AMI. Chem. 1999, 65, 451-456
Determination of Environmental Contaminants Using an Electrospray Interface Combined with an Ion Trap Mass Spectrometer Hung-Yu Lin and Robert D. Voyksner' Analytical and Chemical Sciences, Research Triangle Institute, P.O. Box 12194, Research Triangle Park, North Carolina 27709
A commercial electrospray source was interfacedwith an ion trap mass spectrometer (ITMS)to analyze compounds of environmentalconcern. The systemwas opthlzed to achieve maximum senrltlvky. Ions were injected into the trap, which was held at a heilum pressure of 3 X lo4 Torr, at a 9. value of 0.1-0.15. Coilidonai activatlon of ions generated from pestkldes and dyes war achievedInthe electrospray transport region by adjusting the repeller voHaga The k n trap was also used to provide structurallnformatlonabout the [M H]+ Ions formed. Collklorkduced decomporltlon spectra from both processes were sknllar. Collklonal activation in the eiectrorpray transport rogknrequiredhighsample purity. Tho MS/MS spectra generated in the ITMS were less susceptible to interference from matrix camponem however, the acqukltkn of the MS/MS spectra requkedextwhre lndnment setup. Llquid chromatography/ei~rorpray-ITMSwas used to analyze a spiked water sample directly. F u i k a n ekctrospray CID spectra were obtained on 10-30quantities of materlai, rewitlng in low ppb detection.
+
INTRODUCTION Environmental contaminants are frequently omitted from surveillance because they are not detected by gas chromatography/mass spectrometry (GUMS). Thermal instability and nonvolatility are the main reasons for these failures. The combination of liquid chromatography with MS (LC/MS) has enabled the detection of many new environmentally relevant organic compounds. Techniques such as thermospray and particle beam LC/MS have been used successfully to identify those classes of compounds that are not detected by GC/MS.l-S These LC/MS techniques have certain limitations, however, including the inability to analyze some thermally unstable or nonvolatile compoundsand the lack of sensitivity or specificity.8-8 These limitations have been
* Corresponding author (phone:
919-541-6697). (1)Doerge, D. R.; Miles, C. J. Anal. Chem. 1991,63,1999-2001. (2)Behymer, T.D.;Bellar,T. A,; Budde, W. L. Anal. Chem. 1990,62, 1686-1690. (3)Kim, I. S.;Sasinos, F. I.; Stephens, R. D.; Wang, J.; Brown, M. A. Anal. Chem. 1991,63,819-823. (4)Voyksner, R. D.; Cairns, T. In Analytical Methods for Pesticides and Plant Growth Regulations; Sherma, J., Ed.; Academic Press Inc.: San Diego, CA, 1989; Vol. XVII, Chapter 5,pp 119-164. (5)Voyksner,R.D. In AppIicationofHPLCIMSinClinicaZ Tozicology, Analytical Aspects of Drug Testing; Deutach, D., Ed.; John Wiley & Sons, Inc.: New York, 1989; Chapter 7, pp 173-202. (6)Brown, F.R.; Draper, W. M. Biol. Mass Spectrom. 1991,20,515521. (7)Voyksner, R. D.; McFadden, W. H.; Lamment, S. A. In A Chemical Analysis Series on Application of New Mass Spectrometry Techniques in Pesticide Chemistry; Rosen, J., Ed.; John Wiley & Sons, Inc.: New York, 1987;Vol. 91,Chapter 17,pp 247-258. (8) Voyksner, R. D. In Pesticide Chemistry-Advances in International Research, Development and Legislation; Frehse, H., Ed.; VCH: New York, 1991;Vol. XIV, pp 383-395. 0003-2700/93/0365-045 1$04.00/0
reduced with recent advances in the electrospray interface and quadrupole ion traps. Initial investigationsof an electrosprayinterface combined with an ion trap mass spectrometer (ITMS) have shown its utility with a variety of samples.*ll Electrospray can efficiently desorb ions formed in solution into the gas phase for mass analysie.12-l4 Ion currents generated by electrospray typically have been far greater than those generated by other LC/MS techniques.8 Although electrosprayis a soft ionization technique, structural information can be obtained through collision-induced decomposition (CID) in the ion transport region by adjusting the potentials on the ion extraction elements.l"17 In this way, CID spectra can be generated for all the ions formed by electrospray. The combination of electrospray with ITMS amplifies potential sensitivity and specificity while introducing potential cost effectiveness.1"20 The ITMS has been shown to achieve superior sensitivity over quadrupole instruments and can be operated in the MSn mode to generate additional structural information.21-23 The MSn ability can prove advantageous when chemical noise limits CID capabilities within the electrospray transport region. This paper addresses the determination of environmental contaminants using a commercial electrospray which is interfaced to an ITMS. Optimization of the ITMS and its capability to detect nonvolatile environmental contaminants, includingpesticides, herbicides, and azo dyes are emphasized. In particular, the CID capabilities within the electrospray and within the ITMS are compared for the detection of these environmental contaminants. (9)McLuckey, S. A.; Van Berkel, G. J.; Glish, G. L.; Huang, E. C.; Henion, J. D. Anal. Chem. 1991,63,375-383. (10) VanBerkel, G. J.; Glish,G. L.;McLuckey, S.A.Ana1. Chem. 1990, 62. 1284-1295. (11)Van Berkel, G. J.; McLuckey,S. A,; Glish, G. L. Anal. Chem. 1991, 63,1098-1109. (12)Fenn, 3. B.; Mann, M.; Mong, C. K.; Wong, S. F.; Whitehouse, C. M.Mass Spectrom. Reu. 1990,9,37-70. (13)Whitehouse, C. M.; Dreyes, R. N.; Yamaehita, M.; Fenn, J. B. Anal. Chem. 1985,57,675-679. (14)Mann, M. Org; Mass Spectrom. 1990,25,575-587. (15)Voyksner, R. D.; Pack, T. Rapid Commun.Mass Spectrom. 1991, 5,263-268. (16)Katta, V.; Chowdhury, S. K.; Chait, B. T. Anal. Chem. 1991,63, 174-178. (17)Smith, R. D.; Barinaga, C. J. Rapid Commun. Mass Spectrom. 1990,4,54-57. (18)N o w , B. D.; Cooks, R. G. A d . Chim. Acta 1990,22.8, 1-21. (19)Todd, J. F.J.;Penman, A. D. Int. J.Mass Spectrom. Ion Processes 1991,106,1-20. (20)March, R. D.; Hughes, R. J. Quadrupole Storage Mass Spectrometry, Chemical AnalvaisSeries; John Wiley & Sons, Inc.: New York, 1989;Vol. 102. (21)Kaiser, R. E., Jr.; Cooks, R. G.; Syka, J. E. P.; Stafford, G. C., Jr. Rapid Commun. Mass Spectrom. 1990,4,30-33. (22)Stafford, G. C., Jr.; Kelley, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd, J. F. J. Int. J. Mass Spectrom. Ion. Processes 1984, 60,85-98. (23)Johnson, J. V.; Yost, R. A,; Kelley, P. E.; Bradford, D. C. Anal. Chem. 1990,62,2162-2172. 0 1993 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 4, FEBRUARY 15, 1993
462
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EXPERIMENTAL SECTION Standard. Pesticides were obtained from the Environmental Protection Agency (EPA) Pesticides and Industrial Chemical Repository (Research Triangle Park, NC). Dyes were obtained from the EPA (Las Vegas, NV) and Aldrich Chemical Co. (Milwaukee, WI). Arginine and methionine enkephalin were obtained from Sigma Chemical Co. (St. Louis, MO). Standards were dissolved in 50 % methanol (Baxter,Muskegon,MI) in water with 0.1 % trifluoroaceticacid (TFA)or 1% acetic acid to produce concentrations ranging from 100 to 0.01 ng/pL. The standards were infused into the interface using a Sage 341B syringe pump (Orion Research Inc., Boston, MA) at a flow rate of 1.6 lL/min.
300
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Flgure 4. Electrospray CID spectra for 1 ng of aldlcarb sulfone (MW 222): (A) CID spectrum obtalned In the electrospray transport reglon at a repeller setting of 15 V; (B) CID spectrum of the [M 4- H]+ Ion (mlz 223) obtalned In the Ion trap (qr = 0.3, tickle voltage = 1.5 Vp-p and tickle time = 80 ms).
Electrospray-ITMS. An ITMS (Finnigan MAT, San Jose, CA) was coupled to a Vestec prototype electrospray interface** (Vestec Corp., Houston, TX) (Figure 1) for the experiments described below. To enable couplingof the electrospray interface without changes to the commercial ITMS hardware, the quadrupole ion trap was rotated 180' and moved on the optical rail to the opposite end flange. All wire connections (filament,gate, radio-frequency (rf) electrode) on the front-end flange were extended so that electrical connections could be made when the analyzer was moved to the back-end flange. The extension of the rf electrode wire required that the rf coil be rebalanced by lowering the tap by three coils. The electrospray interface was then mounted in the existing ITMS end cap after the filament (24) Allen, M. H.; Vestal, M. L. J. Am. SOC.Mass Spectrom. 1992,3,
18-26.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 4, FEBRUARY 15,
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and gate electrode were removed. A self-centering lens on the electrospray lens stack mated with the end cap to ensure proper alignment with the ion entrance of the trap. Electrospray Operation. The electrospray was operated with a needle voltage of +2.2 to +2.4 kV, a nozzle voltage of +200 V, a repeller voltage ranging from 5 to 80 V, a source voltage of +10 V, and a lens voltage (lenses 1 and 3) of -70 V. A gate was added to the electrospray (lens 2) to pulse ions into the ion trap at specific time intervals ranging from 10 to 200 ms (Figure 1). This gate was controlled through the normal electron gate pulse of the ITMS electronics. The voltage on the repeller controlled the extent of CID obtained in the electrospray transport regi0n.~4 Higher repeller voltages resulted in more CID product ion formation. The source block temperature of electrospray was set at 260 "C and the ITMS was kept at 110 OC. Two Edwards two-stage pumps Nos. 18 and 12 connected in parallel (Edwards High Vacuum International, Tonawanda, NY) were used at the first stage of the two-stage electrospray interface. An Edwards No. 18two-stagepump was used at the second stage electrospray interface. The thermocouple pressure gauge readings were 2.7 and0.12 Torr for the respective pumpingstagesof the electrospray interface. A 520 L/s turbo molecular pump (Balzers, Hudson, NH) was added to the 170 L/s turbo molecular pump on the ITMS, resulting in an analyzer pressure of 1.0 X Torr. However, the pressure increased to 4.0 X Torr (ionization gauge reading) when helium was added to the ITMS to increase the trapping efficiency of the injected ions. Mass Analysis. The mass-selective instability mode25 was used to eject ions from the ion trap to the multiplier for detection. An ion with a specificmass/chargeratio can be efficientlytrapped as determined by the Mathieu stability diagram.17 When the Mathieu parameter qz of an ion reaches 0.908, the ion cannot be efficiently trapped. By scanning the amplitude of the rf voltage applied to the ring electrode, ions with different mass/charge ratios can be sequentially ejected and detected. Tandem MS (MS/MS) spectra were obtained for particular m/zions on the ITMS. First, ions generated from electrospray were stored in the ion trap by the rf field. The ion of interest was isolated by ejecting all the other ions in the ITMS using a combination of dc and rf fields. Asuitable "tickle" voltage (Vp,,) was then applied to the end caps to provide particular m/z ions with kinetic energy. These excited ions collided with helium gas and converted the kinetic energy to internal energy, forming product ions. The product ions were then ejected from the trap to the multiplier using the mass-selective instability mode of operation. LC/MS. A mixture of propoxur, carbofuran, and aldicarb sulfone spiked into water at 1, 10, and 100 ng/mL was used to demonstrate on-line LC/MS. Separations were peformed on a 25-cm X 1-mm CIS column containing 3-pm particles with a gradient of 100% HzO to 90% CH&N in 25 min at a flow rate of 10pL/min. A balanced column splitter controlled the volume (25) Syka, J. E. P.;Louris, J. N.;Kelley, P. E.;Stafford,G. C.; Reynolds,
W.E.U.S. Pat. 4,736,101,1988.
of column eluant entering the electrospray interface. One end of the splitter was connected to a 25-cm X 4.6-mm column packed with 5-pm Cl8 particles. The other end of the splitter was connected to an injector (Rheodyne Model 8125, Cotati, CA) with a 10-pL injection loop followed by the 1-mm-i.d. column, which was connected to the electrospray needle. The LC pumps (Waters 6000A pump, Milford, MA) provided a 0.6 mL/min flow into the balanced columnsplitter, resulting in a flow rate of about 10 pL/min through the 1-mm-i.d. column.
RESULTS AND DISCUSSION Optimization of Electrospray-ITMS. The initial work performed on the combined electrospray-ITMS system involved the optimization of detectable ion current. Optimization was required because the ion trap was operated in the presence of nitrogen from the electrospray interface together with helium. Also, ions formed by electrospraywere injected into the trap rather than being formed in the trap, as is the case with electron ionization. Parameters requiring optimization included the voltages for gating the electrospray ions into the trap, the time period that ions were gated into the trap, the qz value during ion injection, and the helium bath gas pressure. The electrospray interface required little optimization;ita operation was similarto that of a quadrupole MS.24 Ions formed by electrospray were gated into the trap to minimize space charging (overloading the trap) and the introduction of ions during a mass scan. The normal ITMS gating circuitry was used to drive a gate controller that provides from 12 to 50 V to prevent ions from entering the trap and 0 V to allow ions to be injected into the trap. It was found that negative voltages did not increase the detectable signal and could result in the formation of CID product ions. Voltages between 12 and 30 V were sufficient to prevent ions from entering the trap. The exact voltage used to prevent the ion from entering the trap depended on the ion energy (source block potential) used during the experiment. The length of time that ions could be injected into the trap (gate held at 0 V) was surprisingly long. Typically, good results were obtained from analyzing a 1 ng/pL solution with an ionization period of 200 ms. This period is far longer than the 3-5 ms used under electron ionization conditions. Reducing the ionization period decreased the signal intensity, yet a period of over 200 ms did not increase peak intensity. The longer gating period also resulted in a broader m/z signal, possibly due to limitations imposed by space charge within the trap. The period of time after ionization that ions were stored in the trap was not a critical parameter that required optimization. This stems from the difference in the desol-
454
ANALYTICAL CHEMISTRY, VOL. 65, NO. 4, FEBRUARY 15, 1993
Table I. List of Product Ions Detected for Selected Dyes from MS/MS of the [M Electrospray Transport Region dye Disperse Red X1 [2872-52-8]
MW
MS/MS-ITMS
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314
134 (63)
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255 (3)
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196
+ HI+Ion in the ITMS and from CID in the
119 (12) 156 (62) 196 (100) 247 (12) 336 (58) 353 (43)
315 (100) 119 (30) 156 (59) 196 (48) 353 (100)
tentative identification
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307
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156 (100) 291 (8) 308 (61)
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+ +
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296
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0
235 (13) 252 (58) 267 (38) 279 (16) 297 (100)
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380
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224 (92) 275 (18) 381 (100)
177
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348
Disperse Red X13 [3180-81-21
91 (100) 109 (25) 149 (38) 156 (20) 209 (10) 224 (34) 275 (11) 381 (94)
149 (27) 177 (11) 208 (12) 232 (48) 349 (100)
[M + H - CH30]+ [M + HI+
105 (27) 120 (100) 209 (14)
120 (21)
375 (37)
255 (4) 375 (50)
[M + HI+
251 (8) 363 (77) 463 (100)
251 (23) 363 (100) 463 (38)
[M + H - CisH321+ [M + H - C7H16It [M + HI+
255 Blue X1-Commercial
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ANALYTICAL CHEMISTRY, VOL. 85, NO. 4, FEBRUARY 15, 1993
vation of ions generated by electrospray. Previouslyreported work with electrospray-ITMS systems primarily used collisional warming in the trap to desolvate ions.9-11 The work described in this paper thermally desolvates ions in the electrospray interface, before they enter the ITMS. This method of desolvation does not appear to result in thermal decompositionof the sample and is independent of the ITMS conditions. The addition of helium into the ITMS was necessary to collisionally dampen the electrospray ions injected into the trap to obtain the maximum trapping efficiency (Figure 2). In the absence of helium (ITMS pressure of 1.0 X Torr of air from the electrospray interface), the detected signal for arginine or methionine enkephalin was very weak. The addition of helium up to (2-4) X lo4 Torr helped keep the ejected electrosprayions in the trap. Higher pressure resulted in a decrease in signal intensity. The qr value played a minor role in the signal. The qz values of 0.1-0.15 proved optimal for ion injection. Higher q L values usually resulted in reduced ion current and the formation of some CID product ions, possibly due to increased collisionalenergy of the ions. Trapping ions at lower qr values proved insufficient, resulting in low ion current. Detection Limits. The detection limits of the optimized electropsray ITMS system enabled the full-scan detection of 20-40 pg of material introduced into the system. The mass spectrum of terbutryn (Figure 3) demonstratesthe capability of the system to detect 30 pg of this herbicide. The signal for terbutryn was 2-3 times more intense than that obtained on an electrospray quadrupole system. This enhancement was a factor of 2-3 times less than GCIMS on the trap compared to a quadrupole system. ElectrosprayCID Spectra. The generation of structural informationis a key element in the application of electrospray for environmental contaminant determinations. Identifications cannot be based solely on the presence of the [M HI+ ion. In the electrospray-ITMS system, collisional activation of ions to produce product ions can occur either in the electroepraytransport region or within the ITMS. Collisional activation within the electrospray transport region can occur when the potential on the repeller is increased.24At increased repeller potential, all ions in the electrospray ([M + HI+, fragment ions, and ions from other compounds) are collisionally activated to form product ions. The CID spectrum of aldicarb sulfone at a repeller setting of 5 V, consisting of only an [M HI+ ion at m/z 223, shows significant and interpretable fragments at a repeller setting of 15 V (Figure 4A). Likewise, the ion trap MSIMS spectrum of the [M + HI+ ion of aldicarb sulfone showed identical product ions (Figure 4B). The MS/MS spectrum was also much "cleaner" relative to the CID spectrum generated in the electrospray transport region. Background ions at m/z 33 and 59 and product ions formed from the [M + Nal+ or from other product ions, shown in Figure 4A, were not detected in the MSIMS spectrum obtained from the ion trap. These observations emphasize that the high sample purities through chromatographic separations are required to obtain useful CID spectra in the electrospray transport region. The overall eensitivity and CID efficiency are excellent for both collisional activation techniques. Figure 5 shows the fall in the [M H]+ ion current and the rise in the product ion current as either the repeller voltage (Figure 5A) or tickle voltage (Figure 5B) is increased. Both techniques show less than 10% loss in total ion current across the collisional activation energy range evaluated. Furthermore, better than 80%of the tM + HI+ion current can be converted to product ions with either CID technique. These efficiencies are far superior to magnetic sector or quadrupole ( 6 0 % ) MS/MS techniq~es.2~3~~
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Figure 8. LClMS ion chromatograms from the [M 4-H]+ ion: (A) 0.1 ng of propoxur (m/z 210); (6)0.1 ng of carbofuran (mlr 223); (C) 1 ng of aidlcarb sulfone (mlz 223) spiked in water. Separation was performed using a gradient from 0% acetonitrlle to 90% acetonitrlle in water (0.1% acetic acid) on a C,* 25-cm X l m m column at a flow rate of 10 pLlmin. 100
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200
300
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A number of azo dyes were analyzed to compare the CID spectra generated in the electrospray transport region and in the ITMS (Table I). Although the CID spectra were not identical, they showed certain similarities. Both CID techniques showed the same structurally significant product ions. Often MSIMS in the ITMS resulted in the formation of additional product ions for the compound. Product ions were primarily observed from the cleavage of the azo bond (N-N) and the N-C bonds, as well as from the loss of -OH and alkyl substituent groups. The ability to mass select an ion for CID in the ion trap is often preferred, especially during mixture analysis. However, the MSIMS capability of the trap depends on many variables including tickle voltage (Vpp), tickle frequency (which can vary with space charging), and CID time; the qz value used for trapping and sample concentration must be optimized to acquire an MSIMS spectrum with a high signal1 noise ratio. On the other hand, CID spectra generated within ~~
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(26) Yost, R. A.; Enke, C. G.; McGilvery, D. C.; Smith, D.; Morrison, J. D. Int. J. Mass Spectrom. Ion Phys. 1979,30, 127-136. (27) McLafferty, F. W., Ed. Tandem Mass Spectrometry; John Wiley & Sons, Inc.: New York, 1983.
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ANALYTICAL CHEMISTRY, VOL. 85, NO. 4, FEBRUARY 15, 1993
the electrospray transport region are easily obtained. The repeller voltage is set and a CID spectrum is acquired. But this method is useful only on relatively pure samples or samples showing minimal background, placing more of an emphasis on LC separation. LC/Electrospray-ITMS. On-lineLC/electrospray-ITMS was demonstrated for the detection of three pesticides and herbicides spiked into water. Full-scan acquisition could easily detect 0.1-1-ng quantities of these components (Figure 6). Propoxur could be detected down to 20-30 pg (S/N -4: 1). The ability to acquire full-scan spectra on picogram quantities of material, combined with increased sample loading when injecting in a weak LC elution solvent (direct injection of 10 pL of the water sample into a column equilibrated in 1007% water) enables the direct detection of 1-10-ng/mL (1-10 ppb) levels of contaminants. Structural information to confirm the identity of each component could be obtained by CID in the electrospray transport region by increasing the repeller voltage to 30 V. Because the water sample was relatively clean and the spiked components were separated by LC, no interference ions were detected in the CID spectrum generated in the electrospray transport region.
The CID spectrum a t a repeller setting of 30 V for propoxur (Figure 7) spiked into water a t 6 ng/mL (60 pg injected) provides four ions for conformation. The presence of the [M + HI+ion and three product ions at mlz 168,153, and 111 is sufficient to confirm the identity of this pesticide. Further investigation of the capability of electrosprayITMS LC/MS for the detection of pesticides and dyes in more complex water and soil matrices is in progress.
ACKNOWLEDGMENT The authors wish to thank Dr. L. Don Betowski, EPA, Las Vegas, NV, for dye samples and discussions of this work. Notice: Although the research described in this article has been funded in part by the U.S.Environmental Protection Agency through Contract No. 68-02-4544 to the Research Triangle Institute, it does not necessarily reflect the views of the Agency and no official endorsement should be inferred.
RECEIVEDfor review September 1, 1992. Accepted November 6, 1992.
