Anal. Chem. 1999, 71, 3365-3373
Evaluation of Crown Ether Complexation for Elemental Electrospray Mass Spectrometry Wilson Z. Shou and Richard F. Browner*
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
Formation of various solvent clusters under mild source conditions and the loss of charge state information and lack of sensitivity under harsh source conditions are the major problems encountered in elemental electrospray mass spectrometry (ESI-MS). In this work, the use of crown ether complexation as a means of alleviating and solving these problems was evaluated. The crown ether (18-crown-6) was added postcolumn to the flow injection analysis of various metals including alkali, alkaline earth, and transition metals. In most cases, greatly simplified mass spectra that are free from solvent cluster interference could be obtained under suitable in-source collisioninduced dissociation conditions. Oxidation state information of singly and doubly charged metals was retained in the metal-crown ether complex ions. Alkali metals were detected by two modes: (i) metal-ether complex ions and (ii) metal ions dissociated from the complexes using insource collision-induced dissociation. Alkaline earth and transition metals were detected by monitoring the metalether complexes. Signal improvements in the range of 5-20 times were achieved for the metals studied. Slight improvements in the dynamic range were also observed. Electrospray ionization mass spectrometry (ESI-MS) was first introduced by Dole.1 Following the revolutionary work of Fenn et al.2 and Alexandrov et al.,3 the field of electrospray mass spectrometry for organic species analysis has developed rapidly. As probably the softest method of ionization, it has the ability of producing intact molecular ions (or pseudomolecular ions) directly from solution phase to gas phase. Its ability to generate multiply charged ions from large biomolecules further enables it to characterize those molecules using mass spectrometers with nominally limited mass ranges. Currently ESI has become a routine method for coupling HPLC and other separation methods to mass spectrometry, as well as a common ionization choice for various polar and ionic species in solution.4 While most electrospray applications to date are for large molecules of biological interest, there also has been much interest in using electrospray to characterize inorganic and organometallic species in solution. Metal ions have been used as model com(1) Dole, M. L.; Mack, L. L.; Hines, R. L.; Mobley, R. C.; Fegkuson, L. D.; Alice, M. B. J. Chem. Phys. 1968, 49, 2240-2249. (2) Yamashita, M.; Fenn, J. B. J. Phys. Chem. 1984, 88, 4451-4459. (3) Alexandrov, M. L.; Gall, L. N.; Krasnov, V. N.; Nikolaev, V. I.; Pavlenko, V. A.; Shkurov, V. A. Zh. Anal. Khim. 1985, 40, 1272-1236. (4) Cole, R. In Electrospray ionization mass spectrometry: fundamentals, instrumentation, and applications; Cole, R. B., Ed.; Wiley: New York, 1997; p 1. 10.1021/ac9902143 CCC: $18.00 Published on Web 07/03/1999
© 1999 American Chemical Society
pounds to study electrospray mechanisms from the very early stages of the technique, as demonstrated by Kebarle et al.5 Horlick and co-workers6-10 have published a series of papers discussing electrospray mass spectrometry as a means of characterizing species in metal solutions and evaluating its potential for use in elemental analysis. They demonstrated that different interface conditions can result in widely differing spectra and that simple spectra of metal ions can be obtained under harsh interface conditions. Hieftjie et al.11 reported elemental electrospray on a time-of-flight analyzer with results similar to those obtained with quadrupole instruments. Corr12 and Henion13 interfaced electrospray mass spectrometry to capillary electrophoresis and ion chromatography to separate and analyze mixtures of cations and anions. Considerable effort has also been devoted to using electrospray mass spectrometry in an effort to obtain speciation information in different environmental and biological matrixes.14-16 However, currently there are several major problems encountered in elemental electrospray mass spectrometry. The softness of the ionization becomes a double-edged sword when it comes to the determination of metals in the solution phase. Solvent clusters of metals, rather than the bare metals themselves, are the most commonly observed species in ESI spectra.17,18 Although this phenomenon has limited application in studying solution phase chemistry, it greatly complicates the interpretation of the mass spectra because the spectra are highly dependent on operating conditions, as well as the instruments, solvents, and counterions used. Solvent clusters can be partially eliminated by the use of harsh in-source collision-induced dissociation (CID), as described by Horlick.10 However, under these conditions, ion (5) Blades, A. T.; Jayaweera, P.; Ikonomou, M. G.; Kabarle, P. J. Chem. Phys. 1990, 92, 5900-5906. (6) Agnes, G.; Horlick, G. Appl. Spectrosc. 1992, 46, 401-406. (7) Agnes, G.; Horlick, G. Appl. Spectrosc. 1994, 48, 649-654. (8) Agnes, G.; Horlick, G. Appl. Spectrosc. 1994, 48, 655-661. (9) Agnes, G.; Horlick, G. Appl. Spectrosc. 1995, 49, 324-334. (10) Steward, I. I.; Horlick, G. Anal. Chem. 1994, 66, 3983-3993. (11) Mahoney, P. P.; Guzowski, J. P.; Ray, S. J.; Hieftje, G. M. Appl. Spectrosc. 1997, 51, 1464-1470. (12) Corr, J. J.; Anacleto, J. F. Anal. Chem. 1996, 68, 2155-2163. (13) Huggins, T. G.; Henion, J. Electrophoresis 1993, 13, 531-539. (14) Gwizdala, A. B.; Johnson, S. K.; Mollah, S.; Houk, R. S. J. Anal. At. Spectrom. 1997, 12, 503-506. (15) Olesik, J. W.; Thaxton, K. K.; Olesik., S. J. Anal. At. Spectrom. 1997, 12, 507-515. (16) Zoorob, G.; Brown, F. B.; Caruso, J. J. Anal. At. Spectrom. 1997, 12, 517524. (17) Cheng, Z. L.; Siu, K. W. M.; Guevremount, R.; Berman, S. S. Org. Mass Spectrom. 1992, 27, 1370-1376. (18) Cheng, Z. L.; Siu, K. W. M.; Guevremount, R.; Berman, S. S. J. Am. Soc. Mass Spectrom. 1992, 3, 281-288.
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transmission through the interface tends to be much lower than that under mild conditions. Meanwhile, overall the relatively small size of the metal ions seems to render them inadequately hydrophobic and thus relatively inefficient in the ion evaporation process compared to large organic molecules. This causes inorganic ESI-MS to be generally much less sensitive than organic ESI-MS and is a major reason elemental electrospray in its current stage of development is not really competitive with ICPMS for elemental analysis. Recently, an electrochemical preconcentration technique was used to improve the sensitivity for elemental electrospray in different matrixes.19 However, the problem of low ionization efficiency of metal ions in the electrospray process has still not been solved. Furthermore, under the harsh in-source conditions where elemental ESI is carried out currently, almost all metals, regardless of their charge states in solution, appear in spectra as singly charged ions. The reason for this is that metal ions are too small to stabilize the multiple charges on them and therefore undergo sequential reduction in the gas phase before they reach the mass spectrometer detector.20,21 Consequently, oxidation state information of the metal ions is lost. Therefore, the issues of solvent clusters, relatively low sensitivity, and loss of charge state information must be addressed before electrospray mass spectrometry begins to play a really active role in the field of elemental analysis. ESI-MS has been used to study noncovalent interactions between metals and ligands in the solution phase.22,23 An extensive study of complexation between metals and crown ethers was reported by Colton et al.24 Other workers25,26 have also studied metal-crown ether complexes by ESI-MS, although here the emphasis was more on the MS/MS behavior of the complexes and the determination of affinities between different metals and polyethers. Van Berkel et al.27 were the first to use crown ether complexation in an attempt to solve the problems of solvent clusters in metal electrospray mass spectrometry but only reported results for alkali metals. In the current study, we tried to expand the applicability of this approach to the quantitative analysis of a wide range of different metals. Here we evaluated the use of crown ether complexation as a means to enhance ionization efficiency, preserve charge state information, and increase sensitivity in elemental electrospray analysis for various metals. By the attachment of a bulky organic group (crown ether) to metal ions in the electrospray process, the lack of hydrophobicity of bare metal ions can be alleviated because the metal-crown ether complex usually exhibits organic-like properties in both ionization and ion evaporation processes. Due to the high affinity between polyethers and (19) Pretty, J. R.; Van Berkel, G. J. Rapid Commun. Mass Spectrom. 1998, 12, 1644-1652. (20) Blades, A. T.; Jayaweera, P.; Ikonomou, M. G.; Kebarle, P. Int. J. Mass Spectrom. Ion Processes 1990, 101, 325-336. (21) Blades, A. T.; Jayaweera, P.; Ikonomou, M. G.; Kebarle, P. Int. J. Mass Spectrom. Ion Processes 1990, 102, 251-267. (22) Kohler, M.; Leary, J. A. J. Am. Soc. Mass Spectrom. 1997, 8, 1124-1133. (23) Ross, A. R. S.; Ikonomou, M. G.; Thompson, J. A. J.; Orians, K. J. Anal. Chem. 1998, 70, 2225-2235. (24) Colton, R.; Mitchell, S.; Traeger, J. C. Inorg. Chim. Acta 1995, 231, 87-93. (25) Alvarez, E. J.; Wu, H. F.; Liou, C. C.; Brodbelt, J. S. J. Am. Chem. Soc. 1996, 118, 9131-9138. (26) Alvarez, E. J.; Vartanian, V.; Brodbelt, J. S. J. Am. Soc. Mass Spectrom. 1997, 8, 620-629. (27) Van Berkel, G.; McLuckey, S. A.; Glish, G. Proceedings of the 39th ASMS Conference on Mass Spectrometry and Applied Topics, Nashville, TN, 1991; p 292.
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metals, the complexes become the dominant species over solvent clusters in mass spectra, therefore facilitating the unambiguous assignments of metal ion peaks. Furthermore, the relatively larger size of the crown ether and greater number of sites available for charge stabilization increase the complex stability considerably, thus preserving charge state information. In this paper, the applicability of metal ion-crown ether complexation, followed by ESI-MS, to the analysis of some alkali, alkaline earth, and transition metal cations is demonstrated. Postcolumn addition of crown ethers was used instead of mixing every sample with crown ether individually because it greatly simplifies the sample preparation process and also gives very efficient complexation reactions between metals and crown ethers. This method is also compatible with future possible developments in coupling electrophoresis (CE) or ion chromatography (IC) with electrospray mass spectrometry for separation and analysis of metal ions, since crown ethers may be added postcolumn in IC/ESI-MS or added as makeup liquids in CE/ESI-MS. EXPERIMENTAL SECTION All solvents were of HPLC grade or better, and the acetic acid was of analytical grade. The solvents were ultrasonically sparged before use. The crown ethers were purchased from Sigma Chemical Co. (St. Louis, MO), and stock solutions of 1 × 10-2 M were made in MeOH/H2O (1/1 v/v). Stock solutions of 1 × 10-2 M for the respective metals were prepared from ACS grade AgNO3, BaCl2‚H2O, Cd(NO3)2‚4H2O, CoCl2‚6H2O, CsI, CuCl2‚ 2H2O, FeSO4‚7H2O, KCl, MnCl2‚H2O, Ni(NO3)2‚6H2O, RbBr, SrCl2‚6H2O, and Zn(NO3)2‚6H2O salts in MeOH/H2O (1/1 v/v). Serial dilutions were then carried out to obtain solutions of desired concentrations in MeOH/H2O (1/1 v/v). A Benchmark single-quadrupole mass spectrometer (Extrel, Pittsburgh, PA) equipped with an electrospray ionization source was used. A schematic diagram of the LC/MS system is shown in Figure 1. The solvent (MeOH/H2O (1/1 v/v), 1% acetic acid) was delivered by a 1090 HPLC pump (Hewlett-Packard, Palo Alto, CA) at a flow rate of 100 µL/min. In experiments where no postcolumn addition was used, the flow from the LC was directly introduced into the source. In the postcolumn addition experiments, 18-crown-6 was pumped by another HP 1090 HPLC instrument at a flow rate of 30 µL/min and combined with the column effluent through a zero dead volume “T” union (Alltech Associates, Deerfield, IL) just before the source. The flow introduced into the source was split with a metal tee, and the actual flow to the polyimide-coated fused-silica capillary (50 µm i.d., 150 µm o.d.) needle was approximately 10 µL/min. This flow rate was kept constant in all experiments by adjusting the splitting ratio accordingly through a back-pressure regulator (Alltech Associates, Deerfield, IL). The capillary needle was maintained at +3300 V for all positive-mode electrospray work. Flow injection with a Rheodyne (Cotati, CA) 7010 injector equipped with a 30 µL injection loop was used to obtain electrospray spectra for each metal salt. The mass spectra were acquired by full-profile scanning with background subtraction. In-source collision-induced dissociation conditions were controlled by altering the applied voltage difference between the first and second skimmers. In practice, the second skimmer voltage was usually held constant and the first skimmer voltage was changed. For quantitative experiments, flow injection with a 2 µL loop was utilized and the selected ion
Figure 1. Schematic of the Extrel LC/MS system used and postcolumn addition setup.
monitoring (SIM) mode with a dwell time of 1000 ms for each ion was used. RESULTS AND DISCUSSION For the purpose of comparison, results from electrospray mass spectrometric studies of bare metals without the addition of any ligands, carried out both by others6-10 and by ourselves, are briefly described here. Interested readers are referred to the original papers. Under mild interface conditions, which correspond to the usual conditions used in organic ESI-MS to obtain pseudomolecular ions such as [M + H]+, metal salts typically give various clusters between the metal ions and solvents (MeOH, H2O, ACN, etc.), additives (acetate, formate, etc.), or counterions (Cl-, Br-, NO3-, etc.). This mode of operation was called the “ion cluster” mode by Horlick et al.10 While this cluster formation is not a major problem for alkali metals, for metals other than those of group IA, especially transition metals, it greatly complicates the ESI spectra. Electrospray spectra from even such simple metal salt solutions as CoCl2 or Zn(NO3)2 are quite complex and often difficult to interpret. Furthermore, a change of solvent composition or mobile phase additives may have a dramatic effect on the appearance of the spectra. This causes the elemental ESI results often not reproducible from one instrument to another under the ion cluster mode as a result of the existence of trace amount of additives such as acetic acid, ammonium acetate, formic acid, and ammonium formate which may remain in the solvent delivery systems from previous organic ESI-MS runs. As a matter of fact, almost all early elemental electrospray studies were conducted using essentially 100% methanol, without any solvent additives,
in order to minimize the formation of mixed-solvent clusters and simplify spectra. However, in practice, the vast majority of metalcontaining samples are at least partially aqueous. In addition, background ions from protonated solvents and additives themselves are also known to exist in the low-m/z range in electrospray ionization mass spectra, which may produce further background interferences in the ion cluster mode of operation. As a result, elemental analysis is not practical under the ion cluster conditions. It is possible to break up the ion clusters by conducting the experiments under harsher interface conditions, which was referred to as the “metal ion” mode of operation. Upon an increase in the skimmer-cone (or skimmer-skimmer or skimmerhexpole, depending on instrument designs) voltage differences, the in-source collision-induced dissociation between the ions and neutral gas molecules becomes more energetic and allows metal clusters to be broken. Cleaner spectra can usually be obtained by this means, and most of the elemental electrospray work to date has been carried out in this mode. However, for multiply charged metal ions Mn+, usually only M+ ions are observed in the spectra. As described by Kebarle et al.,20,21 this is the result of gas phase reduction under harsh interface conditions. Furthermore, despite the relative “cleanness” of the spectra, the total ion intensity is quite low, which means that the breakdown of all metal-containing adduct ions in the ion cluster mode does not contribute significantly to the analytically useful M+ ion signal in the metal ion mode. More importantly, the ion transmission efficiency under harsh interface conditions is also considerably lower. To illustrate this point, a quantitative study of the effect of Analytical Chemistry, Vol. 71, No. 16, August 15, 1999
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Figure 2. First skimmer voltage dependence of the Ba+, Co+, and reserpine [M + H]+ ion signals obtained using selected ion monitoring (SIM) of 1 × 10-4 M solutions of BaCl2, CoCl2, and 6 ppm of reserpine (dotted line), respectively. The second skimmer voltage was held constant at 31 V.
interface conditions on some doubly charged metal ion signals was carried out, and the results are shown in Figure 2, in which the signal intensities of Co+ and Ba+ are plotted against the first skimmer voltage, while the second skimmer voltage is held constant. Signal intensities from a model organic molecule, reserpine ([M + H]+ at m/z 609), are also plotted as a reference level for ion transmission through the interface. It is obvious, from examination of the reserpine curve, that the best ion transmission was achieved for a first skimmer voltage in the range 55-105 V. Beyond that range, the transmission decreased dramatically. In other words, the interface conditions for the ion cluster mode correspond to conditions giving the highest transmission efficiency through the interface. On the other hand, the best signals for Ba+ and Co+ ions were obtained at first skimmer voltages well above 120 V, due to the requirement of harsh interface conditions (metal ion mode) to break the clusters. However, under these conditions, the overall ion transmission is much lower than that in the cluster mode. Consequently, total ion signals under the metal ion mode are generally well below the level of ion cluster mode signals. Therefore, the current practice of doing elemental electrospray under the metal ion mode is disadvantageous because of the problems of low sensitivity and loss of charge state information. Our solution to the problem came from the realization that elemental electrospray needs to be carried out under mild interface conditions (ion cluster mode) to ensure good ion transmission and avoid loss of charge state information but, at the same time, also avoid formation of various unwanted clusters. The practical methodology used was to add to the metal solutions of organic ligands, namely crown ethers, in large excess to form strong metal-crown ether complexes, thus eliminating “random” cluster formation between metals and solvents or additives. Morever, it was decided to add the crown ether postcolumn, rather than directly to the samples, to decouple the separation and the 3368 Analytical Chemistry, Vol. 71, No. 16, August 15, 1999
Figure 3. Structures of the crown ethers evaluated.
ionization process. This postcolumn addition (PCA) approach has been demonstrated by many groups in the LC/ESI-MS analysis of various normally “hard to electrospray” compounds.28-31 Several polyethers have been evaluated initially (Figure 3). 18Crown-6 was chosen because it is readily available, is quite soluble in methanol, and, more importantly, has a moderately large size cavity (R ) 1.45 Å) for complexing with a wide range of metals. This “generality”, instead of “specificity”, of the complexing ability of 18-crown-6 is important because real-world samples are almost always mixtures of different metals and the use of a general ligand such as 18-crown-6 (18-C-6) can ensure the detection of most of (28) Chassaigne, H.; Lobinski, R. J. Chromatogr., A 1998, 829, 127-136. (29) Kuhlmann, F. E.; Apffel, A.; Fischer, S. M.; Goldberg, G.; Goodley, P. C. J. Am. Soc. Mass Spectrom. 1995, 6, 1221-1225. (30) Karlsson, K. E. J. Chromatogr., A 1998, 794, 359-366. (31) Kohler, M.; Leary, J. A. Anal. Chem. 1995, 67, 3501-3508.
Figure 4. Background spectrum from a 1 × 10-3 M 18-C-6 solution in 1/1 H2O/MeOH added postcolumn at a flow rate of 30 µL/min at first skimmer voltages of 75 V with a constant second skimmer voltage of 31 V.
the metals, as opposed to the use of a very specific ligand which is capable of only complexing with one or two metal ions. Evaluation of the utility of 18-C-6 complexation first called for an examination of background introduced by the postcolumn addition of the ether. Figure 4 shows the background from 18C-6 under mild collision conditions, over a scan range of 40-400 Da. The spectrum shows [Na + 18-C-6]+ at m/z 287, [H3O + 18C-6]+ at m/z 283, and [H + 18-C-6]+ at m/z 265, as well as peaks at m/z 45, 89, and 133. The last series of peaks is due to the sequential expulsion of C2H4O units and ring cleavage, with hydrogen transfer of the crown ether. Similar behavior has been observed using classical EI mass spectrometry.32 As long as background subtraction is used during mass spectral acquisition, the crown ether spectrum causes few problems. Figure 5 shows the spectra obtained from an equimolar mixture of three alkali metal ions with postcolumn addition of 1 × 10-3 M 18-C-6. The most striking features are the high intensities of the metal-ether complex ions [M + CE]+, typically more than 5 times greater than the intensities of uncomplexed ions, and the subsequent significant improvement of the signalto-noise ratio of the spectra. (It is worth noting that, because of the inherent quadrupole discrimination against higher m/z ions, the signals from the complex ions formed should be even larger than those observed here.) We attribute this increase in signal to the higher ionization efficiency in the ion evaporation process of the metal-ether complexes compared to the bare metals. Increasing collision energy through raising skimmer voltages in the interface results in complex dissociation, with the consequent release of bare alkali metal ions, as shown in Figure 5B. This provides two means for determination of the metal ions in this case of alkali metals: (a) to monitor the metal-crown ether complexes under low collision energy and (b) to monitor the bare metal ions from the dissociation of the complexes under relatively high energy conditions. Both methods are viable, and both were used in this study. Spectra from a CoCl2 solution were recorded with postcolumn addition of 18-C-6, and the results are shown in Figure 6. Under very mild interface conditions, as shown in Figure 6A, [Co + CE]2+ and [Co + 2CE]2+ ions are dominant, with a minor contribution from the [CoCl + CE]+ ion for the mass range studied. It is also possible that complex ions with higher numbers of 18-C-6 units (32) McLafferty, F. W.; Turecek, F. In Interpretation of Mass Spectra, 4th ed.; University Science Books: Sausalito, CA, 1993: p 264.
Figure 5. Electrospray mass spectra of an equimolar (1 × 10-3 M) mixture of KCl, RbBr, and CsI in 1/1 H2O/MeOH with postcolumn addition of a 1 × 10-3 M 18-C-6 solution at 30 µL/min and at first skimmer voltages of (A) 55 V and (B) 95 V while the second skimmer voltage was held constant at 31 V.
Figure 6. Electrospray mass spectra of a 1 × 10-3 M CoCl2 solution in 1/1 H2O/MeOH with postcolumn addition of a 1 × 10-3 M 18-C-6 solution at 30 µL/min and at first skimmer voltages of (A) 55 V and (B) 75 V with a constant second skimmer voltage of 31 V.
exist beyond m/z 400. Again a greatly simplified spectrum and much improved signal intensities are obtained for the complexed ions as compared to the noncomplexed ions. When the voltage difference between the first and second skimmer is increased, [Co + CE]2+ becomes the major ion in the spectrum, as seen in Analytical Chemistry, Vol. 71, No. 16, August 15, 1999
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Table 1. Electrospray Mass Spectrometric Data from Metal Salts with Postcolumn Addition of 18-C-6 (CE) metal salt AgNO3 BaCl2 Cd(NO3)2 CoCl2 CsI CuCl2 FeSO4 KCl MnCl2 Figure 7. Effect of the concentration of 18-C-6 added postcolumn on the SIM signal intensity of [Co + CE]2+ from a 1 × 10-5 M CoCl2 solution at a first skimmer voltage of 65 V and a second skimmer voltage of 31 V.
Ni(NO3)2 RbBr SrCl2 ZnCl2
Figure 6B. In both cases (Figure 6A,B), the 2+ charge state of Co remains intact in the form of metal-crown ether complexes because the Co-18-C-6 complex is large enough to stabilize the two charges on Co2+, instead of undergoing gas phase reduction, as in the case for bare Co2+. The doubly charged nature of these ions can be determined by both the mass-to-charge ratio and the relative narrowness of the mass peaks compared to those of singly charged complex ions such as [CoCl + CE]+. Meanwhile, signal intensities also improve dramatically under these conditions, and this is the combined effect of improved ionization efficiency of the complex ions and the higher transmission achievable under the ion cluster mode than under the metal ion mode. For Co, raising the collision voltage above 75 V resulted in major
ions (m/z) observed [Ag + CE]+ (371, 373) [Ba + 2CE]2+ (333), [Ba + CE]2+ (201), [BaCl + CE]+ (437) [Cd + 2CE]2+ (321), [Cd + CE]2+ (189), [Cd(NO3) + CE]+ (440) [Co + 2CE]2+ (294), [Co + CE]2+ (162), [CoCl + CE]+ (358) [Cs + CE]+ (397) [Cu + 2CE]2+ (296), [Cu + CE]2+ (164), [CuCl + CE]+ (362) [Fe + 2CE]2+ (292), [Fe + CE]2+ (160) [K + CE]+ (303) [Mn + 2CE]2+ (292), [Mn + CE]2+ (160), [MnCl + CE]+ (354), [MnOAc + CE]+ (378) [Ni + 2CE]2+ (293), [Mn + CE]2+ (161), [Ni(NO3) + CE]+ (384) [Rb + CE]+ (349) [Sr + 2CE]2+ (308), [Sr + CE]2+ (176), [SrCl + CE]+ (387) [Zn + 2CE]2+ (296), [Zn + CE]2+ (164), [ZnOAc + CE]+ (387)
dissociation of the complex and, in this case, no detectable yield of Co2+ or Co+ ions. These observations are consistent with the MS/MS results of Brodbelt et al.25 for transition metal-crown ether complexes, and it was proposed that the relative sizes of the crown ether and metals determine the dissociation pattern of metal-polyether complexes. Similar behavior was observed for all the metals studied, with the exception of the alkali metals. Therefore, quantitation of all such singly and doubly charged metals was achieved by monitoring either [M + CE]+ or [M + CE]2+ under relatively mild interface conditions (ion cluster mode), which ensures the preservation of the charge state
Figure 8. First skimmer voltage dependence of the [Ba + CE]2+, [Co + CE]2+, [Rb + CE]+, and Rb+ ion signals obtained using selected ion monitoring (SIM) of 1 × 10-5 M solutions of BaCl2, CoCl2, and RbBr, respectively, with postcolumn addition of 18-C-6 at 30 µL/min. The dotted line was obtained from the signals of 6 ppm of reserpine, as in Figure 2. The second skimmer voltage was held constant at 31 V. 3370 Analytical Chemistry, Vol. 71, No. 16, August 15, 1999
Figure 9. Comparison of signal intensities (SIM) from respective 1 × 10-5 M metal solutions between conventional electrospray operated under the metal ion mode and results obtained by crown ether complexation. See Table 2 for detailed interface and SIM conditions.
information of the metals and meanwhile takes full advantage of the higher ionization efficiency of the complexes, as well as the better ion transmission of the interface. Similar results were obtained from various alkaline earth and transition metals with either +1 or +2 charge states, and in all cases the metal-ether complex ions with the correct charge states were obtained with very good signal-to-noise ratios. Detailed results are summarized in Table 1. In contrast to the existing metal ion mode of operation, in which most metal ions appear at the same mass range as solvent-derived ionic species, the use of crown ether complexation moves the mass range of targeted metalether analyte ions ([M + CE]+ or [M + CE]2+) to above m/z 150, where less interference from solvents or additives exists. This is especially advantageous for first-row transition metals such as Cu, Zn, Ni, Fe, and Co. To assess the quantitative aspects of this technique, optimization of the concentrations of 18-C-6 added postcolumn was carried out using a 1 × 10-5 M CoCl2 solution, and the results are plotted in Figure 7. It can be seen that the optimum signal of [Co + CE]2+ is achieved at an 18-C-6 concentration of 5 × 10-3 M, and hence all quantitative analyses were carried out under these conditions. This concentration is consistent with the result of Karlsson et al. obtained in the postcolumn cationization analysis of carbohydrates.30 On the other hand, changes in the flow rates of the postcolumn addition of 18-C-6 do not affect the signal much in the range from 10 to 100 µL/min; therefore, a flow rate of 30 µL/ min was used throughout the experiments. The effects of interface conditions on the signal intensities of metal-ether complexes were studied using Rb, Co, and Ba, with results shown in Figure 8. As in Figure 2, the second skimmer voltage was held constant while the first skimmer voltage was changed. The signal intensities from [Ba + CE]2+, [Co + CE]2+, [Rb + CE]+, and Rb+ (from the dissociation of [Rb + CE]+) were plotted. Note that, for the complex ions, the best sensitivities were achieved at first skimmer voltages ranging from 65 to 75 V, which is well within the optimum interface transmission range. The
Table 2. Detailed m/z Values (in Parentheses) of Monitored Ions and Respective Interface Conditions Expressed as First Skimmer Voltages (V1st) While the Second Skimmer Voltage Was Held Constant at 31 V in the Quantitative Study regular ESI/metal ion mode salt
ion monitored
crown ether complexation
V1st, V
ion monitored
V1st, V
AgNO3 BaCl2 Cd(NO3)2 CoCl2 CsI
Ag+
(107) Ba+ (138) Cd+ (114) Co+ (59) Cs+ (133)
115 135 115 125 85
CE]+
CuCl2 KCl
Cu+ (63) K+ (39)
115 85
MnCl2 Ni(NO3)2 RbBr
Mn+ (55) Ni+ (58) Rb+ (85)
125 125 85
SrCl2 ZnCl2
Sr+ (88) Zn+ (64)
125 125
75 75 75 65 75 95 75 75 95 75 75 75 95 75 65
[Ag + (371) [Ba + CE]2+ (201) [Cd + CE]2+ (189) [Co + CE]2+ (162) [Cs + CE]+ (397) Cs+ (133) [Cu + CE]2+ (164) [K + CE]+ (303) K+ (39) [Mn + CE]2+ (160) [Ni + CE]2+ (161) [Rb + CE]+ (349) Rb+ (85) [Sr + CE]2+ (176) [Zn + CE]2+ (164)
highest intensity for Rb+ was obtained at 95 V because of the stronger collision energy required to break up the complex, but under these conditions, the ion transmission is still within the optimum range, as denoted by the same reserpine plot as in Figure 2, and is still quite satisfactory. Although the concentrations of the metal solutions in this case are only 1/10th of the concentrations of those solutions used in the experiments of Figure 2, the ion signals generated are comparable; therefore, the sensitivity of the analysis is improved significantly. Comparisons of signal intensities arising both from bare metals under the metal ion mode and from metals with crown ether complexation were carried out using 1 × 10-5 M metal solutions. The results are presented in Figure 9. Again, for alkali metals two methods of quantitation with 18-C-6 complexation exist and both results are listed, while for all other metals the metal-ether Analytical Chemistry, Vol. 71, No. 16, August 15, 1999
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Figure 10. Comparison of dynamic ranges for solutions of BaCl2 with and without 18-C-6 complexation. See Table 2 for detailed interface and SIM conditions.
Figure 11. Comparison of dynamic ranges for solutions of RbBr with and without 18-C-6 complexation. See Table 2 for detailed interface and SIM conditions.
complexes with the correct charge state ([M + CE]+ for Ag and [M + CE]2+ for others) were monitored. To ensure the fairness of the comparison, interface conditions were optimized for each element individually, and the data presented here represent the maximum intensities obtained with the same quadrupole resolution. The detailed m/z values and interface conditions are listed in Table 2. From the graph (Figure 9), it is obvious that improvements of 5-20-fold can be achieved for most metals studied, with the best results obtained from Ba, Sr, and Zn. 3372 Analytical Chemistry, Vol. 71, No. 16, August 15, 1999
Finally, an evaluation of the dynamic range of the complexation technique compared to the noncomplexation method was carried out, and the results for Ba and Rb are shown in Figures 10 and 11, respectively (log - log plots). It can be demonstrated that, because of the improvement in signal intensities, the low end of the dynamic range can be extended beyond 1 × 10-6 M to 1 × 10-7 M on our instrument, thus improving the dynamic range by about 1 order of magnitude. Because the instrument used in the study has a relatively old ESI source and is equipped with only
an analog mode multiplier, the absolute sensitivity is inferior to that of some newer instruments using pulse-counting multipliers. Therefore, the purpose of this study is not to show the absolute sensitivity achievable but rather to demonstrate the improvement in ionization efficiency and sensitivity over those of the existing elemental electrospray techniques carried out on the same instrument. Yet this comparison should still hold for newer instruments. CONCLUSION Postcolumn crown ether complexation is a viable means of alleviating such problems as solvent cluster formation, loss of charge state information, and lack of sensitivity encountered in elemental electrospray mass spectrometry. The improved hydro-
phobicity of the metal-ether complexes facilitates ion evaporation, and the use of a high ion transmission mode in the interface region (ion cluster mode) is enabled because of the soft conditions under which the complexes form. Spectra are thus greatly simplified and signal-to-noise ratios improved. Furthermore, the relatively large size of the complexes stabilizes the 2+ charge state for the alkaline earth and transition metals studied; therefore, the loss of charge state information is avoided. Improvements in sensitivity as well as in dynamic range are obtained.
Received for review February 23, 1999. Accepted May 26, 1999. AC9902143
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