-1457.
W E R OPTIC /
A
SAMPLETIP
FIBER OPTIC GUIDE
B RING ELECT'I?:ODE \
The QITMS was operated in the mass-selective instability mode.16 At the start of each scan, the laser is triggered under computer control, and this is followed by a delay time of 0-500 ms to allow collisional cooling and/or reactions of ions. Finally, the radio-frequencyvoltage applied to the ring electrodeis raised to eject ions of increasing mass onto an externallylocated electron multiplier detector. For thosecompoundswith molecular weights greaterthan 650,theresonant-ejectionmass range extension mode was used.16 Pressure was measured by a Bayard-Alpert type ionization gauge.
RESULTS AND DISCUSSION
Pulsed infrared laser desorption results in formation of ions that typically have an average distribution of kinetic energies from 50 eV for metal ions formed from plasma ignition.26 Thus, it was initially of great importance to evaluate the experimental conditions which optimized the trapping of ions generated externally to the ion trap without requiring the use of an ion gate or lens assembly. The parameters of particular significance included the helium buffer gas pressure, which is usually maintained at 1mTorr for conventional QITMS experiments, the storage time in which ions undergo collisional cooling in the ion trap, the (26) Van der Peyi, G.;Van der Zand, W.;Kiatemaker, P. Int. J. MUSS Spectrom. Ion Processes 1984,62, 51.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER 15, 1992
'1
0
Rb 230
(3-GLYME + K + 3-GLYME)+ 395 I
1
2081
'0
3-GLYME
(
' 0 ,
'"'
ou0/cy
0 -
12-C-4
(o
)
\p
1
10
100
1000
(12-C-4 + K + 3-GLYME)+
10000
Delay Time (ms)
I393
Flgure 2. Plot of Ion abundance vs delay time before detection.
l ~ J L F
m/z 358
C
1
368
378
380
398
408
418
217
I (3-GLYME+K)+
RF Voltage (V 0-p)
Flgure3. Plat of Ionabundancevs rf voltage appliedto the ringelectrode.
position of the probe tip relative to the transmission holes into the ion trap, and the rf voltage applied to the ring electrode which creates the quadrupole trapping field. The systematicsurvey of the effects of these parameters is briefly described below. First, it was determined that the optimal position of the probe relative to the end-cap electrode required alignment of the probe surface with the transmission holes in the endcap. After each laser pulse, the primary plume of desorbed material is ejected 90° from the sample surface. Thus, this plume was aligned to directly pass through the end-cap electrode. Second, a high buffer gas pressure was found to assist in trapping of the desorbed species, and typically 1-4 mTorr of helium was introduced depending on the sample. Presumably, the high helium pressure enhances collisional damping21of the translational energiesof the desorbed species, so that they may be trapped upon admission through the end-cap holes. Related to the kinetic energy damping effect as influenced by the helium pressure is the storage time prior to detection in which ions may undergo considerable additional collisional cooling as they reside in the ion trap. When a high helium pressure is used (i.e. 2 mTorr), a long storage time (i.e. >50 ms) offers modest advantages for effective ion trapping; however, at the lower helium pressures (i.e. I1 mTorr), it appears that a delay of 100-400 ms affords a &fold enhancementof ion detection (likely because ions have time to undergo kinetic cooling to the center of the trap and thus are more effectively ejected and detected during the analytical scan). A plot of delay time vs relative abundance of rubidium ion and a sucrose-related fragment ion (desorbed from a rubidium bromide salt/sucrose mixture) is shown in Figure 2. In general, depending on the combinationof pressure and coolingtime used, the optimum trapping conditionsfor laserdesorbed organic ions correspond to those in which an ion undergoes from 1000 to 5000 collisions prior to detection.
Flgure 4. (A) Complex formation between 12-crown-4 and triethylene glycol dimethyl ether. (B) Isolation of the potassium-bound complex of 12-crown-4 and tri-glyme. (C) Collisional-actlvateddissociation of the adduct showing that triethylene glycol dimethyl ether has a higher potassium ion affinity than 12-crown-4.
Finally, the amplitude of the rf voltage applied to the ring electrode during the laser desorption pulse and ion cooling interval (delay time) has a dramatic effect on the storage and detection of ions. A plot of rf voltage vs signal intensity is shown in Figure 3 for detection of cesium cations desorbed from cesium iodide on the probe. When the rf voltage is less than about 100 V&p or greater than 1000 VeP during this period, the trapping of ions is quenched. Apparently, at very low voltages ( 6 0 V,,), the trapping field strength is insufficient to retain laser-desorbed species. This parallels the trend observed for operation of the QITMS when conventionalionization modes (electron ionization, chemical ionization) are used in which ions are formed inside the trap. At rf voltages >lo00 VWp, desorbed ions apparently cannot penetrate the potential barrier to the quadrupole field or become too highly energetic to be retained in the trapping field. On the basis of these experimental observations, for the laser desorption experiments described below the rf voltage was held at 400 Vwpduring the laser desorption event and cooling period, the helium pressure was maintained at >1mTorr, and a cooling period of 300 ms was used prior to detection. Despite the very different ranges of kinetic energies for the desorbed ions (i.e. alkali-metal ions from salts vs cationized organics vs metal ions by plasma ignition), it was found that the conditionsdescribed above were most suitable for trapping the largest array of ions. This can be rationalized by assuming that the desorbed ions have a range of kinetic energies, and only those with the appropriate overlap of kinetic energies with the established storage conditions will be trapped. For example,the alkali-metal ions formed from salts are retained
2082
ANALYTICAL CHEMISTRY, VOL. 84, NO. 18, SEPTEMBER 15, 1992
164 1
Cr+
.T;'.,
f
Br-
i
)
A
4
1
II
El c d miz
r'a
I
88
"
'
,
IQ6
'
1
128
"
'
l
"
146
(15-C-5 + Br + 15-C-5)-
(M + Fe)+ I
I
B
(M + Cr)+ S Figure 5. (A) Desorption of bromine ions from KBr. (B) Reactions wlth
L
l5-crown-5 to form dimers. under virtually any combination of trapping conditions, whereas the cationized organicstypically required the highest helium pressures and longest delay times for optimal detection. After plasma ignition ionization of metals, it is likely that only the low kinetic energy tail of transition-metal ions is ever effectivelytrapped, and the majority of hotter ions are too energetic to be retained. Several different types of laser desorption experiments can be performed with the fiber optic interface depending on the laser power densities and types of samples. For example, at low power densities (6 X 1Oe W/cm2)F7alkali metal ions and halide ions are formed from desorption of any salt applied to the probe tip. These ions can be trapped and used for subsequent cation or anion attachment experiments with organic substrates introduced into the vacuum chamber. At moderate power densities (107W/cm2), organic ions are formed and can be trapped. Often these are alkali-metalion attached species. At the highest power densities (108 W/cm2), metal ions can be desorbed from the stainless steel sample tip or from foils attached to the tip, and these can be trapped to react with neural substrates. In all cases, both positive and negative ions may be formed, stored, and analyzed. Various types of experiments and the resulting spectra are illustrated in Figures 4-10 to show the versatility of the laser desorption technique with the quadrupole ion trap. Alkali-metal ions are formed very efficientlyby direct laser desorption of an alkali-metal salt applied to the probe tip. These alkali-metal ions, such as potassium cations, may undergo selective attachment to organic substrates, as shown in Figure 4A for formation of adducts of 12-crown-4and triethylene glycol dimethyl ether (designated as MI and M2). After isolation of the potassium-bound adduct (Figure 4B) by applying an appropriate combination of dc and rf voltages (i.e. apex isolation mode9, collisional-activateddissociation of the (Mi + K + Mz)+ion by application of a supplementary resonant voltageZgresults in cleavage of the ether/alkali-metal electrostatic bonds, forming (Mi + K)+ and (Mz + K)+ (27) Verbs, A.; DeWolf, M.; Juhasz, P.; Gijbels, R. Anal. Chem. 1989,
61, 1029.
(28) Weber-Grabau, M.; Kelley, P. E.; Syka, J. E. P.; Bradshaw, S. C.; Brodbelt, J. S. h o c . Ann. Conf. Am. SOC.Mass Spectrom., 35th 1987, 1114. (29) Louris, J. N.; Cooks, R. G.; Syka, J. E. P.; Kelley, P. E.; Stafford, G. C.; Todd, J. F. J. Anal. Chem. 1987,59,1677.
C
(2M+ Fe)+
1
(2M + Cr)+ i
m/z
8
,
250
260
270
2ia
n
290
Figure 6. (A) Desorptlon of transftion-metal ions (Cr+ and Fe+). (B) Metalreactionswith naphthalene. (C)Formationof metabbound dimers of naphthalene.
products (Figure 4C). The abundance5 of the two products give an indication of the relative potassium ion affinities of each ether, and the higher abundance of the triethylene glycol dimethyl ether product shows that it has a greater potassium ion affinity than 12-crown-4. This type of study which utilizes the kinetic method30 is described in further detail in several reporta on the gas-phase host-guest chemistry of macrocy~les.3~93~ The laser desorption method also affords a simple way to form negative ions in the quadrupole ion trap maas spectrometer. In the negative-ion mode, halide anions from the alkali-metal halide salt may be desorbed and detected, aa shown for Br- in Figure 5A. These anions may undergo attachment reactions with organics, as indicated in Figure 5B for 15-crown-5. Negative ions are difficult to form in the QITMS by conventional methods such as electronattachment ionizationbecause there is no ready sourceof thermal electrons (30)McLuckey, S. A.; Schoen, A. E.; Cooks,R. G. J. Am. Chem. SOC. 1982.104.Ma. (31)Mdeknia, S.;Brodbelt, J. J. Am. Chem. SOC.1992,114,4296. (32) Liou, C.4.; Brodbelt, J. J.Am. SOC.Mass Spectrom. 1992,3,543.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER 15, 1992
125 h
(M K)+
(M - H
1
sea
2Na)t
I
(M
-
t
2083
t
H)i
@.
Flgure Desorption of leucine-enkephalin (mol wt 555, in a NaCl mixture).
Flgure 7. Desorption of digltoxigenin (mol wt 374. in a KBr mixture) from the probe.
Gramicidin S
(M + Na)+ I
M+
I
1000
1200
Flgure 10. Desorptionof Gramicidin S (mol wt 1 141, in a KBr mixture). Flgure 8. Desorption of tetraphenylporphine (mol wt 614, in a KBr mixture) from the probe showing fragment ions.
due to the influences of the high rf field on the electron population generated by a filament assembly. At higher power densities, transition-metal ions may be desorbed from the stainless steel probe surface (Figure 6A) or from metal foils attached to the probe tip. These metal species can be used to ionize various organic substrates, such as aromatic compounds, by simple attachment and/or insertion processes. An example of desorption of metal ions with subsequent attachment to naphthalene to form (M + Cr)+ and (M + Fe)+ ions and then metal-bound dimers is shown in Figure 6B,C. At moderate laser power densities, organic ions are formed by laser desorptionlionization. Several representative spectra are shown in Figures 7-10. The organic substrates include digitoxigenin,leucine-enkephalin, tetraphenylporphine, and Gramicidin S. With the mass range extension mode of operation (i.e. application of a supplementary ac voltage of 29 V, at 150 OOO Hz across the end-cap electrodes),the largest molecule that has been analyzed is Gramicidin S, (M + K)+, at 1180Da. Mass assignment is not routine for the extended mass range, highlighting the need for careful calibration with appropriate high molecular weight compounds. As observed in all of the spectra, typically cationized molecular ions are observed with little fragmentation, except for the desorption of tetraphenylporphine in which molecular ions and two fragment ions are seen (Figure 8). The fragment ion at mlz 537 is due to loss of one phenyl group from the molecular ion, but the identity of the other fragment observed at nominally (33) Brown, R. S.; Wilkins, C. L. A d . Chem. 1986,58, 3196. (34) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard,W.G. J.Phys. Chem. Ref. Data 1988, 17 (Suppl. 1).
mlz 473 is not known. A previous report of laser desorption of porphyrins in an FTICR instrument33showed that porphyrins undergo alkali-metal ion attachment at lower laser power densities but instead form molecular ions and fragments at higher power densities. The capability for formation of abundant molecular ions may in part be related to the low ionization potential of porphine-related compounds (Le. ionization potential of porphine is 6.6 eVa) which permits facile electron detachment in the laser plasma. The laser desorption spectrum shown in Figure 9 is unusual in that both protonated and various cationized forms of leucine-enkephalin are observed. Finally, for the desorption of Gramicidin S, a double-pulse sequence enhanced formation of ions. In this type of experiment, the first laser pulse is followed by a 300-ms delay; then a second laser shot is fired, followed by a second 300-ms delay prior to detection. The spectral improvements observed with this sequence may be rationalized by a type of neutral desorptionlpostionization mechanism in which during the first laser pulse sodium and potassium ions are desorbed and stored in the trap. Then during the second laser pulse a plume of neutral Gramicidin S is desorbed and attaches to the alkali-metal ions in the trap. This mechanism is currently under further evaluation.
ACKNOWLEDGMENT The Welch Foundation (F-11551, NIH (R01 GM46723011, NSF (CHE9122699), and donors of the Petroleum Research Fund, administered by the American Chemical Society, are gratefully acknowledged.
RECEIVED for review February 24, 1992. Accepted June 9, 1992.
