Theoretical and experimental comparison of the colloid stability of two polystyrene latexes with different sign and value of the surface charge

 The purpose of this paper is to apply the classical DLVO theory to explain the colloid stability of two model colloids with similar size and different sign and value of the surface charge. For this comparison the hydrodynamic interaction and the presence of hydration forces (extended DLVO theory) have been taken into account. The experimental stability factor and the experimental doublet rate constant in diffusion conditions were compared with those evaluated theoretically. The mathematical treatment permits an easy evaluation and interpretation of the different adjustable parameters such as the Hamaker constant, diffuse layer potential and the hydration layer thickness. The theoretical and experimental comparison shows that the “extended DLVO theory” only permits to explain the stability curves Log[W]/Log[KBr] in a semiquantitative way by using, for the evaluation of the total interaction potential V _T, a value of the Hamaker constant (A) similar to the classical theoretical one for polystyrene particles dispersed in water. In the case of the anionic latex, it was necessary to admit the presence of a hydration layer of a thickness similar to the radius of the hydrated/dehydrated counterion. On the other hand, by using the experimental doublet rate constant in diffusion conditions, we obtain a lower value of the Hamaker constant (A), but within the range of the A values usually found in previous studies. Received: 8 September 1997 Accepted: 8 January 1998     

Studies of laser damage morphology reveal subsurface feature in fused silica

We present a detailed study of the morphology of 355 nm (6 ns) laser‐induced damage in fused silica polished by CeO₂ solution. We see two distinct damage morphologies: the gray haze and the crater. The gray haze, consisting of a high density of pin‐points, appears at the fluence higher than ～10 J/cm², and the crater forms at ～≥ 22 J/cm². The size and depth of the pin‐points are much smaller than the crater. The difference in the two morphologies is attributed to the property of the absorber and its surrounding material in the redeposition layer, which is different from those in the subsurface damage layer. The damage growth characteristics of the two morphologies are measured, and the size of crater increases under successive shots, but the size of the gray haze remains constant. The growth of the crater is attributed to the existence of crack around the absorber, which is observed by SEM imaging. On the basis of the above analysis, the schematic diagram of subsurface feature is discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Interaction between silica nanoparticles in binary liquid mixtures: the effect of polarity on adhesion

In order to explore the relationship between selective liquid sorption and the stability of dispersed silica particles, we carried out studies in ethanol–cyclohexane binary mixtures as well as in partially miscible 1-butanol–water mixtures. In ethanol–cyclohexane, adsorption excess isotherms were determined on hydrophilic and hydrophobic aerosil particles. The volume and the thickness of the adsorption layer were derived from the isotherms. Knowing the layer thickness and Hamaker constants, interparticle interaction potentials were calculated at various mixture compositions. At low ethanol concentration, where hydrophilic surfaces are considerably enriched in ethanol, interparticle interaction is enhanced and the high shear stress calculated from rheological measurements indicates the development of a three-dimensional network of aggregated particles. In contrast, hydrophobic aerosil particles in ethanol–cyclohexane approach each other without ensuing aggregation because interparticle interactions are weak, a fact well demonstrated by rheological measurements. It was also established that interactions between silica particles with hydrophilic surfaces are weak in butanol–water mixtures. Since water is preferentially adsorbed on the surface of hydrophilic particles to the azeotropic composition (\( x_{\rm{1}}^{\rm{a}} \)=0.62), within this wide composition range the Hamaker constant of the interfacial layer is identical with that of water.     

Reversible uptake of water on NaCl nanoparticles at relative humidity below deliquescence point observed by noncontact environmental atomic force microscopy

The behavior of NaCl nanoparticles as a function of relative humidity (RH) has been characterized using non-contact environmental atomic force microscopy (e-AFM) to measure the heights of particles deposited on a prepared hydrophobic surface. Cubic NaCl nanoparticles with sides of 35 and 80 nm were found to take up water reversibly with increasing RH well below the bulk deliquescence relative humidity (DRH) of 75% at 23(°)C, and to form a liquid-like surface layer of thickness 2 to 5 nm, with measurable uptake (>2 nm increase in particle height) beginning at 70% RH. The maximum thickness of the layer increased with increasing RH and increasing particle size over the range studied. The liquid-like behavior of the layer was indicated by a reversible rounding at the upper surface of the particles, fit to a parabolic cross-section, where the ratio of particle height to maximum radius of curvature increases from zero (flat top) at 68% RH to 0.7 ± 0.3 at 74% RH. These observations, which are consistent with a reorganization of mass on the solid NaCl nanocrystal at RH below the DRH, suggest that the deliquescence of NaCl nanoparticles is more complex than an abrupt first-order phase transition. The height measurements are consistent with a phenomenological model that assumes favorable contributions to the free energy of formation of a liquid layer on solid NaCl due both to van der Waals interactions, which depend partly upon the Hamaker constant, A(film), of the interaction between the thin liquid film and the solid NaCl, and to a longer-range electrostatic interaction over a characteristic length of persistence, ξ; the best fit to the data corresponded to A(film)= 1 kT and ξ = 2.33 nm.     

Dispersion behavior of oleic acid in aqueous media: from micelles to emulsions

 Dispersion behavior of aqueous solutions containing oleic acid (RH), sodium oleate (R^-Na⁺), and NaCl was investigated by turbidity and dynamic light-scattering measurements. Changes of the size of scattering particles in solution composed of 1 mM oleic acid and 100 mM NaCl were traced as a function of the degree of ionization α, in terms of radius of the equivalent hydrodynamic sphere. Large associated micelles with a radius of 30 nm appeared by a slight decline of α and existed at α higher than 0.75. They were responsible for the three-phase equilibrium (solution, micelle and aggregated micelle, and acid–soap, (R^-Na⁺)₃RH) characterized by a constant pH of 9.75. The appearance of a new phase, (R^-Na⁺)₃RH, contributed to increase both the turbidity and averaged scattering particle size. As the breakdown of the three-phase equilibrium, radius of scattering particles increased significantly. Finally, oleic acid oil droplets were separated from aqueous phase at low α. When the system was buffered by tris(hydroxymethyl)aminomethane (Tris), scattering particles with a weight-averaged hydrodynamic radius of 75 nm existed in a wide range of α from 0.85 to 0.65. In Tris buffered solution, turbidity formation was induced by the increase in the number of aggregated particles. Received: 12 November 1996 Accepted: 4 April 1997     

Thickness and morphology of polyelectrolyte coatings on silica surfaces before and after protein exposure studied by atomic force microscopy

Analyte–wall interaction is a significant problem in capillary electrophoresis (CE) as it may compromise separation efficiencies and migration time repeatability. In CE, self-assembled polyelectrolyte multilayer films of Polybrene (PB) and dextran sulfate (DS) or poly(vinylsulfonic acid) (PVS) have been used to coat the capillary inner wall and thereby prevent analyte adsorption. In this study, atomic force microscopy (AFM) was employed to investigate the layer thickness and surface morphology of monolayer (PB), bilayer, (PB-DS and PB-PVS), and trilayer (PB-DS-PB and PB-PVS-PB) coatings on glass surfaces. AFM nanoshaving experiments providing height distributions demonstrated that the coating procedures led to average layer thicknesses between 1 nm (PB) and 5 nm (PB-DS-PB), suggesting the individual polyelectrolytes adhere flat on the silica surface. Investigation of the surface morphology of the different coatings by AFM revealed that the PB coating does not completely cover the silica surface, whereas full coverage was observed for the trilayer coatings. The DS-containing coatings appeared on average 1 nm thicker than the corresponding PVS-containing coatings, which could be attributed to the molecular structure of the anionic polymers applied. Upon exposure to the basic protein cytochrome c, AFM measurements showed an increase of the layer thickness for bare (3.1 nm) and PB-DS-coated (4.6 nm) silica, indicating substantial protein adsorption. In contrast, a very small or no increase of the layer thickness was observed for the PB and PB-DS-PB coatings, demonstrating their effectiveness against protein adsorption. The AFM results are consistent with earlier obtained CE data obtained for proteins using the same polyelectrolyte coatings.     

Transition metal-chelating surfactant micelle templates for facile synthesis of mesoporous silica nanoparticles

Highly ordered mesoporous silica nanoparticles with tunable morphology and pore-size are prepared by the use of a transition metal-chelating surfactant micelle complex using Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ ions. These metal ions formed a metal-P123 micelle complex in an aqueous solution, while the metal ions are chelated to the hydrophilic domain such as the poly(ethylene oxide) group of a P123 surfactant. The different complexation abilities of the utilized transition metal ions play an important role in determining the formation of nano-sized ordered MSNs due to the different stabilization constant of the metal-P123 complex. Consequently, from a particle length of 1700 nm in the original mesoporous silica materials, the particle length of ordered MSNs through the metal-chelating P123 micelle templates can be reduced to a range of 180–800 nm. Furthermore, the variation of pore size shows a slight change from 8.8 to 6.6 nm. In particular, the Cu²⁺-chelated MSNs show only decreased particle size to 180 nm. The stability constants for the metal-P123 complex are calculated on the basis of molar conductance measurements in order to elucidate the formation mechanism of MSNs by the metal-chelating P123 complex templates. In addition, solid-state ²⁹Si, ¹³C-NMR and ICP-OES measurements are used for quantitative characterization reveal that the utilized metal ions affect only the formation of a metal-P123 complex in a micelle as a template.     

Application of Thermal Lens Spectrometry to the Study of Chemical Adsorption in a Layer of a Nafion Solid Electrolyte

Thermal lens spectrometry was used to study Langmuir–Blodgett films of a weakly absorbing Nafion polyelectrolyte membrane on the surface of inert polyethylene terephthalate (PET) and glass substrates and to estimate the amount of Nafion (number of layers) using a change in the thermal characteristics of the sample. The sensitivity of thermal lens measurements at the wavelength of the exciting radiation 532.0 nm is comparable to that of solid-state spectrophotometry in the region of the maximum absorbance of Nafion (275 nm). However, the high locality of thermal lens spectrometry (the area of the signal generation zone is 100 μm²) ensures the estimation of the uniformity of the deposition of the polyelectrolyte layer. To increase the absorbance of the layer of the applied polyelectrolyte, the latter is saturated with a colored compound (ferroin). The adsorption of ferroin into the Nafion layer on the PET surface was confirmed; the absorbance of ferroin in the Nafion layer is in the range of 1 × 10–5^–5 × 10^–4 units of absorbance, which corresponds to the surface concentration of ferroin 1 × 10^–11–4 × 10^–10 mol/cm².     

Experimental determination of the static permittivity of extremely thin liquid layers of water dependent on their thickness

 A new method for determining the static permittivity (dielectric constant) of extremely thin liquid interlayers is illustrated. A special condenser, which can be filled with a test liquid, is constructed. Both condenser plates – one planar, the other spherically curved – are made of vitreous carbon and are supplied with a high-grade politure. In order to adjust plate separation distances from 10 nm up to about some μm the planar plate can be easily shifted by a piezoelectric translation stage; the plate separation is monitored by an optical system supported by displacement transducers. The measuring frequency was kept constant at 1 MHz. Water was chosen as the test liquid. At 19.8 °C thin water layers having thickness smaller than 0.3 μm exhibit a decrease of the dielectric constant. The experimental data are consistent with a decay length α^-1 of the order of 1 nm which in view of the underlying crude model must be regarded as approximative. Received: 28 May 1996 Accepted: 30 September 1996     

Diterpenoic Acids Analysis Using a Coupled TLC-Surface-Enhanced Raman Spectroscopy System

Hyphenation of thin layer chromatography (TLC) with surface-based spectral methods requires a homogeneous surface for direct and quantitative analysis on the chromatographic plate after separation. Since most chromatographic materials do not produce strong background signals in Raman spectroscopy (RS) or surface-enhanced RS (SERS), we tested the suitability of two different chromatographic substrates and one interface for coupling SERS with TLC. This was carried out by using a chromatographic thin layer, specially produced for RS measurements, and a monolithic silica thin layer. A typical TLC plate with a modified aluminium backplate foil on one side was used as an interface. Three biologically active diterpenes, namely gibberellic acid (GA), abietic acid (AA) and kaurenoic acid (KA), were used as test analytes. Stock solutions were applied directly onto the surface, followed by the addition of silver colloid and measurements were taken by SERS. The strongest signal (excitation at 514.5 nm) was obtained for GA using a Raman treated thin layer where the enhancement factor value was determined to be 10². Several fundamental Raman bands for GA were found at 1622, 1593, 1570, 1542, 1366 and 1236 cm⁻¹. When the monolithic silica layer was used, no useful SERS signals were observed. The SERS spectra on modified aluminium backplate for AA and GA were quite similar and no SERS spectrum was obtained for KA. Future research will be concerned towards the use of nanostructured surfaces for SERS analysis. An erratum to this article can be found at     

Gas chromatography on 10 um silica particles

Summary Porous silica microparticles designed for modern liquid chromatography have proven effective in gas chromatography. Columns of 35–50 cm gave plate heights as low as 3.3 particle diameters and speeds of 2400 theoretical plates per second or 500 effective theoretical plates per second. Inlet pressures up to 70 atmospheres were required using hydrogen as carrier gas. The particles as received were too retentive for fast chromatography and gave asymmetric peaks. A coating of fluorosilicone oil overcame both problems. Other coatings were less effective. Bonded phases proved less satisfactory on both counts and also gave substantially less efficient columns and greater flow resistance. Column efficiency and flow resistance were sharply dependent on physical properties of the particles. The most efficient packing was clearly spherical particles of 5–10 μm diameter with narrow size distribution, pore diamters about 50 nm, BET surface areas of 25–50 m²/g and surfaces modified with trifluoropropyl silicone. A six-component hydrocarbon sample was separated in 33 s with a resolution of 4 for the most difficult pair and in 2.6 s with a minimal resolution. Performance was limited by end effects and by available pressure so that much better performance can be expected from longer columns and higher pressures.     

Electrophoresis of megaDalton proteins inside colloidal silica

With the growth of the biopharmaceutical industry, there is a need for rapid size‐analysis of proteins on the megaDalton scale. The large pore sizes needed for such separations cannot be easily reached by pushing the current limits of size‐exclusion chromatography or gel electrophoresis. The concept detailed here is the formation of arbitrarily wide pores by packing nonporous colloidal silica in capillaries. This method can be called packed‐capillary electrophoresis, or “pCE”. Electrophoresis of protein standards (11–155 kDa) by pCE, using 345 nm diameter particles in 100 μm diameter capillaries, gives 2x higher resolution than a typical PAGE gel in 1/6 of the time. The electropherograms show that pCE is highly efficient, with half‐micrometer plate heights for all seven standards, giving 10⁵ plates for a 50 mm length. The large pore radius of 65 nm enables baseline resolution of proteins of 0.72, 1.048 and 1.236 MDa in less than 15 min. The short separation time of pCE is attributed to the absence of small pores that restrict protein migration in gels. The pCE separation is applied to the analysis of a stressed pharmaceutical‐grade IgG4 sample, giving unprecedented baseline resolution of monomer, dimer, trimer and tetramer in less than 10 min.     

S-layer templated bioinspired synthesis of silica

The current understanding of the molecular mechanisms involved in the bioinspired formation of silica structures laid foundation for investigating the potential of the S-layer protein SbpA from Lysinibacillus sphaericus CCM 2177 as catalyst, template and scaffold for the generation of novel silica architectures. SbpA reassembles into monomolecular lattices with square (p4) lattice symmetry and a lattice constant of 13.1 nm. Silica layers on the S-layer lattice were formed using tetramethoxysilane (TMOS) and visualized by transmission electron microscopy. In situ quartz crystal microbalance with dissipation monitoring (QCM-D) measurements showed the adsorption of silica in dependence on the presence of phosphate in the silicate solution and on the preceding chemical modification of the S-layer. An increased amount of precipitated silica could be observed when K₂HPO₄/KH₂PO₄ was present in the solution (pH 7.2). Further on, independent of the presence of phosphate the silica deposition was higher on S-layer lattices upon activation of their carboxyl groups with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) compared to native S-layers or EDC treated S-layers when the activated carboxyl groups were blocked with ethylene diamine (EDA). Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy revealed the formation of an amorphous silica gel (SiO₂)_x·yH₂O on the S-layer. The silica surface concentrations on the S-layer was 4 × 10^?9 to 2 × 10^?8 mol cm^?2 depending on the modification of the protein layer and corresponded to 4–21 monolayers of SiO₂.     

Electrophoretic measurement of water charge density and ion hydration

Water exchange between bulk water and water-ion complexes will be at equilibrium when the charge density of the complex surface equals the charge density of bulk water, producing a constant radius water-ion complex. This complex will migrate in an electric field at a velocity proportional to the complex radius. CE velocity is the sum of the complex charge-dependent velocity and the buffer electro-osmotic flow. Simultaneous use of both a base (1.07 mM imidazole) and an acid (1.5 mM MOPS) buffer negates EOF at pH 7.4. Electric fields below 300 V/cm (potassium, calcium) and 400 V/cm (magnesium) yield migration velocities with no dehydration of the water-ion complexes. The number of waters per complex increase with the ion charge density: K⁺ 1.90, Ca⁺⁺ 5.90, Mg⁺⁺ 6.59 waters/ion. The charge densities of the complexes are similar: K⁺ 1.24, Ca⁺⁺ 1.43, Mg⁺⁺ 1.21 e/nm², for an average bulk water charge density of 1.29 ± 0.11 (SD) e/nm². The addition of 0.1% Triton increases the number of waters for Mg⁺⁺ to 25.33 and lowers the charge density to 0.497 e/nm². High electric field dehydration shows that calcium will be fully dehydrated at 638.3 V/cm and magnesium fully dehydrated at 925.5 V/cm, which occur at 6.15 and 5.78 nm from the membrane. Dehydrated magnesium will then bind to calcium channels leading to decreased smooth muscle activation.     

Van der Waals attraction between Stöber silica particles in a binary solvent system

Hydrophilic Stöber silica particles are stable in ethanol, but flocculation may be induced by the addition of sufficient cyclohexane. Low-shear rheological measurements indicated non-Newtonian behaviour beyond the critical cyclohexane concentration. The thickness and composition of the solvation layer around the particles were calculated from the adsorption excess isotherm on the basis of a multilayer adsorption model. The composition dependences of the Hamaker constants of the dispersion medium and the adsorption layer were obtained from optical dispersion measurements. A single-sheet, hard-sphere model predicted a weak van der Waals attraction in the ethanolic regime, but a strong attraction in the cyclohexane-rich region, in good accordance with the rheological properties of the dispersions and visual observations.     

A Three-Dimensional Velocity-Map Imaging Setup Designed for Crossed Ion-Molecule Scattering Studies

In this study, we report the design and simulation of an electrostatic ion lens system consisting of 22 round metal plates. The opening of the extractor plate is covered with metal mesh, which is for shielding the interaction region of the lens system from the high DC voltages applied to all other plates than the repeller and extractor plates. The Simion simulation shows that both velocity-mapping and time focusing can be achieved simultaneously when appropriate voltages are applied to each of the plates. This makes the ion lens system be able to focus large ionic volumes in all three dimensions, which is an essential requirement for crossed ion-molecule scattering studies. A three-dimensional ion velocity measurement system with multi-hit and potential multi-mass capability is built, which consists of a microchannel plate (MCP), a P47 phosphor screen, a CMOS camera, a fast photomultiplier tube (PMT), and a high-speed digitizer. The two velocity components perpendicular to the flight axis are measured by the CMOS camera, and the time-of-flight, from which the velocity component along the flight axis can be deduced, is measured by the PMT. A Labview program is written to combine the two measurements for building the full three-dimensional ion velocity in real time on a frame-by-frame basis. The multi-hit capability comes from the fact that multiple ions from the camera and PMT in the same frame can be correlated with each other based on their various intensities. We demonstrate this by using the photodissociation of CH₃I at 304 nm.     

Glucose diffusivity and porosity in silica hydrogel based on organofunctional silanes

The silica hydrogels prepared at physiological conditions were characterized with respect to the glucose diffusion properties and the porosity by employing various approaches. A diffusion coefficient of glucose in silica hydrogel in the range of 2 × 10⁻¹⁰ m² s⁻¹ was determined by two complementary techniques based on the glucose ingress and egress, respectively. The confocal laser scanning microscopy in a time-lapse imaging mode was employed to measure the ingress of fluorescently labeled glucose analogue inside the hydrogel. In addition, a method for direct glucose release from the hydrogel was established. The simple diffusion model based on the Fickian diffusion and Ritger–Peppas theory were employed for evaluation of diffusion coefficients, respectively. The BET analysis and permeation of fluorescently labeled dextrans of various molecular weights were used to characterize the porosity of silica hydrogel. The radius of pores accessible for diffusion of dextran molecules in prepared silica hydrogel ranges between 1 and 6 nm.     

Evaluation of Thin Film Titanium Nitride Electrodes for Electroanalytical Applications

Titanium nitride is a hard and inert conducting material that has yet not been widely used as electrode material for electroanalytical applications although there are highly developed protocols available to produce well adherent micro and nanostructured electrodes. In this paper the possibilities of using titanium nitride thin films for electroanalytical applications is investigated. Scanning electrochemical microscope (SECM) was used for analysis of the redox kinetics of a selected fast redox couple at thin films of titanium nitride (TiN) in different thicknesses. The investigation was carried out by approaching an amperometric ultramicroelectrode (UME) to the TiN film while the soluble redox couple (ferrocenemethanol/ferrociniummethanol) served as mediator in a SECM configuration. The substrate was biased at a potential so that it rereduces the species being produced at the UME, thus controlling the feedback effect. Normalized current–distance curves were fitted to the theoretical model in order to find the apparent heterogeneous standard rate constant (k°) at the sample. The data are further supported by structural investigation of the TiN films using scanning force microscopy and X‐ray photoelectron spectroscopy. It was found that the kinetics are little influenced by prolonged storage in air. The heterogeneous standard rate constants in 2 mM ferrocenemethanol were (0.73±0.05)×10^?3 cm s^?1 for 20 nm TiN thin layer, (1.5±0.2)×10^?3 cm s^?1 for 100 nm TiN thin layer and (1.3±0.2)×10^?3 cm s^?1 for 300 nm TiN thin layer after prolonged storage in air. Oxidative surface treatment (in order to remove organic adsorbates) decreased the kinetics in agreement with a thicker oxide layer on the material. The results suggest that their direct use for amperometric detection of reversible redox systems in particular at miniaturized configurations may be advantageous.     

Layer-by-layer thin film of reduced graphene oxide and gold nanoparticles as an effective sample plate in laser-induced desorption/ionization mass spectrometry

This work demonstrated a simple platform for rapid and effective surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS) measurements based on the layer structure of reduced graphene oxide (rGO) and gold nanoparticles. A multi-layer thin film was fabricated by alternate layer-by-layer depositions of rGO and gold nanoparticles (LBL rGO/AuNP). The flat and clean two-dimensional film was served as the sample plate and also functioned as the matrix in SALDI-TOF MS. By simply one-step deposition of analytes onto the LBL rGO/AuNP sample plate, the MS measurements of various homogeneous samples were ready to execute. The optimization of MS signal was reached by the variation of the layer numbers of rGO and gold nanoparticles. Also, the small molecules including amino acids, carbohydrates and peptides were successfully analyzed in SALDI-TOF MS using the LBL rGO/AuNP sample plate. The results showed that the signal intensity, S N⁻¹ ratio and reproducibility of SALDI-TOF spectra have been significantly improved in comparison to the uses of gold nanoparticles or α-cyano-4-hydroxy-cinnamic acid (CHCA) as the assisted matrixes. Taking the advantages of the unique properties of rGO and gold nanoparticles, the ready-to-use MS sample plate, which could absorb and dissipate laser energy to analytes quite efficiently and homogeneously, has shown great commercial potentials for MS applications.     

A new porous-layer activated-charcoal-coated fused silica fiber: Application for determination of BTEX compounds in water samples using headspace solid-phase microextraction and capillary gas chromatography

Summary Extra-fine powdered activated charcoal has been used as stationary phase (coating layer) in solid-phase microextraction (SPME). The efficiency and reliability of the prepared device have been investigated for the extraction of some volatile organic compounds such as benzene, toluene, ethylbenzene and xylene isomers (BTEX) from the headspace of water samples. Monitoring of the extracted compounds and further quantitative analysis of the real samples have been performed by capillary GC-FID. Effects of several factors such as temperature, addition of salt, and stirring speed on extraction efficiency and exposure time have been studied. Under optimum conditions, extraction recoveries for these compounds from 50 mL water were >95%. The calibration graphs were linear in the range 5 to 10⁴ pg mL⁻¹ and the detection limit for each BTEX compound was 1.5–2 pg mL⁻¹. The results obtained by use of this porous layer activated charcoal (PLAC)-coated fiber have also been compared with results reported in the literature by use of a polydimethylsiloxane (PDMS)-coated fiber. Presented at the 21^st ISC held in Stuttgart, Germany, 15^th–20^th September, 1996