Vol. 2 (2013), A0015

Mass SPectrometrY DOI: 10.5702/massspectrometry.A0015

Original Article

Cluster Composition Distributions of Pure Ethanol: Influence of Water and Ion–Molecule Reactions Revealed by Liquid-Ionization Tandem Mass Spectrometry Masahiko Tsuchiya,*,1 Haruhiko Fukaya,2 and Yasuo Shida2 2

1 Yokohama National University, Hodogaya, Yokohama, Japan Tokyo University of Pharmacy, Horinouchi, Hachioji, Tokyo, Japan

Studies of clusters in condensed phase at atmospheric pressure are very important for understanding the properties and structures of liquids. Liquid-ionization (LPI) mass spectrometry is useful to study hydrogen-bonded clusters at the liquid surface and in a gas phase. An improved ion source connected to a tandem mass spectrometer provides detailed information about clusters. Mass spectra of pure ethanol (99.5%) observed by the first mass analyzer (Q1) showed neat ethanol cluster ions (C2H5OH)mH+ with m up to 10 and hydrate ions (C2H5OH)m(H2O)nH+ with m larger than 7 and n=1, such as those with m-n=8-1 and 9-1. When the flow rate of ethanol (liquid) was increased, large ethanol cluster ions with m larger than 25 were observed by the second mass analyzer (Q3). It is interesting to note that neat ethanol cluster ions are more abundant than corresponding (with the same m) hydrate ions (n=1), and major hydrate ions contain only one molecule of water. Results indicate that ion–molecule reactions occur between Q1 and Q3, because such mass spectra have never been observed by Q1. Various results indicate that neat ethanol clusters exist at the liquid surface and are ionized to give cluster ions. Keywords: ethanol clusters, liquid-ionization, ion–molecule reactions, clusters at atmospheric pressure (Received December 1, 2012; Accepted January 12, 2013)

INTRODUCTION Studies of clusters in liquid phases under atmospheric pressure are very important for understanding the properties and structures of liquids. Mass spectrometry is the most important method for obtaining information about the distribution of cluster sizes. Clusters in gas and condensed phases have been investigated by mass spectrometry with a variety of techniques. The adiabatic expansion of molecular beams has been widely utilized methods for generating and measuring clusters and cluster ions.1–9) By adiabatic expansion technique, however, the molar ratios of ethanol to water, calculated from observed mass spectra have been much greater than those expected from the concentration of the sample solution.6,7) The association of clusters occurs during the adiabatic condensation processes. The dissociation of clusters, however, occurs during the evaporation in vacuum and also by increasing temperature. By using adiabatic expansion and condensation technique, observed mass spectra must be different from the neutral clusters. The surface compositions of ethanol–water mixtures have been studied using mass spectrometric techniques by sampling the binary vapor in equilibrium with the ethanol–water mixture, and the results have agreed with those obtained from other methods, such as surface tension and neutron reflection analysis.10) El-Shall has found that the limiting attachment energy of methanol in large clusters is similar to condensation energy of ligand methanol, indicating that

clusters may be viewed as a bridge between gas-phase and condensed-phase chemistry.11) Various ion–molecule reactions have been studied by tandem mass spectrometry and selected ion flow tube techniques for small species, such as alcohol ions and base molecules.12,13) An atmospheric pressure ion source utilizing a corona discharge has been used to produce cluster ions.14–17) Propanol and ethanol were observed to form protonated clusters with up to 6 alcohol molecules. The results gave insights about the structures and energetics of the clusters related to the atmospheric pressure gas-phase ion chemistry.14,15) The dependence of ion currents on temperature (ion thermograms) during the thawing of frozen samples of methanol and ethanol have been investigated by monitoring the abundance of different types of ions by low-temperature (LT) secondary ion mass spectrometry (SIMS).18) Correlations between the changes in ion production and the phase transition (melting, boiling, and evaporation) in the sample with temperature increase have been revealed. Meaningful spectrum was absent at the lowest temperature, at which the samples were in the solid state, good-quality spectra appeared in the intermediate temperature range, and disappeared with further rise in temperature. The main positiveions correspond to protonated alcohol (C2H5OH)mH+ with m up to 12, hydrate (C2H5OH)m(H2O)nH+, (due to the presence of water) and ‘fragment’ clusters. It has been suggested that the boiling point of the sample is close to the sharp increase in the abundance of the ions. After this point the cluster ion series terminate abruptly. In ion thermograms,

* Correspondence to: Masahiko Tsuchiya, 4–37–27 Kugayama, Suginami-ku, Tokyo 168–0082, Japan, e-mail: [email protected]

Please cite this article as: M. Tsuchiya, et al., Cluster Composition Distributions of Pure Ethanol: Influence of Water and Ion–Molecule Reactions Revealed by Liquid-Ionization Tandem Mass Spectrometry, Mass Spectrometry, 2: A0015 (2013); DOI: 10.5702/massspectrometry.A0015

© 2013 The Mass Spectrometry Society of Japan

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all types of cluster ions appeared at the same temperature threshold and reach a maximum abundance at about the same point of temperature. The results suggest that all of them originate from the same phase, i.e., the liquid. Secondary ion mass spectra have been measured for a liquid ethanol target bombarded by 2.0 MeV He+ ions.19) A liquid beam from an infusion pump is injected vertically through a nozzle of 20 µm in diameter into the center of the collision chamber. Positive and negative ion spectra exhibit a series of cluster ions of the forms [(C2H5OH)mH]+ and [(C2H5OH)m−H]−, respectively, and both spectra show similar cluster size distributions with almost the same decay slope. The base peak is m=1 and cluster ions with m up to 14 are observed. Secondary ions, CH2OH+, are the dominant fragment ion produced from decomposition of ethanol molecules, while a series of cluster ions are emitted only from the liquid beam target. They have concluded that these cluster ions are produced only from liquid phase ethanol. Ethanol solutions have been measured by using thermospray ionization for producing cluster ions.20) When the solution contains water (mass percentages of ethanol are higher than 50%), large cluster ions containing more than 10 molecules of ethanol are observed (which looks similar to Fig. 4a in this paper). As for pure ethanol, however, neat ethanol cluster ions contain up to only 4 molecules of ethanol. Probably the ionization in vacuum after evaporation of the sample may cause the dissociation of large clusters (m>4) prior to ionization in vacuum.

Liquid ionization (LPI) mass spectrometry

We have developed a soft ionization method, termed liquid-ionization mass spectrometry for analysing nonvolatile organic compounds,21–23) (referred to as LPI since 199822)), in which a liquid sample is ionized by collision with excited argon atoms (Ar*) under atmospheric pressure at ambient temperature (23–25°C). The method (LPI-MS) has been also applied to studies of clusters, such as water, 24) carboxylic acid–water mixtures,25) and reaction products formed in formaline with a small amount of methanol.26) The distribution of cluster sizes of water in the air has been measured at ambient temperature and found that the size distributions were related to the humidity,24) suggesting that neutral clusters of water exist in the air. Alcohol aqueous solutions (20%v/v, methanol and ethanol) have been measured by LPI-MS.27) The molar ratios of alcohol to water calculated from LPI mass spectra have been agreed with the alcohol concentration of the solution, although the flow rate of a sample solution (Fs) and the voltage applied to the needle (VE) should be controlled. It has become clear that the method is appropriate to investigate hydrogen-bonded clusters, which exist at the surface of liquids and also in a gas phase at atmospheric pressure and at ambient temperature. The reasons why the alcohol concentration in vapor was higher than those in the liquid have been verified by LPI-MS. That is, the loss of water molecules in larger clusters is the main processes during evaporation and also the association of small clusters occurs to increase the number of ethanol molecules in clusters, resulting in higher ratios of alcohol to water in the gas phase. These results were consistent with the results obtained by liquid–vapor equilibrium measurements.28) Results also indicate that the composition distributions of cluster ions observed by LPI-MS are likely similar to © 2013 The Mass Spectrometry Society of Japan

Vol. 2 (2013), A0015

the distributions of neutral clusters.27) It has been clear that the position of the needle tip respect to the pinhole is the most critical parameter for obtaining mass spectra showing the cluster compositions at the liquid surface, because the cluster compositions vary very quickly due to evaporation, sample ions produced at the needle tip, and these ions should go through the pinhole up to the detector. The envelope of ion source used previously was a glass cylinder,21–27) so that the image around the needle tip was distorted. In this paper, a glass block was used as the ion source and it is now possible to adjust precisely the position of the needle tip. This paper describes the improved liquid-ionization (LPI) ion source connected to a tandem mass spectrometer, the cluster composition distributions of pure ethanol (99.5 wt%), the influence of a small amount of water, and the association reactions which occur during ion flight between Q1 and Q3. Various results described in this paper indicate that hydrogen-bonded clusters exist at the liquid surface. Liquid-ionization tandem mass spectrometry provides useful information about these natural clusters.

EXPERIMENTAL In previous studies by liquid-ionization (LPI) mass spectrometry, a single quadrupole mass spectrometer has been used.21–27) In this paper, a tandem mass spectrometer (Extrel CMS, TMAX 2000) equipped with an improved LPI ion source was used.

LPI ion source

Because a glass block has a flat face and provides straight observation around the needle tip, a new ion source was made of a glass block (30×60×50 mm). A hole (diameter: 25 mm, length: 50 mm) for installing a device for introducing a liquid sample, a side hole for installing the corona discharge tube and another side hole (diameter: 10 mm) for additional uses (e.g., the entrance for sample vapor) were provided in the glass block (Fig. 1). In addition, a CCD camera (ANMO Electronics Corp., not shown) was used to observe enlarged image of the needle tip. These improvements enabled us to adjust precisely the position of the needle tip respect to the pinhole. The hole diameters of the pinhole, skimmer-1 and skimmer-2, were 0.2, 0.4 and 0.4 mm, respectively. The distances between the pinhole and skimmer-1, and between skimmer-1 and skimmer-2 were 2.0 and 3.0 mm, respectively. In general, appropriate voltages applied to the pinhole electrode, skimmer-1, and skimmer-2 electrodes for measuring cluster ions were 30 V, 20 V, and 5 V, respectively. If skimmer-1 voltage was increased above 25 V, dissociation of cluster ions occurred between skimmer-1 and skimmer-2 due to a collision induced dissociation (CID) with argon molecules (residual gas in vacuum). An additional lens electrode (VLens: diameter of the hole is 2.0 mm) was installed between skimmer-2 and the entrance of the first quadrupole (Q1: first mass analyzer). This VLens electrode is essential to obtain reproducible mass spectra. Without VLens, ion currents obtained at Q1 was in the range of 10 −9–10 −10 ampere, while abundant enough ions (10 −7–10 −6 ampere at Q1) was obtained by applying negative voltage (−60 V~−80 V) to VLens electrode. Output voltage from the electron multiplier Page 2 of 8

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Schematic diagram of a new liquid-ionization (LPI) tandem mass spectrometer, showing an LPI ion source with a device for introducing a liquid sample.

(biased at 1700 V) was in the order of 1 V for the base peak. Mass spectral patterns were nearly the same at VLens between −50 V and −90 V. Therefore, VLens was set at −70 V for the measurement. Besides, mass spectral patterns observed by the first quadrupole (Q1) are almost the same when appropriate voltages are applied to all electrodes between the ion source and Q1. The results indicate that mass spectra observed by Q1 present the composition distribution of cluster ions formed in the ion source. In other words, there is no places for occurring adiabatic condensation in this instrument.27) Several devices for introducing sample solution have been examined and it was found that good results were obtained using a microsyringe needle (Hamilton 7780-01, inner diameter: 0.13 mm). Sample liquid was supplied to the needle tip at a constant flow rate (Fs) by an infusion pump (Kd Scientific, Holliston, USA). Pole bias for Q1, Q2, and Q3 were all to 0 V with respect to ground potential. All mass spectra shown in this paper are the average of 10–30 scans. As described previously, mass spectral patterns are affected by several experimental conditions. The most important is the electric field at the needle tip, which is defined by the voltage applied to the needle (VE) and the distance (d) between the needle tip and the pinhole. The voltage (VE) has three important functions as described previously.21,22) Function 1) is to remove electrons produced by Penning ionization, because the recombination of a positive ion with an electron emitted at Penning ionization is very rapid under atmospheric pressure. Therefore, no ions are observed below about 0.55 kV, while the ion abundances increase sharply with higher VE. Function 2) is to assist the desorption of produced ions by coulombic repulsion. In this paper, it was revealed that the energies for dissociation and light emission were also given by VE. Function 3) is to focus ions through the pinhole to the quadrupole. Therefore, the ion abundances were significantly affected by VE and it was also very important to adjust the position of the needle tip respect to the pinhole. Too high VE initiated the harsh glow discharge © 2013 The Mass Spectrometry Society of Japan

Fig. 2.

Photographs around the needle tip observed by a CCD camera, which show (a): pale light emission and (b): atmospheric pressure glow discharge.

that resulted in the complete dissociation of cluster ions, and only argon ions were observed in the mass spectrum. The distance (d) also affected the electric field. Too long d gave no ions and too short d caused a glow discharge. Generally, d was 0.8–1.0 mm. Appropriate VE depended on the property of a sample, such as ionization energy, volatility and surface tension. Many mass spectra should be measured by varying voltage (VE) at a constant sample flow rate (Fs), because the cluster compositions vary very rapidly at the liquid surface due to evaporation.27)

RESULTS AND DISCUSSION Light emission at the needle tip and influence of VE

Photographs observed by a CCD camera show pale light emission at the needle tip (Fig. 2a) and atmospheric pressure glow discharge (Fig. 2b). Figure 3 indicates the influence of VE on LPI mass spectra of pure ethanol with a small amount of water. Fs was kept constant (20 µL/h). All ions observed in LPI mass spectra were represented as (C2H5OH)m(H2O)nH+ (referred to as m-n). In general, sample ions started to be observed at VE of 0.65 kV, which depended on not only experimental conditions but also the property of a sample. Mass spectral patterns observed at VE Page 3 of 8

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The influence of V E (0.94–1.02 kV) on LPI mass spectra of pure ethanol at a constant sample flow rate (Fs: 20 µL/h). Peak tops in each mass spectrum are connected with one line. Ordinate: relative peak intensities normalized to the base peak. Numbers, such as 3-0 and 4-0, denote m-n of (C2H5OH)m(H2O)nH+.

of lower than 0.94 kV resembled each other comparatively well. At about 0.94 kV, pale light emission started to occur at the needle tip (Fig. 2a). By increasing VE , the ion abundances increased enough to obtain reproducible mass spectra and the emitted light became slightly brighter. At VE of higher than 0.96 kV (Fig. 3), the increase in VE caused the dissociation of larger cluster ions (by coulombic repulsion). Occasionally, beams of the pale light went into vacuum through the pinhole, while mass spectral patterns looked almost the same, although ion abundances increased with VE continuously and decreased continuously by decreasing VE. Accordingly, the species emitting light must be positive ions. At VE of higher than 1.00 kV, the dissociation occurred more significantly. At higher than 1.04 kV, strong discharge (Fig. 2b) happened to occur, resulting in only dominant Ar+ accompanied with Ar2+ (less than 1/3 of Ar+). The results indicate that VE promotes ion evaporation by giving a part of energy for desorption and also provides excitation energies, from which the pale light must be emitted. At VE higher than about 0.96 kV, the excitation energy given to ions may increase with VE and higher VE (e.g., 0.98 kV) causes the dissociation of cluster ions, resulting in the shift of cluster sizes to smaller ones (Fig. 3). Because the most important parameter is considered to be VE , several mass spectra should be recorded over optimum range of VE after adjusting other experimental conditions, such as the position of the needle tip and Fs. An appropriate VE must be the VE at the highest, but without dissociation of cluster ions.

Mass spectra of pure ethanol

A background mass spectrum of LPI-MS showed clusters of water in air, which diffused into the ion source very rapidly as reported previously.24) Because a small amount of water presence in the ambient gas that surround the ion source has significant influence on mass spectra of ethanol, all places causing air leak were shielded with silicone rubber sheets (0.5 mm thick). Argon in a cylinder (High grade G3, 99.999%, Toho Sanso, Yokohama, Japan) used as the ionizing gas (flow rate: 700 mL/min), contained a small amount of water. Mass spectra of argon gas in the cylinder shows water cluster ions, (H2O)nH+, with n=2–14 (n=4 is the base peak), as shown in the inset of Fig. 4a. A mass © 2013 The Mass Spectrometry Society of Japan

Fig. 4.

LPI-mass spectra of pure ethanol (99.5%). (a); With water molecules in an argon cylinder. The inset shows a mass spectrum of water in the argon cylinder. (b); Water in the argon gas was reduced by flowing through a column packed with P2O5 (Sicapent), measured at V E of 1.10 kV and Fs of 20 µL/h. The inset shows a mass spectrum of water reduced in the argon gas. (c); Measured with the least amount of water, measured at V E of 0.70 kV and Fs of 10 µL/h.

spectrum of pure ethanol (99.5 wt%, Wako, Tokyo) measured using the argon cylinder shows ethanol cluster ions, (C2H5OH)mH+ with m up to 10, and many hydrate cluster ions, (C2H5OH)m(H2O)nH+, with larger m (=7–20) and n (=1–5) (Fig. 4a). In order to reduce water in the argon cylinder, a column packed with P2O5 (Sicapent, Merck) was connected to the Ar gas line (polyethylene tube) in front of a flow meter (not shown in Fig. 1), which was connected to the corona discharge tube. A mass spectrum of the ethanol (Fig. 4b) was measured using the argon gas with reduced water (n=1–4), which is shown in the inset of Fig. 4b. Comparing with Fig. 4a, it is likely that most of the hydrate ions (Fig. 4a) are originated from water in the argon cylinder. The background water in the Ar gas was reduced further by continuous use of the P2O5 column. Figure 4c shows one of typical mass spectra of the pure ethanol with the least amount of background water (water cluster ions with n=4 became nearly zero). The abundance of hydrate ions was certainly reduced by reducing water in the background. As described in the Introduction section, ethanol cluster ions, (C2H5OH)mH+, with m up to 12–14 have been observed by both keV ion impact (LTSIMS)18) and 2.0 MeV He+ bombardment.19) Their results are similar to us (Figs. 4b and 4c) and they, both have concluded that these ions are produced from liquid phase ethanol. Page 4 of 8

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LPI mass spectra of the ethanol with the least amount of water measured at V E of 0.86 kV and Fs of 25 µL/h. (a) observed by Q1 and (b) observed by Q3. ○: (C2H5OH)m(H2O)H+.

Hydrate ions, such as 8-1 and 9-1, are clearly observed in Fig. 4 (both a and b), while neat ethanol cluster ions with the same numbers m (2–9) are observed in Fig. 4 (all a, b, and c), in spite of big differences in the intensity of hydrate ions. The concentrations of water in the ethanol calculated directly from all peak heights of each mass spectrum (as described previously27)) are 1.4% (Fig. 4b) and 0.4% (Fig. 4c), respectively. Because purity of the ethanol is 99.5% (0.5% : water), results indicate that hydrate ions (Fig. 4c) may be mainly from the pure ethanol. These data also indicate the existence of neat ethanol clusters with m up to 7 at the liquid surface, because these cluster ions do not contain water molecules.

Mass spectra observed by the second mass analyzer, Q3 When the sample flow rate (Fs) was less than 5 µL/h,

mass spectra measured by the second mass analyzer (Q3) looked similar to those measured by the first mass analyzer (Q1) (Figs. 4b and c). By increasing Fs (>10 µL/h), however, mass spectra observed by Q3 showed much larger cluster ions than those observed by Q1. Figure 5 (a and b) shows mass spectra of pure ethanol with the least amount of water measured in succession at VE of 0.86 kV and Fs of 25 µL/h, observed by Q1 (Fig. 5a) and by Q3 (Fig. 5b), respectively. The mass spectrum shown in Fig. 5a looks similar to that shown in Fig. 4c, although a certain degree of dissociation of cluster ions are recognized, probably due to higher VE. In contrast, the mass spectrum observed by Q3 (Fig. 5b) shows significant increases in the relative abundances of ethanol cluster ions with m larger than 5 and of hydrate ions (with n=1) with m larger than 8 up to around 25. Mass spectra measured at VE of 0.70 kV and Fs of 20 µL/h, were shown in Fig. 6. The mass spectrum shown in Fig. 6a looks again similar to those shown in Figs. 4c and 5a. In contrast, Fig. 6b shows again larger ethanol cluster ions (m>5 up to 25) and hydrate ions (m>7 up to 28, n=1), although the relative abundances of hydrate ions (m=7–22) are smaller than those of corresponding (with the same m) © 2013 The Mass Spectrometry Society of Japan

Fig. 6.

LPI mass spectra of the ethanol with the least amount of water measured at V E of 0.70 kV and Fs of 20 µL/h. (a) observed by Q1 and (b) observed by Q3. ○: (C2H5OH)m(H2O)H+.

ethanol cluster ions (n=0). Besides, these large cluster ions (both of neat ethanol ions and hydrate ions) have never been observed by Q1. Figure 7 shows LPI mass spectra measured at VE of 0.75 kV and Fs of 20 µL/h. Figure 7a observed by Q1 looks again similar to those shown in Figs. 4c, 5a, and 6a, although a certain degree of dissociation of cluster ions are recognized in Fig. 7a, probably due to higher VE. It is interesting to note that by increasing VE , the relative abundances of larger cluster ions (m>5) observed by Q1 become small, and mass spectra observed by Q3 (Fig. 7b) do not show large ions, which look similar to those observed by Q1 (Fig. 7a). Such threshold voltages (VThres) for observing similar mass spectra by both Q1 and Q3, were depending on Fs. For instance, VThres were 0.91 kV with 25 µL/h and 0.78 kV with 20 µL/h, respectively. Page 5 of 8

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results indicate that large cluster ions observed by Q3 were produced by ion–molecule reactions occurred mainly in the collision chamber (Q2). Multiple collisions are possible during long flight path at high pressure (∼10 −2 Pa). Besides, there are many bonds in a large cluster and a large cluster ion. The excess energy given to the produced large cluster ion may be small and easily thermalized by multiple collisions. Such phenomena (large cluster ion formation) were observed also for various compounds, such as water, alcohols, and mixtures of acetonitrile and water. In contrast, cluster ions of compounds like alkanes (hydrophobic) have never been observed by LPI mass spectrometry. Hydrogen bond(s) has an important role for cluster formation.

Mechanisms of ethanol cluster-ion formation

Fig. 7.

LPI-mass spectra of the ethanol with the least amount of water measured at V E of 0.75 kV and Fs of 20 µL/h. (a) observed by Q1 and (b) observed by Q3.

By increasing Fs, the thickness of liquid layer just above the needle tip may increase, resulting in decrease of the electric field at the liquid surface. In contrast, the increase of VE provides higher electric field. With the same electric field, similar ions at the liquid surface may be survived (=observed). In other words, with higher Fs, similar mass spectra may be observed at higher VE.

Influence of sample flow rate (Fs)

It is important to note that Figs. 5a, 6a, and 7a indicate that mass spectra observed by Q1 are likely independent of Fs, while mass spectra observed by Q3 (Figs. 5b, 6b, and 7b) are dependent on Fs. Because Ar flow rate was 700 mL/min and ethanol (liquid) flow rate was 10–30 µL/h, the concentration of ethanol vapor in Ar should be 2–5×10 −4, which is too small for the cluster formation by adiabatic condensation. Besides, the distance between the pinhole and the skimmer 1 was only 2 mm, which is too short for clustering by condensation to form large cluster ions with m up to more than 10. The pressure above the turbo-molecular pump (800 L/s, Shimadzu TMP-803) was 1.3×10 −3 Pa (almost constant during experiments). The pressure in the collision chamber (Q2) estimated from the pumping speed, the conductance of several holes, is assumed to be the order of 10 −2 Pa with higher Fs. It has been known that ion–molecule reactions occur at this pressure. The temperature of the ion flight-path must be ambient (24–25°C). RF frequency of Q1, Q2, and Q3 was the same. Therefore, the ion–molecule (cluster) reactions are likely the processes to produce larger cluster ions (observed by Q3). Even at high VE (0.86 kV), large cluster ions were observed by Q3 (Fig. 5b) with high Fs (25 µL/h). Spectral pattern (shape of size distribution) of ethanol cluster ions observed by Q3 (Fig. 5b) suggests a quasi-equilibrium state. Because similar patterns were often observed with higher Fs (>20 µL/h), the abundance of large cluster ions observed by Q3 were dependent on Fs, indicating that the abundant neutral species (molecules and clusters) existed in the vacuum (above 10 −2 Pa). The primary ions formed in the ion source must be the same as those observed by Q1. The © 2013 The Mass Spectrometry Society of Japan

In the case of 20% ethanol aqueous solution, all ethanol ions have been hydrated (no neat ethanol cluster ions). Neat ethanol cluster ions have been observed only for the vapor (in the gas phase).27) Mechanisms of cluster ion formation in LPI-MS have been described there for ethanol–water binary mixtures. As for the pure ethanol, neat ethanol cluster ions with m b, m′ ∼ m + (C 2H5OH)b′ Metastable Ar atoms (Ar*) produced in corona discharge tube ionize ethanol molecules and clusters at the liquid surface (reaction (1): Penning ionization: ionization energies of ethanol molecule and clusters are lower than internal energy of metastable Ar=11.55 eV). Because cluster ions observed were all protonated, proton transfer reactions like reactions (2) and (3) must follow to produce final cluster ions, (C2H5OH)m′H+, at the liquid surface. Electrons emitted by reactions (1) and (2) should be captured by VE.22,23,27) The ion abundances increase significantly with VE , indicating that emitted electrons must exist close to the liquid surface. Because the pale light goes through the pinhole into the vacuum, the light must be emitted from positive ions as described in the Results section. The dissociation of cluster ions is also observed by increasing VE as seen in Fig. 3. Therefore, all significant influences of VE recognized in Figs. 3 and 5–7 indicate the existence of clusters at the liquid surface. In the case of reaction (3), m should be larger than b, because proton transfer reactions occur from smaller cluster ions to larger clusters, which have larger proton affinities (PA) than the PAs of smaller clusters, although differences in PAs must be small. Because the proton transfer reactions like reaction (3) requires very small energy, the difference between numbers m and m′ may be small, suggesting that the LPI mass spectra represent the size distribution of Page 6 of 8

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neutral clusters at the liquid surface. That must be one of the reasons why the molar ratios of ethanol to water calculated from LPI mass spectra of 20% ethanol aqueous solution have agreed well with the ratio calculated from its concentration.27) It is difficult at present to explain the processes in detail for producing large cluster ions, because multiple collision must occur between Q1 and Q3. These reaction processes, however, should be investigated in future.

CONCLUSION LPI mass spectra observed by Q1 were almost independent of sample flow rates (Fs), and mass spectral patterns were almost the same even when the voltages applied to the focusing electrodes (except for Vskimmer-1 and VE) were varied, although the ion abundances varied with these voltages. The voltage applied to the needle (VE) had significant influence on mass spectral patterns. Pale light emission indicates that such light is emitted from positive ions excited by VE. Adiabatic condensation did not occur in this instrument because of too small concentration of a sample in Ar gas and too short distance between the pinhole and skimmer 2. These data indicate that cluster ions observed by Q1 are almost the same as those formed in the ion source. In other words, LPI mass spectra indicate the size (molecular composition) distribution of clusters at the liquid surface. LPI mass spectra of pure ethanol (99.5%) observed by Q1 present ethanol cluster ions, (C2H5OH)mH+, with m up to around 12. As described in the Introduction section, ethanol cluster ions, (C2H5OH)mH+, with m up to 12–14 have been observed by both keV ion impact (LTSIMS)18) and 2.0 MeV He+ bombardment.19) They, both, have concluded that these ions were produced from liquid phase ethanol. Our results agree well with their results. It is not easy, however, to determine their size distribution in the liquid, because clusters in the liquid are surrounded by many ethanol molecules. With high Fs (>10 µL/h), large cluster ions were observed by Q3. Data indicate that ethanol vapor fly into the vacuum and react with the precursor ions to yield product ions. Relatively abundant hydrate cluster ions contained only one molecule of water and no hydrate ions with m less than 7 (Figs. 5b and 6b) were observed. Large cluster ions observed by Q3 have never been observed by Q1. Various results indicate that neat ethanol clusters exist at the liquid surface and are ionized to give cluster ions observed by Q1.

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J. Q. Searcy, J. B. Fenn. Clustering of water on hydrated protons in a supersonic free jet expansion. J. Chem. Phys. 61: 5282–5290, 1974. P. M. Holland, A. W. Castleman Jr. A model for the formation and stabilization of charged water clathrates. J. Chem. Phys. 72: 5984–5990, 1980. R. J. Beuhler, L. A. Friedman. Study of the formation of high molecular weight water cluster ions (m/e

Cluster composition distributions of pure ethanol: influence of water and ion-molecule reactions revealed by liquid-ionization tandem mass spectrometry.

Studies of clusters in condensed phase at atmospheric pressure are very important for understanding the properties and structures of liquids. Liquid-i...
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