Optical measurement methods, chemical species

Optical sensors for oxygen measurement are attractive since they can be fast, do not consume oxygen and are not easily poisoned. The most common method adopted in construction is based on quenching of fluorescence from appropriate chemical species. The variation in fluorescence signal (I), or fluorescence decay time (x) with oxygen concentration [O2] is described by Stem-Volmer equation91 ... 

Lehaitre et al. [23] have described a fibre-optic spectrophotometric method for the in situ measurement of biological and chemical species. This instrument can measure phytoplanktonic species, and the potential for chemical measurement such as dissolved carbon dioxide is analysed. 

There are few methods suitable for on-line chemical analysis of aerosol particles. Raman spectroscopy offers the possibility of identifying the chemical species in aerosol particles because the spectrum is specific to the molecular. structure of the material, especially to the vibrational and rotational modes of the molecules. Raman spectra have been obtained for individual micron-sized particles placed on surfaces, levitated optically or by an eiectrodynamic balance, or by monodisperse aerosols suspended in a flowing gas. A few measurements have also been made for chemically mixed and poly disperse aerosols. The Raman spectrum of a spherical particle differs from that of the bulk material because of morphology-dependent resonances that re.su It when the Raman scattered photons undergo Mie scattering in the particle. Methods have been developed for calculating the modified spectra (McNulty el al., 1980). 

A second important consideration is how homogeneity is defined and identified. Accurately computing a mixing measure requires quantifying the marker concentration at particular locations in space and/or time. Optical sensing of the concentration field is t3q)ically the method of choice for small-scale systems, although reaction-based schemes allow for relying on the total amount of chemical species produced (or consumed) in the system however, even in these cases optical techniques are typically used to determine concentration. 

Optical (fiber) chemical sensors [1] are capable of measuring a single species in an untreated sample by simply bringing into contact sample and sensor Separation steps or addition of chemical reagents are not required This is the preferred method in case of samples where the matrix does not vary to a large extent, e g blood However, optodes also represent useful detectors for use in chromatography, flow injection analysis... 

The observed mixing ratios of active halogen species in the troposphere is in general 1-100 ppt, and the direct measurements of these species are usually made by using the methods, Long-Path Differential Absorption Spectroscopy (LP-DOAS), Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS), Chemical Ionization Mass Spectrometer (CIMS) and Laser Induced Fluorescence (LIF). As for BrO and lO, satellite observation gives spatial distribution of its tropospheric column. 

Chemical sensors respond to measurands through various chemicals and chemical reactions. They have the ability to identify and quantify liquid or gaseous chemical species. This class of sensors is currently used in many chemical analysis techniques, such as mass spectroscopy, chromatography, infrared technology and others (Fraden, 2010), due to the miniaturization of sensors. There are a variety of chemical sensors with various methods of transduction. The transduction methods of chemical sensors can be organized into three classes based on their modes of measurement (i) electrical and electrochemical properties, (ii) changes in the physical properties, and (iii) optical absorption of the chemical analytes to be measured. 

Optical metiiods, in both bulb and beam expermrents, have been employed to detemiine tlie relative populations of individual internal quantum states of products of chemical reactions. Most connnonly, such methods employ a transition to an excited electronic, rather than vibrational, level of tlie molecule. Molecular electronic transitions occur in the visible and ultraviolet, and detection of emission in these spectral regions can be accomplished much more sensitively than in the infrared, where vibrational transitions occur. In addition to their use in the study of collisional reaction dynamics, laser spectroscopic methods have been widely applied for the measurement of temperature and species concentrations in many different kinds of reaction media, including combustion media [31] and atmospheric chemistry [32]. 

Since 1962, when it was first characterized by pulse radiolysis transient absorption measurements in water, the solvated electron has been widely studied in numerous solvents. The solvated electron, denoted by e, is a thermodynamically stable radical, but like most free radicals, it has a short lifetime due to its high chemical reactivity. The solvated electron is a unique chemical moiety whose properties may be compared in many solvents and are not dependent on the method creating the solvated electron. The solvated electron is an important reactive species as it is the simplest electron donor, its reactions correspond to electron transfer reactions and its reactivity may be used to probe electron transfer properties of acceptors. During the last 40 years, due to its optical absorption properties, the... 

In addition to the shock wave velocity it is necessary, in a system where the density does not uniquely determine the composition (and this includes all but the very simplest chemical systems of interest), to measure some concentration function in order to follow the reaction. This is one of the greatest experimental difficulties associated with the method, since the changes occur so rapidly. Where possible, the concentration change is followed spectrophotometrically. This concentration monitoring is the second function of the observation points in Fig. 3. Especially for species with line spectra, the small optical density change, coupled with the fast response-time necessary, excludes the use of a conventional spectrophotometer. An example of a detection system which has been used for the hydrogen/oxygen... 

---