Radiochemical Methods in Analysis || The Use of Tracers in Inorganic Analysis

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<ul><li><p>Chapter 8 </p><p>The Use of Tracers in Inorganic Analysis </p><p>1. W. McMillan Applied Chemistry Division, AERE Harwell, Didcot, Oxfordshire </p><p>8.1 THE APPLICATION OF TRACERS IN THE INVESTIGATION Page </p><p>OF ANALYTICAL METHODS 297 8.1.1 Introduction, 297 8.1.2 Basic Considerations, 298 8.1.3 Applications, 302 </p><p>8.2 QUANTITATIVE RADIOACTIVE TRACER ME;rHODS 315 8.2.1 Introduction, 315 8.2.2 Isotopic Methods, 317 8.2.3 Non-isotopic Methods, 330 8.2.4 Radiometric Titrations, 334 </p><p>8.1 THE APPLICATION OF TRACERS IN THE INVESTIGATION OF ANALYTICAL METHODS </p><p>8.1.1 Introduction Tracer methods make an important contribution to the investigation of analytical procedures. An analyst seeking a new method for the determina-tion of a particular element or ion in a new matrix may attempt to adapt an established technique or in extreme circumstances institute an entirely new method based on a new reagent or a novel principle. In either case, the investigation of the new procedure can be speeded and clarified by the use of radioactive tracer methods. </p><p>A typical analysis may be broken down into a number of stages: 1. Preliminary treatment; sample collection, concentration, ashing and </p><p>solubilisation. 2. Separation; removal of interfering species by distillation, solvent </p><p>extraction, ion exchange, chromatography, precipitation, isotope exchange, adsorption and electrodeposition. </p><p>3. Measurement; gravimetry, titrimetry, colorimetry, absorptiometry, emission spectroscopy, mass spectrometry, polarography, coulometry and radiometry. </p><p>Throughout the analytical procedure the main aims are quantitative recovery of the analysed species and efficient removal of interferences, </p><p>297 D. I. Coomber (ed.), Radiochemical Methods in Analysis Plenum Press, New York 1975</p></li><li><p>298 J. W. McMillan frequently through separation methods. Radioactive forms of the element or ion being determined and of those interfering, when added to the sample, trace the behaviour of the inactive species throughout the analysis. Measure-ment of the initial tracer activity and its intensity at each stage of the analysis allows the occurrence of losses, the efficiency of individual opera-tions and the overall recovery to be assessed rapidly and accurately. When a particular step is suspect, radioactive tracers can be used with advantage for its study in depth. For instance, in solvent extraction, many parameters may require study and the number of experiments can escalate to a formidable level. Measurement of the distribution of the investigated species between the two phases after each equilibration can be simplified enormously by tracer measurements, as this only entails withdrawal of specimens of each phase and comparison of their radioactivity in a simple counting apparatus. </p><p>Tracer methods can be used successfully not only for the investigation of methods for the measurement of a stable element or ion, but also when the recovery of induced activity is required, as in activation analysis or radio-chemical analysis. In this case separation from other radioactive species with a high degree of decontamination is the main objective. Decontamination factors in excess of 106 are frequently required, much higher than those usually needed in conventional analysis. In these analyses quantitative recovery is sacrificed for the sake of efficient separation, and the recovery of the active species is often ascertained by the addition of a known amount of inactive carrier, the technique being commonly referred to as reverse isotope dilution (see p. 328). </p><p>The principles and techniques involved in the application of tracer methods can be demonstrated by examples drawn from various areas of inorganic analysis. However, a number of basic principles common to all tracer methods are considered first. </p><p>8.1.2 Basic Considerations When planning a radioactive tracer study of an analytical method, or one of its stages, a number of factors must be considered. These include the avail-ability of a suitable isotope, its chemical form, its behaviour in the system studied, the amount of activity required, the form in which it should be counted, and, not least, the health hazards involved. Many of these factors interact and mutually contribute to the overall design of the experiment. </p><p>Today, a comprehensive range of radioisotopes is available commercially [1 J. Unfortunately there are a number of elements for which convenient radioactive isotopic tracers do not exist [2,3], e.g. elements with only isotopes of short half-life such as oxygen esO, t'h = 2.03 min) and boron (12B, t'h &lt; 1 sec). For such elements, tracing with enriched inactive isotopes is a possible alternative, but this lacks the convenience of the radioactive isotope method because of the difficulty of following the progress of the inactive tracer. </p><p>Perhaps the most important property of the tracer is its half-life. The reduction in the activity is large if several half-lives elapse during the course of the experiment so it is preferable to select an isotope with a half-life that </p></li><li><p>The Use of Tracers in Inorganic Analysis 299 is long compared to the duration of the experiment. By doing so the problem of handling unduly large levels of activity at the beginning of the experiment and that of having to make large decay corrections are avoided. Decay corrections can be avoided by measuring both the activity of the recovered tracer and that of the master solution at the end of the experi-ment. A further disadvantage of using short-lived isotopes is that the experi-ments must be carried out near to a source of isotopes; a nuclear reactor or particle accelerator. This problem can be avoided if the isotope of interest is the daughter of a long-lived parent when the daughter can be 'milked' from the parent immediately before use [4], e.g. 90y from 9OSr, or 44SC from 44Ti. One advantage of using short-lived tracers is that their rapid decay eases the problem of ultimate disposal. If a short-lived tracer must be used, the analytical method under study may have to be broken down into short steps each compatible with the half-life of the tracer. </p><p>The purity of the radioactive tracer is of considerable importance. Two principle types of impurity exist. First, radionuclidic impurity, the presence of radioactive nuclides other than that desired. Second, radiochemical impurity, the presence of the nuclide of interest but in a chemical form differing from the one specifically needed. The first type of impurity, if non-isotopic, will lead to errors because its chemical behaviour will probably differ from that of the main tracer. If the impurity is isotopic, it may lead to erroneous decay corrections. The second type of impurity, e.g. the presence of 32P-1abelled pyrophosphate in orthophosphate tracer, could lead to difficulties in orthophosphate tracing studies. Radiochemical purity is normally less important in inorganic than in organic tracer studies where the presence of the tracer in the correct chemical form is absolutely essential. </p><p>The technique of tracer analysis is very dependent on the identical behaviour of the tracer and the inactive element or ion. In order to ensure this, a vital objective is the earliest possible mixing of the tracer and stable species. Naturally, chemical identity of the tracer and stable species is essen-tial and the chemical conditions may need to be adjusted in order to achieve exchange. However, care is needed in those instances where ions rather than elements are being traced in order to avoid unwanted exchange. </p><p>An obvious difficulty arises when the tracer must be added to a solid material, perhaps to study the effects of ashing or dissolution when the results must be interpreted with great caution. The mixing of tracers with liquids and gases poses few problems. A further cause of the non-identical behaviour of radioactive tracers and the stable species is attributable to the difference in mass of the active and inactive isotopes; the isotope effect. This effect is greatest when the ratio of the isotopic masses is greatest, Le. for hydrogen eH) compared to tritium CH), and it decreases with increas-ing atomic number and by incorporation of the isotope into an ion or molecule. Fortunately the effect is small for the separation processes encountered in inorganic analysis, seldom exceeding 1% per stage. While correction for the isotope effect may be necessary for the lightest elements, it can usually be ignored. </p><p>Finally, radiation effects must be mentioned. The chemical nature of a </p></li><li><p>300 1. W. McMillan </p><p>high specific activity tracer may be altered by nuclear decay. For instance, a doubly labelled organic reagent could, through the decay of one or other of the active nuclides in the molecule, produce one of two singly labelled species both of which are radiochemical impurities. It may be advisable to purify such reagents immediately before use. The problem is fortunately rare in inorganic analysis as the majority of tracer experiments are solely concerned with elemental behaviour. The storage of highly radioactive tracer solutions may lead to radiolytic gas production and such solutions must be stored in vented containers. While radiation absorption can produce other chemical changes [5], they are rarely met when using the relatively small amounts of activity needed in inorganic tracer studies. </p><p>Isotopes selected for tracer work should be easily measured, preferably with little or no special preparation before counting. Because of the pene-trating nature of gamma radiation, in most circumstances, gamma emitting nuclides are to be preferred. Sodium iodide well detectors Simplify the measurement of gamma emitters, the geometrical configuration allowing precipitates in centrifuge tubes, small but differing volumes of liquids and many small objects, to be counted with virtually identical efficiency. Gamma emitting radionuclidic impurities can be identified by gamma spectrometry employing either sodium iodide or lithium drifted germanium detectors [6]. The latter are to be preferred because of their superior resolu-tion but are much more expensive than the equipment usually required in a laboratory carrying out tracer studies. Beta radiation is much less penetrat-ing than gamma radiation and beta emitting nuclides normally require separation and some form of source preparation prior to Geiger, propor-tional or scintillation counting [7]. Uquid scintillation counting is particu-larly useful for weak beta emitters because of its high efficiency and the relative ease of source preparation [8]. Because of radiation absorption effects only strong beta emitters are usually detectable in solutions, on chromatographic columns, or adhering to the walls of vessels. However, for techniques such as paper chromatography and thin-layer chromatography both weak and strong beta emitters are detectable by scanning techniques or autoradiography [9, 10]. Due to the continuous nature of beta radiation the detection of radionuclidic impurities by beta counting is difficult though some energy discrimination can be obtained by means of beta spectrometry or the use of absorption foils. An alternative method is half-life discrimination. The absorption of alpha radiation is so marked that the use of alpha emitting tracers should be avoided unless no alternative exists. </p><p>A compromise may need to be made in the selection of a tracer for a particular experiment. Reference to a compendium of nuclear data [1,2] will indicate the nuclear properties of radioactive isotopes and allow a 'reasoned choice for a particular element. The data may show that although a gamma emitting isotope is available, its half-life is too short, and a ,pure beta emitting isotope with a more favourable half-life may have to be chosen, e.g. 37S is a gamma emitter but the half-life is only 5 minutes, while 35S is a pure beta emitter with a half-life of 87 days. </p><p>After selecting a particular radioactive tracer, the amount of activity </p></li><li><p>The Use of Tracers in Inorganic Analysis 301 </p><p>needed for an experiment must be calculated. This will be governed by a number of factors such as the accuracy and precision required, the loss of activity during the tracing study and the efficiency of detection. </p><p>The precision of the measurements is intimately associated with the counting statistics. The standard deviation of the number of counts recorded, C, is equal to ..;c. The precision of the tracer count is also depen-dent on the background count. If T r = tracer count rate, S = tracer + back-ground count in time ts and B = background count in time tb, then </p><p>T=~-.! r ts tb </p><p>and its precision is given by </p><p>If the background is insignificant a relative precision of 1 % is obtained by accumulating 104 counts but if the background counting rate is equal to that of the tracer the same precision would require a tracer + background count of 6 X 104 counts and a background count of 3 X 104 . </p><p>Experimental technique can aid the production of good counting statis-tics. For example, in measuring losses and distribution coefficients, one fraction may contain a relatively small activity compared to the other and the choice may be between measuring a small change in the high activity fraction or a relatively large change in the low activity fraction. The latter procedure is obviously the better approach, providing the background is insignificant, and this can be controlled by starting the experiment with sufficient activity. In tracer studies of radiochemical and activation methods high order decontamination factors are needed. While the precision of measurement required is probably low, limiting values only may be obtain-able unless large initial activities are used. Another reason for having a tracer count well above background level is that it makes characterisation of the activity easier, so ensuring that the activity measured is not due to a nuclidic impurity. </p><p>The major reduction of radioactivity during an experiment is usually caused by deliberate or fortuitous separations of the type described above. Decay losses have already been discussed and can be held at a moderate level by good experimental design. The only remaining factor causing the reduc-tion of activity during a tracer study is dilution, which is usually well defined. </p><p>The final parameter governing the amount of tracer activity required for an experiment is the efficiency of the counting system. In general, the number of counts (C) recorded by a counting system per nuclear disintegra-tion (D) can be defined by an expression of the type </p><p>C = fpDEpG where fp is the fractional emission of particles, of the type being measured </p></li><li><p>302 J. W. McMillan </p><p>per disintegration, e.g. SICr emits only 0.09 gammas of 0.320 MeV per disintegration. Ep is the detector efficiency. G is a factor which takes into account the detector/source geometry, activity reduction attributable to absorption of the radiation in the source and detector housing, backscatter-ing effects etc. </p><p>In most practi...</p></li></ul>


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