molecular dynamics simulations of the electrospray process: formation of nacl clusters via the...

Molecular Dynamics Simulations of the Electrospray Process:Formation of NaCl Clusters via the Charged Residue MechanismLars Konermann,* Robert G. McAllister, and Haidy Metwally

Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada

*S Supporting Information

ABSTRACT: Electrospray ionization (ESI) produces desol-vated ions from solution phase analytes for mass spectrometricdetection. The final steps of gas phase ion formation fromnanometer-sized solvent droplets remain a matter of debate.According to the ion evaporation model (IEM), analytes areejected from the droplet surface via field emission, whereas thecharged residue model (CRM) envisions that ions are releasedupon droplet evaporation to dryness. Exposure of saltsolutions to ESI conditions produces a range of cluster ions.Despite the rich literature on these systems, it is still unclear ifthese salt clusters form via the CRM or the IEM. The currentstudy explores the formation of NanClm

(n−m)+ clusters fromaqueous sodium chloride solution under positive and negative polarity conditions. Molecular dynamics (MD) methods are usedfor simulating the temporal evolution of charged NaCl-containing water droplets. A trajectory stitching approach is developed forcontinuously removing evaporated moieties from the simulation, thereby dramatically reducing computational cost. In addition,this procedure ensures adequate temperature control and eliminates evaporative cooling that would otherwise slow down theprocess. Continuous water evaporation leads to progressive droplet shrinkage, while the emission of solvated single ions ensuresthat the system remains at ca. 90% of the Rayleigh limit. Early during the process all ions in the droplet behave as freely dissolvedspecies, but after a few nanoseconds at 370 K the systems gradually morph into amorphous wet salt aggregates. Ultimately, freeNanClm

(n−m)+ clusters form as the last solvent molecules evaporate. Our data therefore provide direct evidence that sodiumchloride cluster formation during ESI proceeds via the CRM. The IEM nonetheless plays an ancillary role, as it allows the systemto shed charge (mostly in the form of hydrated Na+ or Cl−) during droplet shrinkage. It appears that this study marks the firstsuccessful MD simulation of complete CRM processes.

■ INTRODUCTION

Electrospray ionization (ESI) mass spectrometry (MS) is awidely used analytical technique that covers a diverse range ofapplications.1−3 The ESI process starts with analyte solutionthat is passed through a conductive capillary to which highvoltage has been applied. Redox processes lead to the buildupof positive or negative charge (depending on the polarity used)in the solution as it passes through the capillary.4 This chargeaccumulation induces the formation of a Taylor cone at thecapillary outlet from which a plume of highly charged dropletsis emitted.5 These ESI droplets are exposed to a heated gasenvironment where they undergo rapid evaporation andCoulombically driven jet fission.6 Nanometer-sized progenydroplets generated by these events release analyte ions into thegas phase.5,7 The ions are then sampled by an atmosphericpressure-to-vacuum interface where collisional activationpromotes the final desolvation steps.6,8,9 Ultimately, iondetection by a suitable analyzer produces electrical signalsthat are converted into a mass spectrum.10

The mechanism of gas phase ion formation from chargednanodroplets continues to be a controversial topic. Manyresearchers believe that small analyte ions undergo field

emission from the droplet surface, as envisioned by the ionevaporation model (IEM).5,11−14 Large globular analytes suchas natively folded proteins likely follow the charged residuemodel (CRM) where free ions are formed upon dropletevaporation to dryness.5,13,15−19 However, the distinction ofIEM vs CEM on the basis of analyte size is not universallyaccepted,20,21 and it has been proposed that the IEM can alsoapply to large analytes.22−25 Hybrid models involving elementsof both the CRM and the IEM have been put forward aswell.26,27 In addition, recent data support a chain ejectionmodel (CEM) for disordered polymers such as denaturedproteins.28−30 During the CEM macromolecular chains getexpelled from the droplet by electrostatic and solvophobiceffects.13

Infusion of salt solutions into an ESI source generatescharged clusters such as NanClm

(n−m)+.31−35 It seems possiblethat these species are IEM products, i.e., that cations and anionsassociate within the shrinking droplet prior to cluster ejection

Received: July 29, 2014Revised: September 15, 2014Published: September 22, 2014

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from the surface.36,37 Alternatively, salt clusters might form viathe CRM, following a scenario where ESI droplets evaporate todryness.38−40 An unambiguous differentiation between the twomechanisms on the basis of existing measurements is difficult,considering that the process may depend on the type of salt andthe cluster stoichiometry.41−46 Hence, the mechanism of saltcluster formation during ESI remains an open question.Molecular dynamics (MD) simulations have become an

important tool for probing mechanistic aspects of ESI13,28,47−54

and related phenomena.55−60 An obvious approach forexamining the salt cluster formation process, therefore, wouldbe to model the temporal evolution of salt-containing ESIdroplets. Simulations of this type could help settle the long-standing dispute of IEM vs CRM. Surprisingly, attempts topursue such a strategy have been relatively scarce.MD simulations of the ESI process have demonstrated that

the production of small solvated ions such as Na+ occurs inaccordance with the IEM.13,47,49,51,52 Simulation studies dealingwith CRM events have been much more difficult.13 Onechallenge is related to the time scale of these processes.Whereas IEM events can readily be observed in simulationwindows of less than 1 ns,13,47,49,51,52 droplet evaporation todryness (the hallmark of the CRM) takes orders of magnitudelonger.13 Also, MD simulations on droplet systems are oftenconducted at constant energy where evaporative coolingcontinuously reduces the temperature, thereby bringing solventevaporation to a halt.13,50,51 In ESI mass spectrometers thiscooling is countered by heated background gas.6,61 A recentstudy simulated such a situation directly, by placing hydratedmetal ions in a bath of Ar atoms. Complete loss of the ∼20bound waters took place within ca. 2 ns.54 That study54 markedthe first time that simulations produced a solvent-free analyteion via solvent evaporation. Most ESI mechanistic studies,however, require investigations on droplets comprisingthousands of solvent molecules. The use of an explicit gasenvironment for systems of this size severely increasescomputational cost.62 A more effective approach involves theuse of a thermostat.52,62,63 Nonetheless, to our knowledge thereare no reports in the literature that describe simulations of ESInanodroplet evaporation to dryness. In other words, MDstrategies that are capable of dealing with CRM scenarios haveyet to be developed.The current work devises an approach to overcome the

difficulties outlined above. We conduct temperature-stabilizedMD simulations that culminate in the formation of dryNanClm

(n−m)+ clusters from salt-containing droplets containing∼2420 water molecules. Rapid solvent evaporation from thesedroplets is accompanied by the shedding of charge carriers,mostly in the form of single solvated ions. These chargeemission events are consistent with the IEM. The majority ofthe ions, however, remain contained within the shrinkingdroplet to the very end. Desolvated NanClm

(n−m)+ aggregatesform as the last water molecules evaporate, implying that theCRM represents the dominant mechanism of salt clusterformation.

■ EXPERIMENTAL AND SIMULATION METHODSMass Spectrometry. ESI mass spectra were acquired on a

Synapt G2 time-of-flight instrument (Waters, Milford, MA). 10mM NaCl in water was infused into a standard Z-sprayinterface at a flow rate of 5 μL min−1 using a syringe pump. ESIwas conducted using a desolvation gas (N2) temperature of 473K and a source block temperature of 353 K. The sample and

extraction cones were set to 45 and 4 V, respectively, and theESI capillary was held at 3 kV. Measurements were conductedin positive and in negative polarity. Isotope models weregenerated using the MassLynx software package provided bythe instrument manufacturer.

MD Simulations: General Aspects. Simulations wereconducted on desktop Linux computers using Gromacs 4.6.5 indouble precision for leapfrog integration of Newton’sequations.64,65 Unless noted otherwise, the Amber99sb-ILDNforce field66 was used. A number of MD runs were alsoconducted using the OPLS all-atom (OPLS/AA) force field.67

All simulations employed the widely used three-site TIP3Pwater model.68 Earlier work has demonstrated that the choiceof water model is not critical for droplet simulations, as variousframeworks ranging from simple SPC to five-site TIP5P modelsall exhibit very similar evaporation behavior.51 The use ofpolarizable models in droplet simulations increases computingtimes by a factor of ∼30, and problems with energyconservation have been noted.69 Similar to recent studiesfrom other laboratories,28,47,48,51−57 we therefore restricted thesimulations of the current work to nonpolarizable models.H2O bond distances and angles were constrained using the

SETTLE algorithm.70 Simulations were performed in a vacuumenvironment without cutoffs for electrostatic or Lennard-Jonesinteractions.51 ESI droplets were generated by carving spheresof the desired radius from a pre-equilibrated bulk watercoordinate file. Ions were incorporated into these systems byreplacing random water molecules with Na+ or Cl−. Thedroplets were initially subjected to steepest descent energyminimization using an integration step size of Δt = 0.5 fs.51

Equilibration was conducted with coupling to a velocityrescaling thermostat which employs a modified Berendsenscheme.71 Initial equilibration was performed at 1 K for 10 pswith Δt = 0.5 fs. For all the following steps Δt was increased to1 fs. Equilibration was continued at 150 K for 20 ps andsubsequently at 250 K for 70 ps.52 Production runs were thenstarted with velocity rescaling71 at 370 K, unless notedotherwise. Additional details regarding temperature couplingare provided below. All simulations were conducted withoutbarostat.Both center-of-mass translation and rotation of the system

were eliminated throughout the simulation. The recoilassociated with particle ejection can nonetheless causetranslation and rotation of the residual droplet.69 This effectis particularly pronounced for small droplets, where ejectionevents carry away a relatively high percentage of the systemmass.

Temperature Stabilization by Trajectory Stitching. Asnoted above, a common issue with ESI simulations is theoccurrence of droplet freezing due to the loss of kinetic energybrought about by solvent evaporation.13,50,51 In 2 ns test runsthis phenomenon was explored using droplets with a 2 nmradius consisting of 1081 H2O, 17 Na+, and 8 Cl− at an initialtemperature of 370 K (Figure 1). Simulations conducted atconstant energy (i.e., without thermostat) resulted in a droplettemperature decrease from 370 to 293 K. The droplets lost∼240 water molecules during the 2 ns window. Most of theseevaporation events occurred early during the simulations, whenthe temperature was still high. This behavior is consistent withearlier observations.51

Next, it was attempted to stabilize the droplet temperatureusing a velocity rescaling thermostat.71 The coupling strengthbetween system and heat bath is governed by a user-defined

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time constant τT. Shorter time constants correspond to tightercoupling. τT = 0.1 ps is a common choice,51,52 but we alsotested values of 1 and 0.01 ps. Velocity rescaling stabilized thetemperature around 370 K, when averaged over the entire system(i.e., the droplet with all of the evaporated molecules, Figure1a). However, when considering only the droplet, a considerabletemperature decrease persisted under thermostated conditions.The extent of droplet cooling decreased with decreasing τT(Figure 1a). This trend was accompanied by an increasednumber of evaporated water molecules (Figure 1b). Nonethe-less, it is apparent from these data that even with very tightcoupling the thermostat was not capable of eliminatingevaporative droplet cooling. Similar observations were madewhen using Berendsen72 and Nose−Hoover73 coupling (datanot shown). The Andersen thermostat74 is not implemented inGromacs 4.6.5 and was therefore not tested.Evidently, the persistence of evaporative droplet cooling

renders simulations of CRM processes difficult under any of theconditions in Figure 1. We therefore settled on a schemewhereby long MD trajectories were “stitched together” frommany shorter run segments. The idea behind this strategy isthat evaporative cooling remains almost negligible for shortsimulation windows (e.g., 100 ps). After each of these shortruns all velocities were reassigned at random from a Maxwelldistribution, and the system was recoupled to the velocityrescaling thermostat71 at the desired temperature. In essence,

this strategy is comparable to a crude version of the Andersenthermostat.74 This stitching approach provided temperaturestabilization of the droplet over extended (150 ns) MDtrajectories. Supporting Information Figure S1 demonstratesthat temperature deviations of no more than ±5% wereobserved for the first 4 ns. For later time points the temperaturestandard deviation increases, reflecting the instantaneouskinetic energy fluctuations of the rapidly shrinking system.Nonetheless, Supporting Information Figure S1 clearlyillustrates that trajectory stitching virtually eliminates problemsassociated with evaporative cooling.A second key advantage associated with trajectory stitching is

that evaporated particles can be removed from the system aftercompletion of each MD segment. The continuously decreasingsystem size for consecutive segments dramatically speeds up thesimulations. Complete 150 nm trajectories for 2.6 nm dropletscould be run overnight on a regular quad-core CPU desktopmachine without GPU acceleration. Particles were consideredto be evaporated when they were more than 7 nm away fromthe main droplet center of mass. For trajectory segments withdroplet fission the algorithm was designed to ensure transfer ofthe larger fragment into the subsequent segment. Trajectorystitching was implemented by using a script that makesalternating calls to Gromacs and to a custom-designed dropletcleanup program.

Temperature Profile. The heated gas environment in theion source of typical ESI mass spectrometers promotes solventevaporation and analyte desolvation.6 For mimicking theseconditions the simulations discussed below were run at 370 Kfor the first 50 ns. The tendency of the system to suffer fromevaporative cooling is most pronounced early during theprocess when most of the solvent is lost (see below for details).The need for adequate temperature stabilization thusnecessitated the use of very short (100 ps) MD segments forthe first 4 ns. This was followed by 500 ps segments up to t =10 ns and then 5 ns segments up to t = 50 ns. Electrosprayedanalytes experience collisional activation as they traverse thesampling region and ion guides of the mass spectrometer.Experiments have shown that this process can raise the analytetemperature up to a range of 450−800 K.9,75 We thereforefollowed the initial 50 ns/370 K regime of the simulations by50 ns at 450 K and then another 50 ns at 700 K (both in 5 nssegments), for an overall simulation window of 150 ns(Supporting Information Figure S1).

Droplet Composition. ESI simulations were conducted onNaCl-containing water droplets with an initial radius of 2.6 nmwhich comprised ∼2420 water molecules. Na+ and Cl− ionswere added to these systems at two different concentrations,0.29 M (“low salt”) and 0.38 M (“high salt”), corresponding to13 and 17 Na+/Cl− pairs, respectively. The bulk analytesolutions employed in typical ESI-MS experiments are lessconcentrated. However, the systems simulated in this work aremeant to represent late progeny droplets.5 Solvent evaporationtaking place during the production of these droplets can cause a102−103-fold increase in solute concentration.76 The dropletcomposition used in our simulations, therefore, is in line withexperimental conditions.The upper limit of net droplet charge is given by the Rayleigh

equation5,77

π ε γ=ze

R8

R 03

(1)

Figure 1. Evaporation behavior of positively charged droplets with aninitial radius of 2 nm (1081 H2O, 17 Na+, and 8 Cl−) at an initialtemperature of 370 K. MD simulations were run under fourconditions: without thermostat and with velocity rescaling thermostatusing τT = 1, 0.1, and 0.01 ps, as noted along the x-axis. (a)Temperature of the droplet (black) and temperature of the entiresystem (including evaporated ions and waters, gray) after 2 ns. (b)Cumulative number of evaporated water molecules after 2 ns.



where zR is the number of elementary charges e, R is the dropletradius, and ε0 is the vacuum permittivity. The surface tension γis 0.058 91 N m−1 for water at its boiling point.78 For R = 2.6nm eq 1 predicts a net charge of 15. However, the dropletevolution during ESI typically takes place slightly below theRayleigh limit.5,79−81 For our simulations we therefore chose aninitial charge of 13+ (13− in negative polarity), whichcorresponds to 87% of zR. This excess charge was achievedby incorporation of additional Na+ or Cl−. In ESI experimentsthe excess droplet charge may comprise other species such asH+, OH−, or NH4

+,4,5 but there are also conditions that resultin the ternary H2O/Na

+/Cl− droplet composition used in oursimulations.82 Each of the MD runs (positive and negative, withhigh and low salt) was repeated five times with different initialdroplet structures and different random seeds for velocityassignments and temperature rescaling. Images were renderedin Pymol (Schrodinger), and movies were generated usingVMD.83

■ RESULTS AND DISCUSSION

ESI-MS of NaCl Clusters. Consistent with previousreports,31−35 infusion of 10 mM aqueous NaCl solution intoan ESI source produced a wide range of salt clusters. Whenusing positive polarity, the mass spectrum is dominated bysingly charged Na(n+1)Cln

+ species (Figure 2a). The mostintense signal corresponds to the magic number clusterNa14Cl13

+, which has a cubic 3 × 3 × 3 structure.31,35 Otherprominent magic number peaks include Na5Cl4

+ (planar 3 × 3

× 1) as well as Na23Cl22+ (cuboid 3 × 3 × 5, Figure 2a).31,35

The main peaks observed in negative polarity ESI-MScorrespond to singly charged NanCl(n+1)

− clusters (Figure 2b).Interestingly, the dominance of magic number species is lesspronounced than for positive polarity; e.g., the 3 × 3 × 3Na13Cl14

− cluster41 does not stand out very strongly in Figure2b. The insets in Figure 2 reveal the fine structure of the MSsignals, reflecting the 35Cl/37Cl isotope heterogeneity. Closeinspection of the data also reveals the presence of doublycharged clusters with lower abundance (red distributions inFigure 2).33

Positive Droplet Simulations. We conducted a first set ofESI simulations on NaCl-containing water droplets with excesspositive charge under low salt conditions (26 Na+ and 13 Cl−).Snapshots taken from a typical 150 ns trajectory are shown inFigure 3. The droplet initially maintains an approximatelyspherical shape, but with undulations and short-livedprotrusions (Figure 3, 100 ps). Solvent loss predominantlyoccurs by evaporation of single H2O molecules. During the firstfew nanoseconds this is accompanied by the ejection ofsolvated Na+ ions, with transition states that tend to exhibitconnecting water filaments (Figure 3, 116 ps; SupportingInformation Movie S1). Ions within the droplet initially have notendency to associate with one another; instead they act asfreely dissolved species that are surrounded by their individualhydration shells. With ongoing water evaporation, however,Na+ and Cl− begin to experience encounters. This leads todisordered salt aggregates that undergo rapid dissociation/association within the droplet (Figure 3, 1.4 ns and 3.3 ns).Over time, these ion−ion contacts become more permanent(Figure 3, 4 ns and 4.5 ns). The partially hydrated t = 4.5 nsstructure of Figure 3 has the ion composition Na15Cl13

2+.Emission of one final Na+ produces a cubic Na14Cl13

+ clusterwhich retains bound waters at its sodium corner points (Figure3, 7.5 ns; Supporting Information Movie S2). These remainingwaters are lost within tens of nanoseconds, ultimatelyproducing the desolvated 3 × 3 × 3 species (Figure 3, 150ns) that dominates the mass spectrum of Figure 2a. Althoughthis salt cube undergoes vibrational motions, it preserves itsoverall shape over extended time periods (SupportingInformation Movie S3). Five independent low salt MDsimulations all generated the same Na14Cl13

+ product. Summarystatistics of these runs are provided in Figure 4a,b. Results verysimilar to these Amber99sb-ILDN data were obtained when thesimulations were repeated using the OPLS/AA force field(Supporting Information Figure S2).As a next step we examined the behavior of high salt droplets,

with 30 Na+ and 17 Cl−. The temporal evolution of thesesystems (Figure 4c,d) was similar to that described above, withthe exception that droplet evaporation produced slightly largerNa19Cl17

2+ clusters, comprising a 3 × 3 × 3 core that isdecorated with a 3 × 3 × 1 layer (Figure 5). In contrast to themagic number product generated in low salt simulations(Figure 3, 150 ns), desolvated Na19Cl17

2+ exhibits frequentstructural transitions where two [Na+ Cl− Na+] rows competefor inclusion at one of the cluster core edges (SupportingInformation Movie S4). This behavior is consistent with reportsof facile rearrangements in NaCl nanoclusters.41 None of thepositive droplet simulations in Figure 4 showed any loss of Cl−,as seen from the horizontal green profiles in panels b and d.Thus, the number of Cl− in the positive cluster simulationproducts was determined by the chloride content of the initialnanodroplet.

Figure 2. Mass spectra obtained by electrospraying aqueous NaClsolution using (a) positive polarity and (b) negative polarity ESI.Individual peaks are annotated according to the dominant ion signals.The insets show close-ups of the Na14Cl13

+ and Na13Cl14− magic

number signals along with the corresponding theoretical isotopemodels (blue dots). Red dots refer to the isotope models of doublycharged clusters.



Returning to the key question of this work, we can nowexamine what these MD simulations reveal about themechanism of salt cluster formation during ESI. The ejectionof single solvated Na+ during droplet shrinkage is consistentwith the IEM.5,11−14 However, the positive droplet simulationsdid not show a single instance of cluster desorption from thedroplet surface. Instead, NanClm

(n−m)+ species were alwaysreleased upon droplet evaporation to dryness; i.e., these clustersare CRM products.5,13,38−40

Negative Droplet Simulations. We complemented thesimulations of the preceding section with studies on negativelycharged droplets, both under low salt (13 Na+, 26 Cl−) andhigh salt conditions (17 Na+, 30 Cl−). The summary statisticsof Figure 4e−h reveal that the formation of negative saltclusters generally proceeds along the same lines as discussedabove for positive polarity. In other words, large negativeclusters are CRM products as well.Despite the overall similarities in their behavior, there are

some subtle differences in the temporal evolution of positiveand negative droplets. No solvent persists under negativeconditions for t > 10 ns (Figure 4d,g), whereas a few residualwaters remain attached to positively charged clusters for up to∼60 ns (Figure 4a,d). We attribute this difference to the largerionic radius of Cl− (1.81 Å), which implies a lower charge

density and thus a reduced desolvation enthalpy compared toNa+ (rion = 0.99 Å).78

IEM ejection of hydrated single ions is the dominantmechanism for shedding charge during droplet shrinkage inboth positive and negative polarity (Na+ in Figure 4b,d; Cl− inFigure 4f,h). For negatively charged systems the ejection ofNaCl2

− represents a second pathway for losing charge, althoughthis process is not very common. NaCl2

− can be released ashydrated species from shrinking droplets (Figure 6a, Support-ing Information Movie S5). This process bears strongsimilarities with the ejection of single ions and is thereforeclassified as an IEM event (compare t = 2.174 ns of Figure 6and t = 116 ps of Figure 3).5,11−14 Another mechanism forgenerating NaCl2

− is the fragmentation of desolvated saltclusters. Such fragmentation was only observed for highlycharged (3−) precursors, which undergo extensive structuraldistortions prior to dissociation (Figure 6b, SupportingInformation Movie S6). Both types of NaCl2

− productionevents occurred in 3 out of 10 trajectories. Figure 2b confirmsthat NaCl2

− is produced under experimental conditions.The possible occurrence of NaCl2

− loss in negative polaritysimulations translates into a range of product clusters afterdroplet evaporation to dryness. All of these products weredoubly charged. Na11Cl13

2−, Na12Cl142−, and Na13Cl15

2− were

Figure 3. MD simulation snapshots for the evaporation of a positively charged water droplet with an initial radius of 2.6 nm, containing 2424 H2O(oxygen: red; hydrogen: white), 26 Na+ (blue), and 13 Cl− (green). The time points corresponding to individual frames are indicated. Note that thezoom level increases as time proceeds.



formed under low salt conditions, whereas Na15Cl172−,

Na16Cl182−, and Na17Cl19

2− were seen in high salt simulations.Examples of these product species are depicted in Figure 6 for t= 150 ns. Na12Cl14

2− (Figure 6a) resembles a 3 × 3 × 3 magicnumber cube, but it lacks a central Na+ and therefore representsa hollow structure. Na17Cl19

2− (Figure 6d) is analogous to theNa19Cl17

2+ cluster of Figure 5. In summary, all of the large t =150 ns salt clusters generated from negative droplets representCRM products. Only the “minicluster” NaCl2

− can be releasedfrom shrinking droplets via the IEM, or it can be formed byfragmentation of larger precursor clusters.Temporal Evolution of Droplet Charge. Phase Doppler

interferometry and other experimental techniques revealed thatESI droplets remain slightly below the Rayleigh limitthroughout their life cycle.5,79−81 Hence, the initial dropletcharge used for our 2.6 nm droplets was chosen to be 0.87zR(eq 1). During the simulations droplets shrink because of waterevaporation, while at the same time charge is lost via IEM

events (Figure 4). It is of interest to explore how this ongoingcharge reduction compares to the Rayleigh limit. We will focuson the initial 4 ns window during which the droplets evaporatedown to ∼100 water molecules. For smaller systems it becomesunclear if using a bulk water surface tension in eq 1 remainsadequate.69,84 Rayleigh’s theory applies to spherical systems,5,77

whereas ESI droplets undergo frequent distortions intononspherical shapes. We therefore determined an “effective”droplet radius for each time point, corresponding to that of asphere with the equivalent number of H2O, Na

+, and Cl−.When conducting this analysis, it is seen that the relativedroplet charge remains close to its initial value of 0.87zRthroughout the shrinkage process for all conditions studied(Figure 7). These data compare favorably to experiments,where excursions to z ≫ zR or z ≪ zR are not observed.

5,79−81

■ CONCLUSIONSFor many years it has been unclear whether the ESI-mediatedproduction of salt clusters proceeds according to the CRM orthe IEM.36−46 The current work explored this questions viaMD simulations, using a trajectory stitching approach thatprovides adequate temperature control while at the same timedramatically reducing computational cost. Simulations on thetemporal evolution of salt-containing ESI droplets providedirect evidence that NanClm

(n−m)+ clusters are CRM products.Water evaporation causes gradual droplet shrinkage withoutejection of large salt clusters from the droplet surface. Instead,the final NanClm

(n−m)+ products represent charged residues thatare left behind as droplets evaporate to dryness.Despite the unambiguous conclusion that NanClm

(n−m)+

clusters are CRM products, the IEM still plays a role duringthe formation of these species. Shrinkage of ESI nanodroplets isaccompanied by the ejection of hydrated charge carriers such asNa+ or Cl− (also NaCl2

− on rare occasions). This type ofcharge emission represents an IEM process.5,11−14 Overall, it istherefore concluded that NanClm

(n−m)+ clusters are produced viathe CRM but that the IEM plays an ancillary role duringdroplet shrinkage. This finding is consistent with earlierproposals that were developed on the basis of experimentalobservations.26,27,85

It is gratifying that our simulations readily generate the 3 × 3× 3 magic number cluster that represents the dominant speciesin experimental positive polarity spectra (Figures 2a and 3).

Figure 4. Summary of evaporation kinetics for nanodroplets with aninitial radius of 2.6 nm (∼2420 H2O molecules). The individual panelsdisplay the number of water molecules and ions contained within thedroplet as a function of time for four different initial ion compositions:(a, b) positive droplet, “low salt” with 26 Na+ and 13 Cl−; (c, d)positive droplet, “high salt” with 30 Na+ and 17 Cl−; (e, f) negativedroplet, “low salt” with 13 Na+ and 26 Cl−; (g, h) negative droplet,“high salt” with 17 Na+ and 30 Cl−. Data for each condition wereaveraged over five MD runs using the Amber99sb-ILDN force field.Error bars represent standard deviations.

Figure 5. Na19Cl172+ cluster generated in MD simulations after

evaporation of a high salt droplet with an initial composition of 2416H2O, 30 Na

+, and 17 Cl−. The dotted square indicates the cubic 3 × 3× 3 cluster core.



Our simulations did not generate the corresponding 3 × 3 × 3species in negative ion mode, despite inoculating the initialdroplets with the proper number of Na+. It is intriguing tospeculate that this effect seen in our negative dropletsimulations is related to the low abundance of the 3 × 3 × 3product in the experimental spectrum of Figure 2b. This idea

notwithstanding, it would be unrealistic to presume that simpleMD simulations of the type conducted here can exactlyreproduce experimental mass spectra. Any attempts in thisdirection would require additional insights into the droplet sizedistribution within the ESI plume and the corresponding actualsalt concentrations. Also, some of the experimentally observedclusters may represent gas phase fragments of the initial CRMproducts.31,34 Doubly charged clusters are particularly prone tofragmentation which may help explain the prevalence of singlycharged species in Figure 2.33

In the future it will be interesting to extend the approachdeveloped here to other types of ESI-MS analytes. While thecurrent study provides strong evidence for the formation ofNanClm

(n−m)+ clusters via the CRM, our results do notnecessarily apply to other solutes that undergo ESI-mediatedclustering.41−46 It will also be exciting to test if the CRM holdsfor compact macromolecular analytes as often pre-dicted5,13,15−19 or if the ESI process for these species proceedsvia different pathways.20−25 Work in this direction is currentlyongoing in our laboratory.

■ ASSOCIATED CONTENT*S Supporting InformationFigures S1, S2 and Movies S1−S6. This material is available freeof charge via the Internet at http://pubs.acs.org.

Figure 6.MD snapshots taken at different times during the evaporation of negative droplets. (a) Low salt droplet with an initial composition of 2424H2O, 13 Na

+, and 26 Cl− during ejection of a solvated NaCl2− cluster. Also shown is the final Na12Cl14

2− product obtained in this run at t = 150 ns.(b) Data obtained during another low salt simulation, involving desolvated Na12Cl15

3− which ejects NaCl2− at t = 106 ns. The resulting Na11Cl13

2−

product is shown for t = 150 ns. The two bottom panels display additional examples of t = 150 clusters generated via evaporation of negative highsalt droplets. (c) Na16Cl18

2− and (d) Na17Cl192−. Coloring of elements is as in Figure 3.

Figure 7. Droplet charge z relative to the Rayleigh charge zR duringESI simulations under the four conditions of Figure 4. The timewindow refers to evaporation down to a droplet size of ∼100 watermolecules. Each data set represents an average of five MD runs.



http://pubs.acs.org

■ AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected] (L.K.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

Funding for this work was provided by the Natural Sciencesand Engineering Research Council of Canada (NSERC). Wethank David van der Spoel, Elio A. Cino, and Elias Ahadi forhelpful discussions.

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molecular dynamics simulations of the electrospray process: formation of nacl clusters via the...

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