Detection systems characteristics

A final practical note involves instrument intensity measurement calibrations. The intensity measurement is self-calibrating relative to the incident beam from the source. However, measurements typically have a dynamic range of 10 -10 , and care must be taken to insure the detection system is linear. A method of calibrating the scatterometer is to characterize a diffuse reflector having a known scattering characteristic. For example, a surface coated with BaS04 makes a nearly Lambertian scatterer, which has a BRDF of 1/Jt at all angles. 

Health Elazards - Personal Protective Equipment Self-contained breathing apparatus protective clothing if exposed to liquid. Symptoms Following Etqjosure If concentration of gas in hi enough, may cause asphyxiation. No detectable systemic effects, even at 5 % concentration in air. Treatment for Exposure Remove victim to open air. If the is overcome by gas, qjply artificial resuscitation. Toxicity by Inhalation (Threshold Limit Value) Data not available Short-Term Inhalation Limits Data not available Toxicity by Ingestion Not pertinent Late Toxicity None Vapor (Gas) Irritant Characteristics Vapors are nonirritating to the eyes and throat. Liquid or Solid Irritant Characteristics No appreciable hazard. Practically harmless to the skin because it is very volatile and evaporates quickly. May cause some frostbite. Odor Threshold Data not available. 

Definition and Uses of Standards. In the context of this paper, the term "standard" denotes a well-characterized material for which a physical parameter or concentration of chemical constituent has been determined with a known precision and accuracy. These standards can be used to check or determine (a) instrumental parameters such as wavelength accuracy, detection-system spectral responsivity, and stability (b) the instrument response to specific fluorescent species and (c) the accuracy of measurements made by specific Instruments or measurement procedures (assess whether the analytical measurement process is in statistical control and whether it exhibits bias). Once the luminescence instrumentation has been calibrated, it can be used to measure the luminescence characteristics of chemical systems, including corrected excitation and emission spectra, quantum yields, decay times, emission anisotropies, energy transfer, and, with appropriate standards, the concentrations of chemical constituents in complex S2unples. 

Confirmation of identity of nitrosamines via an independent detection system is desirable since a higher level of confidence is achieved if a different physical property or structural characteristic is measured. Mass spectrometry (MS) has been used... 

One common characteristic of many chemical agents is that they tend to be relatively fast acting that is, victims begin to exhibit symptoms of distress within seconds to minutes after exposure to the agent. This almost-immediate showing of symptoms has implications for defensive strategies based on detection systems, since the chemical agent released in an attack would reach and produce a response... 

The drag is not safe and effective and the approval system rejects it, because it detects these characteristics of the drag. The system works. 

To design a successful high speed fire protection system, an engineer must take many factors into consideration. In addition, each individual installation typically has characteristics that require special attention. Because of the nature of the hazard involved and the need for such extremely fast response, the design of the high speed detection system is best left to a skilled expert. 

Cottrell, J.S. Evans, S. Characteristics of a Multichannel Electrooptical Detection System and Its Application to the Analysis of Lai e Molecules by FAB-MS. Anal. Chem. 1987, 59, 1990-1995. 

Residue chemists would be most interested in comparable confidence bands or confidence bandwidths. These values become a performance characteristic for any detection system. The values indicate the precision of not only the prepared standards but also the precision of the overall operating detection system. It is ultimately envisioned that a given system of a separation column in a chromatograph with a certain type of detector should give bands of standardized values. If a chemist finds he has not met... 

The use of biolc c indicators as an early detection system for severe air pollution episodes or for chronic air pollution problems is of interest to many urban and industrial control officials. Plants served as air pollution detection sterns long before pollution was acknowledged a problem by industry or government. Higher plants serve as useful detection tools, because they develop characteristic symptoms from acute exposure, even though the symptoms are not necessarily specific to cause. Cause and effect can be reliably ascertained, when an air pollution source is identified and symptom patterns are identified within given sensitive species. 

CCTV technology is used to analyze digital images and identify the characteristics of a fire. CCTV can provide both a control action and display the picture in a staffed control room. The latter has a particular benefit as it allows very rapid incident assessment and also reduces the need for personnel to enter the area to investigate the fire conditions. In general terms, the CCTV flame detection system is comprised of three basic components the camera/detection element, the control panel, and the display computers. 

HasweU and Barclay [3] have described a microwave system coupled to an atomic absorption detection system for the analysis of sludges and soils. A major constraint at the present time is that the preferred operation of these types of systems is for sample matrices to be closely matched. A widely varying sample, which exhibits different heating characteristics, wiU either show up as an invaHd result or the time required to cope with this procedure for aU the samples wiU greatly extend the on-Hne analyses time scales. As more of these instrumental systems become Hnked to laboratory information management systems, it wiU become feasible to interact between the control database and the instrumentation so that each sample is treated in an appropriate manner and the optimum time frame is selected for each sample type. When new samples are analysed, the steps could be monitored so that the required time scales are obtained and then stored for future reference. 

Additional important Information on the size of the separated compounds can be obtained easily by comparison with known compounds having similar characteristics furthermore the characterization of the separated compounds is facilitated by the use of multiple detection systems. 

Generally, the performance characteristics of greatest interest for an explosives detection system are sensitivity, selectivity, and response time. As used here, sensitivity is the ability to detect the target analyte in extremely small concentrations, while selectivity is the ability to distinguish the target analyte from other materials that may be present. In combination, good sensitivity and selectivity mean a high probability of detection when the analyte is present and a low false alarm rate when the analyte is not present. 

Figure 6.17 shows a schematic of the LEED system. The sample is bombarded through the left by a beam of electrons. Only radiation or electrons (remember the wave nature of matter ) with the same energy as the incident beam are detected. These electrons are called elastic backscattered electrons. The detection system is a fluorescent screen placed in front of the sample. Holding the screen at a large positive potential accelerates the electrons. Once they reach it, they excite the phosphorus in the screen, marking it with bright spots characteristic of the diffraction pattern. Finally, a camera in front of the screen records the diffraction pattern. 

Thermal emission spectroscopy can be used in middle- and far-infrared spectral regions to make stratospheric measurements, and it has been applied to a number of important molecules with balloon-borne and satellite-based detection systems. In this approach, the molecules of interest are promoted to excited states through collisions with other molecules. The return to the ground state is accompanied by the release of a photon with energy equal to the difference between the quantum states of the molecule. Therefore, the emission spectrum is characteristic of a given molecule. Calculation of the concentration can be complicated because the emission may have originated from a number of stratospheric altitudes, and this situation may necessitate the use of computer-based inversion techniques (24-27) to retrieve a concentration profile. 

Since cyclamate has poor UV absorbing characteristics, HPLC methods for the analysis of this sweetener require specific detection systems, such as indirect photometry or conductivity. Herrmann et al. (24) used indirect photometry for the detection of cyclamate at 267 nm against a UV-absorbing mobile-phase component, p-toluenesulphonate. Biemer (17) and Wu et al. (47) used a conductivity detector for the determination of cyclamate. According to Biemer (17) the use of this detector offers distinct advantages, since compounds coeluting with cyclamate may not exhibit an electrochemical response and, hence, not appear in the chromatogram. 

---