Mary K. Carroll

On-line Determination of Sulfide by the ‘Methylene Blue Method’ With Diode-laser-based Fluorescence Detection

The application of an inexpensive, compact, solid-state, fluorescence-based detector for flow injection analysis to the determination of sulfide by the ‘Methylene Blue Method’, viz., production of Methylene Blue (MB) via the oxidative coupling of sulfide with N,N-dimethyl-p- phenylenediamine (DMPD) in the presence of iron(III), is described. The use of fluorescence-based detection allows the reaction to be performed on-line under less corrosive, albeit sub-optimum, reaction conditions. The detector uses a 670 nm diode laser as the excitation source and inexpensive photodiodes as detector elements; a color filter (used to block scattered laser light) is the only other optical component of the detector. The fluorescence signal resulting from the MB formed on-line is linear over the range 0.75–15.0 mg l–1 injected sulfide, with a limit of detection of 0.08 mg l–1 injected sulfide when 9.0 M H2SO4 is used in the DMPD carrier stream, and 1–2 mg l–1 when less acidic carrier streams are employed. Use of this method for analysis of sulfide unknowns in aqueous solution and in a simulated waste water matrix demonstrates that unknown sulfide samples can be analysed reproducibly.

Hydrophobic Silica Aerogels: Review of Synthesis, Properties and Applications

There are many applications for which a material must be water-resistant. Silica aerogels can have unusual properties, including high surface area, low density, low thermal conductivity, and good optical translucency. This combination of properties makes hydrophobic silica aerogels attractive materials for use in applications ranging from transparent insulation systems to drug delivery platforms. These aerogel materials have been prepared using a wide variety of techniques, including incorporation of silica precursors with non-polar substituents into the sol–gel matrix and surface modification of the matrix following gelation. In this chapter we describe the different aerogel synthesis methods, present a discussion of techniques for measuring hydrophobicity and review the extensive literature on hydrophobic silica aerogels, including information on their physical properties and applications.

Interactions between Methylene Blue and Sodium Dodecyl Sulfate in Aqueous Solution Studied by Molecular Spectroscopy

The interactions of methylene blue (MB, a cationic redox indicator and biological stain) and sodium dodecyl sulfate (SDS, a micelle-forming, anionic surfactant) in aqueous solution have been examined by using Rayleigh scattering, UV-visible absorption, and fluorescence spectroscopy. At SDS concentrations significantly below the critical micelle concentration (cmc), MB forms noncovalent dimers and aggregates with SDS that scatter light but do not fluoresce. For solutions containing 1 μM MB and < 3–5 mM SDS, shifts in the absorption spectrum characteristic of the formation of MB H-aggregates are noted. There appears to be little effect on the fluorescence emission spectrum, indicating that these MB aggregates do not fluoresce appreciably. At and above the known SDS cmc, MB is observed to interact with the micelles. The MB excited-state fluorescence lifetime (380 ps) remains constant until SDS micelles form, then increases to 615 ps. The MB rotational reorientation time similarly increases from 105 to 500 ps between 6 and 8 mM SDS. This finding suggests that the MB is encountering, on average, a microenvironment in the SDS micelles that is 5-fold more viscous than liquid water or the molar volume of the MB/SDS species that is reorienting is 5-fold larger than MB in water.

Flow injection study of the Belousov–Zhabotinskii reaction

It is shown that the Belousov–Zhabotinskii oscillating reaction can be produced and sustained over long periods in a flow system with high precision and without the usual disturbing features of spatial inhomogeneity and gas evolution. However, the reaction can be perturbed, as measured by the induction time, by the addition of species which react with bromide, a critical intermediate in the reaction. The analytical possibilities for the determination of metals at ppb concentrations are demonstrated.

Preparing silica aerogel monoliths via a rapid supercritical extraction method.

A procedure for the fabrication of monolithic silica aerogels in eight hours or less via a rapid supercritical extraction process is described. The procedure requires 15-20 min of preparation time, during which a liquid precursor mixture is prepared and poured into wells of a metal mold that is placed between the platens of a hydraulic hot press, followed by several hours of processing within the hot press. The precursor solution consists of a 1.0:12.0:3.6:3.5 x 10(-3) molar ratio of tetramethylorthosilicate (TMOS):methanol:water:ammonia. In each well of the mold, a porous silica sol-gel matrix forms. As the temperature of the mold and its contents is increased, the pressure within the mold rises. After the temperature/pressure conditions surpass the supercritical point for the solvent within the pores of the matrix (in this case, a methanol/water mixture), the supercritical fluid is released, and monolithic aerogel remains within the wells of the mold. With the mold used in this procedure, cylindrical monoliths of 2.2 cm diameter and 1.9 cm height are produced. Aerogels formed by this rapid method have comparable properties (low bulk and skeletal density, high surface area, mesoporous morphology) to those prepared by other methods that involve either additional reaction steps or solvent extractions (lengthier processes that generate more chemical waste).The rapid supercritical extraction method can also be applied to the fabrication of aerogels based on other precursor recipes.

Convenient Fiber-Optic-Based Sample Cell for Shpol'skii and Low-Temperature Phosphorescence Spectrometry

A sample cell for observing the Shpolskii effect at 77 K is described and analytically assessed. The cell employs fiber-optic light guides to transport excitation and emission radiation. The system is compact, inexpensive, and simple to construct from commercially available laboratory components, and it alleviates several problems inherent in conventional refrigerated-cell designs. Detection limits for anthracene, coronene, and pyrene obtained with the sample cell are 8.8 × 10−8 M, 8.4 × 10−7 M, and 3.5 × 10−7 M, respectively. The linear dynamic range for each compound is 2 to 3 orders of magnitude.

Aerogels as Platforms for Chemical Sensors

Sensing of chemical species in air, in water and in other solvents is important for a wide variety of applications, including but not limited to monitoring chemical species that might have environmental, health, forensic, manufacturing, or security implications. The unusual properties of aerogels – very high surface area, high porosity, low density – render them particularly appealing for sensing applications. In this chapter, we survey the published reports of the application of aerogels to chemical sensing. These include sensors based on silica, silica composite, titania, carbon and clay aerogels, with spectroscopic and conductimetric detection methods.

Diode-Laser-Based Detector for Doublet Peak Measurements in Flow Injection Analysis

Double peaks may be obtained in a single-line flow injection manifold if, by the time the reaction zone reaches the detector, there has been insufficient interspersion of the sample and reagent so that the sample is in excess over the reagent in the center of the reaction zone. The time between the peaks (Δt) in a doublet profile in flow injection analysis (FIA) can be related to other experimental parameters by the equation Δt = (V/Q)ln{Cs[exp(Vi/V) - 1]/CR} (1) for the

Luminescence originating in an optical fiber

Recently, there has been considerable research into the development and applications of fiber-optic spectroscopic sensors. The large number of recent reviews attests to the widespread interest in this area. Fiber optics have several advantages over conventional optics; they can easily be miniaturized, and can be used at a remote distance from the laboratory and/or under harsh conditions. Our research group has been active in the area of fiber-optic chemical sensors. In the course of setting up an instrumental system for sensor research, we discovered some unusual luminescence behavior originating in the optical fiber itself. When a pulsed, high-energy, ultraviolet laser beam was coupled into a fused-silica fiber, the fiber was seen to glow a bright red color. The color was intense enough to be seen with the naked eye in a well-lit room, and appeared to be uniform along the length of the fiber. We proceeded to study this observed emission. This report summarizes our observations.

Solid-State Microprocessor Controlled Detector for Doublet Peak Measurements in Flow Injection Analysis

Abstract A solid-state detector for time-based transmission measurements in flow-injection analysis (FIA) based on a light-emitting diode (LED) source and photodiode (PD) transducer with microprocessor controller is described. The instrument components cost less than US