Milko Novič

On-line thermal lens spectrometric detection of Cr(III) and Cr(VI) after separation by ion chromatography

Abstract The applicability of thermal lens spectrometry for the on-line detection of trivalent and hexavalent chromium species after high-performance ion chromatographic (HPIC) separation was investigated. A collinear dual-beam thermal lens spectrometer was utilized to detect Cr species separated as anions on a Dionex HPIC CS5 column. Precolumn derivitization of Cr(III) by pyridine-2,6-dicarboxylic acid and postcolumn derivatization of Cr(VI) by diphenylcarbazide were necessary for the efficient separation and on-line detection of both Cr species. Under the proposed experimental conditions, i.e. 200-μl sample loop and an argon ion laser operating at 514.5 nm, providing 160 mW power at the sample site, detection limits of 30 and 0.3 ng/ml for Cr(III) and Cr(VI), respectively, were achieved.

Determination of bromide, bromate and other anions with ion chromatography and an inductively coupled plasma mass spectrometer as element-specific detector

An implementation of the Dionex IonPac AS12A analytical column with an element-specific ICP-MS detection is described for the simultaneous determination of halogen and oxyhalogen anions, sulfate, phosphate, selenite, selenate and arsenate. The chromatographic separation was achieved in less than 4 min with an aqueous 11 mM (NH4)2CO3 (pH 11.2, adjusted with aqueous ammonia) as eluent. Special emphasis was given to optimize the ICP-MS detection conditions for the reliable detection (RSD<5%) of bromate and bromide at a bromine concentration level of 1.0 microg l(-1) with 50 microl sample injection volume. In order to achieve the highest detector response for bromine species an ultrasonic nebulizer equipped with a membrane desolvator had to be employed. The detection limits (S/N=3, sample injection volume 50 microl) obtained with the IC-ICP-MS after the optimization were 0.67 microg l(-1) for BrO3-, 0.47 microg l(-1) for Br-, 69 microg l(-1) for ClO2-, 4 microg l(-1) for Cl-, 47 microg l(-1) for ClO3-, 13 microg l(-1) for SO4(2-), 36 microg l(-1) for PO4(3-), 0.4 microg l(-1) for SeO3(2-), 0.3 microg l(-1) for SeO4(2-), and 0.4 microg l(-1) for AsO4(3-).

Iron-catalyzed oxidation of s(IV) species by oxygen in aqueous solution: Influence of pH on the redox cycling of iron

Abstract Redox cycling of Fe(II)/Fe(III) during the catalyzed aqueous S(IV) oxidation by dissolved oxygen in the presence of Fe(II) and/or Fe(III) at an initial pHi of 4.4, often observed in atmospheric waters, was studied in detail. It has been found that the reaction rate is not considerably affected by the oxidation state of iron at the start of the reaction. An equilibrium between Fe(II) and Fe(III) was established a few minutes after the start of the reaction, regardless of the oxidation state of iron at the beginning of the experiment. The prevailing oxidation state of iron in a particular phase of the reaction depends on the concentration of S(IV) in the reaction solution. It has been found that the formation of polymerized hydroxo Fe(III) species is also included in the mechanism of the Fe-catalyzed oxidation of S(IV). The formation of these species was confirmed by the on-line measurement of Fe(II) and Fereac. The results also clearly demonstrate that the pHi of the solution is a major factor, controlling the concentration of Fe(III) ions, the form of S(IV) species, and consequently the reaction rate of S(IV) oxidation by oxygen.

Effect of organic solvents in the on-line thermal lens spectrometric detection of chromium(III) and chromium(VI) after ion chromatographic separation

The effect of methanol, acetone and acetonitrile on the sensitivity, selectivity and the detection limits (LODs) of the determination of chromium species by ion chromatography was investigated. A collinear dual-beam thermal lens spectrometer was used for the direct detection of chromium complexes [pre-column derivatized Cr(III)-pyridine-2,6-dicarboxylic acid, and post-column derivatized Cr(VI)-1,5-diphenylcarbazide] following the ion chromatographic separation on a Dionex HPIC-CS5A solvent compatible column. Different amounts of organic solvents were added directly to the eluent (up to 30%) and to the post-column reagent (up to 60%) to improve the thermooptical properties of the solvents. Consequently, the sensitivity of the technique was increased by a factor of 2-3 and LODs of 0.1 and 10 microg dm(-3) were achieved for Cr(VI) and Cr(III), respectively, when the eluent reaching the detector contained 30% of acetonitrile. The addition of organic solvents also resulted in significant changes in retention times, which improved the Cr(III)/Cr(VI) separation.

Optimization of artificial neural networks used for retention modelling in ion chromatography

The aim of this work is the development of an artificial neural network model, which can be generalized and used in a variety of applications for retention modelling in ion chromatography. Influences of eluent flow-rate and concentration of eluent anion (OH-) on separation of seven inorganic anions (fluoride, chloride, nitrite, sulfate, bromide, nitrate, and phosphate) were investigated. Parallel prediction of retention times of seven inorganic anions by using one artificial neural network was applied. MATLAB Neural Networks ToolBox was not adequate for application to retention modelling in this particular case. Therefore the authors adopted it for retention modelling by programming in MATLAB metalanguage. The following routines were written; the division of experimental data set on training and test set; selection of data for training and test set; Dixons outlier test; retraining procedure routine; calculations of relative error. A three-layer feed forward neural network trained with a Levenberg-Marquardt batch error back propagation algorithm has been used to model ion chromatographic retention mechanisms. The advantage of applied batch training methodology is the significant increase in speed of calculation of algorithms in comparison with delta rule training methodology. The technique of experimental data selection for training set was used allowing improvement of artificial neural network prediction power. Experimental design space was divided into 8-32 subspaces depending on number of experimental data points used for training set. The number of hidden layer nodes, the number of iteration steps and the number of experimental data points used for training set were optimized. This study presents the very fast (300 iteration steps) and very accurate (relative error of 0.88%) retention model, obtained by using a small amount of experimental data (16 experimental data points in training set). This indicates that the method of choice for retention modelling in ion chromatography is the artificial neural network.

Influence of the sample matrix composition on the accuracy of the ion chromatographic determination of anions

Abstract The influence of high concentrations of chloride on the accuracy of the determination of nitrite and nitrate by suppressed ion chromatographic analysis was studied using UV-Vis and conductivity detectors. It was found that in the presence of high concentrations of chloride between 0.1 and 10.0 g/l, the retention times of nitrite and nitrate were significantly increased, and co-elution of the two anions was observed at chloride concentrations greater than 15.0 g/l. The nitrite response (absorbance) was found to decrease with increased chloride concentration, whereas the nitrate response did not change significantly. Calibration graphs for nitrite (1–10 mg/l) in solutions containing chloride were linear but their slopes decreased exponentially with increase in chloride concentration. At the same time the slopes of the calibration graphs for nitrate under similar conditions did not vary significantly. It was found that the decrease in the nitrite peak area was the result of a decrease in the absorptivity (210 nm) caused by the protonation of NO 2 − in the suppressor column. The increase in the retention times of the anions can be explained by the self-elution effect of the chloride present in the sample and on-column changes in eluent composition.

Determination of iron in complex matrices by ion chromatography with UV–Vis, thermal lens and amperometric detection using post-column reagents

Abstract The advantages of thermal lens spectrometry (TLS) and amperometry in comparison to UV–Vis spectrophotometry were tested for Fe 3+ and Fe 2+ detection after their ion chromatographic separation. Iron species were separated on Dionex IonPac CS5A analytical column as anions using a PDCA-based eluent. Non-specific [4-(2-pyridylazo)resorcinol] as well as specific (1,10-phenanthroline in combination with Fe 3+ reducing agent-ascorbic acid) as post-column reagent were tested. Lower detection limits were obtained when selective and colourless post-column reagents were used in combination with TLS detection (25 μg l −1 for Fe 3+ and 5 μg l −1 for Fe 2+ ). Electrochemical detection was also used in some experiments because of its wide linear concentration range and selectivity based on the selection of the applied potential. All three detection systems were tested for the detection of Fe 3+ and Fe 2+ (in synthetic samples) and Fe 3+ (in real samples) in the presence of high concentrations of Cu 2+ and Mn 2+ . Electrochemical detection was found to be the most suitable among the detection systems used for Fe 3+ detection in samples with high Cu 2+ concentration due to the possibility of elimination of Cu 2+ interference by post-column copper masking.

Application of thermal lens spectrometric detection to the determination of heavy metals by ion chromatography

Abstract Dual-beam thermal lens spectrometry (TLS) was tested as a potential detection technique after the ion chromatographic separation of some heavy metals. Before detection, heavy metal ions of interest (Cu2+, Ni2+, Co2+, Zn2+, Cd2+, Pb2+, Fe2+ and Fe3+) were separated on a Dionex HPIC-CS5 separation column. For the separation of Cu2+, Ni2+, Co2+, Zn2+, Pb2+ and Cd2+, oxalic acid was added to form anionic and cationic metal complexes, which were then separated on the same column. For the separation of Fe2+ and Fe3+, pyridine-2,6-dicarboxylic acid was added to the eluent and iron species were separated as anions. 4-(2-Pyridylazo)resorcinol was used as a postcolumn reagent to form absorbing metal complexes characterized by an absorption maximum at 520 nm. For Cu2+ and Co2+ ions, TLS detection at 514.5 nm was found to be advantageous compared with UV-Vis detection at 520 nm (λmax. = 520 nm), whereas for Zn2+ and Ni2+ the detection limits of the two techniques were comparable.

Carbonate interferences by ion chromatographic determination of anions in mineral waters

The high content of hydrogen carbonate anions and dissolved CO2 in some mineral waters interfere with the determination of anions in such samples by ion chromatography. It was found that the irreproducibility of retention times of all anions present in the samples was caused by on-column neutralisation of CO(3)2- to HCO3- by CO2. Microbubbles of CO2 released downstream by the suppressor inhibited exchange of Na+ with H3O+ resulting in lower conductivity and consequently in the decrease of chloride peak. To avoid peak deformation and overlapping caused by carbonate species, they should be removed by acidification followed by CO2 elimination in a gas separation unit. To prevent sample contamination by the anion of the added acid, the electrodialytic clean-up procedure was applied. The liberated CO2 was removed in a microporous Gore-Tex semipermeable tubing connected in series with the dialytic cell. The developed dialytic clean-up procedure was applied to the analysis of some typical mineral waters.

Ion exchange-based preconcentration for the determination of anions by capillary electrophoresis

In the present paper a capillary zone electrophoresis (CZE)-compatible preconcentration technique for anions, based on ion exchange, is described. The described preconcentration approach has found limited use until recently because of the inherent elution step that leads to contamination of the sample with eluent components. In this paper, we describe an improved anion exchange-based preconcentration technique in which contamination of the sample with the eluent constituents, which occurs during anion elution from the preconcentration column, is eliminated by on-line chemical suppression on a packed-bed suppressor column. In the present communication, the basic principles of the proposed anion enrichment system are presented. The system was optimized, resulting in a minimal additional dilution of the eluted sample plug. This was achieved by the use of a computer-controlled, sensing/switching system. The effectiveness of the developed method was later tested on the determination of some anions in a synthetic sample using CE apparatus.