Pavel N. Nesterenko

Adsorption and desorption of methylene blue on porous carbon monoliths and nanocrystalline cellulose

The dynamic batch adsorption of methylene blue (MB), a widely used and toxic dye, onto nanocrystalline cellulose (NCC) and crushed powder of carbon monolith (CM) was investigated using the pseudo-first- and -second-order kinetics. CM outperformed NCC with a maximum capacity of 127 mg/g compared to 101 mg/g for NCC. The Langmuir isotherm model was applicable for describing the binding data for MB on CM and NCC, indicating the homogeneous surface of these two materials. The Gibbs free energy of -15.22 kJ/mol estimated for CM unravelled the spontaneous nature of this adsorbent for MB, appreciably faster than the use of NCC (-4.47 kJ/mol). Both pH and temperature exhibited only a modest effect on the adsorption of MB onto CM. The desorption of MB from CM using acetonitrile was very effective with more than 94 % of MB desorbed from CM within 10 min to allow the reusability of this porous carbon material. In contrast, acetonitrile was less effective than ethanol in desorbing MB from NCC. The two solvents were incapable of completely desorbing MB on commercial granular coal-derived activated carbon.

Ion chromatographic determination of hydrolysis products of hexafluorophosphate salts in aqueous solution.

In this work, hydrolysis of three different hexafluorophosphate salts in purified water was investigated. Aqueous samples of lithium hexafluorophosphate (LiPF(6)), sodium hexafluorophosphate (NaPF(6)) and potassium hexafluorophosphate (KPF(6)) were prepared and stored for different times. Ion chromatography (IC) with UV as well as non-suppressed and suppressed conductivity detection was used for the analysis of the reaction products. For the detection and identification of the formed decomposition products, an IC method using IonPac AS14A 250 mm × 4.0 mm i.d. column and 2.5 mM KHCO(3)-2.5 mM K(2)CO(3) eluent was established. Besides hexafluorophosphate, four other anionic species were detected in fresh and matured aqueous solutions. The hydrolysis products fluoride (F(-)), monofluorophosphate (HPO(3)F(-)), phosphate (HPO(4)(2-)) and difluorophosphate (PO(2)F(2)(-)) were found and were unambiguously identified by means of standards or electrospray ionization mass spectrometry (ESI-MS). It was shown that stability of hexafluorophosphate solutions depends on the nature of the counter ion and decreases in the order potassium>sodium>lithium.

On-line preconcentration of pharmaceutical residues from large volume water samples using short reversed-phase monolithic cartridges coupled to LC-UV-ESI-MS

A simplified preconcentration method for a range of ultra-trace level pharmaceuticals in natural waters has been developed. Solid phase extraction was performed on-line using a micro-reversed-phase monolithic silica column, allowing for very rapid trace enrichment from large volume (500ml) samples with minimal sample handling. Acceptable recoveries of >70% were obtained for the majority of compounds investigated and the monolithic columns could be washed and conditioned on-line with no sample carryover and used reproducibly for up to eight extractions each. The on-line SPE-LC-UV method was coupled to electrospray ionisation ion trap mass spectrometry (ESI-MS) to increase both selectivity and specificity. Detection limits were determined in spiked river and tap water samples and found to lie in the low ng/l region using sample volumes of 500ml, loaded at a flow rate of 10ml/min, and therefore, were suitable for ultra trace analysis.

Investigation of the properties of hypercrosslinked polystyrene as a stationary phase for high-performance liquid chromatography

SummaryThe mechanical rigidity, swelling properties, adsorption selectivity, and chromatographic performance of hypercrosslinked polystyrene (mainly MN-200 or Purosep-200; Purolite, UK) have been studied to evaluate the use of the material as a stationary phase for reversed-phase high-performance liquid chromatography (RPHPLC). By use of inverse size-exclusion chromatography (SEC) this adsorbent, with a high specific surface area of 1500 m2 g−1 was found to have a biporous structure with micropores of ca 1–2 nm and macropores ca 100 nm in diameter. The polymer does not change its volume significantly on changing water for organic solvents. The retention increments for methylene and phenyl groups were calculated and indicated that the mechanism of retention on the hypercrosslinked polystyrene involves π-π interactions and strong hydrophobic interactions. The column performance of the hypercrosslinked polystyrene was found to be acceptable, with reduced plate height increasing very slowly as the linear velocity of the mobile phase increased to high values (up to 20–45 cm min−1). Columns containing hypercrosslinked polystyrene were evaluated for the separation of phenols, dialkyl phthalates, and polyaromatic compounds. On-column preconcentration of trace organic compounds from aqueous media is possible. With smaller particles of hypercrosslinked polystyrene becoming available, this material can be regarded as an alternative to alkylsilica as a hydrolytically stable column-packing material for RPHPLC.

High-performance chelation ion chromatography. A new dimension in the separation and determination of trace metals

A review of so-called high-performance chelation ion chromatography (HPCIC) is presented. The principles of this separation technique are based on exploitation of the chelation effect of stationary phases in the presence of comparatively reduced electrostatic ion-exchange interactions. The common ways to suppress ion-exchange, including increasing the ionic strength and pH of the eluent and the column temperature, are discussed. Unlike the majority of low-efficiency chelation exchangers that are used as preconcentration columns for batch separations, HPCIC can be used for high-efficiency analytical separations of trace metals. The range of applications of chelating exchangers used for the separation and determination of alkaline-earth and transition metal ions by HPCIC as well as their types and properties are considered. The main features of HPCIC are the much greater flexibility in selectivity control compared to ion-exchange and the relative insensitivity to high ionic strengths. These properties make it a more preferable technique for the trace analysis of complex samples, such as seawater, saturated brines, etc.

Separation of uranium(VI) and lanthanides by capillary electrophoresis using on-capillary complexation with arsenazo III

The viability of the separation of lanthanides and uranium(VI) in the form of strongly absorbing complexes with arsenazo III (AIII) was studied with the aim to increase the sensitivity of absorbance detection in determination of these metals by capillary electrophoresis (CE). Special attention was paid to the complexation equilibria in the background electrolyte (BGE). On-capillary complexation provided better peak shapes for lanthanides compared to pre-column complexation. While the BGE composition had very little effect on the peak shape of the kinetically inert uranium(VI) complex, it played a crucial role in the peak shapes of the kinetically labile lanthanide complexes. Addition of a second ligand competing with the metallochromic ligand AIII for the metal ions was found to be critical to achieve good peak shape. The nature and concentration of the competing complexing ligand added to the BGE, the pH, and the concentration of AIII were found to exert a strong influence on the separation selectivity, peak shapes and the detection sensitivity. Several carboxylic acids were compared as BGE competing ligands and citrate provided best selectivity and peak shapes. A citrate BGE at pH 4.7 and containing 0.1 mM AIII was used for the separation of uranium(VI) (350 000 theoretical plates) and LaIII (63 000 theoretical plates) while, to separate most lanthanides and uranium(VI), a similar BGE with a lower (0.03 mM) AIII concentration was used. Using hydrostatic sampling (100 mm for 10 s) detection limits of 0.35 μM (49 ppb) LaIII and 25 μM (60 ppb) UO2 were obtained. Using on-capillary complexation, sample stacking was retained for injection times of up to at least 100 s (ca. 30-mm sample plug) without loss of peak shapes for lanthanides or recovery for LaIII. When this process was used, the detection limit for LaIII was reduced to about 5 ppb. Optimal properties of metallochromic ligands for separation and detection of metals by CE are discussed.

Isocratic separation of lanthanides and yttrium by high-performance chelation ion chromatography on iminodiacetic acid bonded to silica

Abstract The variation of chromatographic retention of lanthanide ions and yttrium on a column packed with iminodiacetic acid bonded to silica was studied. The influence of nitric acid concentration, ionic strength of the eluent and temperature as well as complexing agents (diglycolic, maleic acid) was investigated. It was shown that with nitric acid as eluent in the presence of 0.5 M–1.0 M potassium nitrate the retention of the lanthanides under increased temperature is defined mainly by the stability of the corresponding surface complexes with iminodiacetic acid functional groups. The combination of increased ionic strength of the eluent (0.5 M potassium nitrate) with increased temperature of the chromatographic column (65°C) allowed, under optimal concentration of nitric acid (1.6·10−2 M), the isocratic separation of 14 lanthanide ions and yttrium in 65 min.

Zwitterionic ion-exchangers in ion chromatography: A review of recent developments.

Significant advances within the field of ion chromatography (IC) have often had their roots in research focussed on the development of new phase technologies, aimed to both simultaneously increase efficiency and vary selectivity. To increase selectivity it is necessary to develop new selective ion-exchangers, achieved by varying the nature of functional groups and the matrix of the stationary phase. In this comprehensive review, developments over the past decade in the production and application of zwitterionic and amphoteric ion-exchangers are presented and discussed. Zwitterionic and amphoteric ion-exchangers, where positive and negative charges are located in close proximity, exhibit alternative ion selectivity to standard anion and cation ion-exchangers, such as those traditionally used in IC, and have the potential for selectivity optimisation in IC due to control of the ratio of electrostatic attraction/repulsion forces between analyte ions and ion-exchange groups. This can result in the ability to utilise relatively dilute eluents, which increases detector sensitivity, with further advantages of zwitterionic ion-exchangers including their possible application to the simultaneous separation of cationic and anionic species.

Sorption of noble-metal ions on silica with chemically bonded nitrogen-containing ligands

A study has been made of the sorption of Ir(IV), Rh(III), Pt(IV), Ru(IV), Os(VIII), Pd(II) and Au(III) from aqueous solutions by silica chemically modified with nitrogen-containing organic ligands, as a function of hydrochloric acid concentration, time of contact, concentration of the element and the ionic strength. Sorption of noble-metal ions at pH > 1 on a sorbent containing monoamine groups seems to be due to a complexation mechanism, and to an anion-exchange mechanism at pH < 1. With aminopropyl-silica 1000-fold concentration of Ir(IV) and Rh(III) from their 10(-8)-10(-7)M solutions was achieved and these metals were subsequently determined on the sorbent surface by X-ray fluorescence. Detection limits were 10-20 ng/ml. There was no interference from 1000-fold quantities of non-ferrous metal ions and Fe(III). With the sorbent containing bonded diethylenetriamine groups, 1000-fold concentration of Au(III) was achieved, and it was then determined on the sorbent surface by an atomic-emission method. Conditions for desorption of Au(III) with pyridine and potassium thiocyanate were developed.

Single-column method of chelation ion chromatography for the analysis of trace metals in complex samples

A single-column chelation ion chromatographic system for the preconcentration and separation of trace transition metals is described. The system includes standard chromatographic equipment with a post-column reagent system based on the reaction with 4-(2-pyridylazo)resorcinol followed by photometric detection at 495 nm. Iminodiacetic acid bonded to 5 μm silica (Diasorb IDA) was used as a chelating stationary phase. The strong complexing ability in combination with good kinetics of complexation and ion-exchange selectivity of iminodiacetic functional groups allow both preconcentration of Mn, Co, Cd, Zn, Ni and Cu from waters of high salinity and efficient separation with the same column. The retention characteristics of alkaline-earth and transition metal ions on Diasorb IDA silica (250×4 mm I.D.) column was investigated for a variety of eluents including nitric acid, maleic, malonic, citric, dipicolinic, picolinic, tartaric and oxalic acids. The influence of ionic strength on retention of metal ions involving high nitrate and chloride concentrations was also evaluated. The baseline separation of preconcentrated metals was achieved using a three-step gradient elution scheme which involved first, flushing of the column loaded with the sample with 0.5 M KCl−0.5mMHNO3 for 10 min, followed by 80 mM tartaric acid for 20 min and finally 10 mM picolinic acid for 20 min.