Trace chemical sensor

Bulk sensors certainly have a role in chemical sensing of explosives, but the subject of this book is the other basic type sensor, one that seeks molecules released from the bulk of the explosive material in an object. We will refer to these as trace chemical sensors. They are sometimes called vapor sensors, but that seems a less accurate description when they are applied to explosive molecules, which may not always be found in a vapor state. As we shall see in Chapter 5, that requires us to understand where and how to look for these molecules. It will become apparent upon a little reflection that the two types of sensors are complementary and are best used in different situations. Furthermore, even when trace sensors are used, in some situations sampling of particles of soil or vegetation or sampling from surfaces may prove to be more productive that vapor sampling. For underwater sources the term vapor sensing is also inappropriate. 

Clearly, if there is unbumed explosive material present in an area of interest, then the trace chemical sensor will register a false positive. Similarly, some bulk chemical sensors may provide false positives when they encounter a mass of nitrogen-rich material, such as fertilizer or feces. In either case the rate of false positives is still likely to be less that the false-positive rate of a metal detector. 

Trace Chemical Sensor (Senses presence of specific molecules)... 

The original need that led to development of trace chemical sensors for explosives was the need to restore land that had been abandoned to public or private use. This land was abandoned because of the presence of, or perception of the presence of, mines or unexploded ordnance, often called UXO. These potentially lethal items could be the result of some earlier armed conflict. In that case it is now common to refer to them as explosive remnants of war, or ERW. In some cases the war that left its remnants was concluded many years ago. Dangerous ERW are still found on World War I battlefields, and occasionally on... 

U.S. Army CRRDC also measured surface contamination. In addition, they measured flux of molecules from mines in both air and water. Mines form one of the larger groups of explosive bearing targets for trace chemical sensors. Therefore, these measurements provide some insight into the concentrations at the source... 

The primary purpose for this discussion of EF T of the molecules is to provide the one who employs a trace chemical sensor with means to increase the probability of locating and identifying the source. A thorough understanding of the transport processes presents options to the system designer and the operator to make a system more successful. 

Many sensors commonly used to find objects on the seafloor prove ineffective when the objects become buried. With a more thorough understanding of the EF T of the explosive molecules in these situations, trace chemical sensors may be able to provide more success in locating these objects. 

Trade name for Nomadics, Inc. s Trace Chemical Sensor (not an acronym)... 

In recent years, the evolution of the technological components required for IR sensor systems has been denoted by a significant miniaturisation of light sources, optics and detectors. Essentially, an IR sensor consists of (i) a polychromatic or monochromatic radiation source, (ii) a sensor head and (iii) a spectral analyser with a detector. As sensors where all optical elements can be included in the sensor head are the exception rather than the rule, also various optics, waveguides and filters may form essential parts of IR-optical chemical sensors. Another important building block, in particular when aiming at sensors capable of detecting trace levels, are modifications of the sensor element itself. 

This chapter provides an overview of the basic principles and designs of such sensors. A chemical sensor to detect trace explosives and a broadband fiber optic electric-field sensor are presented as practical examples. The polymers used for the trace explosive sensor are unpoled and have chromophores randomly orientated in the polymer hosts. The electric field sensor uses a poled polymer with chromophores preferentially aligned through electrical poling, and the microring resonator is directly coupled to the core of optical fiber. 

Zhang, J. Tang, X. Dong, J. Wei, T. Xiao, H., Zeolite thin film coated long period fiber grating sensor for measuring trace chemical, Opt. Express 2008, 16, 8317 8323... 

In another study, Darrach et al. [2] reported that samples collected near intact UUXO targets contained traces of explosives at up to parts per billion (ppb) concentration levels. The samples were analyzed in the laboratory, using solid-phase microextraction (SPME) to extract target analytes from the samples. The samples were then processed using a reversal electron attachment detection (READ) technique. If the levels of contamination found in these studies are representative of that emanating from most UUXO, the implication is that sensitive chemical sensors such as the SeaDog may be useful for detecting UUXO. 

Volume14 Analytical Applications of Circular Dichroism edited by N. Purdie and H.G. Brittain VolumelS Trace Element Analysis in Biological Specimens edited by R.F.M. Herber and M. Stoeppler VolumelS Flow-through (Bio)Chemical Sensors... 

The technique requires simultaneous fast and accurate measurements of both the vertical velocity and the trace species in question. Fortunately the technology for the measurement of turbulence with the necessary resolution is available. Sonic anemometers can readily yield air motion data with the required resolution (10). Likewise, the ability to handle the air motion and chemical concentration data with modern computer data systems is well in hand (II). Thus these aspects can be ignored, and the major limitation can be dealt with the availability of appropriate chemical sensors with sufficient time and chemical resolution. 

The factors that influence the chemical resolution of sensors are well understood and are not discussed here. This section reviews the factors that control the temporal resolution of sensors to be used for eddy correlation. In the analysis of the design of chemical sensors to be used for eddy correlation it is instructive to consider the different components of chemical sensor systems separately to determine the influences that they have on the temporal response to variations in the atmospheric concentration of a trace constituent. Of course this analysis is an oversimplification because the total systems operate in a more complex fashion, but it is a useful exercise. 

More information is needed about the surface emission and deposition of trace atmospheric species. These fluxes can often be best measured by the eddy correlation technique with fast chemical sensors in conjunction with micrometeorological instrumentation. As analytical techniques for trace species progress, fast and sensitive sensors are becoming available for field research. Consideration must be given to matching the chemical sensors to the eddy correlation technique. 

In some situations, it may not be possible to use methods based on chemical sensors but necessary to extract a volume of solution for analysis. It is important to ensure the complete separation of the solution from the crystalline phase. Typically, this situation arises when investigating the coprecipitation and inhibition effects of microcomponents, e.g. trace metals and low molecular weight organics and also in monitoring solutions for which suitable electrodes have not been developed. 

Volume 14 Analystical Applications of Circular Dichroism edited by N. Purdie and H.. Brittain Volume 15 Trace Element Analysis in Biological Specimens edited by R.F.M. Herberand M. Stoeppler Volume 16 Flow-through (Bio) Chemical Sensors by M.Valcarcel and M.D. Luque de Castro Volume 17 Quality Assurance for Environmental Analysis edited by Ph. Quevauviller, E.A. Maier and B. Griepink Volume 18 Instrumental Methods in Food Analysis edited by J.R.J. Pare and N.M.R. Belanger... 

Hence, their appUcation field is not only restricted to use in gas separation, pervaporation, and membrane reactors but also appUcable in microscale devices (microreactors and microseparators) and for the preparation of functional materials (adsorbents for trace removal, controUed release capsules, and chemical sensors). 

Biosensors can be defined as chemical sensor systems in which an analyte is detected based on biochemical processes or biochemical utilization. A biosensor is mostly composed of a biological element responsible for sampling and tracing, and a physical element called a transducer responsible for sample transmission and further processing (see also Part V, Chapters 8 and 9). The term biosensor does not really meet the lUPAC definition, in which sensors are defined to be self-containing, perform continuous monitoring and are reversible. For the purpose of this chapter, the term biosensor will not be so strictly used as in the traditional context. 

The problem of explosives detection has not been a typical domain of electronic nose research, mainly since it requires extreme sensitivity to specific chemical compounds (usually a small number) which often occur in trace amounts below the detection limit of available sensors. However, as new and more sensitive technologies for chemical sensor instrumentation are discovered, we should consider how these can be exploited individually and... 

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