Interferent vapors

A critical attribute of a successful chemical sensor is its ability to reject false alarms from background interferent vapors. Sensors that false alarm frequently, whether it be daily, weekly, or even monthly, tend to be ignored with time as the personnel monitoring the devices assume that each new alarm is a false alarm. Sensors which false alarm less frequently (perhaps only once a year or less) tend to be taken more seriously. 

Air containing water vapor at low concentrations (approximately 10% relative humidity) and moderate concentrations (40% - 60% relative humidity) will be used to dilute simulant, agent and interference vapors. 

Approach DPG proposes the use of an evaporative disseminator to generate chemical interference vapors. DPG proposes to use either MIRAN or other organic vapor analyzer to monitor the chemical interferenL which will enable DPG to generate chemical interferent vapors reproducibly. 

Because testing CWAs in the open air is not possible, potential interfering substances are tested in the laboratory at controlled exposure levels. The detector s ability to detect the CWA vapor is tested in combination with potential interfering vapors at the 0.1% and 1% headspace saturation concentration level of interferent vapor, providing that the interferent does not cause the detector to alarm. Such testing reveals whether the detector issues false positive or false negative results. 

Substance vapors that are considered potential interferents should be screened using a controlled generation technique. The headspace vapor of the substance is blended with an airstream to produce approximately 1% concentration of the interferents. If a false alarm occurs, the concentration is lowered to approximately 0.1% and retested. If a detector does not respond to the interference vapor, then the airstream is replaced with a stream of similar air that contains the target CWA in order to assess detection of the target under the influence of the interference vapor. 

Also important is the ability to resist false alarms. False alarms could be expected in high-sensitivity detectors no matter what technologies are used. The key is to assess false alarm rates and the materials that caused the false alarms. If the device responds with false alarms too frequently when exposed to common substances, its usefulness is greatly diminished. Equally if not more important is the ability of a device to function properly while under the influence of interferent vapors. If a relatively large number of interferent vapors affect a device s ability to detect the target chemicals, its usefulness is also limited. 

Once the detector has shown proper response to the subject vapor, it is allowed to clear under the conditioning air. After clearing, the interference vapor is introduced and mixed with the conditioning air to observe the effect on the detector. If the detector responds with an alarm to this interference vapor, it could be misconstrued as the presence of the target toxic chemical. 

If exposure to the interference vapor does not cause a false positive response, the conditioning air is replaced with the subject vapor air. The detector is now... 

When the sample is a solid, a separation of the analyte and interferent by sublimation may be possible. The sample is heated at a temperature and pressure below its triple point where the solid vaporizes without passing through the liquid state. The vapor is then condensed to recover the purified solid. A good example of the use of sublimation is in the isolation of amino acids from fossil mohusk shells and deep-sea sediments. ... 

The flame can become unstable if too large an amount of sample is introduced or if the sample contains substances that can interfere with the basic operation of the plasma. For example, water vapor, air, and hydrogen all lead to instability of the plasma flame if their concentrations are too high. 

Thin-Layer Chromatography. Chiral stationary phases have been used less extensively in tic as in high performance Hquid chromatography (hplc). This may, in large part, be due to lack of avakabiHty. The cost of many chiral selectors, as well as the accessibiHty and success of chiral additives, may have inhibited widespread commerciali2ation. Usually, nondestmctive visuali2ation of the sample spots in tic is accompHshed using iodine vapor, uv or fluorescence. However, the presence of the chiral selector in the stationary phase can mask the analyte and interfere with detection (43). 

Lamination Inks. This class of ink is a specialized group. In addition to conforming to the constraints described for flexo and gravure inks, these inks must not interfere with the bond formed when two or more films, eg, polypropylene and polyethylene, are joined with the use of an adhesive in order to obtain a stmcture that provides resistance properties not found in a single film. Laminations are commonly used for food applications such as candy and food wrappers. Resins used to make this type of ink caimot, therefore, exhibit any tendency to retain solvent vapor after the print has dried. Residual solvent would contaminate the packaged product making the product unsalable. 

Concurrent with requirements for low levels of mercurials in discharge water is the problem of their deterrnination. The older methods of wet chemistry are inadequate, and total rehance is placed on instmmental methods. The most popular is atomic absorption spectrophotometry, which rehes on the absorption of light by mercury vapor (4). Solutions of mercury compounds not stabilized with an excess of acid tend to hydrolyze to form yeUow-to-orange basic hydrates. These frequendy absorb onto the walls of containers and may interfere with analytical results when low levels (ppm) of mercury are determined. 

Ozone in the gas phase can be deterrnined by direct uv spectrometry at 254 nm via its strong absorption. The accuracy of this method depends on the molar absorptivity, which is known to 1% interference by CO, hydrocarbons, NO, or H2O vapor is not significant. The method also can be employed to measure ozone in aqueous solution, but is subject to interference from turbidity as well as dissolved inorganics and organics. To eliminate interferences, ozone sometimes is sparged into the gas phase for measurement. 

When heated with pyrocatechol [720-80-9] copper powder, and alcohoHc sodium hydroxide, carbon tetrachloride gives a blue color that changes to red on addition of hydrochloric acid. This color reaction is not produced by chloroform. Quantitative analysis of carbon tetrachloride may be done by first decomposing the sample free of organic and inorganic chlorides, heating in a sealed tube with alcohoHc potash, and subsequently determining the potassium chloride formed as the silver haHde. The Zeiss interference refractometer has been used to determine the concentration of carbon tetrachloride vapor in air (36). 

Fast detection of trace explosives in air is an urgent analytical problem, which solution will allow one efficiently to perform anti-terrorist measures. The difficulties that one runs into solving this problem are due to low vapor pressure of these compounds and the presence of interference compounds in air. 

Where only about 5% or less of the liquid dow iiflow is to be withdrawn from the column, a special collector box can be installed within the packed bed. This box can remove small quantities of intermediate boiling components that otherwise w ould accumulate in a sufficient quantity to interfere with the fractionation operation. Such a collector box must be designed very carefully to avoid interference with the vapor distribution above it or... 

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