Quantitative Dye Laser Amplified Absorption Spectrometry Robert C. Spiker, Jr.,l and James S. Shirk Department of Chemistry, lllinois lnstifute of Technology, Chicago, 111. 60676
If the sample is included inside the optical cavity of a broad band organic dye laser, it is possible to use weak absorptions for quantitative analysis. For Ho3+ and Pr3+ to M with optical solutions in the range 5 X densities in the range -0.005 to -0.0006, the laser intensity is related empirically to the sample concentration. The method can be used for gases and for atomic absorptions as well as for solutions. The sensitivity of the technique may be increased several orders of magnitude.
Dye laser technology has progressed rapidly. There are now commercially available flashlamp pum$ed dye lasers covering the wavelengt,h range 3500 A into the near infrared. A variety of spectroscopic and analytical uses has been devised for these lasers. Recently we, and others, have reported a laser technique for detecting very weak absorptions (2-4). In this technique, an organic dye laser that is operating with a broad band (10-20 nm bandwidth) output is used. The sample is placed within the optical cavity of the laser. When an absorber is placed in the laser cavity, the laser action is substantially decreased or completely quenched a t the absorbed wavelengths. The wavelength dependent losses introduced by even a very weak absorber decrease the laser output intensity selectively. By directing the output into a monochromator or a spectrograph, it is possible to use a dye laser to detect weak absorptions. Sample absorptions with optical densities (log I o / n equal to 5 x 10-5 or smaller have been detected by this method ( I ) . This technique has been used on gaseous and solution samples. It has also been used to detect atomic absorptions in flames and free radicals produced by flash photolysis. It is clear t h a t this technique can be used to increase the sensitivity of an absorption measurement by two to three orders of magnitude beyond conventional methods, with the possibility of an even greater enhancement. Earlier work demonstrated that this technique can be used for qualitative detection of species with very weak absorptions. For an analytical chemist, there remained the question as to whether the technique could be used for quantitative analysis of molecules with weak absorptions. In this paper, we give results which show that, with the proper calibration, the technique can be used for quantitative analysis. Reliable calibration graphs of laser intensity us. concentration of some rare earth ion (Ho3+, Pr3+) solutions are constructed. Rare earth ion solutions were used for two reasons. The half width of the absorption bands is about 1 nm; thus, it is easy to ensure that the slit width of our monochromator is narrow with respect to the Present address. NIAMD, National Institutes of Health. Bethesda, Md. 20014. (1) R . J. Thrash, H. von Weyssenhoff.and James S. Shirk, J. Chem. Phys., 55,4659 (1971).
(2) N . C. Peterson, M . J . Kurylo, W . Braun, A. M. Bass, and R. A . Kel-
ler, J. Opt. SOC.Arner., 61, 746 (1971). (3) R. A . Keller, E. F. Zalewski. and N . C . Peterson, J. Opt. SOC. Arner.. 62,319 (1972). (4) T . W. Hansch, A. L. Schawlow, and P. E. Toschek, / E € € J. Quantum Electron., 10, 802 (1972)
absorption band width, and the absorption bands are sufficiently weak so that direct spectrophotometry is not applicable to dilute solutions.
THEORY The origin of the sensitivity of the intracavity absorption technique can be understood simply. We define a quantity G = a / L , where a is the unsaturated single pass gain in passing through the dye solution and L is the sum of the losses in the cavity, including the losses due to absorption by the sample. When G ; ; 1, the gain is sufficient to overcome the losses and lasing will occur. In any laser, both G and, consequently, the laser intensity will be wavelength dependent. Figure 1 gives an idealized picture of G us. wavelength for a broadband organic dye laser with a weak absorber inside the optical cavity. A small intracavity absorption increases the cavity losses and thus decreases G where the absorption occurs. This small change in G causes a dramatic change in the intensity of the laser output. By monitoring the wavelength dependence of the intensity of the laser output, it is possible to obtain a spectrum of a very weak absorber. For quantitative studies, it is expected that the intensity of the laser might be a rather complex function of G, and thus also of the losses due to the absorption by a sample inside the cavity. The calculation of the intensity of a dye laser as a function of small additional losses in the cavity is difficult, and the result is not yet available in closed form for a satisfactory model. In this paper, we construct an empirical correlation between sample absorption and laser intensity that is useful for quantitative analysis. We report our data as plots of I o / I us. sample concentration, where IO is the laser intensity with no sample in the cavity and I is the laser intensity with a sample in the cavity. We also use plots of concentration us. AI/Io ( 5 )where II= Io - I. Theoretical treatments of dye laser quenching are available. Hansch, Schawlow, and Toschek (4) gave a description of the quenching of a continuous dye laser by an intracavity absorber. However, they calculated only the ultimate sensitivity of the technique and not the functional form of the dependence of the laser intensity upon the extinction coefficient of the absorber. Keller, Zalewski, and Peterson (3) numerically solved a set of coupled differential equations for a model of a pulsed dye laser. Their results imply that a quantitative relationship between sample absorbance and laser intensity is possible for a particular laser; however, their results are not given in a useful closed form. EXPERIMENTAL Apparatus. Figure 2 shows a diagram of the experimental apparatus. A commercial (Chromabeam 1050, Synergetics Research Inc, Princeton, N . J . ) dye laser was used. In order to obtain a broad band output, the intracavity grating was replaced with a 1.0-m radius of curvature mirror coated for maximum reflectivity in the range 450-650 nm. The dyes used were 7-diethylamino-4methylcoumarin or Rhodamine 6G. both at 4 X 10-5M in etha-
(5( T. P. Belikova, E. A . Sviridenkov, A . F. Suchkov, L. V. Titova. and S. S. Churilov, Sov. P h y s . J E T P , 35, 1076 (1972).
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Energy from the laser power supply (LPS) is discharged through a lamp coaxial with the dye tube (DT). A p u m p ( D P ) circulates t h e dye. Mirrors M , and M2 form the optical cavity of the laser; the sample is contained in a cuvette ( C ) . The laser output passes through a neutral density filter ( N D F ) and via mirrors M3-M5 to a ground glass scatter plate ( G ) in front of the entrance slit to a monochromator ( S ) .The output of a photomultiplier detector ( P M ) at the exit slit is fed to an oscilloscope (0)and a multichannel pulse height analyzer ( P H A )
photomultiplier. The detector output was fed to an oscilloscope for monitoring and to a 100-channel pulse height analyzer where the laser intensity was recorded. Some quantitative studies were made using photographic recording of the laser output. In this case, the output of the laser was directed onto the slit of a 1.5-m Wadsworth spectrograph with a dispersion of 11 A/mm. The spectra were recorded on Kodak SA-1 film. All the spectra of sample solutions, as well as exposures of the laser through neutral density filters for calibration of the film, were recorded a n d developed at the same time. Densitometry of the film was performed on a modified Hilger densitometer. Reagents. Holmium chloride (99.970, anhydrous, Research Organic/Inorganic Chemical Corp.) and praseodymium chloride (hydrated, 99.9'70, Research Organic/Inorganic Chemical Corp.) were used without further purification. Samples of the rare earth chlorides (1 X 10-4-1 X 10-2M in 0.1N HCl) were prepared from stock solutions. Procedure. In a typical experiment, a sample or a blank ( 0 . W HC1) was placed in the laser cavity. T h e xenon flashlamp was discharged manually a t a typical input energy of 61.5 J at 20 kV. This energy was well above the lasing threshold for either dye, Special care was taken to ensure a constant input energy to the flashlamp. The intensity of the laser at a particular wavelength was measured as the peak height of the resulting photomultiplier pulse. Prior to each experiment, the response of the photomultiplier and the electronics were checked for linearity by placing neutral density filters in the laser beam and ensuring that the measured pulse height was indeed proportional to laser intensity.
nol. The laser is pumped by a coaxial xenon flashlamp. The sample solutions were in a rectangular cuvette with a 1-cm light path. The cuvette was placed inside the laser cavity approximately equidistant between the dye cell and the output mirror. The laser output passed through a n 0.d. = 0.5 neutral density filter and onto a ground-glass scatter plate placed directly in front of the entrance slit to a monochromator. This arrangement reduced the laser intensity to prevent overloading the detector and removed the effect of any spatial inhomogeneities in the laser beam. The monochromator was a 0.3-m Hilger Engis Model 600. Slit widths were 5 or 10 pm, ensuring t h a t the slit width was less than the half width of the sample absorptions. The detector was a 1P28A
RESULTS AND DISCUSSION Holmium Determination. Figure 3 shows a plot of' I o / I us. concentration of holmium ions in a O.1N HC1 solution. As can be seen, the plot is nearly linear in the 3 x 1 0 - 4 to 4 X 10-3M concentration range, but curves off at both high and low concentrations. Figure 4 shows a plot of AI/& for the same data. It is linear for low concentrations but falls off a t high concentrations. The graph of Zo/I seems more satisfactory, so Zo/I is used to express our results in the rest of this paper. The error bars in these graphs are the standard error ( = a / d / n ) for all the mea-
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Figure 2. Experimental apparatus
ANALYTICAL CHEMISTRY, VOL. 46, NO. 4, APRIL 1974 * 573
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cavity. Concentrations of holmium ions in the range 0.81 to 5.42 mg/ml were investigated using the coumarin dye. Densitometry of the film provided the data for a calibration curve. This calibration curve could then be used for the analysis of unknown concentrations of holmium ions. The limit of detectability using this technique was not as low as that for the electronic detection, and this technique proved to be more time consuming.
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CONCLUSIONS
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C o n c e n t r a t i o n Pr+*+x I O Figure 5. Laser intensity at 588.3 n m vs. Pr3+ concentration surements on the solutions. These results were reproducible from day to day. Each point represents measurements recorded on from 2 to 5 different days. The graph depicts an easily constructed calibration curve from which unknown concentrations of holmium ions in solution may be ascertained. The calibration curve is, of course, sensitive to the laser parameters. Several important parameters were closely monitored to obtain the observed reproducibility. The spectral slit width was narrow compared to the bandwidth of the holmium absorption. The input energy to the laser was kept constant. The laser was operated well above the lasing threshold of each dye to minimize the effect of variations in the dye. We found that the coumarin dye decomposed slowly and we corrected for it. Finally, we were careful to ensure that we were operating in a linear region of the photomultiplier. With these precautions, we found that for the fiftyfold change in concentration (16 solutions) studied, only two solutions showed relative standard errors of greater than 5%. Such reproducibility is an important factor in making the technique useful for quantitative analysis. Praseodymium Determination. Figure 5 displays the calibration curve obtained for solutions of praseodymium to 5 X chloride ranging in concentration from 1 X 10-3M. The relatively weak 588.3-nm absorption band of praseodymium was used. Again the results were easily reproduced. Each point represents measurements from at least 3 different days. It should be observed that this curve is similar to the Ho3+ calibration curve. For both holmium and praseodymium solutions, concentrations of greater than 10-2M quenched the lasing action enough so that detection of a signal above the noise level was not possible. Photographic Technique. Preliminary to the use of the photomultiplier detector, we used photographic recording of the laser output with Ho3+ solutions inside the laser
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We have demonstrated that it is possible to use the intracavity absorption technique with a dye laser for quantitative analysis. &liable calibration curves have been constructed for our laser for H o ~ +and Pr3+ solutions in the range 5 x I O L 3 to 10-4M. This corresponds to holmium concentrations down to 27 pg/ml (ppm) using the 450.4nm Ho3+ absorption band. Thus, the intracavity absorption technique is ca. 2 orders of magnitude more sensitive than direct spectrophotometry of the solution. Our technique is not, however, as sensitive as atomic absorption or atomic emission methods (6). Even though this technique is not yet the most sensitive technique available for the analysis of H o ~ +and Pr3+ ions, we have demonstrated that it is possible to extend the sensitivity of a particular technique (spectrophotometry of rare earth ion solutions) by two orders of magnitude. We expect that the sensitivity of the atomic absorption technique can be similarly extended by intracavity absorption. In fact, preliminary work in our laboratory with Sr and Na atomic absorptions indicates that this is readily accomplished. We did not choose atomic absorption for these studies because of the narrow slit width required, for absorption studies with atomic lines. More sophisticated detection techniques are necessary in these cases. In this study. the operating parameters of the laser were chosen to ensure reproducible results. Different lasers or changes in the operating characteristics of the same laser will give different calibration curves. It is, for example, possible to increase the sensitivity of the intracavity absorption technique by operating the laser nearer threshold. However, near threshold the laser will become more sensitive to small changes in operating characteristics. The ultimate limit on the sensitivity may arise from instabilities in the laser, although it should be possible to approach the gain of 107 in sensitivity that Hansch e t al. ( 4 ) calculate for the intracavity absorption technique. ACKNOWLEDGMENT We thank Harley Borders for experimental assistance. Received for review August 17, 1973. Accepted November 19, 1973. We thank the Research Corporation and the Illinois Institute of Technology for financial assistance. (6) N. Omenetto, N. N. Hatch, L. M . Fraser, and J, D. Winefordner, Ana/. Chem., 45, 195 (1973).
