2034
Anal. Chem. lg82, 5 4 , 2034-2038
ACKNOWLEDGMENT We wish to thank Fred C. Walls for his expert technical assistance.
Proc. Jpn. SOC.Med. Mass Spectrom. 1981, 6 , 33-40. (16) Barber, M.; Bordoii, R. S.; Sedgwick, R. D.; Tyler, A. N. Nature (London) 1981, 293, 270. (17) Rlnehart, K. L.; Gandioso, L. A.; Moore, M. L.; Pandey, R. C.; Cook, J. C.; Barber, M.; Sedgwick, D.; Bordoli, R. S.; Tyler, A. N.; Green, 8. N. J . Am. Chem. SOC. 1981, 103, 6517-6520. (18) Garrison, B. J.; Wlnograd, N. Science 1982, 216. 805. (19) Kambara, H.; Walls, F. C.; McPherron, R.; Straub, K. M.; Burlingame, A. L. 27th Annual Conference on Mass Spectrometry and Allied TopIce, Seattle, WA, 1979; Paper MPM13, pp 184-185. (20) Barber, M.; Bordoii, R. J.; Sedgwlck, R. D.; Tyler, A. N.; Whally, E. T. Biomed. Mass Spectrom. 1981, 8 , 337. (21) The glycerin matrlx SIMS spectra of kanamycins A, B, and C obtained wlth an Ar' primary beam have recently been published. See Haroda, K.; Suzuki, M.; Tokeda, N.; Tatematsu, A,; Kambara, H. J . Anfiblot. 1982, 3 5 , 102. (22) Krauss, A. R.; Krohn V. E. I n "Mass Spectrometry"; Johnstone, R. A. W., Sr. Reporter; Burllngton House: London, 1981; Vol. 6, Specialist Periodical Reports, pp 118-152.
LITERATURE CITED (1) Beckey, H. D. Int. J . Mass Spectrom. Ion Phys. 1969, 2 , 500. (2) Macfarlane, R. D.; Torgerson, D. F. Science 1978, 191, 920. (3) McNeal, C. J.; Macfarlane, R. D. J . Am. Chem. SOC. 1981, 103, 1609. (4) Burllngame, A. L.; Dell, A., Russell, D. H. Anal. Chem. 54, 1982, 363R. (5) Benninghoven, A.; Jaspers, D.; Slchtermann, W. Appi. Phys. 1976, 11,35. (6) Kambara, H.; HiShlda, S. Org. Mass Specfrom. 1981. 16, 167. (7) Day, R. J.; Unger, S.E.; Cooks, R. G. J . Am. Chem. Soc. 1979, 101, 501. (8) Barber, M.; Bordoli, R. J.; Sedgwlck, R. D. I n "Soft Ionization Mass Spectrometry"; Morris, H. R., Ed.; Heyden: London, 1981, pp 137-152. (9) Barber, M.; Bordoll, R. S.; Sedgwick, R. D.; Tyler, A. N. J . Chem. SOC.,Chem. Commun. 1981, 325. (IO) Surman, D. J.; Vickerman, J. C. J . Chem. Res., Synop. 1981, 170. (11) Surman, D. J.; Vlckerman, J. C. J . Chem. SOC.,Chem. Commun. 1981, 324. (12) Barber, M.; Bardoli, R. S.; Elliott, 0. J.; Sedgwlck, R. D.: Tyler, A. N. Anal. Chem. 1982, 5 4 , 645A. (13) Wiillams, D. H., Bradley, C.; Bojesen, G.; Santikarn, S.; Taylor, L. C. E. J . Am. Chem. SOC. 1981, 103, 5700. (14) Forsberg, L. S.; Dell, A.; Waiton, D. J.; Ballou, C. E. J . Biol. Chem. 1982, 257, 3555. (15) Whitney, J.; Lewis, S.; Straub, K. M.; Thaler, M. M.; Burlingame, A. L.
RECEIVED for review May 7, 1982. Accepted July 14, 1982. Presented in part a t Euchem '82 Conference on Ion Beams, Gregynog, Wales, March 29-April 2, 1982. This work was supported by National Institutes of Health Division of Research Resources Grant RR00719/RR01614. Purchase of the negative ion unit for the Kratos MS-50s was made possible by a grant from the Academic Senate, University of California, San Francisco, 1981.
Thermal Lens Spectrophotometry Based on Pulsed Laser Excitation Kenji Morl, Totaro Imasaka, and Nobuhiko Ishlbashl" Faculty of Englneering, Kyushu University, Hakozaki, Fukuoka 8 12, Japan
A pulsed dye laser Is used as a selective exciting source In thermal lens spectrophotometry. The concentratlons of Cu( I I ) are determlned wlth porphyrln compounds as color reagents, whlch have B relatively sharp Soret band wlth a very large molar absorptivity. On the basis of metal Ion exchange, the direct determlnatlon of Cu(I1) In an aqueous solutlon Is carrled out uslng lead( I I ) tetrakls( N-methyl-4pyrldy1)porphlnetetra-p-toluenesulfonate(TMPyP). The analytical curve Is llnear In the range of (0-2.4) X lo-' M. Analysls Is also done based on solvent extraction Into benzene with meso -tetraphenylporphlnetrlsulfonic acid sulfate (TPPS). The solvent provldes a large blank slgnal because of Its large two-photon absorption cross sectlon ('A,, 'B,"). The present system enables the detection of 1 X lo-' M porphyrin in chloroform, whlch corresponds to an absorptlvlty of 4.7 X The enhancement factors calculated from optlcal parameters are provlded and the advantages of pulsed laser excltatlon are dlscussed.
-
Lasers have been recently used in several analytical applications. In fluorimetry the emission intensity is proportional to the intensity of an exciting source. The laser has usually a large output power so that it can be used advantageously as the exciting source. Sensitivity of conventional spectrophotometry is not determined by its source intensity, but by its source stability. Direct application of the lasers 0003-2700/82/0354-2034$01.25/0
for a light source in conventional spectrophotometry provides few advantages with respect to sensitivity. However, the signal intensity is proportional to the excitation intensity in some spectrophotometry and the use of the laser source enables detection of very small absorption by making the most of its capability. Under irradiation of the strong laser with a Gaussian beam profile, local heating takes place along a laser beam. It leads to a gradient of refractive index in the direction of radius and produces a thermal lens ( I ) . In thermal lens spectrophotometry the signal intensity is proportional to the power of the heating laser source (2); therefore this method is promising for determination of ultratrace samples with no fluorescence. Dovichi and Harris have reported the determination of Cu(I1) with EDTA using a single beam system, including a He-Ne laser as a heating source ( 3 ) . A dual beam system, consisting of a strong heating source and a stable probe beam, has been reported to have great advantages ( 4 ) . This dual beam system promises to provide a considerable enhancement factor (a relative sensitivity to conventional spectrophotometry) and to detect the sample at trace levels (2,5). Dovichi and Harris have also used the Arf laser as the heating source and have shown that the noise level could be reduced down to A = 7 X by measuring transient decay of the thermal lens signal (6). Recently thermal lens spectrophotometry based on image detection is reported to be quite useful for a reliable and accurate measurement of the thermal lens effect (7). This system allows the determination of Fe(I1) in an M and the deteraqueous solution at the level of 2 X Q 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982 n H e - N e Laser
Figure 1.
NZ Laserl
Block diagram of experimental apparatus.
mination of Fe(I1) complex in chloroform at the level of 3 X
M. Porphyrins are well-known to have a Soret band, whose molar absorptivity exceeds lo5 (8). This value is considered to be a theoretical limit for usual organic compounds. Therefore, the porphyrins might be versatile compounds as color reagents for ultratrace analyses. The Soret band has a large molar absorptivity, but it has a narrow band width (10-20 nm). Then, a spectrophotometer with good resolution is necessary in the measurement for the selective determination of the sample. Continuous wave (CW) lasers are currently used in analytical applications. An Ar+ laser has a high average power and good stability. Unfortunately, no line coincides with the Soret band of porphyrins and their metal complexes. Therefore, the laser source, whose wavelength can be changed flexibly, is necessary for the determination of the porphyrin compounds. A CW dye laser pumped by the Ar+ laser is tunable and has good beam coherence. Then, it might be an attractive exciting source for thermal lens spectrophotometry. It is possible to oscillate the dye laser a t the wavelength of the Soret band (near 420 nm), but the Ar+ laser with a large output power in the UV region (333.6-363.8 nm) is required. On the other hand, a pulsed dye laser has a wider tunability range (360-1000 nm), and it has been used for recording thermal lens absorption spectra in the study of the electronic excited states of organic and inorganic molecules (S13). The pulsed laser might be conveniently used as the exciting source in the present application. In this study we applied a nitrogen-laser-pumped dye laser as the exciting source of thermal lens spectrophotometry in the determination of Cu(I1) with porphyrin compounds and investigated various factors which influence the detection limit. We also discuss analytical sensitivity of pulsed-excitation thermal lens spectrophotometry and its advantage as an analytical tool for trace analysis.
EXPERIMENTAL SECTION Apparatus. Figure 1shows a block diagram of the dual-beam thermal lens spectrophotometer with a pulsed dye laser source. The nitrogen laser used as the pumping source of the dye laser has an output power of 0.5 mJ/pulse. The laser dye is 1,4-bis[2-(5-phenyloxazolyl)]benzene(POPOP, ,A = 417 nm) and has an output power of 20 pJ/pulse at 3 Hz.The laser is used in an untuned configuration, since the wavelength of the laser coincides with Soret bands of porphyrin compounds. In order to obtain a laser beam whose profile is close to a Gaussian distribution, we placed a pinhole between the dye cell and the output mirror. This pinhole decreased the laser output power down by half to one-third but improved the coherence of the laser beam. The observed beam profile by a photodiode array was slightly flat at beam center and was not an exact Gaussian. The beam radius at the waist was about 0.1 mm. After being collimated with a lens (Ll,focal length 25 cm), the dye laser beam is split by a quartz wedge. The reflected beam is detected by a photodiode (P.D.l), and the signal is discriminated by an electronic circuit. It is introduced into an analog-to-digital input output interface (Sord, HC AIO) of a microcomputer (Sord, M223, Mark 111) and used as a start signal
2035
for data collection. The probe beam, a He-Ne laser (NEC, GLG2026), is coaxially aligned with the dye laser beam by adjusting a beam splitter (B.S.2) made of a quartz wedge. The laser beams are focused by a lens (L2,focal length 10 cm) into a sample. A sample cell is a conventional 1-cm square quartz cuvette whose position is adjusted to give a maximum thermal lens signal. The heating beam of the dye laser is completely absorbed by a color filter (Toshiba, V-R 63), which is placed far away from the beam waists of the lasers in order to reduce the thermal lens effect induced in this filter. The intensity at beam center of the He-Ne laser passing through the fiter is detected by a photodiode (P.D.2). The photocurrent is amplified by a picoammeter (Keithley 417, response time 4 ms). The background level which corresponds to I , is subtracted by an electronic circuit in the amplifier. The analog signal is introduced into the AI0 of the microcomputer. The data are accumulated during the specified time, and the signals induced by the thermal lens effect are calculated according to programs written by BASIC. Data Processing. The sampling interval of the analog signal is determined by the sampling rate of the microcomputer and adjusted to be 0.5 or 1 ms. The average value of the signal intensity obtained between 50 and 100 ms after laser excitation was subtracted from the value of the minimum intensity (peak value) and used as the thermal lens signal for the construction of an analytical curve. The following data processing is also used in order to improve the sensitivity of the instrument, which is based on the idea to subtract the background signal preceding the thermal lens signal. The pumping source of the nitrogen laser was operated at a constant repetition rate by using a pulse generator which triggers the spark gap switch of the nitrogen laser. The pulse signal from the P.D.1 initiates data sampling. The signal intensity recorded by P.D.2 decreases quickly and recovers gradually. After the specified time the signal intensity decreases quickly again by the following heating pulse. This transient signal of the thermal lens effect is repeatedly recorded and accumulated 10-100 times. Ten successive background signals preceding the thermal lens signal were integrated by the microcomputer and subtracted from the data integrated ten successive thermal lens signals at around the minimum. Reagents. Colorimetric reagents of rneso-tetraphenylporphinetrisulfonic acid sulfate (TPPS), tetrakis(4-N-methylpyridy1)porphinetetra-p-toluenesulfonate (TMPyP), and rnesotetraphenylporphine (TPP) are obtained from Dojindo Laboratory. The extraction reagent of trioctylmethylammoniumchloride (Capriquat) is from Dojindo Laboratory. They are used without further purification. The organic solvents of chloroform and benzene and all inorganic reagents are guaranteed grade. The water is doubly distilled and deionized. Determination in Aqueous Solution. Determination of Cu(I1) in this study is based on the ion exchange reaction of Cu(I1) with Pb(I1)TMPyP. For a preparation of a color reagent solution, 5 mL of 1 X M TMPyP aqueous solution, 5 mL of 1 X M Pb(I1) solution, 10 mL of 0.1 M sodium tartrate, and a buffer solution of 5 mL of 0.5 M sodium borate and 1 M sodium hydroxide were mixed and diluted to 250 mL at the room temperature. It was used as 2 X 10" M Pb(I1) TMPyP solution. In 50-mL beakers 10 mL of this complex solution was added to the sample solutions containing 0, 2, 4, 6, and 8 mL of 2 X lov5M Cu(I1) solution. Each solution was heated to 80-90 "C, and the metal exchange reaction took place almost completely in 2 min. After the solutions were cooled, the samples were filled to 50 mL accurately with water by volumetric flasks. The advantages of the ion exchange reaction in the determination of the metal ion with porphyrin compounds are shown elsewhere in detail (14). Determination by Solvent Extraction. The samples were prepared by mixing the following solution in turn: 0,2,4,6, and 8 mL of 1 X lo4 M Cu(I1) solution, 1mL of 1 M acetate buffer M TPPS aqueous solution. The (pH 4), and 1 mL of 5 X samples were heated on a water bath at 100 "C during 5 min. After the mixtures were cooled, they were transferred to 100-mLseparatory funnels. The 2-mL solution of 2.5 M ammonium sulfate was added and filled up to 50 mL. Furthermore, 10 mL of 5% Capriquat in benzene was added and shaken during 5 min. The aqueous phase was discarded. The solution of 2 mL of 4 M sulfuric acid was added and shaken during 30 s. The aqueous phase was
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
r L ! i
( I ) C W Laser Light
-Time
(*)Thermal Lens Signal
"w 0 (A) CW Laser
Pulsed Laser (3)Light (4) Thermal Signal Lens
,
where I p ( t = a) and I,@ = t ) are the probe beam intensities at beam center just before and after (t = t ) irradiation of the heating laser, respectively, and z is the sample position from the beam waist of the probe laser. The beam radius of the heating laser with a Gaussian beam profile is
where wopis the beam radius at the beam waist, z'is the sample position from the beam waist of the heating laser, b, is the confocal distance, and A, is the wavelength of the heating laser.
-Time
1. +---I--
0 (B) Pulsed Laser
Flgure 2. Signal wave forms of thermal lens signals under CW laser
and pulsed laser excitation. discarded, again. Each leg of the funnels was packed with defatted cotton to remove an aggregate of free TPPS and Capriquat. Organic phases were transferred to the sample cell through cotton, and the sample absorption was measured. The detail of the procedure is shown elsewhere (15).
RESULTS AND DISCUSSION Signal Wave Form. Figure 2 shows schematic wave forms of thermal lens signals obtained by using CW and pulsed lasers as heating sources. In the case of CW laser excitation, the intensity of the probe laser at beam center decreases gradually after irradiation of the heating source by the thermal lens effect, reaching a constant value by equilibrium of heating and decay processes as shown in Figure 2(2). In the case of pulsed laser excitation, the pulse width of the heating laser and the rate of radiationless transition are very fast in comparison with a decay rate of the thermal lens effect. The signal intensity reaches the minimum value instantly and decays with a characteristic time constant as in Figure 2(4). The pulsed thermal lens system has distinct advantages with respect to its large enhancement factor and its background subtraction capability. Quantitative analyses are given by Twarowski and Kliger (16) and Bailey et al. (17) and their experimental verifications are shown elsewhere (9, 18-21). Analysis. In this section we introduce the thermal lens signal under pulsed laser excitation and discuss the effect of mode mismatch of the heating and probe lasers. The focal length of the induced thermal lens by pulsed laser irradiation is given by Twarowski and Kliger as follows for the one-photon absorption process (16) 1 1 - = -(1 2t/t,)-2 (1)
f
fo
+
where f is the time-dependent focal length of the thermal Iens, fo is the focal length just after heating pulse, t is the time after
heating, t , is the characteristic time constant of decay; 1 is the sample length, D is thermal diffusivity, N is the number of molecules, CJ is the cross section of absorption, h is Planck's constant, V , is the frequency of the heating laser, H is the total output energy of the laser, k is thermal conductivity, J is Joule's constant, (dq/dT) is the variation of refractive index with temperature, p is the density, C, is the specific heat, and P(t)is the intensity of the heating laser. When the focal length of the thermal lens is long (f >> l),the signal intensity can be expressed by (16) I p ( t = m) - I p ( t = t ) Z = (4)
s,
I,(t= t )
=
-27
z'=z-zo (6) where zo is the separation of the beam waist positions of the heating and probe lasers. From eq 1-6 we can obtain the thermal lens signal under mode-mismatch conditions.
s, = -2 Z
( l i ? ) ) (1
+ ( Z - z o ) 2 / b , 2 ) 2(7)
For maximum signal intensity the sample position should be adjusted to
+ 3(zO2+ b:)
2z0 f d 4 z o 2 z=
(8)
3
Equations 7 and 8 inform us that mode matching is not essential necessarily in the thermal lens spectrophotometry, but it is rather useful to obtain a larger signal intensity, At different distances on both sides of the beam waist of the probe laser, thermal defocusing and focusing effects are observed. The absolute intensity for the focusing or defocusing effect, which depends on the sign of zo, is larger than the signal intensity obtained in mode-match conditions. The signal intensity given by eq 7 increases monotonically with increasing z under z' = 0. However, it should be noticed that the parabolic assumption used in obtaining the eq 1 becomes poor in this extreme condition. In our experiment the ratio of the signal intensities for the thermal focusing and defocusing effect varied from 0.2 to 5, depending on the beam divergence of the dye laser and the position of the collimating lens (LJ. The effect of mode-mismatch under CW laser excitation is discussed in detail elsewhere (22). Enhancement Factor. Under beam mode-match conditions, the transient signal intensity can be expressed from eq 7 by
The second factor represents the characteristics of the heating laser and can be expressed by
--
- - E,
cw:
A%, :
hv:H
(11)
where Et (J) is the pulsed energy of the heating laser, A, (m) is the wavelength, and mop (m) is the beam radius. The third factor represents the characteristics of the solvent and can be expressed by D(dv/dT)
kJ
(dv/dV =--PCP
(12)
where p (g/m3) is density and C, (J K-l g-l) is specific heat.
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12,OCTOBER 1982
2037
Table I. Enhancement Factor for Pulsed (E,) and Continuous Wave ( E , ) Elxcitation"
io#,
solvent pentane
n -hexane n -hep tane cyclopentane cyclohexane benzene toluene xylene tetralin pyridine aniline nitrobenzene chloroform CCI,
cs,
methanol ethanol propanol butanol acetone water
w m-1 K-I
1130 1230 1260 1280 1230 1440 1330 1320 1300 1670 1720 1510 1150 1030 1510 2010 1670 1570 1520 1590 6110
io-^, J
c
8'1
gm-3
g-l
0.63 0.66 0.68 0.74 0.78 0.88 0.87 0.86 0.97 0.98 1.02 1.21 1.49 1.59 1.26 0.79 0.79 0.80 0.81 0.79 1.00
2.38 2.27 2.25 2.23 1.67 1.74 1.76 1.74 1.65 1.68 2.06 1.52 0.96 0.86
1.00 2.55 2.42 2.40 2.42 2.16 4.19
drll dT x 104, 1/K E, 5.5 5.2 5.0 5.7 5.4 6.4 5.6 4.8 4.3 5.6 5.2 4.6 5.8 5.8 7.7 3.9 3.9 3.9 3.9 5.0 0.8
400 360 340 360 430 430 380 330 280 350 260 260 420 440 640 200 210 210 210 300 20
E, 9.7 8.5 7.9 8.9 8.8 8.9 8.4 7.3 6.6 6.7 6.0 6.1 10.1. 11.3 10.2 3.9 4.7 5.0 5.1 6.3 0.3
m. CW Pulsed laser (E,); E., = 1 mJ, w O p= 0.1X laser (E,); output power = 1 mW. h , = 500 nm; T = 300 (I
K.
The fourth factor represents the characteristics of the sample and can be expressed bly
1Na
1. - lo-*
N
2.30314
(13)
where A is absorptivity of the sample. When absorptivity is small, the signal intensitiy for conventional spectrophotometry is given by
IS
2.3034
The enhancement factor d e f i e d by the ratio of the sensitivity for thermal lens and conventional spectrophotometry is given by
The enhancement factors calculated from the optical and physical parameters according to eq 15 me shown in Table I. The enhancement factors calculated for CW excitation are also included in the last column (2). When the pulsed laser is operated at the repetition rate of 1Hz and operated under the same average power with a CW laser, very large enhancement in the thermal lens signal can be obtained for pulsed laser excitation. tis in the case of CW laser excitation, water is a poor solvent a n d organic solvents such as chloroform, benzene, and carbon disulfide have very large enhancement factors. Then, it is apparent that the use of solvent extraction into an organic phase has a distinct advantage for the sensitive detection of the sample. Transient Signal. Transient curves of thermal lens signals obtained with pulsed laser excitation are shown in Figure 3. The output signal from ithe photodiode is negative, and the signal is inverted in this figure. The signal intensity is proportional to the concentration of the Cu(I1) TPPS complex, and it shows that an analytical curve can be constructed under pulsed laser excitation. The plot of (Sp/Sp(t=o))-l/z vs. t provides a characteristic time constant t,. The experimental result for benzene was not an exact straight line but it gave the t, value of 10-20 ms, which compares favorably to the theoretical
"
g o
6
10
20
30
40
50
Time ( rn 5 )
Flgure 3. Thermal lens signals observed for sample of Cu(I1) TPPS complex in benzene. Sample concentrations are as follows: (1) benzene only, (2) 0.8 X lo-', (3)1.6 X (4) 2.4 X (5) 3.2 X (6)4.0 X M.
value of 53 ms. The thermal lens signal in Figure 3 recovers in 50 ms, and the repetition rate of the laser may be increased up to 20 Hz. The actual repetition rate in the experiment is limited by the accumulation rate of the microcomputer. Determination of Cu(I1) with Pb(I1) TMPyP. TMPyP is one of the most useful porphyrin compound soluble in water and can be used for the determination of various metals. TMPyP forms metal complexes in an aqueous solution, but it has an inherent problem for its analytical use. The wavelength of the absorption band for the Cu(I1) complex (kmm = 425 nm, Ax = 25 nm) is close to that for the ligand molecule of TMPyP (423 nm). It is suggested that a lead(I1)-porphyrin complex be used as a color reagent, since it has an absorption band a t around 476 nm (23). The analytical curve in the determination of Cu(I1) with Pb(I1) TMPyP was constructed based on thermal lens spectrophotometry using the pulsed dye laser. The measurement was carried out by recording the increase of the absorption band a t 425 nm. The analytical curve was linear in the (0-2.4) X lo4 M region. The background signal corresponded to 1.8 X lo4 M of the sample. In order to improve the detection limit, it may be necessary to improve the sensitivity of the spectrophotometer and to reduce background absorption originating from Pb(I1) TMPyP. Determination of Cu(I1) with TPPS Based on Solvent Extraction. The determination of Cu(1I) was also possible using TPPS based on solvent extraction, and a linear analytical curve was obtained in the (0-8) X M range. The background signal corresponded to 8.3 X M of the sample in this procedure. For improved sensitivity of the determination, the Cu(I1) complex is extracted into the benzene solution including Capriquat. Free TPPS providing blank signals cannot be dissolved in both the solutions of the aqueous and organic phases under concentrated H2S0,, so that free T P P S can be removed as an aggregate from the sample solution. The extracted Cu(I1) TPPS complex has an absorption maximum at h = 417 nm (Ax = 11 nm) and can be excited by the POPOP dye laser. This solvent extraction procedure developed by Ishii et al. makes it possible to remove the background absorption by free TPPS (15). Moreover, the thermal lens signal can be enhanced more than 20 times by the use of benzene as the solvent. Then, this analytical procedure may be useful for the sensitive detection of the sample in thermal lens spectrophotometry. Analysis is carried out a t yet lower concentrations in comparison with that in the aqueous phase. The analytical curve was also constructed by using a conventional spectrophotometer. We noticed that the background signal obtained by using a thermal lens spectrophotometer is larger than that obtained by using a conventional spectrophotometer. The background absorption of the solution blank with no Cu(I1) ion increases with decreasing the absorption wavelength in conventional spectrophotometry, and the background is considered to be coming from Capriquat. This background absorption was equivalent
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
to 2.2 X M. In the case of thermal lens spectrophotometry, additional background which corresponds to 6.1 x M originates in the determination of Cu(I1). Even when the blank solution was replaced by the solvent containing benzene only, a large background was observed. Toluene and xylene also provided the strong background signals. The two-photon absorption cross section for these aromatic molecules is not so small at around 417 nm which corresponds to the transition of lAl, lBlu (9). Therefore, the background signal was coming mainly from two-photon absorption of the solvent molecule. For the solvent of chloroform, no appreciable background was observed. Dovichi and Harris have reported a differential technique to reduce the blank signal (24). This technique is also useful for subtraction of the background signal due to two-photon absorption of the solvent molecule as can be understood from eq 7. However, the signal intensities under the thermal defocusing and focusing effects in the dual beam experiment are not necessarily identical as described before. The optical configuration is used under mode-mismatch conditions in this study to increase the enhancement factor. In this case the sample cell should be placed far from the optimum position for the use of the differential technique. So that, the signal enhancement becomes rather poor in comparison with that obtained in mode-match conditions. It may be suggested to use the mode-match configuration for this purpose, but the positions of the beam waists of the heating and probe lasers and the sample and blank cells should be adjusted critically. Sensitivity of Spectrophotometer. In order to examine the sensitivity of the present spectrophotometer, we measured the sample of tetraphenylporphine (TPP, A,, = 418 nm, AA = 11nm) in chloroform a t low concentrations. The detection of the sample with the absorptivity of 4.7 X was possible by accumulating the signal 100 times. The observed SIN ratio was about 2. For improved sensitivity, a He-Ne laser was replaced by one with stable output power, and a photodiode and an amplifier were replaced by a photomultiplier. Furthermore, much improved signal processing shown in the Data Processing paragraphs in the Experimental Section was used. Under these conditions the SIN ratio could be improved even when accumulation was carried out only 10 times. The calculated detection limit in this condition was A = 4.7 X lo", corresponding to 1 X M of porphyrin. For the detection of the samples at yet lower concentrations, the use of a heating laser with a higher output power and a more stable probe laser may be necessary. It is noted that the tunable dye lasers with an output power exceeding 70 mJ/pulse and the probe laser with the stability of 0.01% are already commercially available. The use of a longer accumulation time and signal accumulation equipment with a fast data aquisition rate might be promising. Comparison with CW Excitation. A CW Ar+ laser has been currently used as a heating source in thermal lens spectrophotometry because of its high average power and good beam coherence. Though pulsed dye laser has a low average power and poor beam quality, it has various advantages with respect to its tunability and its sensitivity. The CW argon or krypton ion laser has several discrete emission lines in the UV and VIS regions, but its tunability is considerably limited. It is stressed that pulsed dye laser excitation is useful not only for selective excitation of the sample because of its tunability but also for the sensitive detection of the sample because of its large enhancement factor. Furthermore, the thermal lens signal arises quickly from its constant background level and it can be subtracted precisely and then the very small signal
-
can be detected. As a result, thermal lens spectrophotometry using pulsed lasers may be expected to provide the better detection limit if the average power of the heating source is identical. Using a 4-mW He-Ne laser as a heating source, a minimum detectable absorbance of 1.0 X has been found (3). The detection sensitivity in this condition is calculated to be 4.0 X 10-3/mW. It may be emphasized that the present thermal lens spectrophotometer has enough sensitivity to detect an absorptivity of 4.7 X using a heating laser with only 60 pW in the average power. In this condition the detection sensitivity is 2.8 X lO"/mW. The ratio of the enhancement factors (110) for pulsed excitation (430) under 1mJ/pulse and CW excitation (3.8) under 1 mW almost corresponds to the ratio of the sensitivities (140) of thermal lens spectrophotometry. Dovichi and Harris have applied thermal lens spectrophotometry for flow analysis and reported that the thermal lens signal is sensitive to the flow rate of the stream (25). I t is noted that the response time of the thermal lens spectrophotometer using the pulsed laser is much faster, and the sensitivity of the thermal lens signal is less affected by the flow rate and its variation of the stream. The use of very specific color reagents is essential for the selective and sensitive determination of the sample. We note that many specific colorimetric reagents have an absorption band in the blue or green regions and their metal complexes in the red region. We consider that the thermal lens system including a pulsed exciting source in the red region will play an important role for its practical use in future.
LITERATURE CITED Gordon, J. P.; Leite, R. C. C.; Moore, R. S.;Porto, S. P. S.; Whinnery, J. R. J . Appl. Phys. 1965, 36,3-8. Mlyalshi, K.; Imasaka, T.; Ishibashi, N. Anal. Chim. Acta 1981, 124, 381-389. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1979, 51, 728-731. Long, M. E.; Swofford, R. L.; Aibrecht, A. C. Science 1976, 191, 183-185. Imasaka, T.; Miyaishi, K.; Ishibashi, N. Anal. Chim. Acta 1980, 115, 407-410. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1981, 5 3 , 106-109. Miyaishi, K.; Imasaka, T.: Ishibashi, N. Anal. Chem ., in press. Yothuyanagi, T.; Itoh, J. Kagaku no Ryoiki 1977, 31, 146-153. Twarowski, A. J.; Kliger, D. S. Chem. Phys. 1977, 20, 259-264. Vaida. V.: Turner, R. E.; Casev, J. L.; Colson. S. D. Chem. Phys. Lett. 1978, 54,25-29. Nleman, G. C.; Colson, S. D. J . Chem. Phys. 1978, 68,2994-2996. Colson, S.D.; Cheung, W. Y.; Glownia, J. H.; Riley, S. J. Chem. Phys. Lett. 1980, 76,515-519. Ziegier, L. D.; Hudson, B. S. Chem. Phys. Lett. 1980, 71,113-116. Ishil. H.: Koh. H.: Satoh. K. Nimon Kauaku Kaishi 1980, 1919-1921. Ishii, H.; Satoh, K.; Koh, H. Bbnseki Kigaku 1960, 29, 276-277. Twarowski, A. J.; Kliger, D. S. Chem. Phys. 1977, 20, 253-258. Bailey, R. T.; Crulckshank, F. R.; Pugh, D.; Johnstone, W. J . Chem. SOC., Faraday Trans. 2 1980, 76,633-647. Fuke, K.; Itoh, M. Jpn. J . Appl. Phys. 1980, 49,837-841. Bailey, R. T.; Cruickshank, F. R.; Johnstone, W.; Pugh, D. J . Mol. Struct. 1980, 6 0 , 25-29. Bailey, R. T.; Cruickshank, F. R.; Pugh, D.; Johnstone, W. J . Chem. SOC.,Faraday Trans. 2 1981, 77,1367-1397. Gupta, M. C.; Hong, S.; Gupta, A,; Moacanin, J. Appl. Phys Lett. 1980, 37,505-507. Imasaka, T.; Higashi, T.; Ishibashi. N., unpublished results. Igarashi, S.; Kobayashi, J.; Yotsuyanagi, T.; Aomura, K. Nippon Kagaku Kalshll979, 602-606. Dovlchi, N. J.; Harris, J. M. Anal. Chem. 1980, 52,2338-2342. Dovichl, N. J.; Harrls, J. M. Anal. Chem. 1981, 5 3 , 689-692.
RECEIVED for review November 12, 1982. Accepted July 6, 1982. This research is supported by a Grant-in-Aid for Scientific Research (Grant No. 00547061) from the Ministry of Education of Japan and by a Steel Industry Foundation for the Advancement of Environmental Protection Technology.
