Point detection instruments

A wide variety of commercial equipment is available for detection of hazardous chemicals, including a number of chemical warfare agents. For example, ion mobility spectroscopy is used to detect nerve, blister, and blood agents. The Chemical Agent Monitor is a portable, hand-held point detection instrument that uses ion mobility spectrometry to monitor nerve or blister agent vapors. However, minimum detection limits are approximately 100 times the acceptable exposure limit for nerve agents, and approximately 50 times the acceptable exposure limit for blister agents. 

The CAM uses ion mobility spectrometry to provide a portable hand-held point detection instrument for monitoring nerve or vesicant agent vapors. It provides a graduated readout (low, medium, high). Response time is dependent on concentration but generally takes from 10 to 60 seconds. Minimum levels detectable are about 100 times the AEL for the nerve agents and about 50 times the AEL for vesicants. An obvious drawback to this relative insensitivity to low concentrations is an inability to fully check the efficacy of decontamination efforts, both in the field and subsequently at treatment facilities. 

Elemental composition Cr 42.31%, Cl 57.69%. The metal may be analyzed by AA, ICP, or other instrumental techniques. Chloride may be measured by ion chromatography or by using a chloride ion selective electrode. Because of the blue color of its aqueous solution, end point detection in titrimetric methods may be difficult. 

Improved chemical agent point detection system (IPDS). The IPDS also employs ion mobility spectrometry and is an improved version of a point detection system. In addition to G and VX nerve agents, the IPDS is designed to detect vesicant agent vapors. Because it is a shipboard instrument, it will be much larger and will need more power than portable IMS devices. 

Figure 2 (A) Design of a capillary array instrument with two-point detection and...

Corvari, V. Pry, W.C. Seibert, W.L. Augsburger, L. Wet granulation end-point detection in a high shear mixer instrumented with a capacitive sensor and a strain gaged torque sensor. AAPS Meeting, 1992. 

We often use instruments to detect end points. These instruments respond to properties of the solution that change in a characteristic way during the titration. Among such instruments are colorimeters, turbidimeters, temperature monitors, refractometers, voltmeters, current meters, and conductivity meters. 

An end point in a Karl Fischer titration can be observed visually based on the brown color of the excess reagent. More commonly, however, end points are obtained from electroanalytical measurements. Several instrument manufacturers offer automatic or semiautomatic instruments for performing Karl Fischer titrations. All of these are based on electrometric end point detection. The details of operation of Karl Fischer titrators are discussed in Chapter 22. 

The most common end-point detection method for the Karl Fischer titration for determining water (see Section 20C-5) is the amperometric method with dual polarized electrodes. Several manufacturers offer fully automated instruments for use in performing these titrations. A closely related end-point detection method for Karl Fischer titrations measures the potential difference between two identical electrodes through which a small constant current is passed. 

Cut points specify the beginning and end of the collection of a fraction. Cut points can be based on time, although in practice time is not the most stable parameter, due to possible fluctuations in the flow rate. They can be based on threshold values of the UV-detector signal or on volume of eluent through a mass flowmeter. Cut point locations can also be a combination of a characteristic feature on the chromatogram and volume. Cut point locations for valve switching may also be based on the response of less traditional detection instrumentation such as the on- line measurement of density, conductivity, temperature, spedflc ion detection, near infrared, or refractive index. 

There are numerous automatic titrators that employ potentiometric end-point detection. They usually can automatically record the first or second derivative of the titration curve and read out the end-point volume. The sample is placed in the titration vessel, and the titrant, drawn from a reservoir, is placed in a syringe-driven buret. The volume is digitally read from the displacement of the syringe plunger by the electronic driver. Titrators may also employ photometric detection of indicator color changes. An automatic titrator is shown in Figure 14.7. Automatic titrators make volumetric analyses rapid, reproducible, and convenient. While instrumental methods provide many advantages, classical volumetric analyses are still widely used and are very useful, especially for major constituents, for example, in the pharmaceutical industry. 

Assay Detection instrument Throughput data points per week Gating method Advantages Disadvantages... 

Standard radioactive Material (SRM) solutions are radioactive materials with accurately known radionuclide content and radioactive decay rate or rate of particle or y-ray emission. They are used primarily to calibrate radiation detection instruments and to prepare QC samples that test analytical accuracy. The supplier prepares radionuclide standard solutions in flame-sealed glass ampoules. Other standard radionuclides are in the form of point sources (usually on thin backing) or as solids in configurations that represent actual samples. 

In general practice, gas detection instruments working with electrochemical sensors are very commonly employed. These rather compact devices can be worn on the body, and therefore they are very suitable for monitoring personal exposure. The user is alerted in the case of hazardous concentrations in the working environment by an audible and visible alarm device. Usually, the instruments include a data logger, which enables the recorded data to be evaluated at a PC at a later point of time. The necessary software is normally supphed with the device. 

Continuous and batch processes require different strategies for implementation of process analyzers. For continuous processes, startup must be controlled to quickly reach the optimal process conditions. Constant monitoring is needed for both continuous and batch processes to detect when the process is going out of control so that corrective action can be taken. For batch processes, end point detection is an important control parameter. For all analyses, the simplest method that performs the desired analysis is best. As the complexity of a method increases, changes in the instrument, materials, process, and measurement conditions can cause problems (1). 

New instrumental techniques have been developed simultaneously with new applications to make possible more rapid and convenient methods of analysis. A commercial constant-current source covering a current range of 4.82 to 193.0 ma is used in conjunction with an automatic potentiometric end-point detection device designed specifically for this purpose. It does not require a predetermined end-point potential and requires only two preliminary adjustments prior to titration. A re-... 

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