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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
Simultaneous Determination of Mixtures in Liquid by Laser-Induced Photoacoustic Spectroscopy Shohei Oda," Tsuguo Sawada, Mitsuaki Nomura, and Hitoshi Kamada Department of Industrial Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, Japan
Laser-induced photoacoustic spectroscopy (LIPAS) has been applied to the determination of binary liquid mixtures. Using tar food dyes such as Amaranth, New Coccine, and Sunset Yellow FCF, the additivity of the photoacoustic signal Intensity was examined. The analysis of mixtures of ultra low concentrations of the above dyes in water clearly showed that LIPAS was applicable to the simultaneous determination of mixtures with highly overlapping absorption spectra, the mixtures which have been extremely difficult to analyze by conventional colorimetric and dual-wavelength absorption spectrometry.
Spectroscopy utilizing the photoacoustic effect has been developed in recent years, as exemplified by the use of laser with high monochromaticity and high power, to significantly improve t h e detection sensitivity. Laser-induced photoacoustic spectroscopy (LIPAS) has been used particularly for the determination of trace amounts of several toxic gases in the environmental atmosphere because of its high detection sensitivity and its responsiveness in real time. For instance, a few ppb of NO (I-3), NOz ( 4 ) ,NH3 ( 5 ) ,C2H4( 6 ) ,and other hydrocarbons (7)in air, exhaust gases, and other environments have been measured with this method. Additionally, LIPAS has been shown to be highly sensitive in the measurement of liquid samples ( 8 , 9 ) . T h e authors have successfully applied this method to the determination of ultra trace levels of cadmium in bio-samples where a detection limit of 14 ppt was obtained ( I O ) . This value was about two orders of magnitude lower than the value obtained by the conventional flame atomic absorption spectrometry. The photoacoustic signal is obtained by measuring pressure fluctuations caused by heat generated through radiationless relaxation. The additive property of the photoacoustic signal intensity is not always observed. Consequently, the additive property cannot be expected to exist in some cases: (1)where the heat produced individually by the two components is significantly out of phase with each other because of differences in thermal relaxation times, (2) where intermolecular energy transfer, including chemical reaction of the excited state occurs, or (3) for the case where a fluorescent component is quenched by adding another component. In applying LIPAS to a binary liquid mixture, if the photoacoustic signal is additive with respect to component concentration, then a simultaneous determination of the components can be made in situ by using a combination of lasing lines without resorting t o any prior separation procedures. Since argon ion and krypton ion lasers offer many lasing lines to choose, this new procedure should be now widely applicable, where in the past this type of determination has been extremely difficult to achieve by conventional colorimetric spectrometry because of overlap of the absorption spectra. I n the present investigation, some tar food dyes were chosen as colored samples whose absorption coincided with the lasing line wavelengths of an argon ion laser. The tar food dyes have been used very frequently as coloring agents with dye mixtures 0003-2700/79/035 1-0686$01.OO/O
of two or more. On the other hand, as the toxicity of such dyes has been paid much attention recently as a serious social problem, their trace determination should become important. Therefore, by preparing extremely low concentration dye mixtures, simultaneous determinations were attemped. EXPERIMENTAL Reagents. Three tar food dyes (Amaranth, New Coccine, and Sunset Yellow FCF) used as reagent? were dye standards of the National Institute of Hygienic Science in Japan. Dye solutions, 1.0 mM, were prepared as a stock solution. The stock solutions were diluted with water and adjusted to low dye Concentration. Twice-distilled deionized water was used. Apparatus a n d Experimental Condition. The same apparatus as that described in a previous report (10) was used. As for the photoacoustic sample cell, it was improved in the point that the volume of the previous cylindrical sample cell (length 90 mm, inner diameter 24 mm, and piezoelectric ceramic length 50 mm) was about 40 mL, while that of the new one was about 8 mL (length 50 mm, inner diameter 15 mm, and piezoelectric ceramic length 15 mm). In addition, the piezoelectric ceramic was exchanged for a more sensitive one (NPM, N-21, Tohoku Kinzoku Co. Ltd.). Owing to the modification of the sample cell, the detection sensitivity was improved threefold. An argon ion laser (Spectra Physics Model 164-03), operating in single line modes of 476.5, 488.0, and 5143 nm, was modulated at a given frequency (185 Hz) and the laser power was 300 mW. The absorption spectra of the food dye solutions were measured by a Shimazu Model MPS-5000 spectrophotometer. RESULTS AND DISCUSSION E x a m i n a t i o n of the Additivity of t h e P h o t o a c o u s t i c Signal. Figure 1 shows absorption spectra of the three dyes. The additivity of photoacoustic signal intensity was investigated with Amaranth and Sunset Yellow FCF by adding a given concentration of Sunset Yellow FCF as a n interfering component to various concentrations of Amaranth solution and analytical curves were made. The result is shown in Figure 2 . Using a least squares procedure, the slopes of the curves were (1)0.134, (2) 0.133, and (3) 0.144 a t 488.0 nm and (1) 0.188, (2) 0.201, and (3) 0.189 at 514.5 nm. The variation coefficients among the values of those slopes were 3.6% a t 488.0 nm and 3.1% at 514.5 nm and were within the range of the experimental error of the LIPAS technique. T h e intercepts were 0.788 (488.0 nm) and 0.678 (514.5 nm) for analytical curve 2 and 1.53 (488.0 nm) and 1.36 (514.5 nm) for curve 3. As can be seen, if the concentration of Sunset Yellow FCF was doubled, the value of the intercept increased twofold. Similar relationships were obtained from analytical curves of Sunset Yellow FCF interfered by Amaranth. Based on these results, the additivity of photoacoustic signal intensity with respect to component concentration was found to be valid for this mixture. For the Amaranth-New Coccine mixtures whose absorption spectra substantially overlap each other, additivity also was confirmed. The experimental results are shown in Figure 3. Though the same additivity can be expected for Sunset Yellow FCF-New Coccine mixtures, i t was not examined in the present experiment. S i m u l t a n e o u s D e t e r m i n a t i o n of T w o - C o m p o n e n t Mixtures. In the case where the additivity is valid as 0 1979 American Chemical
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979 476.5 4 8 8 0 5j45
I
I
687
Table I. Results for the Simultaneous Determination of Amaranth-Sunset Yellow FCF Mixtures
8
sample no. namea 1 A S
2
4
5
WAVELENGTH ( n m )
S
A
5.0
S
5.0 4.0
A A
A S
Figure 1. Absorption spectra of tar food dye solutions. (1) Sunset Yellow FCF. (2) New Coccine. (3)Amaranth. Concentration: M; dashed lines: wavelength of argon ion laser
6
A S
7
-/
a
10.0
1.0 10.0 2.0 8.0 4.0
S
3
concentration 10.' M taken found
A S
10.3 2.8
rel. error, ?6 +3.0 + 180.
9.7 2.9
- 3.0
--
45.0 2.5 7- 7.5
7.8
-
4.3 5.4 4.7
+ 8.0
4.4
-6.0 -t 10.0
8.7
8.0 2.0
7.3
2.6
-t 30.0
10.0 1.0
9.6 0.6
-40.0
10.0
10.6
-
-4.0
+ 6.0
A: Amaranth, S: Sunset Yellow FCF.
Table 11. Results for the Simultaneous Determination of Amaranth-New Coccine Mixtures sample no. namea 1 2 CONCENTRATION ( xlO-'Mi)
10.0
10.8
+ 8.0
N
2.0
A
8.0 4.0
3.7 8.7
--15.0 + 8.7
I
I
CONCENTRATION (xlO.'M) Figure 3.
Analytical curves of New Coccine interfering by Amaranth. not added. (2) Amaranth: 5.0 X lo-@M. (3) Amaranth:
(1) Amaranth: 1.0 x 10-7 M
mentioned above, by solving a set of simultaneous equations with two unknowns obtained from measurements a t two wavelengths, the simultaneous determination of binary liquid mixtures becomes possible using LIPAS. Therefore, the calibration graphs were made for the three tar food dyes shown in Figure 1 a t two lasing wavelengths (488.0 and 514.5 nm) M. A lasing wavelength of in the concentration range of 476.5 nm would be also applicable to Sunset Yellow FCF. For M (0.7 Amaranth, the detection limit obtained was 1.1 X ppb), based on a limiting photoacoustic signal-to-noise ratio of 2:l. This value corresponds to an absorptivity of 2.4 X cm-' which is two orders of magnitude lower than that by conventional absorption spectrometry. Using calibration graphs, simultaneous determinations were shown to be possible for various concentration of Amar-
A
5 a
4.2
+ 5.0 --8.0 - 10.0 4- 10.0
4.6 5.5
N
5.0 5.0 4.0 8.0
A
2.0
2.8
+ 40.0
N
10.0
9.4
- 6.0
N 4
Analytical curves of Amaranth interfering by Sunset Yellow FCF. (1) Sunset Yellow FCF: not added. (2) Sunset Yellow FCF: 5.0 X lo-' M. (3) Sunset Yellow FCF: 1.0 X lo-' M
Figure 2.
rel.
error, 7%
A N
3
concentration 10.; M taken found
A
4.4 7.9
-
1.3
A: Amaranth, N : New Coccine.
anth-Sunset Yellow FCF and Amaranth-New Coccine mixtures. The mixtures prepared were of significantly lower concentration ranges than the detection limit obtainable by conventional colorimetric spectrometry. The results are given in Table I and Table 11. In both Amaranth-Sunset Yellow FCF and Amaranth-New Coccine mixtures, the errors produced in the measurements were within 10% of the actual value up to a mixing ratio of 1:2. However for mixing ratios of 1:5 and l : l O , the error for the small portion component of the mixture became relatively large. The errors in the measurement can be regarded to affect the smaller component more than the larger one. However, considering that the concentration range is as low as M, the errors of such an extent are negligible. The above experiments showed clearly that LIPAS was applicable to the simultaneous determination of systems with highly overlapping absorption spectra such as with Amaranth-New Coccine and Amaranth-Sunset Yellow FCF. These mixtures in the past have been extremely difficult to determine without any prior separation procedures using both ordinary absorption and dual-wavelength absorption spectrometry. Kamikura et al. ( I I ) , determined Amaranth-New Coccine mixtures by masking the other component instrumentally with a dual-wavelength absorption spectrometer. They described that the analyzed component in the conM (0.1 ppm) to 1.8 X centration range from 1.7 X M (11 ppm) could be determined under the existence of the interfering component of 8.3 x lo4 M (5 ppm). T h e sole disadvantage of LIPAS is that the objects to be measured are restricted by the use of lasers. However, stable lasers can be obtained nowadays in the visible wavelength
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
region by using dye lasers in conjunction with argon ion and krypton ion lasers. In the future and with the advent of lasers encompassing all wavelengths including the UV region, LIPAS should become a powerful tool for ultratrace analysis. ACKNOWLEDGMENT One of the authors (M. N.)is indebted to Keiichi Furuya of the Science University of Tokyo for his encouragement.
(3) E. G.Burkhardt. C. A. Lambert. and C. K . N. Patel, Science, 168, 1 1 1 1 (1975). (4) P. C. Claspy, C. Ha, and Y. H. Pao, Appl. Opt., 16, 2972 (1977). (5) L. G.Rosengren, E. Max, and S. T. Eng, J. Phys. E.. 7, 125 (1974). (6)L. B. Kreuzer. N. D. Kenyon, and C. K. N. Patel, Scisnce, 177, 347 (1972). (7) L. B. Kreuzer, Anal. Chem., 46, 239A (1974). (8)W. Lahmann, H. J. Luckwig, and H. Welling, Anal. C b m . , 49,549 (1977). (9) S.Oda, T. Sawada, and H. Karnada, BunsekiKagaku, 27, 269 (1978). (10) S. Oda, T. Sawada. and H. Karnada. Anal. Chem., 50, 865 (1978). ( 1 1 ) M. Karnikura, F. Endo, and H. Sasaki, J . FocdHyg. SOC.Jpn., 13, 555 (1972).
LITERATURE CITED (1) L. B. Kreuzer and C. K. N. Patel, Science, 173, 45 (1971). (2) C. K. N. Patel, E. G. Burkhardt, and C. A. Lambert, Science, 184, 1173 (1974).
for review November
273
Ig78. Accepted January
22, 1979.
Laser-Induced Photoacoustic Spectroscopy of Some Rare Earth Ions in Aqueous Solutions Tsuguo Sawada, * Shohei Oda, Hiromichi Shimizu, and Hitoshi Kamada Department of Industrial Chemistry, Faculty of Engineering, The University of Tokyo, 7-3- I , Hongo, Bunkyo-ku, Tokyo, Japan
The photoacoustic spectra of some trivalent rare earth ions in aqueous solutions were obtained by laser-induced photoacoustic spectroscopy (LIPAS). This method was suitable for the spectral measurements of extremely weak absorption such as a forbidden transition of rare earth ions in liquid. Compared with the absorption and emission spectra in the visible region, the photoacoustic spectrum offered complementary spectroscopic information, particularly with respect to the radiationless decay process.
Photoacoustic spectroscopy is currently being used to provide spectral information on a wide variety of solid and liquid samples ( I , 2), which was previously difficult or impossible to measure by ordinary transmission and reflection methods. The availability of dye lasers as an excitation source capable of a high intensity flux within narrow, tunable bandwidths makes photoacoustic detection a promising method for the investigation of the very weak molecular absorption spectra. Dixon et al. ( 3 ) have measured photoacoustic spectra of forbidden transitions in some unstable gaseous sulfur compounds with a tunable CW dye laser. Recently, Lahmann et al. ( 4 ) and the present authors (5)have successfully applied this method to the ultra trace analysis of liquid samples with a n argon ion laser. Laser-induced photoacoustic spectroscopy (LIPAS) has brought a considerable improvement in the detection limit in comparison with conventional absorption spectrometry. I n the present paper, high resolution photoacoustic spectra of some trivalent rare earth ions (Pr3+,Nd”, Eu3+,Ho”, and Er3+) in aqueous solution were measured by the LIPAS, though the lasing wavelength of dyes used limited the measurable region. It is well known that all the lines of rare earth ions at visible and near-infrared regions are forbidden in nature. The intensities of the transitions are, therefore, extremely weak. The LIPAS is expected to be most suitable for the measurements of such forbidden transition in liquid. EXPERIMENTAL Reagents and Procedure. Each rare earth nitrate was prepared by dissolving the oxide (99.9%) in nitric acid. These
solutions were diluted with distilled water and adjusted to a given concentration of rare earth ion with 0.5 N nitric acid. Apparatus. A block diagram of the LIPAS is shown in Figure 1. A tunable dye laser (Spectra Physics Model 375), pumped by an argon ion laser (Spectra Physics Model 164-03), was modulated at about 200 Hz by a light chopper and divided into two beams with a beam splitter. One laser beam was directed into the sample cell and the other, into the reference cell through the collecting lenses cf = 50 cm). The dyes used as pumped media were Rhodamine 110, Rhodamine 6G, and Rhodamine B with lasing ranges of about 540-600, 570-630, and 600-670 nm, respectively. The dye laser operated in the multimode with a bandwidth of 1 cm-’. The wavelength was read with a monochromator. The average dye laser power was about 300 mW. The laser power variation with wavelength was monitored by a calibrated photocell. The pressure fluctuation induced in the solution by absorbed radiation was detected by a piezoelectric ceramic (NPM, N-21 supplied by Tohoku Kinzoku Co. Ltd.). A lock-in amplifier was used to amplify the modulated output signal, which was obtained by subtracting the photoacoustic signal of the reference cell from that of the sample cell with a differential amplifier. The photoacoustic background signal is mainly due to the absorption of the solvent. The absorption coefficient of water in the 500-nm region is very small, that is, approximately cm-’, and might change with wavelength according to purity. However, the photoacoustic signal caused by absorption of the solvent is large. The double-beam operation enabled this background signal to be greatly reduced. Other experimental descriptions have appeared in the previous literature ( 5 ) . The absorption spectrum was obtained with a 90-mm pathlength cell by using a Shimazu MPS-5000 spectrophotometer. All spectral measurements were carried out at room temperature. RESULTS AND DISCUSSION I t is well known that the sharp lines of the rare earth ions are associated with transitions between configurations of the electrons within the inner 4f shell. All of the sharp lines are forbidden to the first approximation as ordinary electric dipole transitions by Laporte’s rule. However, the inversion of center of the electric field of the free ion is removed by the perturbing surrounding field so the lines appear as electric dipole radiation. These transitions, which are so-called forced electric dipole transitions, are all weak, being of about the intensity of natural magnetic dipole transitions. Praseodymium Ion. The photoacoustic spectrum of Pr3+ ion in aqueous solution in the yellow region (-590 nm) is
0003-2700/79/0351-0688$01.00/00 1979 American Chemical Society
