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Total analysis by IBA
516
Nuclear Instruments and Methods in Physics Research B45 (1990) 516-518 North-Holland
TOTAL ANALYSIS
BY IBA
J.R. BIRD ANSTO
Lucas Heights Research Laboratories,
Menai 2234, Australia
Depending on the nature of the sample, a selection of IBA techniques can be used to obtain a total major element analysis plus information on the concentration of a number of trace elements. This approach has been found to be valuable in studies of biological, geological and archaeological materials and the study, layer by layer, of the composition of inhomogeneous surface coatings. There are undoubtedly many other applications. A proton microprobe can be used to measure a complete 3dimensional multi-element distribution and the time dependence of this information. This is a unique capability but more work is needed to develop the concept of total analysis by IBA and especially to streamline data processing methods including the iteration of the calculation of stopping power and other corrections.
1. Total analysis
Total analysis can be defined as the determination of all major and minor elements present in a sample down to a lower limit of concentration which depends on the problem being investigated (usually between 0.1 and 1%). The results should therefore sum to 99-100%. Such an analysis is required for most ion beam analysis work in order to calculate stopping power, X-ray attenuation and other such parameters which enter into the determination of absolute element concentrations. It also constitutes vital information in many fields of investigation and techniques are needed which are able to provide such information quickly and economically. Ion beam analysis (IBA) represents a suite of techniques from which one or more can be selected for the solution of almost any problem in elemental analysis or, in many cases, isotopic analysis. There is a growing use of combinations which provide at least some approximation to total analysis and the technology exists to do this in a much wider range of applications. Furthermore, IBA can be used for spatial scans, depth profiling and in time sequence studies so that the possibility exists for obtaining a total analysis in all these contexts _ a remarkably versatile concept. Scattering (RBS) gives major element information but with poor mass resolution at medium to high masses. It also has limited sensitivity for light element determination in the presence of heavier elements. Forward scattering gives much improved sensitivity but with reduced mass resolution so that it is useful for light-element determination including simultaneous detection of light-mass recoils. X-ray detection (PIXE) is excellent for major and minor element determination at medium atomic numbers and heavier elements when medium elements are absent or selective filters can be used. In some cases, the use of ion-induced X-rays for 0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
XRF (PIXRF) may provide a useful complement to IBA measurements. Elastic recoil (ERA) and nuclear reactions (NRA) - especially involving gamma-ray detection (PIGME) - are most useful for light-element or isotope analysis. However, NRA and activation analysis (PAA) are only useful for selected isotopes and are not particularly suitable for total analysis unless some particularly important elements cannot be determined by other methods.
2. Examples 2.1. Mineral anaIysis It is common practice, in the analysis of minerals by XRF, to assume that each cation is associated with a characteristic amount of oxygen. For PIXE/PIGME analysis, most major cations are readily determined, with the exception of Mg unless this is present at the order of 1% or more. Conversion of element concentrations to the equivalent oxide concentration often gives values which sum to close to 100%. The calculation of an oxide sum is particularly valuable in automated analysis to verify system performance. It provides confirmation that all major elements have been determined or whether light elements including H, C and N may be present in significant quantities. ERA measurements of light elements complete a total analysis as required to obtain a satisfactory understanding of the major element composition of geological samples. In archaeology, for example in obsidian studies, total analysis has the additional advantage of giving immediate information for the identification of unusual samples. For example, charred calcite may be shiny black and be submitted by an archaeologist along with obsidian artefacts.
517
J. R. Bird / Total analysis by IBA 2.2. Metals
2.4. Bioscience and medicine
Backscattering gives a fast indication of the major elements in metals such as bronze, in which the constituents (Cu, Sn, Pb) have such different atomic masses that they are satisfactorily resolved. When the sample is known to be uniform as a function of depth, it is unnecessary to carry out a complete simulation to determine element concentrations. The step height provides this information. Alternatively, a differentiated spectrum can be used in conjunction with a peak fitting routine although the statistical accuracy is lower than for spectrum simulation. The mass resolution from scattering is usually inadequate to allow determination of neighbouring metal elements but additional X-ray or gamma-ray data may help achieve total analysis in this case. The presence of light elements with low scattering cross sections is often obscured in RBS measurements by the continuum from the metal matrix elements. Elastic recoil or nuclear reactions can then be used to determine light elements or advantage can be taken of the high non-Rutherford cross section for proton scattering. In the latter case, careful calibration is necessary to obtain absolute concentrations.
Trace element concentrations can be readily determined by PIXE analysis of organic materials provided that the major element concentrations are available for calculation of stopping power and X-ray attenuation factors. An example is the analysis of hair using PIXE, ERA (for H determination) and RBS (for C, N, 0 and S determination) [3]. The rapid, multi-element capability of PIXE can therefore be best exploited by simultaneous ERA and RBS measurements but again a total data processing system is needed. The use of these methods with proton microprobe scanning and event-by-event storage of data provides a very powerful tool for many studies in bioscience and medicine 141.
2.3. Polymers There are many cases in which the addition of pigments, contamination and other processes produce polymer layers of unknown composition. For example, changes of composition as a result of the weathering of coated steels poses a very difficult analytical problem which can best be solved with IBA [l]. The results of H recoil from 2.5 MeV He bombardment give H depth profiles and 2.5 MeV He RBS data give depth information on elements such as C, 0, Si and Ti which may be contained in a polymer matrix plus pigment material. The raw data show immediately that weathering decreases the amount of H and increases the Ti at the surface. A self-consistent set of depth profiles can be derived from the data but only with tedious iteration of spectrum simulation calculations. These calculations should be automated to derive the profiles without the need for manual intervention. Simultaneous PIXE/PIGME measurements can be used to assess the possibility that other elements may be present and to confirm the concentrations of those observed by RBS. X-ray or gamma-ray yields from the polymer layer can be compared with a multilayer calculation to confirm that the depth profile derived from RBS data is consistent. It is possible to do all such measurements simultaneously [2] but sophisticated data processing techniques must be developed in order to achieve rapid total analysis and produce depth profiles for each element.
2.5. Environmental
science
Widespread use has been made of PIXE for elemental analysis of aerosols but additional information is needed to obtain a complete characterisation of air filters and the materials which they collect. Once again the use of separate detectors for ERA (forward angles) and RBS (backward angles) is successful [5] but a single detector at forward angles can be used with alpha irradiation [6] for the same purpose. The thin target nature of air filters makes it possible to observe separate peaks for each of the light elements and to obtain a sensitivity of the order of 1 pg/cm’ or less for H, C, N and 0. 2.6. Museum
and art objects
External beam PIXE is very useful for answering questions about museum pieces such as: What is it made ofl, Is it a forgery?, How was it made?, How should it be conserved? and many others. PIXE is ideal for nondestructive analysis of tiny regions of pigments to establish the materials used and assess the possible significance of retouching or forgery. For example, a 30 s measurement on a modem acryclic painting gives a high count rate from major elements such as Si, S, Ti and Co from a cobalt blue on a white ground. Other elements are often important so that simultaneous PIGME measurements may be valuable - for example, to determine nitrogen present in Prussian blue but absent in ultramarine, or to verify the presence of Ti which is difficult to resolve from Ba in the PIXE spectrum [8].
3. Conclusion Many applications of ion beam analysis already make some use of the concept of total analysis and this capability should be more widely known. The power of on-line computers should be fully exploited to combine VII. MICRO- AND MILLIBEAMS
518
J.R
Bird / Total analysis by IBA
results from a number of detectors to iterate stopping power and other corrections until a self-consistent total analysis is achieved.
References [l] J.R. Bird, D.D. Cohen and C. Kannemeyer, 5th Austral. Conf. on Nuclear Techniques of Analysis, Lucas Heights (1987) p. 140. [2] C. Janicki, P.F. Hinrichsen, S.C. Gujrath, J. Brebner and J.-P. Martin, Nucl. Instr. and Meth. B34 (1988) 483.
[3] E. Clayton, J.F. Chapman and K.K. Wooller, IEEE Trans.
Nucl. Sci. NS-30 (1983) 1323. [4] G.J.F. Legge, P.M. O’Brien, B.J. Kirby and G.L. Allan, Ultramicroscopy 24 (1987) 283. [5] B.G. Martinsson, Nucl. Instr. and Meth. B15 (1986) 636. [6] T.A. Cahill, Y. Matsuda, D. Shadoan, R.A. Eldred and B.H. Kusko, Nucl. Instr. and Meth. B3 (1984) 263. [7] J.R. Bird, in: Archaeometry: Australasian Studies 1988, ed. J.R. Prescott (University of Adelaide, 1988) p. 136. [8] T. Tuumala, A. Hautojarvi and K. Harva, Nucl. Instr. and Meth. B14 (1986) 70.
Nuclear Instruments and Methods in Physics Research B45 (1990) 516-518 North-Holland
TOTAL ANALYSIS
BY IBA
J.R. BIRD ANSTO
Lucas Heights Research Laboratories,
Menai 2234, Australia
Depending on the nature of the sample, a selection of IBA techniques can be used to obtain a total major element analysis plus information on the concentration of a number of trace elements. This approach has been found to be valuable in studies of biological, geological and archaeological materials and the study, layer by layer, of the composition of inhomogeneous surface coatings. There are undoubtedly many other applications. A proton microprobe can be used to measure a complete 3dimensional multi-element distribution and the time dependence of this information. This is a unique capability but more work is needed to develop the concept of total analysis by IBA and especially to streamline data processing methods including the iteration of the calculation of stopping power and other corrections.
1. Total analysis
Total analysis can be defined as the determination of all major and minor elements present in a sample down to a lower limit of concentration which depends on the problem being investigated (usually between 0.1 and 1%). The results should therefore sum to 99-100%. Such an analysis is required for most ion beam analysis work in order to calculate stopping power, X-ray attenuation and other such parameters which enter into the determination of absolute element concentrations. It also constitutes vital information in many fields of investigation and techniques are needed which are able to provide such information quickly and economically. Ion beam analysis (IBA) represents a suite of techniques from which one or more can be selected for the solution of almost any problem in elemental analysis or, in many cases, isotopic analysis. There is a growing use of combinations which provide at least some approximation to total analysis and the technology exists to do this in a much wider range of applications. Furthermore, IBA can be used for spatial scans, depth profiling and in time sequence studies so that the possibility exists for obtaining a total analysis in all these contexts _ a remarkably versatile concept. Scattering (RBS) gives major element information but with poor mass resolution at medium to high masses. It also has limited sensitivity for light element determination in the presence of heavier elements. Forward scattering gives much improved sensitivity but with reduced mass resolution so that it is useful for light-element determination including simultaneous detection of light-mass recoils. X-ray detection (PIXE) is excellent for major and minor element determination at medium atomic numbers and heavier elements when medium elements are absent or selective filters can be used. In some cases, the use of ion-induced X-rays for 0168-583X/90/$03.50 (North-Holland)
0 Elsevier Science Publishers B.V.
XRF (PIXRF) may provide a useful complement to IBA measurements. Elastic recoil (ERA) and nuclear reactions (NRA) - especially involving gamma-ray detection (PIGME) - are most useful for light-element or isotope analysis. However, NRA and activation analysis (PAA) are only useful for selected isotopes and are not particularly suitable for total analysis unless some particularly important elements cannot be determined by other methods.
2. Examples 2.1. Mineral anaIysis It is common practice, in the analysis of minerals by XRF, to assume that each cation is associated with a characteristic amount of oxygen. For PIXE/PIGME analysis, most major cations are readily determined, with the exception of Mg unless this is present at the order of 1% or more. Conversion of element concentrations to the equivalent oxide concentration often gives values which sum to close to 100%. The calculation of an oxide sum is particularly valuable in automated analysis to verify system performance. It provides confirmation that all major elements have been determined or whether light elements including H, C and N may be present in significant quantities. ERA measurements of light elements complete a total analysis as required to obtain a satisfactory understanding of the major element composition of geological samples. In archaeology, for example in obsidian studies, total analysis has the additional advantage of giving immediate information for the identification of unusual samples. For example, charred calcite may be shiny black and be submitted by an archaeologist along with obsidian artefacts.
517
J. R. Bird / Total analysis by IBA 2.2. Metals
2.4. Bioscience and medicine
Backscattering gives a fast indication of the major elements in metals such as bronze, in which the constituents (Cu, Sn, Pb) have such different atomic masses that they are satisfactorily resolved. When the sample is known to be uniform as a function of depth, it is unnecessary to carry out a complete simulation to determine element concentrations. The step height provides this information. Alternatively, a differentiated spectrum can be used in conjunction with a peak fitting routine although the statistical accuracy is lower than for spectrum simulation. The mass resolution from scattering is usually inadequate to allow determination of neighbouring metal elements but additional X-ray or gamma-ray data may help achieve total analysis in this case. The presence of light elements with low scattering cross sections is often obscured in RBS measurements by the continuum from the metal matrix elements. Elastic recoil or nuclear reactions can then be used to determine light elements or advantage can be taken of the high non-Rutherford cross section for proton scattering. In the latter case, careful calibration is necessary to obtain absolute concentrations.
Trace element concentrations can be readily determined by PIXE analysis of organic materials provided that the major element concentrations are available for calculation of stopping power and X-ray attenuation factors. An example is the analysis of hair using PIXE, ERA (for H determination) and RBS (for C, N, 0 and S determination) [3]. The rapid, multi-element capability of PIXE can therefore be best exploited by simultaneous ERA and RBS measurements but again a total data processing system is needed. The use of these methods with proton microprobe scanning and event-by-event storage of data provides a very powerful tool for many studies in bioscience and medicine 141.
2.3. Polymers There are many cases in which the addition of pigments, contamination and other processes produce polymer layers of unknown composition. For example, changes of composition as a result of the weathering of coated steels poses a very difficult analytical problem which can best be solved with IBA [l]. The results of H recoil from 2.5 MeV He bombardment give H depth profiles and 2.5 MeV He RBS data give depth information on elements such as C, 0, Si and Ti which may be contained in a polymer matrix plus pigment material. The raw data show immediately that weathering decreases the amount of H and increases the Ti at the surface. A self-consistent set of depth profiles can be derived from the data but only with tedious iteration of spectrum simulation calculations. These calculations should be automated to derive the profiles without the need for manual intervention. Simultaneous PIXE/PIGME measurements can be used to assess the possibility that other elements may be present and to confirm the concentrations of those observed by RBS. X-ray or gamma-ray yields from the polymer layer can be compared with a multilayer calculation to confirm that the depth profile derived from RBS data is consistent. It is possible to do all such measurements simultaneously [2] but sophisticated data processing techniques must be developed in order to achieve rapid total analysis and produce depth profiles for each element.
2.5. Environmental
science
Widespread use has been made of PIXE for elemental analysis of aerosols but additional information is needed to obtain a complete characterisation of air filters and the materials which they collect. Once again the use of separate detectors for ERA (forward angles) and RBS (backward angles) is successful [5] but a single detector at forward angles can be used with alpha irradiation [6] for the same purpose. The thin target nature of air filters makes it possible to observe separate peaks for each of the light elements and to obtain a sensitivity of the order of 1 pg/cm’ or less for H, C, N and 0. 2.6. Museum
and art objects
External beam PIXE is very useful for answering questions about museum pieces such as: What is it made ofl, Is it a forgery?, How was it made?, How should it be conserved? and many others. PIXE is ideal for nondestructive analysis of tiny regions of pigments to establish the materials used and assess the possible significance of retouching or forgery. For example, a 30 s measurement on a modem acryclic painting gives a high count rate from major elements such as Si, S, Ti and Co from a cobalt blue on a white ground. Other elements are often important so that simultaneous PIGME measurements may be valuable - for example, to determine nitrogen present in Prussian blue but absent in ultramarine, or to verify the presence of Ti which is difficult to resolve from Ba in the PIXE spectrum [8].
3. Conclusion Many applications of ion beam analysis already make some use of the concept of total analysis and this capability should be more widely known. The power of on-line computers should be fully exploited to combine VII. MICRO- AND MILLIBEAMS
518
J.R
Bird / Total analysis by IBA
results from a number of detectors to iterate stopping power and other corrections until a self-consistent total analysis is achieved.
References [l] J.R. Bird, D.D. Cohen and C. Kannemeyer, 5th Austral. Conf. on Nuclear Techniques of Analysis, Lucas Heights (1987) p. 140. [2] C. Janicki, P.F. Hinrichsen, S.C. Gujrath, J. Brebner and J.-P. Martin, Nucl. Instr. and Meth. B34 (1988) 483.
[3] E. Clayton, J.F. Chapman and K.K. Wooller, IEEE Trans.
Nucl. Sci. NS-30 (1983) 1323. [4] G.J.F. Legge, P.M. O’Brien, B.J. Kirby and G.L. Allan, Ultramicroscopy 24 (1987) 283. [5] B.G. Martinsson, Nucl. Instr. and Meth. B15 (1986) 636. [6] T.A. Cahill, Y. Matsuda, D. Shadoan, R.A. Eldred and B.H. Kusko, Nucl. Instr. and Meth. B3 (1984) 263. [7] J.R. Bird, in: Archaeometry: Australasian Studies 1988, ed. J.R. Prescott (University of Adelaide, 1988) p. 136. [8] T. Tuumala, A. Hautojarvi and K. Harva, Nucl. Instr. and Meth. B14 (1986) 70.