Detection of remote species

The principle appears to be applicable in all regions of the electromagnetic spectrum where conventional heterodyne detection is useful. In the next section (7.4), we consider two versions of the system useful for the detection of remote species. 

Multifrequency Single-Photon Selective Heterodyne Radiometry for Detection of Remote Species... 

We have described a selective heterodyne radiometer potentially useful in the detection of remote species such as pollutants and interstellar molecules. The system operates on the basis of the difference frequency between two radiated lines which, for closely spaced lines, is relatively insensitive to Doppler shift. This allows for the sensitive detection of known species moving at unknown velocities. The two frequencies may be obtained from individual transitions or... 

Lieberman, S. H., Inman, S. M., Stromvall, E. J., Fiber Optic Fluorescence Sensors for Remote Detection of Chemical Species in Seawater , in Chemical Sensors, Tbrner, D. R. (ed.) London Electrochemical Society, 1987. 

The objective ia any analytical procedure is to determine the composition of the sample (speciation) and the amounts of different species present (quantification). Spectroscopic techniques can both identify and quantify ia a single measurement. A wide range of compounds can be detected with high specificity, even ia multicomponent mixtures. Many spectroscopic methods are noninvasive, involving no sample collection, pretreatment, or contamination (see Nondestructive evaluation). Because only optical access to the sample is needed, instmments can be remotely situated for environmental and process monitoring (see Analytical METHODS Process control). Spectroscopy provides rapid real-time results, and is easily adaptable to continuous long-term monitoring. Spectra also carry information on sample conditions such as temperature and pressure. 

Raman spectroscopy, while typically used as a micro-analytical tool, can be conducted remotely. Performance of remote Raman analysis have been recently explored and reahzed for experiments on the surface of Mars (Sharma et al. 2001 Sharma et al. 2003). Raman spectroscopy is a powerful technique for mineralogical analysis, where the sharpness of spectral features of minerals allows for much less ambiguous detection, especially in the presence of mixtures. Visible, near-infrared, thermal, reflectance and in many cases emission spectroscopy of minerals all suffer from broad overlapping spectral features, which complicates interpretation of their spectra. On the other hand, Raman spectra of minerals exhibit sharp and largely non-overlapping features that are much more easily identified and assigned to various mineral species. 

The detection limit for measuring a species by filter collection and subsequent analysis is determined either by the uncertainty in the filter blank, that is, the variability in the amount of a species in an unexposed filter, or by the limit of detection imposed by the analytical method. Most frequently, the limit of detection is determined by blank variability, thus care in preparation and handling of filters is important when using such systems for aircraft sampling, particularly if sampling is conducted in remote areas where concentrations are low. In this regard, it is crucial to establish the blank... 

Sulfate is the ultimate product of the oxidation of SO2. We might expect small concentrations of other species, such as dithionates that are intermediates in the oxidation process, although these have not been detected. Concentrations of sulfate particles in the remote atmosphere are typically at a few nanomoles per cubic meter. In the marine atmosphere the sulfate is found both in coarse particles where it derives from seasalt and in fine particles around a micron in diameter as... 

In both cases, normal or background variability in the distribution and vigour of various species presents complications that need to be taken into account. Factors such as bedrock geology, soil type, slope, soil moisture and climate can have a more pronounced effect than that due to the presence of hydrocarbons (Rock, 1984 Klusman et al., 1992). Nevertheless, numerous accounts have been published of the detection of hydrocarbon-induced vegetation anomalies by remote sensing. 

Vne of the common problems encountered in studies of aqueous geo-chemistry and water pollution is proper identification of a particular species of an element or compound that may be present in the system. The use of electron spin resonance (ESR) spectroscopy to determine the presence and concentration of equilibrium and/or nonequilibrium metal species in natural water systems has not been adequately investigated. Coincidentally, Mn2, one of the easiest elemental species to detect by ESR, is also one of the dissolved species of considerable concern in problems related to heavy metal pollution and aqueous geochemistry. Furthermore, with proper design there exists the possibility of using electron spin resonance as the basis of a remote monitoring system for the detection of appropriate heavy metals in natural water systems. 

The other concern in the 1970s and 1980s was whether or not inorganic lead could be biomethylated naturally, as in the case of mercury. In view of the bulk amounts of organoleads being used anyway at the time, this was not likely to be a key consideration in urban areas, but in pristine locations, natural lead methylation and transport might in some circumstances have led to environmental problems. In fact, there has not been any clear demonstration of such methylation, and it is unlikely to be a big environmental problem. Where organic lead compounds are detected in the remote environment, they are at very low levels, probably accountable by transport of these species. Evidence is considered in the next section. 

Work in the 1980s established concentration levels to be expected in atmospheres remote from the source when TALs were routinely used in gasoline. Typically, these would be at the ngm level. Ionic lead would also be at similar levels. Ionic leads in waters are also very low (at about the ng L level), with TALs being very low. There is little information about organolead species in sediments and soil. The analytical methodologies are discussed in Section 12.13.5. A recent tabular summary of organolead species detected in the various environmental compartments at the levels noted above is given in Ref 149. 

Optical fibers without cladding can be used to monitor the adsorption of chemical species such as proteins on an optical Hber core [87]. Fluorescence is induced by the evanescent wave field of the light propagating in the core. For example, the intrinsic fluorescence of the amino acid tryptophan in immunoglobulin (IgG) and the fluorescence of fluorescein-labelled IgG were detected and used to calculate the amount of protein adsorbed on hydrophilic glass and quartz surfaces. Fluorescent, nonadsorbing dextran was used as a calibration molecule which approximates the diffusion properties of the protein. Remote spectroscopic sensing of the adsorption of Rhodamine B-labelled IgG at the tip of a 600-pm fiber optic has also been described [88, 89]. 

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