Anal. Chem. 1993, 65, 1295-1300
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Operation of a Quadrupole Ion Trap for Particle Beam LC/MS Analyses Brent L. Kleintop, Donald M. Eades, and Richard A. Yost' Department of Chemistry, University of Florida, Gainesville, Florida 32611 -2046
Several operational modes of the quadrupole ion trap mass spectrometer (QITMS) were investigated to perform particle beam (PB) LC/MS analyses. The PB interface was directly coupled to a QITMS without the use of an external ion source; vaporization of the particle beam and ionization by electron ionization (EI) were accomplishedwithin the ion trap. Ions from residual LC solvent initially caused space charging and a significant extent of chemical ionization (CI), which degraded the quality of E1 mass spectra. Ejection of solvent ions prior to mass analysis minimized both space charging and CI, which allowed acquisition of E1 spectra that compared favorably to E1 spectra obtained from pure samples vaporized off of a solids probe and to library E1 spectra. Isocratic LC/PB/QITMS analyses for several pesticides show lower limits of detection (LODs) than those reported for LC/PB/MB analyses on PB systems using quadrupole mass analyzers. Typically nonlinear calibrationcurves are produced, although linear calibrations have been observed for some analytes. INTRODUCT10N Particle beam (PB) interfaces for liquid chromatography/ mass spectrometry (LC/MS), originally known as MAGIC,1>2 have generated recent widespread interest largely because of their ability to produce classical electron ionization (EI) mass spectra for a wide variety of compounds of environmental and biological interest.3-4 The generation of E1 spectra is advantageous because the fragmentation patterns found in these spectra typically reveal structural information,allowing identification of unknown compounds, a typical goal of many LC/MS analyses. E1 spectra also can often be compared to existing library spectra for the identification and confirmation of unknown compounds. Most other LC/MS interfaces (e.g., thermospray and electrospray/ionspray), although better suited for analysis of thermally labile and involatile compounds, produce spectra unique to the interface. This typically limits these techniques to targeted compound analysis, although the use of tandem mass spectrometry for obtaining structural information has been reported.5~6Structural information of electrospray ions has also been obtained 'Address reprint requests to Richard A. Yost, Department of Chemistry, University of Florida, Gainesville, F L 32611-2046. (1)Willoughby, R. C.; Browner, R. F. Anal. Chem. 1984,56,26262631. (2) Winkler, P. C.; Perkins, D. D.; Williams, W. K.; Browner, R. F. Anal. Chem. 1988,60,489-493. (3)Behymer, T. D.; Bellar, T. A.; Budde, W. L. Anal. Chem. 1990,62, 1686-1690. (4)Voyksner, R.D.; Smith, C. S.; Knox, P. C. Biomed.Enuiron. Mass Spectrom. 1990,19, 523-534. ( 5 ) Smith, R. D.; Loo,J. A.; Edmonds, C. G.; Barinaga, C. J.; Udseth, H. R. Anal. Chem. 1990,62,882-899. 0003-2700/93/0365-1295$04.00/0
from collision-induceddissociation (CID)within the transport region of the interface, prior to the mass spectrometer.7~8 The quadrupole ion trap mass spectrometer (QITMS) has also generated widespread interest, first as a low-cast benchtop G U M S instrument, and more recently as a truly versatile high-performance mass spectrometer. The QITMS is an extremely sensitive mass spectrometer, largely due to ita ability to store ions prior to mass analysis and the absence of ion optics. Coupling a PB interface with a QITMS would appear to be an appealing choice for many environmental applicationsemploying LC/MS. The ability of PB to generate E1 spectra that can be compared to library spectra affords the identification of unknown compounds, a typical goal of environmental analyses. The noted sensitivity of the QITMS should also provide the necessary limits of detection (LODs) required for environmental analyses. Unfortunately, the QITMS has seen limited LC/MS applications. Even though ion traps have great potential for low-cost, benchtop instrumentation, most PB systems currently in use employ quadrupole mass analyzers. Ion traps have seen limited use in LC/MS systems primarily because of the large amount of solvent associated with most LC/MS interfaces. Excess solvent ions typically cause space charging and undesired ion/molecule reactions, resulting in poor mass resolution and poor spectral quality. Fortunately, the versatility of the QITMS allows the creation of customized scan functions which eject unwanted solvent ions from the ion trap while efficiently storing analyte ions of interest. The coupling of PB with a QITMS using both direct coupling and injection of ions from an external ion source has been reported;gJo however, currently no information appears in the literature comparing operational modes of the ion trap which minimize space charging by ejecting residual LC solvent ions prior to mass analysis. We have previously reported various methods for massselective ionization in the quadrupole ion trap to eliminate undesired ions of a range of madcharge ( m / z )values.11 These included the use of rf and dc voltages and the use of resonant excitation employing wave forms comprising multiple frequencies. However, these methods required modifications to the ITMS source code. Here we evaluate two modes of ion trap operation implemented with standard ITMS software for PB/LC/MS to eject residual solvent ions from the ion trap prior to mass analysis: (1) elevation of rf voltages to (6)Van Berkel, G. J.; Glish, G . L.; McLuckey, S. A. Anal. Chem. 1990, 62,1284-1240. (7) Loo,J. A.; Edmonds, C. G.; Smith, R. D. Anal. Chem. 1991,63, 2488-2499. (8) Katta, V.; Chowdhury, S. K.;Chait. B. T. Anal. Chem. 1991.63, 174-178. (9)Bier, M. E.; Winkler, P. C.; Lopez, J. T. J.Am. SOC.Mass Spectrom, in press. (10)Bier,M. E.; Hartford,R. E.; Henon, J. R.;Stafford,G. C. Presented a t the 39th ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, T N , May 19-24, 1991;pp 538-539. (11)Eades, D. M.; Yates, N. A,; Yost, R. A. Presented a t the 39th ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, T N , May 19-24, 1991;pp 1491-1492. 0 1993 Amerlcan Chemical Society
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Table I. Compounds Studied, Molecular Weights, Quantitation Ions,and Estimated Full-Scan LODs of Carbamate and Urea-Based Pesticides with LC/PB/QITMS quantitation est LOD compound MW" ion (rn/z) biz) aldicarb sulfone 222 68 20 carbaryl 201 144 25 methiocarb 225 168 40 162 methomyl 105 40 diuron 232 72 5 linuron 248 61 30 monuron 197 72 25 rotenone 394 192 40 ~~
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Flgurr 1. Schematic diagram of the PB/QITMS these studies.
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selectively eject ions below a chosen mlz ratio and (2) application of a supplemental rf signal to resonantly eject a single solvent ion species. Both methods were evaluated when applied either during ionization or during an ion storage period following ionization. The goal of this work was to determine the best way to operate the QITMS to produce librarysearchable E1 spectra for PB/LC/MS analyses. E1 spectra obtained from PBILUQITMS analyses of pesticide mixtures are also compared to library spectra and spectra obtained from solids probe/QITMS analyses (i.e., a "solvent-free" method) of pure compounds. Finally, instrument calibration curves and LODs obtained on this system are reported.
EXPERIMENTAL SECTION Liquid Chromatography. Two LC pumping systems were employed in these studies. Solvent ejection studies utilized an Isco (Lincoln, NE) LC-2600 syringe pump. Chromatographic separations were performed using a Hewlett-Packard (Palo Alto, CA) 1090Lhigh-performance liquid chromatograph. Separations were performed isocratically with a mobile-phase composition of 80/20 acetonitrile/reagent water on a Hewlett-Packard ODS column, 100 X 4.6 mm, packed with 5-pm particles at a flow rate of 0.3 mL/min. The analytical column was thoroughly conditioned by pumping 50/50 acetonitrile/water through the column overnight to remove residual impurities and column bleed. No postcolumn addition of volatile buffersI2was used in any of these studies. PB Interface. The LC/MS interface wasa pr0totypeFinniga.n MAT (San Jose, CA) particle beam interface. This prototype differs from the commercially available interface in that it employs three stages of pumping in the momentum separator region to further reduce the amount of solvent which reaches the ion trap. A thorough description of the interface appears elsewhere.9 The desolvation chamber pressure was monitored by the addition of a pressure gauge (see Figure 1). Addition of He to the desolvation chamber to increase the pressure to -400 Torr resulted in an approximate 2-fold increase in ion signals. The additional He provides a higher thermally conductive medium, which increases the rate of desolvation. These results are consistent with those reported by other laboratories.Ig Supplemental He was also added to the third stage of momentum separation to prevent backstreaming of pump oil from the mechanical pumps caused by low pressures. Typical pressures in the three stagesof the momentum separator were 5-10 Torr, 300 pTorr, and 100 pTorr (adjusted by addition of He), respectively. The interface was inserted into the QITMS via a 'iz-in.-o.d. transfer line probe which allowed insertion and removal without disturbing the high vacuum of the QITMS. The probe tip was positioned 1/4 in. away from the ion trap entrance to prevent high pressures inside the trap. Optimization of PB parameters has been suggested previ~usly.~ Important PB parameters including desolvation chamber temperature, pressure, and nebulizing He flow rate were optimized to provide maximum sensitivity. The interface used in these studies utilized a glass Meinhard (Santa Anna, CA) concentric
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(12) Bellar, T. A.; Behymer, T. D.; Budde, W. L. J. Am. SOC.Mass Spectrom. 1990, I, 92-98. (13) Browner, R. F. J. Microchem. 1989,40, 4-29.
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pneumatic nebulizer in which the position of the nebulizer tip is not easily adjustable. Mass Spectrometer. The mass spectrometer used in these experiments was a Finnigan MAT ion trap mass spectrometer. Fundamentals of ion trap operation, theory, and ion motion are detailed e1se~here.l~ The QITMS vacuum chamber was maintained at 100 "C for all experiments. The filament end cap of the ion trap was modified to allow insertion of two 10-W heater cartridges which were controlled by an external temperature controller (Omega Engineering Model CN 2012, Stamford, CT). This end cap served as the particle beam source target and was operated at a temperature of 250 "C. Electron ionization was employed within the QITMS; detection was accomplished with an electron multiplier setting of -1500 V (lo5gain) and with the conversion dynode at 0 V. Partial pressures of residual solvent Torr (uncorrected) as measured by a were typically -1 X Bayard-Alpert ionization gauge (Granville-Phillips, Boulder, CO) mounted on the vacuum chamber. Helium was added into the vacuum chamber to produce typical QITMS operating pressures Torr (uncorrected). Axial modulation was employed of 1 X during the rf analytical scan to reduce the effects of space charge. A schematic diagram of the PB/QITMS system used in these studies is shown in Figure 1. Samples and Reagents. Pesticide standards listed in Table I were obtained from the US.Environmental Protection Agency (Pesticide Chemical Repository, Research Triangle Park, NC) and used without purification. HPLC-grade methanol and acetonitrile were obtained from Fisher, and reagent water was obtained from a Milli-Q water purification system.
RESULTS AND DISCUSSION Problems Caused by Residual Solvent. A typical profile mass spectrum obtained from flow injection analysis (FIA) of -100 ng of carbaryl (MW = 201 m u ) in 100% methanol mobile phase is shown in Figure 2. The spectrum shown is an expansion of the m/z 25-150 mass range to better illustrate the resolution between adjacent masses; the abundance of the M+ ion at mlz 201 is 4%relative to 145+. The large abundance of methanol solvent ions centered around m/z 33 caused space charging, which resulted in poor mass resolution between adjacent low masses. Notice that resolution improved at higher masses. The inset in Figure 2 illustrates the mass resolution between 144+and 145+. The ITMSgenerates mass spectra by sequentially ejecting trapped ions from low to high mass (mass-selective instability).15 This results in solvent ions being ejected/detected before higher mass ions during mass analysis, which reduced space charging when higher mass ions were detected. The 144+/145+ratio in this (14) March, R. E.; Hugher, R. J. Quadrupole Storage Mass Spectrometry; Winefordner, J. D., Kolthoff, I. M., Eds.; Wiley: New York,
1989. (15) Stafford, G. C.; Kelley, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F. J. Int. J. Mass Spectrom. Ion Processes 1984, 60, 85-98. (16) Eades, D. M.; Kleintop, B. L.; Yost, R. A. Presented at the 40th ASMS Conferenceon Mass Spectrometry and Allied Topics, Washington, DC, May 31-June 6, 1992; pp 129C-1291.
ANALYTICAL CHEMISTRY, VOL. 85, NO. 9, MAY 1, 1993
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spectrum also indicated a large degree of chemical ionization (CI) occurred. The base peaks of the E1 and CI mass spectra of carbaryl are 144+ (M - 57)+ and 145+ (M + H - 57)+, respectively. Although some 145+ from the 13C isotopes of 144+ (CloH80)+ would be expected in the E1 spectrum of carbaryl, the figure shows 145+being -2 times more intense than 144+. The abundant methanol solvent ions have evidently caused a large amount of solvent-CI to occur. Indeed, residual solvent ions have been used to perform CI analyses of pesticides with the same PB/QITMS system.14 However, the large degree of CI was undesirable since a goal of this work was to produce quality E1 spectra. This figure illustrates the need to eject solvent ions prior to mass analysis to minimize space charging and CI to obtain good quality E1 8p ectr a. Solvent Ejection Studiee. These studies investigated twomethods of QITMS operation implemented with standard ITMS software to eject unwanted solvent ions from the ion trap; each method can be applied either during ionization or during the storage time following ionization. Both methods involved operating the ion trap in the rf-only mode (i.e., with the Mathieu parameter14 a, = 0). One method studied involved elevating the rf voltage applied to the ring electrode to eject ions below a chosen mlz, Le., to impose a low-mass cutoff. Since the Mathieu parameter q, is proportional to the rf voltage, elevating the rf voltage increases the q, values of trapped ions. In the rf-only mode, ions with qz values greater than 0.908 will have unstable trajectories and will be ejected from the trap in the axial ( z ) direction. Since q, is inversely proportional to m/z,ions from low molecular weight LC solvents (e.g., CH30H, CHsCN, HzO) can be ejected at a properly chosen rf potential while analyte ions of higher m/z remain stored. In this mode, the ion trap is basically operating as a high-pass mass filter. This mode of operation was investigated when applied both during ionization and afterward, during ion storage. As expected, as the instrument's low-mass cutoff approached each solvent ion species, solvent ions were efficiently (100% ) ejected from the ion trap when the rf was elevated both during ionization and during ion storage. Figure 3 illustrates how carbaryl analyte ion (144+)intensity was affected by elevating the rf level (4,) both during ionization and during ion storage. These plots were obtained by varying either the ionization or storage values of Q~ with a constant amount of carbaryl (2 ng/pL yielding 10 ng/s at 0.3 mL/min.) in 100% methanol flowingthrough the interface. Although the analyte ion signal was relatively unaffected by storage q,, signal decreased
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significantly with increasing ionization qr. As the value of qr increases, so does the initial kinetic energy of the ions formed, resulting in large initial velocities and ion losses due to quasiunstable traje~tories.1~These losses occur when the m a g nitude of an ion's oscillation exceeds the internal dimensions of the ion trap even though mathematical conditions for stability still exist. Although raising an ion's q1 value during ion storage also increases its kinetic energy, the ions are already effectively trapped so ion losses are minimal until the ions become unstable when qr = 0.908. Elevating the ionization value of q, of an ion also decreases the initial ionization volume which limits the region in which ions can be created and remain ~tab1e.l~ The observed decreases in intensity could also be the result of decreased ionization cross sections due to high electron energies a t elevated rf levels.18 Preliminary studies investigating the effects of ionization rf level (4,)of n-butylbenzene ions also showed decreased ion intensities a t higher values of q,.18 There were also some differences in the extent of CI between spectra obtained by ejecting solvent during ionization and during ion storage. Figure 4 shows how the ratio of 144+(E1 fragment) to 145+ (CI fragment plus 13C isotope of 144+)is affected by elevating the rf level during ionization and storage for carbaryl in 100% methanol. The rf level where the methanol solvent ions (mlz 33) are efficiently ejected from the trap is indicated by the dashed line in the figure. In a "pure" E1 spectrum, the 144+/145+intensity ratio would be expected to be 9.0, based on 13C isotopes. The low ratios obtained at low rf levels result from solvent ions causing a large degree of CI to occur. Ejection of solvent ions decreased the amount of CI, resulting in increased 144+/145+ratios; the inability to achieve the expected ratio for a pure E1 spectrum indicated that some CI still occurred in the time before solvent ions were ejected. Also, lower ratios were observed when the rf was elevated during ion storage. This is because solvent ions were stored during the ionization time, which allowed more time for CI to occur before the solvent ions were ejected. (17) Dawson, P. H. Quadrupole Mass Spectrometry and Its Applications; Elsevier,
Amsterdam, 1976.
(18)Pedder, R. E.; Johnson, J. V.; Yost, R. A. Presented at the 40th
ASMS Conference on Massspectrometry and AlliedTopics, Washington, DC, May 31-June 5 , 1992; pp 1761-1762.
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