Anal. Chem. 1993, 65, 1916-1924
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Plasma Source Atmospheric Pressure Ionization Detection of Liquid Injection Using an Ion Trap Storage/Reflectron Time-of-Flight Mass Spectrometer Benjamin M. Chien, Steven M. Michael, and David M. Lubman; Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109
In this work, the capabilities of an ion trap storage/reflectron time-of-flight mass spectrometer (IT/reTOF-MS) combination have been explored for detection of ions generated by liquid injection into plasma source atmospheric pressure ionization mass spectrometry. It is demonstrated herein that the ion trap provides an effective means of storing externally generated ions from 10 ps to 10 s prior to mass analysis via pulsed dc extraction into the reTOF-MS device. The IT/ reTOF storage capabilities provide the potential for nearly 100% duty cycle in converting a continuous ion beam into a pulsed source for TOF. In addition, it is shown that the storage capabilities of the device provide enhanced resolution and sensitivity as the storage time is increased. Further, capabilities were developed for trapping of ions for other external injection sources and ions with up to 300 eV have been trapped and stored for analysis by the IT/reTOF. In addition, the rf voltage was shown to be an effective means of eliminating low-mass background peaks from the trap and, thus, from the TOF mass spectrum obtained. The sensitivity of the device is also demonstrated with liquid injection techniques for a typical sample and found to be in the lowfemtomole range. INTRODUCTION Atmospheric pressure ionization (API) methods have recently received a great deal of attention as ionization sources in mass spectrometry. An important feature of the API technique is its potential for high sensitivity, which is based upon the ionization methods used, i.e., Ni /3 source, corona di~charge,l-~ or plasma source.a16 These techniques utilize
kiloelectronvolt electrons to ionize the components in a buffer gas, which then transfer a proton or charge to a trace analyte through a series of ion/molecule reactions. The large number of collisions at atmospheric pressure make this method a very efficient source for high proton or electron affinity molecules. A further significant property of this atmospheric pressure chemical ionization method is that it results in soft ionization where the molecular ion or protonated molecule, MH+, is observed in the positive mode for easy identification. However, API sources have recently achieved their greatest utility as a convenient method of interfacing liquid chromatography to mass spectrometric detection. The API methodology circumvents the problem of introducing large amounts of liquid effluent directly into the mass spectrometer, thus making it possible to maintain a low pressure for operation with chromatographic i n j e ~ t i o n . ~ ~ ~ 2 ~ API sources have been generally used with quadrupole mass spectrometers, although they have also been interfaced to sector instrument^.^^^ However, in more recent work there has been an effort to interface API sources to time-of-flight mass spectrometers.17-24 The rationale behind this effort is that TOF provides certain distinct advantages over quadrupole devices in several applications. The TOF can provide speed in the rapid acquisition of an entire mass spectrum over an extended mass range. This is particularly important for detection of rapidly eluting species in liquid chromatography applications or in monitoring of transient species in fast reactions. Further, the TOF is capable of detecting and analyzing high-mass ions beyond the range of quadrupoles and most other mass spectrometer~.~5 In addition, TOF-MS can provide relatively high resolution and capabilities for metastable energy analysis via the use of reflectron TOF analyzers.262s A resolution of several thousand can be easily achieved in a rather simple device with no slits, moving parts,
(14) Quimby, B. D.; Sullivan, J. J. Anal. Chem. 1990, 62, 1027-34. (15) Carnahan, J. W.; Gelhausen, J. M. Anal. Chem. 1989,67,674-7. (16) Calzada, M. D.; Quintero, M. C.; Gamero, A.; Gallego, M. Anal. Chem. 1992, 64, 1374. (17) Yamashita, M.; Fenn, J. B. J . Phys. Chem. 1984,88, 4451-59. (18) Bruins, A. P.; Covey, T. R.; Henion, J. D. Anal. Chem. 1987,59, 2642-6. (1) Carroll, D. I.; Dzidic, I.; Horning, E. C.; Stillwell, R. N. Appl. (19) Boyle, J. G.; Whitehouse, C. M. Anal. Chem. 1992, 64, 2084. Spectrosc. Reu. 1981, 17, 337-405. (20) Dawson, J. H. J.; Guilhaus, M. Rapid Commun. Mass Spectrom. (2) Thomson, B. A,; Iribarne, J. V.; Dziedzic, P. J. Anal. Chem. 1982, 1989, 3, 155-9. 54, 2219-24. (21) Dodonov, A. F.;Chernushevich, I. V.; Laiko, V. V. Poster presented (3) Spangler, G. E.; Cohen, M. J. In Plasma Chromatography; Carr, at the 12th International Mass Spectrometry Conference, Amsterdam, T. W., Ed.; Plenum Press: New York, 1984; p 1. (4) Covey, T.R.; Lee, E. D.; Bruins, A. P.; Henion, J. D. Anal. Chem. Holland, August 26-30, 1991. (22) Sin, C. H.; Lee, E. D.; Lee, M. L. Anal. Chem. 1991,63,2897-2900. 1986,58, 1451A-61A. (5) Sakairi, M.; Kambara, H. Anal. Chem. 1988, 60, 774-80. (23) Borle, J. G.; Whitehouse, C. M.; Fenn, J. B. Rapid Commun. (6) Sakairi, M.; Kambara, H. Anal. Chem. 1989, 61, 1159-64. Mass Spectrom. 1991, 5, 400-5. (7) Ketkar,S.N.;Dulak,J.G.;Fite,W.L.;Buchner,J.D.;Dheandhanoo, (24) Ma, C.: Michael, S. M.: Chien. M.; Zhu, J.; Lubman, D. M. Rev. Sci. Instrum. 1992, 63, 139-48. S. Anal. Chem. 1989,61, 260-4. (25) Beavis, R. C.; Chait, B. T. Rapid Commun. Mass Spectrom. 1989, (8) Shen, W. L.; Satzger, R. D. Anal. Chem. 1991, 63, 196C-4. 3, 233. (9) Sofer, I.; Zhu, J.; Lee, H. S.; Antos, W.; Lubman, D. M. Appl. (26) Mamyrin, B. A,; Karataev, V. I.; Shmikk, D. V.; Zagulin, V. A. Spectrosc. 1990, 44, 1391-8. (10) Zhao, J.; Zhu, J.;Lubman, D. M. Anal. Chem. 1992,64, 1426-33. Sou. Phys. JETP 1973,37,45. (27) Boesl, U.; Neusser, H. J.; Weinkauf, R.; Schlag, E. W. J . Phys. (11)Poussel, E.;Mermet, J. M.; Deruaz, D.; Beaugrand, C. Anal. Chem. Chem. 1982,86, 4851. 1988,60, 923-7. (28) Lafortune, F.; Ens, W.; Hruska, F. E.; Sadana, K. L.; Standing, (12) Wilson, D. A,; Vickers, G. H.; Hieftje, G. M. Anal. Chem. 1987, K. G.; Westmore, J. B. Znt. J . Mass Spectrom. Ion Processes 1987, 78, 59, 1664-70. 179-94. (13) Heppner, R. A. Anal. Chem. 1983,55, 2170-4. 0003-2700/93/0365-19 16$04.00/0
0 1993 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993
or scanning fields. TOF devices can also provide excellent sensitivity based upon their high transmission and microchannel plate detectors. TOF-MS has become particularly attractive in pulsed experiments, especiallywith lasers, where a well-defined start time can be established for the time of flight. However, the interface of API sources to TOF becomes more complicated since API generates a continuous ion beam, whereas the TOF requires a pulsed operation. Thus, a method is required to pulse the continuous ion beam, creating discrete ion packets with a sufficiently narrow time profile to provide excellent resolution in the observed mass spectrum. This conversion of a continuous ion beam into a pulsed beam must be performed without a large loss of ions, as this will cause poor sensitivity due to the inherently low duty cycle of the experiment. In addition to the time profile of the pulse, the resolution in TOF depends upon the initial spatial and energy distribution of ions in the acceleration region of the T0F.B The problem here is to take an ion beam which has a spread in space and energy and transform it into an ion packet where both these parameters have been minimized. There have been several attempts to interface API sources to TOF devices. These have involved two basic solutions for converting the continuous ion beam generated into a welldefined pulsed beam. One method, originally developed by BakkerFo uses a beam deflection technique to sweep the ion beam across a narrow slit to produce an ion packet. This method has been interfaced to API by Ma and co-workers24 and to electrospray by Boyle.23 This method was found to provide excellent resolution; however, the duty cycle is poor since the ion beam crosses the slit for only a short period of time in order to achieve that resolution. In other work, pulsed extraction of ions in the acceleration region of the TOF has been used as a means to achieve time resolution of ions produced by continuous ion sources. This technique was first demonstrated by Wiley and McLaren using a continuous E1 s0urce.~9 It has been used with a number of continuous ion beam sources31 and has recently been interfaced to API sources.18-22This method has the potential to achieve a high duty cycle and has shown excellent sensitivity. However, it may also suffer disadvantages in being able to detect only a limited mass range, and all masses may not be detected with the same efficiency due to the transverse velocity component of the ion beam. Although the efficiency for detection can be optimized for a given mass, it cannot be optimized for the entire mass range simultaneously. These issues may become even more problematic with the use of reflectron devices. In this work, we have developed an ion trap storage/reTOF device32as a means of interfacing a continuous ion beam from API into a TOF device. The key feature here is that an ion trap is used to store ions injected from a continuous ion beam source for an extended period of time ranging from several tens of microseconds to several seconds. The ions are subsequently ejected via a dc pulse applied to the exit end cap into the reTOF for analysis. Since the extraction time is short compared to the storage time, the trap can thus convert a continuous ion beam into a pulsed beam with essentially 100% duty cycle. In addition, the properties of traps are such that, in combination with a buffer gas, translationally hot ions can be collisionally cooled and relaxed to the center of the The result is high resolution in a reTOF due (29) Wiley, W. C.; McLaren, I. H. Rev. Sci. Instrum. 1966,26, 1150. (30) Bakker, J. M. B. J. Phys. E. 1973,6, 785. (31) Simpson, R. C.; Emary, W. B.; Lys, 1.; Cotter, R. J.; Fenselau, C. C. J. Chrornatogr. 1991,536, 143. (32) Michael, S. M.; Chien, M.; Lubman, D. M. Rev. Sci. Instrum. 1992.63.4277-84. . - - ,-(33) March, R:; Hughes, R. Quadrupole Storage Mass Spectrometry; Wiley: New York, 1989.
to the excellent energy and spatial resolution that results from using storage in the trap. The resolution is shown to be enhanced even further for a continuous API ion source as the storage time is increased. Most significant though is that the use of the trap as a front end storage device can potentially provide enhanced sensitivity based upon storage and integration of the ion signal prior to TOF mass analysis. It should be noted that pulsed dc ejection of ions from traps has been performed previously by a number of investigators for fundamental studies and has been used to interface ion traps to quadrupole and sector mass spectrometer detectors.3" More recent hybrid devices have seen ion traps interfaced to other ion traps and hybrid BEQ-type devices.39 The quadrupole ion trap is itself a powerful tool for mass analysis and storage of ions over a wide mass range with excellent sample detection limits,99.94vu3 In recent work, various API sources have also been interfaced to the ion trap. In particular, liquid injection and chromatographic sources have been interfaced to API and electrospray/ion trap detection with exquisite sensitivity.44 The trap has been shown capable of analyzing ions in excess of 70 000 u and has been able to achieve extraordinarily high re~olution.~2.@ In addition, an important feature of the trap is its ability to perform multiple stages of tandem mass spectrometry in combination with collisional fragmentation techniques.33~42 The ion trap also has several inherent disadvantages. Although the trap can store high-mass ions, it is often difficult to scan the radiofrequency to a sufficiently large value of the voltage in order to scan out high-mass ions. A technique has been developed known as axial modulation to scan out high mass; however, this usually occurs a t the expense of the accuracy of the mass ~ a l i b r a t i o n . ~In ~ ~addition, ~3 very highmass resolution, i.e., >lo0 OOO, can be achieved in the trap by scanning the rf voltage or the rf frequency very slowly.@ However, the rate a t which the mass range is scanned to achieve this resolution is impractical for many applications. In this work, an ion trap storage/reflectron time-of-flight device has been used as a detector for atmospheric pressure ionization with liquid sample injection. The IT/reTOF is shown to be an effective means of interfacing a continuous ion beam source to a TOF, achieving a high duty cycle even with a low pulse-out extraction rate. It is also shown that the storage properties of the trap act to enhance the sensitivity and resolution of the device. Indeed, a resolution in the lowmass range of -2100 with a peak width of -8 ns fwhm is demonstrated. In addition, the instrumental parameters providing the best ion beam transmission and ion trapping are explored and a means of trapping high-energy ions is also demonstrated. Further, the effects of adjusting the rf voltage on the trap as a unique means of ejecting unwanted background from the reTOF mass spectrum are also shown. The detection limit of the device is also demonstrated with liquid injection techniques for a typical sample and found to be in the low-femtomole range. ~
(34) Ma, C.; Lee, H.; Lubman, D. M. Appl. Spectrosc. 1992,1769,46. (35) Todd,J.F. J.; Waldren,R.M.Int.J. MassSpectrom.IonProcesses 1979,29, 301. (36)Waldren,R.M.;Todd,J.F.J.Int. J.MassSpectrom.IonProcesses 1979, 29, 315. (37) Waldren,R.M.;Todd,J.F.J.Int. J.MassSpectrom. IonProcesses 1979, 31, 15. (38) Mosburg, E. R.; Vedel, M.; Zeraga, Y.; Vedel, F.; Andre, J. Int. J. Mass Spectrom. Ion Processes 1987, 77, 1. (39) Suter, M. J.-F.; Gfeller, H.; Schlunegger, V. P. Rapid Common. Mass Spectrom. 1989, 3,62. (40) Paul, W. Agnew. Chem., Int. Ed. Engl. 1990,29,739. (41) Nourse, B. D.; Cooks, R. G. A w l . Chim. Acta 1990,228, 1. (42) Cooks, R. G.; Kaiser, R. E., Jr. Acct. Chem. Res. 1990, 23, 213. (43) Cooks, R. G.; Williams, J.; Cox, K.; Kaiser, K.; Schwartz, J. Rapid Commun. Mass Spectrom. 1991,5,327. (44)McLuckey, S. A.; Van Berkel, G. J.; Glish, G. L.; Huang, E. C.; Henion, J. D. Anal. Chem. 1991,63, 375-83.
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ANALYTICAL CHEMISTRY. VOL. 65. NO. 14, JVLY 15. 1993
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Flaun 2. Extwnal Ion inlectionllon bap re(lsclmn TOF mass speclrometer: V., API chamber's dc push voltage: V,. V.. plsbap Ehrel lens wltages: Vd radbhsquency potemlab V,. dc exlraction voltage; V, = Vxl. Right tube liner voltage: V, fmuslng voltage: V,. beam steering voltage: VM b n repeller voltage; vR2,b n reRectu voltage. eter interfaced to a quadrupole ion trap storage device (R.M. Jordan Co., Grass Valley, CA)" and a liquid injection aample ionization source. A liquid i n j d o n source was uaed to deliver the sample, dissolved in a solvent, through a heated pneumatic nebulizerassemblytothevaporizationchamber,where thesample is vawrized and the solvent removed. The samnle then Dasses throbph a channel to a separate second chamher and is ionized viaadcplasmasourcein 1 atm He. Theresultingionsareinjected through a pair of differentially pumped skimmers ( - 1.5 Tom). which sample the on-axis component of the ion heam. Alternatively. the vaporized sample can be sampled through the first skimmer and ionized in the differentially pumped region via a PIOW discharge mechanism under reduced pressure. The ions produced were transported into the mass spectrometer region and collimated by a set of Einzel lens into the ion trap device. The ions are stored or accumulated until the trapping potential in nhut off and subsequently an extraction pulse is applied to the exit end cap of the ion trap. The timing and characteristin, of the trappin~iextractionprocesses are descrihed in detail below. Thisertranion pulse triggers the s m for theTOF mamanalpis. Upon leaving the trap. the ion packet enters a field-free drift region 1 m long at the end of which ita velwity is slowed and reversed by the ion reflector. The newly focused ion packet then
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EXPERIMENTAL SECTION The experimental setup is shown in Figure 1-3. It consists of a differentially pumped reflectron time-of-flight mass spectrom-
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, l Q 9 3
retraverses the drift region and is detected by a dual 40-mm microchannel plate detector with a gain of -10s-107. A. Sample Injection System. In order to inject a variety of samples, including samples of relatively low volatility, into the TOF system, a liquid chromatography atmospheric pressure ionization interface (LC-API) using a relatively high current plasma source in 1atm He was employed.1° The system consists of a heated pneumatic nebulizer,a vaporization chamber, a plasma source ionization chamber, and a differentially pumped dualorifice interface. The details of the system are described below (see Figure 1). An LC syringe pump (Varian Model 8500) was used to deliver the sample through a 180-pm fused-silica capillary to the vaporization chamber. The flow rates were typically varied from 30 pL/min to 0.5 mL/min, and the solvent used was methanol degassed in an ultrasonic bath. The 180-pm fused-silicacapillary was inserted through a zero-dead-volumetee into a coaxial '/lein. stainless-steeltube with a0.01-in. i.d. The liquid was converted into fine droplets using a high-velocityjet of helium (1.2 L/minh4 The generated mist was then swept through al/s-in. heated brass tube with another helium gas stream (-200 mL/min) into the vaporization chamber. The helium flow prevents the sample from depositing on the surface of the heated tube and conducts heat to vaporize the solvent. Two fine control flowmeters were used to control the gas flow of the nebulizer in order to achieve the best signal. Al/le-in. thermocoax heater was used to heat the l/S-in. brass tubing. The heated section is a 3/4 in. long, l/z in. in diameter cylindrical brass block tightly wound with l/le-in. thermocoar cable heater through which the l/gin.-tube is inserted. This assembly is directly o-ring sealed to the back end of the vaporization chamber. The vaporization chamber consists of a 1.0 in. by 1.0 in. diameter cylindrical brass chamber and a heated brass block with a 0.5 in. by 0.03 in. diameter channel connected to the ionization chamber. The vaporized samples were transferred through the heated narrow channel into the ionization chamber. The temperatures of the pneumatic nebulizer and vaporization chamber can be controlled independently using a thermocouple and temperature controller. The typical working temperature was 150-250 OC. However, the actual working temperature of the sample is expected to be much lower than the body temperature of the vaporization chamber or pneumatic nebulizer since heat is carried away by the vaporized solvent and carrier gas. B. Ionization. Ionization was produced at atmospheric pressure in the ionization chamber or at reduced pressure in the differentially pumped interface. The atmospheric pressure ionization process provides soft ionization, where the protonated molecular ion generally is the predominant peak observed. In the reduced pressure ionization process,relatively soft ionization or fragmentation can be controlled by altering the experimental conditions. 1. Atmospheric Pressure Ionization. The ionization chamber (see Figure 1)is directly attached to the skimmer and consists of 1.0 in. X 0.8 in. (diameter) brass cell with two l/,-in. Cajon ports with quartz windows for observation and one '/ah. Cajon port for the discharge electrode. The glow discharge is formed with the use of a 0.04-in. diameter tungsten rod which is ground to a sharp tip. The rod is covered with glass or Teflon insulation, and only the tip is exposed to prevent an unstable discharge from occurring on the side of the rod. In order to initiate the plasma source, -1 and -2 kV was applied to the electrode. When a glow was obtained, the actual operating voltage was dropped as much as 100-500 V from the initial breakdown voltage and a current of 0.1-0.6 mA was maintained. The resulting plasma appeared as a white and blue glow extending from the tip of the electrode. The most stable glow was maintained in 1 atm helium. The vaporized sample is ionized via ion/molecule reactions induced by the He dc plasma source as described in previous work.@The ionized sample is transported into a dual-orifice differentially pumped interface. The fiist sampling orifice of this atmospheric pressure interface is 275 pm, and the second orifice inlet to the mass spectrometer is 325 pm. The region between the two orifices is pumped by a 650 L/min mechanical pump to a pressure of -1.5 Torr. The pressure in (46) Willoughby,R. C.;Browner,R.F.A d . Chem. 1984,56,2626-31.
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the mass spectrometer acceleration region under these conditions is 1X 10.8Torr. An ion beam condenser with voltage V,, is placed between the two orifices to enhance the transmission of ions into the mass spectrometer. V p d is a small positive voltage placed on the entire atmospheric pressure interface so that there is a voltage drop between the two orifices. 2. Reduced Pressure Ionization. The vaporization sample traversed through the ionization chamber and was sampled into the differentially pumped region. Instead of initiating a glow discharge at the tip of the electrode in the ionization chamber, -200 to -500 V was applied to the entire API chamber to initiate the glow discharge between the first skimmer and the ion beam condenser (maintained at ground potential). Under these conditions, a bright glow was formed with a current of 0.1-1.0 mA and an operating voltage -100-300 V lower than the initial breakdown voltage. When operated under ambient air conditions, some fragmentation was normally observed in the mass spectrum since higher voltage was required to induce breakdown and maintain the glow. However, if helium is introduced into the system, voltage as low as -60 V is sufficient to maintain a glow and thus soft ionization can be achieved. By increasing the discharge voltage, fragmentation can be induced for structural information. C. Trapping. The ions produced in the ionization source were sampled through the second skimmer and entered the mass spectrometer region. The ions underwent a supersonic jet expansion and cooling and then were collimated by a set of Einzel lens into the ion trap device. The Einzel lens assembly consists of three lenses. The fiist and third lens elements were maintained at the same voltage, VA,while the second element was maintained at a separate voltage, VB. These voltages were optimized with each experimental run and are listed in the figure captions. The Einzel lens assembly normally operates with the voltages on continuously. However, lens B can be pulsed using a positive dc pulse of up to 400 V to deflect ions and produce a pulsed ion beam for introduction into the trap. This pulsing can be performed at various delays so that the ion beam can be injected into the trap for a selected time. The ion trap consists of two end-cap electrodes with a ring electrode between them. These electrodes have hyperbolic surfaces and are configured as shown in Figure 3. The ion trap was completely enclosed with ceramic spacers placed between the ring and end caps except for an inlet and exit aperture 3.1 mm. in diameter on the end caps. A l/Ie-in. stainless-steel tubing with 0.02-in. i.d. was tightly fitted into a hole on the ring electrode in order to introduce helium or other gasesinto the trap to increase the local pressure when needed. A Vernier needle valve was used to finely control the amount of gas admitted into the trap. Typical pressure in the ion trap ranged from 5 X lo-' to 1PTorr. During operation, both end caps are held at 0 V while an rf signal of constant frequency (1.0 MHz) and variable amplitude (0-460 V,,) is applied to the ring electrode. This applied rf field serves to trap ions present within the volume of the trap. Varying the rf amplitudes varies the m/zrange of ions that are stable within the trap. Ions with appropriate m/zfor a particular rf amplitude have a stable trajectory within the trap and, therefore, are t r a p ~ e d . 8The ~ mass range of the ions that will be trapped was approximated by computer simulation." After a selected m / z range has been trapped, a dc pulse was applied to the exit end cap to simultaneously extract all ions from the trap for TOF analysis as detailed below. In some cases, if the energy of the ions from the ionization source is high, the ring electrode and both end caps can be biased with positive voltage to slow down the ions and improve the trapping efficiency. Using this method, ions with energy as high as 300 eV can be injected into and stored in the trap, as will be discussed in more detail later. The trapping, extraction, and detection processes were timed as follows: A Global Specialties Co., 4001 pulse generator (PG) was used to trigger two California Avionics Laboratories, Inc. Model 112 AR (MOD) digital delay generators (DDG1 and DDG2). The DDGl was used to trigger the rf power supply. An R. M. Jordan Co. rf power supply operated at 1.0 MHz, 0-460 V,, was used to trap ions up to m/z of 185,while a modified EA1 rf power supply with variable amplitude 0-2200 Vpp,1.1-MHz output was used to trap ions with mlz greater than 200. The DDGP output pulse triggered the extraction pulser, and this pulser
ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993
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of the ion trap. This extraction pulse was a dc square wave -150 V in amplitude and 2 ps in width with 10-ns rise and fall times. The delay of DDGZ was set to coincide with the delay of DDG1; therefore, extraction of the ions occurred simultaneously with the rf trapping potential being triggered off. The repetition rate of the pulse generator can be set at 0.1-10 KHz, limited by the ions with the longest flight time, so that the repetition rate of the entire experimental cycle varies from 0.1 to 10 KHz. D. TOF Operation. The ions, upon exiting the ion trap, pass through a set of accelerating plates and Einzel lens which serve to focus the ion packet and accelerate it into the field-free flight tube region through a potential difference of --1400 V. A pair of beam deflecting plates are then used to steer the ions toward the ion repellerlreflector assembly, where the ion packet is more tightly focused, reversed in direction, and reaccelerated through the flight tube (with angular displacement from its initial axis of trajectory) onto the 40-mm dual-microchannel plate detector. The reflectron flight tube is pumped by a Varian VHS 4 diffusion pump while the main chamber is pumped by a Varian VHS 6 diffusion pump. A restriction of 1-in. tubing is placed between the flight tube and the main chamber, which produced typical operating pressures of 8 X 10-6 and 1 X 10-6 Torr, respectively. The actual pressure in the ion trap during sample introduction though was between 5 X 10" and 103 Torr. The TOF of the extracted ion packets was measured on the DOSC. Signal averaging was used to enhance the signal-to-noise ratio, and reported spectra are averages of 100 single wave forms unless noted otherwise. In the experiment, DDGBsimultaneously triggered the DOSC and the extraction pulser. The ion signals from the detector were sent to the input of the DOSC, and the time difference between various ion peaks and the trigger (t = 0) reference provides the time of flight of each ion. The TOF spectra in the DOSC can be transferred to a 386 IBM-compatible PC using an RS 232 interface bus established between the DOSC and the computer. A user-written QUICKBASIC program was used to control the transfer processes. The size for each data point was 16 bit for our experiments. The raw data from DOSC were in ASCII form. It was converted into signed decimal form by a user-written program. A study of system resolution performance found that the spectral resolution is limited by the time resolution of the Lecroy 9400A DOSC (10 ns). Later experiments used a Precision Instrument Inc. PI9825 SignalAverager in order to take advantage of its 5 ns time resolution. The PI9825 model is a complete signal averager on two PC-AT cards inserted onto a 3861486 PC motherboard with a maximum digitization rate of 200 mega sampleis. Software accompanying this product provides a graphic control panel used to acquire and store the data on a real-time basis. The stored data in binary form was converted into ASCII form by a user-written program and loaded into commercial spreadsheet software such as EXCEL or LOTUS for further data analysis. Mass calibration was performed by measuring the time of flight ( T ) of afew known masses (such as backgroundwater or methanol cluster ion signal) to find the constant x and y in the empirical equation by linear regression analysis, mlz = X P+ y. All samples were obtained from the Sigma Chemical Co. (St. Louis, MO) and used without further purification. HPLC-grade methanol solvent was purchased from Merck and degassed in an ultrasonic bath prior to use. Sample preparation consisted of merely dissolving a weighed analyte with a measured volume of solvent. All samples were refrigerated when not in use.
RESULTS AND DISCUSSION The capabilities of the ion trap/reTOF for detection of ions produced via the API dc plasma source are demonstrated in Figures 6-9. Figure 6 shows the API mass spectrum of the molecular ion region of pyridine injected into the IT/reTOF using a 10-6 M pyridine solution in methanol. The vaporization and ionization of the liquid sample and the transmission of the resulting ions to the ion trap were performed as described previously. The spectrum obtained reveals three fully resolved peaks at m/z 78,79, and 80, which are assigned to the molecular ion, the protonated molecular ion, and its
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Figure9. Atmospheric pressure glow discharge lonlzatknmassspectra +170 V; VA = -205 V; VB = of pyridine: V w = +220 V; V-150 V; v,, = 460; ,V V,, = -225 V; V, = +345 V; and ail other conditions the same as Figure 8. 13C isotopic peak, respectively. This spectrum was obtained for a storage time of 950 ms and the resulting resolution was >1500. In this case, the R. M. Jordan rf power supply was used to supply rf voltage to the ring electrode. A voltage of -400 V, was used at a frequency of 1.0 MHz in this spectrum. The rf voltage is pulsed off before the ions are ejected into the reTOF, which results in a peak width fwhm of 11ns,and thus relatively high resolution is observed. The sensitivity here is also excellent where only 160 fmol of analyte was used to obtain this signal. The ability to pulse off the rf power supply prior to dc ejection provides better resolution in this spectrum than if the rf is not pulsed off. However, the maximum voltage of 460 V,, available from this supply limits the ability to trap externally injected ions from this dc plasma source to mlz 185. It should be noted that the voltage required
to store the ions may be significantlylower than the rf voltage actually required to trap the externally generated i0ns.M In Figures 7 and 8 are shown the API dc plasma source IT/reTOF mass spectra of amitriptyline and methadone, respectively. The spectrum in Figure 7 was obtained using 1o-B M amitriptyline dissolved in methanol, which wm vaporized and ionized as described previously. The mass spectrum obtained consists of the protonated molecular ion peak at mlz 279 and its 13C isotopic peak at mlz 280. This spectrum was obtained using a modified EA1 rf quadrupole power supply operating a t 1.1MHz and 530 Vpp. This unit (46) Chambers, D. M.; Goeringer, D. E.; McLuckey, S. A.; Glieh, G.L. Anal. Chem. 1993,65,14-20.
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could supply the additional rf voltage, up to 2200 V,,, required to trap mlz > 185. This unit could not be pulsed off before ejection of ions into the reTOF because of the nature of the rf circuit involved. In previous work, this inability to trigger the rf off was shown to result in ion ejection over several cycles with the appearance of doublet peaks and resulting peak broadening.32 Nevertheless, the fwhm width of m/z 279 is -20 ns with a resulting resolution of >E00 using an ion storage time of 90 ms. The spectrum of methadone in Figure 8was taken under conditions similar to those of amitriptyline, except the rf voltage was 630 V,, for optimized signal intensity. In this case a solution of 10-6 M methadone in methanol was used and vaporized and ionized as before. The spectrum shows a protonated molecular ion peak a t m/z 311 and its accompanying 13Cisotopic peak. In addition, a fragment peak due to the loss of a N,N-dimethylamino group at mlz 264 is also observed. The fwhm peak obtained in this spectrum is -20 ns, resulting in a resolution of >1600. In further work, we investigated the effect of the ion storage time on the sensitivity and resolution in the reTOF. This is shown in figure 9a-e for a continuous ion beam where the mass spectrum of pyridine in the IT/reTOF is monitored as a function of trapping time. The pyridine ions were produced using the API dc plasma source as in Figure 6. Figure 9a-e shows that with the use of a continuous ion beam both the resolution and intensity improve with increased trapping time. For example, in trace e, the ions are trapped for only a very short period and the result is a spectrum with relatively poor resolution and weak signal intensity similar to that obtained with no trapping under the same conditions. The next spectrum, d, shows improvement in resolution and signal intensity with only 1 ms of ion trapping while trace c, with a moderate trapping time of 90 ms, shows a marked improvement in both of these signal characteristics where a peak of 16 ns fwhm is observed. Trace b shows an extension to long trapping time (900 ms) and the resultant improvement in resolution and signal intensity where the observed peak is only 10 ns fwhm. Finally, an extremely long trapping time of 9 s in trace a shows some increase in resolution, -2100 (8 ns fwhm), over the previous case with similar signal intensity. The 8 ns fwhm observed begins to reach the time resolution limit of the P I 9825 signal averager. However, the small increase in resolution obtained by using such long storage time ultimately may not justify the extremely long experimental cycle. Thus, an increase in ion trapping time significantly enhances the resolution and signal intensity up to -1 5. The increase in resolution with extended storage time is presumed to be due to the improved spatial and energy resolution of the ions in the trap using a collisional buffer gas. The extended trapping of ions results in their accumulation in the center of the trap which, upon extraction, yields a more spatially resolved ion packet and thus enhances the spatial resolution of the reTOF. In addition, the large number of collisions with the He buffer gas as a function of storage time collisionally relax the translationally hot ions, thus improving the energy component of the resolution. In previous work32 using laser-induced REMPI as a means of producing ions inside the trap, the resolution was found to improve rapidly as a function of storage time. In this previous study the resolution reached its optimal value in less than 100 ps, and little improvement was observed for longer storage times. This indicates that the ions were indeed being rapidly relaxed into the center of the trap. However, these experiments involved ions produced by laser-induced REMPI which resulted in an ion energy distribution of +150V caused a reduced pressure glow discharge to form in the differentially pumped interface region. Vie, served to condense the ion beam emanating from the first skimmer and focus it upon the second skimmer’s orifice. This parameter was also found to be a critical factor influencing the energy of the ions entering the pretrap Einzel lens. The V,, typically was run at +10 to +35V. A setting within this range was found to provide effective ion beam focusingwithout imparting too much energy to the ions. Greater Vie, settings create an ion beam with too much energy to effectively trap ions without a dc bias being applied to the trap. In further work, it was found that high-energy ions could also be stored within the trap. If the first skimmer was changed to 350 wm,the pressure in the differentially pumped region was typically 5-10 Torr, under our operatingconditions. Under these conditions the ion transmission efficiency is very low and only ions with higher energy were sampled into the mass spectrometer region. Indeed, a high voltage on V,
-
400 V
i’
1925
Q
y
I
T l m of flight I usec I
Figuro 11. Reduced pressure glow discharge ionizationmass spectra of dlbenrothiophene (MW, 184; trapping time, 5 ms): (a) V , 3 400 V., (b) V, = 800 V., Ail other conditions were the same as those of Figure 10.
expansion scale. The spectrum also contains a fragment peak due to the loss of a methyl group and two background peaks, the larger of which is off-scale. In Figure lob, for comparison, ambient air is used as the dischargegas and a higher discharge voltage was required to establish an ionizing glow as compared to He. Thisresulta in increased fragmentation peak intensity and formation of additional fragments at the expense of the molecular ion. In addition, the increased voltage required for breakdown of air may also enhance the formation of fragment ions through collision-induceddissociation. Figure 1Oc shows the effect of increasing the discharge voltage on fragmentation, where an increase in discharge voltage significantly increases the degree of fragmentation observed. A potential problem shown in Figure loa-c is the presence of large background ion peaks which may saturate the trap or the detector, especially when attempts to amplify the smaller signal peaks are made. One of the important advantages of the ITIreTOF is the ability of the ion trap to
(47)0, C. 5.;Schueesler, H. A. Int. J. Mass Spectrom. Ion Processes
1981, 40, 67.
(48) 0, C. S.; Schueseler, H. A. Int. J . Mass Spectrom. Ion Processes 1981, 40, 77.
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was needed to efficiently focus ions into the second skimmer. In addition, the He flow rate in the API chamber was found to be critical, where a high He flow rate enhanced the resulting transmission of ions but also appeared to increase the energy of the ions. An ion energy measurement in the mass spectrometer was performed using a blocking grid, and the ion energy was found to be as high as several hundred electronvolts. However, using the technique described in the Experimental Section whereby high-energy ions can be slowed and trapped by using a positive voltage bias on the ring electrode and both end caps, ions of up to 300 eV, could be injected from an external source and trapped. An important potential advantage of the IT/reTOF is the duty cycle of the device. The actual pulse-out time of the device is -2 ps. Thus, if one uses a storage time of >10 ms per cycle for sampling a continuous ion beam, the duty cycle approaches nearly 100%. Other methods have been used to sample continuous ion beams into TOF such as beam modulation and pulsed extraction. In the beam modulation technique an ion beam is swept past a slit to obtain time resolution in the TOF device. In previous work with an API beam, this method was shown to provide excellent resolution in a linear TOFMS;24 however, the sensitivity was rather limited due to the poor duty cycle. It is estimated that even with a beam modulation rate of 10 kHz, a maximum duty cycle of -0.02% can be achieved. An alternative method for interfacing a continuous ion beam to the TOF is the pulsed extraction technique. In this method an ion beam is transmitted between the acceleration plates of the TOF and then rapidly pulsed out with an extraction pulse into the TOF drift tube which is transverse to the ion beam. The duty cycle is limited by the length of the extraction plates and velocity of the ion beam, but has been estimated to be as high as -2.5 % based upon a repetition rate of 10 kHz (19). The duty cycle of these two techniques will decrease when detecting ions with larger m / z where a lower repetition rate must be used. An advantage of the ion trap is that it can achieve an excellent duty cycle for continuous ion beams independent of repetition rate. As shown in this work, a trapping time of 100 ms can be used to enhance the signal and resolution of ions generated by an API source. Under these conditions, a pulse-out rate of only 10 Hz is required, which can easily be processed even with relatively modest electronics and software. When fast spectrum acquisition must be performed, such as applications in GC or LC when detecting rapidly eluting species or monitoring transient species, a 100-ps trapping time and a pulse-out rate of 10 KHz can be used to obtain the same 100% duty cycle. However, the beam modulation and pulsed extraction methods always require a high extraction rate (10 kHz) to achieve a reasonable duty cycle. Although this is possible to process with modern digitizers and software, the use of the low repetition rate provided by the IT/reTOF greatly simplifies data collection and processing. There are other potential advantages of the IT/reTOF over pulsed extraction methods without storage. Simulations performed on the SIMION program show that orthogonal extraction of ions becomes difficult above 50 eV of energy,
where the ions cannot be easily turned around and transmitted down the flight tube. In comparison, the IT/reTOF is capable of slowing down and trapping ions of high energy with resulting high resolution due to the storage properties of the trap. Also, the orthogonal pulsed extraction method will be limited in the mass range that will be observed due to the transverse velocity component of the ion beam. However, the mass storage range of the IT/reTOF is determined by the voltage and frequency applied to the trap and can be made extremely large in conjunction with the use of a buffer gas in the trap. In recent sir nu la ti on^^^ it has been shown that a storage range of several hundred thousand amu should be possible under appropriate conditions. In the present work the mass range is limited by the voltage range of the rf power supplies available and the rf voltage actually required to trap rather than just store the ions. However, storage of ions over arelatively large mass range has been demonstrated in the trap.4z*43The subsequent pulse-out of the ions from the trap into the reTOF occurs on-axis so that any further energy difference in the kinetic energy of ions of different masses in the extraction process is no longer important. In addition, a further possible advantage of the ITIreTOF in these experiments that has not been demonstrated herein is the possibility for MS/MS studies in the trap and for the studies of long-lived metastable decay in the IT/reTOF combination. A measurement of the limit of detection attainable for liquid injection/nebulization into API was examined using pyridine dissolved in methanol solution. An initial solution of 6.55 X 10-6M was prepared and successive dilutions were made down to 6.55 X 10-loM. These samples were run as detailed in this work, but with the storage time optimized for each concentration. A lower limit of detection of 2-3 fmol was determined using a SIN = 3 as our limit of detection criterion. In conclusion, the capabilities of an ion trap storage/ reflectron time-of-flight mass spectrometer combination have been demonstrated for detection of ions generated by liquid injection into plasma source atmospheric pressure ionization mass spectrometry. The ion trap has been shown to be an effective means of storing externally generated ions for up to 10 s, prior to mass analysis in the reTOFMS. The IT/reTOF can provide nearly 10071 duty cycle in converting a continuous ion beam into a pulsed source for TOF. It is also shown that the storage capabilities of the device provide enhanced resolution and sensitivity as the storage time is increased. The detection limit of the device was also demonstrated with liquid injection techniques and found to be in the lowfemtomole range. In addition, the rf voltage was shown to be an effective means of eliminating low-mass background peaks from the trap and, thus, from the TOF mass spectrum obtained.
ACKNOWLEDGMENT We acknowledge partial support of this work by the National Science Foundation under Grant BIR-9223677.
RECEIVEDfor review January 14, 1993. Accepted April 14, 1993.
