Anal. Chem. 1985, 57, 1219-1223 (2) Imasaka, T.; Miyaishi, K.; Ishibashi, N. Anal. Chim. Acta 1980, 115, 407-410. (3) Miyaishi, K.; Imasaka, T.; Ishibaishi, N. Anal. Chim. Acta 1981, 124, 36 1-389. (4) Schrepp, W.; Stumpe, R.; Kim, J. I.; Walther, H. Appl. Phys. 1983, 32, 207-209. (5) Bsrthoud, 1.;Mauchien, P.; Omenetto, N.; Rossi, G. Anal. Chim. Acta 1983, 153, 265-269. (6) Beitz, J. V.; Hessler, J. P. Nucl. Techno/. 1980, 51, 169. (7) Mori, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1982, 54, 2034-2038. (8) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1979, 51, 726-731. (9) Carter, C. A.; Harris, J. M. Anal. Chem. 1983, 55, 1256-1261. (10) Imasaka. T.; Ishibashi, N. Trends Anal. Chem. 1982, 1 , 273-277. (11) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1980, 52, 2336-2342. (12) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1981, 53,689-692.
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(13) Fang, H. L.; Swofford, R. L. J . Appl. Phys. 1979, 50, 6609-6615. (14) Sheldon, S. J.; Knight, L. V.; Thorne, J. M. Appl. Opt. 1982, 27, 1663-1669. (15) Perry, F. W.; Ryabov, E. A.; Zewail, A. H. Laser Chem. 1982, 7 , 9-15. (16) Fang, H. L.; Swofford, R. L. "Ultrasensitive Laser Spectroscopy"; Academic Press: New York, 1963; pp 175-231. (17) Bruhat, G.; Kastier, A. "Cours de Physique Girnirrale (optique)", Masson, ed.; Wiley: New York, 1965.
RECEIVED for review July 16, 1984. Accepted February 1985. This work is partly supported by C.C.E. under No. WAS 83-361-7 and made within contractual collaboration with J.R.C.-Ispra (Euratom).
Thermal Lens Spectrophotometry of Phosphorus Using a Near- Infrared Semiconductor Laser Kazuhiko Nakanishi, Totaro Imasaka, and Nobuhiko Ishibashi* Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812, Japan
A thermal lens spectrophotometer conslstlng of a near-Infrared semlconductor laser was constructed. The theoretlcal sensitivity of the present spectrophotometer uslng a 10-mW laser was 61 tlmes better than the conventional spectrophotometer for the sample In chloroform. When the sample of phosphorus was measured directly In the aqueous phase based on a heteropoly blue method, the detectlon llmlt was 2.2 ppb and 0.7 ppb for single and dual beam methods, respectlvely. When the sample was measured after solvent extractlon Into 2-butanol, the detection llmlts were Improved to 0.2 ppb for both the methods, whlch corresponds to an absorbance of 2 X IO4. The achleved detectlon llmits were about an order of magnltude better than absorptlon spectrometry uslng a conventional l-cm sample cell. The mlnlmum detectablllty was mainly llmlted by blank absorptlon occurring from the solvent. The determlnatlon was also carried out after Ion-palr solvent extractlon with Zephlramlne Into chloroform. The solvent blank was much lower for chloroform, but the detectlon llmlt remalned ldentlcal wlth the prevlous results. The minimum detectablllty was limited by reagent blank In this case.
Several new spectrometric methods have been developed using lasers, but most of them have not yet been used in a commercial instrument. It seems to be due to a large dimension and expense of the laser. Moreover, the laser has some disadvantages with respect to complexity in operation and difficulty in maintenance. However, a semiconductor laser is compact and inexpensive and can be operated with a small power supply. These advantages of the semiconductor laser may greatly simplify the spectrometric system, and therefore one may be able to develop a practical instrument for use in a commercial spectrometer. We have currently studied near-infrared spectrometry using a semiconductor laser and a high-power emitting diode for photoacoustic spectrometry (1) and absorption spectrometry (2). Ohtsu et al. and Chan et al. have applied the semiconductor laser for high-resolution spectroscopy of gaseous molecules such as NH3,HzO,and CHI 0003-2700/85/0357-1219$0 1.50/0
in the near-infrared region (3, 4). Very recently we have reported the determination of surfactants at trace levels using a fluorescent polymethine dye by continuous wave (CW) semiconductor laser fluorometry (5). We have also used pulsed semiconductor laser fluorometry for lifetime measurements (6). A fluorometric method is undoubtedly a most sensitive method, but sensitive absorption spectrometry is essential for the determination of nonfluorescent species. Thermal lens spectrophotometry, which is one of the most sensitive spectrometric methods (7), has recently been developed and has been used for trace analysis (8,9). Most of these investigations are carried out in the visible region, and only a few works in the infrared region (10-12). However, no application has been studied in the near-infrared region. Recently, a sensitive method was required for determination of phosphorus, since eutrophication of lake water causes an environmental problem. Near-infrared spectrophotometry based on a heteropoly blue method is most frequently used for this purpose (13),but this method sometimes suffers from its poor sensitivity especially in ultratrace analysis of the sample in seawater a t parts-per-trillion (pptr) levels. Therefore, preconcentration from a large sample volume is presently necessary. (14, 15). Fujiwara et al. have reported thermal lens spectrophotometry of phosphorus using a continuous wave (CW) dye laser pumped by an argon ion laser as a light source (16). It is useful for the determination of the sample at ultratrace levels (detection limit, 5 pptr), but this method seems to be impractical for routine analysis. The lasers are expensive and have a large dimension, so that they are not useful for field research. Moreover, the wavelength of the CW dye laser (660 nm) does not exactly coincide with the absorption maximum of the heteropoly blue (700-900 nm). In this study we constructed a compact thermal lens spectrophotometer using a near-infrared semiconductor laser (823.9 nm) and applied it to trace analysis of phosphorus at sub-part-per-billion levels.
EXPERIMENTAL SECTION Apparatus. Figure 1A shows a block diagram of the thermal lens spectrophotometer based on a single beam method. A semiconductor laser (Hitachi, HLP-1600, X = 823.9 nm) is used as 0 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
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:&/\ B
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Flgure 2. Absorption spectrum of heteropoly blue of phosphorus (0.5 ppm): (a) aqueous sample: (b) sample extracted into 2-butanol; (c) sample extracted into chloroform with Zaphiramine. For (b) and (c) 10 mL of aqueous sample was shaken with 10 mL of extraction solvent and measured after separation.
Flgure 1. Block diagram of thermal lens spectrophotometer: (A) single beam system: (B) dual beam system.
M hydrazine sulfate solution, and several milliliters of phosphate sample solution. The flask was filled up to 100 mL with water. The solution was transferred to a beaker heated to 80-90 "C for 10 min. After the sample solution cooled, it was transferred to a cuvette and the light absorption was measured. For solvent extraction, 20 mL of sample solution was taken from the beaker to a test tube with a ground stopper and mixed with 10 mL of 2-butanol. It was shaken for 3 min and light absorption of the organic phase was measured. In the case of ion-pair extraction, quaternary ammonium salt such as tetradecyldimethylbenzylammonium chloride (Dotite, Zephiramine) was added before mixing the extracting solvent.
a light source, whose output power was typically adjusted to 10 mW. The laser beam is modulated at 2 Hz by a chopper, which is placed at the beam waist of the laser focused by lens 1 (Kyowa Seimitsu, magnification 40, numerical aperture 0.65). The laser beam is focused again by lens 2 (Asahi Opt. Co., focal length 55 mm) into an 1-cm sample cell. The intensity at the beam center is measured by the combination of a pinhole (1 mm) and a photodiode. The wave form of the signal was measured by a transient recorder (Autnics Co., auto digitizer S210, 1 kwords) and was averaged 32 times by a signal averager (Autnics, signals averager F601). This procedure was repeated ten times, and the average value and the standard deviation were calculated. The transient recorder and the signal averager are controlled by a microcomputer through a GP-IB interface. Figure 1B shows a block diagram of the thermal lens spectrophotometer based on a dual beam method, in which a He-Ne laser (Uniphase, Model 1103) is used as a probe beam. In order to simplify the experimental system, the semiconductor laser was modulated to square waves by using a pulse generator (Hewlett-Packard 8013B) as a power supply. Unfortunately, this technique could not be used in the single beam system, since the modulated laser pulses were slightly deformed from exact rectangular pulses. This effect caused a negative background signal in the single beam system but no appreciable background in the dual beam system. The probe beam is collimated with the exciting beam by a wedged quartz plate and passes through the sample cell without focusing the laser beam. The sensitivity of this configuration has not been clarified by theory, but experimentally the sensitivity is about twice as large as that of the conventional single beam method as described in the later section. The probe beam is expanded by lens 3 (Asahi,focal length 10 cm) and passes through a pinhole to measure the intensity at the beam center and through a band-pass filter (Ditric, 632.8 nm) to block the exciting beam. Reagents and Procedure. The color development procedure is based on the heteropoly blue method currently used for determination of phosphorus (13). A standard sample solution containing 50 ppm phosphorus was prepared by dissolving potassium dihydrogen phosphate (KH,PO,) in doubly distilled and deionized water. For color development, the following solutions were added in turn in a volumetric flask: 10 mL of 0.1 M sodium molybdate (NazMo04-2Hz0) in 10 N sulfuric acid, 4 mL of 0.01
RESULTS AND DISCUSSION Absorption Spectra of Heteropoly Blue. Figure 2 shows the absorption spectra for the heteropoly blue of phosphorus (0.5 ppm) measured by a conventional spectrophotometer. The absorption maximum is located at 830 nm for the sample in the aqueous solution. The extracted heteropoly blue into 2-butanol has an absorption maximum a t 790 nm. The absorption spectrum of the heteropoly blue prepared by ion-pair extraction with Zephiramine into chloroform, which will be discussed in the later section, is also shown in Figure 2. The semiconductor laser used in this study oscillates at 823.9 nm, which coincides with the absorption maximum of the aqueous sample fairly well. However, the laser wavelength does not exactly coincide with the absorption maximum for the sample in 2-butanol, and the molar absorptivity at 823.9 nm is only 70% of the maximum value at 790 nm. Theoretical Sensitivity. The semiconductor laser is expanded in a large angle (20 X 50') because of its small emitting area, and the laser beam is collected by the objective lens with a very short focal length (1-2 mm). Therefore, it may be possible that the laser beam is poorly focused by aberration of the lens and that it provides a lower sensitivity in comparison with the theoretical value. The beam spot size of the semiconductor laser a t the focal point was measured by translating the pinhole (radius, 20 i 5 pm). The observed beam radius was 35 pm x 35 pm, and they were close to the theoretical values of the spot size (20 pm X 35 p m ; error f10 pm) calculated from the beam radius (1.4 mm X 1.7 mm) at lens 2; the error was partly originating from a relatively large diameter of the pinhole and an irregular beam shape of the semiconductor laser at lens 2 since the beam profile was composed of many optical fringes. Interference was coming from superposition of the laser beams reflected by the flat surfaces of a series of components in the objective lens. The present result shows that the laser beam is almost ideally focused by the lens. The enhancement factor, which indicates the relative sensitivity of thermal lens spectrophotometry in comparison with conventional spectrophotometry, was cal-
Pulse Generator
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
culated from theory (17,18). The values for the exciting laser (10 mW) with a circular beam profile are 0.83 for the sample in water, 16 in 2-butanol, and 32 in chloroform. However, beam divergences for perpendicular and parallel directions are different for a semiconductor laser, and the confocal distances (1.5 mm and 4.7 mm) are different for these directions. The optimum sample position is 3liZ times the confocal distance away from the focal point (18),and therefore a difference of the confocal distances may decrease the sensitivity. The short-term noise levels were 0.02% for both the semiconductor and He-Ne lasers. However, the relative sensitivity of the thermal lens spectrophotometric system using a semiconductor laser changed by optical interference, since many optics are required for collimation of the laser beam. Therefore, it has much poor resettability. Mode hopping may be a serious noise in the case of the semiconductor laser in both the intensity and the wavelength, but no such effect was appreciable in the present study after the laser was warmed up about 30 min. The calculated detection limits are 0.35 ppb for the sample in the aqueous solution, 0.024 ppb in 2-butanol, and 0.014 ppb in chloroform, if contribution of the blank signal is negligible. A semiconductor laser with an output power of 100 mW is already commercially available (Spectra Physics, SDL2410-H1), and therefore a theoretical detection limit of 1.4 pptr may be achieved under a theoretical enhancement factor of 320 for the sample in chloroform ( A = 3.8 X lo4). These values are by no means inferior to the sensitivity obtained by using the state of the art laser spectrometry. It is noted that a specially modified commercial spectrophotometer is reported to be capable of being operated with a noise of A = 9 x lo4 (19). Direct Incidence Configuration. In single-beam thermal lens spectrophotometry, the measurement is carried out under the compromized optical conditions. The sample is placed 3liZtimes the confocal distance away from the focal point, at which the beam spot size is twice as large as that at the focal point. Then, the induced thermal lens effect is 4 times smaller in comparison with the case in which the sample is placed at the focal point. Furthermore, the lens effect is less efficiently detected, since the sample is placed at a distance close to the focal point. The most efficient detection may be achieved by introducing the parallel beam; the lens effect would be detected in the infinite sensitivity by placing the pinhole far away from the sample. These compromized conditions cannot be avoided for the single beam system, but the situation is different for the dual beam system. In this study the sample is placed at the focal point for the exciting laser, and a nonfocused laser beam is introduced into the sample to detect the lens effect. In this case the sensitivity is actually limited by diffraction of the probe laser beam, since the beam diameter of the induced thermal lens is so small. The observed enhancement factor of this "direct incidence Configuration" was 1.9 times larger than the value for the single beam system, where the measurement was carried out using an argon ion laser. It is emphasized that this configuration has additional advantages; the optical alignment is very easy since the exciting beam should be focused into a nonfocused (-1 mm) probe beam, and moreover it allows the use of a long sample cell (more than twice the confocal distance) to improve the sensitivity while the width of the sample cell in the conventional thermal lens method is limited to be considerably short (less than one confocal distance). Analytical Curve and Detection Limit. The analytical curve of phosphorus was measured by both the single and dual beam methods. Table I shows the enhancement factor and the detection limit (SIN = 2) of phosphorus achieved by direct
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Table I. Enhancement Factor (E) and Detection Limit (DL) in Determination of Phosphorus solvent extraction E" DL,ppb
aqueous solution E" DL,ppb
method
single beam system 0.5 (0.83) dual beam system 1 (1.6)
2.2 0.7
4 (16) 10 (30)
0.2 0.2
Values in Darentheses are aredicted values.
determination in the aqueous phase and by solvent extraction. The observed enhancement factors are smaller than the calculated values. The discrepancy may originate from the following reasons: (1) The output power of the semiconductor laser is calculated from the manufacturer's data and it is reduced at the sample position by reflection of the laser beam at the surface of the lenses and the cell wall. (2) The exciting laser beam is not 100% collected because of the limited numerical aperture of the lens. (3) Difference of the beam divergence for perpendicular directions reduces the sensitivity. The analytical procedure for the aqueous solution method is simple, but it provides a rather poor sensitivity. The enhancement factor strongly depends on physical parameters of the medium, namely, thermal conductivity and variation of refractive index with temperature. The use of solvent extraction into the organic solvent is advantageous to improve the sensitivity (20),although the procedure is more complicated. The present dual beam method is apparently more sensitive than the single beam method. The detection limit has been improved with increasing the enhancement factor for the direct determination in the aqueous phase. However, no appreciable improvement in the detection limit has been achieved in the determination using solvent extraction, since the background signal determines minimum detectability in this condition. The detection limit achieved by a conventional spectrophotometer (detectability A = was several parts per billion in our study, so that thermal lens spectrophotometry was about an order of magnitude better than the conventional method. Fujiwara et al. have reported a detection limit of 5 pptr for phosphorus using thermal lens spectrophotometry (16). In their experiment the background signal corresponds to 800 pptr of phosphorus, where background absorption by the solvent is already cancelled; the background signal including solvent blank is considered to be several ppb. Then, they could detect 1/1OOO of the signal on the large background. Their ultrasensitive result may be coming from a very careful analytical procedure for color development and from an ultrastable output power of the dye laser. Background Signal. The blank signal in this study corresponded to 12 ppb of phosphorus for the determination in the aqueous phase and - 5 ppb for the solvent extraction method. The source of the background signal was investigated in detail, and it was found that the thermal lens signal was observed even when the solvent of water or 2-butanol was measured. The contributions of the background signal occurring from the solvents were about 65% and 90% of the blank signals for the aqueous phase and solvent extraction methods, respectively. The rest were from reagent blanks. Thermal lens spectrophotometry is capable of measuring solvent absorption directly without using a reference, while it is difficult for conventional spectrophotometry. It may be an advantage of thermal lens spectrophotometry. Light absorption at 823.9 nm was measured in detail for various organic solvents such as alcohols, ketones, chloroform, and carbon tetrachloride using the thermal lens spectrophotometer. Alcohols were found to have a large signal, while chloroform
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Figure 3. Absorption spectra of solvents: (a) water; (b) methanol; (c) 2-butanol; (d) carbon tetrachloride. Carbon tetrachloride was used as a reference solvent.
had a background a t least 6 times smaller than that for alcohols. No signal was observed for carbon tetrachloride within the experimental error. These results are consistent with the previously reported results (21-23). Figure 3 shows the absorption spectra measured by the conventional spectrophotometer for water, methanol, 2-butanol, and carbon tetrachloride; where carbon tetrachloride was used as a reference. The spectrum was also measured for chloroform, but it was almost identical with carbon tetrachloride and was removed from this figure. It is confirmed that water and alcohols have absorption bands a t around 800 nm, and they are not negligible in the spectrophotometric determination of phosphorus. The absorption bands are attributed to overtones of the strong 0-H and C-H vibrational lines, while the overtones for C-Cl vibration locate at yet lower frequency. It is emphasized that carbon tetrachloride and chloroform have very small absorption bands in the near-infrared region, and therefore the use of these solvents is essential for trace analysis by semiconductor laser spectrophotometry. Ion-Pair Solvent Extraction. Heteropoly blue is reported to be extracted as an ion pair into chloroform with a bulky cation such as Zephiramine (24). According to the suggested procedure, the heteropoly blue of phosphorus was extracted and measured by the dual beam method. The achieved detection limit was 0.2 ppb, which was unfortunately identical with the result based on solvent extraction into %-butanol. From the slope of the analytical curve, the enhancement factor was calculated to 23, the theoretical value being 61. The background signal was found to be occurring from reagent blank, which seems to be due to complex formation of the molybdate ion with Zephiramine and succeeding extraction to chloroform. Extraction with other quaternary ammonium salts was investigated further to reduce the blank signal. For tetraethylammonium iodide, tetra-n-propylammonium chloride, and trimethylstearylammonium chloride, the ion pair was located at the boundary of the aqueous and chloroform phases. For tetra-n-butylammonium bromide and tetra-n-amylammonium bromide, the heteropoly blue-quaternary ammonium ion complex was extracted into chloroform, but they gave smaller signals than the complex with Zephiramine. Then, no promising quaternary ammonium ion was found. The use of chloroform is advantageous with respect to the large enhancement factor, but it gives a lower molar absorptivity a t the exciting wavelength (823.9 nm) since the absorption maximum locates at 865 nm as shown in Figure 2. The extraction procedure with chloroform is rather complicated and tedious; bubbles occur in the test tube by the surfactant of Zephiramine. Then, it is concluded that extraction into 2-butanol is, at the present stage, superior than extraction into chloroform. However, it should be stressed that ion-pair solvent extraction into chloroform is essential for more sensitive spectrophotometric determination of
phosphorus. Further Improvements. In order to measure phosphorus at yet lower concentrations, several further improvements seem to be possible. First, a semiconductor laser with a larger output power may be advantageous to increase sensitivity, since the enhancement factor is proportional to the laser power. A 100-mW semiconductor laser is useful for direct determination of phosphorus in the aqueous phase. A semiconductor laser which oscillates a t 780 nm might be more attractive for the method based on solvent extraction into 2-butanol. The molar absorptivity of the heteropoly blue is increased 1.3 times as shown in Figure 2, and the background signal may be slightly decreased as shown in Figure 3. Since a large background noise is superimposed on the sample signal, the laser power should be carefully stabilized to measure a small signal. The package of the semiconductor laser contains a photodiode to measure the output power of the laser. Therefore, feedback control of the output power may readily be achieved, as described in the manufacturer’s technical report. Temperature control of the semiconductor laser promises a further improvement in long-term stability. Another approach for background subtraction may be the use of the reference and sample cells placed before and after the focusing point in the single beam method (25),though this technique cannot be applied to the direct incidence method so that the sensitivity may be decreased. For convenient use of thermal lens spectrophotometry the dual beam method is more preferable, since the signal can be directly read out by a lock-in amplifier. However, a He-Ne laser, which is used as a probe beam in this study, has a large dimension in comparison with a semiconductor laser, and moreover the output power cannot easily be stabilized. It loses the advantage of semiconductor laser spectrometry. It is suggested to use a semiconductor laser as a probe beam which oscillates at the different wavelength (e.g., 700 nm to 1300 nm) because of its small dimension and its capability for feedback control. For breaking through the problems in absorption spectrometry, semiconductor laser fluorometry based on the fluorescence quenching effect may be used in the future. We have found that a near-infrared polymethine dye such as 3,3’-diethyl-2,2’-(4,4;4’5’-dibenzo)thiatricarbocyanine iodide (NK427),becomes nonfluorescent by formation of an ion-pair complex with a phosphomolybdate ion. Semiconductor laser fluorometry is a very sensitive technique ( 5 ) ,and therefore this method has a possibility of being used for ultratrace analysis of phosphorus. Registry No. P,7723-14-0.
LITERATURE CITED Kawabata, Y.: Kamikubo, T.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1983, 55, 1419-1420. Imasaka, T.; Kamikubo, T.; Kawabata, Y.; Ishibashi, N. Anal. Chim. Acta 1983, 753, 261-263. Ohtsu, M.; Kotani, H.; Tagawa, H. Jpn. J . Appl. Phys. 1983, 22, 1553-1 557. Chan, K.; Ito, H.; Inaba, H. Appl. Opt. 1983, 22. 3802-3804. Imasaka, T.; Yoshitake, A,; Ishibashi. N. Anal. Chem. 1984, 56, 1077-1079. Imasaka, T.; Yoshitake, A,; Hirata, K.; Kawabata, Y.; Ishibashi, N. Anal. Chem., in press. Kiiaer. D. S. “Ultrasensitive Laser Spectroscopy”; Academic Press: N ~ W York, 1983; pp 175-232. Harris, J. M.; Dovichi, N. J. Anal. Chem. 1980, 52, 695A-706A. Irnasaka. Ishibashi. N. Trends Anal. Chem. 1982. 7 . 273-277. - -- ., T.:, . . -. Carter, C. A,; Brady, J.’M.; Harris, J. M. Appl. Spectrosc. 1982, 3 6 , 309-314. Higashi, T.; Imasaka, T.; Ishibashi, N. Bunseki Kagaku 1982, 3 7 , 880-68 I . Higashi, T.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1984, 5 6 , 2010-20 13. Boitz, D F.; Meiion, M. G. Anal. Chem. 1947, 19, 873-877. Lueck, C. H.; B o k , D. F. Anal. Chem. 1956, 28, 1168-1171. Miyamoto, M. Bunseki Kagaku 1983, 12, 32-38. Fujiwara, K.; Lei, W.; Uchiki, H.; Shimokoshi, F.; Fuwa, K.; Kobayashi, T. Anal. Chem. 1982, 5 4 , 2026-2029.
Anal. Chem. 1985, 57, 1223-1227 (17) Dovlchl, N. J.; Harris, J. M. Anal. Chem. 1979, 5 1 , 728-731. (18) Sheldon, S. J.; Knight, L. V.; Thorne, J. M. Appl. Opt. 1882, 21, 1663-1669. (19) Kaye, W. Anal. Chem. 1981, 53, 369-374. (20) Miyalshl, K.; Kunitake, M.; Imasaka, T.; Ogawa, 7.; Ishibashi, Anal. Chlm. Acta 1981, 125, 161-164. (21) Stone, J. Appl. Phys. Lett. 1972, 20. 239-240. (22) Stone, J. I € € € J . Quant. Nectron. 1972, QE-8, 386-388. (23) Stone, J. Appl. Opt. 1973, 12, 1828-1830. (24) Ohashi. K.: Suzuki. c.; Yarnamoto, K. Bunsek/ Kagaku w g , 28, 523-526.
(25) Dovichi, N. J.; Harris, J.
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M. Anal. Chem. 1980, 52, 2338-2342.
RECEIVED for review September 6, 1984, Resubmitted February 6,1985. Accepted February 6,1985. This research is supported by a Grant-in-Aid for Scientific Research from the Ministry of &hcation of Japan and by a Kajima Foundation Research Grant.
Multiphoton Infrared Photochemistry for Trace Gas Determination via Visible Chemiluminescence Stanford R. Spurlin' and Edward 5. Yeung* Department of Chemistry and Ames Laboratory, USDOE, Iowa State University, Ames, Iowa 50011
A selectlve method for determlnlng hydrocarbons In the gas phase Is descrlbed. Fluorine atoms are produced In a controlled manner from the multiphoton Infrared dlssoclatlorl of SF,, whlch Is exclted by a pulsed C02 laser. When C2 or CH chemilurnlnescence Is monltored, Interferences are mlnlmlred. The method Is useful for sampling at atmospheric pressure at concentrations between 0.2 and 500 ppm. The contlnuousflow system allows measurements every 2-3 8. The emlsslon shows a characterlstlc delay time before reaching Its maximum. This suggests a radical chaln mechanism In whlch the excited species are quenched by the various radical specles that are produced.
Among the various optical methods for analysis, chemiluminescence is potentially the most sensitive. In principle, each analyte molecule can produce a photon and thus a measurable signal. Unlike fluorescence, the background can be reduced to a negligible level because no light source is used. The most widely used reagent in the gas phase is ozone (atomic oxygen). Unfortunately, chemiluminescence from the reaction with ozone is not very selective (1,2). In almost every case where reaction occurs, emission froh oxygen is observed. Interfering species can thus increase the background emission, degrading the detectability and the quantitative accuracy of the measurement. It is then highly desirable to investigate other chemiluminescence reactions. In particular, if the emitting species is derived from the analyte and not the reagent, a highly selective technique can be developed without sacrificing detectability. For example, we have recently taken advantage of the gas-phase reaction between CIOz and HzSto selectively detect the latter (3). Since the emitting species is S2,interferences are minimized. A basic requirement for chemiluminescence reactions is that the overall exothermicity must be large enough to produce the products in an excited state. To incorporate photoncounting techniques, one further requires that the emission, and hence the excited-state energy, be in the near-infrared or higher frequency region. The choice of chemiluminescence reagents is thus severely limited. This limitation can be relaxed if transient species, such as radicals, can be used, or if the temperature can be substantially raised above room tem*Presentaddress: Department of Chemistry, Clemson University, Clemson, SC 29631.
perature. The use of radicals may also result in sustained chain reactions, so that more than one photon can be derived from each analyte molecule. An example of a chemiluminescence reaction involving radicals is that between atomic fluorine and hydrocarbons (4). Emission can be observed at room temperature from C2and CH bands in the visible region, making it a good candidate for very selective monitoring of hydrocarbons. A difficulty in using radicals as chemiluminescence reagents is their generation. Quantitative accuracy can only be achieved if the radical can be generated in a controlled and reproducible manner. Fluorine atoms have been generated either by an electrical discharge in CF4 or F2-Ar mixtures (4) or by a microwave discharge in CF4 (5). Discharges are often difficult to control because they can be affected by trace amounts of impurities. Interferences may also be present when the impurities are further excited by the discharge to initiate other reaction pathways or to produce emission. When CF4 is used, emission has been reported for the CF and CF2 species ( 4 ) . When Fz is used, O2 and N2 are present as impurities because of the similarity of their boiling points (6). There is also competition between fluorine atomic and molecular reactions, leading to different emission pathways. Over the past decade, there has been a large amount of work in the literature concerning multiphoton infrared photochemistry (7). Many different types of radicals and stable species have been generated in the gas phase (8). Dissociation can occur either directly (unimolecular) from the species absorbing the IR photons, indirectly (bimolecular) through reaction of the absorbing species after excitation to high vibrational states, of thermally (collisional) by excitation transfer from the absorbing species to the gas mixture. In any case, the dissociation extent can be controlled by the proper combination of laser power and laser fluence. Since IR absorption offers spectral selectivity, very few side reactions are likely to be present. Also, since the generated species is typically confined to the immediate vicinity of the optical interaction region, wall effects are avoided (9). Another advantage is that photomultiplier tubes are insensitive to IR radiation, and the low-noise condition of chemiluminescence detection can be preserved. Furthermore, it is known that multiphoton infrared excitation rarely leads directly to visible emission. Chemical selectivity can then be introduced. In this paper, we report the use of multiphoton infrared photochemistry for trace gas determination in a continuous-flow system by monitoring the visible chemiluminescence
0003-2700/85/0357-1223$01.50/0 0 1985 American Chemical Society
