Tracer additives

Several experimental techniques may be used, such as acid/base titration, electrical conductivity measurement, temperature measurement, or measurement of optical properties such as refractive index, light absorption, and so on. In each case, it is necessary to specify the manner of tracer addition, the position and number of recording stations, the sample volume of the detection system, and the criteria used in locating the end-point. Each of these factors will influence the measured value of mixing time, and therefore care must be exercised in comparing results from different investigations. 

Figure 5.131. Response of Sections 1 and 3, corresponding to Fig. 5.130, with an additional tracer addition at T=1.8 to Section 3.

Figure 4. Time course of labeling of POM (particulate organic matter, primarily phytoplankton) with tracer additions of15N in the fertilized and control sides of Lake N2. The solid line represents the predicted labeling, given the turnover time of phytoplankton. Terrestrial detritus remained unlabeled throughout the...

Considerations in Stable-Isotope Experiments. Isotopic tracer additions should label the specific pool of interest quickly, before chemical or biological transformations of no interest distribute the tracer throughout the ecosystem. For calculation of transfers of material from one pool to another the isotope must be well mixed within the ecosystem in the compound it is supposed to trace, and the isotopic content between both pools should be given time to reach equilibrium. 

Tank, J. L., J. L. Meyer, D. M. Sanzone, P. J. Mulholland, J. R. Webster, B. J. Peterson, W. M. Wolheim, and Leonard, N. E. 2000. Analysis of nitrogen cycling in a forest stream during autumn using a 15N-tracer addition. Limnology and Oceanography 45 1013-1029. 

Carrier or Tracer Addition. To quantify the purified final sample that will be measured by a radiation detection instrument (as compared to a mass spectrometer), a carrier or tracer is added to the sample. The carrier usually is the same element as the radioanalyte ( isotopic carrier ) and is standardized, typically at 5-20 mg/mL concentration. The carrier serves two purposes to provide macro quantities so that certain chemical steps (such as precipitation) may be performed on the sample, and to determine the chemical yield, usually by weight. A tracer serves only to determine the chemical yield of the process its nanogram quantities or less, comparable to the radioanalyte in the sample, prevent use as carrier. The tracer is measured by its characteristic radiation at the same time as the radioanalyte. An advantage in alpha-particle spectral analysis is that the activity of the analyte can be calculated from the activity of the tracer without knowledge of the detector counting efficiency, as discussed below. 

The approach is most useful in water samples because complete mixing of the tracer is possible. In sediments, rate measurements are constrained by the inhomogeneous nature of the sample and the dependence of rates on the structure of the environment. In this situation, fluxes between overlying water and sediment cores can be analyzed to obtain areal rates. In conjunction with tracer addition, estimates of nitrification rates can be obtained from the dilution ofN02 or N03 in the overlying water due to its production in the sediments (Capone et ai, 1992). The isotope pairing method for measurement of denitrification (Nielsen, 1992 Rysgaard et ai, 1993) is essentially an isotope dilution approach from which both nitrification and denitrification rates can be calculated. 

Better rate measurements Although the currently available methods for direct assessment of nitrification rates and the geochemical constraints provide internally consistent estimates of the overall rates and distributions of nitrification, improvements in direct rate methods are sorely needed to investigate the effects of environmental variables on rates under realistic conditions. These improvements will likely come with the application of even greater sensitivity isotope tracer methods, which require less substrate perturbation for detection of signal in the product pool. In addition, stable isotope methods that do not require tracer addition may possess the sensitivity to detect small fractionation... 

In the N method, NH4+ or N02 is added to a sample and then the appearance of N label in the N02 or NOa pools is measured over time (e.g., Olson, 1981). The advantage of this technique is that it is a direct, unambiguous measure of the process. One disadvantage is that the increase in NH4+ or N02 concentrations caused by the tracer addition can result in enhanced NH4+ or N02 oxidation rates (e.g., Helder and De Vries, 1983 Ward et ai, 1984). When small additions of N are used, the isotope dilution of the substrate pool must be monitored over time. 

It is worth noting that the term isotope dilution has also been applied to apphcations where the concentration of a compound is determined isotopicaUy rather than by direct chemical determination. The most famihar appHcation is the determination of the magnitude of a blank (pre-existing in an analysis) by adding a known amount of to a sample and determining the decrease from the expected isotopic value of the tracer addition. This concept can also be used in field samples, such as for determining the concentrations of dissolved and combined amino acids. 

Kennedy M. J., Hedin L. O., and Derry L. A. (2002) Decoupling of unpolluted temperate forests from rock nutrient sources revealed by natural Sr/ Sr and Sr tracer addition. Proc. Natl. Acad. Sci. 99, 9639-9644. 

Values are from tracer addition experiments in water from the O2-H2S interface in Saanich Inlet. The oxidation rate was c.2nivih at a substrate concentration of about 0.5 pM and the bacterial catalytic site concentration, [C] = 50 nM, was estimated from zero-oxygen Mn " uptake experiments. 

The inlet concentration most often takes the form of either a perfect pitlse input (Dirac delta function), imperfect pulse injection (see Figure 13-4), or a step input-Just as the RTD function (/) can be determined directly from a pulse input, the cumulative distribution Fit) can be determined directly from a step input. We will now analyze a step input in the tracer concentration for a system with a constant volumetric flow rate. Consider a constant rate of tracer addition to a feed that is initiated at time t = 0. Before this time no tracer was added to the feed, Stated symbolically, we have... 

Special Oil. [Hals Am.] Glyceryl triesters tracer additive to other fats such as dairy butler. rdease agent lubricant in cosmetic emulsions, lipsticks, hair care prods. pigment dispersant for sticks and liners. 

The impulse response is an idealized limit of the pulse response. Consider a family of pulse tests as sketched in Figure 8.2 of shorter and shorter duration At. We maintain constant total tracer addition by spiking the feed with higher and higher concentrations so that the product CfAt == a is constant. The impulse response is the limit of this experiment as At — 0. We call this limiting feed concentration versus time function the delta function, af5(t). It is also called... 

Carriers are added to radionuchde solutions, as discussed in Section 4.7, to perform precipitation separations, avoid radiocolloidal behavior, and to eliminate the need for quantitative recovery by allowing the analyst to calculate the fractional chemical recovery ( yield ). A radioactive tracer (see Section 4.7.3) can be added to the sample in addition to or instead of the carrier to measure yield. Tracer addition may make yield determination easier than carrier addition, or may be necessary if no carrier is available. 

The critical requirement for alpha-particle spectrometry to achieve good characteristic peak resolution, shown in Fig. 9.1 and described in Section 8.3.3, is the preparation of a very thin and uniform source. Less rigorous criteria for resolution can be met if the several radioisotopes to be measured and the tracer have widely different alpha-particle energies, only a single radionuclide is known to be present either initially or after chemical separation, or the measurement is for screening purposes. A sample may be prepared for screening with limited prior separation or none, and without tracer addition, if previous tests have demonstrated that the process causes no losses for all radionuclides of interest. 

A separation procedure with the yield Y below unity is expected in radioanalyt-ical chemistry. For quality assurance purposes, the range of acceptable yields may be set between 0.5 and 1.0. Lower or higher yield fractions suggest occurrence of analytical process problems. Carrier or tracer addition to determine yield is described in Section 6.3. 

One concern is partial or complete loss of the radionuclide before carrier or tracer addition to monitor such loss. Carrier or tracer often are added after these concentration and dissolution processes to achieve complete interchange with the radionuclide in solution (see Section 4.5.2). If, on the other hand, carrier or tracer is added before dissolution, interchange of carrier or tracer with the radionuclide of interest may not occur. 

If the volume of tracer added is known it is a simple matter to calculate the equilibrium concentration, C . The tracer concentration at point A will approach this equilibrium concentration as shown in Figure 8.7. The mixing time may then be defined as the time required from the instant of tracer addition for the concentration at A to reach the equilibrium value. However, this time depends upon the way in which tracer is added and the location of point A. Furthermore, the concentration approaches C, in an asymptotic manner and the end point of the experiment is difficult to detect with precision. 

In the tracer technique, the tracer (usually a pulse of electrolyte solution) is injected into the tank. The tracer concentration is then measured with respect to time at a point, or at several points, within the tank. The mixing time is taken as the time at which the tracer concentration, C, at the measurement location has reached, or has nearly reached, the expected final mean tracer concentration C. If there is no tracer initially present in the tank then a mixing time, can be defined as the time from tracer addition to the time when... 

In most experimental studies 95 is measured, i.e. the time from tracer addition to the time when m=0.05. This is probably the lowest value of m which can be both easily and accurately measured. Experimental evidence suggests that towards the end of the mixing process the rate of mixing is a flrst order process . We can thus relate (93 to any arbitrary mixing time by the relationship... 

Okita and Oyama, using 0.4 m- and 1.0 m-diameter tanks, injected a tracer pulse at the centre of the tank near the liquid surface. They defined the mixing time as the time between tracer addition and the moment at which there was no difference between the concentration measured by two probes, one located near the tank floor at the wall and the other near the liquid surface at the wall. They correlated their data, with an accuracy of 30% for both axial jet and side-entry jet configurations by... 

Continuous tracer addition to a mixed flow reactor... 

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