Sodium Analysis with a Tunable Dye Laser Jurgen Kuhl, Gerd Marowsky,’ and Reimund Torge Carl Zeiss Forschungsgruppe, Oberkochen, Germany
IN ATOMIC ABSORPTION SPECTROMETRY, specific spectral interferences by background absorption and scattering are reported ( I , 2). Although the number of known interferences in the visible range of the spectrum is very small, they may impose a serious limitation on the limit of detectionespecially when highly concentrated sample solutions are analyzed. Background absorption may be caused by molecular absorption or by absorption of impurities aspirated into the flame. To correct these interferences, various experimental setups were applied : continuoussource compensation using a deuterium arc (3), or two light sources (4), or unabsorbed lines of the cathode lamp (5). The tunability of the dye laser permits the scanning of wavelengths in the vicinity of the absorption line within a spectral range of 3 A by tilting the Fabry-Perot interferometer. Compensation for possible background interferences can be achieved with the same light source by measuring the absorption in the line center and adjacent wavelengths. The effectiveness of dye lasers for analytical purposes was demonstrated by Sandford and Gibson ( 6 ) , who determined the sodium distribution in the upper atmasphere with a tunable dye laser lidar system. To test the compensation method and the capability of our laser setup, analytical experiments with a sodium vapor cell were performed. Additional work is being done t o demonstrate the compensation of a variable background absorption by CaO in the range of the sodium resonance lines. EXPERIMENTAL For most analytical applications of organic dye lasers, the bandpidth of the laser emission is rather broad (about 100200 A). Several devices to narrow the frequency output have been described in the literature (7-10). The emission band width of the flashlamp pumped dye laser (rhodamine 6G, 10-4M0in methanol) was internally A by a two-stage filter system. narrowed down to 5 x This system 5onsisted of a narrow-band inierference filter (halfwidth 6 A , peak transmittance a t 5920 A of 84z) and a Fabry-Perot interferometer (separation of the plates 0.3 mm, reflectivity about 8 2 z ) . The laser output was tuned by tilting both frequency-selective elements relative to the resonator axis. Continuous tuning was possible from 5700 1 Present address, Max-Planck-Institut fur Biophysikalische Chemie, Abteilung Laser-Physik, D-34 Gottingen-Nikolausberg, Am Fassberg.
MI
SP
Figure 1. Experimental setup, schematic (MI, microscope; SP, spectrograph; PM, photomultiplier; F, filter; SAC, sodium absorption cell; PC, photocell; DS, diffusing screen; L l . . .L3, lenses, SL, sodium vapor lamp) Lower left: Oscillogram of multiplier (la)and photocell (lo) signals to 6000 A. Figure 1 shows schematically the arrangement of dye laser, spectrograph, sodium absorption cell, photomultiplier, and photocell for oscillographic recording of absorption and reference signals. The coarse adjustment of the laser output to the sodium resonance line could be controlled in the image plane of a high-resolution 3-prism spectrograph. By observation with a microscope, the laser line could be brought to coincidence with a specific absorption line of a sodium reference spectrum of a vapor lamp. Fine adjustment and scanning of the wavelength were done by turning the Fabry-Perot interferometer in steps of some seconds of arc. The mechanical stability of the laser setup permitted reproducible acjustment A interval. of the emission wavelength to within a 2.5 X The laser pulse length was about 1 psec; its output energy was about 1 mJ. [For further details c f . (II).] The absorption of the laser beam in the sodium cell was recorded by a photomultiplier (PM), shielded with a gray filter and a wide-band interference filter for screening of the laser pumping light. To eliminate fluctuations from shot to shot and variations of the output energy caused by the scanning of the wavelength, an aliquot part I , of the incident light was registered by a photocell (PC). Both signals could be observed on a double-beam 50-MHz oscilloscope. RESULTS AND DISCUSSION
(1) S . R. Koirtyohann and E. E. Pickett, ANAL.CHEM., 37, 601 (1965). (2) Zbid., 38, 585 (1966). (3) H. L. Kahn, At. Absorption Newslett., 7, 40 (1968). (4) H. Massmann, Spectrochim. Acta, 23B, 215 (1968). (5) J. B. Willis, “Methods of Biochemical Analysis,” Vol. 11, David Glick, Ed., Interscience, New York, N. Y., 1964. (6) M. C. W. Sandford and A. J. Gibson, J. Atmos. Terr. Phys., 32, 1423 (1970). (7) B. H. Soffer and B. B. McFarland, Appl. Phys. Left., 10, 266 (1967). (8) F. P. Schafer and H. Muller, Opt. Commun., 2,407 (1971). (9) D. J. Bradley, A. J. F. Durrant, G. M. Gate, M. Moore, and P. D. Smith, IEEE J . Quantum Electron., QE-4, 707 (1968). (10) H. Walther and J. I-.Hall, Appl. Phys. Letr., 17, 239 (1970).
To test the stability and tunability of the applied dye laser, an absorption profile of the Dz line was recorded at a vapor density of 500 ng/cm3 in the sodium cell. Figure 2 shows the absorbance ( A ) as a function of the angular position (p) of the Fabry-Perot interferometer corresponding to known variations of the output wavelength. A beam of 1.9-mm diameter was singled out of the enlarged laser beam (6-mm diameter) and sent into the sodium absorption cell of 350-mm length; 20 t o 40 shots were registered for each measuring point. Figure 2 shows the precision data which amount t o (11) W. Schmidt, Laser, 4, 47 (1970).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
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Figure 2. bbsorption profile of sodium D2resonance line at 5890 A
A
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1
I
100
I
300
500
Figure 3. Analytical curve, absorbance ( A ) os. sodium vapor density (c) in ng/cm3
interval from 2 to 500 ng/cm3 by alternation of the heating current. The given numerical values indicate the estimated maximum sodium concentration, since computation of vapor pressure and density were based on the maximum temperature within the cell. The shape of A ( c ) corresponds to Lambert’s law. The limit of detection of about 2 ng of sodium or less may be increased by improving the electronic equipment. In our first experiments, the laser emission was manually adjusted to the line center and t o the adjacent wavelength. The use of a cw laser, periodic fine tuning, and computerized evaluation should improve sensitivity and precision of the method. The main advantage of the described dye laser system for atomic absorption spectrometry brought about is the tunability of this monochromatic light source. By detuning the laser emission, interfering background absorption can be eliminated, and multielement analysis achieved with the same light source, provided the absorption lines of interest lie within the working range of the dye. The resulting sensitivity (2 ng/cm3) is comparable t o the limit of detection (5 to 1 ppb) reported for conventional atomic absorption spectrometry. The problem of absorption measurements, namely the separation of a weak absorption signal and an intensive background signal, could not be solved by using a laser light source instead of a hollow cathode lamp. Variations of the laser output could not be completely compensated by the above described technique. Thus the resulting signal to noise ratio of our experimental arrangement was due to these variations. We could detect a minimum change in transmission of 0.1 by averaging the results of about 50 shots. Application of the dye laser in atomic fluorescence spectrometry considerably increases the sensitivity of the method. A series of analyses has been carried out with an experimental setup similar to the one described above. Results obtained with this setup showed an increase in sensitivity by a factor of lo3 to lo4. Details of this investigation will be published elsewhere (12).
three times the standard deviation of the mean value. The zero point of absorption was adjusted by a 0.1-A detuning of the laser. The absorbance A was obtained by forming the logarithm of the reciprocal transmittance T, defined by:
A similar profile was observed in fluorescence by Walther and Hall (10). They narrowed down the laser output with an electrically tunable birefringent filter (Lyot filter). Figure 3 shows the result of a first quantitative sodium analysis. The sodium vapor density was varied within an
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ACKNOWLEDGMENT
Our thanks are due to R. Baier for his valuable assistance. RECEIVED for review May 18, 1971. Accepted July 6, 1971. The work carried out in connection with this report was sponsored by funds of the Bundesminister fur Bildung und Wissenschaft (Minister of Education and Science), who is not responsible for the correctness and completeness of the contents nor for the protection of the rights of third parties. ~~~
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(12) J. Kuhl and G Marowsky, in preparation.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2. FEBRUARY 1972
