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Enhancement of the fluorescence of the beryllium—morin complex by non-ionic surfactants
Talanra, Vol. 37, No. 9, pp. 947-950, 1990 Printed in Great Britain
0039-914OpO $3.00 + 0.00 Pergamon Press plc
ENHANCEMENT OF THE FLUORESCENCE OF THE BERYLLIUM-MORIN COMPLEX BY NON-IONIC SURFACTANTS WEI FUSHENG, TEN ENJIANG and WV ZHONGXIANG China National Environmental Monitoring Centre, Beijing, 100012, People’s Republic of China (Received 20 April 1989. Revised 18 February 1990. Accepted 28 March 1990)
Summary-The effect of surfactants on the Ruorescence of the beryllium-morin system has been studied. Non-ionic surfactants strongly enhance the fluorescence intensity of the beryllium complex. The addition of Triton X-100 makes possible the fluorimetric determination of submicrogram quantities of beryllium in feebly acidic media (pH 5.8-6.2, hexamine buffer). The fluorescence is excited at 440 nm and measured at 530 nm. The calibration graph is linear up to 10 ng/ml Re and the detection limit is 0.04 ng/ml. The relative standard deviation is 2.2% for beryllium at 0.5 ng/ml concentration and 0.7% for 5.0 ng/ml. The effects of 25 ions commonly found in water have also been studied, Zn*+ and F- give the main interference. The method has been applied to the determination of beryllium in water pollution quality-control samples with satisfactory results.
The reaction with morin’ has been extensively used for determining submicrogram quantities of beryllium in air,*s3 sea-water,4 rocks’ and other samples, but must be applied in alkaline medium. In recent years, surfactants have been successfully used for improving existing analytical methods and in developing new ones. In micellar media, the sensitivity and selectivity of numerous reactions are improved and metal complexes are generally more stable than when formed in the absence of surfactants. Although most of the analytical work done with surfactants deals with their application to spectrophotometric methods,6 they are now also being applied successfully to fluorimetric methods. Their first application in this area was the determination of aluminium with lumogallion in the presence of the non-ionic surfactant poly(ethylene glycol) monolauryl ether.’ Enhancement of the fluorescence of the metalmorin complexes in the presence of surfactants has been extensively studied. It has been found that the fluorescence of the morin complexes of Al 8.gNb,‘O-12Ta,” Pb,13Zn,14 Zr, Hf, Ga, In and Sb&)’ is enhanced by the use of surfactants. Cationic surfactants strongly enhance the fluorescence of the Nb(V)“*” and Tao” complexes, e.g., cetyltrimethylammonium bromide (CTAB) gives an 80-fold increase for the Nb-morin complex. The anionic surfactant sodium lauryl sulphate (SLS) strongly enhances
the fluorescence of the Zr, Hf, Al, Ga, In and Sb complexes (by a factor of 3-13).’ The nonionic surfactant Genpol PF-20 (ethylene oxide propylene oxide condensate) strongly increases the fluorescence of the Al,* Pb13 and Zn14 complexes (by factors of 8, 9 and 75, respectively). In this paper the influences of various surfactants (non-ionic, cationic, anionic and zwitterionic) on the fluorescence intensity of the beryllium-morin complex are studied and the characteristics of the determination of beryllium with morin in feebly acidic media are discussed. EXPERIMENTAL
Apparatus
A Hitachi 850 fluorescence spectrophotometer was used with l-cm silica cells and a 5-nm bandpass for both excitation and emission. Reagents
All reagents used were of analytical grade. Morin solution, 0.04% in absolute ethanol. Hexamine buffer @H 6.2). Dissolve 70 g of
hexamine in 250 ml of water, and adjust the pH to 6.2 with 1M hydrochloric acid. Beryllium stock solution, 100 pg/ml. Dissolve 0.0416 g of Be0 in 5 ml of sulphuric acid (1 + 1) and dilute to the mark with water in a 150-ml standard flask. Dilute this appropriately with water to obtain a 0.01 pgg/ml working solution. 947
WEI FUSHENGet
948
al.
General procedure
0.6
To a solution containing 20 ng of beryllium, add 2.0 ml of buffer, 0.5 ml of ethanolic morin solution and 3.0 ml of aqueous 5% Triton X-100 solution. Dilute to volume in a 25-ml standard flask with water and measure the fluorescence intensity at 530 nm, with excitation at 440 nm.
0.4 Ah 0.2
0
L
.
1.6
1 t%)
CSurfactants
RESULTS
AND DISCUSSION
E#ect of dtrerent surfactants
Figure 1 shows the excitation and emission spectra of the beryllium-morin and berylliummorin-Triton X-100 systems. The fluorescence intensity of the beryllium complex was greatly increased by adding Triton X-100, and the excitation and emission maxima occurred at 426 and 550 nm, respectively. In this study, optimum linearity and sensitivity were achieved with 440 and 530 nm as the excitation and emission wavelengths, respectively. The fluorescence spectra of the beryllium complexes in the presence of other surfactants were also studied. The results show that the addition of 0.6% of non-ionic surfactant (Triton X-100, Tween 80 and/or the emulsifier OP) increases the fluorescence intensity of the beryllium complexes (by at least 5-fold, 4.5-fold and 4-fold, respectively), whereas the presence of gelatin or the anionic surfactant SLS causes little increase. In the presence of the zwitterionic surfactant dodecyldimethylaminoacetic acid (DDMAA) or the cationic surfactant CTAB there is again little increase. Red-shifts (of about 25 nm) were observed in the fluorescence spectra of the beryllium complexes in the presence of Triton X-100, Tween 80, emulsifier OP, CTAB and DDMAA, but not in the presence of gelatin and SLS.
Fig. 2. Effect of non-ionic surfactant concentration: (1) Triton X-100; (2) Tween 80; (3) OP; Be concentration 0.8 ng/ml, morin 0.0008%, pH 6.0, room temperature.
The red-shifts of &,, in the presence of non-ionic surfactants show that the interaction between the beryllium complex and non-ionic surfactants causes the excited state of the complex molecules to change. The beryllium complex molecules may be fixed by the non-ionic micelles, and thus made more rigid and hence the fluorescence quantum yield is raised. EfSect of concentration of non-ionic surfactants
The effect of non-ionic surfactants (Triton X-100, Tween 80 and OP) on the fluorescence intensity of the beryllium complexes is shown in Fig. 2. It was observed that between 0.4% and 1.2% Triton X-100 concentration there was little change in the fluorescence intensity of the beryllium complex. The optimum surfactant concentration was therefore selected as 0.6%. E#ect of pH
In the absence of surfactants the fluorescence of the complex was very weak over the pH range 4.5-9.0. Maximum fluorescence occurred at pH 11.5-12.5, which agrees with the literature.3 The fluorescence intensity of the beryllium complex was strongly enhanced in feebly acidic medium by addition of Triton X-100, but in 0.6
3
r
0.4 2
AL
If
0.2 1
0
I
4
6
I
I
I
I
6
10
12
14
PH Xtnm) Fig. 1. Excitation (I) and emission (II) spectra: (1) Be-morin-Triton X-100; (2) morin-Triton X-100; (3) Be-morin; (4) morin. Be concentration 1 ng/ml, morin 0.0008%, Triton X-100 0.6%, pH 6.0, room temperature.
Fig. 3. Effect of pH. Be concentration 0.8 ng/ml, morin 0.0008%, Triton X-100 0.6%, room temperature. (Note: when the pH was less than 8.0, it was adju’sted with hexamine buffer; when it was more than 8.0, it was adjusted with borax buffer and 1M NaOH).
Fluorescence of Re-moiin complex
949
Table 2. Determination of beryllium in WFCS* Result obtained,
r.s.d.,
nxlml
%
Sample WPCS 4
18.9,17.8,19.9,18.4,18.9,19.4 WFCS 6 856,875,888,894,850,831
3.9 2.8
Certified value, nxlml
16.1-24.9 810-978
*WPCS 6 was diluted RIO-fold with water.
Eflecf of ethanol
I
0
16
32
46
64
EtOH (%f
Fig. 4. Effect of ethanol. (I) Fk-morin-Triton X100-ethanol; (2) Re-morin-ethanol; (3) morin-Triton X100-ethanol; (4) morin-ethanol. J3e concentration 0.8 ng/ml, morin 0.0008%, Triton X-100 0.6%, pH 6.0, room temperature.
alkaline medium the fluorescence was practically unaffected by the surfactant. Eflect
ofmot-in concentration
When the morin concentration was increased, the fluorescence intensities of the beryllium complex and the morin blank were also increased, but the difference between the two intensities was practically constant over the morin concentration range from 0.00032 to 0.0016%, and 0.0~8% was selected as the optimum. Table 1. Effect of co-existing ions on the dete~nation 20 ng of beryllium in 25 ml of aqueous solution Co-existing ions 10 mg K+ 10 mg Na+ 10 mg NO, 10 mg Cl10 mg HCO; 10 mg SOi5 mg As(III) 0.5 mg Ca*+ 0.5 mg Mg*+ 0.1 mg H, PO; 10 Icg CrW) 10 L(RCdz+ 10 jJg co2+ 10 pg N?+ 5 pg V(v) 5 jfg La’+ 5 pg MnZ+ 1 jig CW’ 1 pg A13+ 1 pg Pb2+ 1 pg Fe3+ 1 pg Fe2+ 0.5 pg zp+ 0.1 pg Zn*+ 0.1 pg F1 cg F-
Re found, ng 20.4 20.0 20.6 19.2 20.3 20.3 19.4 18.2 19.8 18.3 19.4 20.5 18.8 18.8 18.7 19.5 19.7 18.5 21.2 19.5 19.5 21.8 18.1 21.0 18.8 16.6
Relative error, % +2 0 +3 -4 Cl.5 +1.5 -3 7; -8.5 -3 +2.5 -6.5 -2.5 ., -1.5 -7.5 +6 -2.5 -2.5 +9 -9.5 +5 -6 -I7
of
Figure 4 shows that in the absence of Triton X-100, ethanol enhances the fluorescence of the beryllium complex, by its solvation effect.“~ls When Triton X-100 is also present the ethanol may destroy the micelles. The fluorescence intensity of the beryllium complex first decreases as the ethanol concentration increases, but when the ethanol concentration is more than 30% v/v, the solvation effect comes into operation and the intensity increases again. Eflect of tem~rature The fluorescence intensities of the beryllium complex and morin increase with decrease in temperature, but the change is small in the temperature range 16-20”. At room temperature, the fluorescence of the beryllium complex and of morin remains almost constant for at least 2 hr. Analytical characteristics The calibration graphs obtained in the presence and absence of Triton X-100 were linear up to 10 ng/ml Be in the final solution, but the slope for the reaction in presence of the surfactant was about 5 times that in its absence. The detection limits found were 0.04 and 0.08 ng/ml in the presence and absence of Triton X-100, respectively, as calculated from 3s/S (where s is the standard deviation of the blank and S is the slope of the calibration graph) (n = 12). The precision was studied at two beryllium con~ntrations, and relative standard deviations (r.s.d.) of 2.2 and 0.7% (11 replicates) were obtained for 0.5 and 5.0 ng/ml Be, respectively. Interferences The effects of 25 ions commonly found in water, on the dete~ination of 0.8 ngjml beryllium were studied. The results are summarized in Table 1. Most do not interfere and only Zn*+ and F- have a serious effect;
WEI
950
kUlENG
Zn2+ increases the fluorescence because of formation of the Zn-morin complex and F- decreases the fluorescence of the beryllium complex because of the formation of beryllium fluoride. Application
The method has been applied to the determination of beryllium in water pollution control samples (WPCS) (supplied by the U.S. Environmental Protection Agency) without prior separation from the matrix, with satisfactory results. The results are shown in Table 2. REFERENCES
1. H. L. J. Zermatten, Prod. Acad. Sci. Amsterdam, 1933, 36, 899. 2. J. Walkey, Am. Ind. Hygiene Assoc. J., 1959, 20, 241. 3. C. W. Sill and C. P. Willis, Anal. Chem., 1959,31, 598.
et al.
4. M. Y. Ishibashi, W. N. Kasamastsu and C. 0. Nishimura, Bull. Inst. Chem. Research Kyoto Univ., 1956, 34, 210. 5. R. May and F. S. Grimaldi, Anal. Chem., 1961, 33, 1251. 6. W. L. Hitue, in Solution Chemistry of Surfactants, K. L. Mittal (ed.), Vol. 1. Plenum Press, New York, 1979. I. N. Ishibashi and K. Kina, Anal. Lett., 1972, 5, 637. 8. J. Mexlina Escriche, M. De La Guardia Cirugeda and F. Hernandez Hernandez, Analyst, 1983, 108, 1386. 9. W. C. Cui, Z. J. Han and H. M. Shi, Chem. J. Chinese Univ., 1986. 7, 612. 10. A. Sanz-Merle1 and J. I. Garcia Alonso, Anal. Chim. Acta, 1984, 165, 159. 11. A. Sanz-Medel, J. I. Garcia Alonso and E. Blanc0 Gonzalez, Anal. Chem., 1985, 57, 1681. 12. A. T. Pilipenko, T. A. Vasilchuk and A. I. Vilkova, Zh. Analit. Khim., 1983, 38, 855. 13. J. Medina Escriche, F. Hernandez Hemandez, R. Marin and F. J. Lopez, Analyst, 1986, 111, 235. 14. F. Hemandez Hernandez, J. Medina Escriche and M. T. Gasco Andreu, Talanta, 1986, 33, 537. 15. H. G. Wang and L. Y. Li, Chem. J. Chinese Univ., 1987, 8, 601.
0039-914OpO $3.00 + 0.00 Pergamon Press plc
ENHANCEMENT OF THE FLUORESCENCE OF THE BERYLLIUM-MORIN COMPLEX BY NON-IONIC SURFACTANTS WEI FUSHENG, TEN ENJIANG and WV ZHONGXIANG China National Environmental Monitoring Centre, Beijing, 100012, People’s Republic of China (Received 20 April 1989. Revised 18 February 1990. Accepted 28 March 1990)
Summary-The effect of surfactants on the Ruorescence of the beryllium-morin system has been studied. Non-ionic surfactants strongly enhance the fluorescence intensity of the beryllium complex. The addition of Triton X-100 makes possible the fluorimetric determination of submicrogram quantities of beryllium in feebly acidic media (pH 5.8-6.2, hexamine buffer). The fluorescence is excited at 440 nm and measured at 530 nm. The calibration graph is linear up to 10 ng/ml Re and the detection limit is 0.04 ng/ml. The relative standard deviation is 2.2% for beryllium at 0.5 ng/ml concentration and 0.7% for 5.0 ng/ml. The effects of 25 ions commonly found in water have also been studied, Zn*+ and F- give the main interference. The method has been applied to the determination of beryllium in water pollution quality-control samples with satisfactory results.
The reaction with morin’ has been extensively used for determining submicrogram quantities of beryllium in air,*s3 sea-water,4 rocks’ and other samples, but must be applied in alkaline medium. In recent years, surfactants have been successfully used for improving existing analytical methods and in developing new ones. In micellar media, the sensitivity and selectivity of numerous reactions are improved and metal complexes are generally more stable than when formed in the absence of surfactants. Although most of the analytical work done with surfactants deals with their application to spectrophotometric methods,6 they are now also being applied successfully to fluorimetric methods. Their first application in this area was the determination of aluminium with lumogallion in the presence of the non-ionic surfactant poly(ethylene glycol) monolauryl ether.’ Enhancement of the fluorescence of the metalmorin complexes in the presence of surfactants has been extensively studied. It has been found that the fluorescence of the morin complexes of Al 8.gNb,‘O-12Ta,” Pb,13Zn,14 Zr, Hf, Ga, In and Sb&)’ is enhanced by the use of surfactants. Cationic surfactants strongly enhance the fluorescence of the Nb(V)“*” and Tao” complexes, e.g., cetyltrimethylammonium bromide (CTAB) gives an 80-fold increase for the Nb-morin complex. The anionic surfactant sodium lauryl sulphate (SLS) strongly enhances
the fluorescence of the Zr, Hf, Al, Ga, In and Sb complexes (by a factor of 3-13).’ The nonionic surfactant Genpol PF-20 (ethylene oxide propylene oxide condensate) strongly increases the fluorescence of the Al,* Pb13 and Zn14 complexes (by factors of 8, 9 and 75, respectively). In this paper the influences of various surfactants (non-ionic, cationic, anionic and zwitterionic) on the fluorescence intensity of the beryllium-morin complex are studied and the characteristics of the determination of beryllium with morin in feebly acidic media are discussed. EXPERIMENTAL
Apparatus
A Hitachi 850 fluorescence spectrophotometer was used with l-cm silica cells and a 5-nm bandpass for both excitation and emission. Reagents
All reagents used were of analytical grade. Morin solution, 0.04% in absolute ethanol. Hexamine buffer @H 6.2). Dissolve 70 g of
hexamine in 250 ml of water, and adjust the pH to 6.2 with 1M hydrochloric acid. Beryllium stock solution, 100 pg/ml. Dissolve 0.0416 g of Be0 in 5 ml of sulphuric acid (1 + 1) and dilute to the mark with water in a 150-ml standard flask. Dilute this appropriately with water to obtain a 0.01 pgg/ml working solution. 947
WEI FUSHENGet
948
al.
General procedure
0.6
To a solution containing 20 ng of beryllium, add 2.0 ml of buffer, 0.5 ml of ethanolic morin solution and 3.0 ml of aqueous 5% Triton X-100 solution. Dilute to volume in a 25-ml standard flask with water and measure the fluorescence intensity at 530 nm, with excitation at 440 nm.
0.4 Ah 0.2
0
L
.
1.6
1 t%)
CSurfactants
RESULTS
AND DISCUSSION
E#ect of dtrerent surfactants
Figure 1 shows the excitation and emission spectra of the beryllium-morin and berylliummorin-Triton X-100 systems. The fluorescence intensity of the beryllium complex was greatly increased by adding Triton X-100, and the excitation and emission maxima occurred at 426 and 550 nm, respectively. In this study, optimum linearity and sensitivity were achieved with 440 and 530 nm as the excitation and emission wavelengths, respectively. The fluorescence spectra of the beryllium complexes in the presence of other surfactants were also studied. The results show that the addition of 0.6% of non-ionic surfactant (Triton X-100, Tween 80 and/or the emulsifier OP) increases the fluorescence intensity of the beryllium complexes (by at least 5-fold, 4.5-fold and 4-fold, respectively), whereas the presence of gelatin or the anionic surfactant SLS causes little increase. In the presence of the zwitterionic surfactant dodecyldimethylaminoacetic acid (DDMAA) or the cationic surfactant CTAB there is again little increase. Red-shifts (of about 25 nm) were observed in the fluorescence spectra of the beryllium complexes in the presence of Triton X-100, Tween 80, emulsifier OP, CTAB and DDMAA, but not in the presence of gelatin and SLS.
Fig. 2. Effect of non-ionic surfactant concentration: (1) Triton X-100; (2) Tween 80; (3) OP; Be concentration 0.8 ng/ml, morin 0.0008%, pH 6.0, room temperature.
The red-shifts of &,, in the presence of non-ionic surfactants show that the interaction between the beryllium complex and non-ionic surfactants causes the excited state of the complex molecules to change. The beryllium complex molecules may be fixed by the non-ionic micelles, and thus made more rigid and hence the fluorescence quantum yield is raised. EfSect of concentration of non-ionic surfactants
The effect of non-ionic surfactants (Triton X-100, Tween 80 and OP) on the fluorescence intensity of the beryllium complexes is shown in Fig. 2. It was observed that between 0.4% and 1.2% Triton X-100 concentration there was little change in the fluorescence intensity of the beryllium complex. The optimum surfactant concentration was therefore selected as 0.6%. E#ect of pH
In the absence of surfactants the fluorescence of the complex was very weak over the pH range 4.5-9.0. Maximum fluorescence occurred at pH 11.5-12.5, which agrees with the literature.3 The fluorescence intensity of the beryllium complex was strongly enhanced in feebly acidic medium by addition of Triton X-100, but in 0.6
3
r
0.4 2
AL
If
0.2 1
0
I
4
6
I
I
I
I
6
10
12
14
PH Xtnm) Fig. 1. Excitation (I) and emission (II) spectra: (1) Be-morin-Triton X-100; (2) morin-Triton X-100; (3) Be-morin; (4) morin. Be concentration 1 ng/ml, morin 0.0008%, Triton X-100 0.6%, pH 6.0, room temperature.
Fig. 3. Effect of pH. Be concentration 0.8 ng/ml, morin 0.0008%, Triton X-100 0.6%, room temperature. (Note: when the pH was less than 8.0, it was adju’sted with hexamine buffer; when it was more than 8.0, it was adjusted with borax buffer and 1M NaOH).
Fluorescence of Re-moiin complex
949
Table 2. Determination of beryllium in WFCS* Result obtained,
r.s.d.,
nxlml
%
Sample WPCS 4
18.9,17.8,19.9,18.4,18.9,19.4 WFCS 6 856,875,888,894,850,831
3.9 2.8
Certified value, nxlml
16.1-24.9 810-978
*WPCS 6 was diluted RIO-fold with water.
Eflecf of ethanol
I
0
16
32
46
64
EtOH (%f
Fig. 4. Effect of ethanol. (I) Fk-morin-Triton X100-ethanol; (2) Re-morin-ethanol; (3) morin-Triton X100-ethanol; (4) morin-ethanol. J3e concentration 0.8 ng/ml, morin 0.0008%, Triton X-100 0.6%, pH 6.0, room temperature.
alkaline medium the fluorescence was practically unaffected by the surfactant. Eflect
ofmot-in concentration
When the morin concentration was increased, the fluorescence intensities of the beryllium complex and the morin blank were also increased, but the difference between the two intensities was practically constant over the morin concentration range from 0.00032 to 0.0016%, and 0.0~8% was selected as the optimum. Table 1. Effect of co-existing ions on the dete~nation 20 ng of beryllium in 25 ml of aqueous solution Co-existing ions 10 mg K+ 10 mg Na+ 10 mg NO, 10 mg Cl10 mg HCO; 10 mg SOi5 mg As(III) 0.5 mg Ca*+ 0.5 mg Mg*+ 0.1 mg H, PO; 10 Icg CrW) 10 L(RCdz+ 10 jJg co2+ 10 pg N?+ 5 pg V(v) 5 jfg La’+ 5 pg MnZ+ 1 jig CW’ 1 pg A13+ 1 pg Pb2+ 1 pg Fe3+ 1 pg Fe2+ 0.5 pg zp+ 0.1 pg Zn*+ 0.1 pg F1 cg F-
Re found, ng 20.4 20.0 20.6 19.2 20.3 20.3 19.4 18.2 19.8 18.3 19.4 20.5 18.8 18.8 18.7 19.5 19.7 18.5 21.2 19.5 19.5 21.8 18.1 21.0 18.8 16.6
Relative error, % +2 0 +3 -4 Cl.5 +1.5 -3 7; -8.5 -3 +2.5 -6.5 -2.5 ., -1.5 -7.5 +6 -2.5 -2.5 +9 -9.5 +5 -6 -I7
of
Figure 4 shows that in the absence of Triton X-100, ethanol enhances the fluorescence of the beryllium complex, by its solvation effect.“~ls When Triton X-100 is also present the ethanol may destroy the micelles. The fluorescence intensity of the beryllium complex first decreases as the ethanol concentration increases, but when the ethanol concentration is more than 30% v/v, the solvation effect comes into operation and the intensity increases again. Eflect of tem~rature The fluorescence intensities of the beryllium complex and morin increase with decrease in temperature, but the change is small in the temperature range 16-20”. At room temperature, the fluorescence of the beryllium complex and of morin remains almost constant for at least 2 hr. Analytical characteristics The calibration graphs obtained in the presence and absence of Triton X-100 were linear up to 10 ng/ml Be in the final solution, but the slope for the reaction in presence of the surfactant was about 5 times that in its absence. The detection limits found were 0.04 and 0.08 ng/ml in the presence and absence of Triton X-100, respectively, as calculated from 3s/S (where s is the standard deviation of the blank and S is the slope of the calibration graph) (n = 12). The precision was studied at two beryllium con~ntrations, and relative standard deviations (r.s.d.) of 2.2 and 0.7% (11 replicates) were obtained for 0.5 and 5.0 ng/ml Be, respectively. Interferences The effects of 25 ions commonly found in water, on the dete~ination of 0.8 ngjml beryllium were studied. The results are summarized in Table 1. Most do not interfere and only Zn*+ and F- have a serious effect;
WEI
950
kUlENG
Zn2+ increases the fluorescence because of formation of the Zn-morin complex and F- decreases the fluorescence of the beryllium complex because of the formation of beryllium fluoride. Application
The method has been applied to the determination of beryllium in water pollution control samples (WPCS) (supplied by the U.S. Environmental Protection Agency) without prior separation from the matrix, with satisfactory results. The results are shown in Table 2. REFERENCES
1. H. L. J. Zermatten, Prod. Acad. Sci. Amsterdam, 1933, 36, 899. 2. J. Walkey, Am. Ind. Hygiene Assoc. J., 1959, 20, 241. 3. C. W. Sill and C. P. Willis, Anal. Chem., 1959,31, 598.
et al.
4. M. Y. Ishibashi, W. N. Kasamastsu and C. 0. Nishimura, Bull. Inst. Chem. Research Kyoto Univ., 1956, 34, 210. 5. R. May and F. S. Grimaldi, Anal. Chem., 1961, 33, 1251. 6. W. L. Hitue, in Solution Chemistry of Surfactants, K. L. Mittal (ed.), Vol. 1. Plenum Press, New York, 1979. I. N. Ishibashi and K. Kina, Anal. Lett., 1972, 5, 637. 8. J. Mexlina Escriche, M. De La Guardia Cirugeda and F. Hernandez Hernandez, Analyst, 1983, 108, 1386. 9. W. C. Cui, Z. J. Han and H. M. Shi, Chem. J. Chinese Univ., 1986. 7, 612. 10. A. Sanz-Merle1 and J. I. Garcia Alonso, Anal. Chim. Acta, 1984, 165, 159. 11. A. Sanz-Medel, J. I. Garcia Alonso and E. Blanc0 Gonzalez, Anal. Chem., 1985, 57, 1681. 12. A. T. Pilipenko, T. A. Vasilchuk and A. I. Vilkova, Zh. Analit. Khim., 1983, 38, 855. 13. J. Medina Escriche, F. Hernandez Hemandez, R. Marin and F. J. Lopez, Analyst, 1986, 111, 235. 14. F. Hemandez Hernandez, J. Medina Escriche and M. T. Gasco Andreu, Talanta, 1986, 33, 537. 15. H. G. Wang and L. Y. Li, Chem. J. Chinese Univ., 1987, 8, 601.