Determination of SCFAs in water using GC-FID. Selection of the separation system

In the present study, an analytical procedure was developed for the determination of short-chain fatty acids (SCFAs) in landfill leachate and municipal wastewater employing injection of aqueous samples to gas chromatograph with flame ionization detector (GC-FID). Chromatographic conditions such as a separation system, injection volume, oven temperature program were investigated and selected. With two columns, one with a polar (polyethylene glycol) and one with a non-polar (dimethylpolisiloxane) stationary phase, good separation of SCFAs, containing from 2 to 8 carbon atoms, was achieved. The sample volume was 2 μL and the temperature program 80 °C (30 s) then 7 °C min⁻¹ to 220 °C (2 min). LOQs values were below 0.25 mg L⁻¹. The concentrations of the acids in the landfill leachate studied ranged from 0.45 ± 0,059 (average ± extended uncertainty) mg L⁻¹ for pentanoic acid to 15.2 ± 0.73 mg L⁻¹ for ethanoic acid. Concentrations of SCFAs in the municipal wastewater were lower than LOQs.     

Analysis of microcystins by capillary zone electrophoresis coupling with electrospray ionization mass spectrometry

In this paper, a rapid and effective method based on capillary zone electrophoresis (CZE) coupled with electrospray ionization mass spectrometry (ESI-MS) was established for the trace analysis of microcystin (MC) isomers in crude algae sample. The experimental conditions including the composition, acidity and concentration of buffer, separation voltage, injection time, and MS detection parameters were investigated in detail. A capillary separation system was as follows: a uncoated fused-silica capillary tube (50 μm i.d. × 90 cm), 40 mmol L⁻¹ ammonium acetate solution (pH 9.86) as running buffer, 25 kV as separation voltage, 20 kV × 3 s water first and 20 kV × 20 s for sample injection. Mass analysis was performed in ESI source, with sheath gas temperature 150 °C, sheath gas pressure 10 psi, and sheath gas flow 6 L min⁻¹. And sheath liquid was 7.5 mmol L⁻¹ acetic acid in 50% isopropanol-water (3 μL min⁻¹). Protonation and ammonium adduct molecular ions m/z 506.9 (MC-LR) and 532.0 (MC-YR) were used for the quantification of MCs. Under these conditions, two MCs were baseline separated within 9 min, the calibration curves were obtained in the range of 0.11-10.0 μg mL⁻¹ and 0.16-10.5 μg mL⁻¹ for MC-LR and MC-YR, respectively. Meanwhile, limits of detection were 0.05 and 0.08 μg mL⁻¹ for MC-LR and MC-YR, respectively. The recoveries for the two MCs were in the range of 95.8-108%. The developed approach had been successfully applied to the analysis of MCs in crude algae samples.     

Determination of ent-kaurene in subcutaneous fat of Iberian pigs by gas chromatography multi-stage mass spectrometry with the aim to differentiate between intensive and extensive fattening systems

This work presents a gas chromatography multi-stage mass spectrometry (GC-MS³) method for the determination of ent-kaurene in subcutaneous fat of Iberian pig, present in adipose tissue of animals due to pasture ingestion (extensive fattening system). The method comprises a saponification and a liquid-liquid extraction of the unsaponifiable fraction, followed by an isolation of the hydrocarbon fraction by high performance liquid chromatography (HPLC) and analysis by GC-MS³ (ion trap) with electronic ionization. The GC-MS³ analysis allows the isolation and characterization of specific fragments from the original (MS¹) molecular structure, and particularly, those fragments originated from the precursor ion (m/z = 229) characteristic of ent-kaurene. The MS/MS product fragment m/z = 213 is used as a further precursor fragment giving rise to a MS³ spectrum specific for ent-kaurene. The limit of detection of the MS³ technique is lower than 0.2 μg kg⁻¹ and a linear regression has been found between 0.2 and 112 μg kg⁻¹. This method is applicable for the determination of the fattening system of the Iberian pig.     

Simultaneous distillation-extraction of high-value volatile compounds from Cistus ladanifer L

The present paper describes a procedure to isolate volatiles from rock-rose (Cistus ladanifer L.) using simultaneous distillation-extraction (SDE). High-value volatile compounds (HVVC) were selected and the influence of the extraction conditions investigated. The effect of the solvent nature and extraction time on SDE efficiency was studied. The best performance was achieved with pentane in 1 h operation. The extraction efficiencies ranged from 65% to 85% and the repeatability varied between 4% and 6% (as a CV%).The C. ladanifer SDE extracts were analysed by headspace solid phase microextraction (HS-SPME) followed by gas chromatography with flame ionization detection (GC-FID). The HS-SPME sampling conditions such as fiber coating, temperature, ionic strength and exposure time were optimized. The best results were achieved with an 85 μm polyacrylate fiber for a 60 min headspace extraction at 40 °C with 20% (w/v) of NaCl. For optimized conditions the recovery was in average higher than 90% for all compounds and the intermediate precision ranged from 4 to 9% (as CV %). The volatiles α-pinene (22.2 mg g⁻¹ of extract), 2,2,6-trimethylcyclohexanone (6.1 mg g⁻¹ of extract), borneol (3.0 mg g⁻¹ of extract) and bornyl acetate (3.9 mg g⁻¹ of extract) were identified in the SDE extracts obtained from the fresh plant material.     

Purge efficiency in the determination of trihalomethanes in water by purge-and-trap gas chromatography

Purge-and-trap gas chromatography-mass spectrometry (PT-GC-MS) has become an accepted method for the analysis of trihalomethanes (THMs) in water. The purge-and-trap technique is based on an efficient transfer of volatile organic compounds from the liquid (contained in the purge chamber) to the gaseous phase by bubbling with an inert gas. The aim of this work was to study the purge system's efficiency by means of several consecutive purge cycles lasting 11 min each of the same liquid sample. The concentration range chosen of THMs was very wide [5-200 μg L⁻¹]. The inert gas flow rate was 40 mL min⁻¹, and experiments were performed at temperatures of 25, 35 and 50 °C. Bromoform (CHBr₃), the least volatile compound, needed 19 cycles to be purged quantitatively at a concentration of 200 μg L⁻¹ and only 7 cycles at 5 μg L⁻¹ for a 25 mL sample at 25 °C. Chloroform (CHCl₃), the most volatile compound, required 4 cycles to be fully extracted at 200 μg L⁻¹ and 2 at 5 μg L⁻¹. Finally, Novak's theoretical model, based on the distribution constant between gas and liquid phases, was used to correlate the THMs purging extraction data.     

A novel analytical system involving hydride generation and gold-coated W-coil trapping atomic absorption spectrometry for selenium determination at ng l level

A novel analytical technique was developed where gaseous hydrogen selenide formed by sodium tetrahydroborate reduction is transported to and trapped on a resistively heated gold-coated W-coil atom trap for in situ preconcentration. Gold coating on W-coil was prepared by using an organic solution of Au. The atom trap is held at 165 °C during the collection stage and is heated up to 675 °C for revolatilization; analyte species formed are transported to an externally heated quartz T-tube where the atomization takes place and the transient signal is obtained. The carrier gas consisted of 112.5 ml min^− 1 Ar with 75 ml min^− 1 H₂ during the collection step and 112.5 ml min^− 1 Ar with 450 ml min^− 1 H₂ in the revolatilization step. The half width of the transient signal obtained is less than 0.5 s. The RSD for the measurements was found to be 3.9% (n = 11) for 0.10 µg l^− 1 Se using peak height measurements.     

Titania-lanthanum phosphate photoactive and hydrophobic new generation catalyst

Titania-lanthanum phosphate nanocomposites with multifunctional properties have been synthesized by aqueous sol-gel method. The precursor sols with varying TiO₂:LaPO₄ ratios were applied as thin coating on glass substrates in order to be transparent, hydrophobic, photocatalytically active coatings. The phase compositions of the composite powders were identified by powder X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). The anatase phase of TiO₂ in TiO₂-LaPO₄ composite precursors was found to be stable even on annealing at 800 °C. The glass substrates, coated with TL1 (TiO₂-LaPO₄ composition with 1 mol% LaPO₄) and TL50 (composite precursor containing TiO₂ and LaPO₄ with molar ratio 1:1) sols and annealed at 400 °C, produced contact angles of 74° and 92°, respectively, though it is only 62° for pure TiO₂ coating. The glass substrates, coated with TL50 sol, produced surfaces with relatively high roughness and uneven morphology. The TL1 material, annealed at 800 °C, has shown the highest UV photoactivity with an apparent rate constant, k_app=24×10⁻³ min⁻¹, which is over five times higher than that observed with standard Hombikat UV 100 (k_app=4×10⁻³ min⁻¹). The photoactivity combined with a moderate contact angle (85.3°) shows that this material has a promise as an efficient self-cleaning precursor.     

An on-line method for pressurized hot water extraction and enzymatic hydrolysis of quercetin glucosides from onions

A novel environmentally sound continuous-flow hot water extraction and enzymatic hydrolysis method for determination of quercetin in onion raw materials was successfully constructed using a stepwise optimization approach. In the first step, enzymatic hydrolysis of quercetin-3,4′-diglucoside to quercetin was optimized using a three level central composite design considering temperature (75–95 °C), pH (3–6) and volume concentration of ethanol (5–15%). The enzyme used was a thermostable β-glucosidase variant (termed TnBgl1A_N221S/P342L) covalently immobilized on either of two acrylic support-materials (Eupergit^® C 250L or monolithic cryogel). Optimal reaction conditions were irrespective of support 84 °C, 5% ethanol and pH 5.5, and at these conditions, no significant loss of enzyme activity was observed during 72 h of use. In a second step, hot water extractions from chopped yellow onions, run at the optimal temperature for hydrolysis, were optimized in a two level design with respect to pH (2.6 and 5.5), ethanol concentration (0 and 5%) and flow rate (1 and 3 mL min⁻¹) Obtained results showed that the total quercetin extraction yield was 1.7 times higher using a flow rate of 3 mL min⁻¹ (extraction time 90 min), compared to a flow rate of 1 mL min⁻¹ (extraction time 240 min). Presence of 5% ethanol was favorable for the extraction yield, while a further decrease in pH was not, not even for the extraction step alone. Finally, the complete continuous flow method (84 °C, 5% ethanol, pH 5.5, 3 mL min⁻¹) was used to extract quercetin from yellow, red and shallot onions and resulted in higher or similar yield (e.g. 8.4 ± 0.7 μmol g⁻¹ fresh weight yellow onion) compared to a conventional batch extraction method using methanol as extraction solvent.     

Multivariate optimization of hydrodistillation-headspace solvent microextraction of thymol and carvacrol from Thymus transcaspicus

In this paper multivariate response surface methodology (RSM) has been used for the optimization of hydrodistillation-headspace solvent microextraction (HD-HSME) of thymol and carvacrol in Thymus transcaspicus. Quantitative determination of compounds of interest was performed simultaneously using gas chromatography coupled with flame ionization detector (GC-FID). Parameters affecting the extraction efficiency were assessed and the optimized values were 5 min, 2 μL and 3 min for the extraction time, micro-drop volume and cooling time after extraction, respectively. The amounts of analyte extracted increased with plant weight. The calibration curves were linear in the ranges of 6.25-81.25 and 1.25-87.50 mg L⁻¹ for thymol and carvacrol, respectively. Limit of detection (LOD) for thymol and carvacrol was 1.87 and 0.23 mg L⁻¹, respectively. Within-day and between-day precisions for both analytes were calculated in three different concentrations and recoveries obtained were in the range of 89-101% and 95-116% for thymol and carvacrol, respectively.     

Utility of copper(II) oxide as a packed reactor in flow injection assembly for rapid analysis of some angiotensin converting enzyme inhibitors

A new simple, sensitive, rapid and precise flow injection (FI) procedure based on the formation of copper complexes with some angiotensin converting enzyme (ACE) inhibitors has been developed and evaluated for the analysis of lisinopril (LN), enalapril maleate (EP), ramipril (RP) and perindopril tert-butylamine (PD). In this method, samples were injected into a flowing stream of distilled-deionized water, carried through the packed reactor of CuO for derivatization followed by ultraviolet (UV) detection. The flow rate was 1.5 ml min⁻¹ and column temperature was ambient (25 °C). Lisinopril was injected directly into the flowing stream and the detector response was measured at 262 nm. The hydrolysis products of enalapril maleate, ramipril and perindopril tert-butylamine in 0.2N NaOH were injected after neutralization with 1N HCl and the detector response was measured at 272, 265 and 252 nm, respectively. The developed method was successfully applied to the determination of tested drugs in pharmaceutical preparations at a sampling rate of 60 samples h⁻¹ and a recovery near 100% for all compounds.     

Determination of unfrozen matrix concentrations at low temperatures using stepwise DSC

The aim of the current study was to determine whether stepwise DSC (SW-DSC) is a suitable method for measuring the unfrozen matrix concentration (C_g) of binary aqueous solutions at temperatures as low as −50 °C. The optimal experimental conditions were determined using water. Reliable heat capacity values were determined at nominal scanning rates between 10 and 100 K min⁻¹, sample weights between 8 and 15 mg, and with the sample completely covering the base of the DSC pan. These conditions were then applied to aqueous solutions of ethylene glycol, glycerol and sodium chloride.The apparent heat is the sum of all heat including latent heat, heat capacity and heat of dilution. The influence of each term on the apparent heat was discussed in detail. The apparent heat values of the frozen samples were then used to calculate the ice fraction in the solution and were expressed as the C_g. The calculated C_g values were similar to previously published values. This study showed that SW-DSC can be used to determine the C_g over a wide temperature range using only one single solution. This technique is advantageous for solutes that are not available in large quantities.     

Improved micro-flame detection method for gas chromatography

A previously developed micro-counter-current flame method is modified to provide both sensitive photometric and ionization detection for gas chromatography (GC). A stainless steel capillary (0.254 mm i.d.) supplying oxygen functions as a burner, which supports a compact flame that burns in a counter-flowing excess of hydrogen. In the “micro-flame photometric detector” (μFPD) response mode, the background emission level is reduced by over an order of magnitude compared to previous experiments using a fused silica capillary burner, resulting in greatly improved detection limits. Chemiluminescent sulfur response in the μFPD is quadratic over 3.5 orders of magnitude, yielding a detection limit of 3 × 10⁻¹¹ gS/s, while that of phosphorus is linear over 5 orders of magnitude down to a minimum detectable limit of 3 × 10⁻¹² gP/s. Tin response is examined for the first time in the μFPD and yields a blue luminescence (ascribed to SnO*) on the surface of the quartz flame enclosure. Although this emission provides a sensitive detection limit near 6 × 10⁻¹⁵ gSn/s, due to the limited surface area within the detector cell it only produces a linear response range of one order of magnitude. Ionization response toward hydrocarbons is also obtained from the hydrogen-rich micro-counter-current flame. A linear response is produced over five orders of magnitude, yielding a detection limit toward carbon of 2 × 10⁻¹⁰ gC/s. Analysis of a simple gasoline sample is used to demonstrate that the device can successfully operate as both a selective and universal GC detector. Results indicate that this micro-counter-current flame method yields comparable performance to conventional flame photometric and flame ionization detectors, making it potentially useful for adaptation to micro-analytical devices and portable GC units.     

Determination of hydroxyls density in the silica-mesostructured cellular foams by thermogravimetry

Thermogravimetry was applied to determine the surface hydroxyls coverage in the mesostructured cellular foams (MCFs) calcined at different temperatures and then rehydroxylated by contacting with water vapor or liquid. The TG measurements were performed by heating MCFs in air stream using a three-step temperature program: (i) at rate of 5 °C min⁻¹ from 25 to 200 °C; (ii) held at 200 °C for 30 min; and (iii) heating at rate of 10 °C min⁻¹ up to 1100 °C. The hydroxyls content was calculated from weight loss during third step. The hydroxyls density appeared to depend strongly on the calcination temperature and the subsequent contact with water vapor. When MCFs were exposed for a short period (ca. 1 min) to moist air the hydroxyls content increased rapidly, more in the samples calcined at 300 °C than 500 °C, to attain surface densities of 4.75 and 1.6 OH nm⁻², respectively. The 2-h contact with water vapor resulted in slower further increase of hydroxyls densities, to values of 5.45 and 2.9 OH nm⁻², for samples calcined at 300 and 500 °C, but longer contacts had no significant effect. A similar trend was also observed when sample was treated with liquid water.     

Determination of ethylphenol compounds in wine by headspace solid-phase microextraction in conjunction with gas chromatography and flame ionization detection

Headspace solid phase microextraction (HS-SPME) was investigated as a solvent-free alternative method for the extraction and determination of 4-ethylphenol (EP) and 4-ethylguaiacol (EG) in red wine by capillary gas chromatography with flame ionization detection (FID) and compared to liquid-liquid extraction.For HS-SPME, better results were obtained with saturated sodium chloride samples, at 55 °C, using a 85 μm polyacrylate fiber. An absorption time of 40 min was needed to reach the absorption equilibrium for EG. This 40-min duration corresponds to the beginning of EP equilibrium and was selected for the experiments. In these conditions, the calibration graphs were linear in the range 5-5000 μg l⁻¹ and the sensitivity was nearly the same for the two compounds. The detection limits were in the low μg l⁻¹ range. In model wine solutions, result obtained with the liquid-liquid extraction method exhibit a linear calibration between 25 and 10,000 μg l⁻¹ with a detection limit of 1 μg l⁻¹, but, the relative standard deviations of the EP and EG result in the low concentration range (<50 μg l⁻¹) are higher than those obtained by HS-SPME (15% compared to 2% for EP and 12% compared to 5% for EG). Taking into account the numerous volatile compounds in wine, HS-SPME is a rapid and valid alternative technique for use in the determination of ethylphenols at trace levels.     

Determination of Se,Pb, and Sb by atomic fluorescence spectrometry using a new flameless,dielectric barrier discharge atomizer

A flameless atomizer for atomic fluorescence spectrometry (AFS), based on an atmospheric pressure dielectric barrier discharge, has been developed for the atomization of hydride-forming elements, such as Se, Sb and Pb. The atomizer (8 mm o.d, 35 mm length) was operated at a power less than 50 W. The discharge was sustained with argon at the flow rate of 0.85 L min^− 1 after optimization. The characteristics of the atomizer and the effects of different parameters (power, gas flow rate, and KBH₄ concentration) are investigated. The most attractive feature of this atomizer is its low operation temperature (~ 52 °C, detected at the outlet of the atomizer by a thermocouple), allowing both the radiation source and the detector to be placed in close proximity with the atomizer. The analytical performance of the atomizer has been evaluated, and detection limits for Se, Sb and Pb obtained with the present technique were 0.08, 0.11 and 0.27 μg L^− 1, respectively. The accuracy of the system was verified by the determination of Se, Sb and Pb in reference material of spinage GBW 10015. The concentrations of Se, Sb, and Pb determined by the present technique agreed well with the reference values (Se: 92 ± 24 mg kg^− 1, Sb: 43 ± 14 mg kg^− 1, Pb: 11.1 ± 0.9 mg kg^− 1). This detector is very promising for field elements detection with portable AFS.     

Glutaraldehyde activated eggshell membrane for immobilization of tyrosinase from Amorphophallus companulatus: application in construction of electrochemical biosensor for dopamine

Tyrosinase from a plant source Amorphophallus companulatus was immobilized on eggshell membrane using glutaraldehyde. Among the three different approaches used for immobilization, activation of eggshell membrane by glutaraldehyde followed by enzyme adsorption on activated support could stabilize the enzyme tyrosinase and was found to be effective. K_m and V_max values for dopamine hydrochloride calculated from Lineweaver-Burk plot were 0.67 mM and 0.08 mM min⁻¹, respectively. Studies on effect of pH showed retention of more than 90% activity over a pH range 5.0-6.5. Membrane bound enzyme exhibited consistent activity in the temperature range 20-45 °C. Shelf life of immobilized tyrosinase system was found to be more than 6 months when stored in phosphate buffer at 4 °C. An electrochemical biosensor for dopamine was developed by mounting the tyrosinase immobilized eggshell membrane on the surface of glassy carbon electrode. Dopamine concentrations were determined by the direct reduction of biocatalytically liberated quinone species at −0.19 V versus Ag/AgCl (3 M KCl). Linearity was observed within the range of 50-250 μM with a detection limit of 25 μM.     

Development and validation of a liquid chromatography-electrosprayionization mass spectrometric method for the determination of dexamethasone in sheep plasma

A quantitative liquid chromatographic-electrospray ionization mass spectrometric method for the determination of dexamethasone in sheep plasma has been developed and validated. The samples were extracted using solid-phase extraction cartridges with mixed reversed-phase materials (oasis-HLB). The chromatographic separation was performed on a reversed-phase XTerrra MS C₁₈ column ( mm; 5 μm) using a mobile phase consisting of 65% methanol in water containing 0.1% (v/v) formic acid, pumped at a flow rate of 0.30 ml min⁻¹. The analyte was detected after positive electrospray ionization using selected ion monitoring (SIM) mode. The probe heater temperature was set at 260 °C, the capillary voltage was set at 3.5 kV and the source block voltage (AQA_max) was set at 30 V. The method was fully validated. Calibration graphs were linear (r better than 0.998, n=11), in concentration ranges 6-1000 ng ml⁻¹ for dexamethasone. The intra- and inter-day RSD values were less than 24.1% (n=6). The limits of detection and quantitation for dexamethasone were found to be 1 and 6 ng ml⁻¹, respectively. The efficiency of the solid phase extraction procedure was found to be 92.4% for dexamethasone. The method was further applied to a pilot kinetic study in order to assess the main pharmacokinetic parameters of dexamethasone in sheep.     

X-ray diffraction study on the thermal expansion behavior of cellulose Iβ and its high-temperature phase

Oriented films of cellulose prepared from algal cellulose were hydrothermally treated to convert them into highly crystalline cellulose Iβ. The lateral thermal expansion behavior of the prepared cellulose Iβ films was investigated using X-ray diffraction at temperatures from 20 to 300 °C. Cellulose Iβ was transformed into the high-temperature phase when the temperature was above 230 °C, allowing the lateral thermal expansion coefficient of cellulose Iβ and its high-temperature phase to be measured. For cellulose Iβ, the thermal expansion coefficients (TECs) of the a- and b-axes were α_a = 9.8 × 10⁻⁵ °C⁻¹ and α_b = 1.2 × 10⁻⁵ °C⁻¹, respectively. This anisotropic thermal expansion behavior in the lateral direction is ascribed to the crystal structure and to the hydrogen-bonding system of cellulose Iβ. For the high-temperature phase, the anisotropy was more conspicuous, and the TECs of the a- and b-axes were α_a = 19.8 × 10⁻⁵ °C⁻¹ and α_b = −1.6 × 10⁻⁵ °C⁻¹, respectively. Synchrotron X-ray fiber diffraction diagrams of the high-temperature phase were also recorded at 250 °C. The cellulose high-temperature phase is composed of a two-chain monoclinic unit cell, a = 0.819 nm, b = 0.818 nm, c (fiber repeat) = 1.037 nm, and γ = 96.4°, with space group = P2₁. The volume of this cell is 4.6% larger than that of cellulose Iβ at 30 °C.     

Development and validation of a chiral HPLC method for rapid screening of allylic alcohol asymmetric epoxidation processes

A simple and rapid HPLC method has been developed using a polysaccharide chiral stationary phase (Chiralpak AD-H) for the resolution of glycidyl tosylate enantiomers. These compounds were obtained by asymmetric epoxidation of allyl alcohol with chiral titanium-tartrate complexes as catalyst after in situ derivatization of the intermediate glycidols. Separations were achieved using two types of mobile phase: a normal-phase (n-hexane), and a polar-phase (methanol or acetonitrile). The influence of the type and concentration of organic modifier in the mobile phase (ethanol or 2-propanol), the flow rate and the column temperature was investigated. In normal-phase mode, the optimized conditions were: n-hexane/ethanol 70/30 (v/v) at a flow rate of 1.2 mL min⁻¹ and 40 °C. In polar-phase mode, the optimized conditions were: methanol at a flow rate of 0.8 mL min⁻¹ and 20 °C. In both cases, analysis time was ≤11 min and the chiral resolution was ≥2. Nevertheless, due to the better Rs obtained in normal-phase mode, only this method was validated to avoid peaks overlapping in real samples. This method was found to be linear in the 5-300 μg mL⁻¹ range (R² > 0.999) with an LOD of 1.5 μg mL⁻¹ for both glycidyl tosylate enantiomers. Repeatability and intermediate precision at three different concentrations levels were below 0.5 and 7.2% R.S.D. for retention time and area, respectively. This method was applied successfully for the determination of glycidyl tosylate enantiomers after in situ derivatization of glycidols obtained in allylic alcohol asymmetric epoxidation processes with chiral titanium-tartrate complexes as catalysts.