Determination of rubidium isotopes by usingsaturated absorption spectroscopy and a see-through hollow cathode glow discharge cell

Kyuseok Song*a, Euochang Junga, Hyungki Chaa, Jongmin Leea, Min-Jeong Kimb andSang Chun Leeb

aL aboratory for Quantum Optics, Korea Atomic Energy Research Institute, P.O. Box 105, Yusong, Taejon, KoreabDepartment of Chemistry, Kyungnam University, Masan, Korea

Saturated absorption spectroscopy using a narrow band diode of Rb by laser-induced fluorescence in a hydrogen–air flameusing diode lasers. However, they could not detect isotopes oflaser was applied to the determination of Rb in a laboratory-

built see-through hollow cathode glow discharge cell. The D1 Rb independently. Borleske et al.13 performed a saturatedabsorption spectroscopic study on Rb with a vapor cell, but(794.7 nm) and D2 (780.1 nm) transition lines were

investigated and the hyperfine structures of 85Rb and 87Rb were the resolution was relatively poor. In this paper, we report thedetection of trace amounts of Rb isotopes in a St-HCGD cell.clearly resolved. The abundance ratio of the two isotopes was

calculated by integrating the signals of the corresponding The observed saturated absorption spectra show well resolvednuclear hyperfine structures of Rb and quantitative analysesisotopes, the ratios being accurate to within 6% compared with

the natural abundance. Aqueous standard solutions of Rb were of Rb isotopes were successfully performed.used to determine the sensitivity of the system. The minimumdetectable amount of Rb was 20 pg or less. The technique is

EXPERIMENTALcomparable to high resolution mass spectrometry.

The experimental set-up for saturated absorption spec-Keywords: Saturated absorption spectroscopy; diode laser;troscopy (SAS) in a St-HCGD cell is shown in Fig. 1. The

rubidium; trace analysis; hollow cathode glow dischargeSt-HCGD cell was constructed with a six-way cross chamber.Input and output ports of the St-HCGD cell were mountedwith rapid access doors to ease sample delivery. These twoSpectroscopic analysis of atomic species has been an interesting

subject for researchers who wish to develop more sensitive and ports were used to deliver laser beams to the sample vapor.The sample was placed on the cathode tube using a micropip-convenient analytical methods. Recently, diode lasers have

been a popular light source for spectrochemical analyses owing ette. Both the cathode and anode were made of stainless steel(diameter: 6 mm), and the distance between the cathode andto their compact size, narrow bandwidth and low price.1,2

Furthermore, high resolution spectroscopies with diode lasers anode was set at 5 mm. Argon was used as the buffer gas forthe discharge, and the pressure of about 1–2 Torr in theare receiving considerable attention and these diode laser based

techniques can be applied to radioactive isotope analyses. chamber was controlled by a mass flow controller (MKS type1259 MFC) and a baratron gauge (MKS baratron type 121).Various laser techniques have been introduced for high reso-

lution spectroscopy since the 1970s, but real analytical laser The discharge current was adjustable between 0 and 200 mAbut was maintained at 150 mA using a high voltage powerspectroscopy with high resolution has not been performed

easily owing to the high operating cost and lack of an efficient supply (Bertan Model 210–10R). The diode laser (EOSI Model2001 ECU, 793–810 nm) had a bandwidth of 1 MHz or lesssample introduction system incorporated with the lasers.3–5

Hence, diode lasers were chosen as a light source for high according to measurements with a spectrum analyzer (TecOptics Model V1000, 300 MHz of free spectral range). Theresolution spectrochemical analysis in conjunction with a see-

through hollow cathode glow discharge (St-HCGD) cell that laser beam was split into three parts. One part was deliveredto the wavemeter (Burleigh Model WA-20A) to measurewas originally designed by Lee and Edelson.6 A St-HCGD cell

has a see-through structure for easy access of laser light as accurately the wavelength of the laser beam. The other twobeams were used as pump and probe beams. The pump beamwell as efficient sample handling and atomization. The glow

discharge has received considerable attention in atomic spec- was presented to the sample in the opposite direction to theprobe beam. The ratio of laser power between the pump andtrochemical analysis during the last two decades, showing

strong analytical potential in a similar manner to the ICP.The St-HCGD cell has been examined as regards its analyticalmerit and shows many advantages over other atomic analyticalmethods in various analytical applications. It can vaporizeatoms with low vapor pressure easily by sputtering with buffergas.7 Its bright emission can be directly applied to traceanalysis.

Rubidium has received considerable attention because of thewidespread interest in atomic clocks and because the D1 andD2 transitions can be easily reached by diode lasers.8 Thehyperfine structure of Rb for the D1 transition has beenobserved with absorption spectroscopy by Suzuki andYamaguchi.9 Several reports have been published on the D2transition of Rb.10,11 These investigations, however, were per-formed for the purpose of studying nuclear hyperfine structuresby the use of an atomic vapor cell containing a high pressure Fig. 1 Schematic diagram of the experimental set-up for saturated

absorption spectroscopy.of Rb vapor. Walters et al.12 have reported on the determination

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probe beams was varied using neutral density filters. The pumpbeam was chopped by a mechanical chopper (HMS Model220A) at a rate of about 1000 Hz. The detection of the saturatedabsorption signal was performed by a fast photodiode(Newport Model 818-BB-21) and the detected signal was sentto a lock-in amplifier (EG&G Model 5210). The analysis ofthe measured signal was performed by a personal computer.The optogalvanic signal of Rb was measured using a commer-cial hollow cathode discharge cell (Cathodeon OG cell,3BNX/Rb) to identify the correct transition wavelengths aswell as the isotope ratio. The discharge current was maintainedat 10 mA and the impedance change by the laser beam wasdetected by a laboratory-built circuit with a lock-in amplifier.

RESULTS AND DISCUSSION

Fig. 3 Optogalvanic spectrum of Rb using a commercial hollowRubidium has two isotopes, both of which have a non-zerocathode lamp. Discharge current, 10 mA.nuclear spin (I=1/2 for 85Rb and I=3/2 for 87Rb). The energy

level diagram and schemes for the D1 and D2 transitions arewell known as depicted in Fig. 2. Four transitions can occur

be measured by integrating the peaks for each isotope. Thein each isotope for the D1 transition, whereas six transitionsisotope ratio (85Rb/87Rb) was measured as 2.68, which is aboutare possible for the D2 transition according to the selectiona 5% deviation from the natural abundance of 2.56.rule. To identify the isotopes of Rb, optogalvanic spectra were

Since optogalvanic measurement of the isotopes of Rb couldmeasured with a commercial hollow cathode lamp and anot clearly resolve the two isotopes, SAS was applied to obtaintunable diode laser. The result for the D1 transition is shownbetter spectral resolution. Since SAS is a Doppler-free spectro-in Fig. 3. Among the four observed peaks in Fig. 3, two peaksscopic technique, better spectral resolution is expected for SASwere identified as representing transitions of 85Rb, whereas themeasurement than for optogalvanic measurement. Fig. 4 showsother two represent those of 87Rb. The spectrum shown inthe saturated absorption spectrum of Rb (100 ppb, 20 ml ) forFig. 3 is not well resolved, so that the hyperfine structures ofthe D1 transition using the laboratory-built St-HCGD cell.each isotope cannot be clearly identified. Each peak, however,This spectrum revealed more complicated structures and hyper-can be fitted to a Gaussian lineshape and the isotope ratio canfine splittings than the spectrum shown in Fig. 3, the optogal-vanic spectrum. The positions and intensities of the peaks werecompared with the published results which were obtained witha highly concentrated vapor cell instead of low-concentratedsolution samples.3 All eight transition lines indicated in Fig. 2were identified. In addition, cross-over signals were alsoobserved. The cross-over signals are useful for the identificationof small intensity signals.

The relative position of each hyperfine structure can becalculated using the following equation:

E={F(F+1)−J(J+1)−I(I+1)}×(A/2)+(BD )/4

where I is the nuclear spin, J the angular momentum and Fthe total angular momentum quantum number. A, B and Dare hyperfine splitting constants. For the 5 2P1/2 state, A is4.03×10−3 cm−1 for 85Rb and 13.65×10−3 cm−1 for 87Rb.14From the hyperfine constant, we can estimate isotope shiftsand the energy difference between the transitions. Table 1

Fig. 4 Saturated absorption spectrum of Rb for D1 transition (100Fig. 2 Optical transition schemes of Rb for (a) D1 and (b) D2 ppb, 20 ml ). Sample was deposited inside the cathode of the cell and

the discharge current was 150 mA.transitions.

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Table 1 Nuclear hyperfine splitting of Rb for the D1 transition

Mass Energy (Ea−Eb)/ Energy (Ea−Eb)/number Transition type GHz (calculated) GHz (measured)

F=2�F∞=2; F=2�F∞=3 0.36 0.2885 F=2�F∞=3; F=3�F∞=2 3.47 3.44

F=3�F∞=2; F=3�F∞=3 0.36 0.36F=1�F∞=2; F=1�F∞=1 0.83 0.86

87 F=2�F∞=2; F=2�F∞=2 5.97 5.99F=2�F∞=2; F=2�F∞=1 0.80 0.80

source of the limitation in detection sensitivity is the evapor-ation procedure. Since sample introduction is made directlyinside the cathode of the glow discharge cell, it takes sometime for the discharge to stabilize. Part of the sample may beevaporated during the stabilization procedure. The detectionof isotopes for 20 pg of Rb and the measured reasonableisotope ratio indicate that isotope ratio measurement withoutusing a mass spectrometer was successful when saturationspectroscopy with a diode laser was employed.

CONCLUSION

The SAS technique was applied to the determination of traceamounts of Rb in a St-HCGD cell with a diode laser. Theminimum amount detected using this technique was 20 pg,and therefore the detection limit was estimated to be much

Fig. 5 Saturated absorption spectrum of Rb for D2 transition (100less than 20 pg. The results of our measurements could beppb, 20 ml ). Sample was deposited inside the cathode of the cell andapplied to the detection of isotopes of other trace elements.the discharge current was 150 mA.The technique might also be suitable for portable high reso-lution spectrochemical analyses, which can be used for rapid

Table 2 Isotope ratios of 85Rb versus 87Rb measured by SAS and on-site measurement of isotope ratios for environmentaloptogalvanic spectroscopy samples. The isotope analysis of rare earth elements is in

progress and the results will be published shortly.Method of measurement Isotope ratio (85Rb/87Rb)

Natural abundance 2.56REFERENCESSAS (D1 transition) 2.46±0.05

SAS (D2 transition) 2.42±0.201 Franzke, J., Schnell, A., and Niemax, K., Spectrochim. Acta Rev.,Optogalvanic spectroscopy 2.68±0.05

1993, 15, 379.(D1 transition)2 Fox, R. W., Weimer, C. S., Hollberg, L., and Turk, G. C.,

Spectrochim. Acta Rev., 1993, 15, 291.3 Groll, H., and Niemax, K., Spectrochim. Acta, Part B, 1993, 48, 633.

summarizes the energy differences between hyperfine structures 4 Niemax, K., Groll, H., and Schnurer-Patschan, C., Spectrochim.for the D1 transition. The measured hyperfine splittings are in Acta Rev., 1993, 15, 349.

5 Zybin, A., Schnurer-Patschan, C., and Niemax, K., Spectrochim.reasonably good agreement with the calculated values.Acta, Part B, 1993, 48, 1713.The measurement of the isotope ratio was also performed

6 Lee, S. C., and Edelson, M. C., In-situ Monitoring of Actinidesusing the SAS signals. The abundance ratio of the two Rband Rare Earth Elements by Electrothermal Hollow Cathode

isotopes (85Rb/87Rb) was determined by integrating the SAS Discharge Spectrometry, US-15, IS-5090, Ames Laboratory Press,peaks belonging to each isotope. The isotope ratio was meas- Ames, IA, 1992.ured as 2.46 by SAS for the D1 transition, which differs by 4% 7 Glow Discharge Spectroscopies, ed. Marcus, R. K., Plenum Press,

New York and London, 1993, and references cited therein.from the isotope abundance ratio found in natural Rb (2.56).8 Moon, H. S., Kim, J. B., Lee, H. S., Yang, S. H., and Kim, Y. B.,The saturated absorption spectrum of the D2 transition was

New Phys., 1995, 35, 191.also measured in the 780 nm wavelength region and the

9 Suzuki, M., and Yamaguchi, Y., IEEE J. Quantum Electron., 1988,spectrum is shown in Fig. 5. The hyperfine structure is not as 24, 2392.well resolved as for the D1 transition owing to the very close 10 Mun, H. S., Kim, H. S., Kim, H. A., Kim, J. B., and Lee, H. S.,location of the hyperfine structure. The unsatisfactory reso- J. Opt. Soc. Kor., 1995, 6, 317.

11 Lee, H. S., Park, H. S., Park, J. D., and Cho, H., J. Opt. Soc. Am.lution of the SAS signal is mainly due to the relatively highB, 1994, 11, 558.pressure inside the discharge cell. The 85Rb and 87Rb isotopes,

12 Walters, P. E., Barber, T. E., Wensing, M. W., and Winefordner,however, can be easily separated in the spectrum and theJ. D., Spectrochim. Acta, Part B, 1991, 46, 1015.

abundance ratio for the two isotopes reasonably measured. 13 Borleske, A., Denius, B., and Haynie, T., http://www.webphysics.The isotope ratio was measured as 2.42, a 5.5% difference davidson.edufrom the natural abundance of Rb. Table 2 lists the isotope 14 Saloman, E. B., Spectrochim. Acta, Part B, 1993, 48, 1139.

ratios measured by SAS and optogalvanic spectroscopy.Several samples with different concentrations of Rb were Paper 7/07606A

studied by using Doppler-free SAS. The minimum detectable Received October 21, 1997amount was determined as 20 pg (20 ml, 1 ppb). The main Accepted December 10, 1997

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