The first application of liquid-phase broadband cavity enhanced spectroscopy (BBCEAS) to the measurement of stopped-flow kinetics is reported. The stopped-flow technique is widely used for the study of the kinetics of fast liquid-phase reactions down to millisecond timescales. UV-visible absorption spectroscopy is commonly used as the detection method. Increased sensitivity can potentially allow reactions which are too fast to be measured, to be studied by slowing down the reaction rate through the use of lower concentration of reactants. A simple low cost BBCEAS experimental setup was coupled to a commercial stopped-flow instrument. Comparative standard absorption measurements were also made using a UV-visible double-beam spectrometer as the detector. Measurements were made on the reaction of potassium ferricyanide with sodium ascorbate under pseudo-first order conditions at pH 8 and pH 9.2 A cavity enhancement factor (CEF) of 78 at 434 nm was obtained whilst the minimum detectable change in the absorption coefficient αmin(t), was 1.35 × 10-5 cm-1 Hz-1/2. The kinetic data at pH 9.2 was too fast to be measured using conventional spectroscopy, whilst the BBCEAS measurements allowed 30 fold lower concentration of reactants to be used which slowed down the reaction rate enough to allow the rate constant to be determined. The BBCEAS results showed a 58 fold improvement in sensitivity over the conventional measurements and also compared favourably with the relatively few previous liquid-phase cavity enhanced kinetic studies which have been performed using significantly more complex and expensive experimental setups.

Submerged macrophytes can provide attached surface for biofilms (known as periphyton) growth. In the present study, the alterations in biofilms formation, and chemical compositions and physiological responses were investigated on leaves of Vallisneria asiatica and Hydrilla verticillata exposed to 0.1 mg L-1(control) or with 10 mg L-1NH4+-N for 13 days. Results from physiological and biochemical indices (content of H2O2, malondialdehyde, total chlorophyll and activity of superoxide dismutase, catalase and peroxidase) showed that high ammonium caused oxidative damage to leaves of two species of plant. Multifractal analysis (based on scanning electron microscope images) showed that for the same plant, the values of width △α (△α = αmax-αmin) of the f(α) and Δf (Δf = f(αmin)-f(αmax)) were smaller on leaves surface of two species of plant treated with 10 mg L-1NH4+-N for 13 days than their controls, suggesting high ammonium treatments reduced morphological heterogeneity of leaf surface and enhanced area of the colony-like biofilms. X-ray photoelectron spectroscopy analysis showed that C, O, N and P were dominant elements on leaves surface of two species of plant and ammonium application increased the percentage of C but decreased that of O. High ammonium increased C1 (C-C or C-H) percentage but decreased C2 (C-O) and C3 (O-C-O or C=O) percentage on leaves surface of two species of plant, indicating that ammonium stress changed the surface chemical states and thus might reduce the capacity of leaves to adsorb nutrients from water column. Our results provided useful information to understand ammonium induced toxicity to submerged macrophytes.

We report on the first detailed use of broadband cavity enhanced absorption spectroscopy (BBCEAS) as a detection system for immunoassay. A vertical R ≥ 0.99 optical cavity was integrated with a motorized XY stage, which functioned as a receptacle for 96-well microtiter plates. The custom-built cavity enhanced microplate reader was used to make measurements on a commercially available osteocalcin sandwich ELISA kit. A 30-fold increase in path length was obtained with a minimum detectable change in the absorption coefficient, αmin(t), of 5.3 × 10(-5) cm(-1) Hz(-1/2). This corresponded to a 39-fold increase in the sensitivity of measurement when directly compared to measurements in a conventional microplate reader. Separate measurements of a standard STREP-HRP colorimetric reaction in microtiter plates of differing optical quality produced an increase in sensitivity of up to 115-fold compared to a conventional microplate reader. The sensitivity of the developed setup compared favorably with previous liquid-phase cavity enhanced studies and approaches the sensitivity of typical fluorometric ELISAs. It could benefit any biochemical test which uses single pass absorption as a detection method, through either the label free detection of biologically important molecules at lower concentrations or the reduction in the amount of expensive biochemicals needed for a particular test, leading to cheaper tests.

The room temperature self- and foreign-continua of water vapor have been measured near 4250 cm(-1) with a newly developed high sensitivity cavity ring down spectrometer (CRDS). The typical sensitivity of the recordings is αmin≈ 6 × 10(-10) cm(-1) which is two orders of magnitude better than previous Fourier transform spectroscopy (FTS) measurements in the spectral region. The investigated spectral interval is located in the low energy range of the important 2.1 μm atmospheric transparency window. Self-continuum cross-sections, CS, were retrieved from the quadratic dependence of the spectrum base line level measured for different water vapor pressures between 0 and 15 Torr, after subtraction of the local water monomer lines contribution calculated using HITRAN2012 line parameters. The CS values were determined with 5% accuracy for four spectral points between 4249.2 and 4257.3 cm(-1). Their values of about 3.2 × 10(-23) cm(2) molecule(-1) atm(-1) are found 20% higher than predicted by the MT_CKD V2.5 model but two times weaker than reported in the literature using FTS. The foreign-continuum was evaluated by injecting various amounts of synthetic air in the CRDS cell while keeping the initial water vapor partial pressure constant. The foreign-continuum cross-section, CF, was retrieved from a linear fit of the spectrum base line level versus the air pressure. The obtained CF values are larger by a factor of 4.5 compared to the MT_CKD values and smaller by a factor of 1.7 compared to previous FTS values. As a result, for an atmosphere at room temperature with 60% relative humidity, the foreign-continuum contribution to the water continuum near 4250 cm(-1) is found to be on the same order as the self-continuum contribution.

Cavity enhanced techniques enable high sensitivity absorption measurements in the liquid phase but are typically more complex, and much more expensive, to perform than conventional absorption methods. The latter attributes have so far prevented a wide spread use of these methods in the analytical sciences. In this study we demonstrate a novel BBCEAS instrument that is sensitive, yet simple and economical to set up and operate. We use a prism spectrometer with a low cost webcam as the detector in conjunction with an optical cavity consisting of two R = 0.99 dielectric mirrors and a white light LED source for illumination. High sensitivity liquid phase measurements were made on samples contained in 1 cm quartz cuvettes placed at normal incidence to the light beam in the optical cavity. The cavity enhancement factor (CEF) with water as the solvent was determined directly by phase shift cavity ring down spectroscopy (PS-CRDS) and also by calibration with Rhodamine 6G solutions. Both methods yielded closely matching CEF values of ~60. The minimum detectable change in absorption (αmin) was determined to be 6.5 × 10(-5) cm(-1) at 527 nm and was limited only by the 8 bit resolution of the particular webcam detector used, thus offering scope for further improvement. The instrument was used to make representative measurements on dye solutions and in the determination of nitrite concentrations in a variation of the widely used Griess Assay. Limits of detection (LOD) were ~850 pM for Rhodamine 6G and 3.7 nM for nitrite, respectively. The sensitivity of the instrument compares favourably with previous cavity based liquid phase studies whilst being achieved at a small fraction of the cost hitherto reported, thus opening the door to widespread use in the community. Further means of improving sensitivity are discussed in the paper.

We analyze the accuracy of the Coulomb energy calculated using the Gaussian-and-finite-element-Coulomb (GFC) method. In this approach, the electrostatic potential associated with the molecular electronic density is obtained by solving the Poisson equation and then used to calculate matrix elements of the Coulomb operator. The molecular electrostatic potential is expanded in a mixed Gaussian-finite-element (GF) basis set consisting of Gaussian functions of s symmetry centered on the nuclei (with exponents obtained from a full optimization of the atomic potentials generated by the atomic densities from symmetry-averaged restricted open-shell Hartree-Fock theory) and shape functions defined on uniform finite elements. The quality of the GF basis is controlled by means of a small set of parameters; for a given width of the finite elements d, the highest accuracy is achieved at smallest computational cost when tricubic (n = 3) elements are used in combination with two (γ(H) = 2) and eight (γ(1st) = 8) Gaussians on hydrogen and first-row atoms, respectively, with exponents greater than a given threshold (αmin (G)=0.5). The error in the calculated Coulomb energy divided by the number of atoms in the system depends on the system type but is independent of the system size or the orbital basis set, vanishing approximately like d(4) with decreasing d. If the boundary conditions for the Poisson equation are calculated in an approximate way, the GFC method may lose its variational character when the finite elements are too small; with larger elements, it is less sensitive to inaccuracies in the boundary values. As it is possible to obtain accurate boundary conditions in linear time, the overall scaling of the GFC method for large systems is governed by another computational step-namely, the generation of the three-center overlap integrals with three Gaussian orbitals. The most unfavorable (nearly quadratic) scaling is observed for compact, truly three-dimensional systems; however, this scaling can be reduced to linear by introducing more effective techniques for recognizing significant three-center overlap distributions.

Sensitive spectroscopic techniques involving high-finesse Fabry-Perot resonators are widely used in the microwave and near-infrared spectral regimes, but hardware limitations have hindered their extension to far-infrared wavelengths. While there is no theoretical limit to the frequency region where cavity-enhanced techniques are practical, the sensitivity of these methods does depend explicitly on the availability of highly reflective optics and, in the case of cavity ringdown spectroscopy, sufficiently fast detectors. Here, we describe a novel high-finesse cavity that uses wire-grid polarizers as the reflective surfaces. Quality factors on the order of 10(5) are achieved at 250 GHz. Based on the optimized cavity design, we investigate the feasibility of extending the cavity ringdown technique to far-infrared wavelengths. With the present commercially available technology, we find spectrometer performance to be limited by both the available optics and detectors. With a 120 cm cavity and a detector response time of ~500 ns, we predict a minimum detectable absorption coefficient, αmin, on the order of 10(-7) cm(-1). Given the sensitivity and noise requirements for the ringdown measurements, faster and more sensitive detectors are needed before implementation of the spectrometer is practical or offers any significant advances to existing methods at far-infrared wavelengths.

We present here the instrument design and first experimental results from a multipass millimeter/submillimeter spectrometer designed to probe dissociative reaction dynamics. This work focuses on benchmarking the instrument performance through detection of the CH3O and H2CO products from methanol dissociation induced by a high-voltage plasma discharge. Multiple rotational lines from CH3O and H2CO were observed when this plasma discharge was applied to a sample of methanol vapor seeded in an argon supersonic expansion. The rotational temperature of the dissociation products and their abundance with respect to methanol were determined using a Boltzmann analysis. The minimum detectable absorption coefficient for this instrument was determined to be αmin ≤ 5 × 10(-9) cm(-1). We discuss these results in the context of future applications of this instrument to the study of photodissociation branching ratios for small organic molecules that are important in complex interstellar chemistry.

A new approach to wavelength-modulation photoacoustic spectroscopy is reported, which incorporates diode lasers in the near infrared and optical fiber amplifiers to enhance sensitivity. We demonstrate the technique with ammonia detection, yielding a sensitivity limit less than 6 parts in 10(9), by interrogating a transition near 1532 nm with 500 mW of output power from the fiber amplifier, an optical pathlength of 18.4 cm, and an integration time constant of 10 s. This sensitivity is 15 times better than in prior published results for detecting ammonia with near-infrared diode lasers. The normalized minimum detectable fractional optical density, alphaminl, is 1.8 x 10(-8); the minimum detectable absorption coefficient, alphamin, is 9.5 x 10(-10) cm(-1); and the minimum detectable absorption coefficient normalized by power and bandwidth is 1.5 x 10(-9) W cm(-1)/square root Hz. These measurements represent what we believe to be the first use of fiber amplifiers to enhance photoacoustic spectroscopy, and this technique is applicable to all other species that fall within the gain curves of optical fiber amplifiers.