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Anal. Chem. 1983, 55,978-981 020,
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Figure 2. Laser tuning curve with etaion inside the laser cavity.
Figure 1. Spectrum of laser output with etaion inside the laser cavity. Three modes are displayed with an intermode frequency spacing of -0.04 cm-'. Free spectral range of spectrum analyzer-0.27 cm-'.
A three-plate birefringent fiiter (Lyot fiiter) placed intracavity yields a line width of 0.67-1.33cm-l depending on pump laser power. Spatial hole burning within the dye jet ( I ) reduced the number of lasing modes so that addition of the 0.5-mm etalon yields the laser spectrum shown in Figure 1 (2, 3). Output is detected at three bands spaced by 0.04 cm-l with the central band being most intense. On occasion it is possible to obtain lasing at only two bands of equal intensity separated by 0.04 cm-l (see Figure 2 in ref 3). Single frequency operation in the commercial Coherent 599 is achieved with two etalons, a Brewster angle plate and the birefringent filter. Scanning the laser wavelength while maintaining single frequency operation requires simultaneous adjustment of the two etalons, the Brewster angle plate and a cavity folding mirror. As was mentioned in the introduction, the single frequency scan range is 1 cm-l. Mode-hopping occurs at the limits of the scan. When operated correctly, the laser power remains constant over the 1 cm-l range. In contrast, the system which uses only a three-plate birefringent filter and 0.5-mm etalon to select laser wavelength is much simpler. Tilting the etalon while keeping the birefringent filter fixed yields the laser power vs. output wavelength results shown as curve A in Figure 2. The dye laser output drops to half power after a wavelength change of 3 cm-l. This represents a 3-fold increase in scan range over the single frequency laser and is effected simply by a single tuning element. During the scan the dye laser bandwidth is 50.08 cm-l. If the etalon is mounted on a motor, reproducible scans with cumulative data averaging are straightforward. We have found that a Scanner Control CCX 102 and motor (General Scanning, Inc.) are particularly well suited for this task. Further improvement is achieved by optimizing the birefringent filter position while scanning the etalon, curve B in
Figure 2. The output power drops by half at 5 cm-l from the starting frequency. It should be emphasized that the etalon and birefringent filter need not be scanned in unison. The etalon can be swept continuously and occasional adjustments made to the birefringent filter to maximize laser power. Cessation of lasing occurs when the etalon tilt becomes so large that walk-off losses exceed the system gain. Note also, that this scan procedure does not alter the dye laser cavity length. Therefore the laser mode hops between cavity modes as the etalon is tilted yielding a scan in discrete jumps of -0.009 cm-l. For the data shown in curve B of Figure 2 the etalon was initially set at 1.2O from the laser beam axis. This corresponded to a frequency of 16929.78 f 0.03 cm-l. The final data point was obtained a t an etalon angle of 5.7". Although we have not explored the system in detail, we have also used this technique to measure the frequency of a commercial ring dye laser with the Burleigh wavemeter in the high resolution mode. As with the procedure used to obtain the data shown in Figure 2,the ring laser frequency is controlled by only the thin etalon and a Lyot filter (i.e., Brewster plate and air-spaced etalon removed). The difference between the ring dye laser and the linear dye laser is that the ring laser always operates single mode. Note, however, the frequency of that mode shifts with time within the 0.08 cm-l band-pass of the etalon. Registry No. Rhodamine 6G, 989-38-8;ethylene glycol, 107-21-1.
LITERATURE CITED (1) Pike, C. T. Opt. Commun. 1974, IO, 14-17. (2) Green, R. B.; Travis, J. C.; Keller, R. A. Anal. Chem. 1976, 4 8 , 1954-1959. (3) Mayo, S.; Keiier, R. A,; Travis, J. C.; Green, R. B. J . Appl. Phys. 1976, 4 7 , 4012-4016.
RECEIVED for review October 8, 1982. Accepted January 17, 1983. Work performed under the auspices of the U.S. Department of Energy.
Photoacoustic Spectroscopy in Gases Based on Wavelength Modulation Bernard C. Yip and Edward S. Yeung* Deparh78nt of Chemistry and Ames Laboratory, Iowa State University, Ames, Iowa 5007 1
With the development of new sources of energy, we must simultaneously strive for "continued improvement in the analytical chemical methods needed to monitor, control and study the environment" ( I ) . Laser spectroscopic methods have gained in popularity because of certain advantages they offer
( 2 , 3 ) ,specifically the coherence needed for long path measurements (4),the power needed for detectability (5),and the monochromaticity needed for selectivity (6). In the infrared region, photoacoustic spectrometry (PAS) is important because of the low detectabilites that are generally possible (7).
0003-2700/83/0355-0978$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983 80 979
Howevler, there are many problems that must be dealt with to minimize 1)ackground contributions to the signal. The ultimate detectable PAS signal is limited by noise in the transducer preamplifier and noise caused by Brownian motion of the molecules (8,9). In practice, this ultimate limit is not achieved because of absorption in the cell windows and cell walls, which also generates an acoustic signal. This background contribution is also proportional to the intensity of the exciting radiation, so that simply increasing the latter to increase the PAS signal of the analyte offers no advantage. Several methods have been suggested for discriminating against contributions from absorption in the cell itself. A differential cell design in principle can compensate for the background (IO),but it is very difficult to match the reference and the sample chambers, both in magnitude and in the phase of the background signal. A resonant cell can be built to create standing acoustic waves inside, such that the windows are at well-defined nodes (11).The nature of the resonance obviously depends on the exact composition of the gas inside and may not be easily controlled from one sample to the next. The length of the cell can be increased to increase the relative signals from the gas vs. the cell or to change the relative phases of the two acoustic signals (12). The extent of improvement is limited, especially since the volume must be increased substantially. I t is possible to use the Stark or the Zeeman effect to modulate the laser wavelength or the absorption wavelength to provide selectivity (13-15). Typical modulation extents are fairly limited because of the magnitudes of these molecular constants, so that the gas sample must be at low pressures to reduce the effect of pressure broadening (16). ‘The PAS signal, however, in certain cases is dependent on the total pressure (17), and detectability may thus be compromised. Furthermore, the large fields needed introduce additional noise in the microphone. A promising scheme for reducing background contribution is wavelength modulation of the light source. In the infrared region, the types of lasers available and the spectral line widths involved do not allow simple adaptation of techniques that work in the visible region (18, 19). Color-center lasers (20) are similar to other visible lasers, but the output wavelengths are broad (ca. 0.1 cm-l) and have yet to span the “atmospheric window” region. Diode lasers (21) can be wavelength modulated by current pulses, but their low powers have thus far limited their use in photoacoustic pollution applications. The spin-flip Raman lasers (22) have potential, but the instrumentation is still too complex for routine use. It is therefore of interest to develop wavelength modulation schemes based on discrete-line moderate-power continuous-wave gas lasers. The inherent widths of these lines are of the order of a few megahertz, so t,hat modulation must involve consecutive vibration-rotation lines rather than be within the normal gain curve of a laser line. The modulation frequency must be of the order of 100 Hz to be compatible with PAS experiments and the modulation must be almost 100% to maintain the sensitivity. We report here an adaptation of a commercial COZ laser for tlhis purpose, as well as the experimental verification of its advantages.
EtXPERIMENTAL SECTION Laser. The laser used is a continuous wave (CW) COz laser of conventional design (Molectron, Sunnyvale, CA, Model C-250) with a flowing gas mixture of about 15 torr He and 2 torr COP Even though the laser is capable of a 50 W output at the main laser lines, only 2-3 W was used in this study. The laser is (=l/d) grating and a piezoelectric equipped with 80 lines/” transducer to fiine-tune the cavity length. Since it is necessary to tune from one rotational line to the next, modulation of the grating angle at a fixed cavity length seems best. To go from, say, h = 10.513 ,um to h = 10.532 ,um, the grating equation for the Littrow arrangement, h = 2d sin 0, shows that the angle must
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Flgure 1. Mechanical arrangement for wavelength modulation: P1,
piezoelectric pusher; P2, coarse adjustment screw; G grating; L, laser cavlty; B, ball bearlng. Pres 5 u w Gouge
Figure 2. Pholoacoustic cell: A through D, vacuum stopcocks; J, O-ring glass joint; F, flexible stainless steel coupling; M, microphone.
change approximately 8.4 X rad. For the grating mount in this laser, this translates to a 113 ,um movement on a 13.5 cm center. So, a piezoelectric pusher (Burleigh,Fishers, NY, Model PZ-44) capable of this range of length of travel is used in place of one of the original mirror-alignment adjustment screws. The actual mechanical modifications are shown in Figure 1. To operate, a square wave of the chosen frequency is derived from a wave generator (Wavetek, San Diego, CA, Model 162) and amplified by a high-voltage operational amplifier (op amp) (Burleigh, Fishers, NY, Model PZ-70). At the same time, the square wave ig used to provide synchronization to the lock-in amplifier. The laser power is monitored by inserting into the light beam a thermopile detector (Coherent Radiation, Palo Alto, CA, Model 210), and therefore only the average power is determined. The wavelength output of the laser is monitored after the sample cells by a specrum analyzer (Optical Engineering, Santa Rosa, CA) which was calibrated with a HeNe laser (Spectra Physics, Montain View, CA, Model 134). Amplitude modulation in the control studies is accomplished by a mechanical chopper (Rofin, Newton Upper F d s , MA, Model 7500) in front of the laser beam. A modulation frequency of 100 Hz was used throughout. The laser beam is smaller than the slots of the mechanical chopper so that a square wave modulation is achieved. PAS Cell. The design of the PAS cell is conventional (nonresonant) and is shown in Figure 2. A condensor microphone (Knowles, Franklin Park, IL, Model BT-1759) is attached to the end of a standard taper joint (14/35) with the wires feeding through a small opening to provide vacuum seal. The microphone is biased as suggested by the manufacturer, and the output is connected directly to a lock-in amplifier (Princeton Applied Research, Princeton, NJ, Model HR-8). A 1-s time constant was used throughout. The cell is isolated from mechanical vibrations in the vacuum system by a piece of flexible tubing and from acoustical noise from the pump by turning off the stopcocks. To isolate the cell mechanically from the work bench, all support is through layers of foam packing material. Gas Handling. Pressures are measured with capacitance manometers with 0-10 torr and 0-1000 torr full scales (MKS Instruments, Burlington, MA, Model 221A). Gas mixtures are prepared from reagent grade gas (Matheson, E. Rutherford, NJ)
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ANALYTICAL CHEMISTRY, VOL.
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without further purification. Mixtures down to 10 ppm were prepared by direct reading on a thermocouple gauge (Varian, Lexington, MA, Model 0531 with 801 electronics). Below that level, mixtures were prepared by expansion of a 10 ppm mixture into calibrated volumes. Care was taken to avoid memory effects in the vacuum line, and sufficient time was allowed for equilibration of the partial pressures before each experiment. To match the conditions as much as possible, all experimentswere performed with a total pressure of 760 torr.
RESULTS AND DISCUSSION The first item of concern is the actual extent of modulation provided by this design. The mechanical movement must be large enough to force lasing in each of the two lines and must be centered so that equal powers are obtained in each half cycle. This is complicated by the fact that adjacent COz laser lines need not possess the same gain. A good approximation is to reduce the wavelength modulation frequency to the order of 1 Hz so that a direct judgment can be made. In looking through the spectrum analyzer, one can in fact see the two laser lines switching alternatively with the applied square wave. Centering of the modulating position is achieved by the mechanical adjustment of the grating mount, plus the bias level on the PZT pusher available through the high-voltage op amp. The approximate center can be located by observing the reading on a power meter with a subsecond response, such that the output level is essentially constant. In addition to these initial adjustments, it is necessary to check the transverse mode structure of the laser beam during modulation. Since the goal is to equalize the unwanted background in the PAS cell, the spatial characteristics of the laser beam must remain unchanged in each of the two half-cycles. This is accomplished by observing the far-field pattern of the beam. When the modulation frequency is increased to the order of 100 Hz, there is no guarantee that the mechanical movement is identical with that at lower frequencies. This is because the mount is under the pushing action of the PZT device on one half-cycle and the pulling action of a spring on the other half-cycle. The time response of the above procedure does not allow a similar evaluation here. We found that the best approach is to use a second, identical cell filled with pure nitrogen gas as a reference. This second cell is located symmetrically with respect to the focal point of the laser on the opposite side of and behind the first (sample) cell. The only PAS signal from the second cell originates from the absorption of the windows or the cell body. I t is then sufficient to minimize this PAS signal by adjusting the bias and the gain on the high voltage op amp that controls the PZT pusher. The magnitude of this residual signal is not expected to be the same as the corresponding one in the sample cell, since the windows are not necessarily matched. However, the fact that the background in the second cell is minimized is an indication that the power levels in the two half-cycles are matched and that the background in the sample cell is also minimized. It is entirely possible that the grating is in some average location which allows both laser lines to be active at all times, without switching from one line to the other. The background, as determined by the second cell, will also be minimized, but the system will then be useless for measurements. The only way to test this is to compare the signal obtained in this manner (with an absorbing gas in the sample cell) with that obtained by using amplitude modulation at the same modulation frequency and modulation function. A good test case is ethylene in nitrogen a t atmospheric pressure. A comprehensive list of absorption coefficients a t various C 0 2 laser wavelengths, measured by PAS, is available in the literature (23). One finds that the absorption coefficient (in atm-l cm-l) for the C 0 2 P(12), P(14), P(20), and P(22) lines in the OOol-lOoO band are, respectively, 4.70, 32.2, 1.50, and 1.42. So, modulation between the first pair of lines should provide
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Anal. Chem. 1983, 55, 981-983
to be intensity independent. The useful range for PAS measurements is thus extended by wavelength modulation. The detectability is comparable to those that were possible only in acoustically resonant cells (24),but without any of the critical requirernents in the operation of those cells. Finally, it may be noted that there are deviations in the experimental points from the line of unit slope in each of the last two figures. This reflects mainly the difficulty in preparing exact mixtures of gases a t thetge low concentrations, even under carefully controlled conditions. In summary, we show that a simple adaptation of a commerical COz laaier permits its operation in the wavelengthmodulated mode for PAS measurements, with good discrimination against background due to window absorption. We expect the same concept to be applicable to long-path absorption as well, with discrimination against scattering and atmospheric turbulence.
LITERATURE CITED “Cleaning Our Envlronment-A Chemical Perspectlve”, 2nd ed.; American Chemlcal Society: Washlngton, DC, 1978. Collls, R. T. H.; Russel, P. B. I n “Laser Monitoring of the Atmosphere”; Hinkley, E. D., Ed.; Springer-Verlag: New York, 1976; Chapter 4. Menzies, R. 1’. I n “Laser Monitoring of the Atmosphere”; Hlnkley, E. D., Ed.; Springer-Verlag: New York, 1976; Chapter 6. Byerly, R. IEEE Trans. 1975, NS-22, 856-869. Kobayaki, T.; Inaba, H. Appl. Phys. Lett. 1970, 17, 139-141. Golden. B. M.; Yeung, E. S. Anal. Chem. 1975, 4 7 , 2132-2135. Kerr, E. L.; Atwood, J. 6.Appl. Opt. 1966, 7 , 915-921.
981
Kreuzer, L. B. J. Appl. Phys. 1971, 4 2 , 2934-2943. Rosengren, L. G. Appl. Opt. 1975, 14, 1960-1976. Deaton, T. F.; Depatie, D. A.; Walker, T. W. Appl. Phys. Lett. 1975, 2 6 , 300-303. Max, E.; Rosengren, L. G. Opt. Commun. 1974, 11 , 422-426. Krltchman, E.; Shtrlkman, S . ; Slatklne, M. J . Opt. SOC. Am. 11378, 6 8 , 1257-1271. Bonczyk, P. A.; Ultee, C. J. Opt. Commun. 1972, 6 , 196-198. Kaldor, A.; Olson, W. B.; Makl, A. G. Sclence 1972, 176, 506-510. Kavaya, M. J.; Margolis, J. S.; Shumate, M. S . Appl. Opt. 1079, 18, 2602-2606. Chang, T. Y.; Morris, R. N.; Yeung, E. S . Appl. Spectrosc. 1981, 3 5 , 587-591. Wake, D. R.; Amer, N. M. Appl. Phys. Lett. 1079, 3 4 , 379-381. Goff, D. A.; Yeung, E. S. Anal. Chem. 1078, 50, 625-627. Castleden, 5. L.; Kirkbright, G. F.; Spiilane, D. E. M. Anal. Chem. 1981, 53, 2226-2231. Welling, H.; L i i n , G.;Beigang, R. I n “Laser Spectroscopy 111”; Hall, J. L., Carlsten, J. L., Eds.; Springer-Verlag: New York, 1977; pp 370-375. Reid, J.; Garside, B. K.; Shewchun, J.; El-Sherbiny, M.; Ballik, E. A. Appl. Opt. 1978, 17, 1806-1810. Patel, C. K. N.; Burkhardt, E. G.;Lambert, C. A. Opt. Quantum Efectron. 1078, 8 , 145-154. Konjevic, N.; Jovicevlc, S. Spectrosc. Lett. 1079, 12, 259-274. Perlmutten, P.; Shtrlkman, S . ; Slatklne, M. Appl. Opt. 1979, 18, 2267-2274.
RECEIVED for review December 6, 1982. Accepted January 28, 1983. The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-eng-82. This work was supported by the Office of Basic Energy Sciences.
Determination of Total Chromium in Seawater by Graphite Furnace Atomic Absorption Spectrometry S. N. Willie, R. E. Sturgeon,* and S. S. Berman Dlvision of Chemistty, National Research Council of Canada, Otfawa, Ontario K1A OR6, Canada
Reported concentrations of chromium in open ocean waters range from 0.07 to 0.96 Fg L-l, with a preponderance of values near the lower limit ( I , and references summarized therein). Methods used for the determination of Cr at this concentration have generally involved some form of matrix separation and analyte concentration prior to determination, such as coprecipitation ( I , 2), chelation-solvent extraction ( 3 , 4 ) ,electroreduction (5, 6), and ion-exchange (7, 8) techniques. Whereas it is desirous to utilize analytical schemes which permit elucidation of the various Cr species, particularlly since Cr(V1) is acknowledged to be a toxic form of this element, it is useful to have the capability of rapid, total Cr measurement where speciation is a matter of secondary importance. Determination of Cr by many of the methods cited earlier is problematic. Variable and nonquantitative recovery with chelation-solvent extraction techniques necessitates use of the method of additions ( 4 ) . Coprecipitation techniques require lengthy processing times and extensive sample manipulation. Ion-exchange suffers from slow uptake and release kinetics, necessitating total destruction and solubilization of the resin (8) or complex apparatus and multicomponent eluting solutions. Problems encountered with the routine determination of even total Cr by oceanographic laboratories are quite evident from the fact that only 7 participants of a total of 40 attempted determination of this element in a recent ICES intercalibration exercise (9). Overall accuracy and precision for Cr were unsatisfactory (9). 0003-2700/83/0355-098 1$01.50/0
This work reports on the use of an immobilized diphenylcarbazone chelating agent to provide a simple, rapid, and quantitative preconcentration procedure for the determination of total dissolved Cr in seawater.
EXPERIMENTAL SECTION Apparatus. A Perkin-Elmer Model 5000 atomic absorption spectrometer fitted with an AS-40 automatic sampler and HGA-500 furnace with Zeeman background correction capability was used for all Cr determinations. Pyrolytic graphite coated tubes were exclusively used with peak height evaluation of signals. Reagents. All reagents were purified prior to use. Concentrated nitric and hydrochloric acids were prepared by subboiling distillation in a quartz still using reagent grade feedstock. A saturated solution of ammonium hydroxide (28%) was prepared by isothermal distillation according to the procedure recommended by Zief and Horvath (10). A saturated solution of SOz-water was prepared by bubbling anhydrous SO, from a lecture bottle through several water traps prior to dissolution in high-purity deionized, distilled water (DDW). Stock standard solutions of Cr(V1) and Cr(II1) were prepared from KzCrz07and KCr(SO,),, respectively. Serial dilutions were made with higb-purity DDW in order to prepare working standards. Silica gel (Fisher, 100-200 mesh) was acid leached with HNOB and HCl and washed with DDW prior to use. Diphenylcarbazone (Fisher) was used as supplied. Coastal seawater samples were obtained from the Atlantic Research Laboratory of the National Research Council, Halifax, Published 1983 by the American Chemical Society
