SPME in environmental analysis

Recent advances in the use of solid-phase microextraction (SPME) in environmental analysis, including fiber coatings, derivatization techniques, and in-tube SPME, are reviewed in this article. Several calibration methods for SPME, including traditional calibration methods, the equilibrium extraction method, the exhaustive extraction method, and several diffusion-based calibration methods, are presented. Recent developed SPME devices for on-site sampling and several applications of SPME in environmental analysis are also introduced.      

Modelling of adsorption kinetics and calibration curves of gaseous volatile organic compounds with adsorptive solid-phase microextraction fibre: toluene and acetone for indoor air applications

Solid-phase microextraction (SPME) with adsorptive Carboxen/PDMS fibre is a powerful sampling device for volatile organic compounds (VOCs) at trace levels in air. However, owing to competitive adsorption, quantification remains a challenging task. In this area, a theoretical model, based on Fick's laws and an extended Langmuir equation, is proposed to deal with the adsorption kinetics of acetone/toluene mixture on SPME fibre under various static extraction conditions. The semipredictive model is first used to determine the axial diffusion coefficients of analytes in the sampling device. The model is then tested with a complex VOC mixture, showing good agreement with experimental data.     

Comparison of two solid-phase microextraction methods for the quantitative analysis of VOCs in indoor air

Competitive adsorption on adsorptive solid-phase microextraction (SPME) fibres implies careful determination of operating conditions for reliable quantitative analysis of VOCs in indoor air. With this objective, two analytical approaches, involving non-equilibrium and equilibrium extraction, were compared. The average detection limit obtained for GC-MS analysis of nine VOCs by the equilibrium method is 0.2 μg m⁻³, compared with 1.9 μg m⁻³ with the non-equilibrium method. The effect of the relative humidity of the air on the calibration plots was studied, and shown to affect acetone adsorption only. Hence, the concentrations that can be accurately determined are up to 9 μmol m⁻³. The methods were then applied to indoor air containing different concentrations of VOCs. The non-equilibrium method, involving short extraction time, can be used for detection of pollution peaks whereas equilibrium extraction is preferable for measurement of sub-μg m⁻³ ground concentration levels.      

Multivariate classification and modeling in surface water pollution estimation

The present study deals with the application of self-organizing maps (SOM) and multiway principal-components analysis to classify, model, and interpret a large monitoring data set for surface water quality. The chemometric methods applied made it possible to reveal specific quality patterns of the chemical and biological parameters used to monitor the water quality (relation between water temperature, turbidity, hardness, colibacteria), seasonal impacts during the long period of observation and the relative independence on the spatial location of the sampling sites (water supply sources for the City of Trieste). Figure The schematic procedure for surface water pollution estimation supported by neural network-based classification and multivariate factor analysis     

Instrumental modification intended to save time,and volumes of sample and reagent solutions,in the atomic fluorescence spectrometric determination of mercury

Use of small membrane pumps, instead of peristaltic pumps, to introduce sample and reagent solutions into the spectrometer has several advantages in atomic fluorescence spectrometric determination of mercury. This simple modification results in a substantial saving in the time required for the measurements and so 90% of reagent solution volumes and 95% of sample solution volumes are saved, with a consequent decrease in the volume of waste generated. The sampling frequency is almost tripled, with no deterioration in sensitivity, which is similar to that obtained by use of peristaltic pumps. The relative standard deviation for ten consecutive measurements of a 1 μg L⁻¹ mercury solution was approximately 2%. Figure Small membrane pumps for the atomic fluorescene spectro metric determination of mercury     

Synthesis of polyacrylamide gel beads with electrostatic functional groups for the molecular imprinting of bovine serum albumin

Synthetic materials capable of recognizing proteins are important in separation, biosensors and biomaterials. In this study, bovine serum albumin-imprinted soft-wet polyacrylamide gel beads were prepared via inverse-phase suspension polymerization, using acrylamide and N,N′-methylene diacrylamide as polymeric matrix components and methacrylic acid as functional monomer. The adsorption study showed, through the imprinting process, that the imprinted gel beads had much higher adsorption capacity than the nonimprinted gel beads, and that the matching of the surface zeta-potential between the templates and the imprinted gel beads can enhance the imprinting effect. Adsorption kinetics indicated that the adsorption process could be described as an apparent first-order kinetic process for the gel beads. From the adsorption isotherm curve, we found that the adsorption of the imprinted gel beads was in agreement with the Langmuir adsorption model. Moreover, selectivity testing of the imprinted gel beads showed that imprinted gel beads exhibited good recognition for BSA as compared to the control protein. We speculate that the formation of complementary shapes and multiple-point electrostatic interactions between the imprinting cavities and the template proteins are the two factors that lead to the imprinting effect.      

Further research on the photo-SPME of triclosan

In this study the photoinduced degradation of triclosan has been investigated by photo-solid-phase microextraction (photo-SPME). In photo-SPME, photodegradation is carried out on the SPME fibre containing the target compound. Triclosan was extracted from aqueous solutions by use of polydimethylsiloxane SPME fibres and these were subsequently exposed to UV irradiation (power 8 W, wavelength 254 nm) for different times (from 2 to 60 min). The photodegradation kinetics of triclosan were investigated, the photoproducts generated were tentatively identified, and the photochemical behaviour of these products was studied by use of this on-fibre approach followed by gas chromatographic–mass spectrometric analysis. Eight photoproducts were tentatively identified, including chlorinated phenols, chlorohydroxydiphenyl ethers, 2,8-dichlorodibenzo-p-dioxin, and a possible dichlorodibenzodioxin isomer or dichlorohydroxydibenzofuran. The main photodegradation mechanisms were postulated and photodegradation pathways proposed. The effect of pH on triclosan degradation and on triclosan-to-dioxin conversion was also investigated. Triclosan degradation occurred, and generation of 2,8-dichlorodibenzo-p-dioxin was confirmed, throughout the pH range studied (from 3 to 9).      

Nitrogen effects in multi-matrix calibrations by radiofrequency glow discharge – optical emission spectrometry

A study about the effect of nitrogen in the calibration curves of a series of analytical emission lines has been carried out in this work. Fifteen reference materials with different matrices (Fe, Al, Zn, Cu and Ni) were used (three of these reference materials contain nitrogen in their composition) and plots of intensity versus the product “sputtering rate times element concentration” were constructed for emission lines of the analytes considered in this work (Al, Fe, Cu, Cr, C, Mo, Zn, Si, Ti and Ni). Two different fits were performed in each plot, first considering only the points corresponding to samples without nitrogen in their composition and secondly including all the points. The results show almost negligible differences in the emission yields calculated. On the other hand, a mixture of Ar containing 0.5% N₂ was employed to check if the nitrogen effect was present at higher concentrations than those expected in analysis when samples with high nitrogen concentrations are used. Differences between the slopes of the calibration curves with the Ar/N₂ and pure Ar discharges were obtained (up to 30%). A study of the molecular bands recorded in the spectra when nitrogen is present in the discharge and determination of the resulting interferences on the analytes have been performed. Figure Glow discharge powered with radiofrequency energy     

Four-way calibration applied to the simultaneous determination of folic acid and methotrexate in urine samples

First-, second- and third-order calibration methods were investigated for the simultaneous determination of folic acid and methotrexate. The interest in the determination of these compounds is related to the fact that methotrexate inhibits the body’s absorption of folic acid and prolonged treatment with methotrexate may lead to folic acid deficiency, and to the use of folic acid to cope with toxic side effects of methotrexate. Both analytes were converted into highly fluorescent compounds by oxidation with potassium permanganate, and the kinetics of the reaction was continuously monitored by recording the kinetics curves of fluorescence emission, the evolution with time of the emission spectra and the excitation–emission matrices (EEMs) of the samples at different reaction times. Direct determination of mixtures of both drugs in urine was accomplished on the basis of the evolution of the kinetics of EEMs by fluorescence measurements and four-way parallel-factor analysis (PARAFAC) or multiway partial least squares (N-PLS) chemometric calibration. The core consistency diagnostic (CORCONDIA) was employed to determine the correct number of factors in PARAFAC and the procedure converged to a choice of three factors, attributed to folic acid, methotrexate and to the sum of fluorescent species present in the urine.      

Solid-phase microextraction gas chromatography-mass spectrometry determination of fragrance allergens in baby bathwater

A method based on solid-phase microextraction (SPME) and gas chromatography-mass spectrometry (GC-MS) has been optimized for the determination of fragrance allergens in water samples. This is the first study devoted to this family of cosmetic ingredients performed by SPME. The influence of parameters such as fibre coating, extraction and desorption temperatures, salting-out effect and sampling mode on the extraction efficiency has been studied by means of a mixed-level factorial design, which allowed the study of the main effects as well as two-factor interactions. Excluding desorption temperature, the other parameters were, in general, very important for the achievement of high response. The final procedure was based on headspace sampling at 100 °C, using polydimethylsiloxane/divinylbenzene fibres. The method showed good linearity and precision for all compounds, with detection limits ranging from 0.001 to 0.3 ng mL⁻¹. Reliability was demonstrated through the evaluation of the recoveries in different real water samples, including baby bathwater and swimming pool water. The absence of matrix effects allowed the use of external standard calibration to quantify the target compounds in the samples. The proposed procedure was applied to the determination of allergens in several real samples. All the target compounds were found in the samples, and, in some cases, at quite high concentrations. The presence and the levels of these chemicals in baby bathwater should be a matter of concern. Baby exposure to fragrance allergens and other cosmetic ingredients through the daily bath     

Recent developments in solid-phase microextraction

The main objective of this review is to describe the recent developments in solid-phase microextraction technology in food, environmental and bioanalytical chemistry applications. We briefly introduce the historical perspective on the very early work associated with the development of theoretical principles of SPME, but particular emphasis is placed on the more recent developments in the area of automation, high-throughput analysis, SPME method optimization approaches and construction of new SPME devices and their applications. The area of SPME automation for both GC and LC applications is particularly addressed in this review, as the most recent developments in this field have allowed the use of this technology for high-throughput applications. The development of new autosamplers with SPME compatibility and new-generation metal fibre assemblies has enhanced sample throughput for SPME-GC applications, the latter being attributed to the possibility of using the same fibre for several hundred extraction/injection cycles. For LC applications, high-throughput analysis (>1,000 samples per day) can be achieved for the first time with a multi-SPME autosampler which uses multi-well plate technology and allows SPME sample preparation of up to 96 samples in parallel. The development and evolution of new SPME devices such as needle trap, thin-film microextraction and cold-fibre headspace SPME have offered significant improvements in performance characteristics compared with the conventional fibre-SPME arrangement. Figure Photo of a high-throughput multi-fibre SPME PAS autosampler     

Optimization of a sensitive method for the determination of nitro musk fragrances in waters by solid-phase microextraction and gas chromatography with micro electron capture detection using factorial experimental design

A solid-phase microextraction method (SPME) followed by gas chromatography with micro electron capture detection for determining trace levels of nitro musk fragrances in residual waters was optimized. Four nitro musks, musk xylene, musk moskene, musk tibetene and musk ketone, were selected for the optimization of the method. Factors affecting the extraction process were studied using a multivariate approach. Two extraction modes (direct SPME and headspace SPME) were tried at different extraction temperatures using two fiber coatings [Carboxen–polydimethylsiloxane (CAR/PDMS) and polydimethylsiloxane–divinylbenzene (PDMS/DVB)] selected among five commercial tested fibers. Sample agitation and the salting-out effect were also factors studied. The main effects and interactions between the factors were studied for all the target compounds. An extraction temperature of 100 °C and sampling the headspace over the sample, using either CAR/PDMS or PDMS/DVB as fiber coatings, were found to be the experimental conditions that led to a more effective extraction. High sensitivity, with detection limits in the low nanogram per liter range, and good linearity and repeatability were achieved for all nitro musks. Since the method proposed performed well for real samples, it was applied to different water samples, including wastewater and sewage, in which some of the target compounds (musk xylene and musk ketone) were detected and quantified. Figure Stardardized Pareto charts for the main effects and interactions     

Analysis of glycans in glycoproteins by diffusion-ordered nuclear magnetic resonance spectroscopy

Enzymatically cleaved glycans from sub-milligram quantities of erythropoietin (EPO) and ovalbumin have been analyzed, without further purification, by two-dimensional diffusion-ordered nuclear magnetic resonance spectroscopy. At NMR sample concentrations below 50 μmol L⁻¹ the major components of the oligosaccharide fractions could be distinguished by their anomeric proton chemical shift and their size-dependent diffusion coefficients. Figure ¹H NMR diffusion decay curves of anomeric protons in the EPO glycan fraction     

Static SPME sampling of VOCs emitted from indoor building materials: prediction of calibration curves of single compounds for two different emission cells

Solid-phase microextraction (SPME) is a powerful technique, easy to implement for on-site static sampling of indoor VOCs emitted by building materials. However, a major constraint lies in the establishment of calibration curves which requires complex generation of standard atmospheres. Thus, the purpose of this paper is to propose a model to predict adsorption kinetics (i.e., calibration curves) of four model VOCs. The model is based on Fick’s laws for the gas phase and on the equilibrium or the solid diffusion model for the adsorptive phase. Two samplers (the FLEC? and a home-made cylindrical emission cell), coupled to SPME for static sampling of material emissions, were studied. A good agreement between modeling and experimental data is observed and results show the influence of sampling rate on mass transfer mode in function of sample volume. The equilibrium model is adapted to quite large volume sampler (cylindrical cell) while the solid diffusion model is dedicated to small volume sampler (FLEC?). The limiting steps of mass transfer are the diffusion in gas phase for the cylindrical cell and the pore surface diffusion for the FLEC?. In the future, this modeling approach could be a useful tool for time-saving development of SPME to study building material emission in static mode sampling.     

A novel microfluidic flow-injection analysis device with fluorescence detection for cation sensing. Application to potassium

A microfabricated device has been developed for fluorimetric detection of potassium ions without previous separation. It is based on use of a fluorescent molecular sensor, calix–bodipy, specially designed to be sensitive to and selective for the target ion. The device is essentially made of a Y-shape microchannel moulded in PDMS fixed on a glass substrate. A passive mixer is used for mixing the reactant and the analyte. The optical detection arrangement uses two optical fibres, one for excitation by a light-emitting diode, the other for collection of the fluorescence. This system enabled the flow-injection analysis of the concentration of potassium ions in aqueous solutions with a detection limit of 0.5 mmol L⁻¹ and without interference with sodium ions. A calibration plot was constructed using potassium standard solutions in the range 0–16 mmol L⁻¹, and was used for the determination of the potassium content of a pharmaceutical pill. Figure Photography of the microfluidic channel showing the ridges in the PDMS substrate at the top of the channel     

Nuclear magnetic resonance of mass-limited samples using small RF coils

Figure Schematic diagram of a typical arrangement used for hyphenating chemical microseparations (e.g. capillary HPLC, CE, or CEC) with microcoil NMR detection     

Gravitation-driven stress-reduced cell handling

We present a simple lab-on-chip device for handling small samples of delicate cells, e.g. stem cells. It uses a combination of sedimentation and dielectrophoresis. The transport of cells is driven by gravitation. Dielectrophoresis uses radio-frequency electric fields for generating particle-selective forces dependent on size and polarisability. Electrodes along the channels hold particles and/or cells in a defined position and deflect them towards different outlets. The absence of external pumping and the integration of injection and sampling ports allow the processing of tiny sample volumes. Various functions are demonstrated, such as contact-free cell trapping and cell/particle sorting. Pairs of human cells and antibody-coated beads, as they are formed for T cell activation, are separated from unbound beads. The cells experience only low stress levels compared with the stress levels in dielectrophoresis systems, where transport depends on external pumping. Our device is a versatile yet simple tool that finds applications in cellular biotechnology, in particular when an economic solution is required. Figure A simple gravitation-driven lab-on-chip device for the separation of mixed populations of microparticles or cells by negative dielectrophoresis.     

Chemosensors in environmental monitoring: challenges in ruggedness and selectivity

Environmental analysis is a potential key application for chemical sensors owing to their inherent ability to detect analytes on-line and in real time in distributed systems. Operating a chemosensor in a natural environment poses substantial challenges in terms of ruggedness, long-term stability and calibration. This article highlights current trends of achieving both the necessary selectivity and ruggedness: one way is deploying sensor arrays consisting of robust broadband sensors and extracting information via chemometrics. If using only a single sensor is desired, molecularly imprinted polymers offer a straightforward way for designing artificial recognition materials. Molecularly imprinted polymers can be utilized in real-life environments, such as water and air, aiming at detecting analytes ranging from small molecules to entire cells. Figure       

Characterization of volatile organic compounds and odors by in-vivo sampling of beef cattle rumen gas, by solid-phase microextraction, and gas chromatography–mass spectrometry–olfactometry

Volatile organic compounds (VOCs) and odors in cattle rumen gas have been characterized by in-vivo headspace sampling by solid-phase microextraction (SPME) and analysis by gas chromatography–mass spectrometry–olfactometry (GC–MS–O). A novel device enabling headspace SPME (HS-SPME) sampling through a cannula was designed, refined, and used to collect rumen gas samples from steers. A Carboxen–polydimethylsiloxane (PDMS) fiber (85 μm) was used for SPME sampling. Fifty VOCs from ten chemical groups were identified in the rumen headspace. The VOCs identified had a wide range of molecular weight (MW) (34 to 184), boiling point (−63.3 to 292 °C), vapor pressure (1.05 × 10⁻⁵ to 1.17 × 10² Pa), and water solubility (0.66 to 1 × 10⁶ mg L⁻¹). Twenty-two of the compounds have a published odor detection thresholds (ODT) of less than 1 ppm. More than half of the compounds identified are reactive and have an estimated atmospheric lifetime of <24 h. The amounts of VFAs, sulfide compounds, phenolic compounds, and skatole, and the odor intensity of VFAs and sulfide compounds in the rumen gas were all higher after feeding than before feeding. These results indicate that rumen gases can be an important potential source of aerial emissions of reactive VOCs and odor. In-vivo sampling by SPME then GC–MS–O analysis can be a useful tool for qualitative characterization of rumen gases, digestion, and its relationship to odor and VOC formation. Figure Modified cannula for rumen gas sampling with SPME     

The dosage of small volumes for chromatographic quantifications using a drop-on-demand dispenser system

A commercially available piezo-driven drop-on-demand dispenser was tested for its suitability for the preparation of analytical calibration standards and in a standard addition approach prior to quantitative ultra performance liquid chromatography (UPLC) analysis of homoserines. The reproducibility of the drop-on-demand dosing system was tested and the verification of the droplet volume was performed by preparing a series of 1.0 mg/L caffeine standard solutions from a 1,000.0 mg/L stock solution and analysis of the concentrations obtained by UPLC. The reproducibility was better than 1% relative standard deviation from measurement to measurement and the highest was 1.6% from day to day. The results were compared with the conventional way of generating standard solutions (pipetting). A gravimetric method and a photography-based method for the determination of the average single droplet volume were compared and found to be in very good agreement. The system was employed for the quantification of N-decanoyl homoserine by standard addition in bacterial culture supernatants containing this analyte. The agreement with conventional quantification techniques was high. The paper shows the feasibility of the approach with advantages in low sample and solvent volume consumption and very good reproducibility and reliability combined with easy usage. Figure Ejected droplet, 60 μs after application of the pulse