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Enhancement of sensitivity of optogalvanic spectroscopy in a flame by laser ionization
1 August 1994
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications
110 (1994) 105-108
Enhancement of sensitivity of optogalvanic spectroscopy in a flame by laser ionization Yuji Oki, Nobutaka Kidera, Mitsuo Maeda Department of Electrical Engineering, Kyushu University, Hakozaki. Fukuoka 8 1.2,Japan Received 28 October 1993; revised manuscript received 15 April 1994
Abstract
In usual optogalvanic spectroscopy in a flame, the signal is obtained by thermal ionization of selectively excited species. The optogalvanic signal in a flame was remarkably enhanced by selective multi-step photo-ionization, when an intense UV laser beam was applied into the flame. The enhancement factor exceeded 100 and a detection limit of 5 ppt (pg/ml) was obtained, when this technique was applied for tracing atomic Ca in water.
1. Introduction
Analytical application of optogalvanic spectroscopy in a flame [ 1,2] is a useful and practical technique for the detection of trace elements in water. This technique is sometimes called laser-enhanced ionization (LEI) spectrometry [ 31. In LEI spectrometry, detection limits of less than 1 ppb (ng/ml) can be obtained for most of the metallic elements in pure water. The change of the ionization current corrected by biased electrodes in a flame atomizer is detected after the excitation of atoms by a pulsed dye laser. At trace element detection, for example, the detection limit of Ca by single-step LEI was reported to be 6 ppt by the excitation of the 227.6 nm line [ 41. In the usual optogalvanic or LEI spectroscopy, the selectively excited atoms are mainly ionized by the collisional process. The dominant ionization process is thermal collision in a flame [ 5 ] and electronic collision in a discharge tube. In LEI spectrometry, the photo-ionization process can be ignored, because the dye laser intensity for the first-step excitation is small. Gorbatenko et al. have reported LEI spectrometry of 0030-40 18/94/%07.00
Ca by two-step excitation. They used two dye lasers with wavelengths of 422.67 nm and 5 18.89 nm and excited the 4s5d ‘D2 level to enhance the ionization [ 61. The detection limit was 20 ppt. Because the power level of the dye laser was very low, ionization by the dye laser was estimated to be ineffective in this case. On the other hand, resonance ionization spectroscopy (RIS) attains more efficient ionization by another intense laser beam. Omenetto et al. have reported a detection limit of 7 ppt by two-step laser induced ionic fluorescence in an ICP [ 7 1. In this paper, we report remarkable enhancement of the optogalvanic signal from the 422.7 nm line of Ca atoms in a flame by the application of another intense UV laser. The dominant optogalvanic signal is generated through selective two-step photo-ionization in this case. Very sensitive detection of Ca atoms in water down to 5 ppt (pg/ml) is attained with a simple setup for optogalvanic spectroscopy in a flame, and the enhancement factor by the ionization laser is more than 100.
0 1994 Elsevier Science B.V. All rights reserved XSD10030-4018(94)00246-Q
Y. Oki et al. /OpticsCommunications110 (1994) 105-108
106
2. Experimental setup
Fig. 1 shows the typical setup of LEI spectrometry in a flame. A premixed burner (slit length: 50 mm) for the atomic absorption spectroscopy is used. Two water-cooled aluminum electrodes with a cross section of 35x70 mm* are placed on both sides of an air-C2H1 flame. The separation of the electrode is 6 mm, and the distance from the top of the burner to the bottom of the electrodes is 5 mm. The - 1.6 kV DC-voltage is applied on both electrodes, and the optogalvanic signal is picked up from the burner. The signal is passed through a pre-amplifier and an active filter which are composed of fast-response amplifiers (National Semiconductor: LH0032C), and processed by a boxcar averager (SRS system: Model SR250). A frequency-tripled (355 nm) Nd: YAG laser (Quanta-Ray: GCR-12, output 70 mJ/pulse, 5-6 ns FWHM, 10 Hz), is used for pumping of a dye laser, and a part of the output beam is used for the ionization of excited atoms. The dye laser is a home-made system with a grazing-incidence grating (2400/mm) and Bis-MSB dye solution is used for a resonant line of the Ca atom at ;1=422.7 nm (4s* ‘S-4s4p ‘PO). The output energy is about 100 uJ and the spectral bandwidth is 0.8 cm-‘. The beam is expanded to 3 mm diameter and positioned at a height of lo- 12 mm above the top surface of the burner. These parameters are found in previous experiments, which have optimized the signal intensity.
The standard CaC12 solution is diluted by pure water that is sampled from a circulating system with two-stage ion-exchange resin filters. The sample water is nebulized into the burner with an approximate speed of 10 ml/min.
3. Experimental results and discussion Fig. 2 shows waveforms of the optogalvanic signal which are directly taken by an oscilloscope, when the 422.7 nm line of Ca is excited. The output energies of the dye laser in Figs. 2a and 2b are 5 uJ and 85 uJ, respectively. In Fig. 2b, the second harmonic generation (SHG, A=532 nm, 108 mJ) of the Nd:YAG laser is simultaneously applied. However, the enhancement of the signal is small even at a large energy of 108 mJ. On the other hand, a remarkable enhancement is observed in the case of the third harmonic generation (THG, A=355 nm, 15 mJ), as shown in Fig. 2a. Fig. 3 shows the energy-level diagram of Ca. SHG cannot ionize the Ca atoms in the 4~4~’ P” level by 1.5r
10 12 TIME [/IS]
Pulsed Nd:YAGLaser
14
16
18
20
(4 1.5,
I
+H.V.
Ejectrode
d
Oscilloscope
-1.0
0
2
4
6
10 12 8 TIME (1~s)
14
16
18
20
(11) Cooling Water
Air
Fig. 1. Setup of photo-enhanced
optogalvanic
spectroscopy.
Fig. 2. Example of an optogalvanic signal at a Ca concentration of 100 ppb with and without ionization laser beam. As ionization laser, THG and SHG of Nd: YAG laser output is used in (a) and (b), respectively. The thick lines mean with ionization laser, and the thin lines mean without.
107
Y. Oki et al. /Optics Communications llO(1994) 105-108 Ionization
6.113eV__&~-a
h r\
2.932eV
SHG
THG
Nd:YAG Laser L:z,
422.7nm Two-photon Ionization -
OeV
Fig. 3. Energy level diagram of Ca and selective ionization by laser.
X=422.7nm DYE:61.5pJ
! :
,//! :,
,
;
dence on the THG energy. This means that the ionization contributing to the noise component is dominantly the two-photon process from the ground state. The optogalvanic signal saturates at an energy of about 20 mJ, as shown in Fig. 4. However, at lower concentrations, another background noise composed from the optogalvanic signal of other species becomes dominant. Therefore, the THG energy of about 20 mJ is obtained for best S/N ratio. Fig. 5 shows the analytical curves taken by changing the Ca concentration, which compares the results with and without THG. The enhancement factor is about 100 in this case for a THG energy of 19.7 mJ. By increasing the THG energy, an enhancement factor of more than 300 can be obtained. In these experiments, the optogalvanic signal intensity was measured from oscilloscope traces by a single laser shot. To improve the S/N ratio, averaging with a boxcar integrator is effective. The window time of the boxcar integrator was set at 4 us. Three hundred shots were integrated and processed by a microcomputer. Fig. 6 shows an example of the signal obtained with the boxcar integrator in the very low Ca concentrations. The dye laser power is 293 uJ and the ionization laser power is 15 mJ approximately. In these concentrations, the dominant component of the background fluctuation is caused by the ionization of particles in the air-CzH2 flame, because it increases by the application of the UV ionization laser without Ca injection. Therefore the S/N ratio can be improved by decreasing the output fluctuation of the ionization laser. The base-line drift is probably caused by the drift of the boxcar integrator. The S/N ratio of
1 0.1 YAk THG
Energy t rfJ]
10s
Fig. 4. Optogalvanic signal intensity as a function of ionization laser power. The concentration of Ca is 30 ppb.
the single-photon process, while THG can. This causes the remarkable difference in the enhancement of the optogalvanic signal in SHG and THG. In the case of THG, the signal intensity is plotted as a function of the THG energy in Fig. 4. At a Ca concentration of 30 ppb, the signal proportionally increases with THG energy. This shows that the ionization is a single-photon process. The noise component in Fig. 4 is the ionization without the dye laser. This component shows approximately square depen-
10-310-2
10-l
1
Concentration
10
2
lo”
104
(ppbj’
Fig. 5. Analytical calibration curves of Ca with and without laser ionization.
108
Y. Okiet al. /Optics Communications 110 (1994) 105-108
4. Conclusion
‘-4 w i
0.2-
tI 0
I
I
I
t
I1
1000
a
I
I)
*
c
I
3000
Time (s) 2ooo
Enhancement of the optogalvanic signal in a flame atomizer is demonstrated by using UV laser ionization. Because the enhancement factor exceeded 100, this technique is useful to improve the detection limit in LEI spectrometry. For Ca atoms in an air-&Hz flame, a detection limit of 5 ppt was attained by this scheme. This sensitivity is one of the best data ever reported for Ca atom detection in water. We are now improving the detection sensitivity of this scheme, and trying the application on other elements.
Fig. 6. Example of the signal of a boxcar integrator at low Ca concentrations.
10 ppt is 4, where the noise is defined as the standard deviation of the background component. The detection limit decided from S/N = 2 is 5 ppt. On the other hand, if the detection limit without THG was 300 ppt, then an enhancement factor of 60 is obtained here. More improvement of sensitivity can be expected, but our experimental equipment used for optogalvanic spectroscopy has the limitation of less sensitivity compared to the equipment used in Ref. [ 4 ] as described in the introduction.
References [ I] R.B. Green, R.A. Keller, G.G. Luther, P.K. Schenck and J.C. Travis, IEEE J. Quantum Elect. QE-13 (1977) 63. [2] D.S. King, P.K. Schenck, KC. Smyth and J.C. Travis, Appl. Optics 16 (1977) 2617. [ 310. Axner and H. Rubinsztein-Dumlop, Spectrochim. Acta 44 B (1989) 835. [ 410. Axner and I. Magnusson, Phys. Ser. 3 1 ( 1985) 587. [ 510. Axner and T. Berglind, Appl. Spectrosc. 43 (1989) 940. [ 61 A.A. Gorbatenko, N.B. Zorov, S.Y. Karpova, Y.Y. Kuzyakov and V.I. Chaplygin, J. Anal. At. Spectrom. 3 ( 1988) 527. [ 71 N. Omenetto, B. Smith, L. Hart, P. Cavalli and G. Rossi, Spectrochim. Acta 40 B ( 1985 ) 14 11.
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications
110 (1994) 105-108
Enhancement of sensitivity of optogalvanic spectroscopy in a flame by laser ionization Yuji Oki, Nobutaka Kidera, Mitsuo Maeda Department of Electrical Engineering, Kyushu University, Hakozaki. Fukuoka 8 1.2,Japan Received 28 October 1993; revised manuscript received 15 April 1994
Abstract
In usual optogalvanic spectroscopy in a flame, the signal is obtained by thermal ionization of selectively excited species. The optogalvanic signal in a flame was remarkably enhanced by selective multi-step photo-ionization, when an intense UV laser beam was applied into the flame. The enhancement factor exceeded 100 and a detection limit of 5 ppt (pg/ml) was obtained, when this technique was applied for tracing atomic Ca in water.
1. Introduction
Analytical application of optogalvanic spectroscopy in a flame [ 1,2] is a useful and practical technique for the detection of trace elements in water. This technique is sometimes called laser-enhanced ionization (LEI) spectrometry [ 31. In LEI spectrometry, detection limits of less than 1 ppb (ng/ml) can be obtained for most of the metallic elements in pure water. The change of the ionization current corrected by biased electrodes in a flame atomizer is detected after the excitation of atoms by a pulsed dye laser. At trace element detection, for example, the detection limit of Ca by single-step LEI was reported to be 6 ppt by the excitation of the 227.6 nm line [ 41. In the usual optogalvanic or LEI spectroscopy, the selectively excited atoms are mainly ionized by the collisional process. The dominant ionization process is thermal collision in a flame [ 5 ] and electronic collision in a discharge tube. In LEI spectrometry, the photo-ionization process can be ignored, because the dye laser intensity for the first-step excitation is small. Gorbatenko et al. have reported LEI spectrometry of 0030-40 18/94/%07.00
Ca by two-step excitation. They used two dye lasers with wavelengths of 422.67 nm and 5 18.89 nm and excited the 4s5d ‘D2 level to enhance the ionization [ 61. The detection limit was 20 ppt. Because the power level of the dye laser was very low, ionization by the dye laser was estimated to be ineffective in this case. On the other hand, resonance ionization spectroscopy (RIS) attains more efficient ionization by another intense laser beam. Omenetto et al. have reported a detection limit of 7 ppt by two-step laser induced ionic fluorescence in an ICP [ 7 1. In this paper, we report remarkable enhancement of the optogalvanic signal from the 422.7 nm line of Ca atoms in a flame by the application of another intense UV laser. The dominant optogalvanic signal is generated through selective two-step photo-ionization in this case. Very sensitive detection of Ca atoms in water down to 5 ppt (pg/ml) is attained with a simple setup for optogalvanic spectroscopy in a flame, and the enhancement factor by the ionization laser is more than 100.
0 1994 Elsevier Science B.V. All rights reserved XSD10030-4018(94)00246-Q
Y. Oki et al. /OpticsCommunications110 (1994) 105-108
106
2. Experimental setup
Fig. 1 shows the typical setup of LEI spectrometry in a flame. A premixed burner (slit length: 50 mm) for the atomic absorption spectroscopy is used. Two water-cooled aluminum electrodes with a cross section of 35x70 mm* are placed on both sides of an air-C2H1 flame. The separation of the electrode is 6 mm, and the distance from the top of the burner to the bottom of the electrodes is 5 mm. The - 1.6 kV DC-voltage is applied on both electrodes, and the optogalvanic signal is picked up from the burner. The signal is passed through a pre-amplifier and an active filter which are composed of fast-response amplifiers (National Semiconductor: LH0032C), and processed by a boxcar averager (SRS system: Model SR250). A frequency-tripled (355 nm) Nd: YAG laser (Quanta-Ray: GCR-12, output 70 mJ/pulse, 5-6 ns FWHM, 10 Hz), is used for pumping of a dye laser, and a part of the output beam is used for the ionization of excited atoms. The dye laser is a home-made system with a grazing-incidence grating (2400/mm) and Bis-MSB dye solution is used for a resonant line of the Ca atom at ;1=422.7 nm (4s* ‘S-4s4p ‘PO). The output energy is about 100 uJ and the spectral bandwidth is 0.8 cm-‘. The beam is expanded to 3 mm diameter and positioned at a height of lo- 12 mm above the top surface of the burner. These parameters are found in previous experiments, which have optimized the signal intensity.
The standard CaC12 solution is diluted by pure water that is sampled from a circulating system with two-stage ion-exchange resin filters. The sample water is nebulized into the burner with an approximate speed of 10 ml/min.
3. Experimental results and discussion Fig. 2 shows waveforms of the optogalvanic signal which are directly taken by an oscilloscope, when the 422.7 nm line of Ca is excited. The output energies of the dye laser in Figs. 2a and 2b are 5 uJ and 85 uJ, respectively. In Fig. 2b, the second harmonic generation (SHG, A=532 nm, 108 mJ) of the Nd:YAG laser is simultaneously applied. However, the enhancement of the signal is small even at a large energy of 108 mJ. On the other hand, a remarkable enhancement is observed in the case of the third harmonic generation (THG, A=355 nm, 15 mJ), as shown in Fig. 2a. Fig. 3 shows the energy-level diagram of Ca. SHG cannot ionize the Ca atoms in the 4~4~’ P” level by 1.5r
10 12 TIME [/IS]
Pulsed Nd:YAGLaser
14
16
18
20
(4 1.5,
I
+H.V.
Ejectrode
d
Oscilloscope
-1.0
0
2
4
6
10 12 8 TIME (1~s)
14
16
18
20
(11) Cooling Water
Air
Fig. 1. Setup of photo-enhanced
optogalvanic
spectroscopy.
Fig. 2. Example of an optogalvanic signal at a Ca concentration of 100 ppb with and without ionization laser beam. As ionization laser, THG and SHG of Nd: YAG laser output is used in (a) and (b), respectively. The thick lines mean with ionization laser, and the thin lines mean without.
107
Y. Oki et al. /Optics Communications llO(1994) 105-108 Ionization
6.113eV__&~-a
h r\
2.932eV
SHG
THG
Nd:YAG Laser L:z,
422.7nm Two-photon Ionization -
OeV
Fig. 3. Energy level diagram of Ca and selective ionization by laser.
X=422.7nm DYE:61.5pJ
! :
,//! :,
,
;
dence on the THG energy. This means that the ionization contributing to the noise component is dominantly the two-photon process from the ground state. The optogalvanic signal saturates at an energy of about 20 mJ, as shown in Fig. 4. However, at lower concentrations, another background noise composed from the optogalvanic signal of other species becomes dominant. Therefore, the THG energy of about 20 mJ is obtained for best S/N ratio. Fig. 5 shows the analytical curves taken by changing the Ca concentration, which compares the results with and without THG. The enhancement factor is about 100 in this case for a THG energy of 19.7 mJ. By increasing the THG energy, an enhancement factor of more than 300 can be obtained. In these experiments, the optogalvanic signal intensity was measured from oscilloscope traces by a single laser shot. To improve the S/N ratio, averaging with a boxcar integrator is effective. The window time of the boxcar integrator was set at 4 us. Three hundred shots were integrated and processed by a microcomputer. Fig. 6 shows an example of the signal obtained with the boxcar integrator in the very low Ca concentrations. The dye laser power is 293 uJ and the ionization laser power is 15 mJ approximately. In these concentrations, the dominant component of the background fluctuation is caused by the ionization of particles in the air-CzH2 flame, because it increases by the application of the UV ionization laser without Ca injection. Therefore the S/N ratio can be improved by decreasing the output fluctuation of the ionization laser. The base-line drift is probably caused by the drift of the boxcar integrator. The S/N ratio of
1 0.1 YAk THG
Energy t rfJ]
10s
Fig. 4. Optogalvanic signal intensity as a function of ionization laser power. The concentration of Ca is 30 ppb.
the single-photon process, while THG can. This causes the remarkable difference in the enhancement of the optogalvanic signal in SHG and THG. In the case of THG, the signal intensity is plotted as a function of the THG energy in Fig. 4. At a Ca concentration of 30 ppb, the signal proportionally increases with THG energy. This shows that the ionization is a single-photon process. The noise component in Fig. 4 is the ionization without the dye laser. This component shows approximately square depen-
10-310-2
10-l
1
Concentration
10
2
lo”
104
(ppbj’
Fig. 5. Analytical calibration curves of Ca with and without laser ionization.
108
Y. Okiet al. /Optics Communications 110 (1994) 105-108
4. Conclusion
‘-4 w i
0.2-
tI 0
I
I
I
t
I1
1000
a
I
I)
*
c
I
3000
Time (s) 2ooo
Enhancement of the optogalvanic signal in a flame atomizer is demonstrated by using UV laser ionization. Because the enhancement factor exceeded 100, this technique is useful to improve the detection limit in LEI spectrometry. For Ca atoms in an air-&Hz flame, a detection limit of 5 ppt was attained by this scheme. This sensitivity is one of the best data ever reported for Ca atom detection in water. We are now improving the detection sensitivity of this scheme, and trying the application on other elements.
Fig. 6. Example of the signal of a boxcar integrator at low Ca concentrations.
10 ppt is 4, where the noise is defined as the standard deviation of the background component. The detection limit decided from S/N = 2 is 5 ppt. On the other hand, if the detection limit without THG was 300 ppt, then an enhancement factor of 60 is obtained here. More improvement of sensitivity can be expected, but our experimental equipment used for optogalvanic spectroscopy has the limitation of less sensitivity compared to the equipment used in Ref. [ 4 ] as described in the introduction.
References [ I] R.B. Green, R.A. Keller, G.G. Luther, P.K. Schenck and J.C. Travis, IEEE J. Quantum Elect. QE-13 (1977) 63. [2] D.S. King, P.K. Schenck, KC. Smyth and J.C. Travis, Appl. Optics 16 (1977) 2617. [ 310. Axner and H. Rubinsztein-Dumlop, Spectrochim. Acta 44 B (1989) 835. [ 410. Axner and I. Magnusson, Phys. Ser. 3 1 ( 1985) 587. [ 510. Axner and T. Berglind, Appl. Spectrosc. 43 (1989) 940. [ 61 A.A. Gorbatenko, N.B. Zorov, S.Y. Karpova, Y.Y. Kuzyakov and V.I. Chaplygin, J. Anal. At. Spectrom. 3 ( 1988) 527. [ 71 N. Omenetto, B. Smith, L. Hart, P. Cavalli and G. Rossi, Spectrochim. Acta 40 B ( 1985 ) 14 11.