Water Clusters in Gas Phases Studied by Liquid Ionization Mass Spectrometry

J. Mass Spectrom. Soc. Jpn. Vol. 52, No. 1, 2004

REGULAR PAPER

Masahiko THJ8=>N6,῎a) Takashi T6H=>GD,b) and Atsushi S=><>=6G6c)

(Received November 4, 2003; Accepted December 6, 2003)

Studies of water clusters under atmospheric pressure are very important. In previous methods, however, clusters existing in vacuum at low temperature have been measured. Therefore, the size distributions of observed cluster ions are likely di#erent from those existing at normal conditions. Liquid ionization (LPI) mass spectrometry we have developed provides information about hydrogen-bonded clusters in a gas phase and also at the liquid surface at ambient temperature under atmospheric pressure. Thus, LPI-MS was applied to study water clusters in gas phases. The results indicated that the sizes of water clusters in vapor increased with the temperature of liquid water and those in the air increased with the relative humidity, while they decreased with increasing the temperature of the gas phase. In general, the size distribution of water clusters shows one smooth envelope, except for a few magic numbered clusters. Adiabatic condensation is negligible to occur in this method and thus, LPI mass spectra give useful information about clusters in gas phases under the normal conditions.

have been studied by using mass spectrometry11) and 1. Introduction ultraviolet (UV) and infrared (IR) spectrometry.12) The 1.1 Clusters in gas and liquid phases cluster sizes are determined by mass spectrometric Studies of clusters in gas and liquid phases under techniques. The clusters just above the liquid surface atmospheric pressure are very important for under- have been measured by this method.11) These optical standing the properties, formation, and structures of spectrometric methods are useful to discuss the struc- liquids and also for understanding chemical reactions ture of isolated clusters. in solutions. Many studies and also reviews1)῍5) have In these methods previously used, however, clusters been reported concerning the reactivity, the associa- are produced in vacuum by adiabatic expansion (low tion and dissociation mechanisms, and the structures temperature). Therefore, the size distribution of clus- of clusters and cluster ions. The theoretical calculation ters is likely somewhat di#erent from those existing at has also been developed to discuss the structure of normal conditions. Besides, optical spectrometric clusters.5) methods do not provide information about the size Clusters in gas and condensed phases have been distribution of clusters. investigated by mass spectrometry with several tech- The nature of water clusters has been long-standing niques.1)῍4), 6)῍10) The supersonic free jet expansions are interest, because water is indispensable for all kinds of the most widely utilized methods for the generation of life, being the most important solvent in which a vari- clusters. Neutral clusters or cluster ions are produced ety of chemical reactions take place. A large number of in vacuum by an expansion of a sample vapor mixed studies have been focused on water clusters. For in- with inert gas at high pressure through a molecular stance, extensive stability of a cluster with 21 water beam nozzle.1), 2), 4), 6)῍8) Another method is the adiabatic molecules (21 mer, magic numbered cluster) have been expansion of a liquid jet, in which a sample liquid is observed with weak features (e.g., 28 mer) by several directly fed to a vacuum system through an injector di#erent experimental methods6), 13)῍15) and its structure nozzle, which is surrounded by a cylindrical gas nozzle, has been estimated as a pentagonal dodecahedron.13), 15) and droplets explode adiabatically into a high The clathrate structure has been suggested experimen- vacuum.9), 10) Produced clusters are ionized by electron tally and a distorted dodecahydron has been suggested ionization or photo-ionization. These methods are by theoretical calculation.15), 17) Results of calculation useful to obtain information about the stabilities and have been reported recently for a variety of water the kinetics of association and dissociation concerning clusters5), 16) and cluster ions.17), 18) neutral clusters and/or cluster ions. Mass spectrometry is only a method which provides Utilizing laser beam, isolated clusters or cluster ions information about the size distribution of clusters, although a mass spectrum shows the size distribution of ῎a) Yokohama National University, Yokohama cluster ions but do not that of neutral clusters. Besides, Present address: 4῍37῍27, Kugayama, Suginami-ku, by previous methods utilizing the adiabatic expansion, Tokyo 168῍0082, Japan $ b) GEOCHTO Ltd., Tokyo it is di cult to measure that what sizes of clusters exist Present address: Mitsui Chemicals Inc. (580῍32 Nagaura, in gas and liquid phases under the normal conditions. Sodegaura, Chiba 299῍0265, Japan) In vacuum, the dissociation of clusters occurs quite c) Hoshi Pharmaceutical College (2῍4῍41 Ebara, Shinagawa- rapidly. Molecular ions of water can be observed by ku, Tokyo 142῍0063, Japan) ordinary electron ionization (EI) under the order of

ῌ 1 ῌ M. Tsuchiya, T. Tashiro, and A. Shigihara

῍3 ῌ 10 Pa, while the sizes of water cluster ions increase less abundant mixed cluster ions, (C2H5OH)m(H2O)nH , with the pressure in the ion source, e.g., chemical ion- have been observed for ethanol῍water mixtures. Small ization (ca.102 Pa) and atmospheric pressure ionization cluster ions were abundant and the ion abundances (ca.105 Pa). By electrospray ionization (ESI), lots of decreased exponentially with increasing the cluster large cluster ions are produced and thus, ESI tech- sizes. The molar ratios of ethanol to water (E/W) niques have been also used to study water clusters19) calculated from observed mass spectra have been too and hydration of biomolecules.20), 21) The results have large, compared with those calculated from the ethanol indicated that some of hydrated ions of biomolecules concentration of sample solutions. In contrast, LPI are the results of solvent evaporation from extensively mass spectra have shown the nearly symmetrical size hydrated ions.21) distribution with one peak top. When the sample We can observe steam rising up on hot water and liquid with an appropriate flow rate was directly suppl- above the top of a chimney. Fine particles of water ied to the sample holder (needle tip), the molar ratios should exist in spray, steam, mist and fog. Because (E/W) calculated from LPI mass spectra became close they are visible, such a visible particle contains more to the molar ratio corresponding to the concentration than 105 molecules of water in it. Fine particles of .30) If the sample liquid, however, was vaporized near water must be produced only by the hydrogen-bonding the needle tip, LPI mass spectra similar to those ob- under the normal conditions. Therefore, a variety of served by the adiabatic expansion methods, e.g., the invisible particles with the sizes between n῎105 and pure ethanol cluster ions were abundant, have been n῎1(n: number of water molecules) should exist in the observed.28) air. Preliminary results by LPI-MS for water clusters in a 1.2 Liquid ionization (LPI) mass spectrometry gas phase25) have shown the influence of the needle We have developed a method of soft ionization, temperature, suggesting that small clusters of water in termed “liquid ionization mass spectrometry,” 22)῍24) in the air go upwards more quickly than larger clusters which a liquid sample is ionized by collision with ex- do. This paper describes the size distribution of water cited argon atoms under atmospheric pressure. The clusters in gas phases, such as steam and those in the liquid ionization (referred to as LPI24), ῎1 mass spectro- air. The results indicate that the sizes of water clusters metry provides information about hydrogen-bonded existing in gas phases under atmospheric pressure are clusters23), 24) as well as non-volatile organic com- dependent on the relative humidity and temperature in pounds. the gas phase and also dependent on the temperature 3 3 Metastable argon atoms (Ar῎, P0:11.55 eV, P2:11.72 of liquid water. eV) are used to ionize a liquid sample on a sample 2. Experimental holder (needle) at atmospheric pressure as follows: 2.1 Conditions for liquid ionization Ar῎ῌnMῐnM ῌ ῌe῍ ῌAr (1) The principle of ionization and the experimental ῐ[kMῌH]ῌ ῌ[(n῍k)M῍H]῍ ῌAr setup for mass spectrometry were almost the same as Eq. (1), in the case of n῎1, has been known as Penning those previously reported,22)῍24) except for the means of ionization. The method had been developed to ionize introducing samples, because the samples were water nonvolatile organic compounds at first,22) but also vapor in steam (evaporated from liquid water) and in applied to studies of clusters, such as those of the air. water,25), 26) of carboxylic acid῍water mixtures,27) and of The flow rates of Ar, from 0.7 to 1.1 L/min, with the water῍ethanol mixtures.24), 28)῍30) Recently, it has discharge currents of 20῍25 mAare appropriate for pro- become clear that the method is appropriate to investi- ducing excited argon atoms (Ar῎)toobtain stable mass gate hydrogen-bonded clusters, which exist in gas spectra. The size distribution of observed cluster ions phases and at the surface of liquids under atmospheric was dependent on the Ar flow rate. Namely, the higher pressure.24), 28)῍30) Furthermore, van der Waals clusters, the flow rate, the smaller the cluster sizes. It is reason- which have been observed by other methods utilizing able, because the abundance of water (number of mole- the adiabatic expansion in vacuum, have been hardly cules) around the needle tip decreases with increasing observed by the LPI-MS. Therefore, the influence of the Ar flow rate. Therefore, the Ar flow rate was kept adiabatic condensation may be negligible in LPI and it constant at 750 (or 800) mL/min during a series of is reasonable that LPI mass spectra are fairly di#erent, experiments. Besides, in the cases for measuring water especially for binary mixtures,29), 30) from previous in the air, the relative humidity in the ion source was mass spectra obtained utilizing the adiabatic expan- always measured in order to estimate the abundance of sion. water in the ion source. For instance, by one of adiabatic expansion meth- Other experimental conditions were almost the same 10) ῌ ods, abundant ethanol cluster ions, (C2H5OH)mH ,and as previously reported for soft ionization. Namely, the distance between the needle tip and the pinhole was 3 ῎1 LI had been used for the abbreviation of this method mm. A needle (made of Mo) was used as a sample 24) untill 1997, but now LPI has been used instead in order holder (shown in Figs. 1 and 2) and ions formed at the to avoid confusion with “laser ionization.” The ioniza- needle tip were observed as a mass spectrum.23), 24) The tion process starts by collision between solute molecules and excited argon atoms (so-called Penning ionization) voltages applied to the needle (VE), pinhole, skimmer-1, followed by ion-molecule reactions occurred at the liquid and skimmer-2 were 1.2῍1.4 kV, 30 V, 15 V, and 0῍1V, surface. Namely, LPI means “Liquid surface Penning respectively. If the voltage applied to the pinhole (VP) Ionization.” or the skimmer-1 was increased, the decomposition of

ῌ 2 ῌ Water Clusters in Gas Phases Studied by Liquid Ionization Mass Spectrometry cluster ions are observed. The temperature of the tween 40 and 65 mL/min. At higher flow rates than 70 needle can be controlled by means of the heater mL/min, larger cluster ions were observed, although current.22)῍24) the reproducibility became poor and the ion abundance Each mass spectrum from m/z 10 to 800 or 1000 was decreased. scanned in 2῍3sand recorded repeatedly about 20῍50 2.3 Water vapor in air (in the ion source) times using a data system (personal computer). Each Mass spectra of water vapor in the gas phase in the mass spectrum shown in this paper is an average of 5῍ ion source were measured by another quadrupole mass 20 mass spectra recorded in succession. spectrometer (ULVAC MSQ 2000, Kanagawa, Japan) 2.2 Water vapor generated by heating water in a equipped with the LPI ion source. Schematic diagram flask of the instrumental setup is shown in Fig. 2. Aquadrupole mass spectrometer (JEOL QH100, The ion source of LPI mass spectrometer was surro- Tokyo) equipped with LPI ion source was used for unded by a big box (50῍60῍80 cm) made of polysty- measuring water vapor in steam, which was evapora- rene plates and the humidity in the box was controlled ted by heating water (20῍60῎)inaflask and trans- by introducing wet air or dry air. Wet air was supplied ferred into the ion source using the device shown in from a wet air generator (Daiichi-Kagaku, Tokyo) as Fig. 1. A glass tube (inner dia.: 1 cm, 15 cm long from shown in the front view of Fig. 2. In the generator (B in the neck of the flask to a side tube of the ion source) for Fig. 2), compressed air flowed through a bubbling tube transferring water vapor was surrounded with a vinyl filled with water. Both the temperature of water and tube, through which temperature-controlled water was the flow rate of air were controlled. Dry air was pro- flowing to keep the temperature of the transfer-tube duced by a similar generator, in which the compressed constant. When keeping the tube at 1῎,the running air flowed through two tubes filled with adsorbents for water was cooled with ice. water, and the flow rate of air was controlled. The air Water was purified with ion exchange resins, filtered from the generator at atmospheric pressure (ῌ1m3/ through fine tubes and distilled. Purified water was min) flowed into the box through a hole (dia.: 10 cm) on poured into the flask equipped with two side tubes as the polystyrene plates (side) near the generator and shown in Fig. 1. One of side tubes was used as a carrier flowed out through another hole on the plate gas inlet and the other tube was used for holding a (backside). Generated air also flowed out through a thermometer. The center tube was connected to the space between the front plate and the side plates of LPI ion source through the transfer-tube. The distance the big box. When measuring water vapor in the air between the end of the transfer-tube in the flask and in the laboratory room, the front plate was open the surface of liquid water was kept at 5 mm in order to without working the generator. observe water clusters present near the surface of The relative humidities and temperatures inside the liquid water. Because the longer the distance, the ion source (A) and the box were always measured using smaller the sizes of observed cluster ions, probably due two hygrometers (Chino Corporation, HN-U2A, Tokyo), to the decomposition of larger clusters during flying respectively. One of hygrometers (C) was installed in upwards from the liquid surface. The flow rate of the side tube of the ion source as shown in the front carrier gas (Ar) was kept at 60 mL/min, because simi- view (not shown in the side view) of Fig. 2. The sensor lar mass spectra were observed at the flow rates be- of hygrometer was placed at about 7 mm from the

Fig. 1. Schematic diagram of the device for generating steam by heating liquid water in a ﬂask and for introducing water vapor (steam) through a transfer tube into LPI ion source. Carrier gas (Ar; 60 mL/min) was used for transferring water vapor. The temperature of liquid water in the ﬂask was controlled by means of a water bath. The transfer-tube was surrounded by a vinyl tube, in which temperature-controlled water was running from another water bath.

ῌ 3 ῌ M. Tsuchiya, T. Tashiro, and A. Shigihara

Fig. 2. Schematic diagrams showing a front view and a side view of the experimental setup for measuring water vapor in the air. LPI ion source (A) was surrounded by a big box (made of polystyrene plate, 50῍60῍80 cm). The humidity inside the box was controlled by humidified air or dry air (100 L/min) flowing into the box from the respective generator (B). The relative humidity and temperature in the ion source were measured using a hygrometer (C). Another hygrometer (not shown in Fig. 2) placed 10 cm below the ion source was used for measuring the humidity and temperature in the box for comparison. needle tip. The other hygrometer (not shown in Fig. 2) at 46ῑ.The “BP” indicated on the top of each Figure was placed in the box, about 10 cm below the ion means the intensity of the base peak, which is propor- source. tional to the output signal from the amplifier connect- The temperature of the needle was ambient (25῍ ed to the electron multiplier. In the case of Fig. 3a, the 30ῑ)for measuring water vapor in the gas phase, base peak intensity (BP ῌ38.5) means abundant ions because the condensation of water on the needle enough to indicate that the condensation of clusters at (sample holder) was not significant at the relative hu- 33ῑ was negligible.23) If the condensation occurred, midity of lower than 60ῒ.Other experimental condi- the BP gave a low value (e.g., BP῏15). tions were nearly the same as those described for mea- The mass spectrum (Fig. 3c) obtained at the needle- suring the steam (2.2). temperature of 65ῑ shows much smaller cluster ions, indicating that the thermal decomposition of clusters 3. Results and Discussion took place at the surface of heated needle (65ῑ). At 3.1 Water clusters in steam needle-temperature of 25῍35ῑ,LPI mass spectra LPI mass spectra obtained using the device shown in looked similar and the total ion abundances were also Fig. 1 are presented in Figs. 3῍5. The e#ects of the similar for them. At the needle-temperatures above needle temperature on LPI mass spectra have been 50ῑ, the sizes of cluster ions intend to decrease with reported for water25) and acetic acid῍water system.27) the temperature, indicating that the thermal decompo- The needle (sample holder) should be heated for less sition took place more at higher temperature (ῐ50ῑ). volatile liquids, such as acetic acid, because the conden- Considering these results and the results already sation took place at ambient temperature and gave obtained for water25) and other liquids,27) the needle- mass spectra showing larger cluster ions than those temperature was kept at 33ῑ for the experiments actually existing in the gas phase. In contrast, too high shown in Figs. 4 and 5. Figures 4 and 5 show the temperature causes the decomposition of clusters to e#ects of temperatures of liquid water in the flask and give smaller cluster ions than actual ones. of the transfer-tube on the size distribution of water Figure 3 shows the e#ects of temperature of the clusters in steam. needle on LPI mass spectra, which present the size Mass spectra of water vapor evaporated from the distribution of water cluster ions. The temperatures of liquid water at 50ῑ are shown in Figs. 4a, b, and c, liquid water in the flask and the transfer-tube (shown which were measured with the tube-temperatures of in Fig. 1) were both 50ῑ.LPI mass spectra observed at 30ῑ a), 50ῑ b), and 80ῑ c), respectively. Mass spec- the needle- temperatures of 33ῑ and 46ῑ (Figs. 3a and trum shown in Fig. 4a (tube temperature῎30ῑ)exhib- 3b, respectively) exhibit water cluster ions ranged from its much larger cluster ions than those shown in Figs. m/z 100 to 700 with the base peak at m/z 379 (21 mer). 3, 4b, and 4c. Although cluster ions larger than mass of The size distribution observed at 46ῑ (b) shifted to 800 must exist, it was unable to observe such large slightly smaller mass numbers than that at 33ῑ (a), clusters because of the mass range limitation of the probably due to the thermal decomposition of clusters instrument used. The base peak at m/z 505 corre-

ῌ 4 ῌ Water Clusters in Gas Phases Studied by Liquid Ionization Mass Spectrometry

Fig. 3. LPI mass spectra of water vapor (steam) Fig. 4. LPI mass spectra of water vapor observed using observed using the device shown in Fig. 1. The the same device (Fig. 1). The temperatures of ῍ ῍ ῍ temperatures of the needle (sample holder) : a) the transfer-tube: a) 30 ,b)50 ,andc) 80 , 33῍,b)46῍,andc) 65῍,respectively. The respectively. The temperatures of the liquid ῍ temperatures of the liquid water and the water and of the needle were kept at 50 and ῍ transfer-tube were both kept at 50῍. at 33 ,respectively. sponding to MHῌ of 28 mer is more intense than that of 4c) exhibits much smaller cluster ions, suggesting that well known 21 mer (m/z 379). The intensity of the base the thermal decomposition took place more signiﬁcant- peak observed in Fig. 4a was about half of those ob- ly in the hotter transfer-tube. served in Figs. 4b and 4c. As described later, if the Mass spectra of water vapor evaporated from the liquid layer of water on the needle was thick, larger liquid water at 20῍ were shown in Fig. 5. The temper- cluster ions with less abundance were observed in LPI atures of transfer-tube were kept at 20῍ a), 50῍ b), and mass spectra.23) In such case, observed size distribution 80῍ c), respectively. The sizes of cluster ions shown in of cluster ions might be to some extent larger than the Figs. 5a, b, and c, are relatively small compared with size distribution of neutral clusters existing in the those shown in Figs. 4a, b, and c, respectively. The vapor. mass spectra (Figs. 5b and 5c) measured at higher The mass spectrum measured at the tube tempera- tube-temperatures (50 and 80῍)show smaller cluster ture of 50῍ (Fig. 4b) exhibits smaller cluster ions than ions than those shown in Fig. 5a. those in Fig. 4a and looks similar to that in Fig. 3a, 3.2 Inﬂuence of the temperatures of liquid water and which was measured under the same experimental con- transfer-tube ditions on a di#erent day. At 80῍,mass spectrum (Fig. The results obtained for water clusters in the steam

ῌ 5 ῌ M. Tsuchiya, T. Tashiro, and A. Shigihara

Fig. 6. Inﬂuence of temperatures of the liquid water in the ﬂask and of the transfer-tube on the average cluster size (N). The temperatures of the transfer-tube (῏:30῍, ῒ:50῍, ῑ:80῍)

and 5, too). Even when the tube-temperature was changed, the absolute amount of water in the tube and in the ion source must be almost the same, because the temperature of liquid water and the flow rate of Ar flowing through the tube were kept constant. If those cluster ions were produced by adiabatic cooling, cluster sizes should be independent on the tube temperature, because the concentration of water in the ion source (ῌin the tube) must be nearly constant. There- fore, the e#ects of the tube-temperature indicate exact- ly that the decomposition of clusters in the gas phase (in transfer-tube) must be promoted by heating and LPI mass spectra show actually the change in the cluster size distribution in the gas phases. It is interesting to note that the e#ects of the temperature of liquid water were opposite to that in a gas phase. This may be also reasonable, however, because larger clusters in the liquid can evaporate at higher temperature, which gives enough energy for evaporation. We can observe steam (extra large clusters) rising Fig. 5. LPI mass spectra of water vapor observed using up from hot water in a kettle by boiling, although large the same device (Fig. 1). The temperatures of clusters observed by the mass spectrometer are too the transfer-tube: a) 20῍,b)50῍,andc) 80῍, small to see by our eyes. respectively. The temperatures of the liquid 3.3 Water clusters in air water and of the needle were kept at 20῍ and For measuring water clusters in the air, the appara- at 33῍,respectively. tus shown in Fig. 2 was used. The humidity in the LPI ion source was controlled by introducing humidified air or dry air into the box (Fig. 2). When the relative under several di#erent conditions, including the res- humidity was lower than 60῎,the condensation of ults shown in Figs. 3῍5were summarized in Fig. 6. The water on the needle was generally negligible, which ordinate means average cluster size, N, calculated from means that the mass spectra show the size distribution each mass spectrum, as defined below. of clusters in a gas phase. If the relative humidity became higher than 65῎,the condensation of fine par- NῌSn I /SI i i i ticles of water began to occur on the wall inside the ion The ni is the number of water molecules in a cluster source. When the condensation occurred at the needle ion corresponding to ith ion peak, and Ii is the intensity tip, the size distribution of observed cluster ions of ith ion peak, which is proportional to the output became larger than those observed without condensa- from the secondary electron multiplier of the mass tion. Therefore, all mass spectra in this paper were spectrometer. These values, niIi and Ii,were summed measured with the relative humidity of lower than up for all peaks in each mass spectrum. 65῎ in order to avoid condensation, and the needle Figure 6 indicates that the average cluster size N (sample holder) and inside the box were kept at ambi- increases with the temperature of liquid water, while it ent temperatures for the experiments shown in Figs. decreases with the tube-temperature (as seen in Figs. 4 7῍9.

ῌ 6 ῌ Water Clusters in Gas Phases Studied by Liquid Ionization Mass Spectrometry

Fig. 7. LPI mass spectra of water vapor in the air in the ion source observed using the experimental setup shown in Fig. 2. The relative humidities in the ion source: a) 52῍,b)56῍,andc) 62῍. The temperature in the ion source: all 27.2ῌ

The relative humidity inside the ion source varied the readings of hygrometer (C) and LPI mass spectra according to the humidity in the room where the in- both indicated that the humidity in the ion source strument was installed. For instance, when the outside increased rather quickly and reached a sort of quasi- humidity was high, like a rainy day, larger cluster ions equilibrium state. were observed (Fig. 7) compared with those observed The mass spectra shown in Figs. 7 and 8 were ob- in a fine less humid day (Fig. 8). The humidity inside tained by opening the front plate of the box. When the ion source was always lower than those in the room opening the front plate, the relative humidities in the (and box), because water vapor was flowing into the ion source and the box changed according to the hu- ion source by di#usion from the air in the room and midity in the room. The humidities in the box and in was diluted with Ar gas (continuous flow for ioniza- the room became almost the same within 20 min. Re- tion). The di#erences in humidity between these two gardless of the room air or the artificial air, similar places were 1/10῍1/5 of the humidity (e.g., when the mass spectra were observed when the relative humidi- relative humidity in the ion source was 45῍,that in ty was the same, although the reproducibility was not the box was 52῍). Considering the fact that water so good. For instance, the mass spectrum shown in Fig. vapor di#used to the needle tip through the open end 7a exhibited large clusters more abundantly compared of the ion source, such di#erences may be reasonable. with that shown in Fig. 8c. In general, the cluster sizes When the generated wet air began to flow into the box, increased with the relative humidity.

ῌ 7 ῌ M. Tsuchiya, T. Tashiro, and A. Shigihara

Fig. 8. LPI mass spectra of water vapor in the ion source observed using the same setup (Fig. 2). Water vapor in the air (shown in a)) was coming from the room where the mass spectrometer was installed. Relative humidities in the ion source: a) 27῎,b)33῎,andc) 50῎,and temperatures there: a) 25.5῍,b)29.3῍,and c) 28.6῍,respectively.

Amass spectrum shown in Fig. 8a was observed When the humidity became lower than 20῎,howev- when the air cooler was working. It is indicated that er, two di#erent size-distributions appeared as shown the cluster sizes of water vapor flowing out from the in Figs. 9a῍9d, which were obtained in succession with cooler may be much smaller than those already exist- decreasing the humidity. One of the distributions con- ing in the room air. sisted of small cluster ions (nῌ2῍8) with the abundance Mass spectra shown in Figs. 9a, b, c, and d were maximum at nῌ4῍6. The other distribution consisted observed by diluting the artificial air. In the beginning of larger cluster ions (nῌ9῍24) with the maximum of the experiment, the humidified air from the genera- around nῌ13῍16. The results suggest that larger clus- tor (B shown in Fig. 2) was flowing into the box. After ters with nῌ20῍25 decompose by dilution with dry air 10῍15 min flowing of the generated air with the same to produce two smaller clusters, one of which mainly humidity, the humidity inside the box became constant consisted of 4῍7 molecules of water and the other con- (50῍60῎). Then, the humidified air stopped flowing sisted of the rest of water molecules. and the dry air started to flow into the box. The With the relative humidity higher than 30῎,the humidities in the box and the ion source decreased. At cluster sizes increased with the relative humidity and the relative humidity higher than 30῎,the cluster size the overall width of the size distribution became wider distribution gradually shifted toward smaller ones, with the humidity. The envelope of the size distribu- holding nearly symmetrical shape of the distribution. tion of cluster ions looked nearly symmetrical, regard-

ῌ 8 ῌ Water Clusters in Gas Phases Studied by Liquid Ionization Mass Spectrometry

Fig. 10. Schematic illustration for the mechanisms of ionization of clusters and of neutralization and desorption (evaporation) of cluster ions at the

needle tip. Ve#:Electric ﬁeld (due to VE: voltage applied to the needle) e#ective to capture electrons.

argon atoms (Ar῏)atthe surface of liquid as shown in

Eq. (3), in which (H2O)k denotes water clusters contain- ing k molecules of water. Because observed clusters are all protonated, the decomposition of primary ions (Eq. (4)) and/or the reactions with water clusters (Eq. (5)) may follow to form protonated clusters. Because proton a$nity (PA) of larger cluster is greater than PA of smaller cluster, proton transfer reactions like Eq. (6) ῌ may take place to form final ions, (H2O)nῐH . ῌ ῍ Fig. 9. LPI mass spectra of water vapor in the ion Ar῏ῌ(H2O)k ῒ(H2O)k ῌe ῌAr kῌ2 (3) ῌ ῌ source observed using the same setup (Fig. 2). (H2O)k ῒ(H2O)fH ῌ(OH)(H2O)k῍f῍1 k῎f (4) ῌ ῌ At first, the relative humidity in the ion source (H2O)k ῌ(H2O)n ῒ(H2O)nῐH ῌ(OH)(H2O)kῐ῍1 was kept constant (e.g., 50ῐ)with the n῎k, nῐ῎kῐ (5) humidified air and then, the dry air was flowing ῌ ῌ (H2O)fH ῌ(H2O)n ῒ(H2O)nῐH ῌ(H2O)fῐ into the box. Relative humidities in the ion n῎f, nῐ῎fῐ (6) source: a) 15ῐ,b)13ῐ,c)10ῐ,andd) 9ῐ. ῌ ῌ Temperatures there: a) 29.7῏,b)29.9῏,c) All final ions, such as (H2O)nῐH and (H2O)fH ,are 30.2῏,and d) 30.2῏,respectively. observed as LPI mass spectra. Figure 10 illustrates the mechanisms of cluster ion formation, of the recombina- tion of a produced ion and an electron, and of the less of the humidity, except for magic numbered clus- desorption (evaporation) of cluster ions at the needle ter ions (21 mers). The results suggest that a quasi- tip. The voltage applied to the needle (VE)has three equilibrium state of the size distribution was attained important functions.23), 24) One of them is to capture in these cases. electrons produced by Eq. (3). Because the recombina- 3.4 Mechanisms of cluster ion formation tion of a positive ion and the electron emitted by Because the mechanism of cluster ion formation by Penning ionization (Eq. (3)) is very quick under atmo- the LPI method has been reported previously for eth- spheric pressure, no ions were observed at VE lower 23), 24) anol῎water binary system, the ionization mecha- than 700 V. The e#ective electric field (Ve#), which is nism for water clusters is described here briefly. supplied from VE as shown in Fig. 10, is required to Water clusters are ionized by collision with excited capture electrons at the needle tip. The voltage VE

῍ 9 ῍ M. Tsuchiya, T. Tashiro, and A. Shigihara

Table 1. Total Energies of A aNeutral Cluster, B Free Molecules, and Their Di#erence (D῏B῍A), and D/n (n῏number of Water Molecules)

A B D῏B῍A D/n Formula (Structure) (H2O)n:[au] n῎H2O: [au] [kJ/mol] [kJ/mol]

H2O ῍76.01075

(H2O)2 ῍152.03046 ῍152.02149 23.5 11.8

(H2O)3 (triangle) ῍228.06000 ῍228.03224 72.9 24.3

(H2O)4 (square) ῍304.09019 ῍304.04299 123.9 31.0

(H2O)5 (pentagon) ῍380.10952 ῍380.05373 159.1 31.8

(H2O)5 (ring, unsym.) ῍380.11433 ῍380.05373 171.3 34.3

(H2O)6 (῎a: hexagon) ῍456.13832 ῍456.06448 193.9 32.3

(H2O)6 (῎b: 2-triangles) ῍456.14229 ῍456.06448 204.3 34.1

(H2O)7 (῎c: heptagon) ῍532.16713 ῍532.07523 241.3 34.5

(H2O)7 (῎d: heptagon) ῍532.16967 ῍532.07523 248.0 35.4

῎a: Hexagon; a quasi-planar cyclic form. ῎b: Each molecule is on a corner of a triangle pole, as shown in Fig. 11. ῎c: Five molecules are in one plane and other two molecules are out of the plane in the same side. ῎d: Five molecules are in one plane and two are out of the plane, but each in the opposite side. appropriate to capture electrons and observe ions those of free molecules. The latter is n times the total under soft conditions is 1.25῍1.4 kV. If the liquid layer energy of one water molecule. The results indicate that on the needle tip became too thick, then no ions were the clusters are more stable than corresponding free observed, because Ve# was unable to reach above the molecules and the cyclic structures are more stable liquid surface. Thus, the ions produced outside the Ve# than the chain type structures. Besides, one large boundary are neutralized and not observed. Therefore, cluster is more stable than two smaller clusters. For only ions produced inside the Ve# boundary are ob- instance, the di#erence between a hexamer (῎binTable served as a mass spectrum. Figure 10 is an illustration 1) and two trimer is, 456.14229῍(228.06000)῎2῏ and does not show actual cluster sizes. 0.02229 (῏58.5 kJ/mol). Figure 11 shows the opti- In the case of measuring clusters in a gas phase, the mized structure of the hexamer, which is the most liquid layer at the needle tip should be very thin (Fig. stable one so far calculated.

10) and the Ve# boundary reaches far from the needle The di#erence D in total energy per a molecule of tip, resulting in much abundant ions compared with water (D/n) shown in Table 1 could be indicative of the the cases of measuring liquid samples.23), 24) Then, LPI stability of clusters. The values of D/n for n῏5῍7are mass spectra (e.g., Figs. 3῍8) should be related to the similar to each other, while the mass spectra shown in clusters existing in the gas phase around the needle tip Fig. 9 indicate that the clusters with n῏4῍6were much

(inside the Ve# boundary, though). abundant than the clusters with n῏7. Considering the ῌ In LPI mass spectra, molecular ions, (H2O)H and structure of ice and magic numbered clusters, such as ῌ (H2O) ,were scarcely observed, except for very dry air. 21 mer and 28 mer, clusters of water may be mainly At first, this reason was thought to be that Ar῎ is consisted of five and six membered rings. Therefore, unable to ionize a water molecule, because the ioniza- the results of calculation and observations are consis- tion energy of a water molecule (12.2 eV) is higher than tent each other and suggest that the decomposition of the internal energy of Ar῎ (11.7 eV). However, when awater cluster (nῐca.25) by dilution with dry air He῎ (metastable He, whose internal energy is 19.7 eV) (giving the least energy) yield two smaller clusters, one was used in place of Ar, almost the same mass spectra of which is mostly a stable cluster with n῏5or6.In were obtained for water clusters, showing no molecular addition, a cluster with n῏4isthought to be an ele- ions. The results suggest that free water molecule mental structure to constitute 6 membered rings exist- exists rarely in the air at atmospheric pressure, except ing in ice. A sort of quasi-equilibrium states may be for very dry air. By adiabatic expansion, larger cluster reached at a certain temperature by heating (thermal ions have been observed in Ar than in He.6), 7) There- collisions) to result in one size distribution (Figs. 3῍5 fore, the agreement between the results obtained with and 7) without any special small clusters (n῏4῍6). Ar and those with He also indicates that the adiabatic 4. Conclusion condensation is likely negligible to occur in our instrument, as previously reported.24) The size distribution of cluster ions changed accord- These results strongly indicate that lots of neutral ing to the temperature of the transfer tube as shown in clusters exist in a gas phase and the sizes of clusters Figs. 4῍6, in spite of almost constant water concentra- increase with the temperature of liquid water and de- tion in the ion source. If the cluster sizes were mainly crease with increasing the temperature of a gas phase. determined by the adiabatic condensation, the cluster 3.5 Theoretical calculation for small clusters sizes should be nearly constant, because the water Optimized structures and their total energies of neu- concentration in the ion source were nearly the same at tral clusters of water were calculated by RHF/6- the same water temperature and the same flow rate of 31G῎.31) The total energies of small clusters (n῏2῍7) so Ar. As mentioned in Introduction section, LPI mass far calculated are shown in Table 1 compared with spectra of binary mixtures, such as ethanol῍water,

ῌ 10 ῌ Water Clusters in Gas Phases Studied by Liquid Ionization Mass Spectrometry

smooth size-distribution of cluster ions (e.g., Figs. 3῍5, 7, 8b, and 8c). The size distribution of a large number of clusters may be in a sort of quasi-equilibrium states (in local) and be observed as an average size distribution under a certain conditions. Heat in a gas phase causes the decomposition of clusters and results in the shift of cluster sizes toward smaller ones with similar shape of distribution, suggesting another quasi- equilibrium state. In contrast, by the addition of dry air, two di#erent size distributions of clusters were observed at relative humidity lower than 20῍ (Fig. 9). Similar mass spectra have been observed by other methods,19), 32) too. By electrospray ionization (ESI),19) one very wide size- distribution ranged from m/z 100 to 2000 (most abundant ions were around m/z 1000) accompanied with one narrow distribution of small cluster ions (nῌ3῍10, abundance maximum at nῌ4) have been observed. The smaller one looked similar to those observed by our method (Figs. 9c and d), indicating that these small clusters may be stable enough to exist in the air. The results shown in this paper strongly indicate that water clusters exist in water vapor (Figs. 3῍5) and also in the air (Figs. 7῍9). In addition, it is certain that the sizes of clusters in the gas phase increase with the relative humidity and with the temperature of liquid water, while they decrease with increasing the temperature of gas phase. The size distribution of clusters in agas phase may be dependent on the absolute number of water molecules in a unit space (ῌpartial pressure of Fig. 11. Optimized structure of a hexamer of water water) and the temperature of that space. with the lowest total energy (῎binTable 1) so Studies of hydrogen-bonded clusters must be one of far calculated. Top: a top view, Bottom: a the most important application of liquid ionization front view. (LPI) mass spectrometry. Although it is still di$cult to determine exact di#erences between the sizes of clusters (n)and those of observed ions (n῏ in Eqs. (5) and (6)), have been fairly di#erent from the other mass many results obtained by LPI-MS indicate that these spectra10) and van der Waals clusters, which are pro- di#erences may be small. duced by the adiabatic condensation in vacuum, have Acknowledgments been hardly observed by LPI-MS. Therefore, it is certain that the adiabatic condensation in our instrument The authors wish to express sincere thanks to late is negligible to occur. The decomposition of large Yutaka Fukase (president of GEOCHTO Ltd.) and late cluster ions occurs by application of high VP (voltage Kotaro Hama for their support in using the instru- applied to the pinhole), but scarcely does under soft ments and Hitoshi Tsuyama for his help. conditions as used in this paper. Besides, It is likely References that water cluster ions once formed are moderately stable during flight from the ion source to the ion 1) V. Hermann, B. D. Kay, and A. W. Castleman, Jr., Chem. collector in vacuum, because we have observed pro- Phys., 72,185 (1982). tonated molecules of various unstable compounds.22)῍24) 2) A. W. Castleman and R. G. Keese, Chem. Rev., 86,589 Therefore, LPI mass spectra described in this paper (1986). gave information related to the cluster sizes of water 3) T. D. Mark, Int. J. Mass Spectrom. Ion Proc., 79,1(1987). existing in the gas phase. 4) J. F. Garvey, W. J. Herron, and G. Vaidyanathan, Chem. The mass spectrum shown in Fig. 4b looks similar to Rev., 94,1999 (1994). that shown in Fig. 3a, because these were measured 5) K. Liu, J. D. Cruzan, and R. J. Saykally, Science, 271,929 (1996). under the same experimental conditions, but on di#er- 6) J. Q. Searcy and J. B. Fenn, J. Chem. Phys., 61,5282 ent days. Slight di#erences observed between them (1974). may be caused by the fluctuation in the size distribu- 7) R. J. Beuler and L. Friedman, J. Chem. Phys., 77,2549 tion of clusters. It has been known that hydrogen (1982). bonds in water clusters change their positions in the 8) D. R. Zook and E. P. Grimsrud, J. Phys. Chem., 92,6374 order of pico-seconds. The sizes and size distribution of (1988). clusters may vary very quickly. Therefore, it is reason- 9) N. Nishi and K. Yamamoto, J. Am. Chem. Soc., 109,7353 able that mass spectra show such fluctuation. In gener- (1987). al, LPI mass spectra observed in gas phases exhibit one 10) N. Nishi, K. Koga, C. Ohshima, K. Yamamoto, U. Naga-

ῌ 11 ῌ M. Tsuchiya, T. Tashiro, and A. Shigihara

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