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Development of a new high-efficiency thermal ionization source for mass spectrometry
ELSEVI E R
International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
Development of a new high-efficiency thermal ionization source for mass spectrometry Yixiang Duan, Edwin P. Chamberlin, Jos6 A. Olivares* Los Alamos National Laboratory, Chemical Science and Technology Division, Los Alamos, NM 87545 USA
Received 3 April 1996; accepted 31 July 1996
Abstract A thermal ionization source for mass spectrometry has been designed and tested. The ion source is based on a tungsten crucible with a deep cavity into which the sample is loaded. The crucible is heated by high energy electron bombardment from a tantalum filament surrounding the crucible. As the sample evaporates inside the crucible, gaseous analyte atoms are produced which interact with the inner surface of the crucible walls to produce positive ions through surface ionization. The ions are extracted from the cavity through a small opening at the end of the crucible. Regulation of the electron emission current makes it possible to control the energy and power applied to the crucible and, therefore, the crucible temperature. A number of elements have been tested in this source. The ionization efficiencies measured using a high transmission isotope separator spectrometer show 10 to 100 times higher efficiency than in conventional surface thermal ionization sources. © 1997 Elsevier Science B.V. Keywords: Thermal ionization; Ion source; High efficiency; Quadrupole; Mass spectrometry; Isotope separation;
Isotope ratios; Instrumentation
1. Introduction Thermal ionization mass spectrometry (TIMS) is one of the most powerful techniques used to determine both the absolute and the relative isotopic abundance of elements [1,2]. Thermal ionization is efficient for elements which have relatively low or moderate first ionization energies, and with sufficient vapor pressure at the source operating temperature. Traditional thermal ionization sources use single, double, and triple, fiat ribbon-type filaments for sample vaporization and ionization. Due to the high sensitivity, precision, and accuracy, TIMS has been broadly employed in geochemistry [3], * Corresponding author,
environmental chemistry [4], materials analysis [5], and the study of nuclear materials [6-8]. Although TIMS is a very sensitive technique, sample utilization efficiency is usually limited in conventional ion sources. For example, the traditional fiat ribbon-type filament thermal ionization sources commonly have a sample utilization efficiency of less than 0.2% for uranium [9]. Thus, techniques that enhance the ionization efficiency for TIMS have been widely explored. In the past, filament shape and geometry has been changed to: 1) increase ionization efficiency, with V-type filaments [10,11]; and 2) improve ion transport through the mass spectrometer by reducing the available filament surface area in order to resemble point sources [12]. Resin beads [13] have been used to improve sample
0168-1176/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0168-1176(96)04434-5
28
Y. Duan et al./International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
loading techniques, specially for the actinides, Ionization enhancers, e.g., silica gel/boric acid [3], have been used to improve the ionization efficiency of a large number of elements, from the alkali and alkaline earth elements to the transition metals. LaB6 glasses have been used to improve negative ionization efficiency [14]. Platinum electroplating of rhenium filaments [15] has been used to provide a high work function surface that improves ionization and provides a controlled method for analyte evaporation and enhanced surface contact by sandwiching the analyte in between platinum layers. Nevertheless, ionization efficiencies have remained, at best, around 10% for the alkali metals, 1% for uranium, and even lower for refractory elements or elements with relatively high first ionization energies, In the early 1970s, a new type of thermal ionization source was developed almost simultaneously by Beyer et al. [16] and by Johnson et al. [17], in which a refractory metal tube resembling a crucible with a deep cavity was used instead of a filament to evaporate and ionize the samples. The crucible was heated to ionizing temperatures, up to 3000°C, by high energy electron impact from a heated filament surrounding the crucible. In this way, the thermal temperature of the sources and the atom-surface interactions can be significantly increased, therefore considerably improving the sample ionization efficiency. These authors reported ionization efficiencies as high as 78% and 93% for Nd and Pm respectively. This thermal ionization cavity (TIC) has been developed extensively for applications to large scale isotope separator online projects (ISOL). Beyer et al. reported [18] ionization efficiencies of 5% for Th, 9% for U, and as high as 80% for Ac by accounting for oxide ions. These authors also observed an inverse correlation between analyte melting point and first ionization potential of the analytes, Karnaukov et al. reported high ionization efficiencies for Cs and Ba using the TIC source [19]. Latuszynski and Raiko demonstrated the
high ionization power of the TIC source by showing ionization efficiencies of 0.5-1% for Hf and Zr [20]. These authors also pointed to the potential application of this type of ion source to mass spectrometry along with on-line isotope separation. Amiel and coworkers showed the application of a TIC type source to the separation and detection of short-lived fission products formed on a target directly mounted on the ion source which was irradiated by neutrons from an online research reactor [21,22]. Kirchner reviewed the main parameters affecting the TIC source performance [23]. In this work Kirchner showed the effect of work function on ionization efficiency by using Ta, W, and Re for the crucible. Kirchner observed that the partial pressure of impurities in the source caused various types of effects, including slight enhancements in the ionization efficiency, by 1.1-1.5, when xenon was added to the cavity region. A number of other TIC source designs followed the work performed in the 70's [24-26], including a design using ZrC as a thermionic source of electrons which, as was stipulated, would cause the plasma potential to increase, thereby increasing the ionization efficiency to 97% and 99% for Ce and Tb respectively [27]. Consequently, both theoretical and experimental studies were carried out on such cavities to better understand the ionization mechanism of the analytes [22,28-30]. The influence on the ionization efficiency from cavity parameters, such as crucible work function, source temperature, plasma density, and partial pressure of crucible materials, was explored. Kirchner finalized the theory on thermoionization in hot cavities with a comprehensive discussion on the effects from ionizer material, ion and neutral densities, temperature, and cavity orifice diameter [31]. The TIC source has been applied solely to isotope seperator systems, although Delmore and his coworkers introduced a similar tube-type source for the study of chemical effects from surface ionization, in which the source is used as an ion gun for secondary ionization mass spectrometry
Y. Duan et al./lnternational Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
[32]. In this paper, we present a new version of the TIC source developed specifically for use in mass spectrometry. The performance of this ion source has been characterized extensively both with the use of an isotope separator and a quadrupole mass spectrometer. A detailed description of the TIC source for mass spectrometry is given along with the performance characteristics observed to date.
2. Experimental 2.1. Ion source design features 2.1.1. Basic structure The design of the new TIC ion source has the primary goal of achieving large sample utilization efficiencies comparable to those achieved by
29
the larger sources used in the ISOL projects. With that goal in mind, the new ion source was designed as shown in Fig. 1. In Fig. la the transverse section view of the ion source coupled to a quadrupole mass spectrometer system is shown. The principle components of the assembly are the adapter flange used to accommodate the ion source on the mass spectrometer or isotope separator, the sample crucible with a cavity where the sample is loaded, and the electron emission filament used to heat the crucible, and electron shield. The ion source can be mounted on an isotope separator or a quadrupole mass spectrometer without modifications. In the configuration shown, the vacuum system must be brought up to air prior to inserting a new sample. A vacuum interlock was not designed into this prototype system, for mechanical simplicity only, and is planned for the future. All of the 1
/ (a)
1
o (b)
1
Fig. 1. Schematic diagram of the new ion source and its coupling with a simple quadrupole system. (a) The ion source mounted on a quadrupole mass spectrometer. (b) A coaxial view of the source: 1, electrodes for power supply; 2, water or air cooling tubing; 3, electron bombardment shielding can; 4, the orifice of the shielding can; 5, crucible holder; 6, crucible; 7, electron bombarding filament; 8, ion lens; 9, quadmpole mass spectrometer.
30
Y. Duan et aL/International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
metal components in the source are made of stainless steel unless otherwise stated,
2.1.2. Crucible A diagram of the crucibles used for this work is shown in Fig. 2. The crucibles used here are made of high-purity ( > 98.5%) tungsten, and are used for loading samples. For isotope separator work, sample loading is usually in the order of 10's of milligrams of material using the crucible shown in Fig. 2a. This crucible consists of two tungsten metal components, the body with a large cavity opening for ease of sample loading, and a cap with a 0.33 mm opening. The cap surface is shaped to minimize the overall magnification of the ion beam in order to get better transport efficiency. For smaller samples ( < 1 mg typically used in mass spectrometry), the crucible shown in Fig. 2b is used. This is a one-piece tungsten metal crucible with a 0.50 mm diameter channel and a 1.25 cm deep cavity. No cap is used for this crucible. Such a crucible can be produced at much lower cost (compared with the one shown in the Fig. 2a), and can be used at a fairly high temperature (3000°C). In both cases (Fig. 2a and b), the ionizing channel opening presents a small apparent object to the ion optic system but is large enough to ensure that a quasi-neutral plasma can be maintained [29]. For isotope
,/
separator work, ion acceleration is achieved by floating the complete ion source and filament power supplies to 50,000 V d.c. On the quadrupole mass spectrometer, the crucible and its base plate can be electrically isolated and operate at 10-40 V d.c.
2.1.3. The filament and shielding can The filament used in this work is very similar to that reported in the literature [16]; it is made from two turns of 0.5 mm tantalum wire. The diameter of the filament's circular opening is set to about 12 mm. The filament is centered near the inside top of the shielding can, and is maintained at negative potential with respect to the crucible to accelerate the emitted electrons towards the crucible. The filament power supply usually floats at - 2500 to - 3000 V d.c. bias voltage, and can deliver up to 40 mA of electron emission current for the quadrupole system and up to 1 A for the isotope separator. Although the power supply system for the isotope separator is capable of regulating the electron emission via computer control of the filament current and voltage, the power supply used for the quadrupole mass spectrometer has no electron emission regulation. Yet, the filament current (and therefore emission current) can be manually controlled and has a stability of better than 5 mA.
2
I
~:~--'-'~~//////////I////I///A¥//I/////I/li~ Fig. 2. Tungsten crucibles used for thermal ionization source. (a) Crucible for isotope separator. (b) Crucible for quadrupole mass spectrometer: 1, ionization channel; 2, crucible cap. See text for further descriptions.
Y. Duan et al./International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
Although the present power supply system is adequate for the work presented in this paper, a regulated emission power supply has been designed and is in construction for future work. While operation of the ion source for isotope separator applications using the large crucible shown in Fig. 2a can require 250-500 W of electron power, for thorium operational temperatures to be reached, the smaller crucible (shown in Fig. 2b) reaches similar operating conditions with a maximum of 100 W. The shielding can used in the source serves several functions. The first one is to shield other parts in the source from bombarding electrons and sputtered tungsten from the crucible, The second is to shield other components from heat produced in the electron bombardment process. The third is as an accelerator for extracting ions produced in the crucible, as the shield is held at the electron bias voltage. Finally, the shielding can eliminates gaseous ions produced by the electron beam by shielding them from the optical system,
2.1.4. Cooling system A cooling device is usually needed to maintain long-term operation of the instrument. Watercooling channels have been designed into the system as shown in the axial view in Fig. 1. Without cooling water flowing, the ion source body may reach temperatures which could damage adjoining equipment or personnel. A fan directing air onto the source body can also be used, providing sufficient cooling for use with the smaller, thinner, crucible,
2.2. Mass spectrometer and isotope separator The quadrupole mass spectrometer used in this work is a Balzers Model QMG 420 system. The ion extraction and focusing lens for the system have been designed in-house, and described previously [33]. It mainly consists of three lenses just in front of the quadrupole to form a simple acceleration-deceleration ion-extraction system,
31
The lenses are assembled and isolated from each other with ceramic insulators. The system pumping is performed by a Balzers 330 L/s turbo-molecular pump, which is sufficient to maintain a base pressure of 10 -s Torr, and < 10 -6 Torr with the source at maximum ternperature. A computer-based data acquisition and control system is used for operating the instrument. The typical experimental parameters used in this study are: bias voltage around - 2500 V d.c.; ion lens adjusted from 10-1000 V in order to achieve maximum ion signals (the extraction voltage was set at about 240 V, and the second ion lens was 70 V); 55 V used for ion accelerating voltage; and maximum filament current less than 10 A. The isotope separator used in this work is a large room-sized instrument, which is routinely used for isotope separation and isotope collection. This instrument has been described previously [34,35]. The ion transmission efficiency in the separator is near unity, which assures good performance for ionization efficiency measurements. An externally-controlled Faraday cup assembly could be attached in front of the collection system, and was used to measure the total ion current produced by the source.
2.3. Sample preparation and running Oxides of the elements of interest are used to examine the source performance. The powder samples are packed into the cavity by pressing the crucible's opening into a small dish containing the sample. A clean tungsten wire is used to make sure the sample is packed onto the bottom of the cavity. The packed crucible is then mounted on the crucible holder and inserted into the source. The crucible is baked in the source at low temperature to remove any volatile contaminants until the vacuum pressure is less than 1 x 10 -6 Torr. The temperature is then increased in steps and the signals from both background contaminants and analytes are monitored.
32
Y. Duan et aL/lnternational Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39 Ion
Current
[E-10A] 39 K
4.0
3.0
2.0
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41K
23Na 0.1
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, 20
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[ainu]
Fig. 3. Background peaks observed in the crucible source. (a) Background peaks of potassium, obtained at emission current of about 2 mA. (b) Background peaks of sodium, obtained at emission current of about 3.5 mA.
Y. Duan et aL/International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
3. Results and discussion
3.1. Behavior of background and analyte elements After the initial bake-out at low temperature, the electron emission is slowly increased. The signal observed in the quadrupole mass spectrometer, during these early stages, is shown in Fig. 3. The background peaks from sodium and potassium (at masses of 23, 39, and 41 respectively) were observed at electron emission currents of 2 to 4 mA. The potassium peaks appear first at relatively low electron emission current and can last for relatively long times (hours). The sodium signal needs a little higher electron emission current (over 3.2 mA) to reach its maximum value (see Fig. 3b), and will quickly diminish, after some running time depending on the power used. At this electron emission current, no analyte signal is observed. Increasing the emission current above 4 mA causes both the potassium and sodium signals to disappear, Therefore, these background peaks do not influence the measurement of the analyte, A mixed sample of the rare earth oxides of europium, samarium and lutetium with a ratio of 1:1:1 was used for testing. The sample amount used in this experiment is about 1 milligram. The selection of elements is based on the first ionization potential of the elements and their vapor pressure, which are considered as the two most important factors in the performance of the thermal ionization source. As indicated in Table 1, the three rare earth elements have similar first
ionization potentials but quite different temperature for achieving 10 -4 Tort vapor pressure, which leads to different behaviors in the source. Fig. 4 shows some results for the selected rare earth elements. When the bias voltage was kept constant ( - 2500 V d.c.), increasing electron emission current means increasing input power or temperature. For this mixed sample, the europium signals can be obtained at electron emission currents of about 20 mA. As is shown in Fig. 4a, at this electron emission level, no signal can be observed for the other analyte elements. Further increasing the electron emission current up to about 23 mA causes the samarium peaks to appear, as shown in Fig. 4b. In this case, only a very small peak for lutetium at mass number 175 can be observed. The large signals for lutetium, seen in Fig. 4c, are obtained at an electron emission current of about 27 mA. However, at this current, the europium quickly evaporates and the signal decreases. Fig. 5a shows the uranium signal obtained from an uranium oxide sample. The electron emission current is about 32 mA. The uranium oxide samples show a high propensity to form oxide ions at mass 254 (for 238U160÷) and 251 (for 2 3 5 U 1 6 0 + ) although a small metal signal is also observed at mass 238. The oxide formation can be reduced by packing the crucible with a small amount of graphite, as shown in Fig. 5b.
3.2. Ionization efficiency In conventional thermal ionization, the sample to be analyzed is placed on a thin, fiat ribbon-type
Table 1 Physical parameter influence in TIMS Element
Eu Sm Lu U
First ionization potential/eV
5.7 5.6 5.4 6.2
33
Temperature to achieve 10 -4 Torr pressure/K 730 850 1550 1850
Emission current used in the TIMS/mA 20 23 27 32
34
Y. Duan et al./International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39 Ion Current
[E-IOA]
1.5
151Eu
1G3Eu
1.0
0.5
0.0
140
145
150
155
(a)
Ion Current
160
165
170
175
180 [amu]
[E-09A] 151Eu 153Eu
1.0
0.5
144-154Sm
0.0 . ...~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 145 150 155 160 165 170 175 (b)
180
[amu]
Fig. 4. (a) Isotope peaks of europium, obtained at emission current of about 20 mA. (b) Isotope peaks of europium and samarium, obtained at emission current of about 23 mA. (c) Isotope peaks of europium, samarium and lutetium. The peaks are obtained at emission current of about 27 mA.
Y.. Duan et al./lnternational Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39 Ion Current
[E-O9A]
35
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[amu] Fig. 4. Continued.
filament, which is heated by passing a current through the filament, much like the filament in a light bulb. The filament is heated to temperatures of up to 1800°C [4], and sample atoms are evaporated from the surface of the filament. Ionization occurs as a result of surface ionization from the interaction between analyte atom and the filament surface. As the filament must be placed in a high vacuum, the atoms evaporated from the filament have limited contact with the filament surface. In fact, once the atoms leave the immediate region near the filament surface, they are not likely to interact with the filament surface again. Therefore, the ionization efficiency for the ribbon-type filament source is very low. In the TIC source, the sample is placed deep inside the small tungsten crucible with a deep cavity. The crucible is then heated by bombardment from electrons emitted from the tantalum filament located around the crucible. As the sample evaporates inside the crucible, the
pressure in the cavity increases, and the gaseous atoms produced interact with each other and with the inner walls of the cavity to produce positive ions of the analytes. Because of the very limited space inside the cavity and relatively large surface area available, the interactions between the gaseous atoms and the surface are considerably increased compared with the ribbon-type filament. Thus, the ionization of analytes in the hot cavity originates from surface ionization at the walls of the cavity. As was described by several authors [29,30], the ionization efficiency for a certain element is strongly dependent on its first ionization potential (described through the SahaLangmuir equation) and the operating temperature. In addition, the ionization efficiency seems also correlated with the ionizer's material and geometry [29] as well as the operating emission current [18]. The theoretical basis for the high ionization efficiency of cavity sources has been
Y. Duan et aL/International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
36 Ion
Current
[E-08]
z38uo
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.
.
230
.
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.
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260 [ainu]
z38U 1.0
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z
(b)
~
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/~ [amu]
Fig. 5. (a) Uranium isotope peaks obtained with uranium oxide at an emission current of about 30 mA. (b) Uranium isotope peaks obtained with uranium oxide at an emission current of about 30 mA after addition of graphite to reduce the oxide ion levels.
Y. Duan et al./International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
debated extensively [22,28-30] and is summarized and finalized in Kirchner's paper [31]. The ionization efficiency for the TIC, 7/, is described from the modified Saha-Langmuir equation for surface ionization at thermal equilibrium, ~, by an amplification factor o~r, i.e. ~/¢
~/= 1-fl(1-coK)
37
Table 2 Ionization efficiency measured with
presentsource Efficiency
Element
Form
Eu
Eu203
72%
Lu u
Lu203 u308
25% 8%
Th Zr
Th02 ZrO 2
2% 2%
(1)
where r is the mean number of wall collisions for an analyte atom in the cavity prior to exiting the cavity, and o~ is the probability of a surfaceionized particle of leaving the cavity as an ion. Using experimental data to correlate ion source behavior to eqn (1), Kirchner concluded that the cavity is far from being at thermal equilibrium for ion formation. According to Kirchner [29], for a tungsten cylinder with 0.2 cm 3 volume and an orifice of 0.7 mm in diameter operated at a temperature of about 2850 IG the atoms make a mean number of wall collisions of about 700 before escaping through the exit hole. Another reason for efficiency improvement is the high operational temperatures achieved with the cavity. With the energy exerted on the crucible, the temperature of the cavity can reach up to 3000°C or even higher [16]. In fact we have often, although undesirably, melted the tungsten crucibles. The thickness of the crucible makes it possible for the cavity to easily survive temperatures of around 3000°C, which the thin ribbon-type ion source can not. Although highly desirable, the current system does not incorporate a pyrometer for temperature measurement due to space constraints, As in traditional TIMS sources, we also found that outgassing of the system and crucible baking are very important for achieving high performance. Using pre-baked tungsten crucibles, baked to temperatures above those used for analysis, the maximum ionization efficiencies are observed. In this work, a series of rare earth and actinide elements have been used to measure the ionization efficiency in the TIC. The results are presented in Table 2, which shows that
ionization efficiencies of up to 72% can be obtained for the rare earths. Uranium follows at 8%. Thorium and zirconium are elements having both high ionization potentials and low vapor pressures, which show a measured ionization efficiency of 2%. These results are similar to the values reported in the early literature [16]. Again, both ionization potential and vapor pressure influence the ionization efficiency in the TIC and can be used to predict behavior for other new analytes. For example, plutonium is expected to have a similar ionization efficiency to lutetium, in the range of 25%. 3.3. Isotope ratio measurements
To evaluate the stability of the TIC source and also to demonstrate the performance of the quadrupole mass spectrometer system, isotope ratio measurements have been carried out with europium and samarium. The isotope peaks at mass 144, 147, 148, 149, 150, 152 and 154 for samarium and 151, 153 for europium were monitored, and the ratios were calculated. Very good performance has been obtained with this system for fifty complete signal acquisitions over a period of 10 minutes and the results are given in Table 3. Although the precision varies with the isotope and its abundance, a typical precision of less than 0.1% can be achieved. Under carefully controlled conditions some measurements with precisions in the 10's of ppm level can also be attained. These are comparable to precisions obtained by single collector magnetic systems [6]. Although not measured yet, the overall efficiency of the quadrupole system is expected
38
Y. Duan et al./International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
Table 3
Isotope ratio measurements and precision Element Eu Sm
Ratio
Average value
151/153 144/152 147/152 148/152 149/152 150/152 154/152
0.9862 0.1750 0.6653 0.4995 0.5852 0.3233 0.8128
to drop two orders of magnitude simply due to the transport efficiency of the quadrupole [36]. Of course, it is expected that the ultimate analytical performance for the TIC source will be achieved on a sector-type mass spectrometer. Nevertheless, these encouraging results reveal the potential applications of the new source w i t h a simple quadrupole system. Background correction and mass discrimination factors have not been accounted for in this work and, therefore, future studies focusing on the more practical applications of the quadrupole system will need to include detailed examinations of these parameters.
SD 0.000034 0.000127 0.00140 0.000891 0.000923 0.000395 0.000145
RSD% 0.0034 0.073 0.21 0.18 0.16 0.12 0.018
Acknowledgements We wish to express our thanks to Mr. Frank Valdez for his help in fabrication of the source, to Mr. Eddie Rios for his prompt technical assistance in instrument set-up. Financial support from the Laboratory Directed Research and Development program at Los Alamos National Laboratory was greatly appreciated. Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy under contract W-7406-ENG-36.
References 4. Conclusion
[1] H.G. Thode, C.C. McMullen and K. Fritze, Advances in Inorganic Chemistry and Radiochemistry, Vol. 2, Academic Press,
We have developed a high-efficiency thermal ionization source that provides o n e to t w o o r d e r s of magnitude improvement in sample utilization efficiency in comparison with the traditional ribbon-type filament ion sources currently used in TIMS. This improved sample utilization efficiency can result in a proportional increase in sample throughput and proportional decrease in analysis time. We expect that the analytical performance of the source will continue to improve a s w e increase our knowledge of its behavior and the effects and constraints of its physical parameters. It is expected that the cavity ion source will reach its ultimate performance when attached spectrometer.
to a
sector-type
mass
New York, 1960, pp. 315-362. [2] L.F. Herzog, Int. J. Mass Spectrom. Ion Phys, 4 (1970) 253. [3] A.M. Volpe, J.A. Olivares and M.T. Murrell, Anal. Chem., 63 (199l) 913. [41 A.G. Adriaens, J.D. Fassett, W.R. Kelly, D.S. Simons and F.C. Adams, Anal. Chem., 64 (1992) 2945. [51 P.J. Paulsen and W.R. Kelly, Anal. Chem., 56 (1984) 708. [61 S.K. Aggarwal, R.K. Duggal, P.M. Shah and H.C. Join, Int. J. Mass Spectrom. Ion Processes, 85 (1988) 137. [7] R.F. Creteila, R.A. Lukaszew, J.G. Marrero and R. Servant, Int. J. Mass Spectrom. Ion Processes, 98 (1990) 99. [8] D. Poupard and A. Juery, Radiochim. Acta, 57 (1992) 21. [91 J.A. Mchugh, Int. J. Mass Spectrom. Ion Phys, 3 (1969) 267. I10] F.A. White, T.L. Collins and F.M. Rourke, Phys. Rev., 101 (1956) 1786. [11] L.A. Dietz, Rev. Sci. Instrum., 30 (1959) 235. [12] J.E. Delmore, J. Phys. Chem., 91 (1987) 2883. [13] D.H. Smith and J.A. Carter, Int. J. Mass Spectrosc. Ion Phys., 50 (1981) 211. [14] M. Shmid, G. Engler, I. Yoresh and E. Skurnik, Nucl. Instr. Meth,, 186 (1981) 349.
Y. Duan et al./International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
[15] D.J. Rokop, R.E. Perrin, G.W. Knobeloch, V.M. Armijo and W.R. Shields, Anal. Chem., 54 (1982) 957. [16] G.J. Beyer, E.Herrmann, A. Piotrowski, V.J. Raiko and H. Tyrroff, Nucl. Instr. Meth., 96 (1971) 437. [17] P.G. Johnson, A. Bolson and C.M. Henderson, Nucl. Instr. Meth., 106 (1973) 83. [18] G.J. Beyer, E. Herrmann, F. Molnar, V.I. Raiko and H. Tyrroff, Radiochem. Radioanal. Lett., 12 (1972) 259. [19] V.A. Kamaukhov, D.D. Bogdanov, A.V. Demyanov, G.I. Koval and L.A. Petrov, Nucl. Instr. Meth., 120 (1974) 69. [20] A. Latuszynski and V.I. Raiko, Nucl. Instr. Meth., 125 (1975) 61. [21] S. Amiel, G. Engler, Y. Nir-el and M. Shmid, Nucl. Instr. Meth., 139 (1976) 305. [22] M. Shmid, Y. Nir-el, G. Engler, and S. Amiel, Nucl. Instr. Meth., 144 (1977) 601. [23] R. Kirchner, Nucl. Instr. Meth., 186 (1981) 275. [24] E.H. Spejewski, Nucl. Instr. Meth., 186 (1981) 317. [25] J. Munzel, H. Wollnik, B. Pfeiffer and G. Jung, Nucl. Instr. Meth., 186 (1981) 343.
39
[26] A. Piotrowski, R.L. Gill and D.C. McDonald, Nucl. Instr. Meth., 224 (1984) 1. [27] E.H. Pilzer and G. Engler, Nucl. Instr. MetE, B26 (1987) 218. [28] V.P. Afanas'ev, V.A. Obukhov and V.I. Raiko, Nucl. Instr. Meth., 145 (1977) 533. [29] R. Kirchner and A. Piotrowski, Nucl. Instr. Meth., 153 (1978) 291. [30] M. Huyse, Nucl. Instr. Meth., 215 (1983) 1. [31] R. Kirchner, Nucl. Instr. Meth., A292 (1990) 203. [32] J.E. Delmore, A.D. Appelhans and E.S. Peterson, Int. J. Mass Spectrom. Ion Processes, 108 (1991) 179. [33] J.A. Olivares, E.M. Garcia and E.P. Chamberlin, Proceedings of the 39th ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, Tenn, May 19-24, 1991. [34] CERN Report 70-3 (1970). [35] E.P. Chamberlin, K.N. Leung, S. Wlathers, R.A. Bibeau, R.L. Stice, G.M. Kelley and J. Wilson, Nucl. Instrum. Meth., B26 (1987) 227. [36] W.K. Huber and G. Rettinghaus, Advances in Mass Spectrometry, Vol. 8B, Applied Science Publishers, 1980, pp. 16381644.
International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
Development of a new high-efficiency thermal ionization source for mass spectrometry Yixiang Duan, Edwin P. Chamberlin, Jos6 A. Olivares* Los Alamos National Laboratory, Chemical Science and Technology Division, Los Alamos, NM 87545 USA
Received 3 April 1996; accepted 31 July 1996
Abstract A thermal ionization source for mass spectrometry has been designed and tested. The ion source is based on a tungsten crucible with a deep cavity into which the sample is loaded. The crucible is heated by high energy electron bombardment from a tantalum filament surrounding the crucible. As the sample evaporates inside the crucible, gaseous analyte atoms are produced which interact with the inner surface of the crucible walls to produce positive ions through surface ionization. The ions are extracted from the cavity through a small opening at the end of the crucible. Regulation of the electron emission current makes it possible to control the energy and power applied to the crucible and, therefore, the crucible temperature. A number of elements have been tested in this source. The ionization efficiencies measured using a high transmission isotope separator spectrometer show 10 to 100 times higher efficiency than in conventional surface thermal ionization sources. © 1997 Elsevier Science B.V. Keywords: Thermal ionization; Ion source; High efficiency; Quadrupole; Mass spectrometry; Isotope separation;
Isotope ratios; Instrumentation
1. Introduction Thermal ionization mass spectrometry (TIMS) is one of the most powerful techniques used to determine both the absolute and the relative isotopic abundance of elements [1,2]. Thermal ionization is efficient for elements which have relatively low or moderate first ionization energies, and with sufficient vapor pressure at the source operating temperature. Traditional thermal ionization sources use single, double, and triple, fiat ribbon-type filaments for sample vaporization and ionization. Due to the high sensitivity, precision, and accuracy, TIMS has been broadly employed in geochemistry [3], * Corresponding author,
environmental chemistry [4], materials analysis [5], and the study of nuclear materials [6-8]. Although TIMS is a very sensitive technique, sample utilization efficiency is usually limited in conventional ion sources. For example, the traditional fiat ribbon-type filament thermal ionization sources commonly have a sample utilization efficiency of less than 0.2% for uranium [9]. Thus, techniques that enhance the ionization efficiency for TIMS have been widely explored. In the past, filament shape and geometry has been changed to: 1) increase ionization efficiency, with V-type filaments [10,11]; and 2) improve ion transport through the mass spectrometer by reducing the available filament surface area in order to resemble point sources [12]. Resin beads [13] have been used to improve sample
0168-1176/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0168-1176(96)04434-5
28
Y. Duan et al./International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
loading techniques, specially for the actinides, Ionization enhancers, e.g., silica gel/boric acid [3], have been used to improve the ionization efficiency of a large number of elements, from the alkali and alkaline earth elements to the transition metals. LaB6 glasses have been used to improve negative ionization efficiency [14]. Platinum electroplating of rhenium filaments [15] has been used to provide a high work function surface that improves ionization and provides a controlled method for analyte evaporation and enhanced surface contact by sandwiching the analyte in between platinum layers. Nevertheless, ionization efficiencies have remained, at best, around 10% for the alkali metals, 1% for uranium, and even lower for refractory elements or elements with relatively high first ionization energies, In the early 1970s, a new type of thermal ionization source was developed almost simultaneously by Beyer et al. [16] and by Johnson et al. [17], in which a refractory metal tube resembling a crucible with a deep cavity was used instead of a filament to evaporate and ionize the samples. The crucible was heated to ionizing temperatures, up to 3000°C, by high energy electron impact from a heated filament surrounding the crucible. In this way, the thermal temperature of the sources and the atom-surface interactions can be significantly increased, therefore considerably improving the sample ionization efficiency. These authors reported ionization efficiencies as high as 78% and 93% for Nd and Pm respectively. This thermal ionization cavity (TIC) has been developed extensively for applications to large scale isotope separator online projects (ISOL). Beyer et al. reported [18] ionization efficiencies of 5% for Th, 9% for U, and as high as 80% for Ac by accounting for oxide ions. These authors also observed an inverse correlation between analyte melting point and first ionization potential of the analytes, Karnaukov et al. reported high ionization efficiencies for Cs and Ba using the TIC source [19]. Latuszynski and Raiko demonstrated the
high ionization power of the TIC source by showing ionization efficiencies of 0.5-1% for Hf and Zr [20]. These authors also pointed to the potential application of this type of ion source to mass spectrometry along with on-line isotope separation. Amiel and coworkers showed the application of a TIC type source to the separation and detection of short-lived fission products formed on a target directly mounted on the ion source which was irradiated by neutrons from an online research reactor [21,22]. Kirchner reviewed the main parameters affecting the TIC source performance [23]. In this work Kirchner showed the effect of work function on ionization efficiency by using Ta, W, and Re for the crucible. Kirchner observed that the partial pressure of impurities in the source caused various types of effects, including slight enhancements in the ionization efficiency, by 1.1-1.5, when xenon was added to the cavity region. A number of other TIC source designs followed the work performed in the 70's [24-26], including a design using ZrC as a thermionic source of electrons which, as was stipulated, would cause the plasma potential to increase, thereby increasing the ionization efficiency to 97% and 99% for Ce and Tb respectively [27]. Consequently, both theoretical and experimental studies were carried out on such cavities to better understand the ionization mechanism of the analytes [22,28-30]. The influence on the ionization efficiency from cavity parameters, such as crucible work function, source temperature, plasma density, and partial pressure of crucible materials, was explored. Kirchner finalized the theory on thermoionization in hot cavities with a comprehensive discussion on the effects from ionizer material, ion and neutral densities, temperature, and cavity orifice diameter [31]. The TIC source has been applied solely to isotope seperator systems, although Delmore and his coworkers introduced a similar tube-type source for the study of chemical effects from surface ionization, in which the source is used as an ion gun for secondary ionization mass spectrometry
Y. Duan et al./lnternational Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
[32]. In this paper, we present a new version of the TIC source developed specifically for use in mass spectrometry. The performance of this ion source has been characterized extensively both with the use of an isotope separator and a quadrupole mass spectrometer. A detailed description of the TIC source for mass spectrometry is given along with the performance characteristics observed to date.
2. Experimental 2.1. Ion source design features 2.1.1. Basic structure The design of the new TIC ion source has the primary goal of achieving large sample utilization efficiencies comparable to those achieved by
29
the larger sources used in the ISOL projects. With that goal in mind, the new ion source was designed as shown in Fig. 1. In Fig. la the transverse section view of the ion source coupled to a quadrupole mass spectrometer system is shown. The principle components of the assembly are the adapter flange used to accommodate the ion source on the mass spectrometer or isotope separator, the sample crucible with a cavity where the sample is loaded, and the electron emission filament used to heat the crucible, and electron shield. The ion source can be mounted on an isotope separator or a quadrupole mass spectrometer without modifications. In the configuration shown, the vacuum system must be brought up to air prior to inserting a new sample. A vacuum interlock was not designed into this prototype system, for mechanical simplicity only, and is planned for the future. All of the 1
/ (a)
1
o (b)
1
Fig. 1. Schematic diagram of the new ion source and its coupling with a simple quadrupole system. (a) The ion source mounted on a quadrupole mass spectrometer. (b) A coaxial view of the source: 1, electrodes for power supply; 2, water or air cooling tubing; 3, electron bombardment shielding can; 4, the orifice of the shielding can; 5, crucible holder; 6, crucible; 7, electron bombarding filament; 8, ion lens; 9, quadmpole mass spectrometer.
30
Y. Duan et aL/International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
metal components in the source are made of stainless steel unless otherwise stated,
2.1.2. Crucible A diagram of the crucibles used for this work is shown in Fig. 2. The crucibles used here are made of high-purity ( > 98.5%) tungsten, and are used for loading samples. For isotope separator work, sample loading is usually in the order of 10's of milligrams of material using the crucible shown in Fig. 2a. This crucible consists of two tungsten metal components, the body with a large cavity opening for ease of sample loading, and a cap with a 0.33 mm opening. The cap surface is shaped to minimize the overall magnification of the ion beam in order to get better transport efficiency. For smaller samples ( < 1 mg typically used in mass spectrometry), the crucible shown in Fig. 2b is used. This is a one-piece tungsten metal crucible with a 0.50 mm diameter channel and a 1.25 cm deep cavity. No cap is used for this crucible. Such a crucible can be produced at much lower cost (compared with the one shown in the Fig. 2a), and can be used at a fairly high temperature (3000°C). In both cases (Fig. 2a and b), the ionizing channel opening presents a small apparent object to the ion optic system but is large enough to ensure that a quasi-neutral plasma can be maintained [29]. For isotope
,/
separator work, ion acceleration is achieved by floating the complete ion source and filament power supplies to 50,000 V d.c. On the quadrupole mass spectrometer, the crucible and its base plate can be electrically isolated and operate at 10-40 V d.c.
2.1.3. The filament and shielding can The filament used in this work is very similar to that reported in the literature [16]; it is made from two turns of 0.5 mm tantalum wire. The diameter of the filament's circular opening is set to about 12 mm. The filament is centered near the inside top of the shielding can, and is maintained at negative potential with respect to the crucible to accelerate the emitted electrons towards the crucible. The filament power supply usually floats at - 2500 to - 3000 V d.c. bias voltage, and can deliver up to 40 mA of electron emission current for the quadrupole system and up to 1 A for the isotope separator. Although the power supply system for the isotope separator is capable of regulating the electron emission via computer control of the filament current and voltage, the power supply used for the quadrupole mass spectrometer has no electron emission regulation. Yet, the filament current (and therefore emission current) can be manually controlled and has a stability of better than 5 mA.
2
I
~:~--'-'~~//////////I////I///A¥//I/////I/li~ Fig. 2. Tungsten crucibles used for thermal ionization source. (a) Crucible for isotope separator. (b) Crucible for quadrupole mass spectrometer: 1, ionization channel; 2, crucible cap. See text for further descriptions.
Y. Duan et al./International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
Although the present power supply system is adequate for the work presented in this paper, a regulated emission power supply has been designed and is in construction for future work. While operation of the ion source for isotope separator applications using the large crucible shown in Fig. 2a can require 250-500 W of electron power, for thorium operational temperatures to be reached, the smaller crucible (shown in Fig. 2b) reaches similar operating conditions with a maximum of 100 W. The shielding can used in the source serves several functions. The first one is to shield other parts in the source from bombarding electrons and sputtered tungsten from the crucible, The second is to shield other components from heat produced in the electron bombardment process. The third is as an accelerator for extracting ions produced in the crucible, as the shield is held at the electron bias voltage. Finally, the shielding can eliminates gaseous ions produced by the electron beam by shielding them from the optical system,
2.1.4. Cooling system A cooling device is usually needed to maintain long-term operation of the instrument. Watercooling channels have been designed into the system as shown in the axial view in Fig. 1. Without cooling water flowing, the ion source body may reach temperatures which could damage adjoining equipment or personnel. A fan directing air onto the source body can also be used, providing sufficient cooling for use with the smaller, thinner, crucible,
2.2. Mass spectrometer and isotope separator The quadrupole mass spectrometer used in this work is a Balzers Model QMG 420 system. The ion extraction and focusing lens for the system have been designed in-house, and described previously [33]. It mainly consists of three lenses just in front of the quadrupole to form a simple acceleration-deceleration ion-extraction system,
31
The lenses are assembled and isolated from each other with ceramic insulators. The system pumping is performed by a Balzers 330 L/s turbo-molecular pump, which is sufficient to maintain a base pressure of 10 -s Torr, and < 10 -6 Torr with the source at maximum ternperature. A computer-based data acquisition and control system is used for operating the instrument. The typical experimental parameters used in this study are: bias voltage around - 2500 V d.c.; ion lens adjusted from 10-1000 V in order to achieve maximum ion signals (the extraction voltage was set at about 240 V, and the second ion lens was 70 V); 55 V used for ion accelerating voltage; and maximum filament current less than 10 A. The isotope separator used in this work is a large room-sized instrument, which is routinely used for isotope separation and isotope collection. This instrument has been described previously [34,35]. The ion transmission efficiency in the separator is near unity, which assures good performance for ionization efficiency measurements. An externally-controlled Faraday cup assembly could be attached in front of the collection system, and was used to measure the total ion current produced by the source.
2.3. Sample preparation and running Oxides of the elements of interest are used to examine the source performance. The powder samples are packed into the cavity by pressing the crucible's opening into a small dish containing the sample. A clean tungsten wire is used to make sure the sample is packed onto the bottom of the cavity. The packed crucible is then mounted on the crucible holder and inserted into the source. The crucible is baked in the source at low temperature to remove any volatile contaminants until the vacuum pressure is less than 1 x 10 -6 Torr. The temperature is then increased in steps and the signals from both background contaminants and analytes are monitored.
32
Y. Duan et aL/lnternational Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39 Ion
Current
[E-10A] 39 K
4.0
3.0
2.0
1.0
41K
23Na 0.1
, . , 16 18
.
, 20
.
, ./~ , 22 24
.
, 26
i
2'8
•
30
i
,
32
v
-
34
i
•
36
,
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38
;0
~'t . . . . 42 44.
(a) Ion
[ainu] Current
[E-IOA] 23Na
3.0
2.0
1.0 39K
41K 0.0
u
16 (b)
•
w
18
,
,
20
,
i
22
•
,
24
-
~
:?6
,
i
28
•
u
30
-
,
32
•
i
34
•
a
36
•
u
38
:~ ,
;
0
.
42
,
.
44
[ainu]
Fig. 3. Background peaks observed in the crucible source. (a) Background peaks of potassium, obtained at emission current of about 2 mA. (b) Background peaks of sodium, obtained at emission current of about 3.5 mA.
Y. Duan et aL/International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
3. Results and discussion
3.1. Behavior of background and analyte elements After the initial bake-out at low temperature, the electron emission is slowly increased. The signal observed in the quadrupole mass spectrometer, during these early stages, is shown in Fig. 3. The background peaks from sodium and potassium (at masses of 23, 39, and 41 respectively) were observed at electron emission currents of 2 to 4 mA. The potassium peaks appear first at relatively low electron emission current and can last for relatively long times (hours). The sodium signal needs a little higher electron emission current (over 3.2 mA) to reach its maximum value (see Fig. 3b), and will quickly diminish, after some running time depending on the power used. At this electron emission current, no analyte signal is observed. Increasing the emission current above 4 mA causes both the potassium and sodium signals to disappear, Therefore, these background peaks do not influence the measurement of the analyte, A mixed sample of the rare earth oxides of europium, samarium and lutetium with a ratio of 1:1:1 was used for testing. The sample amount used in this experiment is about 1 milligram. The selection of elements is based on the first ionization potential of the elements and their vapor pressure, which are considered as the two most important factors in the performance of the thermal ionization source. As indicated in Table 1, the three rare earth elements have similar first
ionization potentials but quite different temperature for achieving 10 -4 Tort vapor pressure, which leads to different behaviors in the source. Fig. 4 shows some results for the selected rare earth elements. When the bias voltage was kept constant ( - 2500 V d.c.), increasing electron emission current means increasing input power or temperature. For this mixed sample, the europium signals can be obtained at electron emission currents of about 20 mA. As is shown in Fig. 4a, at this electron emission level, no signal can be observed for the other analyte elements. Further increasing the electron emission current up to about 23 mA causes the samarium peaks to appear, as shown in Fig. 4b. In this case, only a very small peak for lutetium at mass number 175 can be observed. The large signals for lutetium, seen in Fig. 4c, are obtained at an electron emission current of about 27 mA. However, at this current, the europium quickly evaporates and the signal decreases. Fig. 5a shows the uranium signal obtained from an uranium oxide sample. The electron emission current is about 32 mA. The uranium oxide samples show a high propensity to form oxide ions at mass 254 (for 238U160÷) and 251 (for 2 3 5 U 1 6 0 + ) although a small metal signal is also observed at mass 238. The oxide formation can be reduced by packing the crucible with a small amount of graphite, as shown in Fig. 5b.
3.2. Ionization efficiency In conventional thermal ionization, the sample to be analyzed is placed on a thin, fiat ribbon-type
Table 1 Physical parameter influence in TIMS Element
Eu Sm Lu U
First ionization potential/eV
5.7 5.6 5.4 6.2
33
Temperature to achieve 10 -4 Torr pressure/K 730 850 1550 1850
Emission current used in the TIMS/mA 20 23 27 32
34
Y. Duan et al./International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39 Ion Current
[E-IOA]
1.5
151Eu
1G3Eu
1.0
0.5
0.0
140
145
150
155
(a)
Ion Current
160
165
170
175
180 [amu]
[E-09A] 151Eu 153Eu
1.0
0.5
144-154Sm
0.0 . ...~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 145 150 155 160 165 170 175 (b)
180
[amu]
Fig. 4. (a) Isotope peaks of europium, obtained at emission current of about 20 mA. (b) Isotope peaks of europium and samarium, obtained at emission current of about 23 mA. (c) Isotope peaks of europium, samarium and lutetium. The peaks are obtained at emission current of about 27 mA.
Y.. Duan et al./lnternational Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39 Ion Current
[E-O9A]
35
lrSLu
1.0~
144.154Sm
153Eu
0.5
.
40
.
.
.
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145
.
.
.
.
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.
.
.
.
g
.
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.
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160
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.
.
.
.
,
170
.
.
.
.
,
.
.
.
175
.
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180
[amu] Fig. 4. Continued.
filament, which is heated by passing a current through the filament, much like the filament in a light bulb. The filament is heated to temperatures of up to 1800°C [4], and sample atoms are evaporated from the surface of the filament. Ionization occurs as a result of surface ionization from the interaction between analyte atom and the filament surface. As the filament must be placed in a high vacuum, the atoms evaporated from the filament have limited contact with the filament surface. In fact, once the atoms leave the immediate region near the filament surface, they are not likely to interact with the filament surface again. Therefore, the ionization efficiency for the ribbon-type filament source is very low. In the TIC source, the sample is placed deep inside the small tungsten crucible with a deep cavity. The crucible is then heated by bombardment from electrons emitted from the tantalum filament located around the crucible. As the sample evaporates inside the crucible, the
pressure in the cavity increases, and the gaseous atoms produced interact with each other and with the inner walls of the cavity to produce positive ions of the analytes. Because of the very limited space inside the cavity and relatively large surface area available, the interactions between the gaseous atoms and the surface are considerably increased compared with the ribbon-type filament. Thus, the ionization of analytes in the hot cavity originates from surface ionization at the walls of the cavity. As was described by several authors [29,30], the ionization efficiency for a certain element is strongly dependent on its first ionization potential (described through the SahaLangmuir equation) and the operating temperature. In addition, the ionization efficiency seems also correlated with the ionizer's material and geometry [29] as well as the operating emission current [18]. The theoretical basis for the high ionization efficiency of cavity sources has been
Y. Duan et aL/International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
36 Ion
Current
[E-08]
z38uo
1.0
0.5
Z38 U
0 . 0
.
.
230
.
,
.
235 Ion
(a)
.
.
°
240
.
.
.
.
,
245
. . . . .
,".
250
Current [E-08]
S ' .
.
__
~
.
.
255
.
,
260 [ainu]
z38U 1.0
0.5
z38uo o.o
z
(b)
~
"z o
/~ [amu]
Fig. 5. (a) Uranium isotope peaks obtained with uranium oxide at an emission current of about 30 mA. (b) Uranium isotope peaks obtained with uranium oxide at an emission current of about 30 mA after addition of graphite to reduce the oxide ion levels.
Y. Duan et al./International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
debated extensively [22,28-30] and is summarized and finalized in Kirchner's paper [31]. The ionization efficiency for the TIC, 7/, is described from the modified Saha-Langmuir equation for surface ionization at thermal equilibrium, ~, by an amplification factor o~r, i.e. ~/¢
~/= 1-fl(1-coK)
37
Table 2 Ionization efficiency measured with
presentsource Efficiency
Element
Form
Eu
Eu203
72%
Lu u
Lu203 u308
25% 8%
Th Zr
Th02 ZrO 2
2% 2%
(1)
where r is the mean number of wall collisions for an analyte atom in the cavity prior to exiting the cavity, and o~ is the probability of a surfaceionized particle of leaving the cavity as an ion. Using experimental data to correlate ion source behavior to eqn (1), Kirchner concluded that the cavity is far from being at thermal equilibrium for ion formation. According to Kirchner [29], for a tungsten cylinder with 0.2 cm 3 volume and an orifice of 0.7 mm in diameter operated at a temperature of about 2850 IG the atoms make a mean number of wall collisions of about 700 before escaping through the exit hole. Another reason for efficiency improvement is the high operational temperatures achieved with the cavity. With the energy exerted on the crucible, the temperature of the cavity can reach up to 3000°C or even higher [16]. In fact we have often, although undesirably, melted the tungsten crucibles. The thickness of the crucible makes it possible for the cavity to easily survive temperatures of around 3000°C, which the thin ribbon-type ion source can not. Although highly desirable, the current system does not incorporate a pyrometer for temperature measurement due to space constraints, As in traditional TIMS sources, we also found that outgassing of the system and crucible baking are very important for achieving high performance. Using pre-baked tungsten crucibles, baked to temperatures above those used for analysis, the maximum ionization efficiencies are observed. In this work, a series of rare earth and actinide elements have been used to measure the ionization efficiency in the TIC. The results are presented in Table 2, which shows that
ionization efficiencies of up to 72% can be obtained for the rare earths. Uranium follows at 8%. Thorium and zirconium are elements having both high ionization potentials and low vapor pressures, which show a measured ionization efficiency of 2%. These results are similar to the values reported in the early literature [16]. Again, both ionization potential and vapor pressure influence the ionization efficiency in the TIC and can be used to predict behavior for other new analytes. For example, plutonium is expected to have a similar ionization efficiency to lutetium, in the range of 25%. 3.3. Isotope ratio measurements
To evaluate the stability of the TIC source and also to demonstrate the performance of the quadrupole mass spectrometer system, isotope ratio measurements have been carried out with europium and samarium. The isotope peaks at mass 144, 147, 148, 149, 150, 152 and 154 for samarium and 151, 153 for europium were monitored, and the ratios were calculated. Very good performance has been obtained with this system for fifty complete signal acquisitions over a period of 10 minutes and the results are given in Table 3. Although the precision varies with the isotope and its abundance, a typical precision of less than 0.1% can be achieved. Under carefully controlled conditions some measurements with precisions in the 10's of ppm level can also be attained. These are comparable to precisions obtained by single collector magnetic systems [6]. Although not measured yet, the overall efficiency of the quadrupole system is expected
38
Y. Duan et al./International Journal of Mass Spectrometry and Ion Processes 161 (1997) 27-39
Table 3
Isotope ratio measurements and precision Element Eu Sm
Ratio
Average value
151/153 144/152 147/152 148/152 149/152 150/152 154/152
0.9862 0.1750 0.6653 0.4995 0.5852 0.3233 0.8128
to drop two orders of magnitude simply due to the transport efficiency of the quadrupole [36]. Of course, it is expected that the ultimate analytical performance for the TIC source will be achieved on a sector-type mass spectrometer. Nevertheless, these encouraging results reveal the potential applications of the new source w i t h a simple quadrupole system. Background correction and mass discrimination factors have not been accounted for in this work and, therefore, future studies focusing on the more practical applications of the quadrupole system will need to include detailed examinations of these parameters.
SD 0.000034 0.000127 0.00140 0.000891 0.000923 0.000395 0.000145
RSD% 0.0034 0.073 0.21 0.18 0.16 0.12 0.018
Acknowledgements We wish to express our thanks to Mr. Frank Valdez for his help in fabrication of the source, to Mr. Eddie Rios for his prompt technical assistance in instrument set-up. Financial support from the Laboratory Directed Research and Development program at Los Alamos National Laboratory was greatly appreciated. Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy under contract W-7406-ENG-36.
References 4. Conclusion
[1] H.G. Thode, C.C. McMullen and K. Fritze, Advances in Inorganic Chemistry and Radiochemistry, Vol. 2, Academic Press,
We have developed a high-efficiency thermal ionization source that provides o n e to t w o o r d e r s of magnitude improvement in sample utilization efficiency in comparison with the traditional ribbon-type filament ion sources currently used in TIMS. This improved sample utilization efficiency can result in a proportional increase in sample throughput and proportional decrease in analysis time. We expect that the analytical performance of the source will continue to improve a s w e increase our knowledge of its behavior and the effects and constraints of its physical parameters. It is expected that the cavity ion source will reach its ultimate performance when attached spectrometer.
to a
sector-type
mass
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