Ion-interaction Chromatography Research Articles

EXTENDED THERMODYNAMIC APPROACH TO ION INTERACTION CHROMATOGRAPHY FOR LOW SURFACE POTENTIAL. USE OF THE LINEARIZED POTENTIAL EXPRESSION

The chromatographic behaviour of charged analytes in Ion Interaction Chromatography (IIC) was theoretically investigated. Simplified retention equations were obtained via the linearized potential expression. They can be used to model analyte retention as a function of both the mobile and stationary phase concentration of the Ion-Interaction reagent (IIR), if the surface potential is below 25 mV. Simplified retention equations were compared to those, which can be obtained from two of the most important retention models in IIC. They reduce to stoichiometric or electrostatic retention model equations if the surface potential or pairing equilibria are respectively neglected.

Potentiometric detection of amines in ion chromatography using macrocycle-based liquid membrane electrodes

Potentiometric detection employing solid-state electrodes was applied to the determination of organic amines in cation-exchange, and in ion-interaction chromatography. The amines included nine industrial alkylamines, the biogenic amines putrescine and cadaverine, the neurotransmitter acetylcholine and choline. Three PVC-based liquid membrane electrode coatings were used, incorporating respectively the lipophilic cation-exchanger tetrakis( p-chlorophenyl)borate, a combination of tetrakis( p-chlorophenyl)borate and dibenzo-18-crown-6, or a combination of tetrakis( p-chlorophenyl)borate and a calix[6]arene. The three solid-state electrodes allowed sensitive detection of all amines. The addition of macrocycles with molecular recognition properties resulted in a superior sensitivity when compared to the tetrakis( p-chlorophenyl)borate-based electrode. The effect was most significant for the smaller amines which form the most stable complexes with the macrocycles. Detection limits of the order of 10 −6 M (injected concentration) were measured for mono- and diamines for the macrocycle-based electrodes.

The dipole approach in ion-interaction chromatography of zwitterions

The chromatographic behavior of zwitterions in Ion-interaction chromatography (IIC) is, investigated theoretically for the first time. The modification of the stationary phase in the presence of Ion-interaction reagent (IIR), and adsorption competition between test analytes and IIR for inner layer sites are shown theoretically to change the partition coefficient for zwitterions.

Application of artificial neural networks in multifactor optimization of an on-line microwave FIA system for catalytic kinetic determination of ruthenium (III)

A methodology based on the coupling of experimental design and artificial neural networks (ANNs) is proposed in the optimization of a flow injection system for the spectrophotometric determination of Ru (III) with m-acetylchlorophosphonazo (CPA-mA), which has been for the first time used for the optimization of high-performance capillary zone electrophoresis (J. Chromatogr. A 793 (1998) 317). And since it has been applied in many other regions like micellar electrokinetic chromatography, ion-interaction chromatography, HPLC, etc. (J. Chromatogr. A 850 (1999) 345; J. Chromatogr. A 799 (1998) 35; J. Chromatogr. A 799 (1998) 47). An orthogonal design is utilized to design the experimental protocol, in which five variables are varied simultaneously (Anal. Chim. Acta 360 (1998) 227). Feedforward-type neural networks with extended delta-bar-delta (EDBD) algorithm are applied to model the system, and the optimization of the experimental conditions is carried out in the neural network with 5–5–1 structure, which have been confirmed to be able to provide the maximum performance. In contrast to traditional methods, the use of this methodology has advantages in terms of a reduction in analysis time and an improvement in the ability of optimization. Under the optimum experimental conditions, Ru (III) can be determined in the range 0.040–0.60 μg ml −1 with detection limit of 0.03 μg ml −1 and the sampling frequency of 34 h −1. The method has been applied to the determination of Ru (III) in refined ore as well as in secondary alloy and provided satisfactory results.

Extended thermodynamic approach to ion interaction chromatography.

The chromatographic behavior of charged analytes in ion interaction chromatography (IIC) is theoretically investigated. The chemical modifications of the stationary and mobile phases in the presence of ion interaction reagent (IIR) are theoretically shown to change the partition coefficient for charged molecules. The most reliable literature experimental results concerning retention behavior of charged molecules in IIC were used to test the new theory. Retention equations are compared with those that can be obtained from the most important retention models in IIC. The present exhaustive retention model, which is well-founded in physical chemistry, goes further than the previous ones whose retention equations can be viewed as limiting cases of the present theory. The present extended thermodynamic approach reduces to stoichiometric or electrostatic retention models if the surface potential or pairing equilibria are respectively neglected. Moreover, it is able to quantitatively explain experimental evidences that cannot be rationalized by the existing retention models.

Fast separation of UV absorbing anions using ion-interaction chromatography

Ion-interaction chromatography on a short (30×4.6 mm) 3 μm ODS column has been investigated with the aim of developing fast chromatographic separations of selected inorganic anions. Tetrabutylammonium chloride (TBA-Cl) was used as the ion-interaction reagent in mobile phases that also contained up to 20% methanol. Separations of simple test mixtures of up to eight UV absorbing anions illustrated how excellent efficiencies (>50 000 plates/m) could be obtained under optimized conditions. The use of an optimised mobile phase containing 20 m M TBA-Cl and 20% methanol resulted in the baseline separation of five important anions (iodate, bromate, nitrite, bromide and nitrate) in a separation window of just 28 s, with a shortest total analysis time of 50 s. The method was briefly applied to the rapid analysis of nitrite and nitrate in both a drinking water and a river water sample with a view to future on-line monitoring.

ION INTERACTION CHROMATOGRAPHY OF NEUTRAL MOLECULES. USE OF A POTENTIAL APPROXIMATION TO OBTAIN A SIMPLIFIED RETENTION EQUATION

The chromatographic behavior of neutral molecules in Ion Interaction Chromatography (IIC) was theoretically investigated. The use of a potential approximation to obtain a simplified retention equation was successfully tested. The most reliable experimental literature results, concerning the retention behavior of neutral molecules in IIC, were successfully fitted by the retention equation. The wide variability among them was elucidated on the basis of the developed retention equation. A new separation strategy, to improve the selectivity of the chromatographic method, for difficult to separate mixtures of neutral eluates was proposed.

Ion-interaction chromatography of neutral molecules

The chromatographic behavior of neutral molecules in ion-interaction chromatography (IIC) is investigated theoretically. The physical and chemical modification of the stationary phase in the presence of ion interaction Reagent (IIR) in the eluent, and adsorption competition between test analytes and IIR for inner layer sites are shown theoretically to change the partition coefficient of neutral molecules.

Chemometrically assisted simultaneous separation of 21 aromatic sulfonates in ion-interaction RP-HPLC

A method of ion-interaction chromatography (IIR-RP-HPLC) for the simultaneous separation of 21 polar aromatic sulfonates is developed and optimised by a chemometric treatment. The analytes are: 1-naphthalene sulfonic acid (1-NS), 2-naphthalene sulfonic acid, 2-amino-1-naphthalene sulfonic acid (2-A-1-NS), 5-amino-2-naphthalene sulfonic acid (5-A-2-NS), 8-amino-2-naphthalene sulfonic acid (8-A-1-NS), 1-amino-5-naphthalene sulfonic acid (1-A-5-NS), 4-amino-1-naphthalene sulfonic acid (4-A-1-NS), 6-hydroxy-2-naphthalene sulfonic acid (6-H-2-NS), 4-hydroxy-1-naphthalene sulfonic acid (4-H-1-NS), 6-amino-4-hydroxy-2-naphthalene sulfonic acid (6-A-4-H-NS), 6-amino-1-hydroxy-3-naphthalene sulfonic acid (6-A-1-H-3-NS), 3-amino-2,7-naphthalene disulfonic acid (3-A-2,7-NdS), 7-amino-1,3-naphthalene disulfonic acid (7-A1,3-NdS), 2-amino-1,5-naphthalene disulfonic acid (2-A-1,5-NdS), 1-hydroxy-3,6-naphthalene disulfonic acid (1-H-3,6-NdS), 2-hydroxy-3,6-naphthalene disulfonic acid (2-H-3,6-NdS), 3-nitrobenzene sulfonic acid (3-NBS), 4-phenol sulfonic acid (4-PS), 4-hydroxy-3-nitrobenzene sulfonic acid (4-H-3-NBS), 1,2-benzene disulfonic acid (1,2-BdS), 2,6-anthraquinone disulfonic acid (2,6-AntdS). The factors optimised are: (i) the alkyl chain length of the ion-interaction reagent, (ii) the ion-interaction reagent concentration, (iii) the acetonitrile concentration and (iv) the pH of the mobile phase. An Fractional Factorial design, together with a star design are used to simultaneously study the effect of the variables and of their interactions. The retention times resulted to be not homogeneously distributed in the variable domain: in particular, for all the analytes considered, the use of nonylamine as the IIR caused unexpectedly large retention times. In these conditions, the linear regression algorithm failed in building good predictive models. The problem was solved by using the Box–Cox transformation, that considers other possible dependencies, besides the linear one, between the response (retention time t R) and the variables. In particular, a response 1/ t R and 1 t R permitted to better fit the data and to build reliable predictive models. In turn, the treatment of these models allowed us to locate the experimental conditions to separate, with good resolution, the 21 polar aromatic sulfonates in a total analysis time lower than 37 min.

Spectroscopic Characterization of Ethyl Xanthate Oxidation Products and Analysis by Ion Interaction Chromatography

An ion interaction chromatographic separation method, coupled with UV spectroscopic detection, has been developed for the analysis of ethyl xanthate (O-ethyl dithiocarbonate) and its oxidative decomposition products in mineral flotation systems. The effects of the ion-pairing agent (tetrabutylammonium ion), pH modifier (phosphoric acid), and organic modifier (acetonitrile) in the eluant upon the retention characteristics of the ethyl xanthate oxidation products have been determined. The optimized separation procedure has been successfully applied to the analysis of ethyl xanthate and its oxidation products in a nickel-iron sulfide mineral suspension containing a number of other anionic species, including cyanide complexes of nickel and iron, as well as sulfur-oxy anions. The ethyl xanthate oxidation products investigated in this study have been isolated as pure compounds and characterized by UV-visible, FT-IR, and NMR spectroscopies. The UV-visible and FT-IR spectroscopic properties of these species are discussed in terms of the chemical modifications of the thiocarbonate group.

Determination of metal-cyanide complexes by ion-interaction chromatography with fluorimetric detection

A chromatographic separation/UV-photodissociation/fluorimetric detection system is reported for the determination of cyanide and metal-cyanide complexes. After separation by ion-interaction chromatography, the stable metal complexes are photodissociated with an on-line PTFE reaction coil irradiated by an 8 W standard germicidal lamp. Free cyanide is then detected by reaction with o-phthaldialdehyde and glycine to form a fluorescent isoindole derivative. The method allows the determination of labile cyanide and the cyanide complexes of Fe(II), Fe(III), Ni(II) and Co(II). The detection limits are about 10 μg l −1 cyanide, except for Co(CN) 6 3−, which is less sensitive. The method has been successfully applied to the analysis of cyanide species in spiked river-water samples.

Determination of Niobium(V) and Tantalum(V) as 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP)-citrate ternary complexes in geological materials using ion-interaction reversed-phase high-performance liquid chromatography

A method for the simultaneous separation and determination of Nb(V) and Ta(V) as ternary complexes with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (Br-PADAP) and citrate using ion-interaction reversed-phase high performance liquid chromatography on a C 18 column has been developed. The mobile phase compositions and pre-column complex formation conditions were optimised. Under the optimum conditions, the Nb(V) and Ta(V) complexes were eluted within 4 min with a mobile phase of methanol-water (50% v/v) containing 12 mM tetrabutylammonium bromide and 12 mM citrate buffer at pH 7, with absorbance detection at 600 nm. The resulting % relative standard deviation of retention times, peak area and peak height for Nb(V) and Ta(V) complexes were 0.007, 0.000, 0.67, 2.69, 0.44 and 2.17, respectively. The detection limits (signal to noise ratio = 3) for Nb(V) and Ta(V) were 0.03 ppb and 0.41 ppb, respectively. The method was applied to the analysis of geological samples (granite, basalt and andesite) and the results for Nb(V) obtained by the HPLC method using standard addition calibration agreed well with ICP-MS analysis and with certified values for all materials. For Ta(V), good agreement with certified values was obtained for granite samples using standard addition, but interferences were observed for basalt and andesite which prohibited the determination of Ta(V) in these samples.

Preconcentration and separation of haloacetic acids by ion chromatography

A comparative study was made of the chromatographic behaviour of five haloacetic acids (mono-, dibromoacetic and mono-, di-, trichloroacetic acids). The techniques investigated included reversed-phase ion interaction chromatography with UV detection, suppressed and non-suppressed anion-exchange chromatography. The systems are discussed studying the retention as a function of the mobile phase parameters and the stationary phases used (LiChrospher 100 RP-18, IonPac AS9, AS10 and AS11). A preconcentration step, performed on different substrates, namely LiChrolut-EN and activated vegetal carbon, has been optimized in order to reduce the method detection limits. Results obtained for drinking water samples are shown.

INFLUENCE OF EXPERIMENTAL PARAMETERS ON CHROMATOGRAPHIC BEHAVIOR OF NEUTRAL MOLECULES IN ION INTERACTION CHROMATOGRAPHY

The chromatographic behavior of aprotic neutral molecules in Ion Interaction Chromatography (IIC) was investigated both theoretically and experimentally on a styrene-divinylbenzene based C18 stationary phase. The chemical modification of the stationary phase in the presence of Ion Interaction Reagent (IIR) in the eluent, and adsorption competition between tested analytes and IIR for inner layer sites are theoretically shown to change the partition coefficient for neutral molecules. Experimental evidence confirms that their retention decreases with increasing ion-interaction reagent (IIR) concentration in the eluent. The influence of the mobile phase ionic strength on neutral molecules retention was investigated and the dependence of retention modulus on organic modifier concentration in the eluent was elucidated.

Photodiode Array Detection as a Fingerprinting Tool for Metal Cyanide Complexes in Gold Processing Solutions

Reversed-phase ion-interaction high-performance liquid chromatography with photodiode array detection (PDAD) was used to simultaneously identify and quantify mental cyanide complexes present in gold processing solutions. Identification was achieved using the “fingerprint” or full-scan UV spectrum (190 - 350 nm) obtained from the PDAD. The complexes identified so far were; Cu(CN)4 3− Fe(CN)6 4− Fe(CN)6 3− Co(CN)6 3− Ni(CN)4 2− Au(CN)2 − and Cr(CN)6 3−. Thiocyanate (SCN) was also detected and identified by this technique.

Retention Behavior of Neutral Molecules in Ion Interaction Chromatography

The retention behaviour of aprotic neutral molecules in Ion Interaction Chromatography was investigated. The study demonstrates that their retention, for an aqueous mobile phase (phosphate buffer pH 7.2-methanol, 85:15 v/v) containing the ion-interaction reagent (IIR) in the range 0–25 mM, decreases with increasing IIR concentration. Silanophilic interactions were accounted for by comparison of results obtained with either a silica based or a styrene-divinylbenzene based C18 stationary phase, under otherwise identical conditions. The retention behaviour of neutral molecules was elucidated on the basis of the exhaustive Ion Interaction retention model and evidence was obtained to support the proposition that the decreased retention is the result of an adsorption competition between tested analytes and ion-interaction reagent for inner layer sites on the stationary phase.

Dye-coated stationary-phases: A retention model for anions in ion-interaction chromatography

The separation of inorganic anions (NO3−, NO2−, Cl−, Br−, I−, SO42−, S2O32−) by ion-interaction chromatography mediated with a specific dye has been investigated. Chromatography was performed on a LiChrospher RP-18 colum dynamically coated with crystal violet, using acetonitrile-water buffered with phthalate as the mobile phase. The presence of the dye in the eluent enabled indirect spectrophotometric detection of the analytes, which have no significant UV absorption. Retention data were collected for the different anions by varying the composition of the mobile phase according to a full factorial experimental design. A theoretical model for the retention of singly- and doubly-charged analytes, on the basis of the two main processes of ion-exchange and ion-pair formation, has been proposed and validated with the experimental data.

Electrochemical detection of sulphonated azo dyes and their metal complexes in ion interaction chromatography

An ion interaction chromatographic method coupled to electrochemical detection was used for the separation and the determination of sulphonated azo ligands (SPADNS, Acid Alizarin Violet N and Plasmocorinth B) and their metal complexes. The aim of this work was to verify the compatibility of the separation mechanism with the electrochemical detection of the azo dyes and their metal complexes and to obtain the best experimental conditions for a highly sensitive detection of the analytes considered. The effects of the mobile phase composition (ion interaction reagent, counter-ion, organic modifier, pH) on detection efficiency was investigated and the optimum conditions were identified. The developed procedure, that enabled the determination of metal ions such as copper and iron at low μg/l levels, was applied to the analysis of natural waters.

Monitoring the cyanidation of gold–copper ores with ion-interaction chromatography: Determination of the CN −:Cu(I) molar ratio

The use of reversed-phase ion-interaction high-performance liquid chromatography is reported for process monitoring of cyanide, thiocyanate and metallo–cyanide species in the gold cyanidation process. Thiocyanate and the metallo–cyanide complexes were monitored with a programmable, variable-wavelength UV detector mounted directly after the column, while cyanide, thiocyanate and the Cu(I) and Ag(I) cyanide complexes were monitored with a post-column reaction (PCR) detection system mounted after the UV detector. The eluent composition was optimised to enable separation of the major components of process samples in less than 10 min on a 5 cm column, but a 15 cm column was required for samples that contained Cu(I) at concentrations in excess of 300 ppm. The major emphasis during development of the eluent was to provide baseline resolution of thiocyanate and the Cu(I)–cyanide peaks and the optimal eluent composition consisted of 25% acetonitrile, 10 m M tetrabutylammonium hydroxide, 10 m M KH 2PO 4, 5 m M H 2SO 4 and 80–160 μ M NaCN, adjusted to pH 8.0. The level of cyanide present in the eluent was determined by the degree of separation required between thiocyanate and Cu(I). The PCR detection system enabled the cyanide:copper molar ratio, R, in process samples to be monitored routinely. R was determined from the peak area ratio of the cyanide and Cu(I)–cyanide peaks. The use of the peak ratio eliminated drift in the PCR detection system, providing a stable calibration for R. Good agreement was observed between standard analytical methods and the chromatographic method for determining R. Controlled cyanide leach tests showed that all the cyanide in the samples could be accounted for by the various cyano species detected. The proposed method has been utilised under field conditions on a working gold mine and has been shown to be very useful for determining cyanide losses and for monitoring the gold leaching process when treating cupriferous and pyritic gold ores.

Comparison of prediction power between theoretical and neural-network models in ion-interaction chromatography

The separation by ion-interaction chromatography (IIC) of metal complexes having single and double charges has been studied in order to compare the prediction power of soft (neural-network) and hard modelling (IIC equation). The two approaches have been used to model the retention behaviour as a function of the composition of the mobile phase. With ion-interaction mobile phases, the parameters involved included the concentrations of ion-interaction reagent, organic modifier and ionic strength. From a set of 69 experimental design points (the different mobile phase compositions at which capacity factors are measured), one test set of ten design points and ten training sets, containing from 59 to 11 design points, have been extracted. Chromatographic and chemometric considerations for the selection of the data sets and minimum number of observations required have been discussed. The study showed that the IIC equation predicted more accurately when few experimental data were available, while a similar prediction power was obtained with both models when the number of data was more than 17. Nevertheless the neural-network accounted for a greater versatility without the need to develop an equation.