Luz E. Vera-Avila

Capabilities and limitations of dispersive liquid–liquid microextraction with solidification of floating organic drop for the extraction of organic pollutants from water samples

Dispersive liquid-liquid microextraction with solidification of floating organic drop (DLLME-SFO) is one of the most interesting sample preparation techniques developed in recent years. Although several applications have been reported, the potentiality and limitations of this simple and rapid extraction technique have not been made sufficiently explicit. In this work, the extraction efficiency of DLLME-SFO for pollutants from different chemical families was determined. Studied compounds include: 10 polycyclic aromatic hydrocarbons, 5 pesticides (chlorophenoxy herbicides and DDT), 8 phenols and 6 sulfonamides, thus, covering a large range of polarity and hydrophobicity (LogKow 0-7, overall). After optimization of extraction conditions using 1-dodecanol as extractant, the procedure was applied for extraction of each family from 10-mL spiked water samples, only adjusting sample pH as required. Absolute recoveries for pollutants with LogKow 3-7 were >70% and recovery values within this group (18 compounds) were independent of structure or hydrophobicity; the precision of recovery was very acceptable (RSD<12%) and linear behavior was observed in the studied concentration range (r(2)>0.995). Extraction recoveries for pollutants with LogKow 1.46-2.8 were in the range 13-62%, directly depending on individual LogKow values; however, good linearity (r(2)>0.993) and precision (RSD<6.5%) were also demonstrated for these polar solutes, despite recovery level. DLLME-SFO with 1-dodecanol completely failed for extraction of compounds with LogKow≤1 (sulfa drugs), other more polar extraction solvents (ionic liquids) should be explored for highly hydrophilic pollutants.

Determination of the temperature dependence of water solubilities of polycyclic aromatic hydrocarbons by a generator column-on-line solid-phase extraction-liquid chromatographic method.

An improved dynamic coupled column liquid chromatographic (DCCLC) technique for determining water solubility data of hydrophobic compounds is presented. The technique is based on pumping water through a thermostated generator column in order to generate emulsion-free, saturated aqueous solutions of the compound under study. Through a switching valve system the solute in the aqueous solution is extracted and concentrated by an on-line solid-phase extraction process and subsequently eluted and analyzed by high performance liquid chromatography (fluorescence detection coupled to photodiode array detection). The improvements carried out to the original DCCLC technique have given rise to savings in time for the experimental work and increased sensitivity during the detection and quantification stage. Applicability of the method for studying highly hydrophobic substances is demonstrated by determining water solubility of anthracene and pyrene in the temperature range of 8.9-49.9 and 8.5-32.2 degrees C, respectively. The measured water solubilities are in good agreement with the best available literature data. The method has also been applied to the determination of water solubility of m-terphenyl, 9, 10-dihydrophenanthrene and guaiazulene, in the temperature range of 4.8-49.9, 4.8-25.0, and 4.5-29.9 degrees C, respectively. The uncertainty in the Sw values determined in this work ranged from 0.7% to 4.6%. The experimental water solubility data, as a function of temperature, are fitted to the equation In Sw = A + B/T; where Sw and T are given in mole fraction and Kelvin, respectively.

On-line preconcentration, cleanup and high-performance liquid chromatographic determination of chlorophenoxy acid herbicides in water

Abstract A rapid and simple technique for the determination of six chlorophenoxy acid herbicides in water is described. After extraction and preconcentration of the analytes at pH≈ 1 in a small 20 × 2 mm I.D. polymeric reversed-phase precolumn, an on-line cleanup procedure is performed by a selective transfer of the ionized compounds to a second precolumn packed with an anion exchanger. An acetonitrile-sodium hydroxide (pH 10; 10:90, v/v) solution is used for this step. The anionexchange precolumn is further on-line analyzed by reversed-phase chromatography with gradient elution and UV detection at 230 nm. The recovery of the examined herbicides is nearly 100% with a R.S.D.

Frontal analysis of aqueous phenol solutions in amberlite XAD-4 columns: Implications on the operation and design of solid phase extraction systems

Frontal analysis of aqueous phenol solutions in Amberlite XAD-4 columns was carried out at different experimental conditions. Operating variables such as the concentration, pH and ionic strength of the influent, the presence of competitor solutes, the fluid flow-rate and the column length were considered and their effects on the front profile, the phenol breakthrough volume and the equilibrium parameters were determined. The obtained results may explain some contradictory reported data concerning the recovery of hydrophilic compounds in solid phase extraction (SPE) systems using Amberlite XAD-2 or XAD-4 columns. Furthermore, it is demonstrated that the adsorption parameters derived from a frontal analysis in an XAD-4 column are directly transposable to columns of different sizes (at the analytical level). Therefore, the results of this study may also be used for prediction and/or design of a phenol SPE system adapted to a particular problem.

Determination of carbofuran in surface water and biological tissue by sol-gel immunoaffinity extraction and on-line preconcentration/HPLC/UV analysis.

A selective and simple analytical method for the trace level determination of carbofuran in complex environmental and biological samples was developed based on immunoaffinity extraction (IAE) followed by on-line preconcentration and HPLC/UV analysis of the purified extract. The immunosorbent for IAE was prepared by sol-gel encapsulation of monoclonal anti-carbofuran antibodies, and was fully characterized for capacity, repeatability, binding strength, binding kinetics and cross-reactivity. Method performance was evaluated with two different types of difficult samples: dam water and methanolic extracts of epithelial cervical-uterine tissue. Linear behavior and quantitative recoveries were obtained from the analysis of samples spiked with carbofuran at 0.2-4 ng/mL (dam water, 50 mL samples) and 10-40 ng/mL (biological tissue extract, 2 mL samples). RSD (n=7) and detection limits were, respectively, 10.1% (spike 0.40 ng/mL) and 0.13 ng/mL for dam water; 8.5% (spike 20 ng/mL) and 5 ng/mL for the biological tissue extract. The excellent sample purification achieved with the IAE column allows precise and accurate determination of carbofuran in complex matrices, even when using non-selective UV detection in the chromatographic analysis.

On-line solid-phase extraction and high-performance liquid chromatographic determination of chlorthalidone in urine.

A simple and rapid on-line method for the determination of chlorthalidone in urine is proposed. The sample containing the internal standard is injected in a CN precolumn. After a 2-ml water rinsing, the precolumn is coupled for 30 s to the HPLC column via a switching, valve, allowing the on-line elution of the compounds of interest. Analysis is carried out by reversed-phase chromatography with an acetonitrile-0.01 M phosphate buffer pH 7 (20:80, v/v) eluent, using UV detection at 214 nm. While the LC separation is performed, the precolumn is regenerated and conditioned, and is ready to receive the next sample at the end of the run. Accurate (> 95%) and precise (< 10%) analyses, in the range of 0.1-20 micrograms/ml of chlorthalidone in urine, have been achieved using this method.

ANALYSIS OF CAPSAICIN AND DIHYDROCAPSAICIN IN HOT PEPPERS BY ULTRASOUND ASSISTED EXTRACTION FOLLOWED BY GAS CHROMATOGRAPHY–MASS SPECTROMETRY

A simple method for the analysis of capsaicin and dihydrocapsaicin in peppers by ultrasound assisted extraction (USAE) followed by gas chromatography–mass spectrometry (GC-MS) has been developed. USAE conditions were optimized by experimental design in order to maximize analyte extraction. A full factorial design involving extraction variables such as solvent (ethanol and water), extraction time (5–25 min), extraction temperature (25–50 °C), sample amount (0.25–0.5 g), and ultrasound amplitude (40–80%) was applied. The most significant conditions for capsaicinoid extraction by USAE were solvent type, extraction time, and sample amount. The obtained results were compared with other extraction methods: the official Soxhlet method and a previously reported solid phase microextraction method. Results showed that the extraction efficiency with the application of USAE (98%) was as good as that obtained with Soxhlet and the precision of recovery was less than 5%; in addition, the extraction time was decreased from 5 h to 25 min. The GC-MS analytical method was linear in the range 10–100 μg/mL for capsaicin and dihydrocpsaicin with correlation coefficient r = 0.998 and peak area variability of ∼1% for both capsaicinoids. The method was applied to the analysis of 11 varieties of hot peppers cultivated in México. A large concentration range for capsaicin (101–6800 μg/g) and dihydrocapsaicin (110–2736 μg/g) was found in these pepper samples.

Trace Level Chromatographic Determination of Priority Pollutant Chloro and Nitrophenols in Water with On-Line Sample Preparation

A methodology for the on-line sample preparation and HPLC determination of priority pollutant chloro and nitrophenols at very low concentrations in water was developed. A long precolumn packed with a polymeric reversed phase is used for the extraction and preconcentration of the neutral analytes from an acidified water sample. Then, a selective transfer of the ionized phenols to a small anion-exchange precolumn is carried out by means of an alkaline hydroorganic solution. Finally, the coupling of the second precolumn to the HPLC system, equipped with UV and coulometric detectors, allows the on-line gradient elution, RP-separation and sensitive detection of the compounds of interest. Good recoveries, high precision and detection limits at the sub-μg/L level have been achieved with spiked water samples.

Speciation and transformation pathways of chlorophenols formed from chlorination of phenol at trace level concentration

Trace organic precursors remaining in water after primary treatment can originate a variety of toxic disinfection by-products during chlorination. Therefore, knowledge of conditions leading to their persistence or transformation in chlorinated media is crucial for human health protection. Using phenol as model compound at trace level (50 ppb), the short term formation and degradation of chlorophenols (CPs) in plain water and buffered water (pH 4.8, 7 and 9) treated with typical chlorine doses (1-5 ppm) was investigated. Total phenol consumption and quantitative degradation of formed CPs occurred in ⩽5h with 5 ppm chlorine in plain water and alkaline buffer, and with 1 ppm chlorine in phosphate buffer of pH 7. The enhanced reactivity in this buffer was attributed to high ionic strength (0.18 M). On the contrary, phenol was only slowly transformed to monochlorophenols (MCPs) in acidic media. Analysis of phenol and CPs concentration profiles indicated the coexistence of two competing reaction pathways in neutral and alkaline conditions: 1) successive ortho-para chlorination of aromatic ring up to 2,4,6-trichlorophenol followed by ring cleavage, 2) direct oxidation of MCPs to rapidly degradable oxygenated aromatics (dihydroxybenzenes, benzoquinones). Ionic strength and pH had some influence on preferred pathway but chlorine dose was determinant.

Generator column determination of water solubilities for saturated C6 to C8 hydrocarbons

Water solubilities, S w , of C 6 to C 8 saturated hydrocarbons have been determined at 25°C by using a generator column method. The method is based on pumping water through a bed of a high-surface-area packing, coated with the studied compound. As a result of this process a saturated aqueous solution of that compound is generated, which can be analyzed by gas chromatography. Solubility data are reported for: n-hexane, n-heptane, 3-methylhexane, 2,4-dimethylpentane, n-octane, 4-methylheptane, 2,3,4-trimethylpentane and trans-1,3-dimethylcyclohexane. Experimental S w values determined in this work are in good agreement with reported data determined from diffusion experiments where no stirring steps are involved, and are lower than those determined from batch-stirring methods, which suggest that solutions produced by generator columns are free from emulsions. No experimental S w data have been reported previously for 4-methylheptane and trans-1,3-dimethylcyclohexane. The uncertainty in the S w values determined in this work ranged from ± 1.0 to ± 3.7 %.