- Email: [email protected]
In situ microanalysis—a survey of some major new developments
trends in\analytical chemistry, vol. 5, no. 5,1986
observer
VII
topics of interest in the current literature
ln situ microanalysis - A survey of some major new developments The general trend of developments in the area of in situ microanalysis is directed towards gaining new information about microdomains of a solid, then towards improvement of spatial resolution, detection power, precision and accuracy of analytical techniques and result@. A large number of sophisticated techniques, mainly based on the use of focused particle beams (photons, electrons, ions etc.) is available3. Many of these techniques are constantly being improved and new methods are being developed. Selected new developments of particular importance for materials analysis are presented here. Elemental analysis The analytical task is to determine the type and concentration of elements in microdomains of a solid and to characterize their distribution. Phase analysis
While phase identification in the micrometer domains is already done routinely using electron probe X-ray microanalysis (EPXMA), there have been major new developments in automated distribution analysis with energy dispersive X-ray spectrometers and image processing. A large number of sophisticated systems and programs are now available for automated stereometric analysis including the light elements carbon, nitrogen, oxygen, etc.4. A significant step forward is the incorporation of expert systems which allow ‘intelligent’ phase selection, identification and characterization5. Such techniques will provide more extensive means of quantitative materials characterization and classification of particles (e.g., air pollutants). There is strong emphasis on developing tools for the quantitative characterization of sub-micrometer domains. For ‘nanoanalysis’, analytical electron microscopy (AEM) has be0165-9936/86/$02.00.
come the major tool. This is a result of the development of highly sensitive X-ray detectors and electron energy loss spectrometers. X-ray analysis can now be performed for inclusions as small as 5-10 nm. Mathematical techniques could in principle be applied to enhance lateral resolution6. Transmission electron energy loss spectrometry (TEELS) has also become a quantitative technique on the basis of computer techniques for precise background subtraction. An accuracy of 10% for elemental analysis of phases about 10 x 20 nm in size has been reported7. Great progress could be achieved in TEELS imaging. Applying the technique of scanning the electron beam’ and computer processing of each pixel or by use of laterally resolving electron spectrometers9 high resolution elemental maps can be obtained with nm-resolution or better. Great success of TEELS microscopy is expected for material science and biology * In microstructural imaging, light microscopy with a lateral resolution of 500 nm and transmission electron microscopy (TEM) being developed
Abbreviations AEM ARM EPXMA EXAFS FIM LRMA NEXAFS pixel RHEED SIMS STAM TEELS TEM THEED XRA
all the way to the resolution of individual atoms (‘atomic resolution microscopy, ARM’) are widely spread in the laboratories, but a new technique with intermediate resolution shows great promise: X-ray microscopy (XRM) with focused synchroton radiation allows imaging of thin specimens. The scanning version of XRM presently being developed is expected to achieve a resolution of 10 nm”. Another interesting new technique is scanning transmission alpha microscopy (STAM) measuring the energy loss of a-particles with surface barrier diodes. Transmission micrographs with a resolution of 500 nm have already been obtained”. In the constant search for techniques which are able to analyze nanometer inclusions in a material, even those methods which exhibit a very difficult type of sample preparation come to prominence. The most obvious example is field ion microscopy (FIM), for which every specimen has to be formed to an extremely fine tip. In combination with desorption ionization of atoms from this tip by noble gas or laser light bombardment, and analysis of these ions by a time-of-flight mass spectrometer, FIM has become one of the very powerful techniques for na-
used
analytical electron microscopy atomic resolution microscopy electron probe X-ray micro analysis extended X-ray absorption fine structure (analysis) field ion microscopy laser Raman micro analysis near edge X-ray absorption fine structure (analysis) picture element reflection high energy electron diffraction secondary ion mass spectrometry scanning transmission alpha microscopy transmission electron energy loss spectrometry transmission electron microscopy transmission high energy electron diffraction X-ray microscopy
0 Elsevier
Science Publishers
B .V.
trends in analytical chemistry, vol. 5, ny. 5, I986
VIII
,
noanalysis (‘atomprobe’)“. It has been shown that inclusions as small as a few nanometers can be analyzed quantitatively, the accuracy being limited only by the statistics of registering a very small number of events. Analysis of carbide microprecipitates in steel for five elements measuring a total of 104 atoms (corresponding to a total mass of cu. 10-20 g) has been reported13. Trace analysis in microdomains
Trace analysis in microdomains is of particular interest for geological and biological materials, but can also be used for metals and ceramics. Greatest progress is presently being achieved with secondary ion mass spectrometry (SIMS) which, among the large array of instruments available, has an extremely high detection power. In order to achieve detection limits in the rig/g to ,ug/g level within an analytical domain of about 1 pm’, the yield of signal generation has to be maximized. A successful approach is the measurement of high intensity molecular ions that are generated by a chemical reaction during analysis. An increase in sensitivity by a factor of 1000 has been found: e.g. for phosphorous in W-NiFe alloys? Another approach to increase the secondary ion yield is based on postionization by resonant or non-resonant photon absorption from laser beams. This very sophisticated technique will most likely deliver the ultimate in absolute and relative detection power for in situ micro and surface analysis1’*16. A more elaborate method for trace analysis of microdomains which is developing very rapidly is based on the excitation of X-ray or nuclear radiation by high energy protons or ions (e.g. “N). Microprobes offering a lateral resolution of about l-2 pm enable trace analysis to be carried out with detection limits in the low ,uglg range”. A particularly interesting feature, which also applies to SIMS, is the ability to carry out three-dimensional distribution analysis18. Molecular analysis Molecular in situ microanalysis
is
of great significance in the identification of organic materials, also in combination with inorganic materials. Laser Raman microanalysis (LRMA) is being used routinely for this purpose. New developments of great potential are molecular mapping with laser micro mass spectrometry”, evaluating the fragmentation pattern of molecules, and IR microsc~py*~. A lateral resolution of a few micrometers can be achieved. These techniques significantly enlarge the potential of in situ microanalysis which in the past has been largely confined to elemental analysis.
measurement techniques. The greatest driving force behind the development of this field is the rapid progress of high technology, e.g. microelectronics and high performance materials. Stringent requirements for analytical characterization are posed in the process of development and production of these materials. The techniques for in situ microanalysis provide a wealth of information about advanced materials systems, thus being one of the keys to high technology2s-28.
Structure analysis Structure analysis of small phases is routinely performed by electron diffraction techniques such as transmission and reflection high energy electron diffraction (THEED and RHEED). Information is obtained on structure, lattice parameters, and orientation. However, the minimum size of the crystallites which is necessary to evaluate a diffraction pattern is cu. 20-100 nm. Therefore structural features of dispersed systems cannot be determined. For this purpose extended X-ray absorption fine structure (EXAFS) and near edge Xray absorption fine structure (NEXAFS) analysis offer great potential. Great progress has been made by the use of synchroton radiation due to its high intensity, collimation and pulsed structure. With such techniques, distances between atoms can be determined with an accuracy of 10m3nm. Also information on coordination and symmetry can be derived from the spectra21-23. Time resolved EXAFS is presently being developed to study transient structures of dynamic systems24. The theoretical time resolution to be achieved is in the order of nanoseconds. This opens up exciting new possibilities for the study of condensation, nucleation, phase changes in materials and of reactions occurring at solid-liquid interfaces.
References 1 M. Grasserbauer, Fresenius’ Z. Anal.
Conclusion In situ microanalysis is presently developing at a very rapid pace. New domains are being opened up by new methods and sophisticated analytical
M. GRASSERBAUER
Chem., 322 (1985) 10.5. 2 M. Grasserbauer, Fresenius’ Z. Anal. Chem.,
in press.
3 H. W. Werner and R. P. H. Garten, Rep. Pro-g. Phys., 47 (1984) 221. 4 J. J. McCarthy and J. P. Benson, J. Phys., 45 (1984) C2-215.
5 B. Raeymakers, P. van Espen and F. Adams, Mikrochim. Acta, II (1984) 437. J. B. Vander Sande and A. J. GarratReed, Mat. Res. Sot., 41 (1985) 333. A. Ben Lamine, J. Phys., 45 (1984) c2-709. R. F. Egerton, J. Phys., 45 (1984) C2-423. K. M. Adamson-Sharpe and F. P. Ottensmeyer, J. Microsc. (Oxford), 122 (1981) 309. 10 G. Schmahl, D. Rudolph and B. Niemann, J. Phys., 45 (1981) C2-77. 11 G. F. J. Legge, Nucl. Instrum. Methods Phys. Res., B3 (1984) 561.
12 R. Wagner, Field Ion Microscopy, Springer, Berlin, 1982. 13 K. Stiller, L.-E. Svensson, P. R. Howell, Wang Rong, H.-O. Andren and G. L. Dunlop, Acta Metall., 32 (1984) 1457. 14 P. Wilhartitz, M. Grasserbauer, H. Danninger and B. Lux, Fresenius’ Z. Anal. Chem., 319 (1984) 831. 15 Ch. H. Becker and K. T. Gillen, Anal. Chem., 56 (1984) 167. 16 N. Winograd, J. B. Baxter and F. M. Kimock, Chem. Phys. Lett., 88 (1982) 581. 17 P. Trocellier , Characterization of the superficial area of a solid by means of nuclear analytical techniques, report IUPAC Commission V.2., to be published in Pure Appl. Chem. 18 D. Heck, J. Phys., 45 (1984) C2-245. 19 D. Hercules, Microchim. Acta,
Suppl. 11 (1985) 1.
trendsi~analytica[chembtry,
vol. 5, no. 5,1986
G. Zachmann, 3RD IR-Seminar, Wissenbourg. 21 J. R. Chen, Scanning Electron Microsc., IV (1984) 1483. 22 H. W. Huang, W. Lin and J. A. Buchanan, Nucl. Inst. Meth., 205 (1983) 375. 23 J. Wong and K. J. Rao, Solid State 20
Commun.,
IX Progress in Materials Analysis, Vols.
1 and 2, Springer, Wien-New York, 1983 (Vol. l), 1985 (Vol. 2). 27 Conf. Ser. Inst. Phys., The Institute of Physics, Bristol, London. 28 M. Grasserbauer, G. Stingeder, E. Guerrero and H. Pbtzl, Fresenius’ Z. Anal. Chem., 323 (1986) 421.
45 (1983) 853.
24 P. Largade, Nucl. Instrum. Methods, 208 (1983) 621.
25 Materials Research Society Symposia Proceedings, Philadelphia. 26 M. Grasserbauer et al. (Editors),
Manfred Grasserbauer is Professor of Analytical Chemistry at the Technical University Vienna, Getreidemarkt 9, A1060 Wien, Austria.
meeting reports Euchem Conference ‘Sampling strategies and techniques in envirdnmental analysis*
A report bn the Euchem Conference ‘Sampling Strategies and Techniques in Environmental Analysis’, held in Bilthoven, The Netherlands, 21-24 January, 1986. The Euchem Conference ‘Sampling Strategies and Techniques in Environmental Analysis’ attracted many scientists and analytical chemists engaged in the practice of sampling. During four days, the participants were presented with results of research and practice in the three aspects of the environment that are relevant for analytical chemists: air, water and soil. Much time was devoted to discussion, both around the posters and in special discussion sessions. In the introductory session, ‘Kratochvil , Taylor and Kateman stressed the importance of sampling, good and well-described strategies and protocols. Tools for sampling strategies can be found in statistics, chemometrics and operations research. Monitoring of the air is carried out
fairly intensively. Models based on autocorrelation and wind direction allow interpolation and prediction from the results of monitoring networks. The efficiency of networks can be enhanced by using these techniques, according to Van Egmond and Huygen. Aronds stressed the importance of exact descriptions of sampling methods, especially when monitoring aerosols, and the need for statistically sound conclusions. Sampling techniques were discussed by Thijsse and Ballschmitter. Trapping of trace contaminants requires sophisticated techniques and materials; their analysis is becoming more and more complicated as the number of detectable compounds increases exponentially when the detection limit is lowered. Sampling strategies for water are similar to those for air. Autocorrelation of compounds in masses of water allows efficient sampling plans, if adequately used. Among the many authors who use these methods (a sign of the growing importance of this method of planning), the contributions of Miiskens, Smits and Davis described how these tools can be ap-
A number of articles presented at the meeting will be published in TrAC. The article by John K. Taylor on “The critical relation of sample to environmental decisions” on p. 121 is the first in this series. Two other articles can be found on pp. 124-128 and pp. 128-131. l
plied. They were responsible for quite a large part of the discussions. Deep sea sampling techniques on the contrary do not gain much from statistics. The extraordinary circumstances and high cost allow only single shot samples that require extremely careful preparation to prevent contamination, according to Hillebrand. Kramer and Schoer explained the special problems encountered when planning a sampling strategy for mudflats and estuaries with their rapidly changing flow characteristics, sedimentary speed and oxygen availability. Soil sampling differs from sampling of the other compartments of the environment in that it does not allow the modelling of the component distribution by first order autoregressive models. An interpolation method, known as Kriging, uses a statistical method that is somewhat different from autocorrelation to give a two or three dimensional description of the component distribution. Bouma and De Kwaadsteniet described the use of these techniques in mapping and developing a monitoring network. Van der Gaast described a method of statistically supported simulation models that allows the efficient monitoring of groundwater pollution. Following the important contributions in the past from BenedettiPichler, Visman, Ingamells and Gy, concerning the sampling strategy of particulates, new methods are now becoming available that can be applied to more continuous distributions. Parallel with the methods that use internal correlations for designing strategies, computer-based (simulation) models are emerging. Probably a future conference on this topic will more clearly identify the path that will provide a sound basis for analysis. G. KATEMAN
Professor G. Kateman is at the Laboratorium voor Analytische Chemie, Fakulteit der Wiskunde en Natuurwetenschappen, Katholieke Universiteit, Toernooiveld, 6525 ED Nijmegen, The Netherlands.
observer
VII
topics of interest in the current literature
ln situ microanalysis - A survey of some major new developments The general trend of developments in the area of in situ microanalysis is directed towards gaining new information about microdomains of a solid, then towards improvement of spatial resolution, detection power, precision and accuracy of analytical techniques and result@. A large number of sophisticated techniques, mainly based on the use of focused particle beams (photons, electrons, ions etc.) is available3. Many of these techniques are constantly being improved and new methods are being developed. Selected new developments of particular importance for materials analysis are presented here. Elemental analysis The analytical task is to determine the type and concentration of elements in microdomains of a solid and to characterize their distribution. Phase analysis
While phase identification in the micrometer domains is already done routinely using electron probe X-ray microanalysis (EPXMA), there have been major new developments in automated distribution analysis with energy dispersive X-ray spectrometers and image processing. A large number of sophisticated systems and programs are now available for automated stereometric analysis including the light elements carbon, nitrogen, oxygen, etc.4. A significant step forward is the incorporation of expert systems which allow ‘intelligent’ phase selection, identification and characterization5. Such techniques will provide more extensive means of quantitative materials characterization and classification of particles (e.g., air pollutants). There is strong emphasis on developing tools for the quantitative characterization of sub-micrometer domains. For ‘nanoanalysis’, analytical electron microscopy (AEM) has be0165-9936/86/$02.00.
come the major tool. This is a result of the development of highly sensitive X-ray detectors and electron energy loss spectrometers. X-ray analysis can now be performed for inclusions as small as 5-10 nm. Mathematical techniques could in principle be applied to enhance lateral resolution6. Transmission electron energy loss spectrometry (TEELS) has also become a quantitative technique on the basis of computer techniques for precise background subtraction. An accuracy of 10% for elemental analysis of phases about 10 x 20 nm in size has been reported7. Great progress could be achieved in TEELS imaging. Applying the technique of scanning the electron beam’ and computer processing of each pixel or by use of laterally resolving electron spectrometers9 high resolution elemental maps can be obtained with nm-resolution or better. Great success of TEELS microscopy is expected for material science and biology * In microstructural imaging, light microscopy with a lateral resolution of 500 nm and transmission electron microscopy (TEM) being developed
Abbreviations AEM ARM EPXMA EXAFS FIM LRMA NEXAFS pixel RHEED SIMS STAM TEELS TEM THEED XRA
all the way to the resolution of individual atoms (‘atomic resolution microscopy, ARM’) are widely spread in the laboratories, but a new technique with intermediate resolution shows great promise: X-ray microscopy (XRM) with focused synchroton radiation allows imaging of thin specimens. The scanning version of XRM presently being developed is expected to achieve a resolution of 10 nm”. Another interesting new technique is scanning transmission alpha microscopy (STAM) measuring the energy loss of a-particles with surface barrier diodes. Transmission micrographs with a resolution of 500 nm have already been obtained”. In the constant search for techniques which are able to analyze nanometer inclusions in a material, even those methods which exhibit a very difficult type of sample preparation come to prominence. The most obvious example is field ion microscopy (FIM), for which every specimen has to be formed to an extremely fine tip. In combination with desorption ionization of atoms from this tip by noble gas or laser light bombardment, and analysis of these ions by a time-of-flight mass spectrometer, FIM has become one of the very powerful techniques for na-
used
analytical electron microscopy atomic resolution microscopy electron probe X-ray micro analysis extended X-ray absorption fine structure (analysis) field ion microscopy laser Raman micro analysis near edge X-ray absorption fine structure (analysis) picture element reflection high energy electron diffraction secondary ion mass spectrometry scanning transmission alpha microscopy transmission electron energy loss spectrometry transmission electron microscopy transmission high energy electron diffraction X-ray microscopy
0 Elsevier
Science Publishers
B .V.
trends in analytical chemistry, vol. 5, ny. 5, I986
VIII
,
noanalysis (‘atomprobe’)“. It has been shown that inclusions as small as a few nanometers can be analyzed quantitatively, the accuracy being limited only by the statistics of registering a very small number of events. Analysis of carbide microprecipitates in steel for five elements measuring a total of 104 atoms (corresponding to a total mass of cu. 10-20 g) has been reported13. Trace analysis in microdomains
Trace analysis in microdomains is of particular interest for geological and biological materials, but can also be used for metals and ceramics. Greatest progress is presently being achieved with secondary ion mass spectrometry (SIMS) which, among the large array of instruments available, has an extremely high detection power. In order to achieve detection limits in the rig/g to ,ug/g level within an analytical domain of about 1 pm’, the yield of signal generation has to be maximized. A successful approach is the measurement of high intensity molecular ions that are generated by a chemical reaction during analysis. An increase in sensitivity by a factor of 1000 has been found: e.g. for phosphorous in W-NiFe alloys? Another approach to increase the secondary ion yield is based on postionization by resonant or non-resonant photon absorption from laser beams. This very sophisticated technique will most likely deliver the ultimate in absolute and relative detection power for in situ micro and surface analysis1’*16. A more elaborate method for trace analysis of microdomains which is developing very rapidly is based on the excitation of X-ray or nuclear radiation by high energy protons or ions (e.g. “N). Microprobes offering a lateral resolution of about l-2 pm enable trace analysis to be carried out with detection limits in the low ,uglg range”. A particularly interesting feature, which also applies to SIMS, is the ability to carry out three-dimensional distribution analysis18. Molecular analysis Molecular in situ microanalysis
is
of great significance in the identification of organic materials, also in combination with inorganic materials. Laser Raman microanalysis (LRMA) is being used routinely for this purpose. New developments of great potential are molecular mapping with laser micro mass spectrometry”, evaluating the fragmentation pattern of molecules, and IR microsc~py*~. A lateral resolution of a few micrometers can be achieved. These techniques significantly enlarge the potential of in situ microanalysis which in the past has been largely confined to elemental analysis.
measurement techniques. The greatest driving force behind the development of this field is the rapid progress of high technology, e.g. microelectronics and high performance materials. Stringent requirements for analytical characterization are posed in the process of development and production of these materials. The techniques for in situ microanalysis provide a wealth of information about advanced materials systems, thus being one of the keys to high technology2s-28.
Structure analysis Structure analysis of small phases is routinely performed by electron diffraction techniques such as transmission and reflection high energy electron diffraction (THEED and RHEED). Information is obtained on structure, lattice parameters, and orientation. However, the minimum size of the crystallites which is necessary to evaluate a diffraction pattern is cu. 20-100 nm. Therefore structural features of dispersed systems cannot be determined. For this purpose extended X-ray absorption fine structure (EXAFS) and near edge Xray absorption fine structure (NEXAFS) analysis offer great potential. Great progress has been made by the use of synchroton radiation due to its high intensity, collimation and pulsed structure. With such techniques, distances between atoms can be determined with an accuracy of 10m3nm. Also information on coordination and symmetry can be derived from the spectra21-23. Time resolved EXAFS is presently being developed to study transient structures of dynamic systems24. The theoretical time resolution to be achieved is in the order of nanoseconds. This opens up exciting new possibilities for the study of condensation, nucleation, phase changes in materials and of reactions occurring at solid-liquid interfaces.
References 1 M. Grasserbauer, Fresenius’ Z. Anal.
Conclusion In situ microanalysis is presently developing at a very rapid pace. New domains are being opened up by new methods and sophisticated analytical
M. GRASSERBAUER
Chem., 322 (1985) 10.5. 2 M. Grasserbauer, Fresenius’ Z. Anal. Chem.,
in press.
3 H. W. Werner and R. P. H. Garten, Rep. Pro-g. Phys., 47 (1984) 221. 4 J. J. McCarthy and J. P. Benson, J. Phys., 45 (1984) C2-215.
5 B. Raeymakers, P. van Espen and F. Adams, Mikrochim. Acta, II (1984) 437. J. B. Vander Sande and A. J. GarratReed, Mat. Res. Sot., 41 (1985) 333. A. Ben Lamine, J. Phys., 45 (1984) c2-709. R. F. Egerton, J. Phys., 45 (1984) C2-423. K. M. Adamson-Sharpe and F. P. Ottensmeyer, J. Microsc. (Oxford), 122 (1981) 309. 10 G. Schmahl, D. Rudolph and B. Niemann, J. Phys., 45 (1981) C2-77. 11 G. F. J. Legge, Nucl. Instrum. Methods Phys. Res., B3 (1984) 561.
12 R. Wagner, Field Ion Microscopy, Springer, Berlin, 1982. 13 K. Stiller, L.-E. Svensson, P. R. Howell, Wang Rong, H.-O. Andren and G. L. Dunlop, Acta Metall., 32 (1984) 1457. 14 P. Wilhartitz, M. Grasserbauer, H. Danninger and B. Lux, Fresenius’ Z. Anal. Chem., 319 (1984) 831. 15 Ch. H. Becker and K. T. Gillen, Anal. Chem., 56 (1984) 167. 16 N. Winograd, J. B. Baxter and F. M. Kimock, Chem. Phys. Lett., 88 (1982) 581. 17 P. Trocellier , Characterization of the superficial area of a solid by means of nuclear analytical techniques, report IUPAC Commission V.2., to be published in Pure Appl. Chem. 18 D. Heck, J. Phys., 45 (1984) C2-245. 19 D. Hercules, Microchim. Acta,
Suppl. 11 (1985) 1.
trendsi~analytica[chembtry,
vol. 5, no. 5,1986
G. Zachmann, 3RD IR-Seminar, Wissenbourg. 21 J. R. Chen, Scanning Electron Microsc., IV (1984) 1483. 22 H. W. Huang, W. Lin and J. A. Buchanan, Nucl. Inst. Meth., 205 (1983) 375. 23 J. Wong and K. J. Rao, Solid State 20
Commun.,
IX Progress in Materials Analysis, Vols.
1 and 2, Springer, Wien-New York, 1983 (Vol. l), 1985 (Vol. 2). 27 Conf. Ser. Inst. Phys., The Institute of Physics, Bristol, London. 28 M. Grasserbauer, G. Stingeder, E. Guerrero and H. Pbtzl, Fresenius’ Z. Anal. Chem., 323 (1986) 421.
45 (1983) 853.
24 P. Largade, Nucl. Instrum. Methods, 208 (1983) 621.
25 Materials Research Society Symposia Proceedings, Philadelphia. 26 M. Grasserbauer et al. (Editors),
Manfred Grasserbauer is Professor of Analytical Chemistry at the Technical University Vienna, Getreidemarkt 9, A1060 Wien, Austria.
meeting reports Euchem Conference ‘Sampling strategies and techniques in envirdnmental analysis*
A report bn the Euchem Conference ‘Sampling Strategies and Techniques in Environmental Analysis’, held in Bilthoven, The Netherlands, 21-24 January, 1986. The Euchem Conference ‘Sampling Strategies and Techniques in Environmental Analysis’ attracted many scientists and analytical chemists engaged in the practice of sampling. During four days, the participants were presented with results of research and practice in the three aspects of the environment that are relevant for analytical chemists: air, water and soil. Much time was devoted to discussion, both around the posters and in special discussion sessions. In the introductory session, ‘Kratochvil , Taylor and Kateman stressed the importance of sampling, good and well-described strategies and protocols. Tools for sampling strategies can be found in statistics, chemometrics and operations research. Monitoring of the air is carried out
fairly intensively. Models based on autocorrelation and wind direction allow interpolation and prediction from the results of monitoring networks. The efficiency of networks can be enhanced by using these techniques, according to Van Egmond and Huygen. Aronds stressed the importance of exact descriptions of sampling methods, especially when monitoring aerosols, and the need for statistically sound conclusions. Sampling techniques were discussed by Thijsse and Ballschmitter. Trapping of trace contaminants requires sophisticated techniques and materials; their analysis is becoming more and more complicated as the number of detectable compounds increases exponentially when the detection limit is lowered. Sampling strategies for water are similar to those for air. Autocorrelation of compounds in masses of water allows efficient sampling plans, if adequately used. Among the many authors who use these methods (a sign of the growing importance of this method of planning), the contributions of Miiskens, Smits and Davis described how these tools can be ap-
A number of articles presented at the meeting will be published in TrAC. The article by John K. Taylor on “The critical relation of sample to environmental decisions” on p. 121 is the first in this series. Two other articles can be found on pp. 124-128 and pp. 128-131. l
plied. They were responsible for quite a large part of the discussions. Deep sea sampling techniques on the contrary do not gain much from statistics. The extraordinary circumstances and high cost allow only single shot samples that require extremely careful preparation to prevent contamination, according to Hillebrand. Kramer and Schoer explained the special problems encountered when planning a sampling strategy for mudflats and estuaries with their rapidly changing flow characteristics, sedimentary speed and oxygen availability. Soil sampling differs from sampling of the other compartments of the environment in that it does not allow the modelling of the component distribution by first order autoregressive models. An interpolation method, known as Kriging, uses a statistical method that is somewhat different from autocorrelation to give a two or three dimensional description of the component distribution. Bouma and De Kwaadsteniet described the use of these techniques in mapping and developing a monitoring network. Van der Gaast described a method of statistically supported simulation models that allows the efficient monitoring of groundwater pollution. Following the important contributions in the past from BenedettiPichler, Visman, Ingamells and Gy, concerning the sampling strategy of particulates, new methods are now becoming available that can be applied to more continuous distributions. Parallel with the methods that use internal correlations for designing strategies, computer-based (simulation) models are emerging. Probably a future conference on this topic will more clearly identify the path that will provide a sound basis for analysis. G. KATEMAN
Professor G. Kateman is at the Laboratorium voor Analytische Chemie, Fakulteit der Wiskunde en Natuurwetenschappen, Katholieke Universiteit, Toernooiveld, 6525 ED Nijmegen, The Netherlands.