Screening of volatile composition from Portuguese multifloral honeys using headspace solid-phase microextraction-gas chromatography-quadrupole mass spectrometry

The volatile composition from four types of multifloral Portuguese (produced in Madeira Island) honeys was investigated by a suitable analytical procedure based on dynamic headspace solid-phase microextraction (HS-SPME) followed by thermal desorption gas chromatography-quadrupole mass spectrometry detection (GC-qMS). The performance of five commercially available SPME fibres: 100 μm polydimethylsiloxane, PDMS; 85 μm polyacrylate, PA; 50/30 μm divinylbenzene/carboxen on polydimethylsiloxane, DVB/CAR/PDMS (StableFlex); 75 μm carboxen/polydimethylsiloxane, CAR/PDMS, and 65 μm carbowax/divinylbenzene, CW/DVB; were evaluated and compared. The highest amounts of extract, in terms of the maximum signal obtained for the total volatile composition, were obtained with a DVB/CAR/PDMS coating fibre at 60 °C during an extraction time of 40 min with a constant stirring at 750 rpm, after saturating the sample with NaCl (30%). Using this methodology more than one hundred volatile compounds, belonging to different biosynthetic pathways were identified, including monoterpenols, C₁₃-norisoprenoids, sesquiterpenes, higher alcohols, ethyl esters and fatty acids. The main components of the HS-SPME samples of honey were in average ethanol, hotrienol, benzeneacetaldehyde, furfural, trans-linalool oxide and 1,3-dihydroxy-2-propanone.     

Fungal pathogenic nucleic acid detection achieved with a microfluidic microarray device

Detection of polymerase chain reaction (PCR) products obtained from cultured greenhouse fungal pathogens, Botrytis cinerea and Didymella bryoniae has been achieved using a previously developed microfluidic microarray assembly (MMA) device. The flexible probe construction and rapid DNA detection resulted from the use of centrifugal pumping in the steps of probe introduction and sample delivery, respectively. The line arrays of the oligonucleotide probes were “printed” on a CD-like glass chip using a polydimethylsiloxane (PDMS) polymer plate with radial microfluidic channels, and the sample hybridizations were conducted within the spiral channels on the second plate. The experimental conditions of probe immobilization and sample hybridization were optimized, and both complementary oligonucleotides and PCR products were tested. We were able to achieve adequate fluorescent signals with a sample load as small as 0.5 nM (1 μL) for oligonucleotide samples; for PCR products, we achieved detection at the level of 3 ng.     

A new polyethylene glycol fiber prepared by coating porous zinc electrodeposited onto silver for solid-phase microextraction of styrene

A new polyethylene glycol fiber was developed for solid-phase microextraction (SPME) of styrene by electrodepositing porous Zn film on Ag wire substrate followed by coating with polyethylene glycol sol-gel (Ag/Zn/PEG sol-gel fiber). The scanning electron micrographs of fibers surface revealed a highly porous structure. The extraction property of the developed fiber-to-styrene residue from polystyrene packaged food was investigated by headspace solid-phase microextraction (HS-SPME) and analyzed with a gas chromatograph coupled with flame ionization detection (GC-FID). The new Ag/Zn/PEG sol-gel fiber is simple to prepare, low cost, robust, has high thermal stability and long lifetime, up to 359 extractions. Repeatability of one fiber (n = 6) was in the range of 4.7-7.5% and fiber-to-fiber reproducibility (n = 4) for five concentration values were in the range 3.4-10%. This Ag/Zn/PEG sol-gel fiber was compared to two commercial SPME fibers, 75 μm carboxen/polydimethylsiloxane (CAR/PDMS) and 100 μm polydimethylsiloxane (PDMS). Under their optimum conditions, Ag/Zn/PEG sol-gel fiber showed the highest sensitivity and the lowest detection limit at 0.28 ± 0.01 ng mL⁻¹.     

Headspace solid-phase microextraction of oil matrices heated at high temperature and phthalate esters determination by gas chromatography multistage mass spectrometry

A solvent-free analytical approach based on headspace solid-phase microextraction (SPME) of oil matrices heated at high temperatures coupled to gas chromatography with mass spectrometry detector (GC-ion trap) has been developed for the determination of phthalic acid esters (PAEs) in oil matrices without sample manipulation. For this study, three fibers, i.e., 85 μm-polyacrylate (PA), 50/30 μm-divinylbenzene-carboxen-polydimethylsiloxane (DVB/CAR/PDMS) and 100 μm-polydimethylsiloxane (PDMS) were tested. Variables affecting the SPME headspace composition such as incubation sample temperature, sample incubation time and fiber exposition time were optimized. The optimal values found were 250 °C for sample incubation temperature and 30 min for incubation and extraction time. PA fiber was not suitable for the lightest polar phthalates which showed poor extraction and repeatability values. PDMS fiber had very poor response for some of the heavier and non-polar phthalates, whereas DVB/CAR/PDMS fiber showed the best response and repeatability values for the majority of the phthalates studied. The main benefit of the analytical method proposed is the absence of sample manipulation and hence avoidance of possible contamination coming from glassware, environment, solvents and samples.     

A simple method for preparation of macroporous polydimethylsiloxane membrane for microfluidic chip-based isoelectric focusing applications

A new, simple method was reported to prepare PDMS membranes with micrometer size pores for microfluidic chip applications. The pores were formed by adding polystyrene and toluene into PDMS prepolymer solution prior to spin-coating and curing. The resulting PDMS membrane has a thickness of around 10 μm and macropores with a diameter ranging from 1 to 2 μm measured using scanning electron microscope (SEM) imaging. This PDMS membrane was validated by integrating it with PDMS microfluidic chips for protein separation using isoelectric focusing mechanism coupled with whole channel imaging detection (IEF-WCID). It has been shown that five standard pI markers and a mixture of two proteins, myoglobin and β-lactoglobulin, can be separated using these chips. The results indicated that this macroporous PDMS membrane can replace the dialysis membrane in PDMS chips for the IEF-WCID technique. The preparation method of macroporous PDMS membrane may be potentially applied in other fields of microfluidic chips.     

Rapid screening of precursor and degradation products of chemical warfare agents in soil by solid-phase microextraction ion mobility spectrometry (SPME-IMS)

The use of solid-phase microextraction (SPME) coupled to ion mobility spectrometry (IMS) to detect precursor and degradation products of chemical warfare agents (CWAs) as soil contaminants was investigated. The development and characterization of a system to interface a thermal desorption solid-phase microextraction inlet with a hand held ion mobility spectrometer was demonstrated. The analytes used in this study were diisopropyl methylphosphonate (DIMP), diethyl methylphosphonate (DEMP), and dimethyl methylphosphonate (DMMP). Two SPME fibers with different stationary phases, 100 μm polydimethylsiloxane (PDMS) and 65 μm polydimethylsiloxane divinylbenzene (PDMS/DVB), were evaluated in this study to determine the optimal fiber and extraction conditions. Better results were obtained with the PDMS fiber. SPME-IMS offered good repeatability and detection of the precursor and degradation products in spiked soil at concentrations as low as 10 μg/g. Sample analysis time was less than 30 min for all the precursor and degradation products.     

A simple method for patterning poly(dimethylsiloxane) barriers in paper using contact-printing with low-cost rubber stamps

This paper presents a simple and low-cost method for patterning poly(dimethylsiloxane) (PDMS) barriers in porous support such as paper for the construction of flexible microfluidic paper-based analytical devices (μPADs). The fabrication method consisted of contact-printing a solution of PDMS and hexane (10:1.5 w/w) onto chromatographic paper using custom-designed rubber stamps containing the patterns of μPADs. After penetrating the paper (∼30 s), the PDMS is cured to form hydrophobic barriers. Under optimized conditions, hydrophobic barriers and hydrophilic channels with dimensions down to 949 ± 88 μm and 771 ± 90 μm (n = 5), respectively, were obtained. This resolution is well-suitable for most applications in analytical chemistry. Chemical compatibility studies revealed that the PDMS barriers were able to contain some organic solvents, including acetonitrile and methanol, and aqueous solutions of some surfactants. This find is particularly interesting given that acetonitrile and methanol are the most used solvents in chromatographic separations, non-aqueous capillary electrophoresis and electroanalysis, as well as aqueous solutions of surfactants are suitable mediums for cell lyses assays. The utility of the technique was evaluated in the fabrication of paper-based electrochemical devices (PEDs) with pencil-drawn electrodes for experiments in static cyclic voltammetry and flow injection analysis (FIA) with amperometric detection, in both aqueous and organic mediums.     

Electrophoretic separations of neuromediators on microfluidic devices

In the present work, on-chip capillary electrophoresis for the separation of neuromediators is demonstrated. The influence of separation buffer (composition, pH, SDS additive), on-chip electrokinetic sample stacking, and surface pretreatment of the PDMS-PDMS and hybrid PDMS-glass devices on the electrokinetic characteristics of microfluidics (ν_eo, μ_eo, ζ) and separation performance of on-chip capillary electrophoresis of neuromediators have been investigated. It is demonstrated that for the effective separation of neuropeptides on elastomer-based microfluidic devices, on-chip sample stacking is necessary. Field-amplified sample stacking for electroosmotic flow supported on-chip separations of neuromediators and without special design of the sample injection scheme has been demonstrated. Electrophoretic separations of fluorescently labeled analytes have been achieved within tens of seconds at injection volumes of about 110 pL, with plate numbers varying from <1000 to ∼22,000. These results demonstrate that on-chip separation methods with hybrid PDMS-glass devices are perspective for the analysis of (neuro)peptides in small volumes.     

Pesticide analysis in herbal infusions by solid-phase microextraction and gas chromatography with atomic emission detection

A direct immersion solid-phase microextraction (SPME) procedure was used in combination with capillary gas chromatography with atomic emission detection (GC-AED) for the determination of 10 pesticides (organochlorines, organophosphorus compounds and pyrethrins) in herbal and tea infusions. Ionic strength, sample dilution and time and temperature of the absorption and desorption stages were some of the parameters investigated in order to select the optimum conditions for SPME with a 100 μm PDMS fiber-coating. Element-specific detection and quantification was carried out by monitoring the chlorine (479 nm) and bromine (478 nm) emission lines, which provided nearly specific chromatograms. Calibration was carried out by using a spiked sample infusion. The detection limits varied between 11.9 ng ml⁻¹ for deltamethrin and 0.03 ng ml⁻¹ for p,p′-DDE and p,p′-DDD. The recoveries ranged from 73.5% for deltamethrin to 108.3% for p,p′-DDT in a spiked white tea infusion. Two of the eight samples analyzed contained low levels of some the pesticides considered.     

Preliminary study of a microbeads based histamine detection for food analysis using thermostable recombinant histamine oxidase from Arthrobacter crystallopoietes KAIT-B-007

We designed and prepared a micro biosensing system consisting of a flow through system with a sub-micro liter injection valve and a sub-micro liter volume bioreactor. An electrochemical detector was combined with the reactor for immediate detections. The volumes of the reactor and the sample loop for the injection were 850 nl and 320 nl, respectively. This paper described about the characteristics of the sensing system in the case of histamine detection for food analysis. Histamine oxidase from KAIT-B-007 was prepared by using a gene recombination technique and they were immobilized with chitosan beads (? = 70-105 μm). The detection less than one minute after injection made possible fast analysis for histamine. The biosensing system also showed a high performance for histamine detection in wide range of 1 μM-1 mM. In addition, we practically measured histamine content in raw tuna stored at room temperature and 35 °C up to 96 h. As a result of the comparison between our sensing system and HPLC method, there was good agreement. These results show that our microfluidic biosensing system has the potential to assist miniaturization with small sample volume and short determination time for a sequential food analysis.     

Electropolymerized multiwalled carbon nanotubes/polypyrrole fiber for solid-phase microextraction and its applications in the determination of pyrethroids

A novel solid-phase microextraction (SPME) fiber coated with multiwalled carbon nanotubes/polypyrrole (MWCNTs/Ppy) was prepared with an electrochemical method and used for the extraction of pyrethroids in natural water samples. The results showed that the MWCNTs/Ppy coated fiber had high organic stability, and remarkable acid and alkali resistance. In addition, the MWCNTs/Ppy coated fiber was more effective and superior to commercial PDMS and PDMS/DVD fibers in extracting pyrethroids in natural water samples. Under optimized conditions, the calibration curves were found to be linear from 0.001 to 10 μg mL⁻¹ for five of the six pyrethroids studied, the exception being fenvalerate (which was from 0.005 to 10 μg mL⁻¹), and detection limits were within the range 0.12-0.43 ng mL⁻¹. The recoveries of the pyrethroids spiked in water samples at 10 ng mL⁻¹ ranged from 83 to 112%.     

The use of headspace solid phase microextraction for the characterization of volatile compounds in olive oil matrices

Two different fibre coatings, for solid phase microextraction (SPME) sampling, poly(dimethylsiloxane) (PDMS) and poly(acrylate) (PA), were studied in order to test, for olive oil matrixes, two mathematical models that relate the directly proportional relationship between the amount of analyte absorbed by a SPME fibre and its initial concentration in the sample matrices. Although the PA fibre was able to absorb higher amounts of compounds from the olive oil sample, the equilibrium was reached later then with the PDMS fibre. In both cases, the amount of analyte present affected the time profile or the equilibrium time in two of the concentrations studied, 0.256 μL/kg, 2.56 μL/kg and for 2-ethylfuran, pentan-3-one, pent-1-en-3-one, hexanal, trans,trans-non-2,4-dienal and in the four concentrations studied, 0.256 μL/kg, 2.56 μL/kg, 6.25 μL/kg and 400 μL/kg, for 4-methyl-pent-3-en-2-one, 2-methylbutan-1-ol, methoxybenzene, hexan-1-ol, cis-hex-3-en-1-ol, trans-hex-2-en-1-ol, 2-ethyl-hexan-1-ol and trans,trans-dec-2,4-dienal. Comparing the mathematical models of both fibres, the PA-coated fibre showed direct proportionality between the initial concentration and amount extracted, that allows the possibility of relative quantification in a non-equilibrium state in non-aqueous media. The same was not observed for the PDMS fibre.     

A simple and green analytical method for determination of iron based on micro flow analysis

A simple, inexpensive and reagent-less colorimetric micro flow analysis (μFA) system was implemented in a polymethyl methacrylate (PMMA) micro fluidic manifold. A T-shaped micro channel on a PMMA chip was fabricated by laser ablation and topped with molded polydimethylsiloxane (PDMS). The fabricated μFA system was integrated with the optical components as detector and applied to the determination of iron in water samples. It is based on the measurement of Fe(III)-nitroso-R salt complex at 720 nm formed by the reaction between Fe(III) and nitroso-R salt in an acetate buffer solution pH 5. The proposed μFA consumed very small amount of reagent and sample, it released waste of less than 2.0 mL h⁻¹. The relative standard deviation (R.S.D.) was less than 2% (n = 11) with the recovery of 98.7 ± 0.12 (n = 5). The linear range for the determination of iron in water samples was over the range of 0.05-4.0 μg mL⁻¹ with a correlation coefficient (r²) of 0.9994. The limit of detection (3σ) and limit of quantitation (10σ) were 0.021 μg mL⁻¹ and 0.081 μg mL⁻¹, respectively with a sample throughput of 40 h⁻¹.     

A microflow chemiluminescence system for determination of chloramphenicol in honey with preconcentration using a molecularly imprinted polymer

A novel chemiluminescence (CL) microfluidic system incorporating a molecularly imprinted polymer (MIP) preconcentration step was used for the determination of chloramphenicol in honey samples. The MIP was prepared by using chloramphenicol as the template, diethylaminoethyl methacrylate (DAM) as the function monomer, ethylene glycol dimethacrylate (EGDMA) as the cross-linking monomer, 2, 2′-dimethoxy-2-phenylacetophenone (DMPA) as the free radical initiator and toluene and dodecanol as the solvent. The MIP was pre-loaded into a 10 mm long, 2 mm wide and 150 μm deep channel in a planar glass microfluidic device. When the sample containing chloramphenicol was introduced into the microfluidic device it was first preconcentrated on the MIP then detected by an enhancement effect on the chemiluminescence reaction of tris(2, 2′-bipyridyl) ruthenium(II) with cerium(IV) sulphate in sulphuric acid. A micro-syringe pump was used to pump the reagents. The CL intensity was linear in relationship to the chloramphenicol concentrations from 1.55 × 10⁻⁴ to 3.09 × 10⁻³ μmol L⁻¹ (r² = 0.9915) and the detection limit (3σ) and the quantitation limit (10σ) were found to be 7.46 × 10⁻⁶ and 2.48 × 10⁻⁵ μmol L⁻¹, respectively. This method offered a high selectivity and sensitivity for quantitative analysis of chloramphenicol in the honey samples.     

Development of a long-life capillary enzyme bioreactor for the determination of blood glucose

A long-life capillary enzyme bioreactor was developed that determines glucose concentrations with high sensitivity and better stability than previous systems. The bioreactor was constructed by immobilizing glucose oxidase (GOx) onto the inner surface of a 0.53 mm i.d. fused-silica capillary that was part of a continuous-flow system. In the presence of oxygen, GOx converts glucose to gluconic acid and hydrogen peroxide (H₂O₂). Hydrogen peroxide detection was accomplished using an amperometric electrochemical detector. The integration of this capillary reactor into a flow-injection (FIA) system offered a larger surface-to-volume ratio, reduced band-broadening effects, and reduced reagent consumption compared to packed column in FIA or other settings. To obtain operational (at ambient temp) and storage (at 4 °C) stability for 20 weeks, the glucose biosensing system was prepared using an optimal GOx concentration (200 mg/mL). This exhibited an FIA peak response of 7 min and a detection limit of 10 μM (S/N = 3) with excellent reproducibility (coefficient of variation, CV < 0.75%). It also had a linear working range from 10¹ to 10⁴ μM. The enzyme activity in this proposed capillary enzyme reactor was well maintained for 20 weeks. Furthermore, 20 serum samples were analyzed using this system, and these correlated favorably (correlation coefficient, r² = 0.935) with results for the same samples obtained using a routine clinical method. The resulting biosensing system exhibited characteristics that make it suitable for in vivo application.     

Application of solid-phase microextraction for the determination of organophosphorus pesticides in textiles by gas chromatography with mass spectrometry

A method based on solid-phase microextraction (SPME) and gas chromatography with mass spectrometry (GC/MS) for the determination of 18 organophosphorus pesticides (OPPs) in textiles is described. Commercially available SPME fibers, 100 μm PDMS and 85 μm PA, were compared and 85 μm PA exhibited better performance to the OPPs. Various parameters affecting SPME, including extraction and desorption time, extraction temperature, salinity and pH, were studied. The optimized conditions were: 35 min extraction at 25 °C, 5% NaSO₄ content, pH 7.0, and 3.5 min desorption in GC injector port at 250 °C. The linear ranges of the SPME-GC/MS method were 0.1-500 μg L⁻¹ for most of the OPPs. The limits of detection (LODs) ranged from 0.01 μg L⁻¹ (for bromophos-ethyl) to 55 μg L⁻¹ (for azinphos-methyl) and the RSDs were between 0.66% and 9.22%. The optimized method was then used to analyze 18 OPPs in textile sample, and the determined recoveries were ranged from 76.7% to 126.8%. Moreover, the distribution coefficients of the OPPs between 85 μm PA fiber and simulative sweat solution (K_pa/s) were determined. The determined K_pa/s of the OPPs correlated well with their octanol-water partition coefficients (r = 0.764 and 0.678) and water solubility (r = −0.892 and −0.863).     

Determination of water pollutants by direct-immersion solid-phase microextraction using polymeric ionic liquid coatings

The determination of a group of eighteen pollutants in waters, including polycyclic aromatic hydrocarbons and substituted phenols, is conducted in direct-immersion solid-phase microextraction (SPME) using the polymeric ionic liquid (PIL) poly(1-vinyl-3-hexadecylimidazolium) bis[(trifluoromethyl)sulfonyl]imide as a novel coating material. The performance of the PIL fiber coating in the developed IL-SPME-gas chromatography (GC)–mass spectrometry (MS) method is characterized by average relative recoveries of 92.5% for deionized waters and 90.8% for well waters, average precision values (as relative standard deviations, RSD%) of 11% for deionized waters and 12% for well waters, using a spiked level of 5 ng mL⁻¹. The detection limits oscillate from 0.005 ng mL⁻¹ for fluoranthene to 4.4 ng mL⁻¹ for 4-chloro-3-methylphenol, when using an extraction time of 60 min with 20 mL of aqueous sample. The extraction capabilities of the PIL fiber have been compared with the commercial SPME coatings: polydimethylsyloxane (PDMS) 30 μm, PDMS 100 μm and polyacrylate (PA) 85 μm. The PIL fiber is superior to the PDMS 30 μm for all analytes studied. A qualitative study was also carried out to compare among the nature of the coating materials by normalizing the coating thickness. The PIL material was shown to be more efficient than the PDMS material for all analytes studied. The PIL coating was also adequate for nonpolar analytes whereas the PA material was more sensitive for polar compounds.     

Plastified poly(ethylene terephthalate) (PET)-toner microfluidic chip by direct-printing integrated with electrochemical detection for pharmaceutical analysis

Rapid separation and determination of acetaminophen and its hydrolysate with end-channel electrochemical (EC) detection integrated on a plastified poly(ethylene terephthalate) (PET)-toner microchip capillary electrophoresis (CE) system was investigated. In this separation and detection system, a Pt ultramicroelectrode integrated on a three-dimensional adjustor was used as working electrode. Factors influencing the separation and detection were investigated and optimized. Results show that acetaminophen and p-aminophenol can be well separated within 84 s with R.S.D. < 1% for migration time and R.S.D. < 3.6% for detection current for both analytes. Detection limits for both analytes are determined to be 5.0 μM (S/N = 3). This method has been successfully applied to the detection of trace p-aminophenol in paracetamol tablets. The results demonstrate that the PET-toner microchips can obtain better performance than PDMS microfluidic devices but at much lower cost.     

Microfluidic device capable of sensing ultrafast chemiluminescence

Based on the principle of liquid core waveguide, a novel microfluidic device with micro-scale detection window capable of sensing flashlight emitted from rapid 1,1′-oxalyldi-4-methylimidazole (OD4MI) chemiluminescence (CL) reaction was fabricated. Light emitted from OD4MI CL reaction occurring in the micro-dimensional pentagonal detection window (length of each line segment: 900.0 μm, depth: 50.0 μm) of the microfluidic device with two inlets and one outlet was so bright that it was possible to take an image every 1/30 s at the optimal focusing distance (60 cm) using a commercial digital camera. Peaks obtained using a flow injection analysis (FIA) system with the micro-scale detection window and OD4MI CL detection show excellent resolution and reproducibility without any band-broadening observed in analytical devices having additional reaction channel(s) to measure light generated from slow CL reaction. Maximum height (H_max) and area (A) of peak, reproducibility and sensitivity observed in the FIA system with the microfluidic device and OD4MI CL detection depends on (1) the mole ratio between bis(2,4,6-trichlorophenyl) oxalate and 4-methyl imidazole yielding OD4MI, (2) the flow rate to mix OD4MI, H₂O₂ and 1-AP in the detection window of the microfluidic device, and (3) H₂O₂ concentration. We obtained linear calibration curves with wide dynamic ranges using H_max and A. The detection limit of 1-AP determined with H_max and A was as low as 0.05 fmole/injection (signal/background = 3.0).     

Evaluation of the solid-phase microextraction fiber coated with single walled carbon nanotubes for the determination of benzene,toluene, ethylbenzene,xylenes in aqueous samples

A solid-phase microextraction (SPME) fiber coated with single walled carbon nanotubes (SWCNTs) was prepared by electrophoretic deposition and treated at 500 °C in H₂ stream. In order to evaluate the characteristics of the obtained fiber, it was applied in the headspace solid-phase microextraction (HS-SPME) of benzene, toluene, ethylbenzene and xylenes (BTEX) from water sample and quantification by gas chromatography with flame ionization detection (GC-FID). The results indicated that the thermal treatment with H₂ enhanced the extraction of the SWCNTs fiber for BTEX significantly. Thermal stability and durability of the fiber were also investigated, showing excellent stability up to 350 °C and life time over 120 times. In the comparison with the commercial CAR–PDMS fiber, the SWCNTs fiber showed similar and higher extraction efficiencies for BTEX. Under the optimized conditions, the linearity, LODs (S/N = 3) and LOQs (S/N = 10) of the method based on the SWCNTs fiber were 0.5–50.0, 0.005–0.026 and 0.017–0.088 μg/L, respectively. Repeatability for one fiber (n = 3) was in the range of 1.5–5.6% and fiber-to-fiber reproducibility (n = 3) was in the range of 4.2–8.3%. The proposed method was successfully applied in the analysis of BTEX compounds in seawater, tap water and wastewater from a paint plant.