Trace determination of hydroxyaromatic compounds indyestuffs using cloud point preconcentration

Yu-Chao Wu and Shang-Da Huang*Department of Chemistry, National Tsing Hua University, Hsinchu, 300, Taiwan

Hydroxyaromatic compounds at ppb levels in water weredetermined with cloud point preconcentration followed byhigh-performance liquid chromatography (HPLC) withUV absorption detection. The extraction efficiencies andpreconcentration factors of cloud point extraction wereenhanced by the addition of neutral electrolytes to themicellar solutions. This method was applied to thedetermination of hydroxyaromatic compounds at ppmlevels in commercial dyestuffs. The dyestuff was dissolvedin water and pre-cleaned with a SAX cartridge packedwith an anion-exchange resin; the effluent was thenanalyzed using the proposed cloud-pointpreconcentration, with subsequent determination byHPLC.

Keywords: Hydroxyaromatic compounds; dyestuffs;high-performance liquid chromatography; cloud pointpreconcentration

The isolation and identification of hydroxyaromatic compoundsare of great importance owing to their wide applications inpharmaceuticals, dyes, pesticides and foods. The impact ofhydroxyaromatic compounds on the environment is subject toincreasing attention. The US Environmental Protection Agency(EPA) lists 11 substituted phenols as ‘priority pollutants.’1

Much research has been devoted to these phenolic substances2–8

and many chromatographic separation and detection systemshave been tested. However, the separation and determination ofother toxic substituted hydroxyaromatic compounds9 [e.g.,2,7-dihydroxynaphthalene (2,7-DHNA), 4,4A-dihydroxybiphe-nyl (4,4A-DHBP), 5-hydroxy-1,4-naphthoquinone (5-HNQ),2-hydroxynaphthalene (2-HNA) and 1-hydroxynaphthalene(1-HNA)] have received little attention. D&C Orange No.4(Orange II, Colour Index No. 15510) is a synthetic monoazo dyeprepared by coupling a diazotized sulfonic acid with a2-hydroxynaphthalene. The dyestuff is permitted for use in theUSA in externally applied drugs and in cosmetics.10 Thecontent of 2-hydroxynaphthalene in D&C Orange No.4 islimited to 0.2% by specifications of the Code of FederalRegulation.11 Therefore, for user protection, an accurate andrelatively simple analytical method has to be used to detecthydroxyaromatic compounds in this dyestuff.

Aqueous solutions of non-ionic surfactants become turbidwhen they are heated above the temperature known as the cloudpoint.12,13 The solution is then separated into two isotropicphases, i.e., a surfactant-rich phase and a bulk aqueous phase.The hydrophobic solutes can be enriched into the surfactant-richphase. The small volume of the surfactant-rich phase obtainedwith this methodology permits the design of extraction schemesthat are simple, cheap and less toxic than extraction with organicsolvents and that provides results comparable to those obtainedby other separation techniques. Comprehensive reviews of thetheory and applications of surfactant-mediated separations inanalytical chemistry are available.14,15 The cloud point method-ology has recently been applied to the preconcentration of a

wide range of analytes as a step prior to their determination byHPLC.16–23

It is important to stress that the presence or addition of othermaterials can alter the cloud point of a given non-ionicsurfactant. Many publications14,24,25 have reported the salting-out of the surfactant or lowering of the cloud point by the addedinorganic salts, as would be expected by the competition forwater due to the hydration of the inorganic salts. The effects ofthe cation and anion in the electrolyte are additive and appear todepend on the radius of the hydrated ion, that is, the lyotropicnumber. The smaller the radius of the hydrated ion, the greateris the effect. In this paper, we propose that not only do theneutral electrolytes decrease the cloud point of the surfactantsolutions, but also they can enhance the extraction efficiencyand preconcentration factor of the cloud point methodology.

When cloud point preconcentration prior to HPLC analysis isused, the significant limitations are the high backgroundabsorbance in the UV region and the lengthy operating timerequired for total elution of the surfactant injected. Several waysto overcome this problem have been proposed: Saitoh andHinze.16 used the zwitterionic surfactants 3-(nonyldimethyl-ammonium)propyl sulfate (C9APSO4) and 3-(decyldimethyl-ammonium)propyl sulfate (C10APSO4) which do not absorb atthe customary working wavelengths in HPLC, Pinto et al.17,20

used electrochemical detection owing to the electrodic in-activity of commercially available surfactants such as Triton X-114, and Ferrer et al.23 used a clean-up step with a silica gelcolumn to remove the surfactant before sample injection.

We first investigated the enrichment of five hydroxyaromaticcompounds at ppb levels in water by cloud point preconcentra-tion, with subsequent determination by HPLC with UVabsorption detection. The chromatographic conditions wereoptimized to separate the analyte compounds from the surfac-tant and to shorten the separation time. A simple method todetermine hydroxyaromatic compounds at ppm levels indyestuff samples, based on clean-up with an SAX cartridgepacked with an anion-exchange resin,26 followed by cloud pointpreconcentration and determination by HPLC was developed.

Experimental

Reagents

Analytical-reagent grade 2,7-dihydroxynaphthalene(2,7-DHNA), 4,4A-dihydroxybiphenyl (4,4A-DHBP) and 5-hy-droxy-1,4-naphthoquinone (5-HNQ) were purchased fromTokyo Chemical Industry (Tokyo, Japan); 2-hydroxynaphtha-lene (2-HNA) and 1-hydroxynaphthalene (1-HNA) were ob-tained from Riedel-de Haen (Seelze, Germany). The non-ionicsurfactant Triton X-114 was obtained from Fluka (Buchs,Switzerland) and used without further purification. LC-gradesodium acetate was obtained from Fisher Scientific (Fairlawn,NJ, USA) and acetonitrile from Tedia (Fairfield, OH, USA).The neutral electrolytes, sodium sulfate, sodium chloride andsodium nitrate were purchased from Merck (Darmstadt, Ger-many). De-ionized water was purified in a Milli-Q filtrationsystem (Millipore, Bedford, MA, USA) to obtain LC-gradewater for preparing mobile phases and standard solutions.

Analyst, July 1998, Vol. 123 (1535–1539) 1535

Publ

ishe

d on

01

Janu

ary

1998

. Dow

nloa

ded

on 2

5/10

/201

4 20

:01:

22.

View Article Online / Journal Homepage / Table of Contents for this issue

Apparatus

The HPLC system, assembled from modular components(Waters, Milford, MA, USA), consisted of a Model 600E pump,a Model 486 UV detector and a Model 715 automatic sampler.The analytical column was a 4 mm Nova-Pak Phenyl column (75mm 3 3.9 mm id) (Waters), and a 4 mm Nova-Pak C18 column(15 cm 3 3.9 mm id) (Waters) was used for confirmationexperiments. A Millennium workstation (Waters) was utilizedto control the system and for acquisition and analysis of data.Cloud point preconcentration was carried out in a YSC P610water-bath (Yeong Shin, Hsinchu, Taiwan). A Hettich Tut-tlingen, (Germany) universal centrifuge was used to separatethe surfactant-rich phase from the aqueous phase during cloudpoint preconcentration.

Standards and samples

Stock standard solutions were prepared by weighing thehydroxyaromatic compounds and dissolving them in methanol.A working composite standard solution was prepared bycombining an aliquot of each of the stock standard solutions anddiluting the mixture with water. These solutions were stored indark glass bottles at 4 °C.

Dyestuff samples of three kinds (D&C Orange No.4, SunsetYellow FCF and Tartrazine) were used (Tokyo ChemicalIndustry). The dyestuff (0.1 g) was weighed into a calibratedflask (100 ml), water (about 50 ml) was added and the solid wasdissolved completely with ultrasonic vibration. For recoverytests, a suitable aliquot of the standard solution of thehydroxyaromatic compounds was added to the dyestuff solu-tion.

Procedure

Cloud point preconcentration

Aliquots of 10.0 ml of the cold solutions containing the analytes(1–1000 ng ml21) and electrolytes (0.01–1.20 m) in 0.2% TritonX-114 were used. The contents were mixed well using a vortexmixer for 1 min and then incubated in a water-bath at 40 °C for5 min. The micellar sample solutions were centrifuged at 3500rpm for 5 min (without temperature control). The supernatantaqueous phase was removed with a Pasteur pipette and thesurfactant-rich phase was transferred to the samples vials of theautosampler with a 100 ml micro-transferpettor, then the sample(10 ml) was injected into the chromatographic system by theautomatic sampler.

HPLC operating conditions

The operating conditions for the HPLC system are given inTable 1. The mobile phase was a mixture of acetonitrile and 0.1m acetate buffer (pH 4.66). The gradient elution mode

confirmed that the analytes were eluted and separated in the first10 min and the Triton X-114 was eluted after 10 min.

Determination of hydroxyaromatic compounds in dyestuffsamples

Large amounts of co-existing dyestuff would interfere with theanalysis. A disposable SAX cartridge (3 ml volume tubecontaining 500 mg of SAX sorbent, Analytichem International,Habor City, CA, USA) was used as a clean-up filter. Thedyestuff solution (exactly 10 ml) was passed through the SAXcartridge and the effluent collected. By this means, whereashydroxyaromatic compounds passed unretained through theSAX cartridge, dyestuff components were absorbed. Tominimize losses of hydroxyaromatic compounds caused bypartial absorption on the SAX cartridge, the cartridge was theneluted with 4 ml of 0.1 m acetate buffer (pH 4.66) and the eluatewas collected. This eluate and the earlier effluent werecombined and mixed. An aliquot of the mixed solution (10 ml)was then enriched with cloud point methodology and analyzedin the manner described above.

Results and discussion

Chromatographic behavior of the surfactant

Fig. 1(a) shows the chromatogram obtained by injecting 10 ml ofthe surfactant-rich phase after cloud point separation and usingisocratic elution with acetonitrile–0.1 m acetate buffer (pH 4.66)(22 : 78 v/v). The Triton X-114 shows an appreciable signalafter 230 min, and it is not totally eluted even after 600 min. Ittakes a long time for the chromatographic analysis, although thehydroxyaromatic compounds studied are separated and detectedwithin the first 10 min.

Fig. 1(b) shows the chromatogram of the surfactant-richphase (10 ml) after cloud point separation with gradient elution

Table 1 HPLC operating conditions

Column Nova-Pak Phenyl, 75 mm 3 3.9 mm id, 4 mmFlow rate 1.0 ml21 min21

UV detector 254 nmMobile phase* Gradient elution—

Time/min A (%) B (%) Curve†

22 788 22 78 11

14 100 0 120 22 78 1

* The mobile phase was composed of (A) acetonitrile and (B) 0.1 macetate buffer (pH 4.66). † 1, Immediately goes to specified conditions; 11,maintains start conditions until the next step.

Fig. 1 Chromatograms obtained by the injection of surfactant-rich phasesafter cloud point separation with (a) isocratic elution and (b) gradientelution. For other experimental conditions, see text. Peaks: 1 = 2,7-DHNA;2 = 4,4A-DHBP; 3 = 5-HNQ; 4 = 2-HNA; 5 = 1-HNA.

1536 Analyst, July 1998, Vol. 123

Publ

ishe

d on

01

Janu

ary

1998

. Dow

nloa

ded

on 2

5/10

/201

4 20

:01:

22.

View Article Online

(conditions as specified in Table 1). The hydroxyaromaticcompounds were isolated in the first 10 min and the mainingredient of the surfactant eluted between 12 and 15 min.

Cloud point preconcentration and liquid chromatographicanalysis

The cloud point temperature is roughly constant (23–25 °C) inthe range of surfactant concentration 0.2–5%, thus facilitatingexperimentation. Fig. 2 compares the chromatograms obtainedby injecting hydroxyaromatic compound standard solution (1ppm, 10 ml) and by injecting the surfactant-rich phase (10 ml)which was obtained by cloud point preconcentration of thehydroxyaromatic compound standard solution (1 ppm, 10 ml)with 0.2% Triton X-114. The improvement in sensitivity aftercloud point preconcentration was very significant. The theoret-ical preconcentration factor for the 0.2% Triton X-114 solutionwas 100, which was calculated as the ratio of the volume ofsolution used for extraction to the volume of the surfactant-richphase.27

The influence of the temperature of cloud point preconcentra-tion was studied. The surfactant-rich phase was separatedeffectively from the aqueous phase by centrifuging at 3500 rpmfor 5 min after cloud point preconcentration at 40 °C. When thepreconcentration proceeded at 60 °C, the phase separation wasnot complete even after centrifuging for 15 min. The volumes ofthe surfactant-rich phase were 0.1 and 0.06 ml for extraction at40 °C (centrifuged for 5 min) and at 60 °C (centrifuged for 10min), respectively. The peak area of the analytes in thechromatogram for extraction at 60 °C (centrifuged for 10 min)is approximately 60% of the peak area for extraction at 40 °C(centrifuged for 5 min). The reason for the decreased volume ofsurfactant-rich phase with extraction at 60 °C is partially theincomplete separation of the two phases. These results indicate

that the effectiveness of the phase separation and the enhance-ment factors (the ratio of peak intensities with and withoutpreconcentration) are not improved by increasing the tem-perature of cloud point preconcentration from 40 to 60 °C.Therefore, the preconcentration was performed at 40 °C insubsequent work. This temperature (40 °C) is also thetemperature most commonly used for cloud point preconcentra-tion with Triton X-114 by other investigators.17–20,23

Solution pH is an important factor in those cloud pointextractions involving analytes that possess an acidic or basicmoiety.28 The variation of the extraction efficiency of thehydroxyaromatic compounds is almost negligible when thesolution pH is varied from 3 to 7, where the hydroxyaromaticcompounds are not ionized.

It is well known that the cloud point of a given non-ionicsurfactant decreases on addition of large amounts of electro-lytes.14,16,22,29,30 Some studies28,29 have revealed that there isno difference in the extent of extraction when electrolytes areadded to the micellar solution during cloud point extraction. Onthe other hand, it has been claimed22,30 that the phase separationof the sample solution and the extraction efficiency of theanalytes were improved by adding electrolytes.

Fig. 3 shows that the peak areas of the analytes increase withincrease in the concentration of the electrolytes, and the lowerthe lyotropic number of the electrolytes, the greater is the effect.The anionic lyotropic numbers of NaNO3, NaCl and Na2SO4 are11.6, 10.0 and 2.0, respectively.31 The enhancement factors foraddition of different electrolytes in 0.2% Triton X-114 solutionsare given in Table 2. The amount of hydroxyaromaticcompounds extracted into the surfactant-rich phase depends onthe hydrophobicity of the analytes. The enhancement factorswere improved significantly (more than doubled) with theaddition of 0.25 m Na2SO4 to the surfactant solutions. The

Fig. 2 Chromatograms of hydroxyaromatic compound standards (1 ppm)(a) without and (b) with cloud point preconcentration of 10 ml of the samplewith 0.2% Triton X-114. Chromatographic conditions as described underExperimental. Peak assignments as in Fig. 1.

Fig. 3 Effect of concentration of (a) NaNO3, (b) NaCl and (c) Na2SO4 onthe peak area using cloud point methodology. For other experimentalconditions, see text. Peak assignments as in Fig. 1.

Analyst, July 1998, Vol. 123 1537

Publ

ishe

d on

01

Janu

ary

1998

. Dow

nloa

ded

on 2

5/10

/201

4 20

:01:

22.

View Article Online

enhancement factor for a few compounds is greater than thetheoretical preconcentration factor. The reason for the greaterenhancement factor than the theoretical preconcentration factor(increase in sensitivity) can probably be attributed to themodification of the environment of the mobile phase by thesurfactant when the analytes reach the detector.19,20,27

The linearity of the method was tested by analyzinghydroxyaromatic compounds standard solutions in the range5–1000 ng ml21 with 0.2% Triton X-114. The relationshipbetween peak area and concentration of the analyte was linearover the entire range (the correlation coefficient of theregression lines exceeded 0.9991). The precision of the methodwas calculated with a sample solution spiked at 5 ng ml21 andthe relative standard deviations (RSDs) were between 1.9 and7.4% (n = 5). The method detection limits for thesehydroxyaromatic compounds were 0.8–1.8 ng ml21 (Table 3),and were improved to 0.1–0.7 ng ml21 by adding 0.25 mNa2SO4 to the surfactant solutions (Table 3).

Determination of hydroxyaromatic compounds in dyestuffsamples

A disposable SAX cartridge was used as a clean-up filter,followed by cloud point preconcentration and determination byHPLC. As shown in Table 4, the recovery of hydroxyaromaticcompounds from spiked dyestuffs was excellent (86–106%).The RSD of the mean was < 6%.

The method detection limits of this procedure were 0.6(2,7-DHNA), 0.5 (4,4A-DHBP), 1.2 (5-HNQ), 0.5 (2-HNA) and0.8 mg g21 (1-HNA) and were improved by the addition of 0.25m Na2SO4 to 0.2, 0.3, 0.3, 0.5 and 0.4 mg g21, respectively. Thedetection limits were measured with a concentration equivalentto three times the standard deviation of replicate measurements(n = 7) of the analytes in the dyestuff sample (Sunset YellowFCF). The proportion of hydroxyaromatic compounds indyestuffs for drug and cosmetic use is limited to 0.2% accordingto the US Code of Federal Regulations.11 The proposed method

is capable of determining hydroxyaromatic compounds atconcentrations much lower than the regulatory limits.

The utility of the method was confirmed by the analysis ofD&C Orange No. 4. Representative chromatograms obtainedfrom analysis of a standard (100 mg l21) and sample are shownin Fig. 4(a) and (b), respectively. As the hydroxyaromaticcompounds were well isolated from other dyestuff components,the presence of a peak with the retention time (7.467 min) of ahydroxyaromatic compound (2-HNA) gave a presumptive testfor its presence. The possible presence of 2-HNA in this dyewas further confirmed using a column of varied polarity (C18column), with a retention time of 2-NHA of 20.725 min. Theconcentration of 2-HNA in this dye was 214 ± 5 mg g21. Thisvalue was obtained from triplicate determinations based on acalibration graph. The concentration of 2-HNA in this dyedetermined by the standard additions method was 218 mg g21,which is very close to the value obtained from the calibrationgraph.

Table 2 Enhancement factors* for the different electrolytes in 0.2% TritonX-114 solutions

Compound Addition of electrolyte

None NaNO3† NaCl‡ Na2SO4

§

2,7-DHNA 45 61 105 1034,4A-DHBP 60 77 126 1275-HNQ 15 17 41 422-HNA 60 79 128 1301-HNA 72 94 149 155

* Ratio of peak intensities with and without the preconcentrationstep. † 1.20 m NaNO3 added to the surfactant solutions. ‡ 1.20 m NaCladded to the surfactant solutions. § 0.25 m Na2SO4 added to the surfactantsolutions.

Table 3 Limits of detection* (ng ml21) for the different electrolytes in 0.2%Triton X-114 solutions

Compound Addition of electrolyte

None NaCl† Na2SO4‡

2,7-DHNA 1.3 1.0 0.54,4A-DHBP 1.2 0.3 0.15-HNQ 1.7 0.3 0.72-HNA 0.8 0.5 0.31-HNA 1.8 1.0 0.7

* Calculated as three times the standard deviation of replicate measure-ments (n = 7) of the analytes. † 1.20 m NaCl added to the surfactantsolutions. ‡ 0.25 m Na2SO4 added to the surfactant solutions.

Table 4 Recoveries (%)* of selected hydroxyaromatic compounds added tocommercial dyes†

Compound Sunset Yellow FCF Tartrazine

10 mg g21 100 mg g21 10 mg g21 100 mg g21

2,7-DHNA 98 ± 2 105 ± 1 91 ± 2 88 ± 34,4A-DHBP 100 ± 5 103 ± 1 94 ± 4 87 ± 35-HNQ 88 ± 6 106 ± 3 90 ± 3 86 ± 12-HNA 104 ± 4 104 ± 2 102 ± 1 98 ± 21-HNA 103 ± 3 99 ± 3 103 ± 3 95 ± 2

* Average value and standard deviation of triplicate runs. † Thedyestuff solutions (0.1 g per 100 ml) were processed as described underExperimental.

Fig. 4 Chromatograms of (a) a 100 ng ml21 hydroxyaromatic compoundstandard solution and (b) a dye sample (D&C Orange No. 4), after clean-upon an SAX cartridge, followed the cloud point preconcentration with 0.2%Triton X-114 and separation on a Nova-Pak Phenyl column. For otherexperimental conditions, see text. Peak assignments as in Fig. 1.

1538 Analyst, July 1998, Vol. 123

Publ

ishe

d on

01

Janu

ary

1998

. Dow

nloa

ded

on 2

5/10

/201

4 20

:01:

22.

View Article Online

Conclusions

Hydroxyaromatic compounds in water at ppb levels can bedetermined by cloud point preconcentration with Triton X-114,followed by HPLC with UV absorption detection.

This method was applied to the determination of hydroxyaro-matic compounds at ppm levels in commercial dyestuffs.Because of the presence of dyestuff components at relativelyhigh concentrations, it was necessary to introduce a clean-upstep to remove dyestuff compounds from aqueous samples. Ananion-exchange cartridge (SAX) was used for the clean-up ofaqueous dyestuff samples prior to the cloud point preconcentra-tion and determination procedures.

The addition of the electrolytes to the micellar solutionsenhanced the extraction efficiency and preconcentration factorsof the cloud point methodology; the lower the lyotropic numberof the electrolytes, the greater is the effect.

Cloud point extraction offers a simple alternative to otherseparation and/or preconcentration techniques. It providesseveral advantages, including safety, low cost, the ability toconcentrate a variety of analytes and the possibility of enhanceddetection limits owing to the modification of the micro-environment of the analytes by the surfactant. Optimizing thechromatographic conditions can overcome the limitations of thehigh UV background absorbance and the lengthy operatingtime. The proposed method allows the simple, rapid andsimultaneous determination of hydroxyaromatic compounds,and is expected to be suitable for routine analyses.

This work was supported by the National Science Council of theRepublic of China (NSC 87-2113-M-007-042).

Reference

1 Longbottom, J. E., and Lichtenberg, J. J., Methods for OrganicChemical Analysis of Municipal and Industrial Wastewater, EPA-600/4-82-057, Method 604, US Environmental Protection Agency,Washington, DC, 1982.

2 Brouwer, E. R., and Brinkman, U. A. Th., J. Chromatogr. A, 1994,678, 223.

3 Baiocchi, C., Roggero, M. A., Giacosa, D., and Marengo, E., J.Chromatogr. Sci., 1995, 33, 338.

4 Pocurull, E., Sanchez, G., Borrull, F., and Marce, R. M., J.Chromatogr. A, 1995, 696, 31.

5 Burestedt, E., Emneus, J., Gorton, L., Marko-Varga, G., Dominguez,E., Ortega, F., Narvaez, A., Irth, H., Lutz, M., Puig, D., and Barcelo,D., Chromatographia, 1995, 40, 435.

6 Castillo, M., Puig, D., and Barcelo, D., J.Chromatogr. A, 1997, 778,301.

7 Bernal, J. L., Nozal, M. J., Toribio, L., Serna, M. L., Borrull, F.,Marce, R. M., and Pocurull, E., Chromatographia, 1997, 46, 295.

8 Puig, D., and Barcelo, D., J. Chromatogr. A, 1997, 778, 313.9 Sttig, M., Handbook of Toxic and Hazardous Chemicals and

Carcinogens, Noyes Publications, Park Ridge, NJ, 2nd edn., 1985.10 Code of Federal Regulations, US Government Printing Office,

Washington, DC, 1983.11 Marmion, D. M., Handbook of US Colorants for Food, Drugs, and

Cosmetics, Wiley, New York, 1979.12 Watanabe, H., in Solution Behaviour of Surfactants, ed. Mittal, K. L.,

and Fendler, E. F., Plenum Press, New York, 1982, vol. 2, p. 1305.13 Hinze, W. L., in Ordered Media in Chemical Separations, ed. Hinze,

W.L., and Armstrong, D. W., ACS Symposium Series, No. 342,American Chemical Society, Washington, DC, 1987.

14 Hinze, W. L., and Pramuro, E., Crit. Rev. Anal. Chem., 1993, 24,133.

15 Tani, H., Kamidate, T., and Watanabe, H., J. Chromatogr. A, 1997,780, 229.

16 Saitoh, T., and Hinze, W. L., Anal.Chem., 1991, 63, 2520.17 Pinto, C. G., Perez Pavon, J. L., and Moreno Cordero, B., Anal.

Chem., 1992, 64, 2334.18 Moreno. Cordero, B., Perez Pavon, J. L., Garcia Pinto, C., and

Fernandez Laespada, M. E., Talanta, 1993, 40, 1703.19 Pinto, C. G., Perez Pavon, J. L., and Moreno Cordero, B., Anal.

Chem., 1994, 66, 874.20 Pinto, C. G., Perez Pavon, J. L., and Moreno Cordero, B., Anal.

Chem., 1995, 67, 2606.21 Carabias Martinez, R., Rodriguez. Gonzalo, E., Garcia Jimenez, M.

G., Garcia Pinto, C., Perez Pavon, J. L., and Hernandez Mendez, J.,J. Chromatogr. A, 1996, 754, 85.

22 Sirimanne, S. R., Barr, J. R., and Patterson, D. G., Jr., Anal. Chem.,1996, 68, 1556.

23 Ferrer, R., Beltran, J. L., and Guiteras, J., Anal. Chim. Acta, 1996,330, 199.

24 Nonionic Surfactants: Physical Chemistry, ed. Schich, M. J., MarcelDekker, New York, 1987.

25 Rosen, M. J., Surfactants and Interfacial Phenomena, Wiley-Interscience, New York, 1989.

26 Lu, C. S., and Huang, S. D., J. Chromatogr. A, 1995, 696, 201.27 Wu, Y. C., and Huang, S. D., Anal. Chim. Acta, in the press.28 Frankewich, R. P., and Hinze, W. L., Anal. Chem., 1994, 66, 944.29 Horvath, W. J., and Huie, C. W., Talanta, 1992, 39, 487.30 Akita, S., and Takeuchi, H., Sep. Sci. Technol., 1995, 30, 833.31 Schick, M. J., J. Colloid Sci., 1962, 17, 801.

Paper 8/01372AReceived February 17, 1998

Accepted April 28, 1998

Analyst, July 1998, Vol. 123 1539

Publ

ishe

d on

01

Janu

ary

1998

. Dow

nloa

ded

on 2

5/10

/201

4 20

:01:

22.

View Article Online