Micellar enhanced spectrophotometric determination of organic species

trendsin analytical chemistry, vol. 14, no. I, 1995

29

u31 E. Forghcs, K. Valk6 and T. CserhBti, J. Chromatogr., 631 (1993) 207-213. ~241 E. ForgBcs, T. Cserhati and K. Valkb, J. Chromatogr., 592 (1992) 75-83. v51 T. Cserhhti and H.E. Hauck, J. Chromatogr., 514 ( 1990) 45-55. E. [261 Forghcs and T. CserhBti, J. Chromatogr., 600 ( 1992) 43-49. [271 T. Cserhhti, and E. ForgBcs, J. Chromatogr., 643 (1993) 331-336.

[29] A.Fell,T.A.G.Noctor,J.E.

[281N.W. Smith and D. Brennan, Poster presented at

Esther Forgks and Tibor Cserha’ti are at the Central Research Institute for Chemistry, Hungarian Academy of Sciences, P. 0. Box 17, H1525, Budapest, Hungary,

the 12th Int. Chromatography,

Symp. on Column Liquid Washington, DC, June 19-24,

1988.

MamaandB.J.Clark,

J. Chromatogr., 434 ( 1988) 377-384. [ 301 J. Dolphin, Lab. Pratt., 38 ( 1989) 7 l-82.

Micellar enhanced spectrophotometric determination of organic species J.S. Esteve-Romero * CastelId, Spain

E.F. Simb-Alfonso, M.C. GarciaAlvarez-Coque, G. Ramis-Ramos Valkncia, Spain The use of surfactants to enhance spectrophotometric procedures for the determination of organic species is critically reviewed. Special emphasis is placed on the effects of sutfactants on chromogenic derivatization The following topics are reactions. addressed: modification of acid-base and solubility equilibria, spectral changes, modification of reaction rates, and applications in equilibrium-and kinetic-basedanalyticalprocedures. Applications to flow injection procedures are also briefly reviewed.

been paid to other organized media such as microemulsions or liquid crystals. In the presence of surfactants, equilibrium, kinetic and spectral properties can be modified, and this has been used to improve the characteristics of analytical procedures. The changes are observed over a wide range of surfactant concentrations, below and above the critical micelle concentration (cmc) . Surfactants can induce favourable shifts in equilibrium constants and spectral properties, inhibit undesirable reactions, such as hydrolysis and photolysis, stabilize reaction intermediates, co-solubilize nonpolar and polar samples, derivatization reagents and products, and speed up reactions by means of micellar catalysis. Here we discuss these effects in relation to the enhancement of spectrophotometric procedures for the determination of organic species. 2. Modification of physical and physicochemical properties

1. Introduction The use of surfactants in various areas of analytical chemistry has attracted much interest in the last two decades. Normal or reversed micelles are most frequently used, and much less attention has * Corresponding author.

0 1995Elsevier

Science B.V. All rights reserved

Organic ions and molecules can bind the surfactant assemblies by both electrostatic and hydrophobic interactions. Thus, in a water-continuous micellar solution the non-polar parts of the solute molecule interact hydrophobically with the exposed hydrocarbon chains of the surfactant. If both solute and surfactant are ions or zwitterions, electrostatic interaction is also produced. The con01659936/95/$09.50

trends in analytical chemistry, vol. 14, no. 1, 1995

jugate species of an acid-base pair bind the assemblies with different strengths. Simultaneously, changes in the dielectric constant and surface potentials experienced by the species are produced. This causes the equilibrium constant of the acidbase pair to be shifted. In the same way, other equilibria such as complex formation, redox, extraction and solubility can be modified. Owing to the modification of the protonation constants of the reagents, the optimum pH value at which a reaction takes place could be different for micellar and aqueous solutions. In a common case, one of the species of a conjugate acid-base pair will be active and the other one will not react. A shift of the protonation constant will shift the pH range in which the reaction takes place. As the reagents and intermediates involved in many chromogenic derivatization reactions are ions, a polar medium is required to support the reactions. However, many samples of natural and industrial origin, and many organic dyes that are used to enhance the spectral characteristics of the analytes, have a marked non-polar character, and their solubility in polar media, such as water or alcohol-water mixtures, is limited. Increasing the solubility of the non-polar species without decreasing the solubility of polar and ionic species in an organized medium allows a wider scope of analytes, reagents and samples to be handled. The use of organized media can also improve spectrophotometric methods by providing bathochromic shifts and sensitivity enhancements in the spectra of organic molecules. Spectral changes are produced when the chromophore binds to the assemblies by hydrophobic or electrostatic forces, and result from changes in the microenvironment of the chromophoric group. However, at a given pH, spectral changes can also be caused by a shift in a protonation constant, and when a chromogenic derivatization is used, sensitivity enhancements can be produced by a higher reaction yield. In kinetic procedures or in flow injection (FI) sensitivity enhancements can be procedures, caused by an increase in reaction rates. Frequently, several effects will be produced simultaneously, and experiments should be performed in order to evaluate the contributions of the various causes for the observed spectral changes. The two primary factors that are responsible for catalysis in micellar solutions are the change in reactivity of the reagents upon transfer from the bulk water to the micelle, and the concentration effect. The association of the reagents and intermediate to the assemblies, their

solubilization sites, and orientation play important roles in the catalysis [ 11. The favourable electrostatic stabilization of a charged transition state by an oppositely-charged ionic micelle is often found in micellar catalysis. The effect of orientation at the molecular level can also influence the stereoselectivity of chemical processes, thus altering reaction pathways. Hydrophobic compounds that are electrically neutral are preferentially solubilized by the micelles, regardless of their charge. In this case, the reagents are brought together and locally concentrated in a reduced volume within the solution (i.e., the micelles) and positive catalysis will usually be observed. If a reactant carries a charge opposite to that of the micelle it will be electrostatically attracted to the micelle, and thereby brought closer to any other hydrophobic reagent, thus contributing to acceleration of the reaction. Finally, inhibition or negative catalysis can be expected for reaction between a species which is bound to the micelles and another one which is electrostatically rejected by the aggregates. This repulsion can actually be used with advantage to repel interferences. Micellar catalysis is useful not only in speeding up slow reactions, and thus adapting the time scale of the kinetic experiment to the instrumental requirements, but also in softening the experimental conditions required to carry out the reactions, e.g. by reducing the temperature or a mineral acid concentration. This is of interest when labile analytes or reagents are handled, and for simplification of manual and automated analytical procedures. In addition, an increase in the rate of a chromogenic reaction, and a reduction in the rate of a competing lateral reaction, including inhibition of hydrolysis of the product, can increase the apparent molar absorptivity, and therefore the sensitivity and reliability of the procedure. Some aspects of the use of surfactants apply particularly to FI procedures. Micellar catalysis can be useful for reactions which are slow, and for which either long reaction coils are used to allow sufficient time for the reaction to proceed, or a stopped-flow method is used. When an on-line reaction takes place in FI, two kinetic processes occur simultaneously, i.e., physical dispersion and chemical reaction. If the reaction is slow, sufficient reaction time is necessary to provide a detectable amount of product but, as the length of manifold is increased to provide longer reaction times, the dispersion increases. Eventually, a point is reached where the effect of increased dispersion overcomes

trends in analytical chemistry,

vol. 14, no. 1, 1995

31

[ 2-41. For this purpose, the following association constants were used:

YH-CH,-W-NH,

Wnl

K’=[A] [S,] PA

Scheme 1. Coupling of diazotized avymines (1 -naphthyl)ethylenediamine.

with N-

the increase in signal attained with longer reaction times, and the resulting signal is reduced. A compromise must be reached between these two extremes in order to achieve optimum results from the method. An increase in the sensitivity resulting from the catalytic effect of micelles will yield a lower limit of detection, provided the system noise is constant, or will allow the use of a shorter flow manifold with less dispersion and greater sample throughput. The effects of surfactants on several chromogenic derivatization reactions are examined below. In some cases, a single parameter is modified in order to improve an analytical procedure but, most frequently, there is a simultaneous alteration of various physical properties of the solution and physice-chemical properties of the reagents, intermediates and products. The corresponding analytical procedure would benefit from some of the effects produced, but others will be detrimental.

3. Effect of surfactants on several chromogenic reactions 3.1. Modification of the acid-base properties N-( 1-naphthyl)ethylenediamine azo dyes

of

Diazonium ions, formed by the action of nitrous acid on arylamines, have electrophilic character and couple with activated substrates such as N-( lnaphthyl) ethylenediamine to yield (NED), intensely coloured azo dyes (Scheme 1) . The primary alkylamino group of the NED azo dyes is protonated in weakly alkaline media, and the secondary alkyl-arylamino group protonates in weakly acidic media, giving rise to a bathochromic shift and to a sensitivity increase. Thus, in neutral and acidic media the NED azo dyes are always cations with one or two positive charges. The protonation constant of the secondary amino group of NED azo dyes is modified by surfactants, and this has been studied spectrophotometrically

IHA%

=

c

(2)

[HAI [Snl

where A and HA (charges not indicated) are the monoprotonated and diprotonated azo dye, respectively, the subindex n is the aggregation number, and [S,] is the concentration of the surfactant assemblies. The protonation constant of the assemblies in the presence of the conjugate pair can be defined as: [HASnl

KH=

’

W'l

Wnl

It has been shown that, when an acid solution of a NED azo dye is titrated with sodium hydroxide in the presence of a constant concentration of surfactant, the inflection point of the titration curve is given by: pH1=log

K+ eQcw,l l +Kt[s,I

=log Klp

(4)

where K is the protonation constant of the acidbase pair, in the absence of surfactant, and Kap is the apparent protonation constant, which is a function of the surfactant concentration. At high values of [S,], the limit pH, = log KF is achieved, which has been used to determine log c values. In addition, log Kt can be determined by titration of a buffered solution of the dye with a surfactant solution, Finally, log CA can be calculated from the other constants. The relationship between these constants can be expressed as: KnA (AlogK,,),,=log+=logK;-1ogK C

(5)

which indicates that the difference between log c and log Kis a measure of the selective capability of the surfactant to bind the diprotonated form of the dye with respect to the monoprotonated form. Thus, for instance, the azo dye formed by the diazotized aniline and NED, when titrated in the absence of surfactant gave log K=3.03, and the log Kap changed from this value to log c = 4.15 when the concentration of sodium dodecyl sulphate (SDS) was increased [ 31. Most of the change in log Kap took place almost completely before micellization. In contrast with Triton X-100 most of the

32

change in log &, was produced at concentrations higher than the cmc. N-Cetylpyridinium chloride (NCPC) gave an intermediate situation, with an important part of the change before, and the other part after micellization. The values of log @ =: 1.25 and 0.8 were estimated for Triton X-100 and NCPC, respectively. The values of Kap indicated that the diprotonated azo dye, which has a positive charge on the para-amino group, was more strongly associated to the anionic SDS micelles than the monoprotonated species, whereas for Triton X- 100 and NCPC the monoprotonated species showed a stronger association. Other studies, involving other NED azo dyes, also showed that log @ followed the order SDS > water > TXlOO>NCPC [4]. 3.2. Catalysis of the diazonium ions-NED coupling reactions The coupling reactions of diazotized arylamines with NED are positively catalyzed in an SDS micellar medium. The effect can be explained by association both of the diazonium cations and NED to the anionic micelles. In the acidic reaction medium both the di- and monoprotonated forms of NED should be strongly associated to the micelles by electrostatic and hydrophobic interactions; however, the former should be more strongly associated owing to the extra positive charge. This causes the secondary amino group of NED to be protonated at a higher pH than in water; consequently, in the acidic reaction medium and in the presence of the anionic surfactant the concentration of the monoprotonated form of NED is lower than in water. Since only this form of NED reacts with the electrophilic agent, at a given pH the presence of SDS should reduce the reaction rate. The opposite was observed in the experiments which indicated that, in comparison to the earlier protonation of the secondary amino group of NED, the micellar concentration effect predominated. Both effects, the shift of the protonation constants of the azo dyes to higher pH values and micellar catalysis of the coupling reactions with NED, were used to develop a simplified spectrophotometric procedure for the determination of arylamines [2,4,5]. In a non-micellar solution, the diazonium ions are formed at pH < 1, whereas pH > 4 is necessary for coupling, particularly if the diazonium ion lacks a strong activating substituent. Finally, another modification of the pH is usually made in order to measure the absorbance of the azo

trends in analytical chemistry, vol. 14, no. 1, 1995

R

P-

o-

!

@-

+n+

R

H,C

SH,

Scheme 2. Coupling of diazotized 2,4,6-trimethylaniline with phenols.

dye in its diprotonated form. In the SDS micellar solution, an optimum pH range could be established in which coupling was produced at a sufficiently large rate and, at the same time, the azo dye was protonated [4]. For a large number of diazotizable substances the lower limit, pHmin, was established as the pH where coupling was 99% completed in 1 min, and the upper limit was taken as PH,, = log Z&, - 2, which corresponds to 99% protonation of the dye. In water, an optimum pH range did not exist or was very small. Instead, in the SDS micellar medium, pH,, was higher than pHmi, for all the arylamines tried, with a difference of 1 to 4 pH units. The arylamines could be diazotized and coupled between the values 1.O < pH < 1.6, where all the optimum pH ranges overlapped. Therefore, the coupling and measurement steps could be performed in a 0.06 M HCl solution, which resulted from the addition of the reagents to the 0.15 M HCl solution used to diazotize the arylamine. This simplified procedure was also adapted to FI 161. 3.3. Coupling of diazo tized 2,4,6- trimethylaniline with phenols The use of diazotized 2,4,6_trimethylaniline (TMA) to determine phenol derivatives by coupling in a micellar medium has been proposed [ 71. When phenols are coupled with a diazotized arylamine (Scheme 2), large and unstable values of the absorbance of the blank solution (prepared in the absence of phenol) are frequently observed. The high signal is caused by hydrolysis of the diazonium ion to give the corresponding phenol in a process known as hydroxy-dediazoniation, which is followed by coupling of the produced phenol with excess reagent in the basic medium. The ortho and para positions of the 2,4,6_trimethylphenol which is produced by hydroxy-dediazoniation of the diazotized TMA are blocked by the methyl groups, which hinders its coupling with the excess

trends in analyticalchemistry, vol. 14, no. 1, 1995

33

x -0

+

(‘I-N

G+ 0

OH-

-

Y = (‘I.

Fir

X

+H+

-0$&N+&”

+ Cl-

+ 4H + + 4e-

Scheme 3. Oxidative coupling of phenols with 4-aminoantipyrine. reagent. This allows a very low absorbance for the blank solution, even at high pH values. Because of the hydrophobic methyl substituents of TMA, this reagent is only slightly soluble in water, but the reaction can be carried out in a micellar medium. Strongly positive micellar catalysis was also observed in the derivatization of phenols with the diazotized TMA [ 71. The reaction rates followed the order NCPC > SDS > TX- 100 > water. The positive catalysis in the micellar media can be explained on the basis of hydrophobic association of both the diazonium and phenolate ions to the micelles. However, because of the opposing effects of the electrostatic forces, the diazonium ions are more strongly associated to the anionic SDS micelles, whereas the phenolate ions bind more strongly to the cationic NCPC micelles. Probably for this reason, and the predominance of the hydrophobic forces, micelles of opposite charge did not produce dramatic differences in the micellar catalytic effect. In addition, in the NCPC micellar medium, the protonation of phenolate ions is produced at a lower pH than in water, which should further increase the reaction rate in the presence of NCPC, whereas the opposite occurs in the SDS micellar medium. 3.4. Coupling of phenols with 4-aminoan tipyrine,

2,6-dihaloquinone naphthol

Scheme 4. Coupling of the phenols with 2,6-dihaloquinone chlorimides.

were particularly large in an NCPC medium (30130 nm). In some instances, substrates that are not sufficiently activated to react in water or in alcoholic media can be derivatized in micellar media. Thus, in an aqueous suspension of lead(IV) oxide, lnitroso-2-naphthol coupled with strongly activated substrates (Scheme 5)) including tyrosine (a catecho1 derivative), phenylephrine (a meta-substituted phenol), methoxamine (a para-diary1 ether) and oxprenolol (a or&o-diary1 ether) [8]. The reactions proceeded through the formation of an unstable red intermediate, whose colour gradually faded to give a stable yellow derivative. In an SDS micellar medium less activated substrates, such as the aryl alkyl ethers acebutolol, atenolol and metoprolol, were also derivatized. These substrates did not react in non-micellar media or in a Triton X100 micellar solution. The reagent and the substrates are hydrophobic and should be associated to the micelles, but their orientations in the watermicelle interphase could also play an essential role. 3.5. Coupling of pyrroles with 4-(dimethylamino)benzaldehyde Pyrrole derivatives can be determined spectrophotometrically after chromogenic derivatization

chlorimides and 7-nitroso-Z-

Catecholamines and some phenols couple with 4-aminoantipyrine and with 2,6-dihaloquinone chlorimides to give dyes (Schemes 3 and 4) [ 81. When the reactions were performed in SDS, Triton X-100 or NCPC micellar media, the sensitivity increased by a factor up to 5.7. Also, in these media, the chlorimide derivatives of a number of primary amines, dopamine, L-dopa and norepinephrine, showed small hypsochromic or bathochromic shifts, whereas the derivatives of some more hydrophobic secondary amines, phenylephrine and epinephrine, produced bathochromic shifts, which

NO

0

+HzO

Scheme 5. Oxidative nitroso-2-naphthol.

OH

+2H+

+ 2e

coupling of phenols with l-

34

trends in analytical chemistry, vol. 14, no. 1, 7995

OHC

N(CH&

-

H

CHKW

NW,),

A H+

CH

N(CH&

A

N+(CH,),

Scheme 6. Coupling of pyrroles with 4-(dimethylamino)benzaldehyde.

with 4-( dimethylamino) benzaldehyde (DMBA, Ehrlich’s reagent) to yield a coloured quinonimine (Scheme 6) [9]. The procedure is also applied to the determination of hydroxyproline. After oxidation with chloramine T, this amino acid yields several pyrrole derivatives, mainly pyrrole3-carboxylic acid, which couples with DMBA. When coupling was performed in an SDS micellar medium the sensitivity was enhanced. With Triton X-100 or NCPC the sensitivity enhancement factors were lower, or even negative with some pyrrole derivatives. For a series of pyrrole derivatives in SDS solutions, the enhancement factor ranged from 1.1 to 5.6. Binding of the reagents and product to the anionic micelles was favoured as a result of the hydrophobic character of the molecules and the positive charges of both the protonated electrophilic agent and the quinonimine in the acid medium of the reaction. 3.6. Nitrosa tion of N, N-diethylaniline The effects produced by different micellar systems on the nitrosation of N,N-diethylaniline (DEA) (Scheme 7) have been studied [ lo]. No shifts in the wavelength of maximum absorbance of the derivative, p-nitroso-N,N-diethylaniline, were observed in a cetyltrimethylammonium bromide (CTAB) micellar solution. However, the absorbance band of the intermediate activatedcomplex shifted from 429 nm to 470 nm and the nitrosation rate increased in the presence of CTAB cationic micelles. The initial step is the electrophilic attack by NO+ with the displacement of Hf. This reaction scheme suggests that the nitrosation rate could be

increased in an SDS micellar medium, as the reaction occurs between a neutral hydrophobic and a cationic species. However, the results were contrary to expectation. The presence of the anionic surfactant actually reduced the reaction rate relative to that in the non-micellar medium. However, there was a substantial increase in the reaction rate when a cationic surfactant, such as CTAB, was used. The possible explanation of this behaviour is that, in the acidic media used, much of the substrate was in the protonated form. As the free amine is the active species, any substance in the reaction solution that decreases the protonation constant of the amine will increase both the amount of free amine in solution and the reaction rate. The cationic micelles provided the force for this equilibrium shift. A comparison of the coupling of arylamines with diazonium ions (see the previous section) with the nitrosation of DEA in micellar media reveals that the SDS micelles produced opposite effects on apparently analogous reactions. In both cases a cationic electrophile, i.e., a diazonium ion or NO+, attacks the pm-u position of a free amine. There is no substantial difference between the two arylamines, DEA and NED, which have large hydrophobic parts and undergo analogous protonation equilibria. The difference should be found in the largely different hydrophobicity of the electrophilic agents: the diazonium ions have large hydrophobic parts and are strongly associated to the anionic micelles, whereas NO+ is hydrophilic, and unlikely to be associated to the micelles. Therefore, it is not surprising that, for diazonium ions, the concentration effects were stronger than the effects produced by the shift in the protonation constant of the arylamine, whereas the opposite was observed for NO+. 3.7. Hydrolysis of benzodiazepines

Scheme 7. Nitrosation of N,N-diethylaniline.

Benzodiazepines can be transformed into benzophenones by hydrolysis in an HCl medium at

trends in analytical chemistry,

vol. 14, no. 1, 1995

35

F

NO,

+

K,HNR,

(‘,Ji;

+

-

F-

w&

Scheme 8. Hydrolysis of benzodiazepines.

high temperature (Scheme 8). It has been shown that the selectivity of the spectrophotometric determination of benzodiazepines can be improved by measuring the absorbance of the corresponding benzophenone derivatives in micellar solutions of CTAB or Nemo1 K 1030 (a non-ionic ethylene oxide condensate) [ 111. For example, the addition of Nemo1 K 1030 to a number of benzophenones provided, in some cases, a bathochromic shift of the UV bands, and for some a new band also appeared at a longer wavelength, e.g. 415 nm for the benzophenone produced from oxazepam. The new bands were the same as those obtained in absolute ethanol, which indicated that the micellar microenvironment had similar solvent characteristics to alcohols. In the micellar media the absorbance bands of some benzophenones derived from different benzodiazepines, differed significantly from each other. This is in contrast to the corresponding spectra obtained in non-micellar media, and allows the selective determination of benzodiazepines in binary mixtures. 3.8. Derivatization

of amines with l-fluoro-2,4-

dinitrobenzene

The spectrophotometric determination of amino acids and peptides with 1-fluoro-2,4_dinitrobenzene (FDNB) (Scheme 9), in the presence of CTAB, has been reported [ 121. Micellar catalysis resulted in the amino acids giving slightly higher absorbances than the solutions without surfactant. The more hydrophobic aromatic amino acids gave much larger relative rate enhancements. A procedure for the FI determination of amino acids and other amines using FDNB combined with stopped-flow kinetics has also been described [ 13 1. The suitability of FDNB as a general reagent for routine determinations, especially in pharmaceutical analysis, made automation of the corresponding procedures appropriate. This was important as the FDNB reagent is difficult to handle: it is a vesicant and hydrolyses in alkaline solutions. The analytical reaction is slow, and usually requires heating to speed it up, together with additional steps for hydrolysis of the excess FDNB, and

Scheme 9. Derivatization of amines with 1-fluoro-2,4dinitrobenzene.

extraction of the dinitrobenzene product for measurement. The procedures cannot be applied to coloured samples. These problems associated with equilibrium procedures were overcome in the kinetic procedure where CTAB was used. 3.9. Oxidative coupling of catechol with pphenetidine The effect of micellar media on the bromateinduced oxidative coupling of catechol withp-phenetidine (Scheme IO), which is catalyzed by vanadium(V), has been studied [ 141. At the pH of the reaction,p-phenetidine and catechol are nonionic, and should be associated to micelles of any type by hydrophobic interactions. Positive micellar catalysis was observed with several cationic and non-ionic surfactants, whereas anionic surfactants were precipitated by the bromate and could not be used. After an induction period, the NCPC micelles were capable of catalysing the reaction, even in the absence of vanadium( V) . It was also observed that the catalytic activity of this metal was increased by a factor of 10.6 in the presence of NCPC. Thus, the combined catalytic activity of both vanadium(V) and NCPC micelles had a positive synergistic effect on the reaction rate and, simultaneously, the final absorbance increased by a factor of 3.9. This resulted in an increased sensitivity in the kinetic determination of vanadium(V) . 3.7 0. Reduction of pyridoxal in the presence of cyanide

The determination of pyridoxal (a B6 vitamin) and pyridoxal-5-phosphate can be performed by

f!qH,

‘0

Scheme 10. Oxidative coupling of catechol with p phenetidine.

trendsinanalyticalchemistry, vol.14,no.7,1995

36

+

CN-

HOCH,

A

Scheme 11. Oxidation of pyridoxal in the presence of cyanide.

oxidation in the presence of cyanide to form 4pyridoxolactone (Scheme 11). In a FI method, a 6 m reaction coil and 45°C were required for maximum efficiency in a non-micellar medium. Pyridoxal is sufficiently hydrophobic and cyanide is anionic, so both partition to a CTAB cationic micelle, producing micellar catalysis. Owing to this effect, optimization in the presence of CTAB gave a sensitivity enhancement factor of 1.8 with a 0.2 m reaction coil and 49°C [ 151. It was shown that switching from aqueous to micellar carriers increased the dispersion within the FI system, but the increase in the reaction rates was still sufficiently large to lower the limits of detection.

procedures in organic analysis, to extend the applicability of chromogenic organic reactions to a wider range of substrates and reagents, and to render ionic hydrophilic and hydrophobic compounds and samples compatible. The use of surfactants was initially limited to measurements of equilibrium properties but has been extended to kinetic analytical procedures. The wide variety of organic reactions and the differences in the reactivity and physical properties of reagents, substrates, intermediates and products, suggest that only a very small part of the possibilities has so far been investigated.

Acknowledgements This work was supported Spain, Project PB91/629.

by the DGICYT

of

References 3.7 I. Reduction of 5,5’-dithiobis-(2Atrobenzoic

acid) with sulphur dioxide Sulphite reduces 5,5’-dithiobis-( 2-nitrobenzoic acid) to the intensely coloured 5-mercapto-2-nitrobenzoic acid in a phosphate buffer of pH 6.5 (Scheme 12). The sensitivity of the reaction increased by a factor of 8 in the presence of NCPC, and the effect was used to develop an improved FI procedure for the determination of sulphite and formaldehyde [ 161.

4. Conclusions During the last two decades, organized media have been intensively investigated to help improve analytical procedures in inorganic analysis. More recently, attention has been paid to the use of surfactants in the analysis of organic compounds. We have shown here that surfactants can be used as versatile chemical tools to improve the selectivity, sensitivity and reliability of spectrophotometric

Scheme 12. Reduction of 5,5’-dithiobis-(2-nitrobenzoic acid) with sulphur dioxide.

Martinek, A.K. Yatsimirski, A.V. Levashov and I.V. Berezin, in K.L. Mittal (Editor), Micellization, Solubilization and Microemulsions, Vol. 2, Plenum, New York, 1976, pp. 489-507. [2] G. Ramis Ramos, J.S. Esteve Romero and M.C. Garcia Alvarez-Coque, Anal. Chim. Acta, 223 (1989) 327. [ 31 J.S. Esteve Romero, G. Ramis Ramos and M.C. Garcia Alvarez-Coque, J. Colloid Inter-ace Sci., 141 (1991) 44. [ 41 J.S. Esteve Romero, M.C. Garcia Alvarez-Coque and G. Ramis Ramos, Talanta, 38 (1991) 1285. [5] J.S. Esteve Romero, E.F. Sim6 Alfonso, M.C. Garcia Alvarez-Coque and G. Ramis Ramos, Talanta, 40 (1993) 1711. [ 61 J.S. Esteve Romero, G. Ramis Ramos, R. Forteza Co11 and V. Cerda Martin, Anal. Chim. Acta, 242 (1991) 143. [ 71 J.S. Esteve Romero, L. Alvarez Rodriguez, M.C. Garcia Alvarez-Coque and G. Ramis Ramos, Analyst, 119 (1994) 1381. [8] M.C. Garcia Alvarez-Coque, G. Ramis Ramos and J.S. Esteve Romero, Anal. Lett., 25 (1992) 2059. [9] J.S. Esteve Romero, Ll. Monferrer Pons, M.C. Garcia Alvarez-Coque and G. Ramis Ramos, Anal. Lett., 27 ( 1994) 1557. [ lo] B.F. Johnson, R.E. Malick, B. Ghearing and J.G. Dorsey, Analyst, 117 (1992) 1833. [ 111 M. de la Guardia, M.V. Galdti, J. Monz6 and A. Salvador, Analyst, 114 ( 1989) 509. [ l] K.

trends in analytical chemistry, vol. 14, no. 7, 7995

[ 121 K.A. Connors and M.P. Wong, J. Pharm. Sci., 68 ( 1979) 1470.

[ 131 CA.

Georgiou, M.A. Koupparis and T.P. Hadjiioannou, Talanta, 38 ( 1991) 689. [ 141 M.L. Lunar, S. Rubio and D. Perez-Bendito, Anal. Chirn. Acta, 237 ( 1990) 207.

M.A. Hernandez Tones, M.G. Khaledi and J.G. Dorsey, Anal. Chim. Actu, 201 ( 1987) 67. [ 16 J M.S. Abdel-Latif and G.G. Guilbault, Anal. Lett., [ I5 ]

22 (1989)

37

Dr. J.S. Esteve-Romero is at the Escola Superior de Tecnologia i Cikncies Experimen tab, Universitat Jaume I, 12006 Castelid, Spain. Drs. E.F. SimbAlfonso, M. C. Garcia-AlvarezCoque and G. Ramis-Ramos are at the Departamento de Quimica Analitica, Universita t de Valkncia, Spain.

1355.

Neutron activation analysis: impact on the archaeology of the Holy Land Joseph Yellin Jerusalem,

Israel

Neutron activation analysis (NAA) is one of the most potent scientific methods that has been applied to ancient artifacts in recent years. By means of NAA it is possible to obtain a chemical ‘fingerprint’ of the elemental composition of archaeological artifacts and through this to establish their place of origin. The origin is of paramount importance in reconstructing history from the remains recovered by archaeologists. In this article NAA and its application to archaeology are described. Some recent contributions to archaeology are presented, including new results.

1. Introduction Neutron Activation Analysis (NAA) is one of many scientific techniques available to today’s archaeologists for the analysis of ancient materials. It is the technique that has proven to be most useful for determining the origin of pottery, obsidian and other materials through chemical fingerprinting [ 1-3 1. By a chemical fingerprint we mean a record of the concentrations of the chemical elements. This allows us to determine the geographical origin of pottery or, in for obsidian, the volcanic source from which it was obtained. The technique tells us nothing directly about the chemical form or strucQ 1995 Elsevier

Science

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ture of materials. Other kinds of spectroscopies combined with microscopy are available for studying these and are increasingly employed in the study of ancient technology. NAA refers to the analysis of the results of a process (thermal neutron capture) that makes a measurement possible. Many isotopes formed by neutron capture are unstable and undergo radioactive decay. In the process the nucleus is transformed into a different chemical element. This transformation is often accompanied by the emission of y-rays from the newly formed element which is usually in an excited nuclear state. For example, if the nucleus of a tantalum atom captures a neutron the tantalum isotope formed decays to form an isotope of tungsten (n + Ta-18 1 + Ta182 -+ p- + W- 182 + 7). This is illustrated in Fig. 1. The transformation of Ta to W is accompanied by the emission of y-rays unique to this transformation. The observation of W-l 82 y-rays emanating from the neutron-activated specimen means that there is Ta in the specimen since there is no other way in which these unique rays could be generated except through the disintegration of Ta-182. Because of this, the W-182 y-rays tag Ta, and are often referred to as Tantalum y-rays. Similar terminology applies to the other elements which are tagged by y-rays of elements resulting from radioactive decay following thermal neutron capture. Much of our knowledge about nuclear structure is derived from the observation of y-rays produced by a variety of activation techniques, including 01659936/95/$09.50