LETTER pubs.acs.org/ac
How Are Completely Desolvated Ions Produced in Electrospray Ionization: Insights from Molecular Dynamics Simulations Christopher D. Daub and Natalie M. Cann* Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6
bS Supporting Information ABSTRACT: We apply molecular dynamics (MD) simulations to study the final phase of electrospray ionization (ESI), where an ion loses all of its associated solvent molecules. By applying an electric field to a cluster of H2O molecules solvating an ion and including a surrounding gas of varying pressure, we demonstrate that collisions with the gas play a major role in removing this final layer of solvent. We make quantitative predictions of the critical velocity required for the cluster to start losing molecules via collisions with gas and propose that this should be important in real ESI experiments. Such collisions have heretofore not been explicitly considered in discussions of the ESI process.
E
lectrospray ionization (ESI) is an experimental technique noted for its ability to take protein fragments or other large biomolecules and easily transfer them, in ionized form, from solution into the gas phase. Developed over the course of a few decades,13 today it is routinely paired with mass spectrometry (MS) and in this capacity ESI-MS analysis has revolutionized the quantitative study of proteins and protein fragments. Broadly speaking, ESI is the process in which a solution containing an analyte of interest is passed through a narrow capillary nozzle across a large electrical potential (usually ∼15 kV). The intense electric field at the nozzle tip deforms the liquid into a Taylor cone and jet.4 Ions in the solution concentrate at the tip of the cone and spray out in a fine mist of mutually repelling droplets.5,6 As solvent evaporates from the droplets, the droplet radius is reduced until the high concentration of charge causes the droplets to undergo fission, with the critical size given by Rayleigh’s equation.7 Solvent evaporation and droplet fission continues until the drop reaches a size of ∼10 nm. At this point, there is some disagreement on the remainder of the path taken toward the production of unsolvated ions. In the charged residue model (CRM) developed by Dole, solvent evaporation continues right down until single bare gas phase ions are left.1 By contrast, in Iribarne and Thomson’s ion evaporation model (IEM), ions with an affinity for the liquidvapor interface can directly evaporate into the gas phase,2 perhaps accompanied by a single solvation shell.8 In general, the consensus today seems to be that both models may apply in different situations; for example, the CRM may be more relevant for large ions which are not likely to evaporate directly.9 Molecular dynamics (MD) simulations are well-suited to studying the ESI process and may be able to answer some of the remaining questions about how large ions enter the gas phase. In particular, MD simulations provide a means to directly view r 2011 American Chemical Society
the trajectory of a system of molecules in a way that is still out of experimental reach. In fact, MD simulations have already been used by a few groups to study ESI. Luedtke et al. made a thorough investigation of ESI mechanisms, using classical MD simulations and ab initio computations as well as experiments.10 Their MD simulations focused on the application of an intense electric field, as would be felt at the tip of the Taylor cone, as well as the effect of this field on a NaI-formamide droplet. They found that the droplet gets significantly distorted by the electric field, elongating in the direction of the field.11,12 The droplet will eject single ionsolvent clusters at the tip in a manner similar to the ion evaporation model. The MD work of other groups has concentrated on the charged aqueous nanodroplet regime, where ∼1005000 water molecules are found together with a small number of ions, ranging from protons13 to Na+,14 Ca2+ and Cl,15 H3O+ and diglycine,16 and a larger polyhistidine macroion.17 The earliest work focused on the observation of solvated ion emission similar to the prediction of the IEM.16 Later work studied such aspects as the propensity of ions to reside within a layer of solvent, rather than exposed on the outside of the interface, which can lead to deviations from the Rayleigh limit,14 as can ions with multiple charges.17 Most of this work considers only pure aqueous clusters; very recently, ejection of solvated ions from mixed water/methanol droplets has been studied.18 In this letter, we report on our work using MD to study the ESI process. We have chosen to focus on the final stage of the progression toward bare ions, where either Rayleigh fissions or solvated ion emission have already produced clusters of a single ion surrounded by 1020 water molecules. A water nanodroplet Received: August 10, 2011 Accepted: October 21, 2011 Published: October 21, 2011 8372
dx.doi.org/10.1021/ac202103p | Anal. Chem. 2011, 83, 8372–8376
Analytical Chemistry
LETTER
Figure 1. Snapshots of the 1 Na+ + 20 H2O cluster in 0.5 atm gas with |E| = 0.02 V/nm, showing the progression (left to right) from the equilibrated cluster, to initial solvent loss, and final bare ion formation.
of 10100 molecules should be stable for at least 10100 ns, and the addition of an ion should only decrease the rate of evaporation. Despite this, in ESI experiments it is typically very easy to remove all of the (usually unwanted) solvent before the bare ion is introduced to the mass spectrometer. In experiments where some solvent was observed in the MS, it seems that other mitigating circumstances were in effect. In some cases the electrospray process took place in a vacuum.8,10 In other cases, unusually strong solvent binding to, for example, metal ions19,20 and/or special experimental conditions such as high flow rates and low temperature20 allowed solvent to remain around the ions until after they entered the mass spectrometer. Collision induced or collision activated dissociation (CID or CAD) involves introducing a low pressure of neutral gas into an ion-trap spectrometer, causing complex ions to collide with the gas, with sufficient energy to cause fragmentation.21,22 CID has been used to remove solvent from aqueous metal ion clusters,19,20 and has also been applied at the interface between the ESI apparatus and the mass spectrometer to fragment complex ions. Could a similar process play a part in exposing the bare ion during routine ESI under atmospheric pressure, before introduction into the mass spectrometer? To the best of our knowledge, the present work is the first time the presence of gas has been explicitly included in the MD simulation of ESI. Our findings demonstrate that collisions between gas and the fieldaccelerated charged cluster can contribute significantly to solvent loss. We perform classical MD simulations of clusters of 10, 15, or 20 H2O molecules and one Na+ ion or one Ca2+ ion. A curtain gas is added with pressures of 0.25, 0.5, 1, or 2 atm. For simplicity we have chosen to add argon atoms, while in experiments the typical gas is usually N2 for reasons of cost. The exact nature of the gas will not dramatically affect our conclusions, although as we discuss below the mass of the gas atoms will play a significant role. For the water model, we use the flexible F3C model;23 however, the electric fields we apply (0.002, 0.01, and 0.02 V/ nm) are not large enough to cause significant alterations in water molecule orientations24 or intramolecular dipole moment,25 and we do not anticipate that a rigid model would have led to different results. Interesting effects arising from the interaction of water or other polar liquids with external electric fields, such as electrofreezing26 or elongation of liquid droplets,10,12 do not manifest until fields reach ∼0.5 V/nm; the only meaningful effect of the field in our simulation is to accelerate the charged cluster. Typical field strengths applied experimentally are in the
Table 1. Summary of System Parameters Simulated P/atm
cluster
|E|/ (V/nm)
no. of simulations
1 Na + 10 H2O 1 Na+ + 10 H2O
no gas no gas
no field 0.02
4 8
1 Na+ + 10 H2O
0.25, 0.5, 1
0.01, 0.02
6
1 Na+ + 10 H2O
2
0.01, 0.02
4
1 Na+ + 10 H2O
0.25, 1
0.002
6
1 Na+ + 15 H2O
0.25, 0.5, 1
0.01, 0.02
6
1 Na+ + 15 H2O
2
0.01, 0.02
4
1 Na+ + 20 H2O
no gas
no field
4
1 Na+ + 20 H2O 1 Na+ + 20 H2O
0.5, 1 no gas
no field 0.02
6 8
1 Na+ + 20 H2O
0.25, 0.5, 1
0.01, 0.02
6
1 Na+ + 20 H2O
2
0.01, 0.02
4
1 Na+ + 20 H2O
0.25
0.002
6
1 Ca2+ + 15 H2O
0.5
0.02
6
1 Ca2+ + 20 H2O
0.5
0.02
6
+
kilovolt/millimeter range, comparable to our lowest field and an order of magnitude smaller than our larger fields. It is also known that in ESI experiments the actual electric field is highly nonuniform, being as high as 1 V/nm at the tip of the Taylor cone8,27 or in the vicinity of highly charged droplets; so our simulated field strengths are certainly within the range of experimental relevance. The Na+ and Ca2+ potentials are the well-known models of Smith and Dang.28,29 The Ar Lennard-Jones parameters are from the classic simulation of Rahman.30 In the first 50 ps, the temperature is gradually increased from 30 to 298 K. A snapshot of an initial cluster after equilibration can be seen in Figure 1. An external electric field B E is then turned on, E applied to each charge providing an additional force FBj = qj 3 B qj. The only energy exchange between the cluster and the rest of the system comes from the external field and interactions with the Ar gas. After compensating for the translational energy of the entire cluster, we find that the temperature of the cluster remains quite constant until the cluster begins to break apart (Figure S1 in the Supporting Information). Some further technical and simulation details may be found in the Supporting Information. The cluster moves as the ion comes under the influence of the electric field. We have implemented a shift in reference frame at each time step such that the cluster remains in the center of the cell. To monitor the size of the cluster, a cluster analysis31 is 8373
dx.doi.org/10.1021/ac202103p |Anal. Chem. 2011, 83, 8372–8376
Analytical Chemistry
LETTER
Figure 3. Plot of the average time of initial cluster water loss tbreak versus m/q|E|, with m the cluster mass, q the cluster charge, and |E| the electric field strength. Values are read off the plots in Figure 2 for the lowest pressure available (P = 0.25 or P = 0.5 atm). O are the results with the Ca2+ clusters, are for the Na+ clusters in field E = 0.02 V/nm, 0 are for the Na+ clusters in field E = 0.01 V/nm, and ] are for the Na+ clusters in field E = 0.002 V/nm. Figure 2. Plot of the number of total molecules in the water + ion cluster as a function of time elapsed since application of the electric field. Top: initial cluster 1 Na+ + 10 H2O. Middle: 1 Na+ + 15 H2O. Bottom: 1 Na+ + 20 H2O. Results with |E| = 0.01 V/nm are on the left and with |E| = 0.02 V/nm are on the right. Different pressures P are denoted by the line colors. Dashed lines indicate results with a Ca2+ ion. Error bars indicate the standard error in the determination of the time of initiation of solvent loss at P = 0.25 and 2 atm.
performed every 0.1 ps. A molecule is considered to be part of the cluster as long as it remains within 0.475 nm of any other molecule in the cluster. With this criterion most transient hydrogen bond breaking and remaking events are ignored, but the cluster size can still fluctuate rapidly in some cases. Therefore we smooth the cluster size curves by averaging over 100 total determinations (10 ps) before and after the given time, and further we take the average of several separate simulations to obtain a reliable estimate. Our simulations monitor the effects of changing gas pressure, electric field strength, cluster size, and ion charge. Some simulations were done without gas and/or electric field as controls. We summarize all of our simulations in Table 1. We begin by summarizing the results obtained for our control simulations. First, consider the cluster in the absence of electric field and any surrounding gas. Over several 4 ns trajectories, either zero or one water molecule evaporated from the smallest (10 H2O + Na+) cluster. In similar runs with the largest (20 H2O + Na+) cluster, 02 molecules evaporated. These data agree well with estimates of simulated water evaporation rates at 298 K from nanoclusters in vacuum ∼1 molecule/ns.32 Simulations conducted at higher initial temperatures (up to 700 K) show that solvent evaporation is more probable as expected, but a combination of evaporative cooling and higher binding energy ensures that the last 45 water molecules remain even with an initial temperature of 700 K. Data for an initial temperature of 500 K are shown in Figure S2 in the Supporting Information. Similar results were obtained with no field applied but with a surrounding gas at 298 K; over a dozen 2 ns trajectories (Table 1), only 02 molecules evaporated. Clearly, the presence of gas alone is not sufficient to drive solvent evaporation.
Our final control system is the case where no gas is present but an electric field is applied. In 4 of the 16 runs, collisions of the cluster with a few evaporated water molecules still caused the cluster to lose most of its water during the course of a 4 ns trajectory. However, the large variability in evaporation and collision probabilities makes any quantitative analysis of this case exceedingly difficult. Nevertheless this phenomenon shows that the addition of gas may not be absolutely necessary for collision induced solvent loss to occur, so long as sufficient molecular debris from the ESI process is present. Consider now what happens when both an electric field and a surrounding gas are present. In Figure 1 we display some snapshots from one of these simulations; an animation of the full trajectory may be viewed in the Supporting Information. As the electric field accelerates the cluster, after some time we see that it begins to collide with gas molecules and at some point to lose water molecules. A collision event is shown to highlight the connection between collisions with gas and ejection of solvent. From a detailed analysis of collisions at P = 0.5 atm and |E| = 0.01 V/nm, we find that approximately 40% of collisions lead to the loss of solvent molecules. A histogram of these collision outcomes is shown in Figure S3 in the Supporting Information. In 25% of all collisions there is an immediate (within 5 ps) loss of solvent which we attribute directly to the impact of a gas molecule. In all, roughly 49% of water molecules leave the cluster within 5 ps of a collision and 33% more than 5 ps after a collision, with the remaining 18% leaving the cluster via evaporation. In Figure 2, we plot the cluster size versus the time elapsed after activating the electric field. Decay times are longer for the lower field and these results are provided in Figure S4 in the Supporting Information. From our simulations the average time for the cluster to start losing H2O is fairly insensitive to the gas pressure up to P = 1 atm. At the higher pressure of 2 atm, we see that solvent loss begins somewhat sooner. The time elapsed until solvent loss is initiated shows a clear dependence on the initial cluster mass m and charge q and the field strength |E|, suggesting that the important factor is that the velocity of the cluster vbreak must be fast enough for collisions with gas to impart sufficient energy to eject one or more water molecules from the cluster. 8374
dx.doi.org/10.1021/ac202103p |Anal. Chem. 2011, 83, 8372–8376
Analytical Chemistry
LETTER
Assuming this to be true, we can easily derive some useful equations, Δv v ðtÞ B v ð0Þ FB ¼ qEB ¼ maB ¼ m B ¼ m B Δt t t0 ¼ mvBðtÞ=t
ð1Þ
where FB is the force on the cluster, q is the charge of the cluster, a is the resultant acceleration (assumed constant), νB(t) is the B velocity, and t is the time elapsed since application of the electric field at t0. From here we obtain tbreak ¼ vbreak
m qjEj
ð2Þ
where tbreak is the time at which a cluster is expected to begin to lose water molecules. Equation 2 shows that if vbreak is constant, a plot of tbreak versus m/q|E| should have a constant slope from which vbreak can be determined. Such a plot is shown in Figure 3. From the slope we calculate that vbreak ≈ 1600 m/s. This may seem to be a large velocity, but it is within a factor of 3 of the average velocity of a water molecule in an ideal gas at standard conditions. Moreover, it is known that ion velocities in the drift tube of a mass spectrometer may reach as high as 50 000 m/s or more.33 Results with the Ca2+ clusters show no appreciable difference compared with the Na+ clusters, once the differences in charge and mass are accounted for. Apparently the only effect of increasing the charge on the ion is to accelerate the cluster more quickly, there is no measurable influence of the increased binding energy between water and the ion on vbreak. The estimate of eq 2 assumes that the only force acting on the cluster is the electric field and neglects the influence of friction with the gas; as such it may be considered as an upper bound on the cluster velocity. To get the exact values of vbreak we may simply examine the trajectories and determine the velocity of the cluster at which solvent loss begins (results not shown). These simulation results show that eq 2 is a good estimate for the low pressures, but considerably lower cluster velocity is sufficient to cause solvent loss at high pressure, down to 1000 m/s or less at P = 2 atm of gas. Apparently there are two competing effects at higher pressure; more collisions occur, therefore the probability of solvent loss increases, however the additional collisions also reduce the cluster velocity compared to lower pressures. In the end, solvent loss occurs at nearly the same time, but at lower velocity, and it takes longer to completely desolvate the ion. Another possible factor affecting the high-pressure results is that multiple nearly simultaneous collisions which collectively contribute enough kinetic energy to break hydrogen bonds may occur more often. Our results may now be extrapolated to the real experimental situation. If vbreak is still treated as a constant, then for a typical field strength in an ESI apparatus of 1 kV/mm, from eq 2 we find for the small nanoclusters we study that tbreak ≈ 5 ns or, alternatively, a flight length from the point of single-ion cluster formation until a cluster break up of ≈ 2.5 μm. Both of these numbers fit well within the physical parameters of actual ESI experiments. The inclusion of gas-cluster collisions as an additional mechanism at play in ESI has several interesting implications. For example, only fairly volatile solvents are generally utilized in ESI, due to the lower likelihood of a nonvolatile solvent evaporating.5
Gas-cluster collisions should still be able to remove a nonvolatile solvent molecule in a reasonable time frame, suggesting that at least in this final stage of ESI, nonvolatile solvents could be considered. Generally in ESI experiments, nonvolatile solvents are avoided due to their deleterious effects on the mass spectrometer apparatus; however, some experimentation has taken place using DMSO and DMF to solvate hydrophobic compounds.34 More intriguing in our estimation is the possibility of using a heavier gas (e.g., Xe, SF6) as the sheath gas. The heavier the gas molecule, the lower the critical velocity required to initiate solvent loss via collisions and the less time and flight distance needed for the cluster to lose solvent. Despite possible complications due to condensation of heavier gases,22 we plan to consider this effect in further simulations. Different curtain gases have been considered experimentally,22 but there the focus was on the fragmentation of complex ions due to collisions and not on solvent loss. We have demonstrated via classical simulations how collisions between charged clusters accelerated by an electric field and a sheath gas can remove the water from the cluster. We have made quantitative estimates of the cluster velocity required for this collision induced solvent loss to occur, and we propose that this mechanism should be playing a key role in producing bare ions in the final stage of real ESI experiments. In the previous literature, the sheath gas has been assumed to have only rather minor effects, for example, in maintaining the temperature of the solution droplets as they evaporate.1,35,36 To the best of our knowledge, our work is the first time gas-cluster collisions have been suggested to play such an important role in desolvation in ESI.
’ ASSOCIATED CONTENT
bS
Supporting Information. Further details on simulation methods, supplementary Figures S1S4, and a movie of one of our simulation trajectories. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: +1 (0)613 5332651. Fax: +1 (0)613 5336669.
’ ACKNOWLEDGMENT The authors thank Richard Oleschuk and Graham Gibson for helpful discussions of ESI experiments, NSERC for funding, and Compute Canada, specifically HPCVL and RQCHP, for computing resources. ’ REFERENCES (1) Dole, M.; Mack, L. L.; Hines, R. L.; Mobley, R. C.; Ferguson, L. D.; Alice, M. B. J. Chem. Phys. 1968, 49, 2240–2249. (2) Iribarne, J. V.; Thomson, B. A. J. Chem. Phys. 1976, 64, 2287–2294. (3) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64–71. (4) Taylor, G. Proc. R. Soc. A 1964, 280, 383. (5) Kebarle, P.; Tang, L. Anal. Chem. 1993, 65, 972–986. (6) Kebarle, P. J. Mass Spectrom. 2000, 35, 804–817. (7) Rayleigh, L. Philos. Mag. Ser. 5 1882, 14, 184–186. (8) Thomson, B. A.; Iribarne, J. V. J. Chem. Phys. 1979, 71, 4451–4463. 8375
dx.doi.org/10.1021/ac202103p |Anal. Chem. 2011, 83, 8372–8376
Analytical Chemistry
LETTER
(9) Fernandez de la Mora, J. Anal. Chim. Acta 2000, 406, 93–104. (10) Luedtke, W. D.; Landmann, U.; Chiu, Y.-H.; Levandier, D. J.; Dressler, R. A.; Sok, S.; Gordon, M. S. J. Phys. Chem. A 2008, 112, 9628–9649. (11) Daub, C. D.; Bratko, D.; Leung, K.; Luzar, A. J. Phys. Chem. C 2007, 111, 505–509. (12) Cramer, T.; Zerbetto, F.; García, R. Langmuir 2008, 24, 6116–6120. (13) Ahadi, E.; Konermann, L. J. Phys. Chem. B 2009, 113, 7071–7080. (14) Ahadi, E.; Konermann, L. J. Am. Chem. Soc. 2010, 132, 11270–11277. (15) Consta, S.; Maier, K. R.; Novak, W. J. Chem. Phys. 2003, 119, 10125–10132. (16) Znamenskiy, V.; Marginean, I.; Vertes, A. J. Phys. Chem. A 2003, 107, 7406–7412. (17) Consta, S. J. Phys. Chem. B 2010, 114, 5263–5268. (18) Ahadi, E.; Konermann, L. J. Am. Chem. Soc. 2011, 133, 9354–9363. (19) Jagoda-Cwiklik, B.; Jungwirth, P.; Rulisek, L.; Milko, P.; Roithova, J.; Lemaire, J.; Maitre, P.; Ortega, J. M.; Schr€oder, D. ChemPhysChem 2007, 8, 1629–1639. (20) McQuinn, K.; Hof, F.; McIndoe, J. S. Chem. Commun. 2007, 4099–4101. (21) Smith, R. D.; Loo, J. A.; Barinaga, C. J.; Edmonds, C. G.; Udseth, H. R. J. Am. Soc. Mass Spectrom. 1990, 1, 53–65. (22) Schneider, B. B.; Douglas, D. J.; Chen, D. D. Y. Rapid Commun. Mass Spectrom. 2001, 15, 249–257. (23) Levitt, M.; Hirshberg, M.; Sharon, R.; Landig, K. E. J. Phys. Chem. B 1997, 101, 5051–5061. (24) Bratko, D.; Daub, C. D.; Luzar, A. Phys. Chem. Chem. Phys. 2008, 10, 6807–6813. (25) Yang, K.-L.; Yiacoumi, S.; Tsouris, C. J. Chem. Phys. 2002, 117, 337–345. (26) Svishchev, I. M.; Kusalik, P. G. J. Am. Chem. Soc. 1996, 118, 649–654. (27) Fenn, J. B.; Rosell, J.; Meng, C. K. J. Am. Soc. Mass Spectrom. 1997, 8, 1147. (28) Smith, D. E.; Dang, L. X. J. Chem. Phys. 1994, 100, 3757. (29) Dang, L. X.; Smith, D. E. J. Chem. Phys. 1995, 102, 3483–3484. (30) Rahman, A. Phys. Rev. 1964, 136, A405–A411. (31) Soper, A. K.; Castner, E. W.; Luzar, A. Biophys. Chem. 2003, 105, 649–666. (32) Caleman, C.; van der Spoel, D. Phys. Chem. Chem. Phys. 2007, 9, 5105–5111. (33) Downard, K. Mass Spectrometry: A Foundation Course; RSC Publishing: Cambridge, U.K., 2004. (34) Szabo, P. T.; Kele, Z. Rapid Commun. Mass Spectrom. 2001, 15, 2415–2419. (35) Marginean, I.; Page, J. S.; Kelly, R. T.; Tang, K.; Smith, R. D. Appl. Phys. Lett. 2009, 95, 184103. (36) Chen, L. C.; Mandal, M. K.; Hiraoka, K. J. Am. Soc. Mass Spectrom. 2011, 22, 539–544.
8376
dx.doi.org/10.1021/ac202103p |Anal. Chem. 2011, 83, 8372–8376
