CHAPTER 1 NUCLEAR ANALYTICAL TECHNIQUES FOR ENVIRONMENTAL RESEARCH

Transcription

1 CHAPTER 1 NUCLEAR ANALYTICAL TECHNIQUES FOR ENVIRONMENTAL RESEARCH Peter Bode Delft University of Technology, Interfaculty Reactor Institute, Mekeweg 15 Delft, The Netherlands ABSTRACT A survey is given of various nuclear analytical techniques, particularly in relation to applications in environmental and related life sciences studies. The type of information attainable as well their typical features are compared to those of non-nuclear techniques. The features are: (i) the underlying physically different basis for the analysis, (ii) isotopic rather than elemental determination, (iii) mostly no effect of electrons and molecular structure, and (iv) penetrating character of nuclear radiation. Activation analysis, X-ray fluorescence spectrometry and the radiotracer method are discussed in more detail. Examples are given of typical applications of nuclear analytical techniques in environmental and related life sciences.

2 1 INTRODUCTION The Club of Rome shocked the world in 1972 with the report The Limits to Growth. As a result, man started to realize that the increasing world population and the desire to reach high standards of living would have a noticeable impact on his environment. The report was the awakening that new approaches are absolutely necessary to find a sound balance between the use of the environment to accommodate the world population and to protect the environment at the same time. Various actions were initiated. Not only worldwide studies started to assess the degree of man s impact to the environment, but also new concepts were launched such as sustainable development, renewable energy sources, closed production loops, recycling of disposables etc. Both the studies of the environment, the ecosystems and their natural variability, as well as the development and implementation of the new concepts are substantially based on analytical work, e.g., in monitoring possible changes in the environment and/or developing and testing of new materials. Traditionally a distinction is made between a qualitative analysis (which measurands are present) and a quantitative analysis (how much of them are present). Quite often, this information alone is insufficient for forming sound conclusions. Other information is required such as the chemical form, distribution, structure, kinetics and/or reaction mechanism. Obtaining this information will also be considered as analytical within the frame of this article. Although the majority of analyses are performed via non-nuclear techniques, nuclear techniques and related isotope tracer techniques have become well-established important tools, complementary to non-nuclear methods. These nuclear analytical techniques provide a wealth of information - some of which is difficult or impossible to obtain by other means - on e.g. sources, pathways and effects of many environmental pollutants that are now considered to have a negative impact on biota and particular human health. Before discussing the techniques as such, a short introduction is given on nuclear structure, radioactivity and nuclear reactions. 2 NUCLEAR STRUCTURE, RADIOACTIVITY, NUCLEAR REACTIONS In a simple representation an atom is built-up from a nucleus and electrons, organised into electron shells. The nucleus contains protons and neutrons. In a neutral atom, the number of protons equals the number of electrons, and is denoted with Z, the atomic number. The number of electrons determines the chemical characteristics of the atom; therefore, Z is related to these characteristics. The total number of protons and neutrons is called the mass number, indicated by A. Atoms with the same number of protons but with different numbers of neutrons are called isotopes. Though their mass numbers are different, their chemical behaviour is identical. Some elements have only one stable isotope, e.g. fluorine with 9 protons and 10 neutrons in the nucleus. Most of the elements have two or more stable isotopes (see e.g. Figure 1). For instance, chlorine has two stable isotopes, each of them with 17 protons, but one with 18 neutrons and one with 20 neutrons. The mass number is used to distinguish different isotopes of one element. To this end, the mass number is written as a superscript left sided from the chemical symbol of the element, like 35 Cl and 37 Cl. The fraction, in which a particular stable isotope occurs in the mixture of isotopes of an element, is called isotopic abundance. Not all combinations of protons and neutrons are stable; some are unstable. They lose their excess energy by radioactive decay. An unstable isotope is called radioactive isotope or radioisotope. 2

3 Figure 1. Part of the chart of radionuclides showing stable and radioactive isotopes of the elements. In the above it has been described that a combination of protons and neutrons in an atomic nucleus may have an excess of energy, which makes the nucleus unstable. In such a case it will release its energy by radioactive decay under emission of nuclear radiation. This is also denoted as nuclear disintegration; the unstable nucleus is called radionuclide. Four types of nuclear radiation can be distinguished: (i) α-radiation: emission of a helium nucleus, containing 2 protons and 2 neutrons; the new nucleus will get a mass number being 4 units lower. (ii) β - radiation: emission of (negative) electrons. In the β - decay effectively a neutron transforms into a proton in the atomic nucleus. The mass number remains the same, but the atomic charge changes, and thus an isotope of a different element results from the decay. (iii) β + radiation: positive electrons or positrons. In the β + and EC decay, effectively a proton transforms into a neutron. As an alternative to β + -decay, the nucleus may capture an electron from one of the electrons shells: electron capture or EC. The vacancy in the shell is filled-up via electron transitions from outermost shells and in this process characteristic X-radiation is emitted. In both processes, the mass number remains the same, but the atomic charge changes, and thus the decay results in an isotope of a different element. (iv) γ-radiation: emission of high energetic electromagnetic radiation. In this case the nucleus is at what is called excited level and decays to a less excited level or even direct to the ground level, without changes in either the number of protons and neutrons, and thus in the atomic number. The latter process is also denoted as isomeric transition. Excited levels of an atomic nucleus often result from preceeding α, β -, β + and/or EC decay, and incidentally these excited levels may result directly 3

4 upon activation. Often, the decay via excited levels takes place by subsequent emission of two or more γ's. This is called a γ-cascade; since often the excited level is populated and depopulated in a very short time (e.g. in µs), the related radiations are called to be in coincidence. In a few cases, the excited level has a well measurable lifetime of seconds (s) to hours (h) or even longer. Such a level is considered to be a special radionuclide, and denoted with the symbol m (from metastable), e.g. 116m In. In radioactive decay generally more than one type of radiation is emitted, like β - and γ-radiation. The process of radioactive decay can be derived from a decay-scheme. An example is given in Figure 2. Not every transition in γ-decay results in γ-radiation. Some γ- rays are converted, viz. instead of the emission of γ-rays, electrons are thrown out of the shells. The fraction of the number of disintegrations that practically results in γ-radiation is called the γ-yield. This is also given in the decay scheme. Gamma-radiation is the radiation of choice in e.g. activation analysis and many radiotracer techniques since γ-radiation is mono-energetic and in most cases characteristic for the emitting nucleus. The other advantage of γ-radiation is that it has a high penetrating power, so that it is hardly absorbed in the radioactive material itself. Radioactive decay is a stochastic process, so it can not be predicted when exactly a radioactive nucleus will decay. However, it is measurable which fraction of the original number of radioactive nuclei remains after a given time. The decay rate is proportional to the number of radioactive nuclei: dn - = λ N dt where -dn/dt = the decrease of the number of radioactive nuclei per unit of time, often denoted with the term activity or disintegration rate (s -1 ), N = the number of radioactive nuclei and λ = the decay constant (s -1 ). The disintegration rate is denoted with the S.I.-derived unit Becquerel (Bq), which is the equivalent of 1 disintegration per second. In older literature the earlier unit Curie is used; 1 Curie (Ci) corresponds to 3.7 * disintegrations per second. Instead of the decay constant, the half-life of a radionuclide is the preferred term in characterising radionuclides. The half-life (t 1/2 ) is the time-span in which the number N 0 of radioactive nuclei has been reduced by a factor of 2: = 0.5 N 0 = N exp (- t) N t = t1/2 0 λ from which it follows that λ = ln2/t 1/2. The half-life is characteristic for a radionuclide and may range from a fraction of a second to millions of years. 4

5 days stable Figure 2. Simplified decay scheme of the radionuclide 60 Co. Radionuclides are produced by nuclear reactions that may occur when a nucleus of a stable isotope is hit by nuclear or cosmic radiation, charged particles or high-energy photons. The nuclear reactions also often lead to the production of additional charged particles, neutrons and/or photons. Examples of nuclear reactions are: and 18 O (Z = 8) + 1 H (Z = 1 ) 18 F (Z = 9) + 1 neutron (Z = 0) 23 Na ( Z = 11) + 1 neutron ( Z = 0) 24 Na (Z = 11) + 0 photon (Z = 0) The two reaction equations show that the total number of protons and neutrons does not change during the reaction. The usual notation of nuclear reactions, as an example for these two reactions, is as follows: 18 O(p,n) 18 F and 23 Na(n,γ) 24 Na, or, in general: A (x,y)b with: A as given isotope of element A; x as bombarding particle or photon; y as particle and/or photon emitted upon reaction, also denoted as prompt radiation; B as produced nuclide, being most times radioactive. For y the following notations are used: γ = gamma, 5

6 p = proton, α = alpha particle, n = neutron, and f = fission which implies that the neutron captured splits the nucleus into two or more fragments. The probability that a nuclear reaction of a bombarding particle or photon with an atomic nucleus will take place is expressed as cross section, σ, being a fictitious surface area of the atomic nucleus. The value of the cross section depends on the atomic nucleus, the reaction under consideration, the type of bombarding particle and its energy. For many nuclear reactions, except for the (n,γ) reactions, a lower threshold energy exist below which no reaction takes place. The cross section is often expressed in the unit barn, b, corresponding to m 2. 3 SOURCES OF RADIATION High-energy photons and X-rays are produced with synchrotrons. A synchrotron is also often denoted as a light source. Electrons are accelerated with a linear accelerator to energies up to several GeV and then injected into a storage ring. When the direction of the electrons is changed, they lose their energy by emitting photons of light that can be guided through beam tubes towards experimental facilities. Energetic, charged particles, such as protons, deuterons or α-particles are generated with a cyclotron. The disadvantage of charged particle activation is the limited penetration depth (only several hundred of µm's), and thus it is actually a surface analysis technique. Moreover, the high-energy dissipation upon bombardment sets demands to the radiation and temperature stability of the samples and requires cooling during irradiation. Highenergetic photons with energies of several tens of MeV can be produced as Bremsstrahlung with an electron accelerator. High energetic photons have a good penetration power and thus enable bulk analysis. However, also here problems may arise in respect to heat generation. Furthermore, the cross sections for the related nuclear reactions are rather small. Fast neutrons with energies of several MeV's can be produced with a neutron generator or in an isotopic neutron source. In a neutron generator a nuclear reaction between deuterium nuclei, accelerated up to about 250 kev, and tritium nuclei is evoked, in which neutrons with an energy of about 14 MeV are being produced. Typically the yields are between neutrons s -1. In isotopic neutron sources a nuclear reaction takes place between an α-particle and a low Z-element (e.g. Be) and neutrons with energies of several MeV are produced. The α-particle is provided by radioactive decay of 226 Ra or 238 Pu. Alternatively, the radionuclide 252 Cf decays by spontaneous fission under emission of neutrons of several MeV. The neutron yields of isotopic neutron sources vary typically between 10 4 and 10 9 neutrons per second, depending on the amount of 252 Cf. But it is not as much the neutron yield but merely the neutron fluence rate (also: neutron flux) which is an important parameter for reactor-based NAT s. The neutron fluence rate is the number of neutrons per second per square meter. With neutron generators and isotopic neutron sources the ratio between neutron yield and neutron fluence rate depends strongly on type and construction. The high energy of the neutrons imply that mainly activation reactions of the (n,p), (n,α) and (n,2n) type will take place. Fast neutrons have a high penetration depth but in samples with high concentrations of light elements, in particular hydrogen, moderation will take place. The energy of the neutron reduces, which may result in a final energy lower than the threshold energies for the reactions. The nuclear research reactor is the most common source of neutrons for activation analysis, and typical neutron fluence rates range from m -2 s -1. The majority of the neutrons in a nuclear reactor is in thermal equilibrium with its environment, and therefore denoted as thermal neutrons. With thermal neutrons usually reactions of the (n,γ) 6

7 type result. The gamma radiation emitted during the reaction is called a prompt gamma. Many nuclei have a high cross section for (n,γ) reactions, while other nuclei have a low cross section, e.g. the low Z-elements H, C, N, O, and Si. These elements are in many matrices the major elements; because of their low cross sections neutrons are hardly absorbed in many materials. Since the thermal neutrons are already in thermal equilibrium, no heat is generated upon irradiation. A few elements, like Li, B and Cd have such high cross sections that, if they are present in large quantities, their capture of neutrons may result in a local neutron attenuation or neutron self-shielding. The advantage of isotopic neutron sources and neutron generators is that they are relatively inexpensive, easily transported and generators can be switched off when not in use. Their main application lies with in-field activation (like borehole logging) or industrial analysis of bulk samples. In most cases only the major elements can be determined. Spallation sources are the newest available neutron sources. High energy protons are bombarded to nuclei of high Z-elements like Pb, Ta or Hg) resulting in the release of neutrons from these atomic nuclei; a fundamental different phenomenon as nuclear fission. Several of these sources have already been realised (e.g. in UK and Switzerland) and others are being designed or constructed (Japan, USA). Spallation sources produce pulse neutron beams with fluence rates in the same order as in high flux nuclear reactors. So far, they are mainly used for physics experiments, and the applicability for analytical purposes is still unclear. 4 NUCLEAR ANALYTICAL TECHNIQUES What are nuclear analytical techniques? The narrow definition Nuclear analytical techniques (NAT s) deal with nuclear excitations, electron inner shell excitations, nuclear reactions, and/or radioactive decay, e.g. instrumental neutron activation analysis (INAA), particle-induced X-ray emission (PIXE), X-ray fluorescence (XRF) and radiotracer studies. On the other hand, mass spectrometry and nuclear magnetic resonance (NMR), which do not involve ionizing radiation and are quite often performed as an "in house" technique, are consequently not directly considered as NAT's. What are nuclear analytical techniques? The comprehensive definition In principle nuclear techniques are based on properties of the nucleus itself, compared to non-nuclear techniques which use properties of the atom as a whole, primarily governed by properties of the electrons arranged in shells. However, fundamentally as well as practically no sharp borderline can be drawn between nuclear and non-nuclear techniques. Mass spectrometry deals with ionized atoms, and rarely with the bare nucleus; however, the signal is determined by the mass differences of the nucleus. In this survey mass spectrometry will be considered as nuclear technique. Some nuclear analytical techniques are not only based nuclear properties, but on a combination of nuclear and electronic properties, either within a single techniques, or within a hyphenation of two techniques. Examples of the first category are NMR and Mössbauer spectrometry, where the nuclear signal is fine-tuned by the electron energy levels, giving also chemical information. Examples of the second category are modern chemical separation methods coupled to radioactivity detection or mass spectrometry. PIXE as one of the varieties of ion beam analysis deals with electron shell ionization, and thus is formally a non-nuclear technique. However, since PIXE requires almost the same equipment as other varieties of ion beam analysis, PIXE is mostly incorporated in the domain of the nuclear physicist and practically it is considered here as a nuclear technique. And to a lesser extent this is also a trend for the various new modes of XRF. Whereas in non-nuclear techniques isotopes of the same element generally cannot be distinguished, in nuclear techniques they can, and actually specific isotopes are 7

8 measured instead of elements, generally consisting of a mixture of two or more isotopes. However, since poly-isotopic elements have constant isotope ratios, direct quantitative information on the associated elements is obtained. Since isotopes of a given element may be discriminated, analytical information may be obtained by using elements enriched in respect to a particular stable isotope or labelled with a radioisotope, e.g. in isotope dilution analysis. In addition to analytical information, isotopes studies may also yield kinetic and mechanistic information. Both nuclear techniques and isotope techniques will be comprised by the terminology "nuclear analytical techniques", abbreviated as NAT's. Physical basis and classification of nuclear analytical techniques. Nuclear parameters that serve as basis for NAT s are mass, spin and magnetic moment, excited states and related parameters, and probability of nuclear reactions. When dealing with a radioactive isotope, also the properties of a half-life and the types and energies of the emitted radiation are involved. Table 1 lists a survey of NAT's grouped into nine categories, together with the main type of information generally obtainable within each category. It should be noted that categories overlap, and that some NAT's may be listed in more than one category. This survey aims at "established" NAT's only, viz techniques that have been used and proven to be useful in the field of environmental and related life science studies. Nuclear techniques that have only scarcely been used as analytical tool are considered as "not-established" NAT's sofar. These comprise for instance: analysis via perturbed angular correlations, analysis via positronium chemistry or via muon spin resonance. For the underlying nuclear and other physical principles of the various "established" NAT's, the reader is referred to textbooks and review articles. TABLE 1 Classification of nuclear analytical techniques in categories Category Main types of information obtainable *) Mass spectrometry 1,3 Ion beam analysis 1,2 Nuclear magnetic resonance spectrometry 1,2,3,4 Mössbauer spectrometry 1,3,4 Neutron scattering and diffraction 3,4 Activation analysis 1 Isotope dilution and related analyses 1,3 Stable isotope and radiotracer studies 1,2,3,5,6 Direct radioactivity determinations 1 * 1 = quantitative information (mass or concentration); 2 = spatial information (lateral or depth distribution); 3 = species information (chemical and/or physical form); 4 structural information (electronic and/or structural); 5 = kinetics; 6 = reaction mechanisms; The principles and characteristics of three nuclear analytical techniques relevant for environmental research, viz. neutron activation analysis, X-ray fluorescence spectrometry and the radiotracer method are described in some more detail first. 8

9 5 NEUTRON ACTIVATION ANALYSIS Neutron activation analysis (NAA) allows for the qualitative and quantitative determination of elements. The method is based upon the conversion of stable atomic nuclei into radioactive nuclei by irradiation with neutrons and the subsequent measurement of the radiation released by these radioactive nuclei. Amongst the several types of radiation that can be emitted, gamma-radiation offers the best characteristics for the selective and simultaneous determination of elements. By neutron activation, radionuclides may be produced from all elements present in the sample, albeit at sometimes strongly different production rates. This mixture of radioactivities can be analysed in two different ways: (i) The resulting radioactive sample is chemically decomposed, and by chemical separations the total number of radionuclides is split-up into many fractions with a few radionuclides each: Destructive or Radiochemical Neutron Activation Analysis. This form of NAA will not be discussed in this thesis. (ii) The resulting radioactive sample is kept intact, and the radionuclides are determined by taking advantage of the differences in decay rates via measurements at different decay intervals utilising equipment with a high energy resolution for gamma-radiation: Non-destructive or Instrumental Neutron Activation Analysis (INAA). A procedure in INAA is characterised by (i) activation via irradiation with reactor neutrons, (ii) measurement of the gamma-radiation after one or more decay times and (iii) interpretation of the resulting gamma-ray spectra in terms of elements and concentrations. Nuclear research reactors are the most powerful source of neutrons for activation, typically expressed in the neutron fluence rate or neutron flux, which commonly varies between cm -2 s -1. Activation analysis with reactor neutrons has some attractive aspects. First, generally the probability of getting a nuclear reaction, commonly known as the reaction cross section is high and also high neutron fluence rates are available, so that good detection limits may be achieved for a sizeable number of elements. Second, neutrons penetrate well in materials and relatively homogenous neutron fluence rates may be obtained, so that bulk analyses may be performed. Third, the heat production in the sample is small. Finally, irradiation may be performed as a parasitic activity in a nuclear research reactor and ample irradiation facilities are available or accessible. It has resulted that activation analysis with reactor neutrons has become the most common applied method of activation analysis. An evaluation of the analytical characteristics (see Table 2) also makes clear where to find the niches for NAA. These include: 9

10 TABLE 2 Comparison of analytical characteristics of INAA and other analytical techniques for element determination. --- = poor, +++ = excellent INAA others Sensitivity Absence of blank Non-destructive No chemical matrix effects Multi-element Accuracy, Metrological principles Physically Independent Self-validation RNAA for removal of interferences Determination of chemical yield in RNAA +/- ICP-MS LA-ICP, SS-AAS (+?) +++ LA-ICP, SS-AAS, XRF (+?) +++ ICP, AAS, XRF - + ICP ICP, AAS (+?) +++ XRF XRF + + ICP, AAS ICP, AAS -- - Studies involving samples for which other methods of analysis have difficulties in the calibration step due to chemical matrix effects. This applies particularly to studies in which the matrix composition varies considerably in an unpredictable way, or for which no matrix-matching reference materials are available. - Samples in which the trace element levels are so low that contamination or losses may occur easily during the sample dissolution or digestion step. - Analyses requiring a high degree of accuracy, but even more: reliability, to ensure full comparability of data obtained over a long period of time. - Samples with a high degree of inhomogeneity, requiring the processing of a relatively large analytical portion. - Samples in which the element concentration may vary over several orders of magnitude; here the linearity of NAA pays off. 6 X-RAY FLUORESCENCE SPECTROMETRY X-ray fluorescence spectrometry is one of the oldest nuclear analytical techniques. If a material is being irradiated with electromagnetic radiation of sufficient energy, the atoms resulting in the removal of electrons from the inner orbitals may absorb these photons. The hole will be filled by the transfer of another electron from an outer orbital and the energy difference between the two orbitals will be emitted as secondary or fluorescent X-radiation. Note that the emission of this fluorescence X-rays stops immediately if the primary irradiation is halted. The fluorescence X-radiation is characteristic for each element, well defined (based on the physics of the atomic structure) and thus forms the basis for the high selectivity of X-ray fluorescence spectrometry. 10

11 The primary radiation may come from a radioactive source (usually e.g. 57 Co, 109 Cd, 241 Am) or from an X-ray tube. Commercially available equipment is available as a compact apparatus consisting of an X-ray tube, X-radiation detector and sample changer. The fluorescence X-rays have relatively low to very low energies, which means that they are easily self-absorbed by the sample matrix. X-ray fluorescence spectrometry is therefore more suitable for very thin and/or very flat and homogeneous samples. Analysis of thick samples results in more-or-less semi-quantitative analysis. In general, XRF is less sensitive as INAA. An interesting aspect of XRF is that also very simple, hand-held systems have been developed to be used e.g. for field measurements. Several technical improvements have resulted in enhancement of the sensitivity albeit with concessions to the scope and geometry of samples to be analysed. Total-reflection XRF is a form of XRF that can be applied to very thin samples and for e.g. water analysis. The fluorescence X-rays may also be induced by bombardment of the target atoms with protons or charged particles with energies below the threshold for nuclear reactions: particle-induced X-ray emission spectrometry or PIXE. Charged particles are produced with accelerators and have the advantage that their beam can be focused. Microbeam PIXE facilities exist in which the beam spot is in the order of 1 µm or less, which allows for fascinating position-sensitive analyses. The degree of accuracy is still less good as e.g. in INAA but for most applications rather acceptable. 7 RADIOTRACER METHOD The radiotracer method is a versatile and powerful tool in the study of a wide variety of applications in e.g. chemistry, biology, agriculture, medicine, environmental science and (industrial) technology. The technique is simply based on the adding a known amount of a radioactive substance (ionic solution or radio-labeled compound), closely matching the measurand of interest, to the system under study and the subsequent measurement (e.g. as a function of time, ph, temperature, location in space) of the emitted radiation. The measurement is commonly non-invasive, i.e. outside the system under study. This noninvasive character is a big advantage of radiotracers above e.g. stable isotope tracers, commonly applied in e.g. nutritional studies, since it allows studies of both steady-state and dynamic systems, in equilibrium situations and for transport and exchange phenomena and thus provide information on the chemical and/or physical status of elements. When working with stable isotopic tracers a sub-sample always has to be taken to be inserted in a mass spectrometer, resulting in perturbation of the steady state. A second big advantage of radiotracers is that they can be applied at very low concentrations, which means that, e.g. in environmental research and studies of ecosystems, experiments can be carried out with the tracer at physiologically normal concentrations. The principles of the radiotracer methodology are straightforward and the information unequivocal. The big advantage above, e.g., labeling with fluorescent markers is that these markers are much larger on a molecular scale and may not be representative for the system under study. The radiotracer method does not imply huge equipment investments but rather requires that the four interrelated aspects: experimental designs, data treatment including tracer kinetic analysis and data interpretation are careful considered. 11

12 8 ANALYTICAL CHARACTERISTICS OF NUCLEAR ANALYTICAL TECHNIQUES Within the world of nuclear physicists and most users of NAT's it is now well agreed that NAT's and non-nuclear analytical techniques should be operated as complimentary ones, rather than as complementary. This implies that in this survey the main attention is focused as to features of NAT's as additional analytical possibilities to those of nonnuclear ones as strong points, rather than where the features of NAT's fail in short compared to those of non-nuclear analytical techniques. Compared to non-nuclear analytical methods, NAT's have a totally different physical basis for the analysis. Moreover, the nuclear parameters vary from element to element in a different way compared to the atomic - quite often purely chemical - parameters of the various non-nuclear analytical techniques. This implies that NAT's create an interesting supplementation of analytical possibilities, since strong and weak points of NAT's and non-nuclear techniques are distributed differently over the Periodical Table. For instance, about half of the atoms in living cells are hydrogen, being invisible in X-ray structural analysis, but par excellence accessible for neutron scattering/diffraction and nuclear magnetic resonance. The physical basis of NAT's means that quite often the relationship between measurand and observed signal can be expressed in an exact, mathematical, physics-based equation. Thus, the variables that may affect inaccuracy are identifiable and quantifiable, and contribute to the high accuracy of nuclear analytical techniques. In cases where NAT's and non-nuclear analytical methods may provide in principle the same information, the difference in physical basis between both types of methods make NAT's well suited as independent methods of reference. For instance, in most certification programmes of trace elements in reference materials for environment and related life sciences stable isotope dilution analysis and neutron activation analysis have become indispensable tools. Finally, NAT's may act also as "pioneers" for non-nuclear techniques, where NAT's may disclose analytically new fields of interest. Once the importance of these fields has been recognised, efforts will be spent to get access to these fields with nonnuclear methods as well. Neutron activation analysis and X-ray fluorescence spectrometry were the first multi-element analysis techniques and demonstrated the opportunities of multivariate analysis techniques such as factor analysis for e.g. source apportionment in environmental pollution studies. Once radiochemical neutron activation analysis had disclosed realistic, but rather low, values in environmental and related life science materials, non-nuclear techniques have been improved to meet the increasing demand of reliable determinations of low levels of trace elements, e.g. of chromium and mercury. Another striking example is the present set of immunological assays, which began at the time with the radioimmunological assay of iodine-containing hormones. Now there are several competing immunological assays without radioactive label, e.g. enzyme-linked immunosubstrate analysis or assays using fluorescent labels. Isotopic analysis rather than elemental analysis. Most (over 75%) elements consist of two or more stable isotopes and have several radioactive isotopes. Chemically, all the isotopes behave identically. However, the nuclear parameters of the nuclei in the various isotopes of a given element differ, and thus their responses in NAT's. These characteristics enable tracer experiments, where the tracer and tracee (the compound to be traced) behave chemically similarly, but nuclearphysically differently, so that tracer and tracee can be distinguished. 12

13 This principle is the basis of various NAT's, e.g. isotope dilution analysis and radioimmunoassay. It is also the basis for tracer studies with radioisotopes or with isotopically enriched stable tracers, to get information on spatial, temporal, kinetic, and mechanistic aspects, all four aspects being rather important in the field of the environment and related life sciences. Tracers (labelled compounds) are used in the study of many chemical, biochemical, and biological processes and methods, giving an additional dimension to non-nuclear analytical methods. For instance, the use of 14 C and 3 H (measurement via their radioactivity), 15 N, 18 O and 2 H (measurement via mass spectrometry) and 13 C, 17 O and 1 H (measurement via nuclear magnetic resonance) as a label in a variety of organic compounds has contributed substantially to the increase of knowledge on reaction mechanisms and kinetics. It is important to note that specific isotopes are measured with NAT's rather than elements. There may be pitfalls though. Normally the isotope ratios of elements are identical for samples and standards, thus isotopic analysis is also direct elemental analysis. However, some elements may be artificially enriched or depleted in respect of a particular isotope. This is the case for Li, B, and U, and one should be prepared for such deviations. Using radiotracers, the radioactivity signal is only indicative for the tracee, when the specific activity (the ratio between radiotracer and tracee) is constant in time and uniform over space. However, this ratio can easily change due to exchange with nonlabelled pools of the tracee in the system under study. Determination of specific activities and compartmental modelling may be useful tools in handling this problem. A comparable warning and solution applies to the use of stable enriched isotopes as a tracer. Above it is stated that chemically, isotopes (stable or radioactive) of an element behave identically. This is true, except for small and generally negligible differences due to so-called isotope effects. Particularly with light elements and with "multi-step" environmental and biological processes, a slight isotopic fractionation may occur, which may be revealed by mass spectrometry. The occurrences of fractionations have been used fruitfully for the study of reaction mechanisms and kinetics in some branches of environtent and related life sciences. However, these fractionations are generally too small to be a cause of error in the application of NAT's, except in some cases when dealing with 2 H and 3 H. About 25% of all stable elements are mono-isotopic, which does not enable their working with isotopic enriched elements. Many other elements do not have radioisotopes with a suitable half-life and/or emitted radiation always to work with. Sometimes there is also the problem that the necessary isotopic enriched elements and radioisotopes are not commercially or otherwise available, or do not meet the required specifications. Moreover, quite often these materials are not used as element, but have to be incorporated in specific compounds, which also may be a practical limitation. No or a limited effect of electronic and molecular structure. In many non-nuclear analytical techniques, the signal relevant for detection depends on the chemical state of the measurand, and consequently the occurrence of a measurand in more than one chemical and/or physical state may act as an interference, leading to erroneous results. Since the nucleus is rather insensitive for such effects, NAT's are in principle not sensitive either. This contributes - in addition to the nuclear-physically different base - to possibilities of e.g. activation analysis as methods of reference. On the other hand, this insensitivity may also be a drawback, since quite often information on a particular form of an element is required, e.g. methylmercury rather than total mercury. Therefore, a number of NAT's consists of hyphenated systems, e.g. 13

14 chemical separation methods coupled to e.g. radioactivity detection or mass spectrometry. Generally in these methods the chemical (c.q. biological, physiological, biochemical) part ensures the selectivity, and the nuclear part the detectability and sensitivity. In some NAT's there are specific interactions (coupling) between the energy levels of electrons and nuclei. Although such interactions are rather weak, they may occasionally provide interesting possibilities to give information on electronic and molecular structures. This is the case for analysis via the Mössbauer effect and via NMR. Particular the latter technique has gained substantial importance for structural analysis, since some of the constituting elements in organic materials have suitable nuclei, e.g. 1 H, 13 C and 31 P. However, it should be noted that only a part of the nuclei is suited for NMR, and the Mössbauer effect can only be applied to a rather small number of nuclei. Penetrating character of nuclear radiation. In most NAT's the excitation and de-excitation signals penetrate through matter, thereby enabling non-invasive measurements without disturbing the processes to be studied. This is the case where radio waves and a magnetic field are used (NMR), neutrons (neutron scattering/diffraction, NAA), and gamma radiation (activation analysis, Mössbauer spectroscopy, radiotracer investigations, and radioisotope dilution analysis). Important examples of non-invasive measurements of organisms are: Magnetic Resonance Imaging (MRI), Single-Photon Emission Computerized Tomo-graphy), invivo NAA, NMR localised spectroscopy, and Positron Emission Tomography (PET). The first two methods have proven to be very useful for routine application in medical diagnosis, the third method is also applied in diagnosis, but much less frequently. The last two methods have shown to be extremely useful in physiological studies. For instance: positron emission tomography with 11 C or 18 F labelled compounds has provided much insight in various metabolic and pharmacologic processes, particularly in the - otherwise - rather inaccessible brain. Another non-invasive example is measurement of natural or artificial radioactivity on basis of gamma-emission, e.g. in cases of internal contamination. However, not all NAT's can take advantage of the penetrating character of nuclear radiation. For instance, ion-beam analysis scattering methods, charged particle activation analysis, XRF and beta-counting deal with radiation having a relatively short range in matter, which implies relatively small samples and analyses of surface and near-surface layers only. For the measurement of alpha-radiation a chemical separation of the emitter is necessary. 9 NUCLEAR ANALYTICAL TECHNIQUES IN ENVIRONMENTAL STUDIES Not all nuclear analytical techniques from Table 1 have been applied in environmental research. Some of them, like ion beam analysis and neutron scattering and diffraction found the majority of their applications with solid state chemistry and physics. NMR spectrometry is widely applied in biological and medical research and not much in environmental research. Instrumental neutron activation analysis (INAA) is one of the most common used NAT s in environmental research. The characteristics of the method, in particular its nondestructiveness, make it extremely useful for the analysis of soils, sediments, plant material and air particulate matter. But also man and animal tissue (like hair, nails) and food have been analysed by INAA. After the implementation of European legislation on the reduction of the use of cadmium as a pigment in plastics, INAA turned to be the most appropriate technique for enforcement of this legislation. Plastics mainly consist of C, H, N, and O, elements that hardly produce activation products upon neutron irradiation. 14

15 Hence, the material is virtually transparent for the signals of its contaminants. Moreover, plastics do not have to be dissolved, nor there is a need for dedicated, matrix matching calibrants. Since there are no fundamental limitations to the size of the portion to be analysed by INAA, very large quantities can be directly handled too. A large sample INAA method in which multi-kilogram samples can be handled, was applied to determine the composition of shredded electronic waste, which is very inhomogeneous in nature at the g scale, but for which 1-2 kg samples are more representative [1]. World-wide, INAA is being used to study the trace element content of air particulate matter, particularly in e.g. mega-cities in developing countries (e.g. Jakarta, Beijing, Bangkok, Mexico City). The multi-element character often elements can be quantified simultaneously- allows for multivariate analysis like factor analysis to characterise the sources of emission. But also reverse approaches were followed. In order to study the impact of certain utility vehicles like refuse trucks and school buses, dieseloil was labelled with stable isotopes of elements, usually present at very low levels in the atmosphere, and with good characteristics for determination by INAA like iridium. Thus, the analysis of the air particulate matter along the route of such a truck helped to understand the impact of its emission. Organic halogens, especially the organochlorinated compounds that come from the extensive use of pesticides and herbicides and discharge of wastewater treatment plants are of worldwide concern. Recently, a significant number of studies indicated that traditional analytical methods such as gas chromatography and GC-MS could only provide information about the known organochlorines, which contributed to less than 1-20 % of the total extractable organochlorinated compounds and thus do not reflect the actual contamination. INAA is the only analytical method currently available for simultaneously determining various extractable organohalogens[2] At specialized institutions, NAA is widely used for analysis of samples within environmental specimen banking programmes [3]. The extensive use of NAA in environmental control and monitoring can be demonstrated by the large number of papers presented at two recent symposia organized by the IAEA in these fields: "Applications of Isotopes and Radiation in Conservation of the Environment" in 1992 [4] and "Harmonization of Health-Related Environmental Measurements Using Nuclear and Isotopic Techniques" in 1996 [5]. Similar trends can also be identified from the topics discussed at the last conference on Modern Trends in Activation Analysis (MTAA) and at the symposia on "Nuclear Analytical Methods in the Life Sciences" [6]. The above mentioned proceedings can be considered not only as sources of information on already existing applications, but also as an inspiration for future possible developments. The management of water resources is world-wide being studied by a combination of naturally occurring environmentally stable isotopic forms of water ( 1 H 2 H 16 O and 1 H 2 18 O), cosmic ray produced radioisotopes ( 3 H and 14 C) and reactor produced radioisotopes. Examples of such water management studies are groundwater recharge, salinization mechanisms, seepage situations between basins, sediment transport in sea and harbours, soil erosion and reservoir sedimentation. Obviously, such measurements depend largely on mass spectrometry as an analytical technique. The equilibration of the concentration of pollutants, the process of biological activation and the settling process of sludge have been studied using radiotracers. This made it possible to determine the residence time distribution for individual units of wastewater treatment plants, thus providing insight in the flow pattern of water and sludge [7]. 15

16 Accelerator mass spectrometry (AMS) is intensively used in studies related to global warming, the greenhouse effect. The interest is directed to methane rather than carbon dioxide since methane has a much larger global warming potential. The research is based on the determination of the radiocarbon, 14 C which in extremely small amounts is present in air bubbles trapped in ice. Scientists are using AMS to assess how the methane concentration in the atmosphere varied over the centuries, also before the start of the industrial revolution. The same technique is also currently the best available technique for environmental studies related to the non-proliferation of nuclear weapons and the comprehensive nuclear test ban treaty. The related safeguards require the determination of isotopes like 236 U, 129 I, 36 Cl and 14 C, present in very low concentrations in environmental samples, e.g. the underground waters from the Mururoa Atolls. The last example introduces a quite obvious applied field for NAT s, in particular alpha, beta and gamma-ray spectrometry, for the measurement of the level of natural and man-made radioactivity in the environment and derived systems such as food, surface waters and in the atmosphere. REFERENCES [1] Bode, P., Overwater, R.M.W., De Goeij, J.J.M., J. Radioanal. Nucl. Chem., 216, 5 (1997) [2] Chai, Z.F., Xu, D.D., Zhong, W.K., Mao, X.Y., Ouyang, H., Study of organohalogens in foodstuff and environmental samples by neutron activation analysis and related techniques, presented at the Int. Conf. on Isotopic and Nuclear Analytical Techniques for Health and Environment, Vienna, Austria, 2003, to be published in Int. J. Anal. Bioanal. Chem. [3] Rossbach, M.,Emons, H.,Groemping, A.,Ostapczuk, P., Schladot, J.D., Quality control strategies at the environmental specimen bank of the Federal Republic of Germany, in: Harmonization of Health-Related Environmental Measurements Using Nuclear and Isotopic Techniques, Proceedings Symp., Hyderabad, India, IAEA, Vienna [4] International Atomic Energy Agency, Applications of Isotopes and Radiation in Conservation of the Environment, Proceedings Symposium, Karlsruhe, Germany, IAEA, Vienna 1992 [5] International Atomic Energy Agency, Harmonization of Health-Related Environmental Measurements Using Nuclear and Isotopic Techniques, Proceedings Symp., Hyderabad, India, IAEA, Vienna 1997 [6] Nuclear Analytical Methods in the Life Sciences Chai, Z-F., (ed.), Biol. Trace El. Res.71 (1999), Humana Press, Totowa, New Jersey (1999). [7] Thyn, J., Burdych, J., Blaha, L., Zitny, R., Radiotracer applications in wastewater treatment, in: Applications of isotopes and radiation in conversation of the environment, IAEA proceedings series IAEA-SM-325, 1992, Vienna