Improvement of on-line solid-phase extraction for determining phenolic compounds in water

Summary Modifying the most common design for the on-line coupling of a precolumn to reversed phase LC with diode array detection has resulted in reduction of the broadening of the peaks which results when the compounds of interest are strongly retained by a highly hydrophobic sorbent. The modification consists of the desorption of the analytes trapped on the precolumn solely by the organic solvent used to modify the solvent strength of the mobile phase. Results obtained using this design were compared with those obtained with the conventional design, with C₁₈ and PLRP-S precolumns. The performance of the system was also tested with a highly cross-linked styrene-divinylbenzene copolymer (ENVI-chrom P) precolumn for the determination of phenolic compounds in real samples. The advantages and disadvantages are discussed. Ion-pair solid phase extraction is used in order to increase the breakthrough volumes of more polar compounds, mainly phenol. The use of the new design enables phenolic compounds to be determined at the low μg L⁻¹ level with limits of detection ranging between 0.1 and 2 μg L⁻¹ in tap water when a 10 mL sample was analyzed.     

On-line trace enrichment of phenolic compounds from water using a pyrrole-based polymer as the solid-phase extraction sorbent coupled with high-performance liquid chromatography

A pyrrole-based conductive polymer was prepared and applied as new sorbent for on-line solid-phase extraction (SPE) of phenol and chlorophenols from water samples. Polypyrrole (PPy) was synthesized by chemical oxidation of the monomer in non-aqueous solution. The efficiency of this polymer for extraction of phenol and chlorophenols was evaluated using 35 mg of PPy as the sorbent in an on-line SPE system coupled to reversed-phase liquid chromatography with UV detection. The mobile phase were mixture of phosphate buffer-acetonitrile and compounds were eluted by the mobile phase using a six-port switching valve. High volumes of water, up to 160 ml, could be preconcentrated without the loss of phenols, except for the more polar ones. The R.S.D. for a river water sample spiked with phenol and chlorophenols at sub-ppb level was lower than 7% (n=5) and detection limits of 15-100 and 35-150 ng l⁻¹ for tap and river water were obtained, respectively.     

Determination of phenolic compounds in water samples by on-line solid-phase extraction—supercritical-fluid chromatography with diode-array detection

Summary The eleven Environmental Protection Agency (EPA) priority phenolic compounds have been determined by solid-phase extraction (SPE) coupled on-line to supercritical-fluid chromatography (SFC) with diodearray detection. The variables affecting chromatographic separation were optimized and the analytes were separated at 40 °C in two diol columns connected in series; a gradient of methanol, as modifier, and CO₂ was used as mobile phase. Under these conditions, all the compounds studied were separated to baseline in less than 13 min. PLRP-S and LiChrolut EN were tested as sorbents in a 10×3 mm i.d. laboratory-packed precolumn for solid-phase extraction. An ion-pair reagent, tetrabutylammonium bromide (TBA), was used in the extraction process to increase break-through volumes. The performance of the method was checked with tap and river waters and the pre-concentration of 20 mL of sample in a PLRP-S pre-column enabled phenolic compounds to be determined at low μg L⁻¹ levels with limits of detection ranging between 0.4 and 2 μg L⁻¹. The repeatability and reproducibility between days (n=3) for real samples spiked at 10 μg L⁻¹ were lower than 10%.     

Determination of aromatic sulfonates in coastal water by on-line Ion-pair solid-phase extraction/ion-pair liquid chromatography with UV detection

Summary A method for determining aromatic sulfonates in sea water is presented. Ion-pair solid phase extraction is coupled on-line to ion-pair liquid chromatography with UV detection. In the enrichment step, the recoveries from 100 mL of sea water were higher than 65% for most analytes. Linearity was good and detection limits were between 0.02 and 1 μgL⁻¹. The repeatability of the method, expressed as % of relative standard deviation (n=3) was between 1 and 15%. The method was checked in coastal water near the commercial port of Tarragona, where effluents from the petrochemical industry are discharged. All the samples taken were found to contain 2-naphthalenesulfonate.     

Determination of phenolic compounds in river water with on-line coupling bisphenol A imprinted monolithic precolumn with high performance liquid chromatography

The bisphenol A (BPA) imprinted monolithic precolumn has been prepared by in situ polymerization using 4-vinylpyridine (4-VP) and ethylene dimethacrylate (EDMA) as functional monomer and cross-linker, respectively. The column with good flow-through property was obtained by changing the molar ratio of the porogens (toluene and dodecanol). The selectivity and retention properties of the monolith for the BPA and other phenolic compounds were evaluated. The results show that the hydrophobic and hydrogen-bonding interaction plays important roles in the recognition process. The determination of BPA and other phenolic compounds with on-line solid-phase extraction (SPE) by monolithic precolumn coupled with conventional particulates packed and monolithic reversed-phase columns, respectively, was performed. The method was successfully applied to the analysis of phenolic compounds in river water.     

Measurement of trace levels of antibiotics in river water using on-line enrichment and triple-quadrupole LC-MS/MS

This study presents the development of an automated on-line solid phase extraction (SPE)-liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the determination of 23 antibiotics in environmental water samples. After optimisation of LC-MS/MS conditions, SPE parameters such as sorbent type, sample pH or sample volume were optimised. Antibiotic recoveries ranged from 64% to 98% and compared favourably with those achieved using off-line SPE. Limits of detection were in the range 0.5-13.7 ng L⁻¹.This on-line SPE-LC-MS/MS procedure was applied to the analysis of water samples taken in three rivers within the Seine River basin, near Paris (France). The obtained results revealed the occurrence of 12 antibiotics, including tylosin, erythromycin, tetracycline, amoxicillin, trimethoprim, sulfamethoxazole, oxolinic acid, flumequine, norfloxacin, ciprofloxacin, ofloxacin, and vancomycin (2-1435 ng L⁻¹).     

HPLC study of the efficiency of extraction of phenolic compounds

Summary The rate of extraction of phenolic compounds in two different solvents has been studied by liquid chromatography (HPLC) under reverse phase, gradient elution conditions. The solvents were diethyl ether and ethyl acetate. The method has been applied to two natural samples, a white wine and apple pulp.     

Trace enrichment of polar pollutants from water and the influence of humic substances

Summary A practical liquid chromatographic early-warning system for polar pollutants in water ought to have detection limits at a level below a microgram per litre. Using ultraviolet absorbance detection this normally necessitates trace enrichment of the samples. In this study ten different sorbents have been tested with respect to their enrichment capacity. The hydrophobic polymeric PLRP-S material, in combination with a C-18 modified analytical column, proved to be the best choice. The influence of humic substances, present in surface water, was also studied. Lowering of the sample pH, resulted in a significant increase in the interferences due to the humic substances.     

Reversed-phase liquid chromatography coupled on-line with capillary gas chromatography use of an anion-exchange membrane to remove an ion-pair reagent from the eluent

Summary In order to enable the coupling of reversed-phase liquid chromatography (RPLC) with capillary gas chromatography (GC), the performance of an anion-exchange micromembrane device has been studied to remove the ion-pair reagent methanesulphonic acid from an acetonitrile/water LC eluent. The regenerant in the membrane was tetrabutylammonium hydroxide dissolved in acetonitrile/water, which effects an anion-exchange of methanesulphonate ions for regenerant hydroxide ions. The efficiency of the exchange process was found to be 99.9%. This enabled the direct introduction of the LC eluent, free of ions and with the proper acetonitrile/water ratio, into the GC. The applicability of the on-line LC-micromembrane-GC system has been illustrated for the potential drug eltoprazine, which is quantitatively recovered with a coefficient of variation for standard solutions of 3% at the 150 g/ml analyte level.     

Column liquid chromatography coupled on-line with mass spectrometry using post-column phosphate suppression

The coupling of column liquid chromatography on-line with mass spectrometry using post-column phosphate suppression is described. A membrane suppressor is used to remove the non-volatile phosphate ions from the mobile phase after the chromatographic separation prior to introduction into the mass spectrometer. Using post-column phosphate suppression by a membrane device phosphate concentrations up to 10 mM phosphate, at a mobile phase flow rate of 0.1 ml/min, could be removed for more than 99% without any decay of the sensitivity of the MS detection. Using a new antidepressant tetracyclic basic compound, ORG 4428, and some structure related compounds as model compounds electron impact as well as chemical ionisation spectra of all compounds could be obtained. The set-up could be used for several weeks running, using a phosphate containing mobile phase, without any loss of performance.     

Supercritical fluid extraction-high performance liquid chromatography on-line coupling: Extraction of some model aromatic compounds

Summary An SFE-HPLC system has been tested with a simple model aromatic mixture on spiked silica. The spiked silica was extracted with pure CO₂ and the extract retained in a trap packed with C18 material. Quantitative separation and reproducity have been investigated and are reported.     

Supercritical fluid extraction-high performance liquid chromatography on-line coupling: Extraction of some model aromatic compounds

Summary An SFE-HPLC system has been tested with a simple model aromatic mixture on spiked silica. The spiked silica was extracted with pure CO₂ and the extract retained in a trap packed with C18 material. Quantitative separation and reproducity have been investigated and are reported.     

High-performance liquid chromatographic determination of phenolic compounds in natural water coupled with on-line flow injection membrane extraction-preconcentration

A liquid chromatographic method for determination of trace phenolic compounds has been established, coupled with an on-line supported liquid membrane extraction-preconcentration flow-injection system. Tributyl-phosphate dissolved in kerosene was used as the carrier of the supported liquid membrane. Four phenolic compounds (phenol, catechol, resorcinol and hydroquinone) were chosen as the model compounds and the experiment conditions were optimized. Under the optimum conditions, calibrations were linear in the range of 1-500 microg/L, with good correlation coefficients (r > 0.999). The total analysis time of the system was 22 min, including the membrane extraction, liquid chromatographic separation and equilibration times.     

Comprehensive two-dimensional liquid chromatography in the analysis of antioxidant phenolic compounds in wines and juices

A novel comprehensive two-dimensional liquid chromatographic (LC×LC) system was developed for the quantification of antioxidant phenolic compounds in wine and juice. The system allows parts of the sample that are well separated in the first column to pass directly to the detector after the first column, while the rest of the sample proceeds to the second column. The optimised LC×LC system employed a combination of two C18 columns, the latter column with an ion-pair reagent (tetrapentylammonium bromide). The relative standard deviations (RSD) for the retention times were better than 0.01% in the first dimension and on average 6.3% in the second. The RSD values of the peak volumes varied from 3% (protocatechuic acid) to 13% (caffeic acid, n = 3, 10 μg/ml).     

Retention behaviour of an homologous series of oligodeoxythymidilic acids using reversed-phase ion-pair chromatography

Summary The retention behaviour of an homologous series of phosphorylated oligodeoxythymidylic acid (pd(T)_5–18) oligonucleotides was studied using reversed-phase ion-pair chromatography with isocratic elution conditions. The effects of temperature, pH, eluent ionic strength, percentage organic modifier, concentration and alkyl chain length of the ion-pairing reagent were investigated. The retention behaviour was generally explicable by current theoretical models of ion-pair chromatography. However, the marked effect of mobile phase pH on the retention of the oligonucleotides was unexpected, and this was ascribed to the presence of ionisable residual silanols on the surface of the reversed-phase packing material.     

Analyses of phenolic compounds by microemulsion electrokinetic chromatography

In this study, a microemulsion electrokinetic chromatography (MEEKC) method was developed to analyze and detect 13 phenolic compounds (syringic acid, p-cumaric acid, vanillic acid, caffeic acid, gallic acid, 3,4-dihydroxybenzoic acid, 4-hydroxybenzoic acid, (+)-catechin, (-)-epigallocatechin, (-)-epicatechin gallate, (-)-epigallocatechin gallate, (-)-epicatechin, and (-)-gallocatechin), which are present in many plant-derived foods. The effects of cosurfactant, organic modifier, and oil were examined in order to optimize the separation of these phenolic compounds. The amounts of cosurfactant (cyclohexanol) and organic modifier (acetonitrile) were determined as the major influence on the separation selectivity, while the type of oil partially affected the separation resolution of the phenolic compounds. A highly efficient MEEKC separation method was achieved within 14 min by using a microemulsion solution of pH 2.0 containing 2.89% w/v SDS, 1.36% w/v heptane, 7.66% w/v cyclohexanol, and 2% w/v ACN. Furthermore, the present work could demonstrate that the nature of the oil phase has a significant influence on the separation selectivity of phenolic compounds.     

Optimisation and validation of an automated solid phase extraction technique coupled on-line to enzyme-based biosensor detection for the determination of phenolic compounds in surface water samples

Summary A fully integrated screening system for phenolic compounds was developed incorporating on-line solid phase extraction, fractionation and biosensor detection. Two different types of biosensors, solid graphite and carbon paste electrodes incorporating the enzyme tyrosinase, were compared and used in the screening system. Interfacing of the solid phase extraction and fractionation with the biosensor detection was given special attention since the biosensors were not compatible with the organic modifier used for desorption of phenols from the solid phase extraction step. The system was validated with conventional analytical techniques. Surface water samples from the Ebro river were spiked with 1,10, and 25g L^–1 of catechol, phenol,p-cresol, respectively. Three out of seven samples were spiked and the correct samples were identified, containing phenols equivalent to the spiked concentrations.     

Optimisation and validation of an automated solid phase extraction technique coupled on-line to enzyme-based biosensor detection for the determination of phenolic compounds in surface water samples

Summary A fully integrated screening system for phenolic compounds was developed incorporating on-line solid phase extraction, fractionation and biosensor detection. Two different types of biosensors, solid graphite and carbon paste electrodes incorporating the enzyme tyrosinase, were compared and used in the screening system. Interfacing of the solid phase extraction and fractionation with the biosensor detection was given special attention since the biosensors were not compatible with the organic modifier used for desorption of phenols from the solid phase extraction step. The system was validated with conventional analytical techniques. Surface water samples from the Ebro river were spiked with 1,10, and 25 μg L⁻¹ of catechol, phenol,p-cresol, respectively. Three out of seven samples were spiked and the correct samples were identified, containing phenols equivalent to the spiked concentrations.     

Electrodialytic sample treatment coupled on-line with high-performance liquid chromatography

Summary An electrodialytic sample treatment method coupled on-line with high-performance liquid chromatography (EDIST-HPLC) is discussed in this paper. The performance of EDIST as a function of the donor-phase (sample solution) flow rate, the voltage applied over the electrodialysis block, and the time of dialysis has been studied using the basic drug ephedrine as a model compound. Enrichment of the analyte by a factor of 10–20 was possible. The determination of human plasma spiked with ephedrine is briefly discussed.     

Comparison of adsorbents for on-line solid-phase extraction of polycyclic aromatic hydrocarbons before liquid chromatography with UV detection

Summary On-line solid-phase extraction (SPE) coupled with reversed-phase liquid chromatography and UV detection at 254 nm has been used for the determination of trace-level polycyclic aromatic hydrocarbons (PAH) in soil extracts. Five commercially available adsorbents (C₈, C₁₈, PLRP-S, PRP-1, and Bond-Elut Env) were evaluated. Results showed that recovery of the PAH decreased with increasing molecular weight, because of their poorer solubility. Recovery of high-molecular-weight PAH was significantly improved by addition of 10% (v/v) acetonitrile to the sample before loading of the SPE adsorbent. PAH recovery ranged from 64.0 to 108% when a 50 mL sample spiked with 1 μg L⁻¹ was applied to these adsorbents. Determination of PAH was possible with detection limits below 0.05 μg L⁻¹, which corresponds to 0.2 μg kg⁻¹ soil. The method was successfully used to determine PAH in soil extracts.