or more, and these can be determined accurately by the described procedure. Absorptions due to natural impurities extracted from blood are eliminated by this method. Ephedrine and papaverine, frequently present with theophylline, do not interfere because they show no differential absorption. Barbiturates, which are likely to interfere if present, are completely separated from theophylline using Extraction Procedure 2. Theophylline gives a maximum a t 272 nm when scanned against sodium hydroxide as reference, but it shows maxima a t 285 and 240 nm by differential spectrophotometry. The absorption maximum at 285 nm was used for estimating the amount of theophylline because no
interference by impurities was observed in this region of the spectrum. Of the several common drugs examined, morphine alone interfered with this determination. If morphine and theophylline both are present in a sample, they must be separated by thin layer or column chromatography before quantitation for theophylline can be attempted by this method. However, since morphine disappears from blood so rapidly, only in a very rare instance may the two drugs be found together in a blood sample. Received for review March 29, 1973. Accepted June 4, 1973.
Atomic and Molecular Absorption Measurements by Intra-Cavity Quenching of Laser Fluorescence H. W. Latz, H. F. Wyles, and I?.B. Green Clippinger Graduate Research Laboratories, Department of Chemistry, Ohio University, Athens, Ohio 45701
The interest in laser emission from fluorescent organic compounds, commonly referred to as dye lasers, has been steadily increasing because of the capability of tuning the laser emission over all wavelengths of the normal fluorescence band. Physicists have emphasized the use of these devices as sources of monochromatic radiation for a variety of spectroscopic applications whereas our interest has been in the utilization of the stimulated emission as an analytical signal in itself. Attempts to obtain stimulated emission from naturally occurring coumarins of analytical interest by flashlamp pumping have been unsuccessful to date. This led to a study of solvent effects on the lasing properties of 7-diethylamino-4-methylcoumarin,a compound which has been lased by many workers using flashlamp pumping. The results observed are predictable from normal fluorescence measurements but the effects are amplified by the spectral narrowing achieved with laser fluorescence. These same studies produced some anomalous results which are attributed to a cavity defect and led to a promising analytical technique for measuring trace absorptions of light by atoms and molecules. The purposeful introduction of NO2 into the resonant cavity of our laser produced a well defined absorption spectrum of this gas. A search of the literature revealed that workers a t the National Bureau of Standards had previously observed this phenomenon and had reported on the intra-cavity absorption of a pulsed rhodamine 6G laser by sodium vapor ( I ) and by solutions of E u ( N 0 3 ) ~( 2 ) . In each case, the wavelength absorbed from the broad-band laser emission corresponded to characteristic absorption wavelengths of the metals. More recently, selective absorption of a continuous wave rhodamine 6G laser by iodine vapor was reported by Hansch, Schawlow, and Toschek (3). The results of our studies with NO2 are reported here along with preliminary data on laser quenching due to cavity absorption by metal atoms in flames and by an organic compound in solution. N. C. Peterson, M . J. Kurylo, W. Brown, A . M . Bass, and R. A. Keller, J . Opt. SOC.Amer., 61, 746 (1971). R. A. Keller. E . F. Zalewski, and N. C. Peterson, J. Opt. SOC.Amer., 62, 319 (1972). T. W . Hansch, A . L. Schawlow, and P. E. Electron., QE-8, 802 (1972).
Toschek, I € € € J. Quantum
EXPERIMENTAL A Chromabeam 1050 Laser (Synergetics Research, Inc., Princeton, N.J.) was used for the solvent studies and for molecular absorption measurements. In the latter case, a 10- or 15-cm quartz absorption cell (Cary Instruments, Monrovia, Calif.) was positioned in the resonant cavity of the laser between the laser cell and the output mirror (Figure 1). The laser beam was directed into a spectrograph with a dispersion of 1 2 A/mm (Applied Research Laboratories, Inc., Glendale, Calif.). The spectra were recorded on Kodak Tri-X Pan film, one pulse a! the laser being sufficient, usually requiring a neutral density filter to diminish its intensity. Atomic absorption studies were conducted using a laboratory constructed laser with a co-axial flashlamp based on the design reported by Sorokin et d,( 4 ) . A laminar flow atomic absorption burner (Jarrell-Ash Division, Fisher Scientific Go., Pittsburgh, Pa.) modified to accept a three-slot burner head was positioned before the output mirror within the laser cavity. This provided a flame wider than the laser beam. A 0.5-meter Jarrell-Ash Monochromator was modified to accept a 35-mm camera body for recording of the laser output on Kodak Panatomic-X film. The monochromator has a dispersion of 16 A/mm.
RESULTS Solvent Effects. The typical broad-band laser output of rhodamine S in ethanol is shown in Figure 2. The tuning to lower wavelengths is achieved by lowering the concentration of the lasing compound and thereby reducing the self-absorption at these lower wavelengths. Figure 3 shows the range over which 7-diethylamino-4-methylcoumarin (DEAMC) can be concentration-tuned as well as the significant shift in output wavelength that results from changing the solvent. The shift to longer wavelengths is attributed to increased hydrogen bonding between the solvent and the lasing compound. The total shift is the same as that observed with normal fluorescence but the incremental changes are more obvious due to the narrower bandwidth of the laser emission. This large shift can be achieved continuously by the addition of water to an ethanol solution of DEAMC as shown in Figure 4. The lasing in this case is totally quenched when the water content is approximately 40%. The same shift and subsequent quenching occur when dihydroxy solvents are added to (4) P. P. Sorokin, J. R. Lankard, V. L. Chem. Phys., 48, 4726 (1968)
Moruzzi, and E. C. Hammond, J.
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Figure 5. Lasing characteristics of 3 X 10W3M solutions of 7diethyiamino-4-methyicoumarin in solvents of increasing dieiectric constant
J 546
(1) Propanol. (2) ethanol, (3) methanol. (4) acetonitrile. (5) N,N-dimalhylformamide
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Figure 2. Concentration tuning of rhodamine S in ethanol. The mercury 546.577,and 579 nm lines are also shown
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Figure 6. Laser emission from 7-diethyiamino-4-methyicoumarin I
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Figure 3. The effect of solvent and concentration on the lasing wavelengths of 7-diethyiamino-4-methyicoumarin 0 Propanbl. 0 ethanol. A methanol
i I 577 579
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Figure 4. The lasing wavelength of a 1 X 1 O - W solution of 7diethyiamino-4-methylcoumarin in ethanol-water mixtures 2406
* ANALYTICAL
Figure 7. intra-cavity quenching of a concentration tuned rhodamine 6G laser by increasing concentralions of NO2 The mercury calibration lines appear at the left and three series are shown Starting With spectra 1, 6. and 10 reading from top to bottom
CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
ethanol solutions and it appears that the quenching is due to bridge hydrogen bonding. One might suspect increasing dielectric constant of the solvent as a factor hut, as shown in Figure 5, nonhydrogen bonding solvents with higher dielectric constants shifted the output of the laser to suhstantially lower wavelengths. We have concluded that in the case of the alcohols, the dielectric constant effect is overcome by hydrogen bonding. The lasing behavior of rhodamine S lends further support to this conclusion in that its output wavelength decreases in linear fashion with increasing dielectric constant using the same series of solvents. Rhodamine S is also capable of hydrogen bonding hut it is a cation in solution and the dielectric constant effect predominates. Cavity Defects. The solvent studies revealed that a solution of DEAMC in methanol produced what appeared to be two-hand emission as shown in Figure 6. Deutsch et al., ( 5 ) had previously reported on gaps ohserved in the emission of rhodamine B and suggested the possibility of excited state absorption. In a later report on the same compound, Furumoto and Ceccon ( 6 ) concluded the gaps they observed were due to a transmission window of less than 1%in their totally reflecting mirror which appeared at the same wavelength where the gap was observed. We could not agree with this conclusion since we were not able to detect any transmission window in our totally reflecting mirror. However, the fact that our gap did not shift when the laser was tuned by varying concentration or solvents tended to support the existence of some type of cavity defect as opposed to a chemical effect. It was then found that changing the front mirror reduced the width of the gap and changing the totally reflecting mirror eliminated i t altogether. It therefore appears that the mirrors were responsible hut we have been unable to determine whether it was a particular combination of transmission windows or possibly a result of absorption by the dielectric coatings on the mirrors. The nature of these coatings is unknown to us. This occurrence did suggest that small energy losses within the resonant cavity at discrete wavelengths could result in quenching of a portion of the broad-hand laser emission. Molecular Quenching. The positioning of an ahsorption cell containing NO2 into the resonant cavity of a laser operated with an ethanol solution of rhodamine 6G produced results similar to those shown in Figure 7. The regions of the spectrum where nearly total quenching of laser action occurred corresponded exactly to two well resolved absorption hands of NO2 observed by normal ahsorption spectrophotometry. In addition, some of the fine structure that appeared in the laser quenching spectrum could also he assigned to small shoulder peaks in the normal spectrum. We conclude that the additional fine structure is also due to specific absorption transitions in NOz. A direct photographic comparison of absorption due to intra-cavity laser quenching with normal absorption was obtained by positioning the absorption cell inside and then outside the laser cavity with similar increasing concentrations of NOz. These results are illustrated in Figure 8 and clearly show the increase in detection limits for NO2 that can he realized hy employing the intra-cavity technique. It is also evident that the extent of quenching is directly related to concentration and that quantitation should he possible. At the present time, the limitations of our gas sampling equipment will only permit us to estimate that an NOz pressure of 1 mm of Hg corresponds to a concentration of less than 1 ppm. Absorption at this (5) T. F.
Deutsch.
M. Bass, P. Meyer, and
S. Prolopepa, A w l . Phys.
Len., 11.379(1967). (6) H. Furumotoand H. Ceccon. Appl. Phys. Lett.. 13, 335 (1968)
I I I 581 5 8 6 591 W A V E L E N G T H Inm)
Figure 8. A comparison of intra-cavity quenching by NO2 with normal absorption The top Spectrum was Obtained by lasing through an empty cell within the cavity. This is followed by 6 Spectra resulting from increased NO1 Concentration. The next series of six Spectra were obtained using Similar Concentrations Of NO2 with t h e cell positioned OUtSlde t h e laser cavity
u 577 5 7 9 586
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W A V E L E N G T H (nm) Figure 9. Reproducibility of intra-cavity quenching Top: rhodamine 6G laser spectrum followed by 3 spectra Of the Same NO2 Sample taken at 10-second intervals. Bottom: a fresh sample was introduced in each case
pressure is easily observed using a 10-cm path length cell.
It can also he reported a t this time that an increase in cavity absorption by higher concentrations of NOz results in a shift of the laser output to non-absorbing wavelengths-ie., the resonance energy is channeled into molecular transitions of the lasing compound where the radiation output is not being absorbed. This is evident in Figures I and 8 as increased laser intensity in regions where NO2 exhibits no absorption. Also, Figure 9 reveals
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Figure I O . Microdensitometer traces of rhodamine 6G laser emission with NO? inside the cavity The trace at the right represents a NO2 pressure of 6 mm and the trace at the left 7 mm. At t h e left of each trace are the mercury 5770 and 5791 Alines
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Figure 11. Intra-cavity quenching of a 7-diethylamino-4-methyl. coumarin laser by a solution of p-benzoquinone in hexane
that multiple pulses of the same sample of NOz result in a shift of the lasing wavelengths with absorption in a different region of the NOz spectrum whereas fresh samples provide for excellent reproducibility. Since the shift for the repetitive pulses is into a region of stronger NO2 ahsorption, this implies a lower NO2 concentration. We have concluded that the initial pulse converts NO2 t o NO by photolysis. The quantitative aspects of this technique require further investigation. The data presented here were obtained with rhodamine 6G which lases in a region of the spectrum where NOz absorption is relatively low. Studies are now in progress to determine if a compound that lases in a region where NOz has stronger absorption will be more sensitive to intra-cavity quenching. The photographic detection necessitated by the single pulse mode of operation that is a consequence of flashlamp pumping does not lend itself well to accurate quantitation. Microdensitometers are designed to measure the optical density of an emission line relative to the fairly constant background of unexposed film. In our situation, we are attempting to measure absorption lines relative to a laser intensity that may vary as a result of pumping efficiency or channeling of the output intensity into other wavelengths as a result of absorp2408
WAVELENGTH(nm) Figure 12. Dye laser spectral emission Tap: Rhodamine 80 laser output band. Middle: Band with absorption from 0.01 Ppm Na in an intra-Cavity flame. Bottom: Band with absorption from 20 ppm Na in an intra-cavity flame
tion itself. The microdensitometer traces of NO2 absorption shown in Figure 10 serve to demonstrate that slight changes in NO2 concentration can be measured with a densitometer. Linear plots based on the depth of an absorption peak have been obtained for NO2 over limited concentration ranges where the shift in lasing wavelength is negligible and when laser intensity has been reasonably constant. In one experiment, varying concentrations of pbenzoquinone in hexane were placed in the cavity to determine if condensed phase absorption could be detected. The absorptions were faint but detectable for the absorption band a t 454 nm where the molar absorptivity is 16.6.
ANALYTICAL CHEMISTRY. VOL. 45, NO. 14. DECEMBEIR 1973
Microdensitometer scans were obtained and the laser bandwidth at the absorption maxima was used as a measure of laser intensity. The quantitative results obtained are shown in Figure 11. The ratio of absorption to bandwidth falls off at the higher concentration as total quenching of laser action commences. It is the opinion of the authors that continuous wave laser operation using nitrogen laser pumping in conjunction with photometric detection will be far superior to the preliminary approach described here for quantitative measurements. Atomic Quenching. Our observations of molecular quenching led naturally to a consideration of other absorbing species. The discovery that Peterson et al. (I), had observed sodium vapor quenching using heated glass cells prompted an investigation into the use of an atomic absorption flame within the resonant cavity of the laboratory constructed laser. During our ensuing investigation, it was noted that Ba+ and Sr have been detected as a transient species in an air-acetylene flame within a dye laser cavity (7). Initially, no problems were encountered in achieving normal laser output from rhodamine 6G with an air-acetylene flame in the cavity. Sodium-doublet absorption centered a t 589.2 nm was observed the first time a standard solution containing 60 ppm sodium was aspirated into the flame. Figure 12 shows the typical photographic data obtained in these experiments. Attempts to quantitate Na absorption have met with limited success. Absorptions from the laser band can consistently be seen for a wide range of concentrations if a number of parameters are optimized. These conditions include laser output, film speed, flame conditions, and band position. A linear concentration-absorption relationship has been seen over a narrow, low ppm range, but factors which we believe are inherent to the system itself have prevented definitive quantitation. The possibility of scatter by water droplets in the flame cannot be eliminated as a possible factor contributing to a variable laser output. Sodium absorption has been detected using solutions containing as little as
0.01 ppm sodium but the variability of the laser has prevented the establishment of any relationship between concentration and the extent of absorption. Solutions of mercury and barium salts have also been investigated. No problem has been encountered with barium since its resonance line lies within the range of fluorescein. In initial experiments, absorption has been detected using solutions containing 100 ppm barium. Rhodamine 6G has been used to detect mercury absorption at 577 and 579 nm and the fluorescein compounds have been used for the 546 nm absorption line. Absorption has been noted for only a few of the many trials using 25-0.1 ppm mercury solutions. This is as expected because absorption at these wavelengths is by excited state atoms, whose population is low in an air-acetylene flame. At the present time, the mercury resonance line at 254 nm is beyond the range of available laser fluorescence.
(7) R. J . Thrash, H. von Weyssenhoff, and J. S. Shirk, J. Chem. Phys., 55.4659 (1971).
Received for review February 8, 1973. Accepted May 7 , 1973.
CONCLUSIONS The preliminary data reported here and by the other workers cited strongly indicate that intra-cavity quenching of laser fluorescence by absorbing species placed within the resonant cavity of a laser can be used to significantly enhance absorption measurements for the purpose of trace analysis. We feel this technique is a viable one, generally applicable to the determination of metals, permanent gases, and, very probably, organic vapors at low concentrations. In the specific case of NOz, it could be immediately adapted for air pollution studies. The increased sensitivity, the limit of which has not been established, reduces the need for long path length absorption cells and/or sample concentration and conversion prior to measurement. At the present time, our studies are equipment limited, but we intuitively feel that continuous wave operation of the laser using nitrogen laser pumping and optical tuning will prove superior to the present pulsed mode. The ability to use photometric instead of photographic detection will certainly simplify quantitative studies.
Determination of Fluorine in Petroleum and Petroleum Process Catalysts with a Fluoride Electrode John Nevi1 Wilson and C. 2 . Marczewski The British Petroleum Company Limited, BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middiesex. England
Fluoridation is known to enhance the activity of certain petroleum catalysts as shown, for example, by recent patents on the properties and use of fluorided hydrogenation catalysts (1-3). It may be required to determine fluorine levels in such catalysts during their period of use. Traces of organic fluorine can be produced in the processes, which may adversely affect subsequent operations. Fluo(1) C. Claude, E. E. Neel, and A . P. Anseime, U.K. Patent 1189930 (1970). (2) Robert J . White and Robert J. Houston, U.S. Paten1 3435085 (1969). (3) G. E. Elliot, Jr., J . Salomon, and R. F. Vogei, U.K. Patent 1277998 (1972).
rine must therefore be determined in process streams and products, sometimes at sub-ppm levels. Environmental levels of fluorine are important because the element is an essential trace nutrient, but may cause the fluorosis of living matter if present in excessive concentrations. Data have been published of the fluorine levels in many materials, including soil ( 4 ) and coal ( 5 ) and a variety of foods. Until this present work, data have not been available for crude oils so far as we are aware, although figures for bromine and iodine have been pub(4) H A Shroeder, Arch E n v ~ r o nHeaith 21, 798 (1970) ( 5 ) F V Bethell J lnst Fuel 36,485 (1963)
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