Field Interference Test

The ability to detect a substance in the laboratory under clean, controlled conditions is insufhcient to determine whether a detection device is useful. The units need to be tested in the field with common potential interference substances such as engine exhaust, burning fuels, and other burning materials such as common clothing and building materials. Such tests reveal potential problems under real-world operational conditions. Table 3.14 lists potential field interference substances tested. 

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. 

Even if a particular detector has CWA detection and identification ability, its usefulness is severely diminished if it shows a similar response to too many other substances. Table 3.15 lists common potential interference substances to be tested. Others can be added as necessary. 

Diesel exhaust, revved HTH (supertropical bleach, 107, calcium hypochloride) vapor 

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Table 3.14 List of Potential Field Interferents to be Tested Outdoors...

The polymer injection project consists of 18 producing wells and 4 injectors, distributed among 3 contiguous zones, representing a pore volume of 640 000 m. Four producers were turned into injectors for this purpose, but no additional well was drilled. The choice of the injectors was based on the results of interference tests and the isobar and isopach maps (fig. 2 and 3). Before injection, the cumulative production of the Courtenay field was 332 000 m3 oil, with an average oil cut of about 11 %. 

More recently, Orion has introduced series 93, which uses the same basic principles as series 92, but miniaturized to the size of the electrode tip. Its construction is illustrated in Fig. 28. When the electrode stops functioning, after approximately one-half a year s use according to the manufacturer, the electrode tip is simply unscrewed and replaced with a new, factory-tested one. Air bubbles and electric fields interfere far less with this construction than with the series 92. 

Electromagnetic interference (EMI) testing has become more prevalent for materials that either emit or are affected by EMI. Shielding efficiency (SE) of materials is deterrnined by measuring electric field strength between a transmitter and receiver with or without the presence of the material under test. Several researchers have suggested a correlation between volume resistivity and SE values (300,301). 

Pesticides used on crops grown on the test site in previous seasons may also have an impact on the outcome of a field residue trial. Carryover of prior pesticide applications could contaminate samples in a new trial, complicate the growth of the crop in a trial, or cause interference with procedures in the analytical laboratory. For this reason, an accurate history of what has transpired at the potential test site must be obtained before the trial is actually installed. The protocol should identify any chemicals of concern. If questions arise when the history is obtained, they should be reviewed with the Study Director prior to proceeding with the test site. In most annual crop trials, this will not be a significant issue owing to crop rotations in the normal production practices, because the use of short residual pesticides and different chemical classes is often required for each respective crop in the rotation. However, in many perennial crops (tree, vines, alfalfa, etc.) and monoculture row crops (cotton, sugarcane, etc.), the crop pesticide history will play a significant role in trial site selection. 

Having received the pre-weighed test item, preparation for its use in the field must be made. Ideally, water to be used in the dilution of the test item should be from mains water or a recognized source. The use of water from standing pools, rivers, etc., could potentially lead to problems with interference from contaminants during analysis of the crop samples. Depending on the formulation under test, the test item can be mixed in a variety of ways. First, the required water volume must be accurately measured. Approximately half of this amount can be poured into a clean bucket or similar mixing container. The temperature of the water should be noted at this point... 

Untreated (control) soil is collected to determine the presence of substances that may interfere with the measurement of target analytes. Control soil is also necessary for analytical recovery determinations made using laboratory-fortified samples. Thus, basic field study design divides the test area into one or more treated plots and an untreated control plot. Unlike the treated plots, the untreated control is typically not replicated but must be sufficiently large to provide soil for characterization, analytical method validation, and quality control. To prevent spray drift on to the control area and other potential forms of contamination, the control area is positioned > 15 m away and upwind of the treated plot, relative to prevailing wind patterns. 

Once you have confidence that your method is adequate from the preliminary work in the method tryout, you are ready to begin the method validation. The method validation provides additional data on accuracy and precision, and confirms that there are no problems due to interference. Method validation must be completed before beginning the analysis of the treated samples from the field. The validation should test the detector s response over the expected range of concentrations from the field. 

The analytic principles that have been applied to accumulate air quality data are colorimetry, amperometry, chemiluminescence, and ultraviolet absorption. Calorimetric and amperometric continuous analyzers that use wet chemical techniques (reagent solutions) have been in use as ambient-air monitors for many years. Chemiluminescent analyzers, which measure the amount of chemiluminescence produced when ozone reacts with a gas or solid, were developed to provide a specific and sensitive analysis for ozone and have also been field-tested. Ultraviolet-absorption analyzers are based on a physical detection principle, the absorption of ultraviolet radiation by a substance. They do not use chemical reagents, gases, or solids in their operation and have only recently been field-tested. Ultraviolet-absorption analyzers are ideal as transfer standards, but, as discussed earlier, they have limitations as air monitors, because aerosols, mercury vapor, and some hydrocarbons could, interfere with the accuracy of ozone measurements made in polluted air. 

This method has been used to analyze both symmetrical (C.I. Direct Red 28 and C.I. Direct Blue 6) and unsymmetrical (C.I. Direct Black 38 and C.I. Direct Brown 95) benzidine-based dyes. Based on this work, the application of the method to other benzidine-based dyes should be straightforward. When field samples are submitted for benzidine-based dye analysis, bulk samples of the dyes present in the sample also should be submitted. With these bulk samples, the analyst should be able to determine if this method is applicable to the various dyes submitted and if any interferences are present. The method presently has not been tested on field samples. An existing sampling method (J 3) for azo dyes and diazonium salts should be directly applicable to this method with a change from a cellulose ester to a Teflon filter. This change is necessary to insure quantitative recovery of the sample from the filter. 

The bias determined in the validation referred to the difference between average results of the test method and average results of the independent reference method. However, it was recognized that other sources of bias, e.g. some interferences, may increase the true bias of the method in some unique field situations. 

The most widely accepted method of evaluating the accuracy and precision of an analytical procedure is to sample known concentrations of contaminants in the atmosphere. Thus an important aspect of analytical method development is the generation of test atmospheres that simulate the conditions (i.e., concentration range, humidity, temperature and interferences) found during the field sampling. 

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