A review of chromatographic methods for determination of synthetic food dyes

The purpose of this work was to present a chromatographic methods to analyse synthetic food dyes. The following techniques has been described: thin-layer liquid chromatography (TLC), high performance thin-layer chromatography (HPTLC), traditional column chromatography, high performance liquid chromatography (HPLC), include: ion-pair chromatography (HPLC IP), reversed phase chromatography (RP HPLC) and high performance ion chromatography.     

Ion-pair extraction of tetracyclines

Summary Ion-Pair Extraction of Tetracyclines Tetracyclines form ion-pair complexes with dyes that can be extracted into chloroform and re-extracted into 0.1N hydrochloric acid and finally quantified by spectrophotometry. Of the dyes studied methyl red, methyl orange, bromophenol blue and bromothymol blue form complexes with cationic form of tetracycline (pH 8–9) and crystal violet with the anionic form (pH 9–10). 98% of tetracycline can be extracted from the body fluids and solution containing 10g/ml of tetracycline can be conveniently analyzed.     

Prediction of internal standards in reversed-phase liquid chromatography: IV. Correlation and prediction of retention in reversed-phase ion-pair chromatography based on linear solvation energy relationships

This paper describes the results of the evaluation of a new solvation parameter model for reversed-phase ion-pair chromatography by linear gradient elution. This model is described as . The first six terms are the usual solvation parameter equation for neutral solutes, and the seventh term represents the contribution to retention from solute’s ionization. The last term describes the retention increase due to ion-pair effect. Retention times obtained for 60 solutes (neutral, acidic and basic) in acetonitrile/aqueous mobile phases with different ion-pair reagents (phosphoric acid, trifluoroacetic acid, heptafluorobutyric acid, perchloric acid, and hexafluorophosphoric acid) are used to evaluate the capability of the function. It is concluded that the model describes the retention of ionizable/ionized compounds under ion-pair conditions very well. Accordingly, the function extends the application of linear solvation energy relationships (LSERs) to ionizable compounds in ion-pair chromatography, and allows us to easily predict their retention for chromatographic optimization, including selectivity optimization and internal standard selection. Finally, the conclusion can be extended to ioscratic elution.     

Photochemistry of benzimidazolocyanine dyes in solvents of different polarity

The effect of ion pair formation on the kinetics of the decay of the photoisomers and triplet states of cationic benzimidazolocyanine dyes is studied by flash photolysis. An increase in the rate constant of the reversecis-trans isomerization of the photoisomers is observed when ion pairs are formed (in nonpolar solvents). In the case of benzimidazolocyanine dyes with the I^– anion, ion-pair formation causes an increase in the rate constant of decay of the triplet state. Acceleration of S₁ S₀ internal conversion is discovered for the dyes with I^–1 Translated fromIvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 507–512, March, 1993.     

Separation of synthetic food colourants in the mixed micellar system. Application to pharmaceutical analysis

The paper presents a rapid method for the determination of commonly used synthetic food dyes by micellar electrokinetic capillary chromatography. Detection and separation conditions allowing complete resolution of 15 synthetic food colourants were investigated. The effect of different surfactants on the analytes mobility in relation to their structure was tested. After optimization procedure a dual micellar system was selected. All food dyes were separated in less then 20 min using a fused silica capillary in the borate/dodecylsulfate/deoxycholate buffer containing acetonitrile as organic modifier. The detection wavelength was set at 210nm. The method was successfully validated by determination of linearity ranges, detection limits, precision and repeatability for all colourants tested. In order to apply the method for pharmaceutical analysis a sample pretreatment procedures were found. Liquid pharmaceuticals were used as it or just after dilution with water. From tablets or capsules the colourants were isolated by adsorption on acidic aluminium oxide. The method was used for identification and if possible for quantification the synthetic food dyes in pharmaceuticals. The analytes are detectable at a concentration level 0.3-0.8 microg ml(-1).     

Determination of sulphonated azo dyes in food by ion-pair liquid chromatography with photodiode array and electrospray mass spectrometry detection

An ion-pair liquid chromatography method with on-line photodiode-array and electrospray mass spectrometry detection was developed to determine 10 commonly used sulphonated azo dyes (Tartrazine, Amaranth, New Coccine, Sunset Yellow FCF, Allura Red AC, Ponceau R, Ponceau 3R, Orange I, Orange II and Metanil Yellow) in food. A reversed phase C(18) column with gradient elution was utilized to separate these compounds. Triethylamine was added in the mobile phase as an ion-pair reagent for chromatographic separation. Photodiode-array detection was employed for quantitative determination and electrospray mass spectrometry was used for identification. Good linearity (0.05-10 ppm, r(2)=0.999) and detection limit (<0.01 ppm) were determined with 5 mul injection. In addition, precision and accuracy associated with this newly developed method will be presented. A liquid extraction method was also developed to extract these dyes from different foodstuffs. The application of this method was demonstrated by analyzing sulphonated azo dyes in soft drinks, fruit jam, and salted vegetables.     

Utility of the ion-pair formation for spectrophotometric determination of terfenadine in pure form and in some pharmaceutical formulations

A spectrophotometric procedure for the determination of terfenadine and a number of its pharmaceutical preparations has been developed that offers advantages of simplicity, rapidity, sensitivity and stability indication over the official USP (1995) method. The proposed method is based on the formation of ion-pairs by the reaction of terfenadine with some chromotropic acid mono- and bis-azo dyes. Different variables affecting the ion-pair formation were studied and optimized. At the maximum absorption of 557, 521, 592 and 543 nm, Beer's law is obeyed in the range 0.2–18.6, 0.2–16.4, 0.2–25.0 and 0.2–22.2 g ml^–1 on using reagents I, II, III and IV, respectively. The stoichiometric ratio and stability of each ion-pair were estimated and the mechanism of the reaction is discussed. The molar absorptivity and Sandell sensitivity of the produced ion-pairs were calculated in addition to Ringbom optimum concentration ranges. Statistical treatment of the experimental results indicates that the procedures are precise and accurate. Excipients used as additives in pharmaceutical formulations did not interfere in the proposed procedures. The reliability of the methods was established by parallel determination against the official USP method. The procedures described were successfully applied to the determination of the bulk drug and its pharmaceutical formulations by applying the standard addition technique.     

Retention parameters of some isomeric 2-benzoylbenzoic acids in reversed-phase ion-pair high performance thin-layer and column chromatography

Summary The 2-benzoylbenzoic acid series was investigated by reversed-phase, high-performance, thin-layer and column chromatography using various alkylammonium salts and di(2-ethylhexyl)orthophosphoric acid as polar associating reagents. The effects of the individual substituents on retention were quantified by log k and R_M values. The compounds investigated differing in molecular structure (hydrophilic and hydrophobic substituents) commonly occurring groups in drugs and biologically active substances provide information on molecular interaction in these ion-pair systems. The combined effects on retention of organic modifier and ion-pair reagent concentration were investigated.     

Plain and ion-pair thin-layer chromatography of synthetic dyes on an admixture of silica gel G and barium sulfate in an aqueous ethanol system

An easy, inexpensive, and accurate method for the analysis of synthetic dyes by means of ion-pair thin-layer chromatography (IPTLC) on mixed sorbent phases containing silica gel G and barium sulfate in an aqueous ethanol system is studied. The effect of the composition of the sorbent phases has a major effect on the hundred-fold relative migration rate, as also does the effect of the ion-pairing reagent as the impregnant. Compact and sharp spot application yields very good binary and ternary separations and enables their clear identification. IPTLC has better separations on mixed sorbent phases, which are not possible on plain mixed phases in thin-layer chromatography. The method can be applied for the trace analysis of synthetic dyes in various natural and synthetic samples.     

Separation of aminoacidato ruthenium(II) complexes by ion-pair chromatography

Summary The chromatographic conditions for analytical separation of (⁶-C₆H₆) ruthenium(II) complexes with amino acid and dipeptide ligands have been investigated in dependence on their molecular structure. Different chromatographic phases were tested and the efficiency parameters were determined by the reversed-phase ion-pair technique. For the enantiomeric species L-penicillaminato ruthenium(II) complexes cis- and trans-[(⁶-C₆H₆)Ru(L-Pen)H_–1]₂Cl₂ and the reaction byproducts a complete chromatographic separation has been obtained using sodium dodecylsulphate as an ion-pair reagent. For the purification procedure of the L-penicillaminato ruthenium(II) complexes, in order to characterize the separated pure complexes by ¹H-NMR spectroscopy, a preparative separation has been carried out using the volatile trifluoroacetic acid as an ion-pair reagent.Dedicated to Professor Dr. Wilhelm Fresenius on the occassion of his 80th birthday     

Rapid fluorescence assay for Sudan dyes using polyethyleneimine-coated copper nanoclusters

We report that the intensity of the blue fluorescence of copper nanoclusters coated with polyethyleneimine (PEI) is strongly reduced in the presence of the food dyestuffs Sudan I-IV. This finding was exploited in a label-free fluorescence assay for these Sudan dyes both in ethanol and aqueous solutions. The PEI-capped nanoclusters have an average diameter of 1.8 nm and are displaying, under 355 nm excitation, a blue emission at 480 nm that matches the absorption bands of the Sudan dyes. The clusters are stable in solution for at least 1 month. Under optimum conditions, this assay can be applied to the quantification of the dyes Sudan I, II, III, and IV, respectively, in the 0.1?30, 0.1–30, 0.1–25, and 0.1–25 μM concentration ranges, and the detection limits (3σ/slope) are 65, 70, 45, and 50 nM, respectively. The capability of reducing the fluorescence of the PEI-capped copper nanoclusters is directly related to the number of the functional groups in that Sudan III and IV give lower detection limits. This analytical scheme exhibits a remarkably high selectivity for the Sudan dyes over potentially interfering substances. The method was successfully applied to determine Sudan I, II, III, and IV in hot chilli powder.

Chromatographische Klassifizierung künstlicher Farbstoffe im Mikromaßstab

Zusammenfassung Auch geringe Farbstoffmengen lassen sich auf papier- und dünnschichtchromatographischem Wege klassifizieren. Als chromatographische Techniken kommen normale Chromatographie an Kieselgel und Cellulose, pH-abhängige Chromatographie an acetylierter Cellulose und Reaktionschromatographie am Start von Cellulose zur Anwendung. Bei der pH-abhängigen Chromatographie liegt der Gradient in der mobilen Phase. Dies ist der Unterschied zur Gradient-Dünnschichtchromatographie nach Stahl, bei der sich der Gradient in der stationären Phase befindet. Sehr geringe Farbstoffmengen können zuvor durch normales Erstarren oder Ringofentechnik angereichert werden. In chemischer Hinsicht ist es dabei möglich, hydrophile Substituenten, wie -SO₃H, -COOH, -OH und -NH₂/ -N(CH₃)₂, festzustellen, desgleichen die Anzahl an SO₃H-Gruppen. In färberischer Hinsicht ist es gleichzeitig möglich, die wichtigsten Farbstoffklassen zu erkennen. Als solche kommen in Frage: Leukoküpenesterfarbstoffe, 11- und 12-Metallkomplexfarbstoffe, substantive Farbstoffe, Baumwollaufheller, Reaktivfarbstoffe, Säurefarbstoffe, Naphthol-Komponenten, basische Farbstoffe, Dispersionsfarbstoffe und Küpenfarbstoffe.

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Micro-chromatographic classification of synthetic dyes

Summary Even small amounts of synthetic dyes can be classified by special means of paper chromatography and thin layer chromatography as normal chromatography on silica gel or cellulose, p_h dependent chromatography on cellulose acetate and reaction chromatography at the starting point of cellulose. The method of p_h dependent chromatography has the gradient in the mobile phase. It is therefore different to the gradient TLC by Stahl with the p_h gradient in the stationary phase. Very small amounts of synthetic dyes can be enriched first by normal freezing or ring oven technique. From the chemical point of view it is possible to identify hydrophilic substituents such as -SO₃H, -COOH, -OH, and -NH₂/-N(CH₃)₂ as well as the number of SO₃H-groups. From the coloristic point of view it is possible to identify most classes of synthetic dyes such as leuco vat dyes, 11- and 12-metal complex dyes, substantive dyes, cotton whitening agents, reactive dyes, acid dyes, naphthol components, basic dyes, dispersed dyes and vat dyes.

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Erweiterte Fassung eines Vortrages anläßlich des VI. Internationalen Symposiums für Mikrochemie vom 7. bis 11. 9. 1970 in Graz.     

Color and fastness of natural dyes encapsulated by a sol-gel process for dyeing natural and synthetic fibers

Natural dyes, namely, indigo carmine, cochineal carmine, curcumin and annatto, were encapsulated in silica by a sol-gel method and applied in the dyeing of different textile fibers by exhaustion. For comparative reasons, dyeing using the free (non-encapsulated) bare dyes was also carried out. The hybrid materials were analyzed by a set of techniques to investigate their elemental, structural, textural and morphological properties, and the results showed that it was possible to obtain stable natural dyes for applications in textile dyeing. The silica-structured dyes showed better affinities with the fibers (WO, PA, PAC and PET) in dyeing with cochineal carmine, while cotton (CO) showed better affinities with the encapsulated curcumin and annatto dyes. The performances of the encapsulated dyes were evaluated by color and washing fastness measurements and resulted in improved dye absorption and wash fastness properties. The color change and color transfer measurements of the encapsulated dyes were better (rated at 4–5 on a scale of 1–5) compared to the bare dyes.

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Identification of cosmetic dyes by ion-pair reversed-phase high-performance liquid chromatography

A method based on ion-pair reversed-phase high-performance liquid chromatography with detection at four wavelengths between 400 and 600 nm is reported for the separation and identification of the most common synthetic colour additives in cosmetic products. All the dyes generally employed in the U.S.A. and almost all those in current use in cosmetics in the European Community have been taken into account. The chromatography was performed on a C8 bonded silica packed column, with a 60-min gradient changing from 10 to 95% acetonitrile in water containing 10(-2) M sodium perchlorate (pH 3.0) as mobile phase (flow-rate 2.5 ml/min). Detection limits are in the range 20-100 ng for all dyes investigated. The method has been applied to the analysis of commercial lipsticks.     

Identification of Indigo Dyes in Painting Layers by Pyrolysis Methylation and Silylation. A Case Study: “The Dinner of Emmaus” by G. Preti

Reactive pyrolysis, under methylating or silylating conditions, in combination with gas chromatography-mass spectrometry (GC-MS) was evaluated as an analytical method for the detection of indigo dyes in painting layers. Samples with the addition of tetramethylammonium hydroxide (TMAH) or hexamethyldisilazane (HMDS), for pyrolysis/methylation and pyrolysis/silylation experiments, respectively, were pyrolysed at 600 °C by means of a heated filament pyrolyser and the evolved products were analysed on-line by GC-MS. Methyl and silylated pyrolytic markers, related to 2-aminobenzoic acid and the indoxyl moiety, were established from the analysis of synthetic indigo, neat or as a pigment of artificial painting layers containing a siccative oil. The occurrence of these markers was investigated in a real sample, taken from the painting The dinner of Emmaus by G.Preti (XVII century), in order to identify the blu pigment. Positive identification of indigo was achieved by pyrolysis/silylation and confirmed by RAMAN spectroscopy.     

Determination of aromatic sulfonic acids in industrial waste water by ion-pair chromatography

A method for the identification and quantification of aromatic sulfonic acids in industrial waste water has been developed. The preparation of the samples comprises a clean-up step and an enrichment step, utilizing i) reversed-phase extraction and ii) ion-pair solid phase extraction. The aromatic sulfonic acids are separated by ion-pair chromatography with diode-array detection. An advantage of the developed ion-pair chromatographic method is the separation of linear alkylbenzenesulfonic acids (LAS), lignosulfonic acids and aromatic sulfonic acids in one chromatographic run. Also described is an optimized clean-up procedure for collected fractions of a preparative chromatographic run to identify monosulfonic acids with GC/MS after methylation. The detection limits for aromatic sulfonic acids in industrial waste waters are at the g/l level.     

Chemical Reactions of Surfactants with Acid–Base Dyes and Determination of Imidazolines in the Air of the Working Area

Reactions of acid–base monoazo dyes with polyamine surfactants were studied. The ratios of components in the resulting compounds were determined; the stability constants and molar absorptivities were calculated. A mechanism was proposed for the reaction of monoazo dyes with amine and imidazoline surfactants with the formation of macroheterocycles. Approaches to predicting analytical reactions between monoazo dyes and surfactants were substantiated. The dependence of the extraction of these compounds on the pH of the aqueous phase and the concentrations of NaCl and ethanol was studied. The optimal conditions for extraction separation were found, and a procedure was developed for the spectrophotometric determination of 1-(-hydroxyethyl)-2-heptadecenyl-2-imidazoline (Imidazoline 17) and 1,1"-bis(-hydroxyethyl)-2-heptadecenyl-2-imidazolonium chloride (Imidastat O) in the air of the working area using Methyl Orange as the reagent. The determination limits were 0.025 and 0.12 g/mL, respectively. A sampling procedure was developed for the determination of Imidazoline 17 and Imidastat O in the air with determination limits of 0.05 and 0.25 mg/m³, respectively, for sampling 15 L of air to 60% ethanol.     

Ion-pair extraction and fluorimetric determination of ultratraces of strontium with cryptand 2.2.2 and eosin

A highly sensitive and selective fluorimetric determination of strontium is proposed, based on solvent extraction of the ion-pair formed between the cationic complex of Sr²⁺ with cryptand 2.2.2 and eosinate as counter ion. A linear working range from 0.7 ng/ml (limit of detection) to 500 ng/ml of strontium and a relative standard deviation of 3.5% at the 100 ng/ml level are obtained. The metal: ligand: counter ion molecular ratio in the extracted mononuclear ion-pair is 1 1 1. The equilibrium constants involved in the extraction process were calculated.