Tracer methods

As tracer methods ( English trace to , trace ',' track ') is the developed with the help of tracers processes for tracking and investigating conditions. Tracer methods are used in many areas, including occupational safety and environmental protection, material analysis, and hydrology.

Applications of tracer methods

Determination of the concentration of active substances at the workplace

This method takes place in the field of occupational safety and environmental protection, such as B. to determine the concentration of pesticides . The tracers used here are NaCl (table salt), BSF (brilliant sulfoflavin) and PBM (plant treatment agents). When determining concentrations of active substances or tracer concentrations at the workplace, it is usually a matter of determining substances foreign to the air. The concentration is determined via air samples, as well as their separation, processing with subsequent analysis for pollutants.

There are also proven measuring methods for measuring pesticides. It is examined how the active ingredient gets to the plant as intended. Here, fluorescent dyes are used as tracers, and the transport and action mechanisms of the active ingredients on and in the plant are examined, as well as their residues in the feed and food chain.

Another type of measurement is drift measurement; this measurement method is not about the target plant itself, but about the plants and other organisms in the vicinity. Thus, this method is used to examine how much of the active ingredient is stressing the workplace.

A new task, especially in the field of occupational safety and environmental protection, is to adapt the known measurement methods for pollutants to the special conditions of the workplace, such as the application of pesticides. An essential condition for all methods is the taking of air samples. However, it should be noted that due to the technical application and the weather conditions, temporal and local fluctuations in the active ingredient concentration can occur. Factors such as the direction of incidence of the wind, droplet size and the proportion of active substances in solid, liquid and gaseous aggregate states play a role.

Long-term measurements can be used to make statements about the mean concentration values of an 8-hour working day or the maximum concentration of active substances.

Qualitatively, however, the attempt is made to clarify the causality, i.e. H. what are the mechanisms of action that contribute to plant protection products reaching the workplace, and how to make a statement about how the above-mentioned factors play a role in these mechanisms.

Shorter measurement times are required to become independent of these factors. In practical field use, the measuring times are limited to 5 to 10 minutes by filling the tank. In order to achieve a quantitative concentration with such short times, a large volume flow is necessary when taking the sample.

Standard impinger washing bottles or glass fiber filters with a cold trap are used for the measurements .

In the impinger washing bottle there is a washing liquid which only serves to absorb the separated particles. However, it has been shown that impinger washing bottles also separate gaseous particles if the detergent is matched to the active substance. Due to the large air flow rate of 30 l / min, only solvents with a vapor pressure that is not too low can be used. The impinger wash bottle has the advantage that the active ingredient is already available in dissolved form for further analysis.

The glass filter with cold trap uses liquid oxygen for cooling, which starts to boil at −183 ° C. The glass filter can be used universally for the measurement of all pesticides in the workplace because it separates all solid, liquid and gaseous active ingredients. The liquid oxygen cools the active ingredient components down to −80 ° C; this low temperature of the filter also prevents the active ingredient components that have already been separated from starting to evaporate.

One possibility of error in the glass filter is its large concentration gradient. Care must be taken that during handling and extraction no active ingredients come into the interior of the system from outside and thus falsify the result.

Nevertheless, a more detailed investigation into the degree of recovery and reproducibility is still necessary. A few attempts with impinger washing bottles have produced contradicting results that are still in need of clarification.

Characteristic values and suitability of separators for airborne plant treatment agents and tracers in the workplace. Data according to Lee
		Glass fiber filter	α-cellulose filter	Cold trap + glass fiber filter	Impinger wash bottle *	coated carrier *	Pu foam*
Air flow	m³ / h	until 50	until 50	2.5	to 1.8	17th	45
firmly Liquid separation efficiency gaseous	% % %	> 99 > 99 -	<90 > 99 -	> 99 > 99 70-90	> 95 > 99 13-99	67-100 67-100 67-100
NaCl BSF recovery rate PBM	% % %	> 99 50-99 50-100	(> 99)	70-80	13-99	67-99	79-99
NaCl Reproducibility BSF PBM		++ - (-)	(+)	+	(+)
effort		low	low	big	medium	medium	medium
especially suitable for		Tracer NaCl	Tracer BSF	PBM universal	PBM special	PBM special	PBM special

The table shows the effectiveness of the individual filter methods with different tracers. It can be seen that the glass filter with NaCl as a tracer has a very good degree of separation of the solid and liquid phase, as well as very good reproducibility.

It is therefore necessary to clarify what and how to measure in order to determine the optimal measurement method.

Tracer methods for corrosion testing

Tracers are also used for corrosion studies, radioactive isotopes are used for this. This is used to analyze the distribution on the material surface. Most of the investigations relate to corrosion in aqueous solution. Tracer methods for determining oxygen uptake on surfaces can be divided into three main classes.

The three main classes of corrosion testing

“Post immersion counting” or “counting after immersion”. With this method, the material is dipped into the radioactive solution and then removed, rinsed and dried. The radioactivity is measured with a Geiger counter for β-radiation or a scintillation counter for γ-radiation . This technique can be performed on any sample and is independent of the type and concentration of the radioisotope . A disadvantage of this method, however, is that only the irreversible mass transfer to the surface can be measured by rinsing. In addition, the surface of the metal sample can change when the sample is removed, e.g. B. an oxide layer can arise. The distribution of the masked material on the surface can be determined after the removal of the attacking agent with the help of selective counting of the pulses from individual surface areas or by means of autoradiography . The selective counting gives a quantitative picture of the distribution over different areas of the total surface. It has been shown here that chloride ions are concentrated in the more mechanically stressed areas.
“Depletion technique”: With this technique, the difference in concentration of the tracer is measured before immersion and after immersion in the solution. In this way it can be determined how many radioactive isotopes the material has absorbed. In order to get sufficient reasonable data, the concentration difference should be big enough. A relatively large metal surface with a small volume of solution with a low concentration is used here. The drop in concentration is then measured as part of the solution is withdrawn. Almost any radioisotope can be evaluated with this method.
"Immersion counting method": With this method, the amount of tracer that is on the metal is measured directly while it is still in the solution. For this purpose, however, the background radiation emanating from the solution must be kept as small and constant as possible so that the radiation from the tracer can be measured. One possibility of limitation is to only use radioisotopes that release very low-energy β radiation. The immersion counting method is a suitable method for the simultaneous measurement of electrochemical quantities and adsorption . This procedure creates a better understanding of how the corrosion process takes place on the metal surface.

Economic benefit

The investigation of the wear of metals, such as corrosion, friction or lubrication, is a not unimportant factor of industrial nations. In 1966 the damage caused by wear and tear was around 6 billion euros. Because of this, the optimization of the factors influencing wear is a not inconsiderable reason for increasing the service life and quality .

Tracer methods in process optimization

Tracer methods are also popular in the process optimization of liquid reactors. With the tracer, the flows and the residence time in the individual areas of the reactor can be displayed. For this purpose radioactive nucleotides are used as tracers. For this purpose, the solvent of the tracer is applied to the inlet of the reactor in the form of a pulse in order to then measure the pulse rate over time at the outlet.

Tracer Methods in Hydrology

In this area of hydrology , attempts are made to determine the “age of the groundwater” or the time the groundwater remains in the subsurface, i.e. its relative flow velocity in the aquifer . The age of the groundwater can also be determined directly from the distribution of certain isotopes, which in turn decay underground or are also produced there at a certain rate.

The problem with many different isotope methods is that there is a method-specific dwell time for each. Various tracers (geosciences) are used in the field of hydrology .

Individual evidence

↑ ^a ^b Gerd-Jürgen Mejer: On measurement technology including tracer methods for determining the concentration of active substances at the workplace . In: Basic agricultural technology . tape 34 , no. 2 . Braunschweig-Völenrode 1984, p. 72-76 .
↑ RE Lee Jr. (Ed.): Air pollution from pesticides and agricultural processes. CRC Press, Boca Raton FL 1976.
↑ ^a ^b ^c ^d J. GN Thomas: Tracer methods for corrosion testing . No. 11 . Teddington July 8, 1968, p. 957-960 .
↑ G. Polzer: Examples of the economic importance of wear tests using radioactive nuclides . In: Isotopes in Environmental and Health Studies . 1969, ISSN 0021-1915 , doi : 10.1080 / 10256016908621789 .
↑ W. Pippel, HJ Oelmann, M. Klötzer: Development of a reactor for liquid phase reactions on the basis of residence time measurements with the help of tracer methods . In: Isotopes in Environmental and Health Studies . 1986, ISSN 0021-1915 , pp. 321-322 , doi : 10.1080 / 10256018608623682 .
↑ Petra C. Blaser: Tracer Methods in Hydrology . Ed .: Ghent University. Gent 2007, p. 11-12 ( ugent.be ).

[Mejer-1] Gerd-Jürgen Mejer: On measurement technology including tracer methods for determining the concentration of active substances at the workplace . In: Basic agricultural technology . tape 34 , no. 2 . Braunschweig-Völenrode 1984, p. 72-76 .

[2] RE Lee Jr. (Ed.): Air pollution from pesticides and agricultural processes. CRC Press, Boca Raton FL 1976.

[Thomas-3] J. GN Thomas: Tracer methods for corrosion testing . No. 11 . Teddington July 8, 1968, p. 957-960 .

[4] G. Polzer: Examples of the economic importance of wear tests using radioactive nuclides . In: Isotopes in Environmental and Health Studies . 1969, ISSN 0021-1915 , doi : 10.1080 / 10256016908621789 .

[5] W. Pippel, HJ Oelmann, M. Klötzer: Development of a reactor for liquid phase reactions on the basis of residence time measurements with the help of tracer methods . In: Isotopes in Environmental and Health Studies . 1986, ISSN 0021-1915 , pp. 321-322 , doi : 10.1080 / 10256018608623682 .

[6] Petra C. Blaser: Tracer Methods in Hydrology . Ed .: Ghent University. Gent 2007, p. 11-12 ( ugent.be ).