Anal. Chem. lg83, 55, 1075-1079
valuable discussions and a critical reading of the manuscript.
LITERATURE CITED Colin, H.; Guiochon, G. J. Chromatogr. 1977, 747, 289-312. Engelhardt, ti.; Ahr, Ci. Chromatographia 1981, 74, 227-233. Cox, C. 8. J. Chromatogr. Sci. 1977, 75, 385-392. Colin, H.; Guiochon, G. J. Chromatogr. 1978, 756, 183-205. Lochmuller, C. H.; Wilder, D. R. J. Chromatogr. 1979, 77, 574-579. (6) Berendsen, 12. E.; de Galan, L. J. Chromatogr. 1980, 796,21-37. (7) Horvath, C.; Melandor, W.; Molnar, I.J. Chromatogr. 1976, 725, 129- 156. (8) Karch, K.; Subestlan, I.; Halasz, I.J. Chromatogr. 1976, 122, 3-16, (9) Hemetsbergor, H.; Behrensmeyer, P.; Henning, J.; Rlcken, H. Chromatographia (979, 72, '71-76. (IO) Gilpin, R. K.; Squires, J. A. J. Chromatogr. Sci. 1981, 79,195-199. (11) Gllpln, R. K.; Gangoda, M. E.; Krlshen, A. E. J. Chromatogr. Sci. 1982, 20, 345-348. (12) Synder, L. R.; Ward, J. W. J. Phys. Chern. 1966, 7 0 , 3941-3952. (13) Scott, R. P.; Kucera, P. J. Chromatogr. 1979, 777, 37-48. (14) Berendsen, G. E.; de Galan, L. J. Liq. Chromatogr. 1978, 7 , (1) (2) (3) (4) (5)
561-566. (15) Hemetsbergrr, H.; Mnasfeld, W.; Rlcken, H. Chromatographia 1976, 9 ,303-310. (16) Scott, R. P. W.; Traiman, S . J. Chromatogr. 1960, 796,193-205. (17) Snyder, R. Oi. J. Chem. Phys. 1967, 4 7 , 1316-1360. (18) Maroncelli, M.; Qi, S. P.; Strauss, H. L.; Snyder, R. G. J. A m . Chem. Soc. 1982. 704, 6297-6247. (19) Maroncelll, hd.; QI, S. P.; Snyder, R. G.; Strauss, H. L. Science 1981, 274, 188-190. (20) Zerbi, G.; Megni, R.; Gussonl, M.; Moritz, K. H.; Bigotto, A,; Diorlikov, S. J. Chem. Phys. 1981, 75, 3175-3193. (21) Cameron, D. G.; Casnl, H. L.; Mantsch, H. IH. Biochemistry 1980, 79, 3665-3672.
1075
(22) Mendelson, R.; Dluhy, R.; Taraschi, T.; Cameron, D. G.; Mantsch, 11. H. Biochemistry 1981, 20, 6699-6706. (23) Mantsch, H. H.; Martin, A.; Cameron, D. G. Biochemistry 1981, 20, 3138-3145. (24) Berendsen, G. E.; Pikaart, K. A,; de Galan, L. J. L i q . Chromatogr. 1980, 3, 1437-1464. (25) Flory. P. J. "Statistical Mechanics of Chain Molecules": Interscience: New York, 1969. (26) Jernigan, R. L.; Flory, P. J. J. Phys. Chem. 1969, 5 0 , 4165-4177. Strobel, G. R.; Dettenmaier, M.; Stamm, M.; Steidel, N. (27) Fischer, E. W.; Faraday Ditrcuss. Chem. SOC. 1979, 6 8 , 26-45. (28) Clark, A. T.; Lal, M. J. Chem. SOC.,Faraday Trans. 2 1978, 7 4 , 1857-1869. (29) Nagle, J. F. Annu. Rev. Phys. Chem. 1980, 37, 157-195. (30) Snyder, R. G. J. Chem. Phys. 1979, 7 7 , 3229-3235. (31) Scott, R. P. W.; Kucera, P. J. Chromatogr. 1977, 742, 213-232. (32) Hennion, M. C.; Picard, C.; Combellas, C.; Caude, M.; Rosset, R. J. Chromatogr. 1981, 270, 211-228. (33) McCormick, R. M.; Karger, B. L. J. Chromafogr. 1980, 799,259-273. (34) Tanaka, N.; Sakagami, K.; Araki, M. J. Chromatogr. 1980, 199, 327-337. (35) Marcelja, S. Biocfiim. Biophys. Acta 1974, 367, 165-176. (36) Seellg, J.; Nlederberger, W. Biochemistry 1974, 73, 1585-1588.
RECEIVED for review August 23,1982. Resubmitted February 18, 1983. Accepted February 18, 1983. This work is part of the PbD. Thesis of Lane C. Sander, University of Washington, Seattle, WA, 1982. The FT-IR instrument was obtained with the aid of a grant from the National Science Foundation (CHE-8211516).
Determination of Nitrogen Dioxide by Pulsed Thermal Lens Spectrophotometry Kenji Mori, Totaro Imasaka, and Nsbuhlko Ishibashl" Faculty of Engineering, Kyushu University, Hakozakl, Fukuoka 8 12, Japan
A thermal lens spectrophotometrlc system, consisting of a pulsed dye laser source, a photornultlplier, and a wave memory controlled by a mlcrocomputer, Is used for a determlnatlon of atmospheric NO,. l h e enhancement factor for the sample In varlous medla Is callculated by uslng the theoretical equation and the optlcal parameters and compared with the observed value for the gaseous and llquid phase samples. The agreement of tlhe observed value wlth the theoretlcal value was fair. For a laser pulse energy of 1 mJ, a slgnal enhancement of I O 3 is expected in comparison with a conventlonal spectrophotometer. The determlnatlon of NO, Is carried out by using a dye laser with a pulse energy of 20-40 pJ, and a detectlon llmllt of 0.8 ppm was achleved. Verlflcatlon of the theoretlcal equritlon and advantages of 8 pulsed laser source for the determiriation ad the gaseous sample are dlscussed In comparison wllh a CW laser sxcltatlon method.
Nitrogen oxides, especially in the form of NOz, are very important in air pollution, since the interaction of NOz in the atmosphere with sunlight and unsaturated hydrocarbons produces hazardous photochemical oxidants. Therefore, a sensitive and reliable means for trace analysis of NOz is required. Several analytical methods are currently proposed, and some instruments such as a chemiluminescent NO2 monitor are used in practical applications (1). However, the fingerprinting assignment by a spectrometric method is essentially impossible for the determination of NO2. Many laser
spectrometric methods are proposed to overcome this problem. Laser fluorimetry using Ar+ and He-Cd lasers has been investigated, and its analytical detection limits are reported to be 1-3 ppb and 0.6 ppb, respectively (2-4). A fluorimeter with a tunable dye laser and a fluorescence monochromator may have a potential for the fingerprinting determination of NOz. The use of the fluorescence monochromator enables normalization of the fluorescence intensity of NOz with the Raman bands of nitrogen and oxygen in the atmosphere (5). The detection limit is reported to be 600 ppb for the direct determination and 20 ppb for the trap method. Barnes et al. applied a tunable dye laser to the measurement of the excitation spectrum of NOz in flames (6). The detection sensitivity in that study was on the order of 200 ppm. Spectrophotometry is one of the most reliable methods, but a main disadvantage may be its poor sensitivity. In order to remove this limitation, a few laser spectrophotometric methods have been investigated. Latz et al. have proposed the idea of using intracavity quenching of laser emission (7), and Atkinson et al. have obtained a linear analytical curve in the 5-100 mtorr range of NO2 (8). Angus et al. have used photoacoustic spectrometry for the detection of atmospheric NOz (9). Using a continuous wave (CW) rhodamine 6G dye laser with an output power of 250 mW, they have obtained the detection limit of around 20 ppb. Claspy et al. have used a flashlamp-pumped pulsed dye laser with an output energy of 7 mJ/pulse (average power, 210 mW) and demonstrated the detection of 2 ppm NOz in air a t 10 torr total pressure with a S I N ratio of 100 at an average power of 100 mW (IO). The
0003-2700/83/0355-1075$01.50/00 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
He- Ne Laser
n
Ne Lasar
Pinhole
I -
1
1“ P.D.
$83’
--yM0
Trig, input
Wave
Memory
Interface
Figure 1. Block diagram of experimental apparatus.
’
environmental level of NOz is regulated to be less than 40-60 ppb in Japan, and a much more sensitive method is needed for the direct determination of atmospheric NOz. Recently an ultrasensitive detection of NOz has been demonstrated by using an IR-tunable lead salt diode laser and 1-m multiple pass White cell (total path length, 40 m) with a detectability of less than 0.1 ppb a t reduced pressure (11). Very sensitive spectrometry which can operate a t atmospheric pressure is required as an practical means. When a laser beam is focused onto the sample, local heating takes place along the laser beam. This basic phenomenon is known as the thermal lens effect (12) and is used for the detection of very small absorption (13, 14). This effect is adequately described by theory (15, 16) and is used for spectroscopic applications (14). The intensity of the thermal lens effect is proportional to the absorbed radiant energy. Therefore, it provides a very sensitive means for ultratrace analysis by using a strong exciting source. A dual-beam thermal lens technique under CW laser excitation is well illustrated (17-19). This technique is very useful for ultratrace analysis because of its high sensitivity (20-23). The trace analysis of samples in the liquid phase has been reviewed elsewhere (24,25). However, very few analytical applications of thermal lens spectrophotometry are reported for the determination of a gas-phase sample. Bailey et al. have introduced the theoretical response curve of the thermal lens signal and have also investigated experimentally the transient behavior of the thermal phenomenon with line-tunable COz laser excitation (26-28). Colson e t al. have used thermal lens spectroscopy for studies of the electronic excited states of trans-butadiene and SO2 (29-31). These studies imply that thermal lens spectrophotometry is quite useful to determine the characteristics of molecules in the gaseous phase. However, these studies have not demonstrated the analytical sensitivity of the thermal lens method. In this study we calculated enhancement factors for pulsed and continuous wave (CW) laser excitation for gaseous samples and verified these results by comparing them with experimental values. We also used this spectrophotometric technique for the determination of NOz and discussed its advantage as an analytical tool for trace analysis. EXPERIMENTAL S E C T I O N Apparatus. Figure 1 shows a block diagram of the thermal lens spectrophotometer, consisting of a nitrogen-laser-pumped dye laser as the heating source, a He-Ne laser as the probe beam, a photodetector, and data processing equipment. The dye laser operated in an untuned configuration and has an output energy of 20 pJ/pulse. The optical configuration and beam quality of the dye laser are shown elsewhere (32). The collimated beams of the dye and He-Ne lasers are focused into the sample cell by the lens Lz (ASAHI) with a focal length of 80 cm. A 20-cm sample cell with two Brewster windows is used for trace analysis of NOz. An 1-cm sample cell was also used for the measurements of transient thermal lens signals to improve the response time. In this case lens Lzwas replaced by a lens (ASAHI) with a focal length of 10 cm. The dye laser heating source is blocked by a
band-pass interference filter (Ditric Optics),which transmits only the 633-nm emission of the He-Ne laser. The distance between the sample cell and the pinhole in front of the photomultiplier should be long enough to allow the proper measurement of the thermal lens signal (33). In this study the probe laser beam is simply expanded for convenience by lens L3 of focal length 10 cm. The probe beam passes through a 1-mm pinhole and is detected by a conventional side-on photomultiplier. The color filter is used to remove the room light and any transmitted heating laser beam. The thermal lens signal is introduced to a wave memory (NF Circuit Design Block, WM 852, bandwidth 5 MHz, sampling frequency 20 MHz) controlled by a microcomputer (Sord, M 223, Mark 111) through a GP-IB interface (NF Circuit Design Block, GP-1). The wave memory is triggered by a signal obtained from the dye laser pulse. By use of a pretrigger mode the signal before and after the trigger pulse is recorded, and the complete transient signal can be observed. In order to attenuate dye laser emission a pair of crossed polarizers (Kento) were used. The output energy of the dye laser was monitored by a joule meter (Molectron,5-3 DW). The sample was prepared by a stepwise dilutions in a vacuum line equipped with diffusion (Tokuda, 2”) and rotary (Tokuda, 50 L min-l) pumps. The pressure of the sample gas was measured by an oil manometer. Reagents. The NOz contained in a 1 L cylinder was obtained from Takachiho and used after several distillations. The laser dyes of 4,4”’-bis(butyloctyloxy)-p-quaterphenyl (BBQ, from benzene (POPOP, Nakarai, 387 nm), 1,4-bis[2-(5-phenyloxazolyl)] from Kishida, 419 nm), Coumarin 500 (C-500 from Exciton, 505 nm), and Rhodamine 6G (Rh-6Gfrom Daiwa, 588 nm) were used without further purification. R E S U L T S A N D DISCUSSION
Thermal Lens Signal. When the thermal lens effect is small, the intensity and transient decay of the. thermal lens signal, S,, for pulsed laser excitation is given by (32, 33)
s, = Sp(t=O)(l + 2t/t,)-2 Sp(t=O)
= 2.303&&,
(1) (2)
(3) where Sp(t=O) is the signal intensity a t t = 0 after excitation, t is the time, t, (=wlp2/4D) is the characteristic time constant, where wlp is the beam radius of the heating laser a t the sample and D is the thermal diffusivity, Et is the pulsed energy of the heating laser, X, is the wavelength, wop is the beam radius at waist, p is density, C, is specific heat, (dv/dT) is the variation of refractive index with temperature, and A is the absorbance of the sample. The enhancement factor in thermal lens spectrophotometry is defined as a relative sensitivity in comparison with conventional spectrophotometry for CW laser excitation (34). E, may be similarly defined as an enhancement factor for pulsed laser excitation. The beam radius, alp, a t an optimum sample position can be calculated by (33) w:l
= 4w0;/3
(4)
The characteristic time constant, t,, is obtained by mop2
t = - -3-0= -
WOPZPCP
(5) 312 The signal intensity is related to the transmitted intensities It and I , a t time t = t and t = 03 by
The calculation of enhancement factors from the physical parameters provides us information about the sensitivity of thermal lens spectrophotometry. The calculated values of E, are shown in Table I for a heating dye laser with a pulse energy
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
Table I. Calculated Enthancement Factor for Samples in Various Gases under Pulsed ( E P) and CW ( E , ) Excitationa gas air N, 0 2
Ar He H, c12
co
CO, HCl 3"
NO
N2O NO2
so2
CH4 CZH, C,H, c, H2 c, H6 a
104k, w rn-' K" 261 260 267 177 1500 1815 89 252 166 145 246 259 174 165 96 343 218 204 213 104
10
3P,
C
J
&-I
1.17 1.14 1.30 1.62 0.16 0.08 2.88 1.14 1.79 1.48 0.69 122 1.79 1.87 2.60 0 65 122 1.14 1.06 3 17
Pulsed laser ( E p ) ,E,, = 1 mJ, m o p = 0.1 X
1.01 1.04 0.92 0.52 5.23 14.4 0.48 1.04 0.84 0.80 2.09 0.99 0.90 0.82 0.62 2.24 1.76 1.56 1.69 1.05
1077
-
10b(dq/dT), K-I
EP
E,
0.88 0.90 0.82 0.86 0.11 0.42 2.44 1.02 1.38 1.37
770 790 710
67 69 61 97 2 5 5 50
1.18 0.9
1.55 1.6 2.10 1.35 2.35 2.2 1.8 5.31
1100
140 380 1800
890 950 1200 850 770 1000 1100 1400 960
81
1100
170 190 96 70 180 190 440 79 220 220 170
1700
1000
1100
1300
106tC,s
151 152
149 159 19 21 518 157 302 272 195 155 309 310
560 141 3 28 291 280 1067
m; CW laser ( E c ) , output power = 1 W, h p = 500 nm, T - 300 K.
of 1 m J and a beam waist size of 0.1 mm. The calculated characteristic time constants are also listed in the last column. The enhancement factor obtained with a CW laser source (E,) is given by
where Po is the output power of the CW heating laser, X, is the wavelength of the probe laser, and k is the thermal conductivity (34). The calculated values of E, are also listed in Table I for comparison with pulsed laser excitation. As shown in previous papera, the enhancement factors for normal organic solvents are 3000-11000using 1W of the CW laser source and 200-600 using 1m J of the pulsed laser source (20,32). It was pointed out that the usle of the pulsed laser is obviously advantageous if thie average power of the heating source is identical at 1 Hz. But since the CW laser has generally a high average power, the CW laser provides much more sensitive detection in most applications. However, Table I shows that the CW heating source provides a relatively low enhancement factor for gaseous samples even for a thermd lens system using 1 W of laser power. On the other hand, the pulsed thermal lens system has very large enhancement factors and is quite useful for sensitive determinations of the trace gaseous samples. It is noted that even if the average power of the pulseld laser is less than times a t 1 Hz in comparison with the CW laser, the enhancement factor is 10 times larger for the samples in air. The reason that the large enhancement factor can be obtained for pulsed laser excitation is that the gas sample has a very low density and has a small heat capacity per unit volume. Therefore, the temperature increase is much larger in comparison with a condensed phase sample. The small enhancement factor for CW laser excitation for a gaseous sample is due to a small variation in refractive index with temperature. Furthermore, the thermal diffusivity of air is larger than that of liquids because the air is less dense, and the induced thermal lens effect disappears rapidly. Therefore, the equilibrated value of the enhancement factor, E,, becomes very small. Sensitivity of thermal1 lens spectrophotometry, which can be defined as a relatives amplitude of the signal to that of conventional spectrophotometry, is proportional to the enhancement factor. The detection limit may be improved by increasing the enhancement factor; therefore a heating laser
with a large pulse energy is essential for trace analysis. However, the detection limit is also determined by precision of the observed .value. A low-repetition-rate pulsed laser with a large pulse energy gives a large enhancement factor, but it gives relatively poor precision. On the other hand, the CW laser modulated at a high frequency gives a small enhancement factor because of its small pulse energy, but precision of the data can be much more improved by repetitive signal accumulation. The ideal heating laser for pulsed thermal lens spectrophotometry may have following characteristics: (1) a large pulse energy, (2) short pulse widths in comparison with a characteristic time constant (t, = 151 ps in air), (3) a long duration time between laser pulses in comparison with t,, (4) a high repetition rate for signal accumulation. Excitation Intensity. Equations 1-3 imply that the signal intensity is proportional to the pulse energy of the heating dye laser when absorption occurs through a one-photon process. A linear relationship between the pulse energy and the thermal lens signal was experimentally observed, and it indicates that the thermal lens effect originates through an one-photon absorption process at this power level. This result may come from the fact that the exciting wavelength of the dye laser coincides with a strong one-photon absorption band of NOz and the pulse energy of the heating laser is too small to induce two-photon absorption. Twarowski and Kliger pointed out that one- or two-photon processes should be J for some readily measurable with a pulse energy of molecules, and signals from a moderate two-photon absorption would be larger than those from a weak one-photon absorption (33). The linear relationship also shows that a strong heating source is essential for the sensitive determination of NOz, and no saturation effect takes place at this power level. At yet larger signal intensities this line deviates from linearity because of the small quadratic dependence of the thermal lens signal. Enhancement Factor. The theoretical calculation predicts that pulsed laser excitation provides a very sensitive means for the determination of gaseous samples. In this section this estimation is experimentally verified. The observed enhancement factors for NOz in air are shown in Table 11. The experimental results for liquid phase samples are also included for comparison with the gas-phase sample. The theoretical values calculated a t the specified excitation wavelength are shown in the last column. The observed enhancement factor is sensitive to the alignment of the heating and probe lasers and the position of the sample cell. Furthermore, the re-
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ANALYTICAL CHEMISTRY, VOL. 55,
NO. 7, JUNE
1983
-
Table 11. Enhancement Factor for Samples in Various Gases and Liquids under Pulsed Laser Excitationa sample obsd theoretl
I 1 0.16
il
.
- 0.12 ~
NO, in air
(1)670 (760)
920
Cu(V1I in water
(2) 670 (810) (1)32 (2j 12 (3) 1 3
24
I, in CCI, VI)
0.08
35Ob
20 440
120
130
in water-methanol mixture
nm, output power, corrected to 1 mJ; the measurements are carried o u t independently for (l),(2), and (3); parentheses indicate value extrapolated to t = 0 ; sample cell, 1 cm. h = 505 nm. a
c
h = 420
fractive index of the gaseous and condensed samples is different, and this induces a slight difference in the path of the laser beams. In order to reduce the relative error between the liquid standards and a gas-phase sample, the optical configuration is slightly readjusted to obtain the maximum signal intensity. Moreover, in order to reduce the systematic error, the optical configuration was completely reconstructed and aligned for the individual samples. The absolute error may be estimated to 0.3-3 times of the observed value. The experimental values agree fairly well with the theoretical values, and the present results show that pulsed thermal lens spectrophotometry gives a larger signal intensity for the gaseous sample in comparison with the liquid solvent such as CC4, which is the solvent with the largest enhancement factor in the condensed phase. It is noted that this experimental result should be considered to be obtained by partly compensating the positive (signal increasing) effect and negative (signal decreasing) effect. The slight misalignment of the laser beam which is unavoidable in the dual beam experiment may reduce the signal intensity. The heating dye laser is assumed to have a Gaussian beam profile in the calculation of the enhancement factor, but the observed beam profiie is slightly flat at the peak of the profile. Its deviation from a Gaussian distribution decreases the thermal lens signal (35). The positions of the beam waists are not completely identical in the dual beam experiment. Depending on the relative positions, either the thermal defocusing or focusing effect gives a much larger signal. In the calculation of the enhancement factor, the effect of mode mismatch is not taken into account, and therefore the observed enhancement factor may be different from the calculated value. Under this mode mismatch condition, the observed signal can be much larger than the calculated signal assuming the mode match condition. The effect of mode mismatch is discussed in detail elsewhere (32, 35). Sample Pressure. The signal response of thermal lens effect was investigated under reduced pressure. The results are shown in Figure 2. The spike immediately after excitation is radio frequency interference from the nitrogen laser. The transient decay curve is given by eq 1when the thermal lens signal is small. The plot for the calculated square root of It/(Im - It) vs. t should give a straight line. The results given in Figure 2B are consistent with the theory. From the intercept - 10)]-1/2 = 2.5 A 0.06. The beam of this straight line [Io/(Im diameter of the dye laser is assumed to be 0.6 X m when the laser beam is focused by the lens with the focal length of 80 cm. The thermal diffusivity ( D ) of air is (0.22 X X 760/P [W m2 torr J-l] where P(torr) is the total pressure. The slope (SL) of the straight line can be calculated from the optical parameters by
8 Y
0.04 0
T i m e (use c 1
Timehsec)
Flgure 2. (A) Transient decay curve of thermal lens signal. Strong RFI noise appears just after excitation. ( 6 )Plot of (Ii/(Im - It))-”’ vs. f . The total pressure of the sample denoted by the number on the rlght-hand side of the figures is as follows: (1) 7.8 torr; (2) 15.3 torr, (3) 22.7 torr, (4) 29.5 torr, (5) 36.7 torr, (6) 43.6 torr, (7) 49.5 torr. Partial pressure of NOp was 1 torr; excitation wavelength was 420 nm.
Table 111. The Calculated and Observed Slopes of the Plot (Sp(t))-1’2 vs. t pressure, torr 7.8 15.3 22.7 29.5 36.7 43.6 49.5
calcd
obsd
1 2 x 104 6.1
11 x 104
4.1
3.2 2.5 2.1 1.9
5.9 4.0 3.1 2.4 1.9 1.6
where p is the density. The theoretical and observed slopes are given in Table 111. The agreement of the observed data with the theoretical values is excellent. Similar results are also obtained for samples with total pressures of 130-760 torr. The present result shows that the thermal lens signal is adequately described by the present theory. It can be confirmed that the signal intensity immediately after excitation is independent of the total pressure and the characterstic time constant (t,) is proportional to the total pressure of the sample. Bailey et al. have used a pulsed C 0 2 laser and measured transient decay of the thermal lens signal (28). From the decay curve the thermal conductivity coefficients ( k = pDC,) of Ne, Ar, Kr, and Xe are obtained by using thermal lens spectrophotometry. Analytical Curve. The analytical curve of NO2 in air was constructed by using thermal lens spectrophotometry. The signal was accumulated 10 times for the data above 50 ppm and 30 times for the sample below 20 ppm. From the data of the 3.4 ppm and 0 ppm samples the detection limit was calculated to be 0.8 ppm. When the output impedance of the photomultiplier is adjusted to 1 kB, the signal reaches a maximum value 30 ps after excitation. For a 1MB impedance, the signal reaches a maximum value 250 ys after excitation because of stray capacitance of the cable and the equipment. The former gives a 60% greater peak height but a somewhat noisier decay curve. There is no appreciable difference in the detection limits for these two methods. From the background noise level, the present spectrophotometer was found to be capable of detecting signals as small as of the probe beam intensity. In conventional spectrophotometry an absorptivity of 10-3-10-4 can be readily measured, and the detection limit may be 1-10 ppm for a 20-cm sample. The present result implies that thermal lens spectrophotometry is 2-20 times more sensitive than conventional spectrophotometry. The calculated enhancement factor is about 20 in this condition for a sample with 1 cm length. Even if a sample cell 20 cm long is used, no increase in the signal intensity can be observed, since the theory is valid only for a thin sample and an effective sample length is much shorter than the length of the sample cell. The enhancement
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
factor for the 20 cm long sample is calculated to be almost unity. The use of a lens with a long focal length and a long sample cell was found to improve the signal stability. When the laser beam is tightly focused on the sample, fluctuation of the signal increases. This fluctuation may be originating from the scatter of the laser beam by small particles in the gas. This effect could lbe reduced by a weaker focusing of the laser beams and by using the longer sample cell. The observed enhancement factor from the analytical curve is 0.5 in this case and is consistent with the calculated value. Thus the improvement in the detection limit of NO2 is mainly due to the improvement of the signal detection system. In this study a homemade dye laser with a low pulse energy (20 d)is used as the exciting source. We would like to stress that a high power laser with pulse energy exceeding 20 m J is already commercially available. Therefore, pulsed thermal lens spectrophotometry may be expected to be a very useful technique in the future for trace analysis of gaseous samples in the atmosphere. A CW laser is a less efficient exciting source for the gaseous samples as shown in this study. However, a converitional CW laser commercially available has a much higher average power. It is noted that 5 ppb of NO2 has been readily detected by using a CW argon ion laser as the exciting source (36).
ACKNOWLEDGMENT The authors wish to thank Taketoshi Sonoda for his technical assistance in the construction of the vacuum line and the sample chamber. Registry No. NOz, 10102-44-0.
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RECEIVED for review July 20, 1982. Resubmitted January 6, 1983. Accepted February 1,1983. This research is supported by a Grant-in-Aid for Scientific Research (Grant No. 00547061) from the Ministry of Education of Japan and by the Steel Industry Foundation for the Advancement of Environmental Protection Technology.
