Tracer materials

Tracer materials are defined as any product included in the test substance that can be recovered analytically for determining the drift from the application. This may be the active ingredient in an actual tank mix, or it may be a material added to the tank mix for subsequent detection. The selection of an appropriate tracer for assessing deposition rates in the field is critical to the success of a field study. Tracer materials such as low-level active ingredient products, colored dyes, fluorescent dyes, metallic salts, rare earth elements and radioactive isotopes have been used with varying degrees of success in the field. An appropriate tracer should have the following characteristics  

When a tracer is considered, it is important to evaluate its performance with respect to these criteria, especially stability during exposure and storage/analysis. Normal practice involves conducting weathering tests where field collectors are treated with known amounts of the tracer and an assessment is made of weathering, extraction and storage stability under conditions pertaining to the intended use. The characteristics of the tracer allow it to be applied uniformly over the application area. Typically, application monitors are used to verify both the application rate and the uniformity of the application. 

Active ingredient tracers or test substances can be quantified using gas chromatography (GC) and high-performance liquid chromatography (HPLC) analyses. 

Colored and fluorescent dyes have the advantage of being relatively cheap and easy to use. Standard procedures are available for detection of the dyes using colorimeters and fluorimeters. Some of these instruments can be used in the field to analyze samples as they are collected following exposure to the dyes. Fluorescein has been widely used for studying spray deposition within and outside canopies.  

A problem that has been encountered with many dyes is that they tend to degrade in sunlight. The SDTF studied fluorescent dyes in various laboratories and found that Eosine OJ and Tinopal CBS-X were relatively stable. However, when the same dyes were used in the field, it was discovered that they were not stable under warm and humid conditions owing to aqueous photolysis. The SDTF therefore decided to use dilute active ingredient and metal tracers that were more stable. 

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Polyester composition can be determined by hydrolytic depolymerization followed by gas chromatography (28) to analyze for monomers, comonomers, oligomers, and other components including side-reaction products (ie, DEG, vinyl groups, aldehydes), plasticizers, and finishes. Mass spectroscopy and infrared spectroscopy can provide valuable composition information, including end group analysis (47,101,102). X-ray fluorescence is commonly used to determine metals content of polymers, from sources including catalysts, delusterants, or tracer materials added for fiber identification purposes (28,102,103). 

Kinds oi Inputs Since a tracer material balance is represented by a linear differential equation, the response to anv one kind of input is derivable from some other known input, either analytically or numerically. Although in practice some arbitrary variation of input concentration with time may be employed, five mathematically simple input signals supply most needs. Impulse and step are defined in the Glossaiy (Table 23-3). Square pulse is changed at time a, kept constant for an interval, then reduced to the original value. Ramp is changed at a constant rate for a period of interest. A sinusoid is a signal that varies sinusoidally with time. Sinusoidal concentrations are not easy to achieve, but such variations of flow rate and temperature are treated in the vast literature of automatic control and may have potential in tracer studies. 

A practical method of predicting the molecular behavior within the flow system involves the RTD. A common experiment to test nonuniformities is the stimulus response experiment. A typical stimulus is a step-change in the concentration of some tracer material. The step-response is an instantaneous jump of a concentration to some new value, which is then maintained for an indefinite period. The tracer should be detectable and must not change or decompose as it passes through the mixer. Studies have shown that the flow characteristics of static mixers approach those of an ideal plug flow system. Figures 8-41 and 8-42, respectively, indicate the exit residence time distributions of the Kenics static mixer in comparison with other flow systems. 

Identify the flow pattern of the prototype system by subjecting it to an impulse, step, or sinusoidal disturbance by injection of a tracer material as reviewed in Chapter 8. The result is classified as either complete mixing, plug flow, and an option between a dispersion, cascade, or combined model. 

Evaluation can be performed by measuring capture efficiency using real contaminants and applying the real process or by substituting with tracer materials. A simpler, but qualitative, method of evaluation is the visualization of the airflow. If the relationship between capture efficiency and airflow rate is known, a measurement of the airflow rate can be used for frequent evaluation. See Section 10.5. 

Capture efficiency is the fraction of generated contaminant that is directly captured by the hood. Measurement of capture efficiency involves measuring concentration of process-generated contaminant or a tracer material. Using process-generated contaminant requires use of instruments suited to each specific contaminant and its conditions (temperature, pressure, concentration, form, etc.). In order to facilitate these measurements, a tracer is often substituted for the process-generated contaminant. The tracer is usually a gas (sulfur hexafluoride, nitrous oxide, helium, or similar), but an aerosol (particles) can also be used (potassium iodide, polystyrene particles, microbiological particles, etc.). The chosen tracer should be as similar to the real contaminant as possible, but at the same time should... 

Neuroanatomists have taken advantage of the phenomenon of fast retrograde transport to locate remote nerve cell bodies in the CNS of an experimental animal that are connected to an identified axonal fiber tract whose origin is uncertain. The tracer material [purified horseradish peroxidase (HRP) enzyme] is injected in the region of the axon terminals, where it is taken up by endocytosis and then is carried by retrograde axonal transport over a period of several hours to days back to the nerve cell body. The animal is sacrificed, and the enzyme tracer is localized by staining thin sections of the brain for peroxidase activity. 

Purification of the radioactive tracer was modified to include a fractional sublimation before a single extraction—recrystallization cycle to conserve the tracer material. Microgram samples were prepared in melting point capillaries for assay by mass spectroscopic analysis (Table III), made by direct probe injection of the sample into the ion source (18). The probe was heated rapidly to 200°C, and mass spectra were obtained during vaporization of the sample. Tri-, tetra-, and pentachlorodibenzo-p-dioxins vaporized simultaneously with no observed fractionation. 

Sensitive analytic procedures enable detection and measurement of very low tracer levels. In tracer studies, an identifiable tracer material is injected through one or more injection wells into the reservoir being studied. Water or other fluid is then injected to push the tracer to one or more recovery wells in the reservoir. The output of the recovery wells is monitored to determine tracer breakthrough and flow through the recovery wells. Analysis of the breakthrough times and the flows yields important information regarding how to perform the secondary or enhanced recovery processes. 

Nuclear medicine scans Method of body imaging that uses a radioactive tracer material (e.g., technetium and gallium) to produce body images. For example, bone scans detect uptake and cellular activity in areas of inflammation. 

Consider a small volume of fluid q a entering the vessel virtually instantaneously over the time interval dt at a particular time (t = 0). Thus q 0 = qdt, such that q V and dt t. We note that only the small amount q 0 enters at t = 0. This means that at any subsequent time t, in the exit stream, only fluid that originates from q is of age f to t + dt all other elements of fluid leaving the vessel in this interval are either older or younger than this. In an actual experiment to measure E(t), q g could be a small pulse of tracer material, distinguishable in some manner from the main fluid. In any case, for convenience, we refer to q 0 as tracer, and to obtain E(t), we keep track of tracer by a material balance as it leaves the vessel. Note that the process is unsteady-state with respect to q 0 (which enters only once), even though the flow at rate q (which is maintained) is in steady state. 

This shows how the amount of tracer material in the vessel, q that originated from q 0, varies with time t. ... 

Table 19.4 gives the calculations of E(t), t, and of based on the histogram method. Column (4) lists the values of c j calculated from (A). Column (5) gives At, required for the calculations in subsequent columns, from equations 19.3-12 to -14. Column (6) gives values for the tracer material-balance (see below). Column (7) gives values of E t) from equation 19.3-4 with C(t) = E(t). Columns (8) and (9) give values required for the calculation of Tin equation 19.3-7, and of of in 19.3-8, respectively. 

For the tracer material-balance relation, equation 19.3-3 is written as... 

Since a tracer material balance is represented by a linear differential equation, the response to any one kind of input is derivable from the response to some other known input either analytically or numerically. This is evident from a comparison of transformed equations. Take for instance,... 

Reactors sometimes conform to some sort of ideal mixing behavior, or their performance may be simulated by appropriate combinations of ideal models. The commonest ideal elements are stated following, together with their tracer material balances. Initial values, boundary conditions and solutions of the equations depend on the kinds of inputs and are stated with individual solved problems. 

Residence time distributions can be determined in practice by injecting a non-reactive tracer material into the input flow to the reactor and measuring the output response characteristics in a similar manner to that described previously in Section 2.1.1. 

An injected slug of tracer material flows with its carrier fluid down a long, straight pipe in dispersed plug flow. At point A in the pipe the spread of tracer is 16 m. At point B, 1 kilometer downstream from A, its spread is 32 m. What do you estimate its spread to be at a point C, which is 2 kilometers downstream from point A ... 

The experimental technique used for finding this desired distribution of residence times of fluid in the vessel is a stimulus-response technique using tracer material in the flowing fluid. The stimulus or input signal is simply tracer introduced in a known manner into the fluid stream enter-... 

In a technique known as medical imaging, tracers are used in medicine for the diagnosis of internal disorders. Small amounts of a radioactive material, such as sodium iodide, Nal, which contains the radioactive isotope iodine-131, are administered to a patient and traced through the body with a radiation detector. The result, shown in Figure 4.11, is an image that shows how the material is distributed in the body. This technique works because the path the tracer material takes is influenced only by its physical and chemical properties, not by its radioactivity. The tracer may be introduced alone or along with some other chemical, known as a carrier compound, that helps target the isotope to a particular type of tissue in the body. 

To determine the retention time of water in this vessel, a tracer was mixed in water and injected into the flowstream before it entered the gas boot. Samples were taken at the water outlet and then were analyzed for tracer material. As indicated in Fig. 4, the peak retention time was 19 minutes. 

For many cases in which the RTD cannot be calculated theoretically, experimental techniques have been developed to measure it. Such techniques are used by introducing a tracer material into the system and recording its concentration at the exit.9 These methods are discussed in great detail in the literature. In general, a step change in tracer concentration results directly in the F(t) function, and an impulse type of tracer injection results directly in the/(f) function. 

It should be noted, however, that most of these techniques assume a plug-type inlet flow into the system. If this is not the case, special care must be taken in introducing the tracer material (e.g., in an impulse signal, the amount of tracer must be proportional to the local velocity, otherwise complex corrections may be required). 

Suppose that a quantity m of tracer is injected at the inlet of a system during a period of time, which is very short compared to the mean residence time, t. The concentration of this tracer material is measured in the outlet stream as a function of time. Assume the following conditions ... 

Plug Flow Reactor The tracer material balance over a differential reactor volume dVr is... 

The nature of the tracer dictates the detection system. For the liquid phase, quite often the tracers (e.g., NaCl, H2S04, etc.) are such that the detection probe is directly inserted into the reactor and continuous monitoring of the concentration at any fixed position is obtained by means of an electrical conductivity cell and a recorder. In this case, no external sampling of liquid is necessary. If the tracer concentration measurement requires an analytical procedure such as titration, etc., sampling of the liquid is required. For the solid phase, a magnetic tracer is sometimes used. The concentration of a solid-phase tracer can also be measured by a capacitance probe if the tracer material has a different dielectric constant than the solid phase. In general, however, for solid and sometimes gas phases, some suitable radioactive tracer is convenient to use. The detection systems for a radioactive tracer (which include scintillation counters, a recorder, etc.) can be expensive. Some of the tracers for the gas, liquid, and solid phases reported in the literature are summarized in Table 3-1. 

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