Colorimetric chemical detectors

Colorimetric qualitative detectors should not be used as the sole means for the detection of toxic vapors. However, these devices are suitable for use in low-risk settings where they can be worn on the exterior of PPE. Military M9 and M256 kits are examples of kits that wiU detect gases. The military M8 kit is an example of a kit that will detect chemical liquids. 

Indicator (Colorimetric) Tubes — The use of solid chemical detectors (indicating tubes) is common practice. They are simple devices to operate which tends to cause many users to ignore their limitations which must be recognized if they are to be useful in evaluating potential hazards due to air contaminants. This apparent simplicity increases the number of people who attempt to use them, which in turn results in wide variations in both individual competence and the accuracy of the data obtained. 

Portable or fixed multipoint colorimetric detectors are available which rely on paper tape impregnated with reagent. A cassette of the treated paper is driven electrically at constant speed over a sampling orifice and the stain intensity measured by an internal reflectometer to provide direct readout of concentration. Such instruments are available for a range of chemicals including those in Table 9.7. 

Brinkley et al. demonstrated89 a simple to use, easy to interpret, low cost, and environmentally friendly colorimetric detector of the chemical warfare agent - mustard gas (HD, l,l-thiobis(2-chloroethane)). An optically transparent xerogel encapsulating Cu(II) acetate was fabricated to detect HD analogues and can serve as the optical sensor based on metal-ligand charge-transfer mechanism. 

Acoustic wave sensors are also used to detect nerve and blister agents. The surface acoustic wave chemical agent detector (SAW Mini-CAD) is a commercially available, pocket-sized instrument that can monitor for trace levels of toxic vapors of sulfur-based mustard agents (e.g., distilled mustard) and G nerve agents (e.g., tabun, sarin, soman) with a high degree of specificity. Colorimetric tubes are the... 

Whether eluted from columns or from thin-layer plates, the quantitative determination of sugars was traditionally based on colorimetric reactions involving the use of chemical reagents, e.g., anthrone. These detection methods have been largely replaced in modem HPLC by the refractive index detector, although ultraviolet detectors are also employed. Recently we have also seen the introduction of other types of detector (e.g., the mass detector), as will be discussed later. 

Keen visual observation is the most important task during a site reconnaissance. Flowever, some basic in-held chemical testing equipment is useful. Commercial kits are available, mainly based on colorimetric reactions. These kits can be used to check for the presence or absence of common contaminants and to measure soil pH. However, the results obtained using these test kits should only be regarded as qualitative, not least because samples taken at the surface may not represent the bulk of materials on the site. For site reconnaissance, the use of in-held instrumental testing is not justihed. An exception is hand-held equipment to test for the presence of gaseous and volatile contamination—photo-ionisation and other detectors are useful where such contaminants are anticipated. While a Phase la study does not normally include intrusive investigation, a small number of samples may be taken for laboratory analysis. These can be important when waste and other potentially contaminated materials of unknown composition have been spread, deposited or are stored on the site. 

The M18 detection kit and the M256A1 kit are mihtary items. The M18 is a colorimetric device for measuring the concentration of selected airborne chemicals. The Ml8 comes with detector tubes for cyanide, phosgene. Lewisite, sulfur mustard, and nerve agents GA, GB, GD, and VX. 

Zone penetration is an ideal tool for measuring selectivity coefficients, since the method allows readouts to be taken (1) at that vertical slice of the composite zone where the dispersion coefficients are equal (Da = Db at point M Fig. 2.26, bottom) and (2) at the peak maximum of the pure A component. The concentrations within such a composite zone and position of the point M at a time tM are readily established by an experiment where zone A is first injected alone and peak A is recorded by a detector of choice. (If chemical reactions are involved, like a reaction with a suitable reagent for colorimetric detection, a suitable manifold and colorimetric detector are used—cf. Section 4.5.2). Next, zone B is injected alone and peak B is recorded. Provided that the same solution of analyte is injected in both runs A and B, and that the detector responds linearly to the injected species, this experiment yields, (a) the isodispersion point M within the time concentration rnatrix, which is identified via time delay tM, and (b) the peak height Ha for the response of the pure species A at the time Im, because it equals the horizontal distance between point M and the baseline (Fig. 2.26). 

Much of today s technology has been developed into commercially available detection equipment, however, and this equipment should allow first responders, whether they be police, fire, Hazmat, or EMS units, to detect the presence or absence of CWA. This equipment is available, reasonably priced, and will detect a wide array of chemical agents. The M9 paper and the M256 kit are simple and inexpensive devices that enable responders to rapidly detect classical CW agents. The photo-ionization detector, the ion mobility detector, the surface acoustic wave detector, and the colorimetric tubes give medical personnel an ability to deal with a wider array of chemicals. As a market evolves for these items of detection equipment, modifications for the civilian community will be made to simplify their usage and the costs associated with their acquisition and maintenance should decrease. 

Colorimetric or colour indicator (detector) tubes should be used as in the manufacturer s directions and thrown out on the use-by date. Some tubes give a colour with another chemical (called cross sensitivity ), so the manufacturer s notes should be checked. A new type as noted earher still uses detector chemicals but measures changes electronically, not by colour. 

Operation costs should include equipment purchase, maintenance, and consumables costs. Cost comparisons should be based on the cost per analysis per chemical. For example, the purchase price of certain colorimetric detectors may seem relatively low but each analysis will incur additional consumables costs. Thus, the final cost of an operation could be relatively high compared to automatic continuous types of devices due to the cost of required consumables. Thus, although an automatic detector may cost tens of thousands of dollars initially, it does not require large amounts of consumables, it detects a number of chemicals at once, and it has a very long operational life. 

EC sensors are relatively sensitive, as they react to chemical vapor concentrations at the low parts-per-million level. However, EC sensors are not as selective as colorimetric detectors (see Chapter 10). They may respond to various chemicals simultaneously without differentiation capability. This is because the oxidation-reduction reaction between the chemicals in the sample and the electrolyte controls the detection. Any chemicals contained in the sample that will react with the electrolyte on the working electrode surface will generate electrical current and are detected together with those from targeted chemicals. Using a chemical filter may reduce or eliminate some of the chemical interference potential. The nse of a second working electrode that responds to different sets of chemicals from the first working electrode within the same sensor may also lead to better selectivity. 

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