Anal. Chem. 1983, 55, 1907-1910
1907
Thermal Lens Spectrophotometry with Argon Laser Excitation Source for Nitrogen Dioxide Determination Tatsuji Higashi, Totaro Imasaka, and Nobuhiko Ishibashi* Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812, J a p a n
I n order to Improve the !sensltlvlty of thermal lens spectrophotometry, a new type aJ optical conflguratlon was studled. An Ai+ laser for excltatlon was focused Into a sample cell as usual, but a probe laser was Introduced directly without focuslng the beam. The laser beams were colllmated and Isolated with two prlsms to ellminate optical Interference of the laser beams. A part of tlhe probe laser passing through tho sample cell was split out Iby a quartz wedge and the intensity at the beam center was detected by a photodiode. The slgnal was measured by a digital lock-In ampllfler system. The detection limit of NO, was !j ppb, using the 488-nm line of the Ar+ laser (700 mW). The dynamic range of the linear analytlcal curve was obtalned over 4 orders of magnitude. The present method has a distinct advantage over a photoacoustlc method wlth respect to the capabllity of continuous monltorlng of atmospheric NO,.
Nitrogen dioxide plays an important role in photochemical reactions in the at,mosphcre; therefore a sensitive and reliable monitoring method is strongly required. Conventional ablsorption Spectrometry is useful for the determination of NO12 above the part-per-million level, but it suffers from poor sensitivity in a part-per-lbillion level. Several spectrometric methods such as fluorescence and chemiluminescence techniques are currently used for the determination at such lower concentrations. These methods are sensitive but have several disadvantages with respect to the reliability in the observed values and to the complexity in an operational procedure. There are few molecular species in the air which absorb visible emission except for NO,; therefore a spectrophotometric method is quite reliable and practical. Recently new types of laser spectrophotometry have been investigated, and one of the most sensitive methods might be thermal lens spectrophotometry. When a laser beam is introduced into a sample, local heating along a beam takes place and leads to a change of refractive index in the vicinity of the laser beam (1). Spectrophotometry based on this thermal lens effect is advantageous because of its very high sensitivity, since the thermal lens signal is proportional to the intensity of the exciting laser beam. Several analytical applications to liquid phase samples have been reported (2-12), but very few applications are studied for gas-phase samples (13,149. We have developed a new optical configuration for ultratrace determination of ambient NOz in this work. With optimization of the experimental conditions (excitation wavelength, modulation frequency, temperature of the sample, pinhole radius in the detection system), we demonstrate ultratrace analysis of NO, in air and discuss the advantage of thermal lens spectrophotometry over photoacoustic spectrometry (15-1 7). EXPERIMENTAL SECTION Apparatus. Figure 1 shows a block diagram of the optical configuration of the thermal lens spectrophotometer developed
in this study. The exciting laser beam is an Ar+ laser (NEC, GLG3200), and the probe laser beam is a He-Ne laser (Spectra Physics 236-138-02). The exciting laser beam specified by a broken line is modulated by a beam chopper and is focused by a lens 1 (focal length; f = 80 cm) into a flow cell (length = 1m, diameter = 22.5 mm), whose windows are made of quartz plate. The probe laser beam, whose polarization is adjusted to be perpendicular to the exciting laser, passes through the sample cell without focusing the beam and is split out by a wedged quartz plate placed at the Brewster angle for the Ar+ laser. This optical configuration allows efficient extraction of the probe laser beam without losing the output power of the exciting laser. The unfocused probe beam passes through an interference filter, which blocks the exciting beam and room light. The distance between the sample cell and a pinhole should be long enough, but the laser beam is conveniently expanded in this study by lens 2 (f = 9 cm). The intensity at the beam center is measured by using a pinhole and a photodiode 1 (Hamamatsu, S780-8BQ). Prisms 1and 2 (60' prism) are used for collimation and isolation of the exciting and probe laser beams. When prism 1was removed, the He-Ne laser beam was reflected by the output mirror of the Ar+ laser and was fed back to the He-Ne laser. In this condition the small vibration of the optical table induced a serious background noise in the output power of the He-Ne laser, so that detection of the small thermal lens signal was difficult. The situation was identical for the Ar+ laser and prism 2. Photodiode 2 detects the reflected exciting beam. This signal is delayed and used as the reference signal of a counter (NF, PC-545A). The direct current component of the signal is removed by passing through an RC filter circuit (7 = RC = 0.47 s). The thermal lens signal was amplified by an operational amplifier (Burr-Brown, 3421K), and converted t o frequency. The succeasive pulses are integrated by the counter and displayed by a digital printer (NADA, DP102). Procedure. The value of Io, which corresponds to the probe beam intensity under the exciting beam being intercepted by a beam chopper, was measured by using the Noise Mode of the counter. When the (chopper was opened, the exciting beam changes the probe beam intensity to I,. The signal intensity of (Io- I - ) was integrated 100 times by using the Data Subtraction Mode of the counter. It took about 6 s for signal accumulation. These procedures were repeated five times, and the S I N ratio was calculated from the standard deviations of the signal intensities for the background and the sample of minimum concentration (11 ppb). Reagents. The air for sample dilution was supplied with an air pump (IWAKI, AP.2402). The nitrogen dioxide gas, the water vapor, and suspended particulates in the room air were removed by passing through columns containing silica gel, activated charcoal, and cotton wool. The standard gas of NO, was generated continuously by a Permeater (KITAZAWA PD-lB, permeation tube), and its concentration was adjusted by controlling the flow rate of the diluent air and the temperature of the permeation tube. RESULTS AND DISCUSSION Enhancement Factor. A new theoretical model, in which an aberrant nature of the thermal lens was taken into account, was recently proposed by Sheldon et al. (18). It is verified to give more precise estimation than the previous model for the signal intensity of the thermal lens effect (19). The total fractional change in the signal intensity is given by eq 1under optimized conditions, in which the sample cell is assumed
0003-2700/83/0355-1907$01.50/0 0 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
C
0
20
40
60
Time (ms)
Figure 2. Transient thermal lens signal. ” 10 [
TIME ( m s ) 2
10 5
Figure 1. Block diagram of experimental apparatus.
to be sufficiently thin and is positioned a t 3liZconfocal distances (2,).
It - -- I , I,
-
1 - 0 tan-’ {0.577/(1 + t , / t ) } -1 1 - 0.5240
(1) 1
where
0 100
1000
500 l / t (sec-’)
+
Figure 3. Relationship between 0.577/tan [((i (0.5248 - i ) I f / I m ) / 8 ] and l / t . t, =
.“pP
4k
(3)
Equation 1is valid only when 8 is less than 0.1. P [W] is the exciting power, A [dimensionless] is the absorbance of the sample, X [cm] is the wavelength of the probe beam, k [J s-l cm-l K-’1 is the heat conductivity,dn/dT [K-l] is the variation in refractive index with temperature, t, [SI is the characteristic time constant, w [cm] is the beam radius at the sample cell, wo [cm] is the beam radius at the beam waist, C, [J g-I K-l] is the specific heat, and p [g ~ m - is ~ the ] density of the sample. Equation 1can be written as eq 4 using the steady-state signal intensities, Io and I ,
I-, - I , - 2.303-( 0.227 P zd n) A
Xk
10
I , - 1,
IO
2.303EA
(6)
The value of E represents an enhancement factor which indicates relative sensitivity of thermal lens spectrophotometry to conventional absorption spectrometry. It equals 0.227 times the old enhancement factor, P(dn/dT)/Xk, previously defined by Dovichi and Harris (2). This new enhancement factor resolved discrepancy between the theoretical and experimental values (2-10,19-21). The value of (Io- Im)/Iois proportional to absorbance of the sample as shown in eq 5. This equation is similar to the form of normal absorption spectrometry, (IO - I)/Io, rather than the old form of thermal lens spectrophotometry, (Io- I,)/I,. The probe laser beam is not focused in the present optical configuration shown in Figure 1,since it provides a large signal intensity for the sample between -31/2Z, and +31/2Z,. On the other hand, the optimum sample position is limited to the small region a t around -3112Z, or +31/zZ, in the normal optical configuration. Then, the present system allows the use of a longer sample cell and improves sensitivity several times in comparison with the normal configuration. Equations 1and 5 may not predict exact behavior of the thermal lens signal in this study but will provide us with a guideline for the qualitative discussion.
Transient Signal. The transient curve of the thermal lens signal is shown in Figure 2. The beam radius w can be expressed as follows for the laser beam with a Gaussian intensity profile:
+
u2 = oo2(1 (Z/ZC)’}
(7)
where
2, =
nwo2/X
(8)
2 [cm] is the distance between the beam waist and the sample cell, and 2, [cm] is the confocal distance. The beam radius at the waist, w,, and the confocal distance, Z,, were calculated from eq 7 and 8 to be 0.023 cm and 33 cm, respectively, under the present experimental conditions (A = 488 nm, w = 0.060 cm at Z = 80 cm). The characteristic time constant, t,, could be calculated from eq 3 to be 0.58 ms for the sample in air (w = 0.023 cm, C, = 1.01 J g-l K-l, p = 1.18 X g ~ m - k~ , = 2.60 X lo4 J s-l cm-l K-l). On the other hand, the values oft, are relatively long for the liquid samples (t, = 88 ms for water and t, = 170 ms for CClJ. Equation 1can be modified to the expression of
tan[(l
+ (0.5240 - l)It/Im}/O] = t,(l/t) + 1
+
(9)
The plot of 0.577/tan [(l (0.5248 - l)It/Im)/8]vs. l / t from the observed data is shown in Figure 3. The obtained curve is not a straight line and the agreement between the experimental and theoretical results is not excellent. In the theoretical model the laser beam is assumed to be focused, and w should be constant within the thin sample cell. On the other hand, the probe laser beam is not focused in this experiment and a cell length of 100 cm is used for efficient detection of the thermal lens effect. Therefore, the assumptions in the theory are not necessarily satisfied. But, the theory still describes the experimental result fairly well as shown in Figure 3. Exciting Power. The intensity of the thermal lens signal is expected to be proportional to the power of the heating laser from eq 5. An excellent straight line was obtained with a correlation coefficient of 0.999 and a slope of 0.101 W-l for a sample containing 13.5 ppm of NOz. The present experimental result is consistent with eq 5 even for this modified optical configuration.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
190'9
Table I. Molar Absorptivity of NO, Art laser wavelength, marc power,a nm mW
__--
457.9 476.5
76 ?: 22 7 1 ? 16 60?:4 71?: 12 3 9 t 11 33i 4
140 330 7 00 300 150
488.0 496.5 501.7 514.5
molar absorptivity, L mol-' cm-'
800
Output power of GLQ3200 Art laser used in this study. a
Ratio of Pinhole Radius and Beam Radius 0.L
-
0.01
0%
I
2
3
Pinhole Radius (mm9
Flgure 4. Effect of pinhole radius on signal intensity.
Temperature. The enhancement factor depends upon physical parameters of heat conductivity and the change of refractive index as shown in eq 5. These parameters are known to vary with the temperature. From the theory the variation of the signal intensity is estimated to be O.G%/deg at 30 "C. The observed variations were at least less than 10% of the signal intensity in the range of 25-30 "C and 30-35 "C. This result informs us that the effect of the temperature change is negligible in environmental monitoring of atmospheric NOz. Excitation Wavelength. The absorption spectrum of NO2 in the visible region is composed of sharp structures and a continuum absorption band. The Ar+ laser used as the exciting source has several llines in the visible region from 457.9 nm to 514.5 nm. Molar absorptivities of NOz at the individual lines were determined to find a best experimental condition. The results are Bhown in Table I with the available output power of the laser. The values of the absorptivities in the range from 457.9 nm to 496.5 nm are almost identical. On the other hand, those at 501.7 nm and 514.5 nm are found to be relatively smaller. Then, the excitation wavelength was adjusted to 488 nm, since it has both the large output power and the large molar absorptivity. Pinhole Radius. In the present optical configuration the signal intensity is measured as the increase of a depth of concave occurring at the peak of a Gaussian profile of the probe laser. Then, the signal intensity is more sensitive to the pinhole radius than the conventional configuration of thermal lens spectrophotometry. The relationship between the signal intensity and the pinhole radius was investigated to optimize experimental conditions. When the unfocused beam radius of the probe beam was 11 mm a t photodiode I, the pinhole radius was changed from 0.5 mm to 2.4 mm. The observed signal intensit,y is shown in Figure 4. When the pinhole radius is decreased, the signal intensity increases as expected. However, the S I N ratio in this range was almost identical, since the noise increased gradually with decreasing pinhole radius. The source of noise in this condition may be coming from particulates suspended in the atmosphere, so that the use of a large aperture may average out the effect of
Concentration( ppm)
Flgure 5. Analytical curves of NO2 at high concentration.
occasional light scattering and reduce the noise in the signal detection. The pinhole radius of 1.15 mm is used in the other experiments in this study. Effect of Gas Velocity. Dovichi and Harris have used thermal lens spectrophotometry as a detector for flowing liquid samples (20). I t is pointed out that the thermal lens signal is strongly affected with turbulent mixing of the sample at a high flow rate. This effect increases thermal conduction and reduces the signal intensity. Moreover, fluctuation in the pumping velocity induces a serious noise in the measurement. The behavior of turbulent mixing effect was investigated for a gas phase sample under a flowing condition. The intensity was constant in the range from 0 to 6.6 m mi&, and no effect of turbulent mixing was observed. Since the characteristic time constant is 100-1000 times smaller for the gas phaEie sample (15),the signal response is relatively faster than that for the liquid phase sample. Furthermore, the sample is flowing in the same dlirection to the laser beams, so that this axial flow induces little effect in the signal intensity. However, the signal would serilously decrease when the sample flowed perpendicularly to the laser beams. Modulation Frequency. The thermal lens signal decreased 20% with increasing a modulation frequency from 7 Hz to 25 Hz, but the fast modulation frequency might be preferable for reduction of the noise signal in the lock-iin amplifier system. The optimum modulation frequency was investigated to obtain the largest S I N ratio, but no distinct optimum frequency could be found in this range. The modulation frequency Was tentatively adjusted to 17 Hz in the other experiments. Analytical Curve a n d Detection Limit. The analytical curve in the part-per-billion range was constructed, and a straight line was obtained from 0 to 300 ppb. The background signal corresponded to 40 ppb of NO,. This background may originate from light absorption by the cell windows and by residual NO, and particulates in the air. The detection limit of 5 ppb is achieved for ambient NOz, using the 488-nm line of the Ar+ laser withi an output power of 700 mW a t 25 "C. This detection limit corresponds to an absorbance per unlit length of a! = 1.3 X cm-l. This value is much lower than environmental regulation limits of 40-60 ppb in Japan. The analytical curves in the parts-per-million range are shown in Figure 5. The dependences of (I,, - I J / I m and (Io- I m ) / I o are linear below 0.1, but these curves deviate from a straight line as the signal intensity increases. Hu and Whinnery have derived the expression of the thermal lens signal above 0.1 (19), which is based on the earlier model and is given by
Io - -- I , - - e + - 8 2 I, 2 8 = P(l - 10-A)(dn/dT)/Xk e 2.303PA(dn/dT)/Xk (11) The plot of -0 vs. concentration of NOz is also shown in Figure 5 . In this case a linear relationship is observed at least up
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
Table 11. Comparison of Spectrophotometry in Detection of NO, detection method limit, ppb ref conventional (cell 100 cm) photoacoustic (pulse) pho toacous tic (CW ) photoacoustic (CW) thermal lens (pulse) thermal lens (CW) a Estimated ( A = nm). This work.
*
4000 40 20 2 80 0 5
from the data
(E
a 16 15 17 14 b
= 60, h = 488
Is____
to 240 ppm, which is an upper limit of standard gas generation. I t may be concluded that eq 10 cannot predict the absolute intensity of the thermal lens signal, but it is practically very useful to construct a longer linear analytical curve for the cases of the 0 values above 0.1. The dynamic range using eq 11 exceeds more than 4 orders of magnitude. The detection limit using conventional spectrophotometry is estimated to be 4 ppm or slightly lower with a 100-cm cell ( A 5 Therefore, this thermal lens spectrophotometry is about 1000 times more sensitive than conventional spectrophotometry. The very low detection limit may be partly coming from a large enhancement factor and partly coming from good precision of the signal detection and processing system. The calculated enhancement factor from eq 5 and 6 was 11.3 (A = 488 nm, P = 700 mW, 25 "C). The actual experimental value calculated from the slope of the analytical curve was 7.0. The present optical configuration is known to provide about two times larger enhancement factor than the theoretical value, when a thin sample cell is used (22). In this study a much larger enhancement factor could be obtained by using a much shorter sample cell (e.g., 10 cm), but it would decrease the signal intensity and give a poor detection limit. Photoacoustic spectrometry has been used for the determination of NOz in the air. The reported detection limits are listed in Table 11. Angus et al. have detected 20 ppb of NOz by using a longitudinally resonant gas cell and a visible CW dye laser (15). Claspy et al. have measured 40 ppb of NO2 by using a flashlamp pumped pulsed dye laser a t 600 nm (16). Very recently Poizat and Atkinson have demonstrated the detection of NOz with a specially designed resonant cell and a CW Kr+ laser, and have achieved the detection limit of 2
ppb. The present result is comparable to these low detection limits. Since the pressure change occurring by light absorption is detected in photoacoustic spectrometry, therefore this method is sensitive to the environmental vibration and essentially requires a closed static cell installed in a sound-tight shield case. Furthermore, the flowing sample may associate with acoustic noise (17).Therefore, it seems to be difficult to detect a very small photoacoustic signal when an open cell is used. It is emphasized that thermal lens spectrophotometry requires no static cell and is not affected by the sample flow, so that it is practically very useful for continuous monitoring of atmospheric NOz. Registry No. Nitrogen dioxide, 10102-44-0. LITERATURE CITED (1) Gordon, J. P.; Leite. R. C. C.; Moore, R. S.;Porto, S. P. S.;Whinnery, J. R. J . Appl. Phys. 1985, 3 6 , 3-8. (2) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1979, 51,728-731. (3) Harris, J. M.; Dovlchi, N. J. Anal. Chem. 1980, 52,695A-706A. (4) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1980, 52,2338-2342. (5) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1981, 53, 106-109. (6) Imasaka, T.; Miyaishi, K.; Ishibashi, N. Anal. Chim. Acta 1980, 175, 407-410. (7) Miyaishi, K.; Imasaka, T.; Ishibashi, N. Anal. Chim. Acta 1981, 124, 38 1-389. (8) Imasaka, T.; Ishibashi, N. Trends Anal. Chem. 1982, 1 , 273-277. (9) Mori, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1982, 54, 2034-2038. (10) Miyaishi, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1982, 54, 2039-2044. (11) Haushaiter, J. P.; Morris, M. D. Appl. Specfrosc. 1980, 3 4 , 445-447. (12) Fujiwara, K.; Lei, W.; Uchiki, H.; Shimokoshi, F.; Fuwa, K.; Kobayashi, T. Anal. Chem. 1982, 5 4 , 2026-2029. (13) Higashi, T.; Imasaka, T.; Ishibashi, N. Bunseki Kagaku 1982, 31, 680-68 1. (14) Mori, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1983, 55, 1075-1079. ( 1 5 ) Angus, A. M.; Marinero, E. E.; Colles, M. J. Opt. Commun. 1975, 14, 223-225. (16) Ciaspy, P. C.; Ha, C.; Pao, Y. Appl. Opt. 1977, 16,2972-2973. (17) Poizat, 0.; Atkinson, G. H. Anal. Chem. 1982, 54, 1485-1489. (18) Sheldon, S. J.; Knight, L. V.; Thorne, J. M. Appl. Opt. 1982, 21, 1863-1669. (19) Hu, C.; Whinnery, J. R. Appl. Opt. 1973, 12,72-79. (20) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1981, 5 3 , 689-692. (21) Imasaka, T.; Shimanoe, K.; Ishibashi, N. J . Chem. Phys., in press. (22) Imasaka, T.; Higashi, T.; Ishibashi, N., unpublished work, 1982, Hakozaki, Japan.
RECEIVED for review March 28,1983. Accepted June 28,1983. This research is supported by a Grant in Aid for Scientific Research (Grant No. 547061) from the Ministry of Education of Japan and by a Steel Industry Foundation for the Advancement of Environmental Protection Technology.
