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Ion-emitting molten glasses — silica gel revisited
Mass Spectrometry
ELSEVIER
and Ion Processes
International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 5 14
Ion-emitting molten glasses - - silica gel revisited T. Huett, J.C. Ingram, J.E. D e l m o r e * Idaho National Engineering Laboratow Box 1625. Idaho Falls. ID. 83415-2208, USA
Received 14 November 1994; accepted 20 January 1995
Abstract
A Bi" ion emitter has been developed and studied which is modeled on the silica gel matrices that have been used to produce ions from a variety of elements such as Pb, Ag and Te for isotope ratio measurements. Studies with this model system demonstrate that this ion emitter is a liquid glass and that ion emission originates from the surface of the liquid glass. The large difference in ion emission observed with the use of different refractory metal filaments to support this silica gel type matrix is shown to be due to the extent to which the refractory metal is dissolved by the liquid glass, altering the properties of the glass, which in turn alters the ion emission properties. It is not due to the variation in work function of the refractory metal support filaments, as had been suspected. Additional types of study will have to be conducted in order to understand the mechanism of ion emission from the surfaces of these molten glasses. Kevwords: Ion emitter; Ion imaging; Molten glass; Silica gel; Surface (thermal) ionization
1. Introduction Silica gel technology, first reported in 1969 for producing Pb ions [1], has found wide use in the isotope ratio community for producing ions from many elements for the surface ionization mass spectrometry of solids. There are many variations on this technology, but most methods entail mixing the sample with silica gel and either phosphoric acid or boric acid, heating to dryness, mounting in the mass spectrometer and heating to an appropriate temperature (the reported temperature ranges vary considerably). Variations on these matrices have been reported with the addition of A1203 :": Dedicated to the memory of Professor Alfred O. Nier. * Corresponding author.
[2] and GeO 2 [3]. In discussions with various developers and users of these methods, general frustration has emerged with the lack of understanding of why these methods produce ions. The purpose of the study reported here was to develop a better understanding of how the various silica gel systems work by developing a model system which (1) was close enough to the silica gel systems in use for isotope ratio analyses to be a reasonable model thereof, (2) was easily handled and reproduced, and which emitted ions, (3) was readily usable for materials studies to understand better those features necessary for good and reproducible ion production. Substrates were chosen which were known to give major differences in ion emission properties for silica gel type matrices to magnify the factors affecting ion emission.
0168-1176/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved S S D I 0168-1 176(95)04184-2
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T. Huett et al./International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 5-14
Re and Ta substrates were chosen for this study because they were known t o give large differences in ion emission. This type of model system can never hope to be an exact mimic of an analytically useful system since we add high concentrations of the element to be ionized, which alters the material. The element to be ionized was added in relatively large amounts so that pre- and post-concentrations could more easily be measured, and it was hoped that the oxidation state could be determined in a postanalysis. An analytically useful system would have the element in very low concentrations. To the knowledge of the authors, an extensive bibliography of the elements that can be ionized by silica gel technology has never been compiled. The majority of elements reported in the literature of which we are aware as having been analyzed by this technique comprise a block in the periodic table ranging from a VIB element (Cr) [3] on the left, to a VIA element (Te) [4] on the right, with a wide range of variations on the methods reported in Ref. [1]. Included in this group are reports of Sn, Pb, Ag, T1, Te, Bi, Cd, Au, Fe, Ru, Ni, Cr and Po [1-9]. There are also references to IIA ions being formed by this technique [10]. Although there have been no reports of the use of anions for analyses from silica gel matrices, the perrhenate (ReO~-) anion was observed during the course of this work. The wide variation in the chemistry of these different elements indicates that there are multiple ion emission mechanisms. It is hoped there will be sufficient common ground among the various methods for these different elements that the model system reported here can offer some guidance for the next generation of experimentation to study actual ion formation chemistry, and for the operation of systems used for isotope ratio analyses.
2. Experimental The method used in these studies was
adapted from an unpublished method from the Los Alamos National Laboratory [9] for producing Bi +, and used bismuth nitrate (or Bi metal) with silica gel and boron oxide at an operating temperature of 1173 K. Larger amounts of the ion-emitting material were prepared and melted in a vacuum furnace on Re or Ta sheets for subsequent analysis. Analyses of the off-gases produced during initial heating were performed by electron impact mass spectrometry. The cooled materials were analyzed by the following methods: dissolution followed by ICP-AES, SEM, XPS and X-ray diffraction.
2.1. Sample preparation Bismuth borosilicate (BBS) was produced by two methods, both with high bismuth concentrations. These two methods were chosen because it was believed they would yield similar products with the exception of the relative oxygen content. Bi was added as the nitrate in one sample, and as the metal in the other sample. If the relative oxygen content in the final matrix is important, it is hypothesized that these two preparative methods will produce differences in ion emission properties. In the first method, 1.755 g of boric acid (Argent, reagent grade) was mixed with 0.867 g of ethyl silicate (Fischer, 98% pure) in an argon filled glove box. 1.19 g of bismuth nitrate (J.T. Baker, reagent grade) was then added to the resulting mixture. Silica gel has a high and variable water content, and had it been used for the synthesis of the starting material there would have been considerable uncertainties in the ratios of the starting components. This is the reason for maintaining an anhydrous system up to this point. Approximately 10 ml of a 0.05 M nitric acid solution was added to the sample outside the glove box. This was then heated and stirred overnight. The second method is much like the first in that 0.867 g of ethyl silicate, 1.755 g of boric acid and about
T. Huett et al./International Journal of Mass Spectrometry and hm Processes 146/147 (1995) 5 14
10 ml of 0.05 M nitric acid were used, but in this method 0.51 g of finely powdered elemental bismuth (Johnson Matthey, 99.999% pure) was used in place of the bismuth nitrate. The dilute nitric acid would probably have dissolved much or all of the metallic Bi, but this would then have reduced the nitrate content of the slurry, resulting in a net reduction in the oxidative strength. BBS was placed on either tantalum or rhenium substrates using a slurry of BBS and water. Metal (Re or Ta) ribbons were used as supports in the mass spectrometric studies, Re tubes were used for the imaging studies, while Re or Ta sheets were used to produce the materials studied in bulk. The filament and tube configurations are described in detail in Ref. [11]. The metal support was gently heated in air to drive excess water from the slurry. When the slurry was dry, the BBS was adhered to the substrate. The samples were then ready to be mounted in the ion source. Samples described as heated were taken to 1173 K for 3 min, then cooled to room temperature, all at about 1 × 10 -s Torr pressure. 2.2. Ion emission mass spectrometry
The bismuth ion currents, and the positive and negative ion spectra from the different samples were recorded with a 60 ° magnetic sector mass spectrometer designed and built by NIST. The samples were placed in the spectrometer and heated to 1173 K for the positive ion spectra and the bismuth ion current measurements, and to 1273 K for the negative ion spectra. The higher temperature was required to produce a measurable perrhenate current, although prolonged operation at this higher temperature shortened the useful life for Bi + emission. 2.3. Electron impact mass spectrometry
The pressure in the mass spectrometer was
7
observed to increase as the samples were heated. In order to explore this effect, a separate experiment was devised with a sample filament that could be heated in the same vacuum chamber as an Extrel Model C50 quadrupole mass spectrometer with electron impact ionization. Gases which were non-condensable were readily observed with this configuration, including reactive gases such as nitrogen oxides. No attempt was made to quantify these measurements. Condensable vapors such as Bi-containing species were not observed, although such an obervation would not be expected, since there was no direct line of sight between the sample and the ionizer. 2.4. Ion source imaging
An ion source imaging (ISI) instrument was designed and built in our laboratory [11] for the purpose of determining the origin of ions emitted from a thermal surface ionization source. This instrument focused the ions with an electrostatic lens onto a microchannel plate. The image produced was an ion micrograph of the ion emitter. The ISI instrument has " )a detection limit of 7.45 × 10 3 ions s i m m and a spatial resolution of about 3 #m, but no mass discrimination. The lack of mass discrimination was not a problem for this study since the ion beams consisted of about 99% of the ion of interest. The ion emitters used in this instrument were heated to the same temperatures as used in the magnetic sector experiments. The images could be photographed with a still camera, which required exposures of several minutes. Alternatively, the images could be recorded with a video camera and either stored on a video cassette recorder or in the computer memory. Once the images were in the computer m e m o r y they could be processed and printed using a laser jet printer. The images recorded in this investigation were obtained with the video system, since the ion image changed much too rapidly to be
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T. Huett et al./International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 5-14
recorded with the still camera. This was presumably due to churning of the molten glass on the substate. A single video frame was obtained in 1/30 s, and the images did not change significantly within this time. The video images had lower resolution than the images from the photographic camera, but since there were no sharp details that could be observed from these liquid sources, this was inconsequential.
2.5. X-ray diffraction The X-ray diffraction studies were performed With a Phillips Electronic Instruments PW 1729 X-ray generator, an A P D 3720 diffraction control unit and a type 150 100 00 wide range goniometer with theta compensating slit. The samples were heated to 1173 K at 1 x 10-8 Torr on their respective metal supports, cooled to room temperature, removed from the vacuum, transferred to the X-ray instrument and then analyzed.
2.6. Inductively coupled plasma atomic emission spectroscopy The bulk concentrations of Bi were measured in all the samples, those of Re measured in the two bismuth borosilicates on Re, and those of Ta in the two bismuth borosilicates on Ta. After the samples had been heated to 1173 K and cooled to room temperature at 1 × 10 -8 Torr pressure, the glass was cracked from the substrate, weighed, and dissolved in a mixture of aqua regia and hydrofluoric acid. Concentrations were measured using a Fisons ARL3410 inductively coupled plasma atomic emission spectrometer (ICP/AES).
2.7. Scanning electron microscopy Electron micrographs were generated from the samples after they had been cooled and
coated with a thin layer of gold. The samples consisted of the deposit (BBS) and its metal substrate. The electron micrographs were obtained with an Amray 1830 scanning electron microscope (SEM). The electron beam energy used was 20 kV.
2.8. X-ray photoelectron spectroscopy The bismuth borosilicates on their respective metal supports were analyzed with a Perkin-Elmer PHI model 5400 small spot (1.1 ram) X-ray photoelectron spectroscopy (XPS) system. The excitation was supplied by Mg Ko~ X-rays (1253.6 eV) with the source operating at 15 000 V and 300 W. All of the data were obtained with the samples at room temperature. Sample charging was observed due to the insulating nature of these samples at room temperature. Compensation for this charging was achieved by normalizing to the carbon peak.
2.9. Visual observation and electrical conductivity In order to observe visually the ion-emitting material at the operating temperature, an experiment was devised in a vacuum chamber with a wire loop about 3 mm in diameter, mounted close to a viewport. This loop was loaded with the ion emitter while it was still a powder, electrically heated to obtain good adhesion to the wire, and mounted in the vacuum chamber. This permitted visual observation of the material as the temperature w~/s increased. A permutation of this experiment was set up with a wire threaded through this loop, and while the loop itself was heated with an electric current, the conductivity between the ground leg of the supply heating the loop and the threaded wire was measured, This permitted an estimation to be made of the electrical conductivity of the ion emitter at the operating temperature.
T. Huett et al./International Journal of Mass Spectrometry and Ion Processes 146/147 ~1995) 5 14
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3. Results and discussion
The emission of Bi + from the four molten glass ion emitter combinations was measured at 1173 K. Large samples (about 100 rag) were melted on sheets of the respective substrate, broken from the substrate, dissolved, and analyzed by ICP/AES for Bi and either Re or Ta, depending on the substrate. The results are tabulated in Table 1. When scanning the anion spectrum, the perrhenate ion was observed, but only for one sample type, the BBS prepared from Bi nitrate and melted on Re, and then only at 1273 K. The substrate effect is illustrated to be very real by the data for BBS prepared from Bi nitrate, which shows that the Re substrate provides a sensitivity two orders of magnitude higher for Bi + formation than does the Ta substrate. There are two explanations for this effect: either the substrate is changing the nature of the deposit, or the substrate is the ion formation surface. The reported work function of Re is about 5.0 eV, while that of Ta is about 4.1 eV [12], so if the ions were to originate from the filament surfaces, it would be expected that the ion formation efficiency would be much greater from the Re surface, as was observed. A reasonable test to differentiate between these two scenarios is to determine the origin of the ions within the source, i.e. to find out whether they originate from the metal substrate or from the deposit, In order to test this concept, several Bi + emitters were imaged in Re tubes, and all
Fig. 1. Ion micrograph of Bi + ions from BBS prepared from Bi nitrate in an Re tube at 1173 K.
showed ion emission from the deposits only. Fig. l is an image of the total cation emission from a typical emitter mounted in a tube type ion source [11]. When this ion emitter assembly was examined under a microscope, it could be seen that the main deposit was in the face of the tube opening, with small residues around the lip of the tube. This corresponds to what is observed in the image. Since the spectra collected on the mass spectrometer showed the cation beam to be about 99% Bi+, this image corresponds to Bi + emission from the deposit. Fig. 2 is an anion image from the same tube at a higher temperature (1273 K vs. 1173 K), but at a different time since the electronics had to be shut down in order to reverse the polarity. Since the anion spectra collected on the mass spectrometer showed that the beam was 100% perrhenate, this image corresponds to perrhenate emission from the deposit. With ion
Table 1 Relative Bi + intensities and concentrations of Bi and the substrate metal in the glass residue Bi as
Filament material Relative Bi + intensity Substrate in glass ( m g g t) Bi conc ( m g g l)
Nitrate
Metal
Metal
Nitrate
Re 100 1.7 (Re) 9.8
Re 50 1.2 (Re) 34.8
Ta 10 7.3 (Ta) 194
Ta 1 6.3 (Tal 148
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T. Huett et al./International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 5-14
Fig. 2. Ion micrograph of ReO4 ions from BBS prepared from Bi nitrate in an Re tube at 1173 K.
emission only coming from the face of the deposit, the work function of the Re filament (or tube) cannot have a direct effect on ion emission, and the ion emission effects from mounting the ion emitter on an Re substrate must be related to Re altering the molten glass. The observation of perrhenate anions clearly demonstrates that the BBS prepared with Bi nitrate is corroding the Re substrate, and that Re is present in the deposit. Another feature of the emitters that was readily observed during imaging was the speed with which the features changed, as though the deposit was a viscous liquid. In order to explore the possibility of the deposit being molten, an experiment was devised using an approximately 3 mm wire loop loaded with ion emitter, mounted in a vacuum system with a viewport. An electric current was passed through the wire, heating the specimen. The material was observed to melt and churn. X-ray diffraction of the material after it had been cooled and removed from the vacuum showed no features, indicating it was a glass. Several different materials were melted and examined with X-ray diffraction (after cooling), and all appeared to be glasses. Hence, it is concluded that these materials are molten glasses at operating temperature and, from the imaging experiments,
that the ions are emitted from the surface of the molten glass. None of the images demonstrated sharp details, and it was uncertain if this was due to a lack of detail observable on the surface, or if the samples were electrically charging and blurring the focusing via electrostatic defocusing. The electrical conductivity experiment was to clarify the issue of electrostatic charging. The glasses were all insulators when cool, but upon melting they all showed 0.5-2 9t resistance across about 1 mm of the molten material. This indicates that the samples were not electrically charging during operation in the imaging instrument, and that the lack of detail in the images is a result of the surface being a liquid. Evidence of the effect of the Re substrate on the ion emission properties of the matrices was given by the observation ofperrhenate (ReO4) in the anion spectrum of the ion emitters prepared from Bi nitrate and melted on Re. This demonstrates that the molten glass was corroding the Re substrate and that Re was being oxidized to its highest oxidation state, +7. Perrhenate was only observed from the samples prepared from Bi nitrate and melted on Re; the samples prepared from Bi metal and melted on Re did not produce perrhenate in concentrations observable by mass spectrometry. This observation indicates that an oxidizing agent (excess nitrate) is required in order for perrhenate to be observable. Analysis of the off-gases using the electron impact ion source mass spectrometer showed that the only gases to be given off in appreciable amounts were water and nitrogen oxides (H20, NO and NO2). The nitrogen oxides produced from heating the material could have come from two sources: thermal decomposition of nitrate, and oxidation of Re to perrhenate by nitrate. After the observation of perrhenate, larger quantities of all of the glasses were prepared and analyzed, and the results presented in Table 1 demonstrate that all of the glasses contain significant quantities
T. Huett et al./International Journal o f Mass Spectrometry and Ion Processes 146/147 (1995) 5 I4
[l
Fig. 3. Scanning electron micrograph at 1000x and 20 kV of the gold-covered BBS prepared from Bi nitrate and heated on Ta to 1182 K.
Fig. 5. Scanning electron micrograph at 1000x and 20 kV of the gold-covered BBS prepared from Bi nitrate and heated on Re to 1182 K.
of metal from the substrate. The two types of deposit melted on Re had comparable amounts of Re incorporated into the glass, and although one was obviously more oxidized than the other, as indicated by perrhenate ion emission, the emission of Bi ~ intensity was within a factor of two (Table 1). This indicates that Bi + emission is not highly sensitive to the relative state of oxidation of the matrix for the Re system, although the
opposite trend was observed with the Ta substrate. Significant quantities of the substrate are incorporated into the glass, as can be seen from the data in Table 1, and the samples melted on Ta had several times as much substrate as the samples melted on Re. The materials melted on Re appeared to be much smoother than those melted on Ta, as observed by SEM imaging of the cooled
Fig. 4, Scanning electron micrograph at 1000x and 20 kV of the gold-covered BBS prepared from Bi metal and heated on Ta to
Fig. 6. Scanning electron micrograph at 1000x and 20 kV of the gold-covered BBS prepared from Bi metal and heated on Re to 1182 K.
1161 K.
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T. Huett et al./International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 5 14
material. Figs. 3-6 are SEM images of these four materials. SEM studies were only performed on one set of samples, although optical microscopy was used to examine many others, and the same types of differences were observable. Examination of these images demonstrates that each of the four types of glasses are substantially different from the others. The cooling rates for these materials were not controlled; the heating current was simply turned off. This was an oversight at the time, and may have caused unnecessary cracking; however, all samples were treated equally. Thus, the conclusion is made that the addition of these heavy metals at the few parts per thousand level (from a partial dissolution of the substrate) imparts substantially different properties to the glasses, affecting both the physical appearance and the ion emission properties. It is surmised from the measured ion current that this heavy metal content is undesirable. The four combinations were blended to give the same concentration of Bi, but as can be seen from Table 1, the Bi concentration after 3 min heating varied considerably among the four sample types. It also decreased dramatically for all samples, and in particular for the samples melted on Re. The best Bi + emitters had the lowest Bi content after heating. The Bi either evaporated or alloyed with the substrate. A new series of experiments is required to advance further our understanding of these effects. The observation of ReO4 from one sample demonstrates that at least some Re leaves the deposit via evaporation. There have been reports [13] that rhenium oxide has a very high work function, although quantitative measurements could not be found in the literature. In order to see if rhenium oxide in the emitter material made a significant difference, an experiment was devised where a small amount of barium perrhenate was added to the nitrate based glass, which was melted on Re. Ba perrhenate was chosen since it has a much lower vapor pressure than any of the
Re oxides. The emission of Bi + was severely depressed, indicating that the excess perrhenate (or possibly the Ba) poisoned the ion-emitting properties of the glass. Thus, if Re oxide does raise the work function of the glass surface, the effect on ion formation is negated by the heavy metal poisoning effect of either perrhenate or Ba. This is consistent with the fact that the measured concentrations of Ta (the poorer ion emitter) were several times higher than the concentrations of Re (the better ion emitter) in the respective materials. The original Los Alamos method called for the use of Pt filaments. Pt substrates were not tested in this study, but it is possible that they may have less chemical interaction with the molten glass, and consequently less heavy metal content with possibly superior ion emission properties. XPS analyses were performed on the four materials. Measurable amounts of Bi 3+ were observed in the samples melted on Ta, but not on those samples melted on Re. XPS has rather high detection limits, in the order of 0.1-0.2%, and hence the low concentrations of Bi in the emitters melted on Re could not be observed by XPS. No elemental Bi was observable during the initial phase of XPS analyses on any of the materials, although it was observed to grow with X-ray exposure for those samples initially containing Bi 3+. If any other Bi species was present, the concentration was too low to be measured. This suggests that the presence of Bi 3+ in the emitter deposit alone is not sufficient for Bi + emission, and the species that is responsible for ion emission is at too low a concentration to be detected by XPS. An analytical technique capable of detecting the Bi species in the condensed phase matrix that is responsible for Bi + emission is required.
4. Conclusions
There are numerous issues needing further
T. Huett et al./lnternational Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 5 14
study, but some of the conclusions made can help guide additional experiments. The most certain of these is that the emitters are molten and highly viscous glasses at ion emission temperatures, and that ions originate from the surface of this molten glass. It has been known since the original studies [1] that the filament material is very important for ion formation efficiency, with Re filaments giving superior ion formation to Ta filaments. It was generally assumed this was due to the higher work function of Re, and that ions originated from the Re surface. The results of this study show that ions do not originate from the filament surface. They further indicate that Re is a better filament material as it is less reactive towards the molten glass, and less Re than Ta dissolves in the molten glass. It appears that heavy metals poison the ion emission process. It is possible, although not proven, that the concentration of Bi in the glasses of this model system altered the ion emission properties, just as the metals from the substrate altered the ion emission process. Bi either migrates into the filament material in appreciable quantities (particularly Re filaments), or has a much higher vapor pressure in the more favorable matrices, since most of the Bi added to these materials was lost when the glass was heated to 1173 K. Little can be said concerning the actual mechanism of ion production from these molten glasses at this time. It is unlikely that a single mechanism is responsible for ion emiss i o n for all the elements that can be ionized by this technology. XPS data demonstrated that Bi 3+ was the predominant oxidation state of Bi in the cooled glasses, at least for the less efficient ion emitters. Since Bi 3+ is at much higher concentrations in the poor ion emitters, it is unlikely to be the oxidation state responsible for ion emission, unless there is some kind of complexation issue which is not understood. Different analytical techniques are obviously required to identify the active species in the
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molten glass responsible for Bi+ ion emission. Bi has three oxidation states, 0, +3, and +5, with reports of non-stoichiometric states of less than +3 for certain solid state materials [14]. The +5 state is probably not active in this system owing to its requirement for a very highly oxidizing environment. The +3 state is not a sufficient requirement for Bi + emission either, as shown by the XPS data. This leaves the 0 oxidation state, and nonstoichiometric states as the alternatives. A better model system using an element with less complex chemistry would offer an advantage in these types of study. Two systems that will be considered in future work are Ag [8] and Cd [6], both of which have been analyzed by silica gel technology. Ag has only 0 and +1 oxidation states, which greatly simplifies the number of possibilities for the active species responsible for ion emission. Cd has only 0 and +2 oxidation states, and has excellent sensitivity with nuclear magnetic resonance analysis, which might help to define the oxidation state responsible for ion emission. Another mechanism possibly affecting ionization is the level of complexation of the element, as opposed to the oxidation state. N M R would be useful for determining the state of complexation in the ion emitter, which is another reason why Cd is considered to be a model choice of system. Additionally, the choice of Re and Ta as substrates was based on the premise that major differences in ion emission would more readily allow discernment of the factors affecting ion emission. Now that we are aware of the heavy metal poisoning effect, a re-evaluation of substrates for inertness (particularly Pt) is in order.
Acknowledgments This work was supported by the Chemistry Division of the Office of Basic Energy
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T. Huett et al./International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 5-14
Sciences, US Department of Energy, under grant 3ED102.
References [1] A.E. Cameron, D.H. Smith and R.E. Walker, Anal. Chem., 41 (1969) 525. [2] K.J.R. Rosman, R.D. Loss and J.R. De Laeter, Int. J. Mass Spectrom. Ion Processes, 56 (1984) 281. [3] J.L. Birck and G.W. Lugmair, Earth Planet. Sci. Lett., 90 (1988) 13!. [4] R.D. Loss, K.J.R. Rosman and J.R. De Laeter, Talanta, 30 (1983) 831. [5] F. Tera and G.J. Wasserburg, Anal. Chem., 47 (1975) 2214.
[6] K.J.R. Rosman and J.R. De Laeter, Int. J. Mass Spectrom. Ion Processes, 16 (1975) 385. [7] A. Shukolyukov and G.L. Lugmair, Science, 259 (1993) 1138. [8] W.R. Kelly, F. Tera and G.J. Wasserburg, Anal. Chem., 50 (1978) 1279. [9] D.J, Rokop, Los Alamos National Laboratory, personal communication (1991). [10] T. Lee, D.A. Papanastassiou and G.J. Wasserburg, Geochim. Cosmochim. Acta, 41 (1977) 1473-1485. [11] J.E. Delmore, A.D. Applehans and J.E. Olson, Int. J. Mass Spectrom. Ion Processes, 140 (1994) 111. [12] V.S. Fornenko and G.V. Samsonov (Eds.), Handbook of Therrnionic Properties, Plenum, New York, 1966. [13] T. Fusii, Int. J. Mass Spectrom. Ion Processes, 87 (1989) 343. [14] M.R. Chandrachood, S.R. Sainkar, I.S. Mulla and S. Badrinarayanan, Mater. Lett. 8 (1989) 343.
ELSEVIER
and Ion Processes
International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 5 14
Ion-emitting molten glasses - - silica gel revisited T. Huett, J.C. Ingram, J.E. D e l m o r e * Idaho National Engineering Laboratow Box 1625. Idaho Falls. ID. 83415-2208, USA
Received 14 November 1994; accepted 20 January 1995
Abstract
A Bi" ion emitter has been developed and studied which is modeled on the silica gel matrices that have been used to produce ions from a variety of elements such as Pb, Ag and Te for isotope ratio measurements. Studies with this model system demonstrate that this ion emitter is a liquid glass and that ion emission originates from the surface of the liquid glass. The large difference in ion emission observed with the use of different refractory metal filaments to support this silica gel type matrix is shown to be due to the extent to which the refractory metal is dissolved by the liquid glass, altering the properties of the glass, which in turn alters the ion emission properties. It is not due to the variation in work function of the refractory metal support filaments, as had been suspected. Additional types of study will have to be conducted in order to understand the mechanism of ion emission from the surfaces of these molten glasses. Kevwords: Ion emitter; Ion imaging; Molten glass; Silica gel; Surface (thermal) ionization
1. Introduction Silica gel technology, first reported in 1969 for producing Pb ions [1], has found wide use in the isotope ratio community for producing ions from many elements for the surface ionization mass spectrometry of solids. There are many variations on this technology, but most methods entail mixing the sample with silica gel and either phosphoric acid or boric acid, heating to dryness, mounting in the mass spectrometer and heating to an appropriate temperature (the reported temperature ranges vary considerably). Variations on these matrices have been reported with the addition of A1203 :": Dedicated to the memory of Professor Alfred O. Nier. * Corresponding author.
[2] and GeO 2 [3]. In discussions with various developers and users of these methods, general frustration has emerged with the lack of understanding of why these methods produce ions. The purpose of the study reported here was to develop a better understanding of how the various silica gel systems work by developing a model system which (1) was close enough to the silica gel systems in use for isotope ratio analyses to be a reasonable model thereof, (2) was easily handled and reproduced, and which emitted ions, (3) was readily usable for materials studies to understand better those features necessary for good and reproducible ion production. Substrates were chosen which were known to give major differences in ion emission properties for silica gel type matrices to magnify the factors affecting ion emission.
0168-1176/95/$09.50 ~ 1995 Elsevier Science B.V. All rights reserved S S D I 0168-1 176(95)04184-2
6
T. Huett et al./International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 5-14
Re and Ta substrates were chosen for this study because they were known t o give large differences in ion emission. This type of model system can never hope to be an exact mimic of an analytically useful system since we add high concentrations of the element to be ionized, which alters the material. The element to be ionized was added in relatively large amounts so that pre- and post-concentrations could more easily be measured, and it was hoped that the oxidation state could be determined in a postanalysis. An analytically useful system would have the element in very low concentrations. To the knowledge of the authors, an extensive bibliography of the elements that can be ionized by silica gel technology has never been compiled. The majority of elements reported in the literature of which we are aware as having been analyzed by this technique comprise a block in the periodic table ranging from a VIB element (Cr) [3] on the left, to a VIA element (Te) [4] on the right, with a wide range of variations on the methods reported in Ref. [1]. Included in this group are reports of Sn, Pb, Ag, T1, Te, Bi, Cd, Au, Fe, Ru, Ni, Cr and Po [1-9]. There are also references to IIA ions being formed by this technique [10]. Although there have been no reports of the use of anions for analyses from silica gel matrices, the perrhenate (ReO~-) anion was observed during the course of this work. The wide variation in the chemistry of these different elements indicates that there are multiple ion emission mechanisms. It is hoped there will be sufficient common ground among the various methods for these different elements that the model system reported here can offer some guidance for the next generation of experimentation to study actual ion formation chemistry, and for the operation of systems used for isotope ratio analyses.
2. Experimental The method used in these studies was
adapted from an unpublished method from the Los Alamos National Laboratory [9] for producing Bi +, and used bismuth nitrate (or Bi metal) with silica gel and boron oxide at an operating temperature of 1173 K. Larger amounts of the ion-emitting material were prepared and melted in a vacuum furnace on Re or Ta sheets for subsequent analysis. Analyses of the off-gases produced during initial heating were performed by electron impact mass spectrometry. The cooled materials were analyzed by the following methods: dissolution followed by ICP-AES, SEM, XPS and X-ray diffraction.
2.1. Sample preparation Bismuth borosilicate (BBS) was produced by two methods, both with high bismuth concentrations. These two methods were chosen because it was believed they would yield similar products with the exception of the relative oxygen content. Bi was added as the nitrate in one sample, and as the metal in the other sample. If the relative oxygen content in the final matrix is important, it is hypothesized that these two preparative methods will produce differences in ion emission properties. In the first method, 1.755 g of boric acid (Argent, reagent grade) was mixed with 0.867 g of ethyl silicate (Fischer, 98% pure) in an argon filled glove box. 1.19 g of bismuth nitrate (J.T. Baker, reagent grade) was then added to the resulting mixture. Silica gel has a high and variable water content, and had it been used for the synthesis of the starting material there would have been considerable uncertainties in the ratios of the starting components. This is the reason for maintaining an anhydrous system up to this point. Approximately 10 ml of a 0.05 M nitric acid solution was added to the sample outside the glove box. This was then heated and stirred overnight. The second method is much like the first in that 0.867 g of ethyl silicate, 1.755 g of boric acid and about
T. Huett et al./International Journal of Mass Spectrometry and hm Processes 146/147 (1995) 5 14
10 ml of 0.05 M nitric acid were used, but in this method 0.51 g of finely powdered elemental bismuth (Johnson Matthey, 99.999% pure) was used in place of the bismuth nitrate. The dilute nitric acid would probably have dissolved much or all of the metallic Bi, but this would then have reduced the nitrate content of the slurry, resulting in a net reduction in the oxidative strength. BBS was placed on either tantalum or rhenium substrates using a slurry of BBS and water. Metal (Re or Ta) ribbons were used as supports in the mass spectrometric studies, Re tubes were used for the imaging studies, while Re or Ta sheets were used to produce the materials studied in bulk. The filament and tube configurations are described in detail in Ref. [11]. The metal support was gently heated in air to drive excess water from the slurry. When the slurry was dry, the BBS was adhered to the substrate. The samples were then ready to be mounted in the ion source. Samples described as heated were taken to 1173 K for 3 min, then cooled to room temperature, all at about 1 × 10 -s Torr pressure. 2.2. Ion emission mass spectrometry
The bismuth ion currents, and the positive and negative ion spectra from the different samples were recorded with a 60 ° magnetic sector mass spectrometer designed and built by NIST. The samples were placed in the spectrometer and heated to 1173 K for the positive ion spectra and the bismuth ion current measurements, and to 1273 K for the negative ion spectra. The higher temperature was required to produce a measurable perrhenate current, although prolonged operation at this higher temperature shortened the useful life for Bi + emission. 2.3. Electron impact mass spectrometry
The pressure in the mass spectrometer was
7
observed to increase as the samples were heated. In order to explore this effect, a separate experiment was devised with a sample filament that could be heated in the same vacuum chamber as an Extrel Model C50 quadrupole mass spectrometer with electron impact ionization. Gases which were non-condensable were readily observed with this configuration, including reactive gases such as nitrogen oxides. No attempt was made to quantify these measurements. Condensable vapors such as Bi-containing species were not observed, although such an obervation would not be expected, since there was no direct line of sight between the sample and the ionizer. 2.4. Ion source imaging
An ion source imaging (ISI) instrument was designed and built in our laboratory [11] for the purpose of determining the origin of ions emitted from a thermal surface ionization source. This instrument focused the ions with an electrostatic lens onto a microchannel plate. The image produced was an ion micrograph of the ion emitter. The ISI instrument has " )a detection limit of 7.45 × 10 3 ions s i m m and a spatial resolution of about 3 #m, but no mass discrimination. The lack of mass discrimination was not a problem for this study since the ion beams consisted of about 99% of the ion of interest. The ion emitters used in this instrument were heated to the same temperatures as used in the magnetic sector experiments. The images could be photographed with a still camera, which required exposures of several minutes. Alternatively, the images could be recorded with a video camera and either stored on a video cassette recorder or in the computer memory. Once the images were in the computer m e m o r y they could be processed and printed using a laser jet printer. The images recorded in this investigation were obtained with the video system, since the ion image changed much too rapidly to be
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T. Huett et al./International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 5-14
recorded with the still camera. This was presumably due to churning of the molten glass on the substate. A single video frame was obtained in 1/30 s, and the images did not change significantly within this time. The video images had lower resolution than the images from the photographic camera, but since there were no sharp details that could be observed from these liquid sources, this was inconsequential.
2.5. X-ray diffraction The X-ray diffraction studies were performed With a Phillips Electronic Instruments PW 1729 X-ray generator, an A P D 3720 diffraction control unit and a type 150 100 00 wide range goniometer with theta compensating slit. The samples were heated to 1173 K at 1 x 10-8 Torr on their respective metal supports, cooled to room temperature, removed from the vacuum, transferred to the X-ray instrument and then analyzed.
2.6. Inductively coupled plasma atomic emission spectroscopy The bulk concentrations of Bi were measured in all the samples, those of Re measured in the two bismuth borosilicates on Re, and those of Ta in the two bismuth borosilicates on Ta. After the samples had been heated to 1173 K and cooled to room temperature at 1 × 10 -8 Torr pressure, the glass was cracked from the substrate, weighed, and dissolved in a mixture of aqua regia and hydrofluoric acid. Concentrations were measured using a Fisons ARL3410 inductively coupled plasma atomic emission spectrometer (ICP/AES).
2.7. Scanning electron microscopy Electron micrographs were generated from the samples after they had been cooled and
coated with a thin layer of gold. The samples consisted of the deposit (BBS) and its metal substrate. The electron micrographs were obtained with an Amray 1830 scanning electron microscope (SEM). The electron beam energy used was 20 kV.
2.8. X-ray photoelectron spectroscopy The bismuth borosilicates on their respective metal supports were analyzed with a Perkin-Elmer PHI model 5400 small spot (1.1 ram) X-ray photoelectron spectroscopy (XPS) system. The excitation was supplied by Mg Ko~ X-rays (1253.6 eV) with the source operating at 15 000 V and 300 W. All of the data were obtained with the samples at room temperature. Sample charging was observed due to the insulating nature of these samples at room temperature. Compensation for this charging was achieved by normalizing to the carbon peak.
2.9. Visual observation and electrical conductivity In order to observe visually the ion-emitting material at the operating temperature, an experiment was devised in a vacuum chamber with a wire loop about 3 mm in diameter, mounted close to a viewport. This loop was loaded with the ion emitter while it was still a powder, electrically heated to obtain good adhesion to the wire, and mounted in the vacuum chamber. This permitted visual observation of the material as the temperature w~/s increased. A permutation of this experiment was set up with a wire threaded through this loop, and while the loop itself was heated with an electric current, the conductivity between the ground leg of the supply heating the loop and the threaded wire was measured, This permitted an estimation to be made of the electrical conductivity of the ion emitter at the operating temperature.
T. Huett et al./International Journal of Mass Spectrometry and Ion Processes 146/147 ~1995) 5 14
9
3. Results and discussion
The emission of Bi + from the four molten glass ion emitter combinations was measured at 1173 K. Large samples (about 100 rag) were melted on sheets of the respective substrate, broken from the substrate, dissolved, and analyzed by ICP/AES for Bi and either Re or Ta, depending on the substrate. The results are tabulated in Table 1. When scanning the anion spectrum, the perrhenate ion was observed, but only for one sample type, the BBS prepared from Bi nitrate and melted on Re, and then only at 1273 K. The substrate effect is illustrated to be very real by the data for BBS prepared from Bi nitrate, which shows that the Re substrate provides a sensitivity two orders of magnitude higher for Bi + formation than does the Ta substrate. There are two explanations for this effect: either the substrate is changing the nature of the deposit, or the substrate is the ion formation surface. The reported work function of Re is about 5.0 eV, while that of Ta is about 4.1 eV [12], so if the ions were to originate from the filament surfaces, it would be expected that the ion formation efficiency would be much greater from the Re surface, as was observed. A reasonable test to differentiate between these two scenarios is to determine the origin of the ions within the source, i.e. to find out whether they originate from the metal substrate or from the deposit, In order to test this concept, several Bi + emitters were imaged in Re tubes, and all
Fig. 1. Ion micrograph of Bi + ions from BBS prepared from Bi nitrate in an Re tube at 1173 K.
showed ion emission from the deposits only. Fig. l is an image of the total cation emission from a typical emitter mounted in a tube type ion source [11]. When this ion emitter assembly was examined under a microscope, it could be seen that the main deposit was in the face of the tube opening, with small residues around the lip of the tube. This corresponds to what is observed in the image. Since the spectra collected on the mass spectrometer showed the cation beam to be about 99% Bi+, this image corresponds to Bi + emission from the deposit. Fig. 2 is an anion image from the same tube at a higher temperature (1273 K vs. 1173 K), but at a different time since the electronics had to be shut down in order to reverse the polarity. Since the anion spectra collected on the mass spectrometer showed that the beam was 100% perrhenate, this image corresponds to perrhenate emission from the deposit. With ion
Table 1 Relative Bi + intensities and concentrations of Bi and the substrate metal in the glass residue Bi as
Filament material Relative Bi + intensity Substrate in glass ( m g g t) Bi conc ( m g g l)
Nitrate
Metal
Metal
Nitrate
Re 100 1.7 (Re) 9.8
Re 50 1.2 (Re) 34.8
Ta 10 7.3 (Ta) 194
Ta 1 6.3 (Tal 148
10
T. Huett et al./International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 5-14
Fig. 2. Ion micrograph of ReO4 ions from BBS prepared from Bi nitrate in an Re tube at 1173 K.
emission only coming from the face of the deposit, the work function of the Re filament (or tube) cannot have a direct effect on ion emission, and the ion emission effects from mounting the ion emitter on an Re substrate must be related to Re altering the molten glass. The observation of perrhenate anions clearly demonstrates that the BBS prepared with Bi nitrate is corroding the Re substrate, and that Re is present in the deposit. Another feature of the emitters that was readily observed during imaging was the speed with which the features changed, as though the deposit was a viscous liquid. In order to explore the possibility of the deposit being molten, an experiment was devised using an approximately 3 mm wire loop loaded with ion emitter, mounted in a vacuum system with a viewport. An electric current was passed through the wire, heating the specimen. The material was observed to melt and churn. X-ray diffraction of the material after it had been cooled and removed from the vacuum showed no features, indicating it was a glass. Several different materials were melted and examined with X-ray diffraction (after cooling), and all appeared to be glasses. Hence, it is concluded that these materials are molten glasses at operating temperature and, from the imaging experiments,
that the ions are emitted from the surface of the molten glass. None of the images demonstrated sharp details, and it was uncertain if this was due to a lack of detail observable on the surface, or if the samples were electrically charging and blurring the focusing via electrostatic defocusing. The electrical conductivity experiment was to clarify the issue of electrostatic charging. The glasses were all insulators when cool, but upon melting they all showed 0.5-2 9t resistance across about 1 mm of the molten material. This indicates that the samples were not electrically charging during operation in the imaging instrument, and that the lack of detail in the images is a result of the surface being a liquid. Evidence of the effect of the Re substrate on the ion emission properties of the matrices was given by the observation ofperrhenate (ReO4) in the anion spectrum of the ion emitters prepared from Bi nitrate and melted on Re. This demonstrates that the molten glass was corroding the Re substrate and that Re was being oxidized to its highest oxidation state, +7. Perrhenate was only observed from the samples prepared from Bi nitrate and melted on Re; the samples prepared from Bi metal and melted on Re did not produce perrhenate in concentrations observable by mass spectrometry. This observation indicates that an oxidizing agent (excess nitrate) is required in order for perrhenate to be observable. Analysis of the off-gases using the electron impact ion source mass spectrometer showed that the only gases to be given off in appreciable amounts were water and nitrogen oxides (H20, NO and NO2). The nitrogen oxides produced from heating the material could have come from two sources: thermal decomposition of nitrate, and oxidation of Re to perrhenate by nitrate. After the observation of perrhenate, larger quantities of all of the glasses were prepared and analyzed, and the results presented in Table 1 demonstrate that all of the glasses contain significant quantities
T. Huett et al./International Journal o f Mass Spectrometry and Ion Processes 146/147 (1995) 5 I4
[l
Fig. 3. Scanning electron micrograph at 1000x and 20 kV of the gold-covered BBS prepared from Bi nitrate and heated on Ta to 1182 K.
Fig. 5. Scanning electron micrograph at 1000x and 20 kV of the gold-covered BBS prepared from Bi nitrate and heated on Re to 1182 K.
of metal from the substrate. The two types of deposit melted on Re had comparable amounts of Re incorporated into the glass, and although one was obviously more oxidized than the other, as indicated by perrhenate ion emission, the emission of Bi ~ intensity was within a factor of two (Table 1). This indicates that Bi + emission is not highly sensitive to the relative state of oxidation of the matrix for the Re system, although the
opposite trend was observed with the Ta substrate. Significant quantities of the substrate are incorporated into the glass, as can be seen from the data in Table 1, and the samples melted on Ta had several times as much substrate as the samples melted on Re. The materials melted on Re appeared to be much smoother than those melted on Ta, as observed by SEM imaging of the cooled
Fig. 4, Scanning electron micrograph at 1000x and 20 kV of the gold-covered BBS prepared from Bi metal and heated on Ta to
Fig. 6. Scanning electron micrograph at 1000x and 20 kV of the gold-covered BBS prepared from Bi metal and heated on Re to 1182 K.
1161 K.
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material. Figs. 3-6 are SEM images of these four materials. SEM studies were only performed on one set of samples, although optical microscopy was used to examine many others, and the same types of differences were observable. Examination of these images demonstrates that each of the four types of glasses are substantially different from the others. The cooling rates for these materials were not controlled; the heating current was simply turned off. This was an oversight at the time, and may have caused unnecessary cracking; however, all samples were treated equally. Thus, the conclusion is made that the addition of these heavy metals at the few parts per thousand level (from a partial dissolution of the substrate) imparts substantially different properties to the glasses, affecting both the physical appearance and the ion emission properties. It is surmised from the measured ion current that this heavy metal content is undesirable. The four combinations were blended to give the same concentration of Bi, but as can be seen from Table 1, the Bi concentration after 3 min heating varied considerably among the four sample types. It also decreased dramatically for all samples, and in particular for the samples melted on Re. The best Bi + emitters had the lowest Bi content after heating. The Bi either evaporated or alloyed with the substrate. A new series of experiments is required to advance further our understanding of these effects. The observation of ReO4 from one sample demonstrates that at least some Re leaves the deposit via evaporation. There have been reports [13] that rhenium oxide has a very high work function, although quantitative measurements could not be found in the literature. In order to see if rhenium oxide in the emitter material made a significant difference, an experiment was devised where a small amount of barium perrhenate was added to the nitrate based glass, which was melted on Re. Ba perrhenate was chosen since it has a much lower vapor pressure than any of the
Re oxides. The emission of Bi + was severely depressed, indicating that the excess perrhenate (or possibly the Ba) poisoned the ion-emitting properties of the glass. Thus, if Re oxide does raise the work function of the glass surface, the effect on ion formation is negated by the heavy metal poisoning effect of either perrhenate or Ba. This is consistent with the fact that the measured concentrations of Ta (the poorer ion emitter) were several times higher than the concentrations of Re (the better ion emitter) in the respective materials. The original Los Alamos method called for the use of Pt filaments. Pt substrates were not tested in this study, but it is possible that they may have less chemical interaction with the molten glass, and consequently less heavy metal content with possibly superior ion emission properties. XPS analyses were performed on the four materials. Measurable amounts of Bi 3+ were observed in the samples melted on Ta, but not on those samples melted on Re. XPS has rather high detection limits, in the order of 0.1-0.2%, and hence the low concentrations of Bi in the emitters melted on Re could not be observed by XPS. No elemental Bi was observable during the initial phase of XPS analyses on any of the materials, although it was observed to grow with X-ray exposure for those samples initially containing Bi 3+. If any other Bi species was present, the concentration was too low to be measured. This suggests that the presence of Bi 3+ in the emitter deposit alone is not sufficient for Bi + emission, and the species that is responsible for ion emission is at too low a concentration to be detected by XPS. An analytical technique capable of detecting the Bi species in the condensed phase matrix that is responsible for Bi + emission is required.
4. Conclusions
There are numerous issues needing further
T. Huett et al./lnternational Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 5 14
study, but some of the conclusions made can help guide additional experiments. The most certain of these is that the emitters are molten and highly viscous glasses at ion emission temperatures, and that ions originate from the surface of this molten glass. It has been known since the original studies [1] that the filament material is very important for ion formation efficiency, with Re filaments giving superior ion formation to Ta filaments. It was generally assumed this was due to the higher work function of Re, and that ions originated from the Re surface. The results of this study show that ions do not originate from the filament surface. They further indicate that Re is a better filament material as it is less reactive towards the molten glass, and less Re than Ta dissolves in the molten glass. It appears that heavy metals poison the ion emission process. It is possible, although not proven, that the concentration of Bi in the glasses of this model system altered the ion emission properties, just as the metals from the substrate altered the ion emission process. Bi either migrates into the filament material in appreciable quantities (particularly Re filaments), or has a much higher vapor pressure in the more favorable matrices, since most of the Bi added to these materials was lost when the glass was heated to 1173 K. Little can be said concerning the actual mechanism of ion production from these molten glasses at this time. It is unlikely that a single mechanism is responsible for ion emiss i o n for all the elements that can be ionized by this technology. XPS data demonstrated that Bi 3+ was the predominant oxidation state of Bi in the cooled glasses, at least for the less efficient ion emitters. Since Bi 3+ is at much higher concentrations in the poor ion emitters, it is unlikely to be the oxidation state responsible for ion emission, unless there is some kind of complexation issue which is not understood. Different analytical techniques are obviously required to identify the active species in the
13
molten glass responsible for Bi+ ion emission. Bi has three oxidation states, 0, +3, and +5, with reports of non-stoichiometric states of less than +3 for certain solid state materials [14]. The +5 state is probably not active in this system owing to its requirement for a very highly oxidizing environment. The +3 state is not a sufficient requirement for Bi + emission either, as shown by the XPS data. This leaves the 0 oxidation state, and nonstoichiometric states as the alternatives. A better model system using an element with less complex chemistry would offer an advantage in these types of study. Two systems that will be considered in future work are Ag [8] and Cd [6], both of which have been analyzed by silica gel technology. Ag has only 0 and +1 oxidation states, which greatly simplifies the number of possibilities for the active species responsible for ion emission. Cd has only 0 and +2 oxidation states, and has excellent sensitivity with nuclear magnetic resonance analysis, which might help to define the oxidation state responsible for ion emission. Another mechanism possibly affecting ionization is the level of complexation of the element, as opposed to the oxidation state. N M R would be useful for determining the state of complexation in the ion emitter, which is another reason why Cd is considered to be a model choice of system. Additionally, the choice of Re and Ta as substrates was based on the premise that major differences in ion emission would more readily allow discernment of the factors affecting ion emission. Now that we are aware of the heavy metal poisoning effect, a re-evaluation of substrates for inertness (particularly Pt) is in order.
Acknowledgments This work was supported by the Chemistry Division of the Office of Basic Energy
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T. Huett et al./International Journal of Mass Spectrometry and Ion Processes 146/147 (1995) 5-14
Sciences, US Department of Energy, under grant 3ED102.
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[6] K.J.R. Rosman and J.R. De Laeter, Int. J. Mass Spectrom. Ion Processes, 16 (1975) 385. [7] A. Shukolyukov and G.L. Lugmair, Science, 259 (1993) 1138. [8] W.R. Kelly, F. Tera and G.J. Wasserburg, Anal. Chem., 50 (1978) 1279. [9] D.J, Rokop, Los Alamos National Laboratory, personal communication (1991). [10] T. Lee, D.A. Papanastassiou and G.J. Wasserburg, Geochim. Cosmochim. Acta, 41 (1977) 1473-1485. [11] J.E. Delmore, A.D. Applehans and J.E. Olson, Int. J. Mass Spectrom. Ion Processes, 140 (1994) 111. [12] V.S. Fornenko and G.V. Samsonov (Eds.), Handbook of Therrnionic Properties, Plenum, New York, 1966. [13] T. Fusii, Int. J. Mass Spectrom. Ion Processes, 87 (1989) 343. [14] M.R. Chandrachood, S.R. Sainkar, I.S. Mulla and S. Badrinarayanan, Mater. Lett. 8 (1989) 343.