Detailed Study on the Use of Electrospray Mass Spectrometry To

Jun 28, 2007 - Silicate speciation in aqueous solutions containing tetraalkylammonium hydroxide as template is examined by electrospray mass spectrome...
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Anal. Chem. 2007, 79, 6005-6012

Detailed Study on the Use of Electrospray Mass Spectrometry To Investigate Speciation in Concentrated Silicate Solutions Stefan A. Pelster, Ferdi Schu 1 th, and Wolfgang Schrader*

Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mu¨lheim, Germany

Silicate speciation in aqueous solutions containing tetraalkylammonium hydroxide as template is examined by electrospray mass spectrometry. A thorough study has been carried out to define and optimize the conditions of analysis for a highly concentrated reaction solution of inorganic speciessin this case, silicate oligomerssby using different ion source designs. The results reveal specific advantages with respect to the detected species. Condensation of monomers leads to oligomeric units that condense further in different reaction steps to larger species. Potential gas-phase reactions that can disturb characterization of the formed silicate species were intensively investigated and characterized. One reaction that plays a key role is the alkoxylation of Si-OH groups that is caused by two different reactions: first, a reaction of methanolate from the solvent and, second, by the involvement of the template salt, tetramethylammonium hydroxide. Solid-state formation from solution is one of the most important chemical reactions. It includes natural processes, such as biomineralization, technical processes, such as the synthesis of pigments or catalysts, or single-crystal synthesis as needed for protein structure determination. The macroscopic properties determine the performance in the target applications, but they emerge from the microscopic properties of the individual crystals, like the surface-to-volume ratio. Both the macroscopic and microscopic properties are strongly affected by the processes occurring during solid-state formation.1,2 Even the earliest stages of particle formation from solution can be of great relevance with respect to the resulting solid. Although these early stages are often discussed in terms of a nucleation event, in most cases, it is probably a complex reaction sequence leading to the formation of the solid. The better understanding of how molecules come together and form larger aggregates, which finally lead to solid particles, could be the way of controlling and directing these processes. Despite the great variety of systems that crystallize from solution, major emphasis has been placed on the investigation of microporous crystals due to their technical importance in catalysis. In particular, work is focused on silicate nucleation and crystal* To whom correspondence should be addressed. Phone: +49 (208) 3062271. Fax: + 49 208 306 2982. E-mail: [email protected]. (1) Mann, S.; Ozin, G. A. Nature 1996, 382, 313. (2) Kulmala, M.; Pirjola, L.; Ma¨kela¨, J. M. Nature 2000, 404, 66. 10.1021/ac0706729 CCC: $37.00 Published on Web 06/28/2007

© 2007 American Chemical Society

lization, which results in such important materials like zeolites or mesoporous structures.3,4 The long-known zeolites play a major role in catalysis, e.g., for cracking of heavy hydrocarbons or gasoline production from methanol, as ion exchangers, or in other molecular sieve applications like sulfur elimination from gas streams.5 The classical syntheses of these materials use a silicate or alkoxysilane in alkaline solutions, often with organic additives that act as structure-directing agents that induce the formation of a specific zeolite structure. Especially the structure directing effect of different tetraalkylammonium ions (TAA) should be emphasized here.6-10 These tetraalkylammonium salts have a strong structuredirecting effect on silicates in solutions to form different structures depending on the length of the alkyl group. The use of tetramethylammonium ions (TMA) leads to the preferred formation of a silicate octamer that has the structure of a cube, where Si atoms occupy the corners and oxygen atoms are located at the bridges in between. Although different species are present in solution, the cubic octamer is the most prominent species after a certain reaction time. Tetraethylammonium ions (TEA) direct the silicates to preferentially form a prismatic hexamer, while the structuredirecting characteristics of tetrapropylammonium ions (TPA) leads to the formation of higher species that already can build particles in solution. A reaction scheme that illustrates a major pathway and the formation of some species that develop in solution from the silicate precursor in the presence of the template is displayed in Figure 1. The first steps are the hydrolysis of the silicate precursor and the first condensation leading to the formation of the dimer. Very quickly, higher organized species develop with the octamer and hexamer as the main structures, depending on the template. After the main species are formed, the reaction is in an equilibrium state, where the species are in stable condition, and that is changed only after external influences, like dilution or a change of concentration of either the template or the precursor compound. (3) Corma, A. Chem. Rev. 1995, 95, 559. (4) Akporiaye, D. E. Angew. Chem., Int. Ed. 1998, 110, 2594. (5) Ookushi, T.; Onaka, M. Chem. Commun. 1998, 2399 (6) Schoeman, B. J.; Regev, O. Zeolites 1996, 17, 447. (7) Burkett, S. L.; Davis, M. E. J. Phys. Chem. 1994, 98, 4647. (8) Burkett, S. L.; Davis, M. E. Chem. Mater. 1995, 7, 920. (9) Burkett, S. L.; Davis, M. E. Chem. Mater. 1995, 7, 1453. (10) Pelster, S. A.; Schrader, W.; Schu ¨ th, F. J. Am. Chem. Soc. 2006, 128, 4310.

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Figure 1. Simplified reaction scheme leading to the formation of silicate species in the presence of a structure-directing template, Left: with TMA leading to a cubic octamer as the major species. Right: with TEA leading to a prismatic hexamer as the leading species. It has to be noted that additional species are present at the end of the reaction, but all are minor in concentration. For details, see Pelster et al.10

However, the nature of the silicate-template interaction and the condensation pathways are unknown. At this point it is still under discussion whether growth proceeds by a monomer addition sequence or the assembly of preformed building blocks. Thus, the synthesis of zeolitic materials is often based on empirical findings or the exploration of parameter spaces by trial and error. Deeper insights into the mechanisms by which solids form from solution is hampered by the shortcomings present analytical methods have. There are a number of methods available to analyze both precursor molecules as well as resulting solids. However, the observation of the nucleation process is still problematic due to the lack of adequate techniques to directly monitor formation of molecules of about 100-1000 atoms, which is the size scale on which many of the relevant processes occur. The most often used and very powerful technique is 29Si NMR spectroscopy, which has superseded the previously used GC analysis, and which is possibly the only method revealing structural information of the detected 6006

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units. 29Si NMR spectroscopy was one of the first analytical methods used to investigate silicate species and to elucidate the structure of the formed species.11-15 Further fundamental studies concerning silicate species in aqueous solutions found that nucleating silicate solutions contain a limited number of small, highly condensed molecules in dynamic equilibrium.16-23 These (11) Lippmaa, E. T.; Alla, M. A.; Pehk, T. J.; Engelhardt, G. J. Am. Chem. Soc. 1978, 100, 1929. (12) Hoebbel, D.; Garzo, G.; Engelhardt, G.; Ebert, R.; Lippmaa, E. T.; Alla, M. A. Z. Anorg. Allg. Chem. 1980, 465, 15. (13) Lippmaa, E. T.; Ma¨gi, M.; Samoson, A.; Engelhardt, G.; Grimmer, A. R. J. Am. Chem. Soc. 1980, 102, 4889. (14) Engelhardt, G.; Hoebbel, D. Z. Chem. 1983, 23, 33. (15) Hoebbel, D.; Garzo, G.; Engelhardt, G.; Vargha, A. Z. Anorg. Allg. Chem. 1982, 494, 31. (16) Knight, C. T. G. J. Chem. Soc., Dalton Trans. 1988, 1457. (17) Harris, R. K.; Knight, C. T. G. J. Mol. Struct. 1982, 78, 273. (18) Harris, R. K.; Knight, C. T. G. J. Chem. Soc., Faraday Trans. 2 1983, 79, 1525. (19) Harris, R. K.; Knight, C. T. G. J. Chem. Soc., Faraday Trans. 2 1983, 79, 1539.

studies exclude the existence of embryonic precursor species with a specific structure.16,17,24 In contrast, such a precursor species was suggested by Kirschhock et al., who also used 29Si NMR spectroscopy for the analysis of soluble silicates.25-28 However, this suggestion was severely challenged by other groups.10,23,24,29-31 Other techniques that have been applied for the investigation of the later stages of solid-state formation, such as diffraction, require the existence of a long-range order, which typically develops only after the initial, crucial processes are finished. Scattering methods like DLS are in principle able to address the size scale relevant in the early solids formation stages, but the information on the species present in solution is limited to the size and in fortunate cases the shape of the particles. However, using a combination of diffraction, scattering, and electron microscopy techniques on nucleating silicate solutions allowed us to obtain results that suggest the direct transformation of a colloidal phase to crystalline structures in some cases.32-35 As the very few publications of mass spectrometric applications indicate, this technique is still quite uncommon for the investigation of silicate solutions. 10,36-38 In previous work, we have proven the general applicability of electrospray mass spectrometry (ESIMS) to investigate condensation reactions in aqueous silicate solutions.10,36 ESI-MS is one of the few techniques that is able to come close to detect species during the early stages of solids formation and approach it from the side of precursors in solution. This method allows the observation of the reaction steps in the oligomerization of silicates. (20) Kinrade, S. D.; Knight, C. T. G.; Pole, D. L.; Syvitski, R. T. Inorg. Chem. 1998, 37, 4272. (21) Kinrade, S. D.; Knight, C. T. G.; Pole, D. L.; Syvitski, R. T. Inorg. Chem. 1998, 37, 4278. (22) Kinrade, S. D.; Donovan, J. C. H.; Schach, A. S.; Knight, C. T. G. J. Chem. Soc., Dalton Trans. 2002, 1250. (23) Knight, C. T. G.; Wang, J.; Kinrade, S. D. Phys. Chem. Chem. Phys. 2006, 8, 3099. (24) Knight, C. T. G.; Kinrade, S. D. J. Phys. Chem. B 2002, 106, 3329. (25) Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965. (26) Kirschhock, C. E. A.; Ravishankar, R.; Van Looveren, L.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4972. (27) Ravishankar, R.; Kirschhock, C. E. A.; Knops-Gerrits, P.; Feijen, E. J. P.; Grobet, P. J.; Vanoppen, P.; De Schryver, F. C.; Miehe, G.; Fuess, H.; Schoeman, B. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4960. (28) Kirschhock, C. E. A.; Kremer, S. P. B.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 2002, 106, 4897. (29) Kragten, D. D.; Fedeyko, J. M.; Sawant, K. R.; Rimer, J. D.; Vlachos, D. G.; Lobo, R. F.; Tsapatsis, M J. Phys. Chem. B 2003, 107, 10006. (30) Ramanan, H.; Kokkoli, E.; Tsapatsis, M. Angew. Chem., Int. Ed. 2004, 43, 4558. (31) Davis, T. M; Drews, T. O.; Ramanan, H.; He, C.; Dong, J.; Schnablegger, H.; Katsoulakis, M. A.; Kokkoli, E.; McCormick, A. V.; Penn, R. L.; Tsapatsis, M. Nat. Mater. 2006, 5, 400. (32) Mintova, S.; Olson, N. H.; Valtchev, V.; Bein, T. Science 1999, 283, 958. (33) Mintova, S.; Olson, N. H.; Bein, T. Angew. Chem., Int. Ed. Engl. 1999, 38, 3201. (34) Mintova, S.; Petkov, N.; Karaghiosoff, K.; Bein, T. Microporous Mesoporous Mater. 2001, 50, 121. (35) Mintova, S.; Olson, N. H.; Senker, J.; Bein, T. Angew. Chem., Int. Ed. 2002, 41, 2558. (36) Bussian, P.; Sobott, F.; Brutschy, B.; Schrader, W.; Schu ¨ th, F. Angew. Chem., Int. Ed. 2000, 39, 3901. (37) Eggers, K.; Eichner, T.; Woenckhaus, J. Int. J. Mass Spectrom. 2005, 244, 72. (38) Sobott, F.; Schunk, S. A.; Schu ¨ th, F.; Brutschy, B. Chem. Eur. J. 1998, 4, 2353.

In this study, we investigated condensation reactions of aqueous/alcoholic silicate solutions that are commonly used for zeolite synthesis. To follow the different steps in solid-state formation, the experiments were carried out under “clear solution” conditions where a tetraalkoxysilane was used as silicon source and tetraalkylammonium hydroxide (TAAOH) as organic template.39 This reaction was chosen because it has been investigated using 29Si NMR and can be used as a model reaction. However, mass spectrometric analysis of these oligomers is very difficult due to the high salt content and high pH, and therefore, a detailed study with respect to the mass spectrometric parameters was needed. The technical circumstances and conditions for measurements of highly concentrated solutions are described. Special interest is being taken in the occurrence of disturbing gas-phase reaction and clustering of the silicates in the analytical system. Additional results indicate the validity of the approach, and a comparison to 29Si NMR data shows the advantages that ESI-MS can bring to the analysis of inorganic compounds. EXPERIMENTAL SECTION All reagents were obtained from commercial sources and used as received. Tetramethoxysilane (TMOS, 99%, Fluka, Steinheim, Germany) was used as a silicon source for most experiments. Additionally, for measurements of hydrolysis effects, tetraethoxysilane (TEOS, 99%, Fluka), tetra-n-propoxysilane (TPOS, 97%, Lancaster, England), and tetra-n-butoxysilane (TBOS, 97%, Fluka) have been applied. Tetramethylammonium hydroxide (TMAOH, 25% in water, purum, Fluka), tetraethylammonium hydroxide (TEAOH, 35% in water, Aldrich), and tetra-n-propylammonium hydroxide (TPAOH, 40% in water, Alfa Aesar) were used as the organic templates for the reactions. For a series of experiments, deuterated TMAOD‚5D2O (d13, 98%, Eurisotop GmbH, Saarbru¨cken, Germany) was used as a template. Methanol was purchased from Merck KGaA (Darmstadt, Germany). Water was triply distilled. For 29Si NMR spectroscopy, a small amount of D2O was added to the samples (Deutero GmbH, Kastellaun, Germany). All samples were prepared in 30-mL polypropylene bottles equipped with magnetic stirrers. The molar composition of the samples was SiO2:TAA2O:H2O:MeOH ) 1.00:1.12:52.63:4.00. For investigations regarding methoxylation of silicates, a series of measurements was performed with varying water/methanol ratios, the TMOS/TMAOH ratio unaffected. The samples were prepared by dissolving TAAOH (0.0038 mol) in the corresponding amount of water and methanol. The mixture was stirred for 5 min at 500 rpm and TMOS (0.0033 mol) was added slowly while stirring was continued. For these studies, the reaction mixtures were used after the reaction to the main species had been finalized, causing the silicates to exist in an equilibrium state. Under these conditions, the preferred species are stable and can be analyzed using ESI-MS. In order to find the optimum operating conditions for the higher concentrated and easy to nucleate reaction solutions, it was possible to employ four different mass spectrometers with different analyzers and, more important, different ion source designs. The quadrupole instruments were (i) a Hewlett-Packard (39) Schoeman, B. J.; Sterte, J.; Otterstedt, J. E. J. Chem. Soc., Chem. Commun. 1993, 994.

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HP 598B and (ii) a Fison-VG-Plattform II spectrometer. The HP instrument was equipped with an Analytica of Bradford ion source that employs an on-axis sprayer system, while the Fisons ion source uses a Z-spray alignment. The electronic ion trap was a Bruker Esquire 3000 with an Agilent orthogonal ion source. The magnetic ion trap was an Apex III Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) from Bruker again equipped with the Agilent orthogonal ion source. Additional experiments concerning gas-phase reactions have been carried out using an Applied Biosystems Q-Trap 4000, equipped with a Turbo Ion source. The samples were injected by direct infusion and measured with an ESI voltage of 4 kV. The average flow rate was between 60 and 120 µL/h, the most often applied 100 µL/h. The desolvation temperature was set to constant 250 °C. CapExit voltage as well as cone-skimmer 1 and cone-skimmer 2 voltages, delay and accumulation times, and other technical parameters were optimized for each spectrum to achieve the best signal-to-noise ratio. Mass calibration from 100 to 3000 Da was performed using standard calibration solution (Agilent Technologies, Bo¨blingen, Germany). 29Si NMR spectra were recorded with an AMX 400 Bruker spectrometer operating at 79.483 MHz. Samples containing D2O for the deuterium locking frequency were placed in a 10-mm PTFE NMR tube liner without a glass sample tube to minimize the silicon background from the glass. The pulse sequence used to acquire the spectra was a 29Si{1H} inverse-gated experiment with a waltz16 composite pulse program for proton decoupling. Generally, for the 29Si determination, a 13.6-µs pulse with 8-s relaxation delay was used between each acquisition. The number of acquisitions ranged from 15 000 to 30 000 depending on the concentration of the solutions. RESULTS AND DISCUSSION The difficulty of analyzing aqueous silicate solutions to investigate their behavior in regard to oligomer speciation lies in the complexity of the sample. The interesting effects about the formation of higher oligomers are only observed at high concentrations of both the silicate precursor and the template ions, which makes the ESI-MS analysis more challenging. For a better understanding of the effects in solution and in order to understand and eliminate the influence of the analytical system, a detailed study of analysis of silicate species has been carried out using different mass spectrometers, as can be seen in Figure 2. Here, spectra recorded with four different mass spectrometers are displayed, allowing a good comparison of the different instruments. The obtained data correlate well with already reported data from MS and 29Si NMR.36 The silicate precursor in this case is TMOS, reacting with the template TMA, resulting predominantly in the formation of a cubic octamer at m/z 551, which can be seen in all of the spectra. One of the most critical parameters turned out to be the electrospray. Due to the high salt content and the potential for contamination, the flow needs to be as small as possible. It was found that the spray direction is of major importance for the results. Using an on-axis alignment leads to severe incrustations of the whole aperture system of the spectrometer caused by precipitating SiO2 and to a distinctive memory effect regarding 6008

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the detected species. Between measurements, the instrument has to be flushed with solvent for at least a couple of hours. An onaxis sprayer geometry also leads to results with a low signal-tonoise ratio, showing more satellite signals and generally a higher background that is caused by contamination due to the high salt content (Figure 2a). The exchange of an on-axis for a Z-sprayer alignment results in a much better spectrum (Figure 2c). The best results can be achieved with an off-axis geometry as it is realized with the ion trap MS and the FT-ICR (Figure 2c,d). The spray direction is arranged orthogonal to the analyzer direction, so that only very little precipitation can be found on the aperture. There is nearly no memory effect, which facilitates sample handling and the optimization processes. Additionally, another important parameter concerning a calm and constant spray is the sprayer itself. It was found that the properties regarding nebula generation are different among the used sprayers. The best sprayer was a triple-layer (CE) sprayer with a stainless steel spray capillary. In comparison to a fused-silica capillary that works well for other applications, here the viscosity and amount of charged ions in solution did not make for a good sprayer performance. The dynamic characteristic of a quadrupole analyzer in comparison to an ion trap allows the analysis of higher concentrated samples. The concentration necessary for formation of defined oligomeric silicate species is already close to the upper limit of the capacity of an ion trap because interactions like spacecharge effects of large ion loadings can reduce the effectiveness of these analyzers. Therefore, the risk of overfilling increases with the concentration of solute in a sample. One effect that was observed during these studies is the formation of satellite peaks next to the silicate species. Accurate mass detection using FT-ICR MS (see Figure 2d) allows us to assign these signals. From calculation of the elemental composition, it can be shown that the difference of the satellite peaks is +14, +28, and +42 Da, corresponding to a difference of CH2, C2H4, and C3H6, respectively. One explanation would be the exchange of an OH group with a methoxy or an ethoxy group. One alternative method for analyzing silicates in solution is the application of 29Si NMR, which allows the observation of highly concentrated silicate solutions in the liquid phase.36 Accordingly, 29Si NMR was used again to investigate the satellite peaks in the mass spectrum. The mass difference of 14 Da can result from a methoxylation by the methanol solvent. In a series of experiments, the use of alcohol was changed from a methanol/water mixture (see Figure 3a) to a mixture of ethanol/water (Figure 3b), to a mixture of ethanol/methanol and water (Figure 3c), to an alcoholfree solvent using purely water (Figure 3d). The results show that when using methanol as a solvent in addition to the signal from the Q0 signal from Si(OH)4 a signal with a chemical shift of 0.32 ppm was found that represents the methoxylated species. When using ethanol as a solvent, the chemical shift for the ethoxylated species is 1.29 ppm. These data are in excellent agreement with data from the literature.40 Both signals of methoxylated and ethoxylated species are present when using both methanol and ethanol as solvent. When no alcohol is present, neither methoxylated nor ethoxylated species are detect(40) Schach, A. S.; Sloan, T. A.; Knight, C. T. G. J. Chem. Soc., Dalton Trans. 1999, 3149.

Figure 2. Comparison of different sprayer instrument conditions for analysis of silicate species using the cubic octamer as an example.

Figure 3. 29Si NMR spectra of Si1 species in different alcoholic solutions. Methoxylation of silicate species partially depends on the alcohol in solution. The sample in (a) contains methanol, in (b) ethanol, in (c) both methanol and ethanol, in (d) no alcohol but pure water.

able. These results indicate the involvement of the alcoholic solvent in the reaction. A similar series of experiments has been carried out with ESIMS using the cubic octamer as an example. Here the methanol/ water ratio was changed from 2:1 (see Figure 4a) to 1:10 (Figure

4b) to pure water (Figure 4c). The difference between the first two mixtures can be seen in the reduction of intensity of the satellite peak. But surprisingly, the signal at m/z 565 still exists when no alcohol is present in solution. Here, an additional mechanism needs to be involved, since the only other carbon Analytical Chemistry, Vol. 79, No. 15, August 1, 2007

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Figure 4. ESI-MS spectra of cubic octamer from TMOS/TMAOH reaction. The ratio of methanol/water was changed from (a) 2:1 to (b) 1:10 and (c) and (d) 0%. In (d), TMAOH was substituted by TMAOD-d13.

Figure 5. Reaction scheme as an explanation of methoxylation in the gas phase.

source available is the tetramethylammonium salt. To clarify the reaction an isotopically labeled ammonium salt was used (TMAOD-d13‚5D2O). The result can be seen in Figure 4d, where two different reactions took place. One was an H/D-exchange reaction between the hydrogen atoms of the hydroxyl groups at the corners of the octamer and deuterium from D2O. Second, the difference between the cubic octamer and the satellite peak was 17 Da instead of 14 Da. This leads to the conclusion that the methoxylation is caused by two reactions. In the given pH range of ∼14, the alcohol resulting from the hydrolysis of the silanes partially exists in the form of its alcoholate, which in turn is able to substitute hydroxyl groups via nucleophilic attack. Under these conditions, equilibria between the different species exist in solution:

ROH + OH- h RO- + H2O R3SiOH + RO- h R3SiOR + OHAdditionally, a reaction in the gas phase in the mass spectrometer involves the template salt where hydrogen from the hydroxyl groups is exchanged against a methyl group from the template (see Figure 5). These results show that some species are formed in the liquid phase and some minor species are formed in the gas phase. For a better understanding of the experimental conditions, some experiments were carried out to detect and eliminate artifacts that are formed in the analytical system. For these experiments, a triple-quadrupole mass spectrometer with a linear ion trap as the third sector (Q-Trap) was used. The results are displayed in Figure 6, again using the TMOS/TMAOH reaction as a model system, which forms the cubic octamer as the main species. In the top spectrum, the major signals of the cubic octamer are present, as are some minor compounds. This spectrum was 6010

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recorded by scanning the first quadrupole (Q1). The middle spectrum shows the results from a Q3 scan, where the ions are passing through Q1 and Q2 and are scanned with Q3. Here, all major signals are still present, but overall, the spectrum looks much simpler and the number of peaks is reduced. The lower spectrum was recorded using Q3 as an ion trap, thus, trapping an amount of ions in the analyzer before scanning and sending them out to the detector. The major difference in these scanning modes is the gas pressure in the collision cell (Q2) of the triple-quadrupole MS. This pressure is increased for a Q3 scan and is even higher for an ion trap scan in Q3. Especially, for the ion trap scan, the gas pressure in Q2 needs to be increased to allow a cooling of the ions before trapping them in Q3. Additionally, the ions need a longer live span to survive the procedure of trapping, scanning, and detection. This reduces the signal even further due to the fact that only the most stable ions survive, while unstable clusters are reduced or even fully eliminated. This observation allows us to identify signals resulting from the formation of silicate species under defined equilibrium conditions and to dismiss unwanted cluster signals. For a better understanding of structural components of silicate species, studies using collision-activated dissociation (CAD) were carried out by isolating the desired ion species and subjecting them to collision inside of the ion trap as are shown in Figure 7. These experiments revealed that the main fragmentation reaction is an elimination of water first on geminal OH groups, which leads to SidO double bonds, and second on two adjacent Q3 silicon atoms each exhibiting an OH group (vicinal silanols). Accordingly, the Si-O-Si bond is broken and the whole molecule gets destabilized. In the case of the cubic octamer, two such elimination processes and the removal of a SiO2 group are sufficient to convert it into the heptamer (see Figure 7). Figure 8 shows possible fragmentation pathways of the species at m/z 551. The heptamer (m/z 509) in turn loses three water molecules and is transformed into the hexamer (m/z 413). Considering all data of the CAD experiments, they clearly show the connection of all different silicate species. Unfortunately, CAD studies are not a good tool for structural elucidation of silicate species, because this study showed that the resulting fragments

Figure 6. MS spectra of TMOS/TMAOH reaction using a triple-quadrupole mass spectrometer: (a) Q1 scan, (b) Q3 scan, (c) ion trap scan.

have a tendency to rearrangement reactions that lead to the formation of stable species, which are similar to those that are formed directly in the liquid phase. Therefore, characterization by CAD studies can only lead to tentative structural assignments. CONCLUSIONS The results show that electrospray mass spectrometry is a wellsuited method to investigate speciation of silicate species on the

way to form solid structures. Potential gas-phase reactions were investigated, and the results show that the major silicate species detected reflect the liquid-phase situation while some minor gasphase products were identified. Satellite peaks with mass differences of +14, +28, and +42 Da are correlated with two different reactions: one with the alcoholic solvent that leads to an alkoxylation. Second, an additional gas-phase reaction with the tetramethylammonium ion can take place. Possible conditions that Analytical Chemistry, Vol. 79, No. 15, August 1, 2007

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Figure 7. CAD studies of silicate species from m/z 551. Multiple water elimination and transformation to other known structures is observed.

Figure 8. Possible fragmentation mechanism for the cubic octamer at m/z 551. By water elimination on two adjacant silicon atoms the bonding Si-O-Si bridge breaks enabling fundamental structural changes within the molecule and leading to the heptamer at m/z 509.

can lead to the formation of cluster ions formed in the gas phase have been investigated and could be eliminated.

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The major advantage of MS in comparison to other potential methods like NMR is that MS allows a much faster generation of data, and therefore, time-dependent analysis on a scale of some seconds is possible instead of some hours, as in NMR. While in MS time-dependent analysis can be performed after 30 s, it takes between 12 and 24 h to generate a 29Si NMR spectrum.10 Unfortunately, silicate species are not the ideal sample for electrospray MS, since the main goalsthe observation of particle formationsalso is the biggest drawback for the analysis due to the always present risk of precipitation or sedimentation within the instrument. It could be shown here that these problems can be taken care of and that with proper adaptation and optimization the method gives valuable data in a relevant field of chemistry. Overall, the use of an “organic” mass spectrometer for an inorganic application shows the advantages and versatility that electrospray mass spectrometry offers. ACKNOWLEDGMENT The authors thank Dr. Kristin von Czapiewski (Applied Biosystems) for some Q-Trap measurements.

Received for review April 5, 2007. Accepted May 31, 2007. AC0706729