Determination of photophysical rate constants for the non-protected fluid room temperature phosphorescence of several naphthalene derivatives

Spectrochimica Acta Part A 57 (2001) 1261– 1270 www.elsevier.nl/locate/saa

Determination of photophysical rate constants for the non-protected fluid room temperature phosphorescence of several naphthalene derivatives Long-Di Li a,*, Wen-Qing Long b, Ai-Jun Tong a b

a Department of Chemistry, Tsinghua Uni6ersity, Beijing 100084, People’s Republic of China Department of Chemistry, Jinggang Shan Normal College, Jian 343009, People’s Republic of China

Received 1 September 2000; received in revised form 21 November 2000; accepted 22 November 2000

Abstract The determination of kinetic parameters for luminescence processes is very important in understanding the phosphorescence process and the mechanisms of the heavy atom effect (HAE). In our previous work, we reported that room temperature phosphorescence (RTP) emission of many naphthalene derivatives can be induced directly from their aqueous solution without using any kind of protective medium, and the name Non-Protected Fluid Room Temperature Phosphorescence (NP-RTP) is suggested for this new type of RTP emission. In order to further understand this kind of luminescence phenomenon, the influence of heavy atom perturber (HAP) concentration on RTP lifetime of several naphthalene derivatives was studied in detail in this paper. The possibility of determination of photophysical parameters for emission of NP-RTP was explored based on the definition on the phosphorescence lifetime and the relation with the concentration of HAP in this paper. A static Stern– Volmer equation for phosphorescence was derived and the luminescence kinetic parameters were calculated. The results obtained by two different ways proved that photophysical parameters for RTP emission can be determined based on the changes of the RTP lifetime. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Luminescence kinetic parameters; Naphthalene derivatives; Non-protected fluid room temperature phosphorescence; Potassium iodide; Thallium nitrate

1. Introduction The heavy atom perturber (HAP) can enhance the intersystem crossing rate of T1 – S0, so the rate of radiation and radiationless from T1 to S0 state was enhanced simultaneously. Radiationless deac* Corresponding author. E-mail address: [email protected] (L.-D. Li).

tivation processes includes quenching processes and radiationless deactivation processes of T1 to S0. RTP intensity is proportional to the population of the excited triplet phosphors. Only an appropriate heavy atom effect (HAE) is useful to enhance RTP emission and to increase analytical sensitivity. Despite this the HAE is very important for emission and enhancement of RTP, at present, which remains still at the qualitative de-

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scription level, the selecting of the HAP for a given luminescent system still relies on repetitive experiments on different HAP in practice work. There have been some studies on the kinetic parameters of solid substrate room temperature phosphorescence (SS-RTP) [1]. But up to now, only a few investigations have been reported on the kinetic parameters for fluid RTP emission. Shtykov et al. [2] determined and discussed the decay rate constants for the sensitized room temperature phosphorescence of polycyclic aromatic hydrocarbons (PAHs) triplet states in aqueous micelle solution, as well as the quenching rate constants of these states by thallium ions.Wei Yansheng et al. [3] determined P0 (defined as pre-exponential factor of the RTP decay curve) and t-value of several PAHS in SDS micelle solution. Phosphorescence life t indicates the probability of the triple deactivation. Therefore, it is reasonable that the possible ISC(S1-T1) process of a phosphor, which reflects heavy atom induced effect on ISC of phosphors, can be expressed qualitatively by obtained direct parameters with t. As indicated above, because HAP can promote the rates of radiation and radiationless processes of triplet state, the phosphorescence lifetime would be shortened. Thus, the measurement of t-value may open up a possibility to obtain the kinetic parameters. Huang [4] determined the luminescence kinetic parameters of several amino acids and proteins at low temperature (77 K) by the measurement of t. However, the kinetic parameters on NP-RTP emission, which is a new kind of RTP phenomenon discovered recently [5– 13], have not been reported. In this paper, the influence of HAP on RTP intensity and lifetime of several naphthalene derivatives was studied in detail, the possibility of determination of photophysical parameters for NP-RTP emission was explored based on the definition on the phosphorescence lifetime and relationship with the concentration of HAP.

2. Experimental

2.1. Apparatus All RTP spectra and lifetimes were measured on a Perkin –Elmer LS-50B luminescence spectrome-

ter, the RTP lifetime was performed with Obey– Decay application program and by using a cell holder whose temperature was kept at 259 0.1°C by circulating water. Excitation and emission slits were 15 and 20 nm, respectively. The gate time 2.0 ms and delay time 0.1 ms were kept constant for all of the phosphorescence measurements. All absorption spectra were recorded on a Shimadzu UV 2100S spectrophotometer.

2.2. Reagents Table 1 shows the naphthalene derivatives used in this paper and their respective codes. They were purchased from Beijing Chemical Plant, Shahai Chemical Plant (China), Fluka and E.Merk company respectively. 5× 10 − 4mol/l aqueous solutions were prepared for the code 10, 11 (addition of a little of dilute NaOH solution.) and 12 compounds, 5×10 − 4mol/l acetone–water (4:96,v:v) solutions were prepared for the code 2, 7, 8, 9 compounds, 2.0× 10 − 3mol/L acetone–water (25:75) solutions were prepared for the code 5 and code 6 compounds and 2.0× 10 − 3mol/l acetone–water (30:70) solutions were prepared for the rest of the compounds. KI(AR) and Na2SO3(AR) were obtained from Beijing Chemical Plant (China) and a 1.0 mol/l solution (or solid) of KI and a 0.15 mol/l solution of Na2SO3 were used in experiments. Na2SO3 solutions were prepared when used and kept in a tightly stoppered container. All of the organic solvents used were AR grade, the water used was prepared by twice sub-boiling distillation.

2.3. General procedure A 1 ml aliquot of naphthalene derivative solution was added to a 10 ml quantitative test tube (1 ml acetonitrile was added again for code 3–6 compounds). Different amount of HAP (KI or TlNO3) solution and 1 ml aliquot of 0.1 mol/l Na2SO3 were added in turn, then the solution was filled to the mark with water. After the solution was shaken, the phosphorescence spectrum and lifetime were measured under the constant temperature of 259 0.1°C with a 1 cm quartz cell. Meanwhile, the absorption spectra were recorded using regent blank as a reference.

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Table 1 Chemical structure, name and code of the 12 derivatives studied

Code

Name (grade) and structure

Code

Name (grade) and structure

1 2 3

Naphthalene(AR) R1–R8 = H 1-Naphthol(AR) R1 = −OH 1-Naphthalene chloride(CP) R1 = −Cl 1-Naphthalene bromide(CP) R1 = −Br 1-Naphthol ethyl ether(AR) R1 = −OCH2CH3 2-Naphthol ethyl ether(CP) R2 = −OCH2CH3

7 8 9

1-Naphthylacetic acid(CP) R1 =−CH2COOH 1-Naphthoxyacetic acid(CP) R1 =−OCH2COOH 2-Naphthoxyacetic acid(CP) R2 =−OCH2COOH

10

2-Naphthalenesulfonic acid sodium(CP) R2 =−SO3Na

11

2-Amino-1-naphthalenesulfonic acid sodium (AR) R2 =−NH2, R1 =−SO3Na 1-Amino-4-naphthalenesulfonic acid sodium; R1 =−NH2, R4 =−SO3Na

4 5 6

12

3. Results and discussion

3.1. NP-RTP spectra of naphthalene deri6ati6es Under the favorable conditions, all of the naphthalene derivatives tested can emit strong and stable RTP signals in the absence of a protective medium. For compounds without internal heavy atom, RTP signals can not be induced even in the presence of Na2SO3 as deoxygenator, which means that the intersystem crossing rate constant kisc from S1 to T1 state for these compounds is very small. When external HAP was added to the system, a strong and stable RTP signal can be observed immediately due to the kisc was increased obviously. The HAE of TlNO3 is stronger than that of KI. As shown in Figs. 1– 3, for all of the derivatives tested, the emission spectra exhibits two peaks and their form have not changed with change of HAP. The excitation spectrum of naphthalene exhibits a single peak, whereas two peaks were found for rest compounds. At the same time, the ratio between peak intensity at shorter wavelength and that at longer wavelength for excitation spectrum was changed with change of HAP.

3.2. Absorption spectra of naphthalene deri6ati6es in the presence of external HAP. Figs. 4–7 are absorption spectra of 1-naphthalene bromide and 1-naphthylacetic acid in the presence of external HAP (KI or TlNO3) in different concentration. The following are noteworthy: (1) When external HAP was introduced into luminescent system, whether it was KI or TlNO3, the absorption peak located at short-wavelength

Fig. 1. The NP-RTP spectra of naphthalene (code 1), C = 2.0 ×10 − 4 mol/l(acetone 3%). 1: CTl =0.04 mol/l; 2: CKI = 0.4mol/l.

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Fig. 2. The NP-RTP spectra of 2-naphthalene ethyl ether (code 6), C = 2.0× 10 − 4 mol/l (acetone 12.5%). 1: CTl =0.08 mol/l; 2: CKI =0.6 mol/l.

range (near 230 nm) disappeared. (2) The mechanism of heavy atom effect (HAE) is different for KI from for TlNO3. When KI was used as HAP, the maximum absorption wavelength was shifted to slightly longer wavelength, which indicated that a ground state complex was formed between I− and phosphor. But for TlNO3, the maximum absorption wavelength was keep constant. (3) The effect of HAP on absorption spectrum is different for different phosphors, for example, the absorbance of 1-naphthalene bromide was increased slightly with the addition of external HAP, but for 1-naphthylacetic acid, the absorbance was decreased slightly.

3.3. NP-RTP lifetime of naphthalene deri6ati6es obtained at different concentration of HAP

Fig. 3. The NP-RTP spectra of 2-naphthoxyacetic acid (code 9), C = 5.0 × 10 − 5 mol/l (acetone 0.4%). 1: CTl = 0.05 mol/l; 2: CKI =0.1 mol/l.

As described above, because the mechanism of heavy atom effect (HAE) is different for TlNO3 and KI, we studied only the effect of KI on lifetime and calculated photophysical parameters for RTP emission in this paper. According to the experimental procedure, the RTP lifetime obtained at different concentration of KI for the naphthalene derivatives was shown in Tables 1 and 2, respectively. For the derivatives containing internal heavy atom, such as code 3 (1-ClNP) and code 4 (1-BrNP), we can measure t0 directly in the absence of external HAP. We can see from Table 1 that the lifetime t0 of 1-ClNP and 1-BrNP was 1.059 and 0.3047 ms, respectively. But for the rest of the compounds tested, only when an external HAP was presented, can the measurement of RTP lifetime be carried on. We also found that the RTP lifetime was shortened with the increase of HAP concentration as was expected.

3.4. Calculation of photophysical parameters for RTP emission

Fig. 4. Absorption spectra of 1-naphthalene bromide in the presence of different concentration of KI. Down to up, CKI = 0, 0.04, 1, 1.5, 3 mol/l (the maximum absorption wavelength lmax was shifted from 284.3 to285.5nm).

3.4.1. The principle for calculation of photophysical parameters based on the change of RTP lifetime Base on the absorption spectra of naphthalene derivatives in the presence of different concentration of KI, we suppose that I− reacts with phosphor to form a ground state complex in a

CKI 0 0.01 0.02 0.04 0.05 0.06 0.08 0.1 0.12 0.16 0.2 0.3 0.4 0.6 0.8 1 1.2

Code 1

Code 2

Code 3

Code 4

0.9941

0.3047 0.2819 0.2633 0.2338

1.059 0.9854 0.9101 0.7982

0.2091 0.1978 0.1798 0.1667

0.7142

1.331 1.306

1.211 1.134

0.9863 0.8166 0.6871

Code 5

Code 6

Code 7

Code 8

Code 9

Code 10

Code 11

Code 12

2.145

0.4109

0.4370

2.015

1.799

0.3847

0.4123

1.108

0.9454 1.379

2.456

0.9293

0.3218

1.282

1.833

0.8083 0.7027 0.6605 0.5892 0.5258

0.3092 0.2902

1.183 0.995 0.9505 0.8533

1.539 1.224

0.2668 0.2572

0.8668 0.7824

0.8980 0.9065 0.7452 0.6234 0.5191 0.4346 0.3822

0.2898 1.060 0.9770 0.8114

0.2214 0.1988

0.3846 0.3602 0.3334 0.3099

1.699 1.505 1.294 1.125 0.9962

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Table 2 The RTP lifetime t (ms) of naphthalene derivatives in the presence of different concentrations of KI

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stoichiometry of 1:1. When the compounds without an internal heavy atom, the intersystem crossing rate constant kisc of the system is expected to be very small, the RTP signal can not be observed. Once the external HAP was added to the system, a strong and stable RTP signal can be observed immediately due to the kisc which was obviously increased. Thus the photophysical process in the presence of KI for phosphorescence emission of naphthalene derivatives can be expressed simply as

Fig. 6. Absorption spectra of 1-naphthalene bromide in the presence of different concentration of TlNO3 down to up, CTl =0, 0.04, 0.12 mol/ (lmax =284.3nm).

The phosphorescence lifetime in the presence of KI equals to the sum of 3P·I* and 3P* because of summability of phosphorescence lifetime. However, 3P* is the phosphor essentially, KI changes only the intersystem crossing rate constant kisc of the system. According to the definition of phosphorescence lifetime, we have ~0 =1/kp

(1)

where kP is a rate constant for phosphorescent decay, which is mainly dependent on the chemical structure of phosphor; ki is a rate constant for triplet state radiationless deactivation from T1 “ S0, which is mainly dependent on environmental factors; the x is the molar concentration of HAP. From these relations above, we have got following relation

~= 1/[kp + kix]

(2)

~0/~= [kp + kix]/kp = 1+ ki/kpx

Fig. 5. Absorption spectra of 1-naphthylacetic acid in the presence of different concentration of KI. Up to down CKI = 0, 0.1, 0.6, 2, 3 mol/l (the maximum absorption wavelength lmax was shifted from 280.8 to282.7 nm).

(3)

Fig. 7. Absorption spectra of 1-naphthylacetic acid in the presence of different concentration of TlNO3. Up to down,CTl =0, 0.04, 0.12, mol/l (lmax =280.8 nm).

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This is a Stern– Volmer equation, the slope ki/kp = ki × ~0 =KSV. The KSV is the static Stern– Volmer constant. When ~0/~ was plotted as a function of the concentration in KI, we have got a linear relationship. On the other hand, as described above, because the HAP can promote the intersystem crossing and the rate of radiative processes of triplet state, the phosphorescence lifetime would be shorten. Thus, ~=~0e − kx

(4)

where x is the concentration of KI (mol/l), k is the Fig. 10. The plot of ln t vs CKI for 1-naphthol ethyl ether(C = 2 ×10 − 4 mol/l, 12.5% acetonitrile); 2-naphthalenesulfonic acid sodium(C = 5 ×10 − 5 mol/l) and 2-amino-1-naphthalenesulfonic acid sodium(C = 1 ×10 − 4 mol/l); CNa2SO3 =0.01mol/ l.  2-amino-1-naphthalenesulfonic acid; 2-naphthalenesulfonic acid sodium; 1-Naphthol ethyl ether.

decreasing rate of lifetime (ms·mol − 1·l). When lnt was plotted as a function of the concentration in HAP, we have also got another linear relationship: ln~ =ln~0 − kx.

Fig. 8. The plot of ln t versus CKI for 1-BrNp and 1-ClNp. C1-BrNp =C1-ClNp = 2× 10 − 4 mol/l (13% acetonitrile); CNa2SO3 = 0.01mol/l.

Fig. 9. The plot of t0/t versus CKI for 1-BrNp and 1-ClNp. C1-BrNp =C1-ClNp = 2× 10 − 4 mol/l (13% acetonitrile) CNa2SO3 = 0.01mol/l.

(5)

3.4.2. Luminescence kinetic parameters of naphthalene deri6ati6es for RTP emission According to the RTP lifetime obtained experimentally above, we calculated luminescence kinetic parameters of naphthalene derivatives tested by extrapolating curves based on the above relationship. The results are shown in Figs. 8–11 and Tables 3 and 4. We can see from Tables 3 and 4, for the BrNP and the ClNP, which contained an internal heavy atom, the RTP lifetime t0 obtained by extrapolating curves of 1/t = 1/t0 + ki x and lnt =lnt0 −kx (ref. Figs. 8 and 9) were in good agreement with the t0 value obtained in the absence of external HAP, which proved that the RTP lifetime method can be used to determine photophysical parameters for RTP emission; Because the internal HAE of Br atom is stronger than that of Cl atom, the rate kp for phosphorescence emission of BrNP is higher than that for ClNP, so the t0 of the former is shorter than that of the latter. At the same

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Fig. 11. The plot of t0/t versus CKI for 1-naphthol ethyl ether (C= 2× 10 − 4 mol/l, 12.5% acetonitrile), 2-naphthalenesulfonic acid sodium (C = 5× 10 − 5 mol/l) and 2-amino-1-naphthalenesulfonic acid sodium (C = 1× 10 − 4 mol/l). CNa2SO3 = 0.01mol/l.  2-naphthalenesulfonic acid sodium; 2-amino-1-naphthalenesulfonic acid; 1-naphthol ethyl ether.

time, the radiationless deactivation ki of the former is larger than that of the latter.

3.5. The test of reasonableness The reasonableness of results obtained above was tested in two ways. One of them, the t0 value obtained by extrapolating curve of lnt –C was used in the equation of t0/t =(kp +kiC)/kp = 1+ kiC/kp = 1+ KSVC, making a plot of t0/t versus

C; another is the t0 value obtained in the absence of external HAP (for 1-naphthalene chloride and 1-naphthalene bromide) was used in the plot of t0/t versus C, then test whether the intercept B is equal to 1? As results shown in Tables 5 and 6, all of the intercept B obtained by two ways were approaching to the limit 1. The result proved not only that the RTP lifetime method can be used to determine photophysical parameters for RTP emission, but gave us an inspiration that the t0 value obtained in the absence external HAP for the compounds contained internal HAP can be used directly in the determination of photophysical parameters for RTP emission. We can see from Figs. 12 and 13, when we make a plot using t value obtained in two ways versus CKI, respectively, two curves were overlapping each other in the range of lower concentration of KI, which means that in order to obtain an accurate result the concentration of HAP should be as lower as possible.

3.6. Conclusion The measurement of t value opens up a possibility to obtain the luminescent kinetic parameters. The results obtained by two ways proved that the RTP lifetime method can be used to determine luminescent kinetic parameters for RTP emission. For the compounds containing internal heavy atoms, we can directly use the measured ~

Table 3 Luminescence parameters obtained by plot of ln v =ln t0−kx, KI as HAP Luminephor

lex/lem (nm)

t0 (ms)

kp (kp =1/t0)

k (ms·mol−1·l)

ra

Code Code Code Code Code Code Code Code Code Code Code Code

284/514 294/529 282/519 294/519 286/524 277/505 279/521 288/523 277/503 285/503 287/521 286/522

1.350 1.016 1.050 0.3012 0.3362 1.436 2.377 1.065 2.219 0.4378 0.4485 2.184

0.7407 0.9843 0.9524 3.320 2.974 0.6964 0.4207 0.9390 0.4507 2.284 2.230 0.4579

1.673 0.5518 6.590 6.202 0.2283 0.5320 0.9843 0.8795 0.8509 0.6697 0.3708 0.6526

0.9992 0.9968 0.9979 0.9981 0.9971 0.9910 0.9825 0.9953 0.9937 0.9994 0.9984 0.9988

a

1 2 3 4 5 6 7 8 9 10 11 12

r is the correlation coefficient of linear fit.

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Table 4 4. Luminescence parameters obtained by plot of 1/t= 1/t0+kix, KI as HAP Luminephor

lex/lem (nm)

t0 (ms)

kp (kp =1/t0)

ki (ms−1·mol−1·l)

KS

ra

Code Code Code Code Code Code Code Code Code Code Code Code

284/514 294/529 282/519 294/519 286/524 277/505 279/521 288/523 277/503 285/503 287/521 286/522

1.398 1.061 1.060 0.3038 0.3398 1.482 2.838 1.183 2.449 0.4693 0.4556 2.406

0.7153 0.9425 0.9431 3.292 2.943 0.6748 0.3524 0.8453 0.4083 2.131 2.195 0.4156

1.753 0.7689 7.657 24.83 0.7970 0.4940 0.7822 1.419 0.6605 2.379 1.013 0.4684

2.451 0.8158 8.116 7.543 0.2708 0.7321 2.220 1.679 1.618 1.116 0.4615 1.127

0.9938 0.9957 0.9998 0.9999 0.9982 0.9925 0.9977 0.9941 0.9959 0.9983 0.9985 0.9906

a

1 2 3 4 5 6 7 8 9 10 11 12

r is the correlation coefficient of linear fit.

Table 5 The test for the parameters of phosphor obtained by way 1 Luminephor

t0

kp

ki

Ksv

B

ra

Naphthalene 1-Naphthalen chloride 1-Naphthalen bromide 1-naphthol 1-naphthol ethyl ether 2-naphthol ethyl ether 1-naphthylacetic acid 1-naphthoxyacetic acid 2-naphthoxyacetic acid 2-Naphthalenesulfonic acid sodium 2-amino-1-naphthalenesulfonic acid 1-Amino-4-naphthalenesulfonic acid sodium

1.350 1.050 0.3012 1.016 0.3362 1.436 2.377 1.065 2.219 0.4378 0.4485 2.184

0.7404 0.9524 3.320 0.9843 2.974 0.6964 0.4207 0.9390 0.4507 2.284 2.230 0.4579

1.753 7.657 24.83 0.7689 0.7970 0.4940 0.7822 1.419 0.6605 2.379 1.013 0.4684

2.367 8.040 7.479 0.7812 0.2680 0.7094 1.859 1.511 1.466 1.041 0.4541 1.023

0.9658 0.9903 0.9916 0.9573 0.9894 0.9689 0.8374 0.9006 0.9061 0.9331 0.9850 0.9079

0.9938 0.9998 0.9999 0.9957 0.9982 0.9925 0.9977 0.9941 0.9959 0.9983 0.9985 0.9906

a

r is the correlation coefficient of linear fit.

Table 6 The test for the parameters of 1-BrNp and 1-ClNp obtained by two ways Phosphor

1-BrNp (KI) 1-ClNp(KI)

t0/t =B+KsvCKI (t0obtained by without external HAP)

t0/t= B+KsvCKI(t0 obtained by lnt–C) t0

kp

B

Ksv

kp

B

Ksv

r

0.3012 1.050

3.320 0.9524

0.9916 0.9903

7.479 8.040

3.331 0.9479

0.9883 0.9950

7.454 8.078

0.9999 0.9998

value in the absence external HAP in the calculation of the kinetic parameters. We also found that the strict temperature control and deoxi-

dization from luminescence systems are the key to obtain a accurate result, and the high concentration of external HAP is not suitable.

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.

Fig. 12. The change of t with CKI in two ways for 1-BrNp (we can see that the optimum concentration of HAP is CKI =0  0.1 mol/l.

Fig. 13. The change of t with CKI in two ways for naphthalene (we can see that the optimum concentration of HAP is CKI = 0  0.4 mol/l.

Acknowledgements

[5] L.D. Li, Y.L. Chen, A.J. Tong, Hua Xue Tongbao (Chemistry) 6 (1996) 3. [6] L.D. Li, Y. L. Chen, Y. Zhao, A.J. Tong, Anal. Chim. Acta 341 (1997) 241. [7] L.D. Li, Y. Zhao, Y.G. Wu, A.J. Tong, Talanta 46 (1998) 1147. [8] L. Mou, X.K. Chen, L.D. Li, Chem. J. Chinese Universities 20 (1999) 214. [9] X.K. Chen, L. Mou, L.D. Li, Chem. J. Chinese Universities 20 (1999) 1052. [10] L. Mou, X.K. Chen, L.D. Li, Chin. J. Anal. Chem. 27 (1999) 509. [11] A.S. Carretero, C.C. Blanco, A.F. Gutierrer, J. Agric. Food Chem. 46 (1998) 3683. [12] A.S. Carretero, C.C. Blanco, B.C. Diaz, et al., Anal. Chim. Acta 361 (1998) 217. [13] A.F. Gutierrer, A.S. Carretero, B.C. Diaz, et al., Appl. Spect. 53 (1999) 741.

This work was supported by the Chinese National Natural Science Foundation (grant No.29775013)

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