Detection subsystem

Detection - Accepts a sample from the preconcentrator and makes an analysis. Several types of detectors are currently incorporated in trace portals. These detectors include ion mobility spectrometers or mass spectrometers. The detection subsystem needs to be reliable and rugged to perform hundreds to thousands of analyses per day with the necessary sensitivity and specificity required in the trace portal system. Most detectors are concentration-sensitive devices. 

Suitable instrumentation is available for on-line or at-line monitoring of petroleum fuels. The sensor is a small, low-cost plug-in component, leading to a greatly simplified detection subsystem compared to those of other techniques. 

The protection system was designed to avoid any unsafe condition. It was subdivided into two subsystems, the nuclear detection subsystem and the interlock subsystem. The nuclear detection subsystem is used to monitor neutron flux level and period. It is composed of 8 nuclear channels to monitor the neutron flux from start up to 100% of full power (100 watts), including comparators and isolation ampliBers. Three channels are used in the start-up region, and the others in the intermediate and power regions. In each region we have three measurements of the neutron flux (power) and three measurements of the period. The nuclear channels are complemented by two linear channels, used (alternatively) to control the reactor in automatic mode. Figure 5 shows the relative location of the detectors, and the operational interval of them. 

Historical DataBase Subsystem We have discussed the use of on-hne databases. An historical database is built similar to an on-line database. Unlike their on-line counterparts, the information stored in a historical database is not normally accessed directly by other subsystems for process control and monitoring. Periodic reports and longterm trends are generated based on the archived data. The reports are often used for long-term planning and system performance evaluations such as statistical process (quality) control. The trends may be used to detect process drifts or to compare process variations at different times. 

Each of the subsystems can, apart from the others, make a significant diagnostic contribution. For example, the instrumental cell isolation and sample handling component could be used with DNA-based or other non-MS systems for detection and/or identification. As another example, the principles underlying pattern drift compensation can apply to MALDI MS and even non-MS detection systems such as capillary GC of fatty acid methyl esters. 

The male moth s pheromone-analyzing olfactory subsystem is composed of pheromone-specific antennal ORCs projecting to the similarly specialized, anatomically defined MGC in the AL and MGC output neurons that project to olfactory foci in the protocerebrum. This subsystem is an example of a labeled-line pathway (18). Its specialization to detect, amplify, and analyze features of sex-pheromonal signals and its consequent exaggeration of common olfactory organizational principles... 

Steps 1 and 2 are repeated on the reduced matrix and each maximal loop that is isolated from the others as it is detected is placed sequentially in the ordered sequence for solving the subsystems of simultaneous equations. The procedure is continued until all of the columns of the matrix P are removed. 

The determination of explosives in soils has been mostly commonly associated with the detection of unexploded ordnance such as land mines (both anti-personnel and anti-tank). Chambers et al. [70] designed sampling subsystems for soil/vapor sampling. A probe was used to extract and concentrate vapors of explosives in the pore volume of soil in the vicinity of land mines with sub-part-per-biUion detection limits for TNT and related explosive munitions compounds [70]. As an... 

Another enhancement in current development is preconcentration subsystems appropriate for Fido robot-deployed and handheld explosives detection systems. The primary Fido operational concept does not incorporate preconcentration, but clearly there are scenarios and applications where preconcentration will contribute to performance. To this end, integrated systems that will tightly couple AFP-based detection with MEMS-based preconcentration are under development. 

As previously stated, model-based FD consists on detection, isolation, and identification of faults in the components of a system from the comparison of the system measurements with a priori information given by the mathematical model. The discrepancies between the real system behavior and the behavior predicted by the model are taken into account via quantities called residuals. Then, the residuals are processed by a decision making system (Fig. 6.2) whose aim is to generate alarms and/or directions for other subsystems (e.g., the control system). 

The trace detection of explosives on a person is not a trivial task. Current trace portals are sophisticated analytical systems designed to detect a wide variety of explosives. These portals incorporate many subsystems to accomplish explosive detection. Three subsystems (sample collection, preconcentration, and detection) have significant influence on the ability of the portal to detect explosives and are as follows ... 

Sample collection - Removes the explosive (vapor and/or particulate) from the person and transports the sample to the preconcentration subsystem. Current trace portals use non-contacting means of pulsating air jets, commonly referred to as puffers , and sampling airflow (tens to hundreds of liters of air per second) to accomplish the removal and transport of particles and vapor. The sample collection area needs to be designed properly to remove, contain, and transport explosives from the entire body, from head to feet. Sample collection is important in trace explosives detection because of the limited amount of explosive residue available to extract from a person [8],... 

Trace detection portal systems can be mathematically modeled [8], The model provides the basis for describing trace portal subsystems and is independent of the specific sampling technique, preconcentration technology, or detector. Through the model, a relative measure of a trace portal system can be determined, which provides a means for comparing trace detection portals. 

Equation 1 shows the relationship of sample collection, preconcentration, and detection in the portal. This equation contains measurable parameters such as (i) sample removal and transport from a person, (ii) collection (adsorption) and release (desorption) of the explosive in the preconcentrator, (iii) additional preconcentration, if applicable, (iv) a term that accounts for flow mismatches between the detector inlet and the preconcentrator subsystem outlet, and (v) a proportionality term relating signal strength to concentration at the detector. 

Although it may appear that 17C is part of the preconcentration term and not sample collection, this sample collection is a function of both the sample collection subsystem and preconcentrator. The sample collection subsystem is designed to efficiently remove explosives from the person and transport the explosives to the preconcentrator. Thus, 17,. accounts for both the removal and the transport. The preconcentrator then traps or collects the explosives (17J for subsequent detection. Sample collection and preconcentration must be integrated for optimal performance in a trace portal detection system. In other words, the preconcentrator should be designed to accept the entire volumetric airflow of the sampling system. Any mismatch in these subsystems will decrease the ability of the portal to detect explosives. 

The mathematical model demonstrates the importance of sample collection (r/T), preconcentration (tjc), concentration (C), and detection (k) in a complete trace portal detection system. Of these three subsystems, the detector is the most understood. Considerable information is available that quantifies the sensitivity, specificity, and limits of detection (LOD) for a particular detection method when used for trace explosives detection [13-15], For trace detection portals, the selection of the detection method is based on performance, initial cost, and maintenance issues. The remaining subsystems (sample collection and preconcentration) are the most variable and least understood for their contribution to trace portal performance. Optimizing the explosive removal and transport in sample collection along with preconcentration will enhance the performance of the entire trace detection system. The sensitivity of the detector will help determine the performance needed from the sample collection and preconcentration. 

In general, trace detection portal systems are recognized more by the type of the detector rather than the method of sample collection and preconcentration. Emphasis on the detector occurs because the detection method tends to be the high-tech part of the portal system. But, as has been shown above, the detector is no more important than the collection and preconcentration subsystems. All subsystems must be appropriately considered and optimized in a trace detection portal system. A very sensitive detector is of no consequence if the explosive is not removed, transported, and delivered. 

All of the currently available commercial trace portals, which will be described later, utilize the same underlying theme in the sample collection subsystem for explosive removal and transport. Specifically, the most frequently implemented sample collection concept can be categorized as non-contacting or non-invasive . This definition implies that no part of the portal physically contacts the person to collect an explosive sample. All of these non-contacting trace detection portals use air jets to dislodge explosive particles from the person and use airflow to transport explosives away from the person. 

As mentioned earlier, all subsystems of a trace portal must be integrated to facilitate explosives detection. After removal and transport of the explosive, the explosive must be collected for eventual delivery to the detector. The preconcentrator receives the explosive from the sample collection subsystem. A well-designed preconcentrator will enhance a marginal sample collection system. 

The preconcentrator is a critical subsystem in a trace detection system because of its ability to collect minute amounts of explosive material. Without preconcentration, the amount of material available to detect would be well below the LOD for many detectors. In addition, the preconcentrator serves as an impedance-matching device between the airflow of the sample collection subsystem and the airflow to the detector. The characteristics of a well-designed preconcentrator include... 

Ap, weight gain and ESEM, and electron dispersive spectra (EDS) for species detection, and (vi) filtration subsystem development and optimization. 

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