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Negative surface ionization mass spectrometry of atmospheric iodine
International Journal of Mass Spectrometty and Zon Processes, 85 (1988) 301-318 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
301
NEGATIVE SURFACE IONIZATION MASS SPECI’ROMETRY OF ATMOSPHERIC IODINE *
HIROSHI KISHI Department of Industrial Chemistry, Oyama Technical College, 771 oaza Nakakuki, @ama 323 (Japan) HIROYUKI Department Matsuyama
KAWANO
**
of Chemistry, Faculty of Science, Ehime University, 2-5 BunkyLGchij, 790 (Japan)
(First received 12 January 1988; in final form 12 April 1988)
ABSTRACT In order to detect the trace amount of iodine molecules included in air, a negative surface ionization-type ion source employing La& as an ionizing surface material was connected to a commercial mass spectrometer with an electron multiplier. Under the experimental conditions that the surface temperature and the sample gas pressure around the ion source were about 1400 K and 30 ptorr, respectively, the work function of the ionizing surface was about 2.7 eV and the ionization efficiency of the iodine molecule was governed practically by its sticking probability to the surface. A calibration plot of the iodine concentration in air vs. the negative iodide ion current measured by the mass spectrometer was nearly straight in the range between 50 and 0.1 p.p.m., thereby suggesting that the present method may be employed to monitor radioactive iodine molecules included in the off-gases of nuclear fuel reprocessing facilities.
INTRODUCTION
When a sample gas of halogen or halide, for example, is directed onto a negatively biased hot solid surface with a low work function, negative halide ions are usually emitted together with thermal electrons. This phenomenon is generally called negative surface ionization (NSI) or thermal negative ionization and has long been utilized as a simple and convenient method for
* This work was supported in part by a Grant-in-Aid for Scientific Research No. 60880026 from the Ministry of Education, Science and Culture of Japan. * * To whom correspondence should be addressed. 0168-1176/88/$03.50
0 1988 Elsevier Science Publishers B.V.
302
various purposes such as isotope ratio determination, neutral beam detection, negative ion beam generation, and nuclear fission product.’ analysis [l-5]. Recently, NSI mass spectrometry has been proposed as a useful method for the very sensitive detection of the trace amount of iodine molecules included in off-gases of a nuclear fuel reprocessing plant [6]. In order to develop the method for practical use, however, many fundamental studies must be made on the selection of experimental conditions (ionizing surface material, surface temperature, and sample gas pressure), on the stability of work function (+) of the hot surface, and on the durability of the surface material .employed. It should be noted that #I is usually anticipated to be greatly increased by adsorption of oxygen because the sample gas introduced into the ion source region includes about 20% of oxygen, a concentration which is more than lo5 times as large as that (usually less than 1 p.p.m.) of iodine molecules therein. From the viewpoint mentioned above, we have manufactured a small negative ion source using La$ as an ionizing surface material and also measured the negative iodide ion current (i-) as a function of the concentration (C) of iodine included in a sample gas. This paper summarizes the experimental data on (a) + in a high vacuum, (b) its change due to adsorption of sample gas molecules, (c) the dependence of i- upon T or the sample gas pressure ( p,) in the ion source, and (d) the calibration plot of ivs. c. THEORETICAL
When negative iodide ions are produced from iodine molecules impinging upon an ionizing surface after attaining equilibria (1) and (2), Eqs. (3)-(8), which are readily derived from our theory on NSI [3,7], hold in general. I ,+21
(1)
I+e-*I-
(2)
1‘-
= 2gqeSaNc-y
(3) (4
-
(Y-sn= no
1 l-r-4 -p
A-+ exp [
y2=
I.01 X 106NA(1 + (Y-)
l--Y
UN [~~/ART]“* =3.35X
1024~~~J)
(5)
,?&‘- I
exp[y]
exp[$]
exp[-$1
exp[-g]
(6)
303
N=
=
1.33 X 103N,pi [ 217MRT,] 1’2
2.20 x lOZip, T1/2 s
/3- = i-/2gqeSN
= m-y
(7) (8)
Here, i- is the multiplied current of I- incident upon the first dynode of an electron multiplier, g is the gain of the multiplier, n is the collection efficiency of I- by the dynode, e is the elementary electric charge, S is the ionizing surface area, (I is the sticking probability of I, to the surface and hence the remainder, (1 - a), represents the fraction of the I, molecules externally reflected from the surface without attaining equilibria (1) and (2) on the surface, N is the flux of I, impinging on unit surface area per unit time, y is the degree of dissociation of those I, molecules sticking on the surface, E- is the degree of ionization of I, n- and no are the fluxes of Iand I emitted from unit surface area per unit time, respectively, and hence the sum of the two is equal to ayN when the internal reflection coefficients (r- and r”) of I- and I are zero, a- is the ionization coefficient of I, A is the electron affinity of I, T is the surface temperature, $ is the work function of the surface, NA is Avogadro’s number, p is the reduced weight of I, per mole, AS is the dissociation entropy of I,, D is the dissociation energy of I,, pi is the partial pressure (in torr) of I, in the ion source region, M is the molecular weight of I,, T, is the temperature of the sample gas, usually equal to room temperature, and p- is the ionization efficiency of the iodine molecules impinging upon the surface. The above theory includes the following implicit postulations. (a) I; ions are not produced by any means such as NSI or gas-phase electron capture (GPEC) under the present experimental conditions. (b) I- ions are not formed by GPEC. On the ionizing surface alone, they can be produced from those iodine molecules (UN) sticking to the surface. (c) The internal reflection coefficient ( rm) of undissociated iodine molecules [ uN( 1 - y)] on the surface is virtually zero. (d) Neither on the surface nor in the gas phase, do those molecules [N(l - a)] externally reflected from the surface attain equilibria (1) and (2). Postulations (a) and (b) are verified by the present experiment, as will be described below. Postulation (c) may be valid at high temperatures such as T z 1000 K. Even if rrn B 0, it has practically no effect upon our theoretical conclusions because uN(1 - y) is essentially equal to zero since y = 1 under the conditions in this work. Postulation (d) requires the correction for the difference in total flux between the equilibria on the surface and those in the gas phase in the vessel illustrated in Fig. 1 of ref. 7. It is u that corrects this
304
difference. This is because the overall degree of dissociation of the total iodine molecules (N) including both the sticking (alv) and the reflected molecules [N(l - o)] is not equal to y but to ay. This is the reason why Eq. (3) includes u. These considerations lead to the conclusions that postulations (a)-(d) are reasonable and hence that the above theory is valid for analyzing the experimental data obtained in this work. In order to detect I, with a high efficiency, it is desirable to adopt the condition that both y and c- are essentially unity. With respect to I, at T = 400-1700 K, the following values are available from a thermochemical table [8]; AS = 24.4-25.7 cal mol-’ K-‘, D = 1.57-1.62 eV, and A = 3.13-3.35 eV. In the high temperature range above - 1000 K usually employed in NSI, both r- and r” are considered to be zero in general [3]. Judging from the experimental result that u of Cl, to Hf or W at 1700-2200 K is less than lob2 [9], u of halogen molecules seems to be much smaller than unity. However, it is very difficult, in general, to evaluate u for a given gas-solid system under a specified experimental condition [3]. Initially, therefore, in this section the effective flux (UN) will be tentatively employed instead of the actual flux (N). Then, it is very easy to find theoretically the experimental condition (+ and T) under which the desirable condition (y 1: 1 and E- = 1) is fulfilled for a given value of UN. Lines (l)-(4) in Fig. 1 correspond to E- = 0.99, 0.90, 0.50, and 0.1, respectively, which are independent of UN. Curves (5)-(7) correspond to y = 0.99 for UN = lOlo, 1012, and 1014 molecules cme2 s-l , respectively. If u = 1, then, UN = 1014 molecules cmv2 s-l corresponds to pi 2: 1 ptorr, which corresponds to such an extremely high concentration (C) of lo5 p.p.m. when the total pressure ( p, = pi/C) of a sample gas introduced into the ion source region is - 10 ptorr, a value often adopted in our experiments. Accordingly, selection of + and T in the area belonging to the right-hand side of curve (7) is sufficient to satisfy the condition of y > 0.99 because C is generally much smaller than lo5 p.p.m. In other words, y = 1 at N s 1014 molecules cmm2 s-l is attained for any value of $ so long as T > 650 K. This is mainly because the value of D for I, is as small as 1.6 eV. In this way, the lower limit of T satisfying y = 1 is successfully evaluated without knowing the accurate values of u for our I, gas-La$ surface system to be operated under various experimental conditions. Line (8) shows the boundary temperature above which both r” and r- are considered to be essentially zero. When a set of T and $I included in the dotted area in Fig. 1 is selected, therefore, the condition of y = 1 and c- > 0.9 is satisfied for any value of u, even when N is as large as 1014 molecules cmT2 s-l. Consequently, such a surface of + 5 2.8 eV should be adopted at T > 1000 K in order that both y and E- may become practically unity. When the sample gas of p, = 30 ptorr including less than 200 p.p.m. of iodine is directed onto an La$ surface at
314
and (2), respectively, in Fig. 8. It should be noted that i-(1.08)/i-(27.2) is not equal to the concentration ratio (1.08/27.2 = 0.040) but varies from 0.079 to 0.028 according as the sample gas pressure increases from 3.0 to 30 ptorr [see curve (3) in Fig. 81. This result indicates that the percentage of ioriginating from background iodine becomes larger with a decrease in ps since u is regarded as virtually i ependent of C although u strongly depends upon both T and ps. e?onsider that p- given by Eq. (8) at T = const. is decreased to l/X by the increase of the sample gas pressure from 3.0 to 30 ptorr, that the incident fluxes of iodine molecules from the supplied sample gas of C = 1 p.p.m. at 3.0 ptorr and from iodine contaminants on the sample inlet system are Y and n, respectively, and also that n is independent of C and p,. Then, Eqs. (13) and (14) are derived with curves (1) and (2) in Fig. 8, respectively, by a similar method used for Eqs. (11) and (12). i-(30 ptorr, 1.08 p.p.m) i-(3.0 ptorr, 1.08 p.p.m.)
~ 3.6 = (n + 10.8V)/X 49 pA n+l.OSv
03)
i-(30 ptorr, 27.2 p.p.m.) i-(3.0 ptorr, 27.2 p.p.m.)
0.13 nA = (n + 272v)/X n + 27.2~ E 0.62nA
(14
Here, i-(30 ptorr, 1.08 p.p.m.), for example, is the ion current at ps = 30 ptorr and C = 1.08 p.p.m. In this way, n is determined to be 3.4~ (and hence X = 43) because n = 85v (and hence X = 15) is unlikely in this work. This result means that the incident flux (n) due to background iodine at ps = 3.0 ptorr is about 3 times as large as that (v) due to supplied sample gas and hence that n at 30 ptorr becomes - 0.3~. Accordingly, even when both of the values of C calculated from Eq. (9) are 1 p.p.m. at 30 and 3.0 ytorr, the actual values of C are concluded to be - 1.3 and 4.4 p.p.m., respectively. This is the main reason why the ratio i-(1.08)/i-(27.2) increases remarkably with a decrease in ps, as shown by curve (3) in Fig. 8. At p, > 15 ptorr, however, the ion current ratio remains constant at - 0.03, nearly equal to C(1.08)/C(27.2) = 0.04. In the pressure range, therefore, the value of C calculated from Eq. (9) may be accurate with about 25% error, which is mainly due to the iodine contaminant inside the sample inlet system so long as C 2 1 p.p.m. The result of X= 43 means that /3- is decreased to l/43 by a pressure increase from 3.0 to 30 ptorr and hence that /3-(30)/p-(3.0) = a(30)/a(3.0) decreases to 0.023 since both y and e- remain essentially unity. This figure is nearly equal to the value of 0.021 evaluated from Eq. (12), thereby indicating that it is reasonable to adopt n = 3.4~ instead of n = 85v as a unique solution for Eqs. (13) and (14). In conclusion, a larger value of p, is favorable for decreasing the error in the determination of C.
315
Calibration curve
On the basis of the above results and discussion, T and p, were selected to be 1400 K and 30 (or tentatively 3.0) ptorr, respectively, and i- was measured as a function of C, according to Eq. (9). The calibration curves thus obtained are shown in Fig. 9, the abscissa at the top of which indicates rf corresponding to C. Employment of a needle valve made of stainless steel at p, = 30 ptorr yields curve (l), where i- does not exhibit a sharp dependence on r,. This result suggests that the inside surface of the valve is readily contaminated with iodine. This valve was therefore replaced with a glass needle valve with which curves (2)-(4) were obtained. Change of rf at p, = 3.0 ptorr yields curve (2), which is far from the result to be expected from Eq. (9), especially in the range below - 1 p.p.m. This is probably because the relative value of n to v becomes larger with an increase in r,, as already described with respect to the data in Fig. 8. Such a tendency is also observed at a much higher pressure (p, = 30 ptorr), as shown by curve (3) although it is much better than curves (1) and (2) in Fig. 9. Air flow rate, rf (m3 s-l) 100
50
I
’
I 0.1
10
5
1
I
0.5
P
I
0.5
1
5
I
I
10
50
Iodineconcentration, C (PMI)
Fig. 9. Calibration curves achieved at T = 1400 K. Curve (1) is obtained with a steel needle valve at pS= 30 ptorr. The use of a glass needle valve yields curves (2) and (3) at ps = 3.0 and 30 ptorr, respectively. Curve (4) is obtained at ps = 30 ptorr after passing air alone through the sample inlet system and the ion source heated to - 200 o C for two days in order to remove the iodine contaminants due to many long runs including the data for curves (2) and (3).
316
In order to remove the iodine contaminant inside the system, air alone was passed through the system and the ion source heated to - 200 OC for two days at p, = 30 ptorr. By this treatment, the background ion current (ii) of I- at C = 0 was sucessfully reduced from 11 to 1.5 pA, which is much smaller than that (8 PA) usually observed in previous work (see Fig. 7 in ref. ll), thereby suggesting that a glass needle valve is more advantageous than a metal aperture plate such as employed previously in order to decrease ii as easily as possible. After the above treatment, curve (4) in Fig. 9 is obtained at p, = 30 ptorr. Although the detection sensitivity of iodine decreased considerably compared with that in the case of curve (3), curve (4) shows more clearly the dependence of i- upon C in the range below 1 p.p.m. Deduction of i; from each data point on curve (3) or (4) in Fig. 9 yields line (3’) or (4’) in Fig. 10, respectively, which shows that the net current Air flow rote, rf km3 s-l)
/-
I d
Avo*05 0.1
I
I
I 0.5
1
5
10
50
Iodineconcentration. C (PIMI)
Fig. 10. Relation between the iodine concentration in air and the corrected ion current of iodine. Curves (3) and (4) are cited from Fig. 9. Lines (3’) and (4’) are obtained by deduction of the background ion current at C = 0 from each data point on curves (3) and (4), respectively.
317
(i- - i;) is proportional to C in the range down to 0.1 p.p.m. and also that the slopes of both lines (3’) and (4’) are slightly greater than unity. The latter is probably because iodine contamination becomes more serious as rf decreases. In consequence, our experimental apparatus and method should be improved to prevent the contamination more effectively. CONLUSIONS
The results and discussion described above lead to the following conclusions. (1) The negative ion source of the La$-Re type developed in this work has work function as low as about 2.7 eV in a high vacuum. (2) Even when air without or with iodine molecules below - 200 p.p.m. is introduced into the ion source up to - 30 ptorr, + remains below about 2.8 eVsolongasT213OOK. (3) Under the conditions described in (2), both y and E- are essentially unity and hence /3- is, in practice, governed only by (I since both r o and rat T B 1000 K are considered to be negligibly small compared with unity. (4) The value of u is much smaher than unity and decreases sharply with decrease in T and/or an increase in p, in the respective ranges ( - 1150-1600 Kand - 2-30 ptorr) covered in this work. (5) At V, = 100 V, the most favorable temperature is about 1400 K, above which i- is decreased by the space-charge effect due to a much stronger electron emission current and above which the lifetime of the La$ layer may be shortened more rapidly by a reaction between La$ and 0, forming BO- and BO;. (6) The concentration of background iodine is estimated to be usually about 0.3 p.p.m. when the value of C calculated from Eq. (9) is 1 p.p.m. and when ps is 30 ptorr. In order to minimize the relative concentration (n/y) of background iodine, p, should be kept as large as possible. (7) In order to keep u as large as possible and also to minimize the oxidation reaction mentioned in (5), on the other hand, a smaller value of p, is favorable for detecting atmospheric iodine by the present method. (8) Consideration of (2)-(7) described above indicates that T and p, should be selected to be about 1400 K and 30 ptorr, respectively. (9) Under the experimental condition selected according to (8), a calibration plot of i- - ia vs. C yields a straight line throughout the range of 50-0.1 p.p.m. The slope of the calibration line, however, is greater than unity. This is mainly because n/v becomes larger as r, decreases. (10) Similarly to the previous work, the most serious problem encountered in this work is how to minimize the iodine contamination inside the apparatus during operation, especially at r, < 10 cm3 s-l.
318
To the best of our knowledge, this is the first study to achieve such a successful result, i.e. that the detection limit of atmospheric iodine by negative ion mass spectrometry is as low as - 0.1 p.p.m. Further work, however, is now in progress in order to decrease ii and also to find better surface materials. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
H. Kawano, J. Vat. Sot. Jpn., 23 (1980) 1. H. Kawano, J. Vat. Sot. Jpn., 23 (1980) 229. H. Kawano and F.M. Page, Int. J. Mass Spectrom. Ion Phys., 50 (1983) 1. H. Kawano, Y. Hidaka and F.M. Page, Int. J. Mass Spectrom. Ion Phys., 50 (1983) 35. H. Kawano, Y. Hidaka, M. Suga and F.M. Page, Int. J. Mass Spectrom. Ion Phys., 50 (1983) 77. H. Kawano, S. Matsuoka, T. Fujii, M. Tsuchiya, H. Fumoto and H. Kishi, Mass Spectrosc., 34 (1986) 249. H. Kawano, Int. J. Mass Spectrom. Ion Processes, 69 (1986) 97. D.R. Stull and H. Prophet, JANAF Thermochemical Tables, U.S. National Bureau of Standards, Washington, DC, 2nd edn., 1971. M.L. Shaw and N.P. Carleton, J. Chem. Phys., 44 (1966) 3387. L.J. Favreau and D.F. Koenig, Rev. Sci. Instrum., 38 (1967) 841. H. Kishi and H. Kawano, Int. J. Mass Spectrom. Ion Processes, submitted for publication. H. Kawano, T. G&la, H. Takeichi, T. Kenpa and Y. Hidaka, Int. J. Mass Spectrom. Ion Phys., 47 (1983) 261. A.E. Dabiri, Surf. Sci., 49 (1975) 77. H. Kawano, S. Itasaka, S. Ohnishi and Y. Hidaka, Int. J. Mass Spectrom. Ion Processes, 70 (1986) 195. H. Kawano, S. Itasaka and S. Ohnishi, Int. J. Mass Spectrom. Ion Processes, 73 (1986) 145.
301
NEGATIVE SURFACE IONIZATION MASS SPECI’ROMETRY OF ATMOSPHERIC IODINE *
HIROSHI KISHI Department of Industrial Chemistry, Oyama Technical College, 771 oaza Nakakuki, @ama 323 (Japan) HIROYUKI Department Matsuyama
KAWANO
**
of Chemistry, Faculty of Science, Ehime University, 2-5 BunkyLGchij, 790 (Japan)
(First received 12 January 1988; in final form 12 April 1988)
ABSTRACT In order to detect the trace amount of iodine molecules included in air, a negative surface ionization-type ion source employing La& as an ionizing surface material was connected to a commercial mass spectrometer with an electron multiplier. Under the experimental conditions that the surface temperature and the sample gas pressure around the ion source were about 1400 K and 30 ptorr, respectively, the work function of the ionizing surface was about 2.7 eV and the ionization efficiency of the iodine molecule was governed practically by its sticking probability to the surface. A calibration plot of the iodine concentration in air vs. the negative iodide ion current measured by the mass spectrometer was nearly straight in the range between 50 and 0.1 p.p.m., thereby suggesting that the present method may be employed to monitor radioactive iodine molecules included in the off-gases of nuclear fuel reprocessing facilities.
INTRODUCTION
When a sample gas of halogen or halide, for example, is directed onto a negatively biased hot solid surface with a low work function, negative halide ions are usually emitted together with thermal electrons. This phenomenon is generally called negative surface ionization (NSI) or thermal negative ionization and has long been utilized as a simple and convenient method for
* This work was supported in part by a Grant-in-Aid for Scientific Research No. 60880026 from the Ministry of Education, Science and Culture of Japan. * * To whom correspondence should be addressed. 0168-1176/88/$03.50
0 1988 Elsevier Science Publishers B.V.
302
various purposes such as isotope ratio determination, neutral beam detection, negative ion beam generation, and nuclear fission product.’ analysis [l-5]. Recently, NSI mass spectrometry has been proposed as a useful method for the very sensitive detection of the trace amount of iodine molecules included in off-gases of a nuclear fuel reprocessing plant [6]. In order to develop the method for practical use, however, many fundamental studies must be made on the selection of experimental conditions (ionizing surface material, surface temperature, and sample gas pressure), on the stability of work function (+) of the hot surface, and on the durability of the surface material .employed. It should be noted that #I is usually anticipated to be greatly increased by adsorption of oxygen because the sample gas introduced into the ion source region includes about 20% of oxygen, a concentration which is more than lo5 times as large as that (usually less than 1 p.p.m.) of iodine molecules therein. From the viewpoint mentioned above, we have manufactured a small negative ion source using La$ as an ionizing surface material and also measured the negative iodide ion current (i-) as a function of the concentration (C) of iodine included in a sample gas. This paper summarizes the experimental data on (a) + in a high vacuum, (b) its change due to adsorption of sample gas molecules, (c) the dependence of i- upon T or the sample gas pressure ( p,) in the ion source, and (d) the calibration plot of ivs. c. THEORETICAL
When negative iodide ions are produced from iodine molecules impinging upon an ionizing surface after attaining equilibria (1) and (2), Eqs. (3)-(8), which are readily derived from our theory on NSI [3,7], hold in general. I ,+21
(1)
I+e-*I-
(2)
1‘-
= 2gqeSaNc-y
(3) (4
-
(Y-sn= no
1 l-r-4 -p
A-+ exp [
y2=
I.01 X 106NA(1 + (Y-)
l--Y
UN [~~/ART]“* =3.35X
1024~~~J)
(5)
,?&‘- I
exp[y]
exp[$]
exp[-$1
exp[-g]
(6)
303
N=
=
1.33 X 103N,pi [ 217MRT,] 1’2
2.20 x lOZip, T1/2 s
/3- = i-/2gqeSN
= m-y
(7) (8)
Here, i- is the multiplied current of I- incident upon the first dynode of an electron multiplier, g is the gain of the multiplier, n is the collection efficiency of I- by the dynode, e is the elementary electric charge, S is the ionizing surface area, (I is the sticking probability of I, to the surface and hence the remainder, (1 - a), represents the fraction of the I, molecules externally reflected from the surface without attaining equilibria (1) and (2) on the surface, N is the flux of I, impinging on unit surface area per unit time, y is the degree of dissociation of those I, molecules sticking on the surface, E- is the degree of ionization of I, n- and no are the fluxes of Iand I emitted from unit surface area per unit time, respectively, and hence the sum of the two is equal to ayN when the internal reflection coefficients (r- and r”) of I- and I are zero, a- is the ionization coefficient of I, A is the electron affinity of I, T is the surface temperature, $ is the work function of the surface, NA is Avogadro’s number, p is the reduced weight of I, per mole, AS is the dissociation entropy of I,, D is the dissociation energy of I,, pi is the partial pressure (in torr) of I, in the ion source region, M is the molecular weight of I,, T, is the temperature of the sample gas, usually equal to room temperature, and p- is the ionization efficiency of the iodine molecules impinging upon the surface. The above theory includes the following implicit postulations. (a) I; ions are not produced by any means such as NSI or gas-phase electron capture (GPEC) under the present experimental conditions. (b) I- ions are not formed by GPEC. On the ionizing surface alone, they can be produced from those iodine molecules (UN) sticking to the surface. (c) The internal reflection coefficient ( rm) of undissociated iodine molecules [ uN( 1 - y)] on the surface is virtually zero. (d) Neither on the surface nor in the gas phase, do those molecules [N(l - a)] externally reflected from the surface attain equilibria (1) and (2). Postulations (a) and (b) are verified by the present experiment, as will be described below. Postulation (c) may be valid at high temperatures such as T z 1000 K. Even if rrn B 0, it has practically no effect upon our theoretical conclusions because uN(1 - y) is essentially equal to zero since y = 1 under the conditions in this work. Postulation (d) requires the correction for the difference in total flux between the equilibria on the surface and those in the gas phase in the vessel illustrated in Fig. 1 of ref. 7. It is u that corrects this
304
difference. This is because the overall degree of dissociation of the total iodine molecules (N) including both the sticking (alv) and the reflected molecules [N(l - o)] is not equal to y but to ay. This is the reason why Eq. (3) includes u. These considerations lead to the conclusions that postulations (a)-(d) are reasonable and hence that the above theory is valid for analyzing the experimental data obtained in this work. In order to detect I, with a high efficiency, it is desirable to adopt the condition that both y and c- are essentially unity. With respect to I, at T = 400-1700 K, the following values are available from a thermochemical table [8]; AS = 24.4-25.7 cal mol-’ K-‘, D = 1.57-1.62 eV, and A = 3.13-3.35 eV. In the high temperature range above - 1000 K usually employed in NSI, both r- and r” are considered to be zero in general [3]. Judging from the experimental result that u of Cl, to Hf or W at 1700-2200 K is less than lob2 [9], u of halogen molecules seems to be much smaller than unity. However, it is very difficult, in general, to evaluate u for a given gas-solid system under a specified experimental condition [3]. Initially, therefore, in this section the effective flux (UN) will be tentatively employed instead of the actual flux (N). Then, it is very easy to find theoretically the experimental condition (+ and T) under which the desirable condition (y 1: 1 and E- = 1) is fulfilled for a given value of UN. Lines (l)-(4) in Fig. 1 correspond to E- = 0.99, 0.90, 0.50, and 0.1, respectively, which are independent of UN. Curves (5)-(7) correspond to y = 0.99 for UN = lOlo, 1012, and 1014 molecules cme2 s-l , respectively. If u = 1, then, UN = 1014 molecules cmv2 s-l corresponds to pi 2: 1 ptorr, which corresponds to such an extremely high concentration (C) of lo5 p.p.m. when the total pressure ( p, = pi/C) of a sample gas introduced into the ion source region is - 10 ptorr, a value often adopted in our experiments. Accordingly, selection of + and T in the area belonging to the right-hand side of curve (7) is sufficient to satisfy the condition of y > 0.99 because C is generally much smaller than lo5 p.p.m. In other words, y = 1 at N s 1014 molecules cmm2 s-l is attained for any value of $ so long as T > 650 K. This is mainly because the value of D for I, is as small as 1.6 eV. In this way, the lower limit of T satisfying y = 1 is successfully evaluated without knowing the accurate values of u for our I, gas-La$ surface system to be operated under various experimental conditions. Line (8) shows the boundary temperature above which both r” and r- are considered to be essentially zero. When a set of T and $I included in the dotted area in Fig. 1 is selected, therefore, the condition of y = 1 and c- > 0.9 is satisfied for any value of u, even when N is as large as 1014 molecules cmT2 s-l. Consequently, such a surface of + 5 2.8 eV should be adopted at T > 1000 K in order that both y and E- may become practically unity. When the sample gas of p, = 30 ptorr including less than 200 p.p.m. of iodine is directed onto an La$ surface at
314
and (2), respectively, in Fig. 8. It should be noted that i-(1.08)/i-(27.2) is not equal to the concentration ratio (1.08/27.2 = 0.040) but varies from 0.079 to 0.028 according as the sample gas pressure increases from 3.0 to 30 ptorr [see curve (3) in Fig. 81. This result indicates that the percentage of ioriginating from background iodine becomes larger with a decrease in ps since u is regarded as virtually i ependent of C although u strongly depends upon both T and ps. e?onsider that p- given by Eq. (8) at T = const. is decreased to l/X by the increase of the sample gas pressure from 3.0 to 30 ptorr, that the incident fluxes of iodine molecules from the supplied sample gas of C = 1 p.p.m. at 3.0 ptorr and from iodine contaminants on the sample inlet system are Y and n, respectively, and also that n is independent of C and p,. Then, Eqs. (13) and (14) are derived with curves (1) and (2) in Fig. 8, respectively, by a similar method used for Eqs. (11) and (12). i-(30 ptorr, 1.08 p.p.m) i-(3.0 ptorr, 1.08 p.p.m.)
~ 3.6 = (n + 10.8V)/X 49 pA n+l.OSv
03)
i-(30 ptorr, 27.2 p.p.m.) i-(3.0 ptorr, 27.2 p.p.m.)
0.13 nA = (n + 272v)/X n + 27.2~ E 0.62nA
(14
Here, i-(30 ptorr, 1.08 p.p.m.), for example, is the ion current at ps = 30 ptorr and C = 1.08 p.p.m. In this way, n is determined to be 3.4~ (and hence X = 43) because n = 85v (and hence X = 15) is unlikely in this work. This result means that the incident flux (n) due to background iodine at ps = 3.0 ptorr is about 3 times as large as that (v) due to supplied sample gas and hence that n at 30 ptorr becomes - 0.3~. Accordingly, even when both of the values of C calculated from Eq. (9) are 1 p.p.m. at 30 and 3.0 ytorr, the actual values of C are concluded to be - 1.3 and 4.4 p.p.m., respectively. This is the main reason why the ratio i-(1.08)/i-(27.2) increases remarkably with a decrease in ps, as shown by curve (3) in Fig. 8. At p, > 15 ptorr, however, the ion current ratio remains constant at - 0.03, nearly equal to C(1.08)/C(27.2) = 0.04. In the pressure range, therefore, the value of C calculated from Eq. (9) may be accurate with about 25% error, which is mainly due to the iodine contaminant inside the sample inlet system so long as C 2 1 p.p.m. The result of X= 43 means that /3- is decreased to l/43 by a pressure increase from 3.0 to 30 ptorr and hence that /3-(30)/p-(3.0) = a(30)/a(3.0) decreases to 0.023 since both y and e- remain essentially unity. This figure is nearly equal to the value of 0.021 evaluated from Eq. (12), thereby indicating that it is reasonable to adopt n = 3.4~ instead of n = 85v as a unique solution for Eqs. (13) and (14). In conclusion, a larger value of p, is favorable for decreasing the error in the determination of C.
315
Calibration curve
On the basis of the above results and discussion, T and p, were selected to be 1400 K and 30 (or tentatively 3.0) ptorr, respectively, and i- was measured as a function of C, according to Eq. (9). The calibration curves thus obtained are shown in Fig. 9, the abscissa at the top of which indicates rf corresponding to C. Employment of a needle valve made of stainless steel at p, = 30 ptorr yields curve (l), where i- does not exhibit a sharp dependence on r,. This result suggests that the inside surface of the valve is readily contaminated with iodine. This valve was therefore replaced with a glass needle valve with which curves (2)-(4) were obtained. Change of rf at p, = 3.0 ptorr yields curve (2), which is far from the result to be expected from Eq. (9), especially in the range below - 1 p.p.m. This is probably because the relative value of n to v becomes larger with an increase in r,, as already described with respect to the data in Fig. 8. Such a tendency is also observed at a much higher pressure (p, = 30 ptorr), as shown by curve (3) although it is much better than curves (1) and (2) in Fig. 9. Air flow rate, rf (m3 s-l) 100
50
I
’
I 0.1
10
5
1
I
0.5
P
I
0.5
1
5
I
I
10
50
Iodineconcentration, C (PMI)
Fig. 9. Calibration curves achieved at T = 1400 K. Curve (1) is obtained with a steel needle valve at pS= 30 ptorr. The use of a glass needle valve yields curves (2) and (3) at ps = 3.0 and 30 ptorr, respectively. Curve (4) is obtained at ps = 30 ptorr after passing air alone through the sample inlet system and the ion source heated to - 200 o C for two days in order to remove the iodine contaminants due to many long runs including the data for curves (2) and (3).
316
In order to remove the iodine contaminant inside the system, air alone was passed through the system and the ion source heated to - 200 OC for two days at p, = 30 ptorr. By this treatment, the background ion current (ii) of I- at C = 0 was sucessfully reduced from 11 to 1.5 pA, which is much smaller than that (8 PA) usually observed in previous work (see Fig. 7 in ref. ll), thereby suggesting that a glass needle valve is more advantageous than a metal aperture plate such as employed previously in order to decrease ii as easily as possible. After the above treatment, curve (4) in Fig. 9 is obtained at p, = 30 ptorr. Although the detection sensitivity of iodine decreased considerably compared with that in the case of curve (3), curve (4) shows more clearly the dependence of i- upon C in the range below 1 p.p.m. Deduction of i; from each data point on curve (3) or (4) in Fig. 9 yields line (3’) or (4’) in Fig. 10, respectively, which shows that the net current Air flow rote, rf km3 s-l)
/-
I d
Avo*05 0.1
I
I
I 0.5
1
5
10
50
Iodineconcentration. C (PIMI)
Fig. 10. Relation between the iodine concentration in air and the corrected ion current of iodine. Curves (3) and (4) are cited from Fig. 9. Lines (3’) and (4’) are obtained by deduction of the background ion current at C = 0 from each data point on curves (3) and (4), respectively.
317
(i- - i;) is proportional to C in the range down to 0.1 p.p.m. and also that the slopes of both lines (3’) and (4’) are slightly greater than unity. The latter is probably because iodine contamination becomes more serious as rf decreases. In consequence, our experimental apparatus and method should be improved to prevent the contamination more effectively. CONLUSIONS
The results and discussion described above lead to the following conclusions. (1) The negative ion source of the La$-Re type developed in this work has work function as low as about 2.7 eV in a high vacuum. (2) Even when air without or with iodine molecules below - 200 p.p.m. is introduced into the ion source up to - 30 ptorr, + remains below about 2.8 eVsolongasT213OOK. (3) Under the conditions described in (2), both y and E- are essentially unity and hence /3- is, in practice, governed only by (I since both r o and rat T B 1000 K are considered to be negligibly small compared with unity. (4) The value of u is much smaher than unity and decreases sharply with decrease in T and/or an increase in p, in the respective ranges ( - 1150-1600 Kand - 2-30 ptorr) covered in this work. (5) At V, = 100 V, the most favorable temperature is about 1400 K, above which i- is decreased by the space-charge effect due to a much stronger electron emission current and above which the lifetime of the La$ layer may be shortened more rapidly by a reaction between La$ and 0, forming BO- and BO;. (6) The concentration of background iodine is estimated to be usually about 0.3 p.p.m. when the value of C calculated from Eq. (9) is 1 p.p.m. and when ps is 30 ptorr. In order to minimize the relative concentration (n/y) of background iodine, p, should be kept as large as possible. (7) In order to keep u as large as possible and also to minimize the oxidation reaction mentioned in (5), on the other hand, a smaller value of p, is favorable for detecting atmospheric iodine by the present method. (8) Consideration of (2)-(7) described above indicates that T and p, should be selected to be about 1400 K and 30 ptorr, respectively. (9) Under the experimental condition selected according to (8), a calibration plot of i- - ia vs. C yields a straight line throughout the range of 50-0.1 p.p.m. The slope of the calibration line, however, is greater than unity. This is mainly because n/v becomes larger as r, decreases. (10) Similarly to the previous work, the most serious problem encountered in this work is how to minimize the iodine contamination inside the apparatus during operation, especially at r, < 10 cm3 s-l.
318
To the best of our knowledge, this is the first study to achieve such a successful result, i.e. that the detection limit of atmospheric iodine by negative ion mass spectrometry is as low as - 0.1 p.p.m. Further work, however, is now in progress in order to decrease ii and also to find better surface materials. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
H. Kawano, J. Vat. Sot. Jpn., 23 (1980) 1. H. Kawano, J. Vat. Sot. Jpn., 23 (1980) 229. H. Kawano and F.M. Page, Int. J. Mass Spectrom. Ion Phys., 50 (1983) 1. H. Kawano, Y. Hidaka and F.M. Page, Int. J. Mass Spectrom. Ion Phys., 50 (1983) 35. H. Kawano, Y. Hidaka, M. Suga and F.M. Page, Int. J. Mass Spectrom. Ion Phys., 50 (1983) 77. H. Kawano, S. Matsuoka, T. Fujii, M. Tsuchiya, H. Fumoto and H. Kishi, Mass Spectrosc., 34 (1986) 249. H. Kawano, Int. J. Mass Spectrom. Ion Processes, 69 (1986) 97. D.R. Stull and H. Prophet, JANAF Thermochemical Tables, U.S. National Bureau of Standards, Washington, DC, 2nd edn., 1971. M.L. Shaw and N.P. Carleton, J. Chem. Phys., 44 (1966) 3387. L.J. Favreau and D.F. Koenig, Rev. Sci. Instrum., 38 (1967) 841. H. Kishi and H. Kawano, Int. J. Mass Spectrom. Ion Processes, submitted for publication. H. Kawano, T. G&la, H. Takeichi, T. Kenpa and Y. Hidaka, Int. J. Mass Spectrom. Ion Phys., 47 (1983) 261. A.E. Dabiri, Surf. Sci., 49 (1975) 77. H. Kawano, S. Itasaka, S. Ohnishi and Y. Hidaka, Int. J. Mass Spectrom. Ion Processes, 70 (1986) 195. H. Kawano, S. Itasaka and S. Ohnishi, Int. J. Mass Spectrom. Ion Processes, 73 (1986) 145.