Anal. Chem. 1994,66, 4432-4436
Negative Surface ionization Mass Spectrometry for Real-Time Monitoring of iodine Molecules in Process Off -Gas Hiroshi Kishi
Department of Materials Chemistry & Bio-Engineering, Oyama National College of Technology, 771 Nakakuki, Oyama 323, Japan Toshihiro Fujii*
National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, lbaraki 305, Japan
Negative surface ionization mass spectrometry is being developed for continuous measurement of iodine found in the atmospheric environmentas a result of nuclear fuel reprocessing. Studies have been made on a series of low work function surface materials (CaC03-SrC03-CaC03, BaC03-SrC03, m 6 ) with the intent of developing improved methods for iodine analysis, particularly at the environmental level (ppb concentration range). The results demonstrate the feasibility of performing real-time measurements of the trace amount of iodine encountered in process off-gas by use of a CaC03-SrC03-CaC03coated rhenium filament emitter. This emitter surface with thermoelectronic work function (p at 2.0 eV gives 37.5 times greater sensitivity than the established m 6 emitter for iodine analysis. Iodine129 is a volatile fission nuclide product that is found in the atmospheric environment as a result of nuclear reactor operation and nuclear fuel reprocessing. The interest in accurate, rapid, and relatively lowcost analytical methods for the determination of iodine has strongly increased in recent years for the main purpose of meeting the regulation requirement of abatement facilities of radioiodine in nuclear fuel reprocessing plants.'s2 There is a need for reliable methods that are capable of measuring iodine129 in real time, at or below the maximum permissible concentration of 0.1 part per million (ppm) (v/v), in the off-gas.2 There have been a number of methods for developing iodine monitors. Fernandez et aL3 developed a technique that includes cryogenic sample collection, gas chromatography, and isotope dilution mass spectrometry and a successful measurement on trace amounts of iodine. However, this method is not an on-line analysis. Some on-line measurement instruments based on laser spectroscopy have been tested on a laboratory scale. Both extracavity fluorescence4and intracavity absorption5~~ configurations have been used to detect 1-129in the ppb (v/v) concentration (1) United States Regulations 40 CFR 190, 1977. (2) Hebel, W., Cottone, G., Eds. Management Modes for Iodine-129 Harwood Academic Publishers: New York, 1982. (3) Femandez, S. J.; Rankin, R A; McManus, G. J.; Nielson, R A. Proc. 18th DOA Air Cleaning Conj 1985,2, 1318. (4) BaranavsLi, A P.; McDonald, J. R NRL Memo. Rep. 1977,No. 3514. (5) Peterson, N. C.; Kurylo, M. J.; Braun, W.; Bass, A. M.; Keller, R A. J. Opt. SOC.Am. 1971,746, 61. (6) Hohimer,
4432
J. P.; Hargis, P. J., Jr. Anal. Chem. 1979,51, 930.
Analytical Chemistry, Vol. 66, No. 24, December 15, 1994
range. The main disadvantage of these methods may be interference. The best sensitivity for measurement of environmental1-129 has been provided by inductivelycoupled plasmaatomic emission spectroscopy (ICP-AES) in a vacuum ultraviolet r e g i ~ n .This ~ meets the requirements for a real-time, sensitive monitor. Scaling estimates indicate that a concentration level of 0.19 ppb (v/v) in air may be achieved. There is no doubt that the mass spectrometer system would be very helpful in determining the amount of specific compounds known to be present in air. The first mass spectrometric method for measuring the iodine isotopic ratio was reported in 1965 by McHugh and Scheffield,8using a sputter ion source in a doublefocusing mass spectrometer. Due to interference of oxygenated hydrocarbons with 1-129, this method is not adequate for the measurement of 1-129in many environmental materials. Negative surface ionization mass spectrometry, with an ion source employing a LaB6 emitter, has been applied for I2 detectionsgThe results indicate that a calibration plot is nearly straight in the range between 50 to 0.1 ppm. This is not satisfactory enough to meet regulation limits. Historically, negative surface ionization (NSJ) has been the preferred ionization process for performing analysis of halogen compounds. If electronegativechemical species are adsorbed on an incandescent metal surface, negative ions occur. The surface ionization efficiency /3 for the thermodynamic equilibrium is expressed by
where p is the work function of the surface at which ionization occurs at temperature T,k is the Boltzmann constant, and E, is the electron aftinity of the emitting chemical species. gdg- is the ratio of the statistical weights of negative ions and the neutral species. Equation 1shows that the conditions require the use of elements with high electron affinities (Ea) on low work function surfaces. If E, - p > kT (the ionization process for the formation of negative ions is exothermic), p 1, and theoretically, the ion yield should decrease with a higher temperature. (7) Fujii, T.; Uehiro, T.;Nojiri, Y.;Mitsutsuka, Y.;Jimba, H. Anal. Chem. 1990,
62, 414. (8) McHugh, J. A; Schffield, J. C. Anal. Chem.
1965,37, 1099. (9) Kishi, H.; Kawano, H. Int. J. Mass Spectrom. Ion Processes 1988,85, 301. 0003-2700/94/0366-4432$04.50/0 0 1994 American Chemical Society
mass spectrometer
NV -
mass analysis
E+
tIA/
source houslng
/ 'sample diffusion tube (1 ? ) cell
I
v
I
pulse countlng
Figure 1. Schematic diagram of the apparatus: A, air cylinder; B, flowmeter; C, electron multiplier.
Various investigators10have studied NSI as a physical phenomenon and found that ionization efficiencies for the halogens can be raised by several orders of magnitude by using low work function ionizers in place of high work function pure metals. In earlier work," stable yields of 1.5%for C1 on the thoriated W surface were claimed. Later, oxide-coated filaments such as BaO/ W, SrO/W, or ThOz/Mo were successfully applied to negative ion emission. For example, high rates up to 50%were obtained for chlorides by use of a W-0-Ca surface as reported by Mueller and Wassmuth.12 Rareearth borides (La&) of a porous disk type have also been utilized.13 NSI of iodine, the present target species, has good potential for sensitive determination due to the relatively high electron affinity (3.3 eV) of the iodine atom. An emitter material suitable for this application would need to have a work function in this range. The working ionizer temperature must be high enough, at a given iodine pressure, to prevent excessive halogen surface coverage of the ionizer,whose work function would increase above the electron affinity value. Moreover, it must be high enough to obtain complete dissociation of the halogen vapor. In addition, it must be refractory and chemically inert toward the species being ionized. Among possible candidates, a promising one is lanthanum hexaboride. w6,which has been studied extensively,is a stable, refractory compound having a low work function. The work of Radhidi et al.14 is of particular interest. In this paper, high ion production efficiencies for atomic iodine ions (Ifrom -)gaseous IZusing porous were reported. D e l m ~ r ehas ' ~ used negative surface ionization mass spectrometry (NSIMS) to measure fission product iodine in which the 1-129 content was greater than the 1-127. He also measured the background level in the mass spectrum of natural 1-127. Stoffels16reported, in 1982, the NSIMS with a solid iodine sample coating on the sample filament. The ability to measure loi atoms (2 x 10-15 g) of 1-129in a chemically pure sample has been demonstrated. (10)Kawano, H.;Hidaka, Y.; Page, F. M. Int. j . Mass Spectrom. Ion. Processes 1983,50,35. (11) Persky, A;Greene, E. F.; Kuppermann, A 1.Chem. Phys. 1968,49,2347. (12)Mueller, R;Wassmuth, H. W. Nucl. Instrum. Methods 1975,118, 329. (13)Pelletier, J.; Pomot, C.; Cocagne, J. /. Appl. Phys. 1979,50, 4517. (14)Rachidi, I.; Monte, J.; Pelletier, J.; Pompt, C.; Rinchet, F. Appl. Phys. Left. 1976,28,292. (15) Delmore, J. E.Int. J. Mass Spectrom. Ion Phys. 1982,43,273. (16)Stoffels, J. J. Radiochem. Radioanal. Lett. 1982,55, 99.
The alkaline earth compounds may also be good ionizers for NSI. They exhibit an unusual combination of low work function and low volatility. Thus, these materials have attracted interest as thermionic cathodes. The BaO-type cathode, in fact, is in everyday use. It appears that these emitters can be good ionizers for negative surface ionization. We have developed a direct, real-time monitoring method for 1-129 emitted from the nuclear facilities that is expected to significantly improve these areas of 1-129 measurement technology. From these considerations, we have developed the method, based on negative surface ionization mass spectrometry with the surface ionizer prepared from CaC03-SrC03-CaC03, BaC03%COS,or hB6. This paper reports for these surfaces (1) the work function measurement and its increase with adsorption of oxygen, (2) the optimization of the surface ionizers to improve the efficiency of NSI of iodine, and (3) the response characteristics of the surface from the complex of CaC03-SrC03-CaC03 for the analysis of iodine in the dilution gas of air. EXPERIMENTAL SECTION
Apparatus. The experimental arrangements are shown in Figure 1. All experiments were performed in a vacuum system with a mass spectrometer. The mass spectrometer used is a 60" sector magnetic analyzer (Hitachi M-52) with a resolution of about 800 in the range of 1-1500 amu. After a 200 "C bake, a vacuum in the range of 2 x BaC03-SrC03 was established. The negative ion signal was measured by either a dc amplifier or a pulse counting device. A small computer (NEC, PC9801VX) controlled and cordinated these functions. Surface Preparation (Ion Source). A rhenium ribbon filament, sprayed-coated with an ionizing material in an organic binder, was prepared as the ionizer. The materials tested were BaC03-SrCO3, BaC03-SrC03-CaC03, and powder. The temperature of the rhenium ribbon was measured with an optical pyrometer (Model 502, Hamada Electric Works, Tokyo). The technique employed to generate an ionizing layer on the Re surface was similar to that employed elsewhere.li The experimental sequence was as follows. (a) For BaCOs-SrCOs/Re: (1) washed with redistilled water five times, (2) dried for 1 h at 110 "C, (3) mixed with binder (nitrocellulose 2.7 wt %, ethanol 1.4 wt (17) Dushman, S.Rev. Mod. Phys. 1930,2,381.
Analytical Chemistry, Vol. 66, No. 24, December 75, 1994
4433
%, amyl acetate 52.4%,and carbonate with the composition of
SrC03and BaC03 at 42.6%and 57.4%),(4) coated on the 0.025 x 0.75 x 14 mm Re ribbon, (5) removal of nitrocellulose binder under low vacuum (around 1 Torr) and at 500 "C, (6) decomposition of carbonate to oxide for a few hours under high vacuum (less than lo-* Torr) and at 850 "C, (7) partial reduction of oxide into free barium under high vacuum (less than Torr) and at 1200 "C, at least for several minutes. (b) For CaC03-SrC03CaC03/Re: preparation procedure is the same as BaC03-SrC03/ Re with the exception of the composition (CaC03,9.5%,SrCO3, 42.5%,and BaC03, 48 wt %). (c) For LaBs/Re: coated 80/100 mesh, 99.99%pure LaB6 on the Re ribbon heated to lo00 "C for a few minutes under high vacuum (about Torr). The methods reported by Favreau et al. were used for the preparation of the ionizer.1s Standard Iodine Gas. Standard iodine gases were prepared by using the diffusion cell methodlgdeveloped previously? Briefly, the gas inlet system consisted of a diluent air gas cylinder, flowmeter, and stainless steel diffusion cell with a sample tube including iodine. The prepared gas was admitted to the source housing through a needle valve 0. Procedure. Iodine vapor can be delivered to the surface ionizer, where iodine evaporates as ions that are extracted by a potential of 4 kV between the ionizer and a mass spectrometer. The electrons emitted by the ionizer can be separated from the ions by a transverse magnetic field. The sensitivity optimization was performed by varying the surface material and the operating conditions.
I
I
7.0
7.5
-12
I 8.0
I
8.5 ( 1 1 ~ x) 104
I 9.0
I
-12
9.5
Figure 2. Richardson plot for the CaCOa-SCO3-CaCOdRe ionizer. Table I. Characteristics Summary of All Tested Ionizers
ionizer Ba-Sr-Cab Ba-Sf MSd
2.00 2.15 2.67
sensitivitp (A)
optimum ionizer temp 0
3 x 10-10 9 x 10-11 8 x 10-12
1320 1320 1400
Sensitivity after background substruction under the conditions that sample gas pressure is 3 x 10-5Torr and [I21 = 0.1 ppm. If sensitivity is expressed in units of amperes per Torr, the listed ionizers have sensitivities of 100, 30,and 7.7A/Torr, respectively. BaC03:SrC03: CaCO3/Re. BaCO3:SrCOs/Re. k&/Re.
RESULTS AND DISCUSSION
Ideally, a NSI surface with a sufficiently low work function should be chemically inert, able to withstand repeated exposure to sample gases and handling, exhibit low material evaporation (long life), and produce stable, high ionic current. Optimization of Analysis. 1. Work Function Determination. The k s t experiments undertaken were measurements of the work function. A low-y, surface material must be found. This requirement translates to maximizing electron emission. Thermionic work function measurements were made using ribbon filaments of a known emitting area and collecting thermionically emitted electron current on a ring collector plate in a simple diode arrangement.20 The current-voltage characteristics of the cathodes were obtained as a function of temperature by placing the cathode approximately 10 mm away from the anode. The saturation electron current was derived for thermionic work function. The standard method of determining the thermionic work function of a specimen is to apply the Richardson equation
1,= A p exp(-cp/kT)
(2)
Here Jo is the saturation electron emission current density (A/ cm9. The electron current densityJ, collected on the anode, is plotted for different values of the anode voltage (VJ, For each temperature, a plot of In J vs JV, is made and the straight-line (Schottky) portion is extrapolated to zero field; yielding the zerofield current densityld'l. Then a plot in InV,/m vs l/Tgives (18) Favreau, L. J.; Koenig, D.F. Rev. Sci. Instrum. 1967,38,841. (19)Miguel, A H.; Natusch, D.F. S. And. Chem. 1975,47,1705. (20) Eckstein, B. H.; Forman, R J. Appl. Phys. 1962,33, 82.
4434 Analytical Chemistry, Vol. 66,No. 24, December 15, 1994
a slope (-rp/k) and intercepts A from which the so-called Richardson work function, rp, and preexponential factor, A, may be determined. Figure 2 shows the Richardson plot for the CaC03-SrC03CaCOdRe surface. Table 1 summarizes the pl values at the limited temperature range for the three tested surfaces. CaC03SrCO3-CaCO3/Re shows a clear work function superiority among the three. The effect of stoichiometry on CaCO3-SrCO3-CaCO3/ Re work functions has not yet been investigated systematically, so the minimum attainable values of rp may be lower than those shown in Table 1. 2. Decrease in ~p with Adsorption. Well-known experimental resultsz1 have indicated that the adsorption of oxygen atoms on a metal surface produces a change in the work function of the surface. Since the surface coverage changes with the partial pressure (PJ of supplied air samples, especially at low surface temperatures, we have treated the diode data somewhat differently to calculate the work function increase, A q . Instead we have measured it at each temperature according to
where J and JO are the electron emissions at P, = 0 and e Torr, respectively. This working definition allows us to find the variation of Ay, with P, of the sample gas. (21) Zandberg, E. A; Rasulev, U. Kh.; Khalikov, Sh. M. Sou. Phys. Tech. Phys.
1976,21, 483.
,
+
'
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I
'
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'
T(K) 1500 1400 1300
surface temperature -C+-
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1100
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5.0 10 20 50 100 200 INTRODUCED AIR PRESSURE (p torr)
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Figure 3. Increase of work function of CaC03-SrC03-CaC03/Re with introduced air at three different temperatures.
t
10.~5-
0 W
-nn
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0
A graph of Ag, vs P, is shown in Figure 3 at three different surface temperatures for the CaC03-SrC03-CaC03/Re surface. This figure reveals that the introduction of an air sample leads to a surface coverage, presumably due to oxygen atoms. This would produce an increase in the work function and an exaggerated decrease in the ionization efficiency. Furthermore, the change in surface coverage as a function of temperature is evident, possibly causing a decrease in ionization efficiency, especially at lower temperatures. The stability of the surface to iodine exposure was tested. The change in work function was also observed after exposure of the air sample including the 100 ppm iodine in air. The variation of work function behaves almost the same as in Figure 3, suggesting that the small amount of iodine in air has little effect on 9. 3. Sample Gas Pressure Dependence of Iodine Ion Current. The ion current is proportional to the flux of neutral particles that strike the ionizer. So, the principal requirement for surface ionization sources producing the highest ion fluxes is to keep the gas sample pressure as high as possible. However, one of the problems encountered with an increase in sample gas pressure is that the electric discharge takes place in the ion source. Furthermore, the previous coverage studies showed that the surface work function is a sensitive function of the surface coverage. Seemingly minor changes in surface coverage give rise to changes in the ionization efficiency, which results in corresponding changes in sensitivity. Sample introduction that provides a condition with. P, = 3 x Torr as sample pressure was used for all the following studies. 4. Surface Temperature. I- emission current as a function of ionizer temperature is shown in Figure 4 for the 12 concentration of 5 ppm (at a diffusion cell temperature of about 300 K). As has been shown for increasing temperatures, ion current characteristics vs reciprocal temperatures present an exponential emission increase followed by an abrupt decrease without being stabilized at the maximum. Two regions appear on the curve. An exponential increase is mainly due to the increase in the thermal decomposition of iodine with increasing temperature. The d e crease region can be explained partly by the following descrip tions: (1) increasing emission of electrons from the ionization filament with higher temperatures forms a negative electron cloud around the filament and prevents the iodine ion emission; (2) the adsorption time of neutral iodine atoms at the metal surface decreases with increasing temperature. This decrease leads to a lower ionization probability.22 The optimum temperature (To) (22) Heumann, K G.; Schindlmeier,W. 2. Anal. Chem. 1981,306,245.
W
5-
a
2-
L+
5-
10."
21LuddAL 7,0
8.0
9,o
10.0
11,o
(1IT) x 104 Figure 4. Negative ion current as a function of ionizer temperature under the conditions that [I21 = 5 ppm (v/v) and f s= 3 x 10-5 Torr.
giving a maximum ion current was found around 1370 K; from Figure 4. For increasing fluxes, To shifts a little to higher temperatures. Response Characteristics. 1. Sample Pressure vs Negative Ion Signal (Calibration Curve). The major ions observed on sample introduction were atomic iodine ion (I-). The 12- ion was not observed, indicating that the dissociation of I2 molecules occurs fully under the present conditions. Figure 5 shows the iodine concentration (ppm) vs I- signal under the conditions of T = 1370 K and P, = 3 x Torr. The straight calibration curves were obtained for all the surfaces by subtracting the background ion current (see next section). As can be seen from Figure 5, the relative sensitivities of iodine from the various ionizers were CaC03-SrC03-CaC03/Re > BaCOs-SrCOs/Re > LaBG/Re. As expected, the sensitivity increased with decreasing work function. The sensitivity was measured for all the ionizers. These data are also summarized in Table 1 along with the optimum ionizer temperature and the work function. The CaC03-SrC03-CaC03/Re ionizer was chosen for more detailed sensitivity study based on high ionization efficiency. 2. Sensitivity Consideration. Figure 6 shows single-ion monitoring of I- (m/z 127) with the CaC03-SrC03-CaC03/Re ionizer at T = 1320 K. The x-axis is time (min). Other experimental conditions are an air sample gas pressure of 3 x 10+ Torr and an iodine concentration of 0.01 ppm. This figure leads to a minimum detection concentration limit of 3 ppb (concentration of iodine producing a peak 2 times higher than noise level). This performance level is to be considered only as an indication of the potential performance of this source which, in addition, must be tested with a real off-gas sample. Analytical Chemistry, Vol. 66,No. 24, December 15, 1994
4435
5
10-I 2
5
loo 2
5 10’
2
~
current 10 4IA
io-’
5 2 Backgroundcurrent
5
2 10.~ 5 2 lo+
5L’ 2
tlme(min.) Figure 6. Single-ion monitoring of iodine atom ions (mlz 127) with the tricarbonatelRe ionizer at T = 1320 K.
10-l0
/
1-21 ( ppm ( V W ) Figure 5. Calibration plot of iodine for three surfaces: (A) CaCO3SrCO3-CaCO$Re (T=1320 K); (0)BaCO3-SrCOdRe (T=1320 K); (0)LaBs ( T = 1400 K).
The limitation on measurement of 1-127found in this work was a spectral background at mass 127 arising from sample impurities. Since a similar level of background was observed at masses 125, 126,128, and 130, the background is attributed to hydrocarbons. With no sample in the compresser’s air gas, a spectral background ion current on the order of 10-10 A was typically observed. Chloride was always present in the full-scan mass spectrum as a major, but noninterfering ion. 3. Comparison. The data of Table 1 suggest that both CaC03-SrC03-CaC03/Re and BaCOs-SrCOs/Re may be viable low work function materials, as well as LaBs, which already has substantial practical application. These carbonate materials provide at least 1 order of magnitude greater sensitivity for measuring of 1-127 than does the LaBs material. Unfortunately, neither of these materials has yet been studied as much as LaBs, to show, in particular, whether the surface is thermally stable or withstands air exposure for an extended period. Concluding Remarks. The present study demonstrates the feasibility of measuring the iodine in air by generating ions from NSI in a mass spectrometer. Instrumental sensitivity was such
4436 Analytical Chemistry, Vol. 66, No. 24, December 15, 1994
that 3 ppb iodine atoms in air introduced into the instrument could be detected. Sensitivity in this range warrants real-time monitoring in the process off-gases. Moreover, there are low-background, very selective ionhtion and relatively high ion beams using NSI technique. The CaC03-SrC03-CaC03/Re ionizer with a work function of 2.0 eV, which can be conveniently formed from the complex of these materials, was considered to be most appropriate for environmental applications due to its superior ionization efficiency. All iodine analyses reported here were made on pure materials. The situation with respect to environmental 1-129 remains to be determined. For pure materials, it appears that the CaC03SrCO3-CaCO3/Re surface provides at least 1order of magnitude greater sensitivity for measurement of 1-129 than does the LaBs surface. ACKNOWLEDGMENT
This work was supported in part by a Grant-in-Aidfor Scient& Research (60880026) from the Education Ministry in Japan. We are grateful to Tom McMahon at Alabama Language Academy for manuscript preparation. Received for review May 9, 1994. Accepted September 13, 1994.@ @
Abstract published in Advance ACS Abstructs, October 15, 1994.
