Environmental measurement techniques direct monitoring

However, application of environmental scanning electron microscope (ESEM) [26] has made real-time measurements possible. With this technique the structural transitions at the crack tip can be directly monitored during deformation [20, 27-29], ESEM is especially useful for investigation of fracture mechanisms [29] such as crazing and cavitation. However, there is still no work reporting the transitions of the semi-crystalline structure during deformation studied by ESEM. The reason can be the fact that the amorphous phase is eroded from the surface of the uncoated polymer sample by the scanning electron beam [30]. 

In human health risk assessment, direct and indirect methods of exposure assessment are distinguished. The direct method involves measurements of exposure at the point of contact or uptake, for instance, by monitoring chemical concentrations in humans or the environments they are exposed to (food, air, water). The indirect methods use modeling and extrapolation techniques to estimate exposure levels (Fryer et al. 2006). Also in environmental exposure assessment, these 2 ways to assess exposure may be applied. 

Electrochemical analysis methods assure, generally, the most reliable analytical information because of the simplicity of the sampling process which includes (1) sample dissolution in water or in organic solvents and (2) the possibility of measuring directly and continuously the activity of the species present in the solutions. The preconcentration step is not necessary, because of the sensitivities and limits of detection that characterize the electrochemical methods. The determined species are not necessary to be converted to other measurable species. The electrochemical methods can be successfully used for in vivo monitoring. Spectrometric analysis methods, on the other hand, nearly always require a complex sampling process because of the presence of interfering species. Therapy is necessary to adopt the best separation techniques that can assure, for each analytical method, the most reliable analytical information. Nondestructive techniques are used especially for environmental analysis, and surface analysis assures the best reliability of the analytical information. 

The first of the separation techniques to be used in process measurement was gas chromatography (GC) in 1954. The GC has always been a robust instrument and this aided its transfer to the process environment. The differences between laboratory GC and process GC instruments are important. With process GC, the sample is transferred directly from the process stream to the instrument. Instead of an inlet septum, process GC has a valve, which is critical for repetitively and reproducibly transferring a precise volume of sample into the volatiliser and thence into the carrier gas. This valve is also used to intermittently introduce a reference sample for calibration purposes. Instead of one column and a temperature ramp, the set up involves many columns under isothermal conditions. The more usual column types are open tubular, as these are efficient and analysis is more rapid than with packed columns. A pre-column is often used to trap unwanted contaminants, e.g. water, and it is backflushed while the rest of the sample is sent on to the analysis column. The universal detector - thermal conductivity detector (TCD)-is most often used in process GC but also popular are the FID, PID, ECD, FPD and of course MS. Process GC is used extensively in the petroleum industry, in environmental analysis of air and water samples" and in the chemical industry with the incorporation of sample extraction or preparation on-line. It is also applied for on-line monitoring of volatile products during fermentation processes" ... 

Compared with methods for the workplace, methods for diffusive sampling of non-occupational indoor air and ambient air have been less well developed. The range of concentrations and environmental conditions used to evaluate samplers for workplace monitoring is not directly applicable to non-occupational environments. However, diffusive monitors are Ending increasing use in non-occupational environments. This chapter discusses the principles governing diffusive sampling and the factors that can influence sampler performance, and reviews studies that have applied the technique for the measurement of VOCs in indoor air. 

As already stressed, these techniques involve many analytical steps such as extraction, derivatization, separation and detection, which should be performed in such a way that decay of the unstable species does not occur. However, the control of the quality of measurements is often hampered by the lack of suitable reference materials for speciation analyses. Research is hence directed towards the development of new (if possible simple) analytical methods, the production of reference materials, and the monitoring of chemical species for various purposes (environmental risk assessment, toxicity studies, biogeochemical cycles of trace elements, etc.). 

Direct evidence for accumulation of PAHs by species that are able to metabolize these compounds is often difficult to obtain because of the rapid rate of biotransformation to metabolites that are not routinely detected by standard analytical techniques. If appreciable metabolism is occurring, then the bioaccumulation study needs to be performed using radiolabeled compounds to allow a mass-balance accounting of parent compounds and metabolites. Hence, for metabolically active species that accumulate very little parent compound, it may be more appropriate to monitor the metabolic products for evidence of accumulation. Two techniques have been developed to measure PAH metabolites one utilizes HPLC fluorescence detection of metabolites in bile of teleosts and the other uses P-postlabeling for DNA adducts that occur as a result of the interaction of metabolites and cellular DNA. Both techniques have displayed a highly positive correlation with environmental concentrations, making them useful for monitoring populations in our coastal areas. 

Monitoring the evolution of the features over time allows, in principle, to detect structural damage. In practice, however, this needs to be applied with care because of two reasons. Firstly, many features cannot be measured directly, but they have to be estimated from measured data using system identification techniques. Modal characteristics, for instance, can be estimated from vibration response data such as accelerations or strains, but this introduces estimation errors (Reynders et al. (2008) Reynders 2012). Secondly, nearly all features are not only sensitive to structural damage but also to changes in temperature, relative humidity, wind speed, operational loading, etc. This means that both the accuracy of the estimated features and the environmental and operational influences should be accounted for. 

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