Jointly p~tblished by Elsevier Science S. A., Lausanne a,wl A "kad~miai Kiad6, Budapest

J.Radioanal.Nucl.Chem.,Letters 212 (4) 313-320 (1996)

BUFFER GAS EFFECT ON THE DETECTION OF IODINE BY LASER INDUCED FLUORESCENCE

Qian Lin, Xiannian Liu, Zhong-kao Jin, Qike Zheng*

Institute of Laser Chemistry, Fudan University, 220 Handan Road, Shanghai, 200433,

People's Republic of China

Received 20 December 1995 Accepted 3 January 1996

Laser-induced fluorescence (LIF) coupled with photon-counting technique to detect molecular iodine at ultratrace level is reported. Electronic quenching rate con- stants for N 2, NO 2 and H20, as well as for the mixture of NO 2 and H20 has been measur- ed. The application of the LIF method to monitoring 129I 2 in spent fuel reprocessing off-gas streams is evaluated.

INTRODUCTION

A real-time, on-line method for the determination of 129

I has long been required to monitor the concentra-

tion of radioactive iodine in spent fuel reprocessing

off-gas streams, and to observe accidental large re-

lease of radio-iodine I . It has been difficult to moni-

tor 129I directly by conventional counting techniques

because of its extremely low concentration and the low

*Author to whom correspondence should be addressed.

(123o.5 731/96/US $12.0 313 Cop)wlg ht (69 1996 A "lcad[,,~ai Kiadd. B tgtapest A II rights reserved

L1N et al.: DETECTION OF IODINE BY LASER INDUCED FLUORESCENCE

energy of its decay products. Laser-induced fluor-

escence (LIF) is a promising candidate for its ultra-

sensitivity and its application to the determination

of 129I has been studied by some groups It was first 2

proposed by Baronavski and McDonald I , and a 12 detec-

tion limit of 2x1011 molecules cm "3 was obtained with

a He-Ne laser. The effects of several buffer gases

(He, Ar, N2, 02, CO2) were also exam• The iodine

concentration they studied ranged from 9x1013 to Ix1015 ~3, .

molecules cm in air at atmospheric pressure. Goles

et al. 2 detected 129I at 2xi09 molecules cm -3 using a 3 580 nm CW dye laser. Yokoyama and Fujisawa studied

the effect of N 2 on the fluorescence of 127I 2 and 12912

using a He-Ne laser, and their detection limit in N 2

at atmospheric pressure was 5xi012 molecules cm -3

They had also reported that NO 2 interferes with the

detection of 12. Xiannian Liu and Zhilin Wang 4 obtain-

ed a detection limit of 7xID 9 molecules cm -3 of 127I 2

with a He-Ne laser, and studied the 12 fluorescence

quenching by He, Ar, N2, 02 and CO 2. All previous

studies show that the LIF technique is applicable even

in the presence of air or N 2 at atmospheric pressure

if the concentration of iodine is more than 1012 mole- -3

cules cm . As we know, in addition to air and NOx,

H20 exists in the reprocessing off-gas, which may

either react with NOx, or quench the 12 fluorescence.

The effect of H20 on LIF detection of I2 need to be

examined.

In this work, quenching effects Of N2, NO 2 and H20

on iodine fluorescence were studied separately with a

He-Ne laser. The effect of a mixture of NO 2 and H20

in different proportions was also examined. The depend-

ence of iodine fluorescence intensity on the concen-

314

LIN et al.: DETECTION OF IODINE BY LASER INDUCED FLUORESCENCE

TO VQCUUm

I2 sample

M "

L

~ MonochromQtor~

J He-Ne laser ,J I I

Fig. I. Schematic diagram of the apparatus

tration of 12 was measured. Based on these quenching

rate constants, the detection limit of 12 in the pres-

ence of these buffer gases was evaluated.

EXPERIMENTAL

Figure I depicts the schematic of the experimental

setup. The 632.8 nm radiation, at a total output of

30 mW, from a He-Ne laser was used as the excitation

source. The fluorescence of iodine was focused onto

the entrance slit of a monochromator with a lens. A

red-sensitive photomultiplier tube (HAMAMASU R928)

at the exit slit was coupled to photon counting elec-

tronics. A semiconductor electric-thermal cooling sys-

tem was applied to cool the photomultiplier tube to

keep the temperature of PMT at -30 ~ The sample cell

used was made of a glass tube with two flat optical

windows fused on each end. The windows were perpendi-

cular to the laser beam. The vapor pressure of iodine

was controlled by keeping solid iodi~e in different

315

LIN et al.: DETECTION OF IODINE BY LASER INDUCED FLUORESCENCE

7 30

25

20-

15

10 I

i / / J oN02 5 ~ ,,H20

0 ~ - 1 I I 0 5 10 15 20

Pressure,torr

Fig. 2. Stern-Volmer plots oZ iodine fluorescence quenched by N2, NO 2 and H20

slush baths I . Molecular iodine and other reagents used

were of analytical grade. NO 2 gas made by mixing NO

with 02, was purified through several thaw-pump cycles.

The 12 fluorescence detection wavelength was set at

658.6 nm.

RESULTS AND DISCUSSION

Our results show that the quenching effects of N2,

NO 2 and H20 obey the Stern-Volmer equation5:

F 0 - I + K [Q]

F sv

where F 0 and F are fluorescence intensities in the ab-

sence and presence of quencher respectively. K is sv

the Stern-Volmer quenching rate constant. The plots

316

LIN et al.: DETECTION OF IODINE BY LASER INDUCED FLUORESCENCE

7 25

el,, 20

15

10

5

I i I i 1 t 2 4 6 8

Pressure, t,orr

Fig. 3. Quenching effects of the mixture of NO 2 and H20 on iodine fluorescence, a - fixing the pressure of H20 at 1.3 torr: b - fixing the pressure of NO 2 at 2.0 torr

of N2, NO 2 and H20 are shown in Fig. 2. The Ksv values

of N2, NO 2 and H20 are 1.0, 2.2 and 3.1 torr -I (7.5xi0 -3,

1.6xi0 -2 and 2.3xi0 -2 pa-1), respectively. It shows

that quenching effects on iodine fluorescence are in

the order of H20>NO2>N 2. Due to the main components

of the off-gas are N 2 and 02, their quenching on the

12 fluorescence must be severe. But they are easy to

remove from the system. The concentrations of NO 2 and

H20 are less than those of N 2 and 02, but their removal

from 12 is difficult. So we studied the quenching of

the mixture of NO 2 and H20 on iodine fluorescence.

Figure 3 illustrates the total effect of the mix-

ture of NO 2 and H20 on iodine fluorescence, which was

obtained by fixing the pressure of one of them and

change the other one's pressure. NO 2 and H20 may react

317

Lib/et al.: DETECTION OF IODINE BY LASER INDUCED FLUORESCENCE

3

2

1

1011 I 1 1

1012 1013 1014 1015 Iodine concentration, molecules, r -3

Fig. 4. Fluorescence intensity as a function of iodine pressure

tO form HNO 3 and HNO 2. If the reaction of NO 2 and H20

takes place, the total effect of the mixture should

deviate from the effects of NO 2 and H20. We calculated

the quenching effects of NO 2 and H20 with the Ksv ob-

tained to compare with the experimental data. The cal-

culations are also shown in Fig. 4. Z t is surprising

to notice that the calcuIated data are so close to

the o~es 'oStained from experiments. This implies that

the reaction between NO 2 and H20 is not significant

under our experiment conditions.

We also measured the dependence of fluorescence

intensity on iodine concentration. The results are

shown in Fig. 4.

The curve illustrates a linear relationship between

fluorescence intensity and iodine concentration in the

range from 1.7xi011 to 9.7x1014 molecules cm -3. The

detection limit is 6xi09 molecules cm -3 if two times

318

LINT et al.: DETECTION OF IODINE BY LASER INDUCED FLUORESCENCE

the standard deviation of the background is used as

the limit. Using the KsvS obtained from our experi-

ments, we can evaluate the possible detection limit

in a system containing 30 torr NO2, 30 torr H20, and

700 torr N2, that will be about 6xi012 molecules cm -3

The experimental results of other groups I'3 are in

good agreement with our estimation. Since the detec~

tion limit depends on the presence of quenching gases,

applying the LIF technique for the detection of ultra-

trace amounts of iodine would be feasible if the re-

moval of H2, 02, NO 2 and H20 from 12 were achieved.

CONCLUSION

The quenching effects of N2, NO 2 and H20 on the

laser induced fluorescence of I were examined. Their 2

-I quenching rate constants are 1.0, 2.2 and 3.1 torr

The quenching of the mixture of NO 2 and H20 on 12 flu-

orescence was found to be the sum of their respective

effects. In a gas mixture of 700 torr N2, 30 torr NO 2

and 30 torr H20 , quenching by those gases reduces the

sensitivity of the LIF method by three orders of magni-

tude. The LIF method is applicable if the off-gas

stream is pretreated.

The authors thank Mr. Zhilin Wang for his expert

technical assistance in performing these experiments.

319

LIN et al.: DETECTION OF IODINE BY LASER INDUCED FLUORESCENCE

REFERENCES

I. A.P. Baronavski, J.R. McDonald, Proc. 15th DOE Nuclear Air Cleaning Conference, I (1979) 971.

2. R.W. Goles, R.C. Fukuda, M.W. Cole, F.P. Brauer, Anal. Chem., 53 (1981) 776.

3. A. Yokoyama, G. Fujisawa, T. Sakurai, K. Suzuki, Spectrochimica Acta, 47A (No. 5) (1991) 567.

4. Xiannian Liu, Zhilin Wang, Xin Chen, Jiukuan Shun, Qike Zheng, Fudan Xue Bao Zi Ran Ke Xue Ban (Jour- nal of Fudan university, Nature Science), 25 (1986) 449.

5. J.R. Lakowicz, Principles of Fluorescence Spectro- scopy, Plenum Press, New York, 1983.

320