In Situ Liquid Secondary Ion Mass Spectrometry: A Surprisingly Soft

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In Situ Liquid SIMS: A Surprisingly Soft Ionization Process for Investigation of Halide Ion Hydration Yanyan Zhang, Wenjuan Zeng, Liuqin Huang, Wen Liu, Endong Jia, Yao Zhao, Fuyi Wang, and Zihua Zhu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05804 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 6, 2019

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Analytical Chemistry

In Situ Liquid SIMS: A Surprisingly Soft Ionization Process for Investigation of Halide Ion Hydration Yanyan Zhang,a,b,c Wenjuan Zeng,a,c Liuqin Huang,b Wen Liu,b Endong Jia,d,c Yao Zhao,a Fuyi Wang,a,c,* and Zihua Zhub,* Beijing National Laboratory for Molecular Sciences, National Centre for Mass Spectrometry in Beijing, CAS Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. b Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99354, USA c University of Chinese Academy of Sciences, Beijing 100049, China d The Key Laboratory of Solar Thermal Energy and Photovoltaic System, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, China a

ABSTRACT: The understanding of ion solvation phenomenon is of significance due to their influences on many important chemical, biological and environmental processes. Mass spectrometry (MS) based methods have been used to investigate this topic with molecular insights. As ion-solvent interactions are weak, ionization processes should be as soft as possible in order to retain solvation structures. An in situ liquid secondary ion MS (SIMS) approach developed in our group has been recently utilized in investigations of Li ion solvation in non-aqueous solution and detected a series of solvated Li ions. As traditionally SIMS has long been recognized as a hard ionization process with strong damage occurring at the sputtering interface, it is very interesting to study further how soft in situ liquid SIMS can be. In this work, we used halide ion hydration as an example to compare the ionization performance of in situ liquid SIMS approach with regular electrospray ionization MS (ESI-MS). Results show that although ESI has been recognized as a soft ionization method, nearly no solvated halide ions were detected by ESI-MS analysis, which acquired only strong signals of salt ion clusters. In contrast, in liquid SIMS spectra, a series of obvious hydrated halide ion compositions could be observed. We further evaluated the hydration numbers of halide ions and revealed the effects of the ion size, charge density and polarizability on the hydration phenomenon. Our findings demonstrated that the in situ liquid SIMS approach is surprisingly soft, and expected to have very broad applications on investigation of various ion-solvent interactions and many other interesting chemical processes (e.g., the initial nucleation of nanoparticle formation) in liquid environment.

Ion solvation almost always occurs in either aqueous or nonaqueous media, determining the structure, properties, and intermolecular interactions of compositions in solutions, which would have a strong impact on the fundamental mechanisms and dynamics of related systems.1-3 As an important solvation phenomenon, halide anion hydration exerts significant effects on chemical, environmental, biological, and health-related processes.4-12 For example, the fluoride-water adduct plays an important role in stabilizing certain hemoproteins such as sperm whale myoglobin.13 To date, the investigation of halide ion hydration phenomenon has received widespread attention due to the great significance for the understanding of related processes. Besides, comparative valuation of hydration structures of halide ion series has been a paradigm for studying the effects of ion size, charge density and polarizability on the ion-solvent interactions. The elucidation of halide ion hydration is a complex picture which needs the combination of computational and experimental studies. Extensive computational studies including ab initio calculations14,15 and molecular dynamics (MD) simulation approaches16-20 have been devoted to studying

ion-solvent interactions by assessing the structural, energetic, and spectroscopic properties of halide-containing water clusters. Due to the employment of semi-empirical and approximation methods, great conflicts exist among the computational results. An important reason is lacking of suitable experimental methods to test computational results. For example, as computational studies of halide ion solvation are generally based on quantum calculations of small solvated ion structures17,18, it’s of high importance to experimentally test the quantum calculation results due to their further influences on the reliability of related MD simulations. To date, a number of experimental techniques, including nuclear magnetic resonance spectroscopy13, X-ray diffraction21, neutron diffraction22, photoelectron spectroscopy23 and infrared spectroscopy24 have been applied to elucidate ion solvation process. However, these techniques are used only for bulk phase analysis and it is difficult to test small solvated structures25. Moreover, most of them only provide indirect chemical shift information, which cannot be directly compared to computational results,24,26 and various assumptions have to be made about the character of the ion-solvent interactions25.

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In this regard, mass spectrometry possesses unique advantages of providing molecular information with more straightforward insights into small solvated ion structures. As ion-solvent interactions are much weaker than common chemical bonds, soft ionization processes are required in order to maintain the solvation structures for detection. Electrospray ionization mass spectrometry (ESI-MS) is generally considered as a soft ionization technique27-33, and has been recently employed in the investigation of Li+ ion solvation in non-aqueous lithium salt electrolytes.34-36 The results explicitly supported the preferential solvation of cyclic over acyclic carbonate molecules by Li+ ion in mixed solvents, and also provided molecular information of the coordination number of Li+ ion in its solvation sheath.36 Traditionally, secondary ion mass spectrometry (SIMS) has long been recognized as a hard ionization process with strong damage occurring at the sputtering interface. However, we recently developed an innovative in situ liquid SIMS approach37-41 and have successfully utilized it to evaluate ionsolvent interactions in three classic non-aqueous electrolytes in Li ion batteries42. We detected a series of solvated Li ions, which allowed us to compare the preferential solvation of Li ions and to examine the stable coordination number of Li ions with organic solvents. Apparently, the ionization process of in situ liquid SIMS is not hard as considered. Therefore, it is of great interest to evaluate how soft in situ liquid SIMS can be. To address this issue, in this work we evaluated the weak interactions of a series of halide ions (F-, Cl-, Br- and I-) with water solvents by utilizing both in situ liquid SIMS and ESIMS techniques for comparison. Interestingly, clear signals of solvated halide ions could be observed in liquid SIMS spectra; while a series of salt cluster ion peaks dominated the ESI-MS spectra with very weak solvated halide ion signals. Therefore, our results show a surprisingly soft ionization process in in situ liquid SIMS approach. In addition, we evaluated hydration numbers of the halide ion series in vacuum by using the in situ liquid SIMS approach, and revealed the effects of ion size, charge density and polarizability on the halide ion hydration. Our study demonstrated in situ liquid SIMS analysis is actually a soft ionization technique for liquid samples.

EXPERIMENTAL SECTION ESI-MS measurements. Negative ion electrospray ionization mass spectra of 2 mM KX (X=F, Cl, Br, I) aqueous solutions were all obtained on a Xevo G2 Q-TOF mass spectrometer (Waters, Manchester, UK). Typical source conditions were as follows: capillary voltage, 2.0 kV; sample cone, 40 V; extraction cone, 4.0 V; source temperature, 373 K; desolvation temperature, 523 K; desolvation gas (N2) flow rate, 600 L h−1. The mass spectra were acquired in the range of m/z 30-2000, except for that of KF solution in the range of 15-2000. MassLynx (ver. 4.1) software was used for all the analysis and post processing. Fabrication of the microfluidic device. We fabricated the high-vacuum compatible microfluidic device according to our previously reported method with minor adaptations.38,42 Briefly, a silicon frame (5.0 mm (L) × 5.0 mm (W) × 0.2 mm (H)) with a thin silicon nitride (SiN) membrane (100 nm in thickness) immobilized beneath it was sealed on top of the liquid chamber which was previously machined with a size of 3.0 mm (L) × 3.0 mm (W) × 0.3 mm (H) on a polyether ether ketone (PEEK)

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block. After introducing a sample of interest through two liquid channels, the ends of two liquid channels were sealed, and then the microfluidic device was loaded into the main chamber of the ToF-SIMS instrument for in situ liquid SIMS measurements. In situ liquid SIMS measurements. We performed in situ liquid SIMS measurements of 2 mM KX aqueous solutions and a series of KBr solutions at different concentrations (0.02 M, 0.2 M, 2 M, 5.49 M) on a ToF-SIMS 5 instrument (ION-TOF GmbH, Münster, Germany) in both positive and negative ion modes. The conditions of in situ liquid SIMS measurements were modified on the basis of our previous work39,42 and delay extraction mode was applied in in situ liquid SIMS measurements to achieve better mass resolution (detailed in Figures S1 and S2 and related discussions in the Supporting Information). Briefly, we focused a 25 keV pulsed Bi3+ primary ion beam at a frequency of 10 kHz to a diameter of ~350 nm, the pulse width of which was 100 ns. The primary ion beam was scanned on a small circular area of 2 μm in diameter on the SiN membrane. As soon as a small hole was created, signal intensities of the solution within the liquid chamber dramatically increased (such as K+ ions in the positive ion mode and halide ions in the negative ion mode). After collecting reasonable signal intensities, we stopped the measurements and reconstructed mass spectra from a time region with stable liquid signals (about 50 s). The pressure in the main chamber during the measurements was 1-2×10-6 mbar.

RESULTS AND DISCUSSION Comparison of ESI-MS and in situ liquid SIMS spectra. To evaluate how soft an ionization technique can be, it is very important to know interactions at what energy level can survive during ionization process. We applied both regular ESI-MS and our developed in situ liquid SIMS methods to evaluate halide ion-water interactions. ESI-MS spectra of 2 mM KX (X=F, Cl, Br, I) aqueous solutions in the negative ion mode were shown in Figures 1, S3, S4, and S5, respectively. A series of peaks assigned to salt cluster ions (KnXn+1-) dominate the spectra, and we barely observed any signal of solvated halide ions (X(H2O)n). It is well known that general chemical bonds (several hundreds kJ/mol) can survive during ESI process. However, in this work, the undetected hydrated halide ion signals suggested that halide ion-water interactions (40-100 kJ/mol)43 are hard to survive during regular ESI ionization process. In sharp comparison, we obviously detected a series of solvated halide ion peaks (X(H2O)n-) by using in situ liquid SIMS under negative mode as shown in Figures 2, S6, S7, and S8, suggesting that halide ion-water interaction at the energy level of 40-100 kJ/mol can survive relatively easily during in situ liquid SIMS ionization process. These results surprisingly show that in situ liquid SIMS is more suitable for investigation of halide ion hydration phenomenon than regular ESI-MS. The underlying reasons could be explained as follows. During the ESI process, the charged electrospray droplets undergo a series of solvent evaporation and Coulomb fission, eventually forming "dry" gas-phase ions and entering into the mass analyzer and detector in vacuum.33,44,45 The intrinsic feature of the ESI process might cause the frequently occurrence of desolvation phenomenon for the hydrated halide

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Analytical Chemistry ions; thus, the finally detected gas-phase ions couldn’t reflect information of the original hydrated compositions (Figure 3a). More importantly, during ESI process, salt solutions may form droplets of different sizes that contain several cations and anions but with only one net charge, such a droplet would not split into smaller droplets during evaporation, leading to formation of salt cluster ions (Figure 3a).

fragmentation (soft) ionization technique and produces molecular ions.50-52 In addition, secondary ions can go through the time-of-flight tube and be detected in a short time range (10200 µs) after formation. During this process, minimum collisions and evaporation can occur, maintaining solvated halide ions. Water cluster emission was usually reported in previous literatures from water or aqueous samples.42,53 In this work, a series of water cluster ions OH(H2O)n- were also clearly observed as labeled in the negative liquid SIMS spectra of 2 mM KX solutions (Figures 2 and S7 – S8) except for KCl solution (Figure S6), in the SIMS spectrum of which the water cluster ion series were overlapped with 35Cl(H2O)n- ion series. These verified that a soft ionization process indeed occurred during our liquid SIMS investigations on halide ion hydration as shown in Figure 3b.

Figure 1. Negative ESI-MS spectra of 2 mM KF aqueous solution in the m/z range of (a) 15-2000 and (b) 15-100. Signal intensities were normalized to the maximum one, respectively.

Figure 2. Reconstructed negative in situ liquid SIMS spectra of 2 mM KF aqueous solution in the m/z range of (a) 0-100, (b) 100-300 and (c) 300-600. The spectra were normalized to the sputter time. In (b), blue arrows point to OH(H2O)5-9- ions, and red ones refer to F(H2O)5-9- ions labeled with representative m/z values.

Although traditionally SIMS as a “hard” ionization method has a disadvantage of severe secondary ion fragmentation due to the bombardment of the high-energy primary ion source on solid samples,46 our previous studies have demonstrated that ion fragmentation in liquid SIMS analysis could be alleviated due to the continuously updated liquid surface around the hole via diffusion as well as the flexible liquid environments41,47,48, which facilitates formation of intact molecular ions. Actually, the ionization process in in situ liquid SIMS seems very similar with that in fast atom bombardment,49 which is a relatively low

Figure 3. A schematic comparison of ionization and detection processes of (a) regular ESI-MS and (b) in situ liquid SIMS. In regular ESI, it takes about 10-100 ms for the tiny liquid droplets from the ambient-vacuum interface to become ions and reach a detector. During these processes, a lot of collisions and serious evaporation can occur, and most solvent molecules were stripped from the ions. Besides, in electro-spaying of KX aqueous solutions, it seems easy to form tiny droplets of different sizes that contain several K+ and X- ions but only with one net charge, as shown in (a). During evaporation, such a droplet cannot split into several smaller droplets due to only a single net charge, resulting in formation of KnXm+/- salt cluster ions. As a comparison, the secondary ions generated from in situ SIMS sputtering (b) can travel to a detector within 10-200 µs, and minimum collisions and much less evaporation can occur during this process. Therefore, hydrated halide ions can be detected.

Apparently, as described above the regular ESI-MS was not suited for the investigation of halide ion hydration. Actually, most of ESI sources are designed to enhance desolvation. For example, heated sheath gas is regularly used to remove solvent molecules. In an early study, a variant electrospray ionization method attempted to address this issue, which was named as field evaporation of ions out of solution (FEIS).25 They did

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extensive modifications on the instrument by applying cryogenic pumping and electrospraying the electrolyte solutions into a vacuum instead of traditional ionization source at atmospheric pressure to avoid possible interference.25 In addition, it has been reported that some proper instrument modifications and careful adjustment of experimental parameters were necessary in the ESI-MS study of the solvation phenomenon of Li+ ions in the mixed solvents of ethylene carbonate (EC) and dimethyl carbonate (DMC), a classic nonaqueous electrolyte for lithium-ion batteries36, the results of which are in good agreement with our recent in situ liquid SIMS investigations42. In this aspect, our in situ liquid SIMS approach is much simpler with no need of instrumental modifications and as a surprisingly soft ionization process it is anticipated to be widely used for investigations of various ion-solvent interactions. Hydration phenomenon of halide ions. Then, to examine the hydration phenomenon of halide ions, we extracted the signal intensities of solvated halide ions X(H2O)n- (n=1- 6) from the negative ion SIMS spectra of 2 mM KX aqueous solutions (Figures 2, S6, S7, and S8), respectively, and normalized them to the signal intensities of corresponding naked halide ions as shown in Figure 4.

Figure 4. A comparison of signal intensities of X(H2O)n- ions normalized to that of X- ions (X=F-, 37Cl-, 79Br- and I-, respectively) in the negative liquid SIMS spectra of 2 mM KX solution (see Figures 2, S6-S8). Note: intensities of F(H2O)n- ions normalized to that of F- ions were calibrated in normal mode (see details in Figure S2 and related discussions in the Supporting Information).

In Figure 2 and 4a, we found that the ion signal intensity of F(H2O)3- (m/z 73) is much larger than those of other solvated fluoride ions, such as F(H2O)- (m/z 37), F(H2O)2- (m/z 55) and F(H2O)4- (m/z 91). This indicates fluoride ion combined with three water molecules is the dominated form among the hydrated fluoride ions. In the SIMS spectrum of 2 mM KCl aqueous solution (Figure S6), it should be noted that the series of water cluster ion peaks could not be distinguished very well from those of 35Cl(H2O)n- ion series. Hence we used the signal intensities of the series of 37Cl(H2O)n- ions to evaluate the solvated chlorine ions by water molecules as shown in Figure 4b. Due to possible interference, however, the ion intensity of the peak 37Cl(H2O)4- (m/z=109) was still abnormally high, the

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intensity ratio of which to 35Cl(H2O)4- (m/z=107) is inconsistent with the natural isotopic abundance of chlorine (35Cl%=75.78%, 37Cl%=24.22%). Therefore, we omitted the data at the solvation number of 4. It turned out that the signal intensity of 37Cl(H2O)2- (m/z 73) is the strongest as compared to other solvated chloride ions such as 37Cl(H2O)1- (m/z 55), 37Cl(H O) - (m/z 91), indicating that the solvated chloride ion 2 3 by two water molecules has the largest proportion in the solvated chloride ions (Figure 4b). It is worthy of pointing out that in spite of the interference of the series of 35Cl(H2O)n- ion intensities from those of water clusters, the 35Cl(H2O)2- (m/z 71) ion intensity is also the highest as shown in Figure S6. In the SIMS spectrum of 2 mM KBr aqueous solution (Figure S7), the intensity ratios of 79Br- to 81Br-, and 79Br(H2O)n- to 81Br(H2O)nions in the in situ liquid SIMS spectra were observed to be very close to the natural abundance of bromine element isotopes (79Br%=50.69%, 81Br%=49.31%). Here, we used the series of 79Br(H O) - ions as representatives for comparison, among 2 n which the ion intensity of 79Br(H2O)- was the strongest (Figure 4c), suggesting bromide ion bound with a water molecule is the most abundant hydration form. Figure 4d presents the relative intensities of a series of solvated iodide ions I(H2O)n- extracted from the spectrum in Figure S8 of 2 mM KI aqueous solution. Similar to the results of Br- ion solvation, the signal intensity of the iodide ion with one water molecule I(H2O)- (m/z 145) is also the highest. Besides, the difference between signal intensities of I(H2O)- and I(H2O)2-6- is more distinct than that observed between Br(H2O)- and Br(H2O)2-6- ions, indicating the larger dominance of I(H2O)- among the hydrated iodide ion series. To summarize the above results observed by liquid SIMS, hydrated F-, Cl-, Br-, and I- anions are the most abundant when they bind to 3, 2, 1, and 1 water molecules, respectively. The larger the radius of the anions, the smaller the charge density and the weaker the ability of the halide ions to form hydrogen bonds with water molecules. This trend is consistent with our obtained hydration numbers of halide ion series. In addition, we did not observe significant salt cluster ion peaks KnXn+1- (X=F, Cl, Br) in the SIMS spectra of 2 mM KF, KCl, and KBr (shown in Figures 2, S6, and S7); while, a series of obvious KnIn+1cluster ion peaks were shown on that of 2 mM KI solution (Figure S8). These phenomena indicate that I- anions are more likely to form salt clusters with K+ ions than other halide anions due to the less stable solvation layer of I-. Ab initio calculations have been used to assess the microscopic properties of halide ion-water clusters and revealed the influence of polarizability on the halide ion hydration structures, the value of which is only 0.76 Å3 for fluoride but much higher for larger halide ions in the range of 3-6 Å3.26 During the initial hydration there is no water-water hydrogen bonding through F(H2O)1-5- with the anion at or near the cluster ion center of mass due to the strong interaction of the fluoride ion with the bonded water molecules.26 In sharp comparison, when the first water molecule binds to the larger halide ion, due to the less strong H-O-H…X- interactions (X=Cl, Br, I) than the H-O-H…F- interaction, the spherical charge distribution of the anion could be significantly distorted and thus additional water molecules would coordinate on the same side of the anion. As a consequence, significant hydrogen bonding between water molecules is characteristic for X(H2O)n- ions (X=Cl, Br, I).14 Therefore, our liquid SIMS results with these calculation studies together contribute to reveal the influences of ion size,


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Analytical Chemistry charge density and polarizability of halide ions on the hydration phenomenon. The hydration numbers of halide ions observed using FEIS (a modified ESI-MS technique) were 2.5 of F-, 2.0 of Cl-, 1.5 of Br- and 1.0 of I-, respectively,25 in good accordance with our liquid SIMS observations. Here, it should be noted that as in all mass spectrometers ions are transmitted and detected in vacuum environments, both of these results may reveal the stability of hydrated halide ion compositions in vacuum. MD simulations conducted by Xantheas et al determined the coordination numbers of 6 for fluoride ion19, 5.1 for chloride and 4.7 for iodide ion in aqueous solutions, respectively.20 The trend of the effects of polarizability on coordination numbers is in agreement with these MS results in vacuum, but each value is larger. This discrepancy might be ascribed to the inherent limitations of each of these methods, such as the use of semiempirical and approximate methods in MD simulations, as well as the difficulty of MS methods in completely revealing the properties of hydrated ions in solutions which might have some difference from those in the gas phase. To date, the experimental evaluation of ion-solvent interactions in aqueous solutions with direct molecular evidences has still not been achieved yet not only because of the substantial technical challenge but also the dynamic and much more complicated situation within solutions. In situ liquid SIMS spectra of potassium halides at different concentrations. When analyzing high-concentration or high salt solutions using ESI-MS, the spraying nozzle might get blocked arising from the high viscosity of these electrolytes.36 Therefore, it is difficult for ESI-MS to be applied for investigations of high-concentration solutions. In this aspect, in situ liquid SIMS is a more versatile technique as the concentrations of the analytes could be in a wide range. For example, we successfully applied in situ liquid SIMS technique in the examination of ion-solvent interactions in non-aqueous electrolytes at both low (1.0 M) and high (4.0 M) concentrations in Li-ion batteries.42 Besides, it has advantages of no bias on the detection of positive and negative ions.42 In this work, we performed in situ liquid SIMS on a series of KBr solutions at different concentrations (20 mM, 200 mM, 2 M, and 5.49 M, respectively) in negative ion mode, and spectra are shown in Figures S9-S12. We extracted the intensities of the mono-hydrated bromide ions 81Br(H2O)- relative to 81Br- ions from the spectra of varied concentration of KBr solutions as shown in Figure 5a. It can be seen that the relative intensity was weaken with the increasing concentration. When the concentration was higher than 2 M, the hydrated ion peaks of bromide ions were nearly negligible. Similarly, in positive ion spectra of 2 mM and 20 mM KBr solutions (Figures S13 and S14), the apparent solvated potassium ion peaks K(H2O)n+ were observed. As the concentration further increases (Figures S15, 16, and 17), the signal intensities of the series of K(H2O)n+ peaks, such as 39K(H2O)+ were too weak to be detected (Figure 5b). In addition, water cluster ions, OH(H2O)n- and H(H2O)n+ ions in the negative and positive spectra, respectively, were only clearly observed in spectra of dilute solutions (Figures S7, S9, S13, S14). For the high concentration of KBr solutions, the signals of salt ions and salt cluster ions became strong, suppressing the signals of water cluster ions (Figures S10-S12 and S15-17).

Moreover, in both positive and negative liquid SIMS spectra of 2 mM KBr solution, we barely detected salt ion cluster peaks. At 20 mM, we found a series of salt ion clusters KnBrn+1- (n=14) with clearly isotopic distributions in the negative ion mode and Kn+1Brn+ in the positive ion mode. Here, it should be noted that a 20 mM solution is still dilute (about 1400 water molecules per ion). This means that at 20 mM aqueous solution K+ and Brions should still be well separated by the large amount of water solvent molecules. Therefore, the sharply increased salt ion clusters indicated that water evaporation does occur at the in situ SIMS sputtering interface. At a higher concentration of 200 mM, normalized intensities of cluster ion peaks such as 39K81Br - ions in the negative mode and 39K 81Br+ ions in the 2 2 positive mode further increased (Figures 5c and 5d). With the concentration further increased to 2 M and 5.49 M, the signal intensities of salt ion cluster peaks decreased instead. This was ascribed to the much lower mobility of the highly concentrated electrolytes which caused relatively more damaged materials accumulated at the sputtering interface.42 However, as shown in the shadowed area in both positive and negative depth profiles of 5.49 M solution (Figure S18), after punching through the SiN membrane window signals from liquids quickly reached a stable state, indicating the liquid surfaces exposing through the small aperture were self-renewable even at the high concentration.

Figure 5. A comparison of normalized signal intensities of (a, b) hydrated ions and (c, d) salt ion clusters in the positive liquid SIMS spectra (Figures S13-17) of KBr solutions at different concentrations. (a) 81Br(H2O)- and (c) 39K81Br2- ions normalized to that of 81Br- ions in the negative liquid SIMS spectra (Figures S7 and S9-12); (b) 39K(H2O)+ and (d) 39K281Br+ ions normalized to that of 41K+ ions.

In situ liquid SIMS, a complementary tool to ESI-MS. The first generation of liquid SIMS (LSIMS) was developed at the beginning of 1980s. It was one of only a few ionization techniques that can generate molecular ions at that time.49 It was initially named fast atom bombardment (FAB) as an energetic primary neutral beam was used.54,55 Later, common SIMS primary ion beam approved to be feasible, too.56,57 This technique had been extensively used for about two decades. However, it became quiet after 2000 because of development of ESI-MS. Compared to these two techniques, our newly-

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developed in situ liquid SIMS has the following advantages in mass spectrometric study of liquid samples. First, no extra materials (such as glycerol in FAB/LSIMS) are needed. Secondly, our new approach is more sample friendly. For example, ESI-MS generally cannot be used to test highconcentration or highly salty or relatively sticky samples, but our in situ liquid SIMS can deal with such samples readily.42,58 Also, a low vapor pressure liquid is required for FAB/LSIMS analysis, but our in situ liquid SIMS could also be applied to analyze liquids with high vapor pressure under vacuum, such as aqueous samples.47,59-61 This advantage not only makes sample preparation simple, but also can keep the original chemical information as much as possible. More interestingly, a unique advantage of in situ liquid SIMS is that it can provide molecular information at solid-liquid interfaces under operando conditions, which has been difficult for any other techniques so far. For example, although mass spectrometry has been introduced into the electrochemistry field for several decades, gas or liquid that is close to electrode surface has to be introduced into a mass spectrometer for analysis. Such an approach usually fails to provide nanometer-scale depth resolution as well as the evolution information of electrode surfaces. As a comparison, our in situ liquid SIMS approach can not only provide very decent depth resolution to analyze evolution of electric double layer (which is in nanometer range), but also provide real-time molecular information of electrode surface and electrolyte simultaneously.38,40,62-64 Therefore, in situ liquid SIMS can be a very powerful complementary tool for ESI-MS analysis.

CONCLUSIONS In this work, we used halide ion hydration as an example and compared the spectra of dilute potassium halide aqueous solutions by regular ESI-MS and our in situ liquid SIMS techniques. Results show that the solvated ion peaks of halide ions were clearly observed in the in situ liquid SIMS spectra; while, in the ESI-MS analysis, almost no solvated ion peaks were detected with a series of salt cluster ion peaks dominating the spectra. These suggest that compared with regular ESI-MS, a surprisingly soft ionization process occurred in our in situ liquid SIMS which was demonstrated to be a more suitable analysis tool to evaluate the weak interactions of halide ions with water molecules. It should be noted that our liquid SIMS results suggest that the interactions with an energy level of 2040 kJ/mol can be reasonably survived during in situ SIMS ionization process for mass spectrometric analysis. On the basis of data obtained by in situ liquid SIMS, we determined the hydration numbers of halide ions in the gas phase, and revealed that the ion-solvent interactions by water molecules are strongly influenced by the ion charge density and polarizability of the central ions. Our liquid SIMS studies could contribute with computational efforts to fundamental understanding of a clearer picture of various ion-solvent interactions. Considering other advantages of in situ liquid SIMS, e.g. sample friendly and the feasibility of molecular studies of solid-liquid interface under operando condition, we anticipate that in situ liquid SIMS will be a very promising complementary technique for ESI-MS for investigations of various ion-solvent interactions, molecular structure of complex liquids, initial nucleation of nanoparticle formation, and many relevant chemical and biological processes in the future.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional ESI-MS, liquid SIMS spectra and related discussions (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by an FY2016 open call LDRD fund of the Pacific Northwest National Laboratory (PNNL). In situ liquid SIMS was performed at EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at PNNL. We would appreciate Mao Su for his helpful discussions. F.W., Y.Z and Y.Z. acknowledge the NSFC (Grant Nos. 21127901, 21575145, 21621062) for support.

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In Situ Liquid Secondary Ion Mass Spectrometry: A Surprisingly Soft

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