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Flame-fluorescence detection of Mg, Ni, and Pb with a frequency-doubled dye laser as excitation source
Volume
7, number
3
OPTICS COMMUNICATIONS
March
FLAME-FLUORESCENCE
DETECTION
OF Mg, Ni, AND Pb
WITH A FREQUENCY-DOUBLED
DYE LASER
AS EXCITATION
1973
SOURCE
J. KUHL and H. SPITSCHAN Carl Zeiss, Forschungsgruppe, Received
D-7082 Oberkochen,
29 January
Germany
1973
The determination of traces of Mg, Ni, and Pb by atomic fluorescence spectrometry laser emission for excitation is reported. Low detection limits and long linear analytical the qualification of this method for quantitative analysis.
1. Introduction The application of atomic fluorescence spectroscopy as a widely employed analytical tool for trace element analysis is mainly delayed by the lack of intensive excitation sources for a lot of elements. During the last two years several groups have studied the possible use of dye lasers as sources of excitation in atomic fluorescence flame (or non-flame) spectrometry [l-3]. Today generation of stimulated emission from dyes is limited to the spectral range between 340 and 1200 nm. As it seems difficult to find a highly efficient laser dye for wavelengths shorter than 300 nm due to strong absorption by the molecules in the excited singlet states [4], direct excitation of the sensitive spectral absorption lines below 300 nm (especially those of the metal atoms, which play an important role in the environmental pollution problem, e.g., Pb, Ni, Hg) by dye laser emission will be impossible. These difficulties can be overcome, however, taking advantage of the well known techniques of frequency mixing in non-linear crystals. Intensive, frequency tunable light sources for the wavelength region between 26 1 and 360 nm have been realized by frequency doubling of visible dye laser radiation within ADP and KDP crystals [ 51. If the beamspread of the laser is reduced by mode selection, high peak power conversion efficiencies can be achieved even for moderate laser output power. Therefore, tunable narrowband UV line sources yield256
using frequency doubled dye working curves demonstrate
ing several kW of peak output the basis of flashlamp-pumped results of atomic fluorescence excitation sources for Mg, Ni, will be reported here.
power are supported on dye lasers. Some initial measurements with such and Pb in C2H2/air flames
2. Experimental The experimental configuration used for the investigations is shown in fig. 1. Compared with earlier work [6], the layout of the flashlamp-pumped dye laser system was changed. The rhodamine-6G solution was passed through a quartz dye cell having a length of 10 cm
GRA TING SPECTROGQAPH
,
Fig. 1. Schematic
diagram
of the experimental
set-up.
Volume
7, number
3
OPTICS
COMMUNICATIONS
and an internal diameter of 3 mm. Efficient excitation of the dye was achieved by optical pumping with a 4 inch linear flashlamp (Xenon Corp., Novatron 701 CR, pump energy 15-20 W, pump pulse length 1.3 I.tsec). As the laser system allows pulse repetition rates up to at least 10 set-l, rnost of the experiments were done with 1 to 3 laser shots per second*. The output of the laser was narrowed by a two stage wavelength selector consisting of an interference filter and a Fabry-Perot interferometer and frequency doubled in a 38 mm long KDP crystal. ‘The resulting bandwidth of the UV line was less than 0.02 A. Design and construction of the wavelength selector and details about the second harmonic generation have been described extensively[5, 61. Correct tuning of the generated W radiation to the resonance line of interest was achieved by observing the emission spectrum of a hollow cathode lamp, and the generated W line in the focal plane of a 1.5 m Czerny-Turner grating spectrograph by means of a UV image converter tube. A conventional nebulizer burner system (Zeiss Flame Emission Photometer PF5) was used for aspiration of the solutions in the laminar premixed C,H,/air flame and evaporation of the analyte. The fluorescence photons perpendicular to the direction of the exciting beam were detected by a fast response photomultiplier tube (RCA IP 28) amplified and integrated by a boxcar integrator (Princeton Applied Research, Model 160) and recorded on a strip chart recorder for evaluation. The fluorescence signal was separated from the broadband flame background radiation by- means of a prism monochromator v/l0 aperture, dispersion 75 A/mm for h = 300 nm). In all cases a slit width of 0.5 mm was chosen. The wavelengths used were 285.2 nm for Mg, 283.3 nm (absorbing line) and 405.8 nm (observed fluorescence line) for Pb, and 305.1 nm for Ni.
* For these experimental conditions more than 2 X 10’ flashes are possible without any destruction of the flashlamp. Using different dyes and different interference filters for spectral narrowing and tuning, the output wavelength of this dye laser could be continuously varied between 440 and 700 nm yielding at least 8- 10 kW peak output power and a spectral bandwidth of less than 0.4 nm. Further spectral narrowing with a Fabry-Perot interferometer should be possible without serious loss (less than a factor 2).
March
1973
3. Results and discussion The efficiency and capability of the described setup for spectroscopic investigations is demonstrated in fig. 2. The recorded fluorescence profile of the Ni line h = 305.1 nm under typical flame conditions was scanned by tuning the UV wavelength generated from the repetitively pulsed dye laser across the Ni absorption line. This spectral tuning was accomplished by continuous tilting of the Fabry-Perot interferometer within the laser cavity by means of a servomotor as a scan drive system. The position of the interference filter and the orientation of the non-linear crystal does not have to be changed for tuning intervals as small as 0.2 A. Never. theless, the UV power was constant for the whole tuning range as shown in the lower part crf the figure.
Fig. 2. Fluorescence line profile of the Ni line h = 305.1 nm scanned by the 0.02 A wide, frequency doubled output of the dye laser.
257
Volume
7. number
3
OPTICS
COMMUNICATIONS
March
K) ANALYTE
Fig. 3. Typical recordings of the fluorescence signal and the background radiation near the limit of detection for the Mg line h = 285.2 nm.
Typical recorder traces for Mg solutions near the detection limit are presented in fig. 3. The noise level of the blank signal, which is mainly caused by intensity fluctuations of the laser outputt, severely limits the sensitivity. The analytical curves for Mg, Ni, and Pb plotted in fig. 4 show fluorescence signal as a function of analyte concentration. The limit of detection has been taken as the concentration producing a signal to noise ratio of 2. These detection limits are summarized in table I together with some pertinent experimental conditions. The values for Mg and Pb are comparable with or better than the previously published atomic flame fluorescence spectrometric data obtained with different types of excitation sources [7-121. For the detection of Pb, the sample was irradiated with UV-radiation (A = 283.3 nm) and the fluorescence of the blue line h = 405.8 nm was monitored. Thereby, the fluorescence signal could be separated from the strong Rayleigh and Mie scatter. Whereas the intensity of the UV beam was too small to produce a population of the excited atomic level comparable with the saturation limited value for Ni and Pb,
Fig. 4. Analytical curves for laser excited of Mg, Ni, and Pb in a CaHz/air flame.
Detection Element
Mg
258
Wavelength
285.2 305.1 283.3 405.6
Ni Pb
18”
lf
lo’
I&dmll
atomic
fluorescence
Table 1 limits for Mg, Ni, and Pb in the CzHa/air
(rim)
I
t The fluctuations of the generated UV radiation which could be controlled by means of a beam splitter and a photocell amounted to about + 10% of the peak power.
CONCENTRATION
(absorption) (fluorescence)
d
$3
1973
flame
Approximate irradiance (kW/cm’)
Detection limit
2 6
0.0003 0.1
1*5
0.03
(ppm)
$7’ IRRADIANCE
1 (kW/cm2)
Fig. 5. Fluorescence signal observed in fhe Mg resonance line (h = 285.2 nm) as a function of exciting irradiance, demonstrating the occurrence of saturation.
Volume
7,
number 3
OPTICS COMMUNICATIONS
the influence of saturation phenomena [ 13- 151 on the Mg fluorescence signal was easily observable. The fluorescence signal for Mg observed for different values of excitation irradiance, which was lowered stepwise by neutral density filters over at least 4 orders of magnitude, is plotted in fig. 5. From this diagram it may be derived that the application of an irradiance of about 2 kW/cm2 for the Mg resonance line (h = 285.2 nm) should distinctly reduce the influence of laser fluctuations on the fluorescence signal. Therefore, the stronger increase in sensitivity for Mg detection becomes explainable. Due to the reabsorption effects within the flame, the fluorescence signal for the 100 ppm solution is smaller than that for 10 ppm solution (fig. 5) if the irradiance level was kept below 0.1 kW/cm2. If the irradiance is raised, the ground state of the sample is bleached, thus reducing reabsorption. For Ni atomic fluorescence detection limits of 3 ng/ ml have been reported for excitation in a H,/air flame [ 161. The poor value which was gotten from the fluorescence excited by the frequency doubled dye laser may be explained by the lower sensitivity of the line X = 305.1 nm compared with the normally used strong absorption line X = 232.0 nm and by the higher fluorescence quenching rate within the C2H2/air flame. Nevertheless, the value given here is at least 5 times smaller than the detection limit which was measured for fluorescence excitation of the 305.1 nm line in the Hz/air flame by a high intensity hollow cathode lamp ]161. The results for Mg indicate that an increase in sensitivity may be expected for other elements too, if the irradiance level may be raised. Certainly, the laser excited atomic fluorescence limits of detection could be improved by increasing the stability of the laser output or application of improved signal processing techniques. Recording the ratio of the fluorescence signal and the laser output power and compensation of un-
March 1973
specific background scatter by measuring the signal for two slightly different wavelengths, one of which is tuned to the center of the absorption line, the other outside of the absorption profile, should result in a 10 to 100 fold improvement in detection limits.
Acknowledgement The work on which this report is based was supported by the Bundesminister fur Bildung und Wissenschaft as part of the technology program of his ministry. References ] 1 ] L.M. Fraser and J.D. Winefordner, Anal. Chem. 43 (197 1) 1963; 44 (1972) 1444. [2] M.B. Denton and H.V. Malmstadt, Appl. Phys. Letters 18 (1971) 485, [3] J. Kuhl and G. Marowsky, Opt. Commun. 4 (1971) 125. [4] K.H. Drexhage, VII. International Quantum Electronics Conference, Montreal (1972). IS] J. Kuhl and H. Spitschan, Opt. Commun. 5 (1972) 382, and references therein. (61 J. Kuhl, G. Marowsky, P. Kunstmann and W. Schmidt, Z. Naturforsch. 27a (1972) 601. Acta 25B L71 D.G. Mitchell and A. Johansson, Spectrochim. (1970) 175. IS1 T.S. West and X.K. Williams, Anal. Chim. Acta 45 (1969) 27. 191 D.L. Manning and D. Heneage, At. Absorption Newsletters 6 (1967) 124. Acta 25B IlO1 D.H. Cotton and D.R. Jenkins, Spectrochim. (1970) 283. Llll V. Sychra and J.P. Matousek, Talanta 17 (1970) 363. [121 R.F. Browner, R.M. Dagnall and T.S. West, Anal. Chim. Acta 50 (1970) 37.5. J. Kuhl, S. Neumann and M. Kriese, to be published. E.H. Piepmeier, Spectrochim. Acta 27B (1972) 431. E.H. Piepmeier, Spectrochim. Acta 27B (1972) 445. J.P. Matousek and V. Sychra, Anal. Chem. 41 (1969) 518.
2.59
7, number
3
OPTICS COMMUNICATIONS
March
FLAME-FLUORESCENCE
DETECTION
OF Mg, Ni, AND Pb
WITH A FREQUENCY-DOUBLED
DYE LASER
AS EXCITATION
1973
SOURCE
J. KUHL and H. SPITSCHAN Carl Zeiss, Forschungsgruppe, Received
D-7082 Oberkochen,
29 January
Germany
1973
The determination of traces of Mg, Ni, and Pb by atomic fluorescence spectrometry laser emission for excitation is reported. Low detection limits and long linear analytical the qualification of this method for quantitative analysis.
1. Introduction The application of atomic fluorescence spectroscopy as a widely employed analytical tool for trace element analysis is mainly delayed by the lack of intensive excitation sources for a lot of elements. During the last two years several groups have studied the possible use of dye lasers as sources of excitation in atomic fluorescence flame (or non-flame) spectrometry [l-3]. Today generation of stimulated emission from dyes is limited to the spectral range between 340 and 1200 nm. As it seems difficult to find a highly efficient laser dye for wavelengths shorter than 300 nm due to strong absorption by the molecules in the excited singlet states [4], direct excitation of the sensitive spectral absorption lines below 300 nm (especially those of the metal atoms, which play an important role in the environmental pollution problem, e.g., Pb, Ni, Hg) by dye laser emission will be impossible. These difficulties can be overcome, however, taking advantage of the well known techniques of frequency mixing in non-linear crystals. Intensive, frequency tunable light sources for the wavelength region between 26 1 and 360 nm have been realized by frequency doubling of visible dye laser radiation within ADP and KDP crystals [ 51. If the beamspread of the laser is reduced by mode selection, high peak power conversion efficiencies can be achieved even for moderate laser output power. Therefore, tunable narrowband UV line sources yield256
using frequency doubled dye working curves demonstrate
ing several kW of peak output the basis of flashlamp-pumped results of atomic fluorescence excitation sources for Mg, Ni, will be reported here.
power are supported on dye lasers. Some initial measurements with such and Pb in C2H2/air flames
2. Experimental The experimental configuration used for the investigations is shown in fig. 1. Compared with earlier work [6], the layout of the flashlamp-pumped dye laser system was changed. The rhodamine-6G solution was passed through a quartz dye cell having a length of 10 cm
GRA TING SPECTROGQAPH
,
Fig. 1. Schematic
diagram
of the experimental
set-up.
Volume
7, number
3
OPTICS
COMMUNICATIONS
and an internal diameter of 3 mm. Efficient excitation of the dye was achieved by optical pumping with a 4 inch linear flashlamp (Xenon Corp., Novatron 701 CR, pump energy 15-20 W, pump pulse length 1.3 I.tsec). As the laser system allows pulse repetition rates up to at least 10 set-l, rnost of the experiments were done with 1 to 3 laser shots per second*. The output of the laser was narrowed by a two stage wavelength selector consisting of an interference filter and a Fabry-Perot interferometer and frequency doubled in a 38 mm long KDP crystal. ‘The resulting bandwidth of the UV line was less than 0.02 A. Design and construction of the wavelength selector and details about the second harmonic generation have been described extensively[5, 61. Correct tuning of the generated W radiation to the resonance line of interest was achieved by observing the emission spectrum of a hollow cathode lamp, and the generated W line in the focal plane of a 1.5 m Czerny-Turner grating spectrograph by means of a UV image converter tube. A conventional nebulizer burner system (Zeiss Flame Emission Photometer PF5) was used for aspiration of the solutions in the laminar premixed C,H,/air flame and evaporation of the analyte. The fluorescence photons perpendicular to the direction of the exciting beam were detected by a fast response photomultiplier tube (RCA IP 28) amplified and integrated by a boxcar integrator (Princeton Applied Research, Model 160) and recorded on a strip chart recorder for evaluation. The fluorescence signal was separated from the broadband flame background radiation by- means of a prism monochromator v/l0 aperture, dispersion 75 A/mm for h = 300 nm). In all cases a slit width of 0.5 mm was chosen. The wavelengths used were 285.2 nm for Mg, 283.3 nm (absorbing line) and 405.8 nm (observed fluorescence line) for Pb, and 305.1 nm for Ni.
* For these experimental conditions more than 2 X 10’ flashes are possible without any destruction of the flashlamp. Using different dyes and different interference filters for spectral narrowing and tuning, the output wavelength of this dye laser could be continuously varied between 440 and 700 nm yielding at least 8- 10 kW peak output power and a spectral bandwidth of less than 0.4 nm. Further spectral narrowing with a Fabry-Perot interferometer should be possible without serious loss (less than a factor 2).
March
1973
3. Results and discussion The efficiency and capability of the described setup for spectroscopic investigations is demonstrated in fig. 2. The recorded fluorescence profile of the Ni line h = 305.1 nm under typical flame conditions was scanned by tuning the UV wavelength generated from the repetitively pulsed dye laser across the Ni absorption line. This spectral tuning was accomplished by continuous tilting of the Fabry-Perot interferometer within the laser cavity by means of a servomotor as a scan drive system. The position of the interference filter and the orientation of the non-linear crystal does not have to be changed for tuning intervals as small as 0.2 A. Never. theless, the UV power was constant for the whole tuning range as shown in the lower part crf the figure.
Fig. 2. Fluorescence line profile of the Ni line h = 305.1 nm scanned by the 0.02 A wide, frequency doubled output of the dye laser.
257
Volume
7. number
3
OPTICS
COMMUNICATIONS
March
K) ANALYTE
Fig. 3. Typical recordings of the fluorescence signal and the background radiation near the limit of detection for the Mg line h = 285.2 nm.
Typical recorder traces for Mg solutions near the detection limit are presented in fig. 3. The noise level of the blank signal, which is mainly caused by intensity fluctuations of the laser outputt, severely limits the sensitivity. The analytical curves for Mg, Ni, and Pb plotted in fig. 4 show fluorescence signal as a function of analyte concentration. The limit of detection has been taken as the concentration producing a signal to noise ratio of 2. These detection limits are summarized in table I together with some pertinent experimental conditions. The values for Mg and Pb are comparable with or better than the previously published atomic flame fluorescence spectrometric data obtained with different types of excitation sources [7-121. For the detection of Pb, the sample was irradiated with UV-radiation (A = 283.3 nm) and the fluorescence of the blue line h = 405.8 nm was monitored. Thereby, the fluorescence signal could be separated from the strong Rayleigh and Mie scatter. Whereas the intensity of the UV beam was too small to produce a population of the excited atomic level comparable with the saturation limited value for Ni and Pb,
Fig. 4. Analytical curves for laser excited of Mg, Ni, and Pb in a CaHz/air flame.
Detection Element
Mg
258
Wavelength
285.2 305.1 283.3 405.6
Ni Pb
18”
lf
lo’
I&dmll
atomic
fluorescence
Table 1 limits for Mg, Ni, and Pb in the CzHa/air
(rim)
I
t The fluctuations of the generated UV radiation which could be controlled by means of a beam splitter and a photocell amounted to about + 10% of the peak power.
CONCENTRATION
(absorption) (fluorescence)
d
$3
1973
flame
Approximate irradiance (kW/cm’)
Detection limit
2 6
0.0003 0.1
1*5
0.03
(ppm)
$7’ IRRADIANCE
1 (kW/cm2)
Fig. 5. Fluorescence signal observed in fhe Mg resonance line (h = 285.2 nm) as a function of exciting irradiance, demonstrating the occurrence of saturation.
Volume
7,
number 3
OPTICS COMMUNICATIONS
the influence of saturation phenomena [ 13- 151 on the Mg fluorescence signal was easily observable. The fluorescence signal for Mg observed for different values of excitation irradiance, which was lowered stepwise by neutral density filters over at least 4 orders of magnitude, is plotted in fig. 5. From this diagram it may be derived that the application of an irradiance of about 2 kW/cm2 for the Mg resonance line (h = 285.2 nm) should distinctly reduce the influence of laser fluctuations on the fluorescence signal. Therefore, the stronger increase in sensitivity for Mg detection becomes explainable. Due to the reabsorption effects within the flame, the fluorescence signal for the 100 ppm solution is smaller than that for 10 ppm solution (fig. 5) if the irradiance level was kept below 0.1 kW/cm2. If the irradiance is raised, the ground state of the sample is bleached, thus reducing reabsorption. For Ni atomic fluorescence detection limits of 3 ng/ ml have been reported for excitation in a H,/air flame [ 161. The poor value which was gotten from the fluorescence excited by the frequency doubled dye laser may be explained by the lower sensitivity of the line X = 305.1 nm compared with the normally used strong absorption line X = 232.0 nm and by the higher fluorescence quenching rate within the C2H2/air flame. Nevertheless, the value given here is at least 5 times smaller than the detection limit which was measured for fluorescence excitation of the 305.1 nm line in the Hz/air flame by a high intensity hollow cathode lamp ]161. The results for Mg indicate that an increase in sensitivity may be expected for other elements too, if the irradiance level may be raised. Certainly, the laser excited atomic fluorescence limits of detection could be improved by increasing the stability of the laser output or application of improved signal processing techniques. Recording the ratio of the fluorescence signal and the laser output power and compensation of un-
March 1973
specific background scatter by measuring the signal for two slightly different wavelengths, one of which is tuned to the center of the absorption line, the other outside of the absorption profile, should result in a 10 to 100 fold improvement in detection limits.
Acknowledgement The work on which this report is based was supported by the Bundesminister fur Bildung und Wissenschaft as part of the technology program of his ministry. References ] 1 ] L.M. Fraser and J.D. Winefordner, Anal. Chem. 43 (197 1) 1963; 44 (1972) 1444. [2] M.B. Denton and H.V. Malmstadt, Appl. Phys. Letters 18 (1971) 485, [3] J. Kuhl and G. Marowsky, Opt. Commun. 4 (1971) 125. [4] K.H. Drexhage, VII. International Quantum Electronics Conference, Montreal (1972). IS] J. Kuhl and H. Spitschan, Opt. Commun. 5 (1972) 382, and references therein. (61 J. Kuhl, G. Marowsky, P. Kunstmann and W. Schmidt, Z. Naturforsch. 27a (1972) 601. Acta 25B L71 D.G. Mitchell and A. Johansson, Spectrochim. (1970) 175. IS1 T.S. West and X.K. Williams, Anal. Chim. Acta 45 (1969) 27. 191 D.L. Manning and D. Heneage, At. Absorption Newsletters 6 (1967) 124. Acta 25B IlO1 D.H. Cotton and D.R. Jenkins, Spectrochim. (1970) 283. Llll V. Sychra and J.P. Matousek, Talanta 17 (1970) 363. [121 R.F. Browner, R.M. Dagnall and T.S. West, Anal. Chim. Acta 50 (1970) 37.5. J. Kuhl, S. Neumann and M. Kriese, to be published. E.H. Piepmeier, Spectrochim. Acta 27B (1972) 431. E.H. Piepmeier, Spectrochim. Acta 27B (1972) 445. J.P. Matousek and V. Sychra, Anal. Chem. 41 (1969) 518.
2.59