Understanding spectroscopy with a view to rationalizing spectrochemical analysis: an abysmal adventure or a realistic ideal?

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hmica Ada Vol. 46B,No. 6/7,pp. 711-739, 1991

0564-8547/91 $3.00t .oo Pergamon Pressplc.

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Understanding spectroscopy with a view to rationalizing spectrocbemical analysis: an abysmal adventure or a realistic ideal? t * P.W.J.M. BOUMANS Philips Research Laboratories, P.O. Box 80.000, 5600 JA Eindhoven, The Netherlands

(Received 28 November 1990; accepted as camera-ready copy: 15 March 1991)

Abstract-This paper presents a view on the creation, transfer, and use of knowledge that constitutes or should constitute the basis of spectrochemical analysis methods. This view is projected on a communication network of “spheres” of which physics, spectrochemical physics, methodology development, instrument development, and applications development form the nuclei. According to the author’s definition, the knowledge created in the sphere of methodology development is based partly on parametric studies of sources or atomizers, partly on analytical or spectroscopic approaches inherent in and characteristic of spectrochemical methodology development. On the other hand, methodology development is understood to derive a rational backing from an indispensable interaction with spectrochemical physics and physics. Methodology development is argued to occupy a crucial and central position in the communication network because it searches for concrete answers to generally analytical or spectrochemical questions and provides these answers by building up systematic knowledge that is directly usable in both instrument and applications development. This philosophy and the pertinent question raised in the title of this paper are discussed in a concrete context by reviewing and analyzing the developments in three domains of spectrochemical analysis: inductively coupled plasma (ICP) atomic emission spectrometry (AES), spark AES, and glow discharge (GD) spectroscopy. The pivot position of methodology development is illustrated by the developments of both ICP-AES and GD-AES, where methodology development has substantially contributed to the implementation of rational approaches. This situation is contrasted with the development of spark AES, where a vast reservoir of profound fundamental knowledge has been acquired but has also been locked up in an ebony tower, because there has been little or no demand from the side of instrument manufacturers for a rationalization of spark-AES analysis. This has created a “spark-AES methodology gap”, which, for various reasons discussed, is not likely to be bridged in the future. GD spectroscopy is shown to cope with a rather fundamental handicap: the lack of local thermal equilibrium in GDs. The further rationalization of GD spectroscopy is concluded therefore to require an essential expansion of spectrochemical physics using basic input from physics and the implementation of approaches not yet very common in the spectrochemical field. On the whole, the author expresses his concern not so much about the unavailability of knowledge that would help rationalizing spectrochemical analysis, but about the proper transfer of this knowledge to the recipients. In particular, the author emphasizes the need for making fundamental knowledge in the circuits of methodology development and spectrochemical physics better consumable, not only to applied analysts but also to chemometricians in order to prevent the proliferation of artificial intelligence that rests on practice only but lacks a really rational basis.

1. INTR~OUCTION THE PREPARATIONof this award address could have been a very comfortable job if I had given way to the temptation of composing an overview of my own recent and current work. Audience and readers might have considered it an interesting presentation, but, undeniably, one would have added: “He is growing old!“. Award address given in the Symposium: “The Shaping, Moulding, and Perpetual Reshaping of Analytical Atomic Spectroscopy”, on the occasion of the presentation of the 1990 Spectrochemical Analysis Award of the Analytical Division of the American Chemical Society (sponsored by Perkin-Elmer, Norwalk, CT) to Paul Boumans, organized by Alexander Scheeline during the 200th Meeting of the American Chemical Society, Washington, DC. 27-31 August 1990. This article was published in the Special Boumans Festschrift 711

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Alternatively, I could have granted a glance at my memoirs - which still have to be written - and again one might have found the presentation interesting, but then one would have noted: “He is old”. My share in the preparation of this Award Symposium would have been much easier altogether, had I confined it to letting the invited speakers tell their stories, but I would certainly have regretted to have missed a chance of assigning myself the task of moulding a train of conscious or unconscious thoughts that have plagued me for a great many years. The essence of these thoughts is condensed in the title of this address, a thesis followed by a question. The meaning of the thesis: “Understanding spectroscopy with a view to rationalizing spectrochemical analysis” seems obvious, in particular if seen in the light of HIEFTJE’S paper “Toward improved understanding and control in analytical atomic spectrometry” [ 11. Answering the subsequent question in the title of the present paper may provide more difficulties. However, giving a detinitive answer is not the purpose of this address. The aim is to make the reader conscious of a problem: the existence of an abyss,‘a gap between fundamental, academic research, on the one hand, development and application on the other. Perhaps I am harping herewith on the same string as WALTER SLAVIN did as recipient of the Anachem Award [2], but I shall look at the problem from a somewhat different angle of incidence, with less emphasis on the axis academic researcher-instrument manufacturer. My concerns may be expressed by expanding the title into a few further questions: - Do not we waste precious resources on fundamental research by only sterilizing the results in the ebony towers of academic theses and scientific journals? - Is it possible to bring the fruits of fundamental directly profit of them?

research closer to a public that could

- Do we want to understand spectroscopy for the mere sake of understanding something or do we want to understand it in order to exploit this understanding - or let it be exploited by others - to improve spectrochemical analysis methods? - Is not the fundamental route too long for the rational solution to be still of use for the hurried, analytical empiricists who are often compelled to find in irrational and awkward ways solutions to practical problems when implementing new methods? - How do we want to define fundamental? What is “understanding” spectroscopy, what a “rational approach” to spectrochemical analysis?

and

- What is an abysmal adventure and what a realistic ideal? I shall deal with these questions by considering examples rather than by presenting a rigorous and dry philosophy. I have gleaned the examples chiefly from my own experience, since this permits me to pronounce assessments with a minimum risk of collisions! Thus, yet memoirs? A bit, yes, but only as means justified by the end. Recounting a few stories may also make it easier to digest the philosophy, and, perhaps the reader might enjoy them in their own right. Possibly one will conclude: “He has lost his wild hair, but not entirely!” 2. THE START OF A CAREER IN ATOMIC SPECTROSCOPY My first assignment in instrumental analysis as a student of 21 has been the determination of cesium and rubidium in the mineral beryl using the d.c. carbon arc, for which I should exploit the cathode layer effect. This approach dates back to a German physicist, MANNKOPFF in Giittingen, where it has been explored in the early 1930s by geochemists, such as GOLDSCHMIDT and PREUSS. An American spectroscopist, LESTER STROCK also happened to work in MANNKOPFF’S laboratory and published a booklet on this method in 1936 [3]. This booklet as well as the German publications of the MANNKOPFF group [4-61 formed the literature with which I started to attack the problem. The instrumental outfit was scrap.

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Paramount is that I was at once fascinated by the physics of the phenomenon of the “cathode layer effect”, which is due to transport of ions in the longitudinal field of the dc arc, in competition with convection and diffusion. Another point is that the prominent lines of Cs and Rb accessible with my spectrograph are located in the cyanogen band region. Reducing the CN band intensity demanded the use of a temperature suppressant, excess alkali metal, which, in turn, destroyed the cathode layer effect as a result of ionization suppression. Thus, at once two desires were born: - Understanding the plasma physics of the arc. - Describing the cathode layer phenomenon by mathematical equations which would permit a rigorous statement about optimum analysis conditions as to the type and amount of alkali additive to be used for analytes of different ionization potential. That is what the young man considered to be the only rational approach to emission spectroscopic analysis! All the rest could be no more than pedestrian, empirical fiddling! Fortunately for him (and spectroscopy!) the young man soon discovered the Physical School at Utrecht University, where they had been exploring arcs since the early 1930s. They needed these arcs to produce atomic spectra for which they wanted to verify the sum rules, and this entailed the measurement of transition probabilities. This, in turn, required knowledge of the physical conditions in the arc. The young man thus landed in paradise and went home with piles of physical dissertations of the Utrecht Physical School, among which a complete treatise “The production and measurement of high temperatures up to 7000 “K” by J.A. SMIT [7]: 300 pages with a great many in small type! It was what the Germans call: “gefundenes Fressen”, a godsend. In principle, the components for building a vast bastion were available: Boltzmann, Saha, Maxwell, Planck, temperatures of various kinds, electron pressure or concentration, descriptions of ionization, recombination, excitation, de-excitation, detailed balancing, radial and axial distributions, Abel inversion, ambipolar diffusion, diffusion from a point source, and transport in an electric field. The brick-layer got to work and attacked a complex of mathematical equations with a log table as the 1954 pocket calculator. It was a long way to find optimum analysis conditions in the only rational way! My first paper was presented (in German) at the 6th CSI in Amsterdam: “Concentration distribution of metal vapour in the cathode layer arc” and later published in the Proceedings [B]. To my great surprise, the analytical spectroscopy community was more or less - actually more than less - stupefied about the fact that spectrochemical analysis could be projected on a basis of sound physics. They had never yet realized that ionization in a plasma is essentially the same as ionization in an aqueous solution, that pH and n, are similar concepts, the only difference being that the ionization constant for a plasma, known as the Saha equation, is given as a function of temperature, whereas nobody bothers about the temperature-dependence of the hydrogen-ion concentration in a solution. - Even today, some 35 years later, I would not like to cater to the analytical spectroscopists who still await the enlightening! Subsequently I kept working on the analysis of the physics of the dc arc with a view to devise rational rules for optimizing spectrochemical analysis, not just the cathode layer arc, and even not primarily. This work led to my dissertation (in Dutch): “Some fundamental aspects of the dc carbon arc for spectrochemical analysis” [9], published in 196 1. Thereafter I have reworked, on invitation, the dissertation and brought it into the form of a textbook: “Theory of Spectrochemical Excitation” [lo], published in 1966. Since this book is now well known in the spectrochemical community, I need not recount the story behind it. However, in the present context, I wish to extract from the Prefaces of the book and dissertation a few statements concerning the rationalization of spectrochemical analysis.

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The thesis began with a quotation from WILLIAM MEGGERS’ “Principals and Principles of Spectrochemical Analysis” [ 1l] concerning the spectrochemical literature up to 1948: ‘Even a superficial glance at this literature gives an impression of a supertluity of “methods”, an excessive multiplicity of light sources and operating conditions, and a purely empirical solution of most problems. It appears that the applied spectroscopists (excluding astrophysicists) have scarcely taken any notice of the great advances in knowledge concerning the origin and interpretation of atomic spectra which the fundamental physicists brought forth in the period of decline of spectrochemical analysis, 1885-1925.’ We then look at a few statements in the Preface of the book [IO]: ‘Practical knowledge and experience are indeed important requisites for successfully exploiting the [emission] spectrochemical method in the field of analytical chemistry. Since the method is essentially empirical, it is, in principle, a simple one, provided that we succeed in exciting all samples in an identical manner . . .. . . .. . .. . .. . .. . However, creating the required constancy of excitation conditions is hampered by the very nature of the sample, whose composition profoundly influences the excitation characteristics of the light source. Therefore, spectrochemists are inevitably involved in all the processes that determine the radiation output of the light source for a given sample.’

‘Dealing with this ensemble of processes is the object of this book. The reader will seek in vain for enumerations of practical rules that would tell him how to tackle a particular analysis problem. What he will find is a detailed and specified exposition on the laws that govern the excitation of samples in the d.c. carbon arc. This should enable him to derive the rules for performing particular case.’

an analysis rationally in a

‘Detailed knowledge of the conditions in one source will give sufficient insight into general fundamentals, and brings specialized literature on other sources easily within reach. The fundamental approach attempted here unfortunately involves the risk that the book is doomed to remain “the voice of one crying in the wilderness”, since many topics must be depicted in a more complicated manner than spectrochemists are accustomed to.’ Finally, from the Preface of the thesis: ‘Spectrochemical analysis begins where this thesis ends’. Here we may raise two questions: - How does fundamental knowledge proliferate, where is it used, and by whom? - Which are the profits that spectrochemical analysis ultimately derives from it? These questions may be discussed in the scope of a series of “spheres” that touch spectrochemical analysis: - physics, - astrophysics, - plasma physics, - spectrochemical physics, -

basic methodology development,

electronics/information science, instrument R & D, spectrochemical methods development, standard equipment development, cookbook methods development, - cookbook analysis,

-

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Understandingspectroscopy - automated cookbook analysis, - artificial intelligence. Most of these spheres will not require elucidation,

except for the two printed in italics.

Spectrochemical physics. I coined this term in the book on “spectrochemical excitation” [lo] and think that the term is still useful in the present context. I would define spectrochemical physics as the complex of fundamental studies of physical and chemical processes in sources and atomizers aimed at (i) understanding why analysis does not proceed as ideally as desired, in other words, explaining “strange” observations, (ii) devising “remedies” on the ground of the resulting insights into physical-chemical processes, (iii) defining optimum analysis conditions for the source or atomizer under study, and (iv) proposing modifications or innovations.

In short, the goal of spectrochemical physics is the shaping or reshaping of spectrochemical analysis methods as the fruit of rational thinking and experimenting rather than as the result of empirical trial and error fiddling. However, it is inherent in the approach that spectrochemical physics tends to isolate itself into an ebony tower following the motto of “l’art pour l’art” rather than being directed towards the improvement of spectrochemical analysis. We want to understand, but once a point is understood, we leave the implementations to others, who often do not have the interest and/or the capabilities to pick up the thread, or pretend to have no time for this. Before getting further into this matter, let me also give a definition of the adjacent sphere. Basic methodology development. I would define it as the complex of fundamental

studies analysis, thus all the stages

related to any or all of the steps implied in a spectrochemical “from sample handling to data processing”, with these goals: (i) establishing generally usable rules for optimizations of various types, (ii) devising universally applicable approaches for the detailed development of practical methods, (iii) easing comparisons of data obtained with different equipments and/or methods, and (iv) facilitating the exchange and transfer of methods and data between equipments and laboratories.

Clearly, according to this definition, basic methodology development is closer to spectrochemical analysis than spectrochemical physics in that it operates with concepts, such as detection limit, precision, dynamic range, signal-to-noise, and signal-tobackground, which directly relate to analysis. Methodology development also has a wider coverage in that it extends to all steps of an analysis, not just the excitation or atomization. It is, however, closely linked to spectrochemical physics and ideally should use the insights gained in that field. However, methodology development will definitely emphasize the generalization of the results of parametric studies rather than the detailed physical interpretation of these results. The above definitions reflect a personal “a posteriori philosophy” deduced from past developments, and possibly usable as an “a priori philosophy” for future developments. I shall elucidate this view by projecting the spheres on the developments of inductively coupled plasma (ICP), spark, and glow discharge (GD). I have centred my arguments about these excitation sources; however, other sources, such as direct current plasmas or microwave plasmas could have been chosen as well.

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4. BASIC METHODOLOGYDEVELOPMENTAND SPECTROCHEMICAL PHYSICS IN THE ICP ERA The position of spectrochemical physics in the mid 1960s might by characterized 1966 welcome to the Theory of Spectrochemical Excitation” [lo]:

by “The

- Arc: moribund. Did no more need it! - AAS: booming. Did not want to need it! They did not excite but were excited! - ICP: cradled. Did not yet need it! If an author sees this as the main outcome of more than ten years of hard work, he can only say: “Happy New Year!” Fortunately, emission spectroscopists were soon to leave the catacombs and to enter not just a “Happy New Year”, but a “Happy New Era”. To cite VELMER FASSEL [12], “this was presaged in a number of ways, for example, by provocative titles in several scientific communications”. Thus FASSEL used “Optical Emission Spectroscopy: Stagnant or Pregnant?” [ 131 *), while BOUMANSused “Multielement Analysis by Optical Emission Spectroscopy - Rise or Fall of an Empire? [ 151. After the initial pioneering work by FASSEL and GREENFIELD in the 196Os, as recounted in their reviews [12,16-231, t the true ICP boom began in the early 1970s. The first priority then was to establish the analytical characteristics in a systematic way. This work is in the sphere of basic methodology development, which means that the 1970 ICP spectroscopists were primarily involved in (i) parametric studies with the various analytical performance characteristics as criteria: - detection limits (prominent lines), - matrix effects (multiplicative interferences), - precision, - dynamic range; (ii) “optimization” (iii) comparisons

and “compromises”; of the ICP with arc and flame.

The second priority source, i.e., (i)

was in the sphere of spectrochemical

physics: understand

the new

explain the trends found in parametric studies: - dependence of line and background on the principal ICP parameters;

(ii) explain “anomalies”: - ionic line advantage, - low level of multiplicative interferences, - feasibility of compromise conditions. Fortunately, the triumphs of the “poor man’s spectroscopy of the early 197Os”, AAS, did not yet encourage the belief in the renaissance of emission spectroscopy, so that basic methodology development in connection with instrument research got a chance before the situation could be spoilt by an uncontrolled applications boom. I shall not embark here upon recounting the details of the parametric studies: the essentials have been summarized in the section: “Optimization of the ICP operating conditions” in Part 1 of my ICP book [24, pp.193-2361. - Additional information can be found in other chapters of this work, in particular the chapters on applications in Part 2 [25], and in MONTASERand GOLIGHTLY’SICP book [26]. - What I shall do is to give a view on these and the subsequent developments by a stepwise build-up of a block diagram, which may be taken to represent the history of the shaping of “ICP knowledge circuits”. *) With an acknowledgment to A. WALSH for being the first to use the descriptive “Stagnant or pregnant” phraseology in a paper entitled: “Atomic Absorption Spectroscopy: Stagnant or Pregnant?” [14]. t

See also section 3.6: “Review of reviews” in [24, pp.92-941.

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In that context, I shall refer to a few results of the parametric arguments.

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Fig. 1. Evolution of the ICP knowledgecircuit: the start. Thus, the initial “assignment” (Fig. 1) was to establish the analytical characteristics of the ICP via parametric studies and to start the development of ICP apparatus and sample introduction devices. Concurrently, those involved in this work naturally attempted to explain the findings on the basis of the existing fundamental knowledge in the field of spectrochemical physics. The latter did not really work, however (Fig. 2): the complex spatial structure of the ICP seemed to put up a barrier, and, altogether, ICP behaviour could not be simply explained by what we knew about arcs and flames. Yet both instrument and methodology development have benefited a lot from these systematic parametric studies and the venturous or adventurous explanations.

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Fig. 2. Evolution of the ICP knowledgecircuit: the first barrier encountered. Among my own early observations in an argon ICP was the dependence of the intensities of a variety of spectral lines on carrier gas flow (CT Fig. 4.40, p.206 in [24]), of which a broad range could be covered and this was feasible because we used an ultrasonic nebulizer [27,28]. We tried to explain the maxima on the ground of T and n, measurements derived from intensity ratios of Zn and Mg lines, thus the application of what I had done formerly with respect to the dc arc [9,10]. However, this was unsuccessful [29], and understandably so, in particular, if one looks at the transition from a “smooth” analytically useful ICP with a low carrier gas flow, to an ICP that is broken open by too high a carrier gas flow, with the consequence that the carrier gas passes as a virtually independent body through the

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annulus. t We simply recognized that this “high-flow configuration” could not be of analytical interest, primarily because further experiments showed that multiplicative interferences (“matrix effects”) could become horrible at high carrier gas flows (c$ Fig. 4.44, p.210 in [24]). We therefore did not further examine that configuration. For relatively low carrier gas flows we generally found smooth dependencies of line intensities on the principal ICP parameters: power, carrier gas flow, and observation height [28,32]. We also found distinctive features between the behaviour of ionic lines, on the one hand, and atomic lines of elements of relatively low ionization potential, on the other. A further analysis of the dependence of line and background intensities on power [32] led to the idea of discussing such dependencies in terms of “soft” and “hard” lines [33], a concept coupled to “norm temperatures” (cJ [24, pp.19&201] and [34]), which were computed to be relatively low for soft lines and relatively high for hard lines. The approach implied, of course, a smooth transition between the two extremes. The discussion of the results of parametric studies in terms soft and hard lines was useful to classify the observed trends and to recognize the systematics of line intensity behaviour in an argon ICP, which was smooth and clear in many respects. The entire approach enabled us to define optimum conditions for achieving low detection limits, which have been called “compromise conditions”, but in fact hardly implied compromises, if the alkali metals as analytes were kept out of sight. Curiously, also the matrix effects caused by alkalis could be relatively well mastered under the same compromise conditions. Explaining why they could be mastered and why they spoilt the ICP, for example, at relatively high carrier flows, remained in the darkness, and still does so, I fear, although, in the meantime, many dark points have been partly or wholly clarified.

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Fig. 3. Evolution of the ICP knowledge circuit: extension of parametric studies to molecular gas and mixed gas ICPs, as well as ICP mass spectrometry.

Also illustrative for the usefulness of parametric approaches as parts of the basic methodology development are the more recent studies (Fig. 3) by the groups of BARNES, MAGYAR, MERMET, MONTASER, HORLICK, and others, with respect to molecular gas, mixed gas, and He ICPs, as both excitation sources for atomic emission spectrometry (see, e.g., the reviews [35,36], the literature cited therein, and [37-391) and ion sources for mass spectrometry (see, e.g., [4045]). Such studies have considerably enriched our knowledge, and their results provide concrete guidelines for analytical optimization. On the other hand, however useful and indispensable parametric studies have been and still are for basic methodology development, they only provide a descriptive picture, not an explanation, the latter being a picture based on general fundamental knowledge, which, strictly speaking, is descriptive, too, but descriptive on a higher level. t A study of some physical characteristics of the “low-flow ICP” versus the “high-flow ICP” has been later on undertaken by KORNBLUM and DE GALAN[30,31].

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Fig. 4. Evolution of the ICP knowledge circuit: entry of the “work horse” and extension of methodology development into the domain of spectroscopy itself.

Before considering the fundamental picture, let us first look at the extension of the block diagram (Fig. 4) with the creation of the “work horse”, via “instrument development” and “sample introduction”. This advance would have been impossible without a substantial expansion of basic methodology development into the domain of spectroscopy itself. This research is characterized here by the labels, “spectral interferences”, “resolution”, and “noise”. More labels might be added, e.g., “line widths” [46-571. Essential is that this sphere induced many crucial instrumental developments, in particular with respect to the spectrometer and the handling of spectral data (see, e.g., [58-65]). To mention a few: - improved straylight characteristics - holographic gratings, - echelles, - background correction in polychromators, - improved line selection, - sequential slew-scan monochromators, - use of internal standards, - Fourier transform spectrometry. The diagram does not contain an allusion to the development of the ICP itself, such as ICPs with reduced gas consumption (see, e.g., [66-681) or ICPs operated at less conventional frequencies (see, e.g., [66,69-741). I do not think that anything really essential has changed since the early 1970s. For example, we started in 1971 with a 50 MHz ICP using a free-running generator [27] and stuck to it all over the years. At present everyone hails the “free running” approach and “40 MHz” (which is analytically the same as 50 MHz, but more convenient in view of government regulations). Also, the macro-, mini- and micro-torch affairs seem to have passed like a storm in a glass of water, not to talk about enormities like the “laminar flow torch” for the argon ICP [75-80]!

The extension of the diagram (Fig. 5) shows, in the sphere of methodology development, the items “detection limits”: breakdown of detection limits [55,81,82], development of the concept of true detection limit [55,83,84], application of second derivatives [85], chemometric approaches (see, e.g., [86]), in particular Kalman filtering to reduce the disastrous effect of spectral interference on the true detection limit [87-891, and “spectrum simulation” [55,90-941. In this context I should also mention the measurement and analysis of noise power spectra (see, e.g., [95-971 and the literature cited therein), the development of the signalto-noise (SNR) theory and the “SBR-RSDB approach” (the approach using signal-to-

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Fig. 5. Evolution of the ICP knowledge circuit: the barrier in the domain of spectrochemical physics bypassed and methodology development extended to the breakdown of detection limits and simulation of emission spectra.

background ratio and relative standard deviation of the background), as well as an effort to connect the SNR and SBR-RSDB approaches (see [98] and the literature cited therein). A further inspection of the diagram also reveals an appreciable expansion in the sphere of spectrochemical physics, where previous barriers were bypassed by increased use of Abel inversion [99-1031, generalized tomographic image reconstruction techniques [104,105], high-speed motion picture photography [106], and, generally, a variety of diagnostic techniques (see, e.g.,[ 107-l 1l]), including pulse interrupted or power amplitude modulation approaches [ 112- 1171, Thomson and Rayleigh scattering [ 118-1231, and relaxation time [124-1271 or Langmuir probe measurements [128], as well as computer simulation and modelling of ICPs [129-1321. On the whole, the research work of various groups of prominent scientists has substantially enriched our knowledge in the field of the spectrochemical physics of the ICP (see, e.g., [34,99-1581). The knowledge gained in this fundamental circuit has been coupled to some extent into the circuit of basic methodology development, and has thus exerted some indirect influence on instrument development and jmalysis [ 11. If we look at the further evolution of the block diagram (Fig. 6), we see it being extended with “cookbook applications” and “methods development”. At the same time we observe a sharply increased emphasis on sample preparation and sample introduction techniques (see, e.g., [159-2031). Note the white area and the barriers at the bottom centre. The filling of this spot tends to lead to the birth of a “grey circuit of comfortable knowledge” (Fig. 7). We see a few “blackouts” and the disappearance of barriers. The diagram symbolizes the risks we run when methods development detaches itself from the existing “white” knowledge circuits and embarks upon establishing again what is stored in the literature, while adding “comfortable explanations”. Should those involved in methods development be crushed between the barriers of the “white” knowledge circuits ? Or can they live comfortably (Fig. 8) with methods development between “white” and “grey”? The methods developer cannot be saddled with all the knowledge available in the “white” circuits. On the other hand, methods development does profit of this knowledge in that

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Fig. 6. Evolution of the ICP knowledge circuit: the booming of the ICP as a practical tool including the risk that methods development isolates itself from the existing “white” knowledge circuit.

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Fig. 7. Evolution of the ICP knowledge circuit: incarnation of the risk of methods development isolation, resulting in “blackouts” and in the creation of the “grey” circuit of “comfortable knowledge”.

part of it is implemented in the “work horses”. More profits could still be obtained by direct and more intense interaction between methods development and basic methodology development. A prerequisite for this is to make the basic knowledge better consumable. In fact, ICP spectroscopy is in a very favourable situation in that it has a vast and sound circuit of basic knowledge available. It must be relatively easy to translate and transfer that

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1 Det. limits

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Fig. 8. Evolution of the ICP knowledge circuit: making basic knowledge consumable in order that the indispensable methods and software developers (including chemometricians) be enabled to their jobs rationally and can live comfortably. [with acknowledgements to PAULC&ANNE (183~1906) for creatinghis “cardplayers”and makingthemavailablefor thisdiagram].

knowledge to the analysis and methods development circuits, which, in turn, would result in a further rationalization of spectrochemical analysis. Perhaps one might note that this is no longer needed in view of an increasing incorporation of artificial intelligence into analytical instruments. This may be partly true. However, I fear that this development may easily take a wrong direction because chemometricians, on the one hand, tend to shut up themselves in apparently unassailable ebony towers of untransparent mathematics, and, on the other hand, tend to mimic actual analysts’ behaviour without critically comparing this behaviour with the rational rules of the basic methodology, which, as appears sometimes, neither analysts nor chemometricians are always familiar with. It should be repeated that ICP spectroscopy is in a very favourable situation in that basic methodology development has been in the limelight all over the years. There is something This situation essentially contrasts with spark emission available in this sphere. spectroscopy. 5. BASIC METHODOLOGY DEVELOPMENTAND SPECTROCHEMICALPHYSICS IN THE SPARK DOMAIN

Spark atomic emission spectroscopy (AES) may be featured by the block diagram shown in Fig. 9. When scrutinizing this diagram we do not need binoculars to discover the big gap in the centre of the picture: a gap which represents what I would call the “analytical spark-AES methodology gap”. I coined this term a few years ago when discussing this issue at some length with ALEXANDER SCHEELINE.This discussion was induced by a series of questions from one of our development departments, where they were interested in a more systematic approach to spark emission analysis. SCHEELINE’Sanswer was significant: ‘So far as I know, this is, with one exception, the first time that any instrument company asked the right questions about the spark. To some of these questions, I can give you definitive answers. However, all the work I have done, and before me, all the work that WALTEKS’S group did was highly qualitative in nature. I

Understanding

I

1

I

H

Sample preparation IL

I Work

4

wdb’n”tErlm Gas-jet stabilization

horse

723

spectroscopy

N Ii

Space and time resolution SophisRicated diagnostic techniaues

I

Fig. 9. Status of spark emission spectroscopy: analysis concentrated in the work horse and spectrochemical physics encaged in an ebony tower, with basic methodology development as the missing link in between.

cannot say, “if you do this, the improvement factor will be . . . “. The best I can do is say, “if you do this, here’s what will happen and why.“’ The main conclusions from the discussion may be summarized as follows. There is an enormous gap between the fundamental knowledge available in the literature and the application of this knowledge to practical analysis. t There is no bridge between the results of fundamental spark research and practical spark-AES analysis. There has never been a systematical development of an analytical spark-AES methodology, which uses results and knowledge from the fundamental spark research circuit, on the one hand, and fundamental concepts and approaches inherent in the general AES methodology, on the other hand. In other words: applied spark analysts do not understand the fundamentalists and there is not a group of methodologists in between.

In this respect there is a considerable contrast between spark-AES and ICP-AES: the development of the ICP primarily roots in the work of analytical methodologists and in t This literature on spark emission is adequately covered by two books [204,205], a chapter [206], and the reviews by WALTERS[207,208], BELCHER[209], D~~LAN and BELCHER[210], LAQUA [211], and SCHEELINE[212]. Among the research papers of special interest in the present context, I mention in particular Ref.[213] for information on repetition rate, and Refs [214-2201 dealing with analyses of the temporal and spatial behaviour of the spark. These papers include a study on the exploitation of time gating for increasing signal-to-background ratios [218]. Much of the outcomes of the latter work was presaged in the earlier works of KAISER, LAQUA.BARDOCZ.and others (cJ [205-2121). The most recent work, by MORK and SCHEELINE[220], deals with the use of a charge-coupled device (CCD) for detection of the spark emission in combination with time-resolution techniques. This enabled those authors to observe, for the first time, line shape for emission from single sparks. and to monitor the change of line shape as a function of time and space. The CCD results, while guided by pre-existing theories, have led to alternative scenarios for describing spark emission. The work confirmed some earlier observations and suggests explanations for some phenomena which previously were unobservable or which could not be definitely interpreted.

P. W. J. M. BOUMANS

724

the interaction between methodology development and practical analysis, whereby some input from fundamental research was channeled via methodology development to applied analysis. Therefore researchers have raised, pursued, and answered questions such as: - “For which parameter setting do I obtain the best signal-to-background ratio (SBR)?” - “How do I achieve a low background RSD?” - “How can I improve the detection limit?” - “Which conditions provide for the lowest RSD in the net line signals?” This has never been done, at least not in a systematic way, for the spark. The diagram illustrates this state of affairs with only a small hole in the barrier to let some information pass from right to left, such as know-how about waveform control and gas-jet stabilization. On the whole, however, the unique fundamental knowledge on the spark has been locked up in an ebony tower, populated by scientists such as WALTERS, SCHEELINE,COLEMAN, GOLDSTEIN, and others. The diagram shows at the same time how

an ebony tower looks! Is it perhaps the transformation lighting?

Sample preparation -

e

(Fig. 10) of the windmills we are

w~f%krlm Gas-jet 4 stabilization R

I

Work

horse

Space and time resolution Sophisticated diagnostic techniques

I

Fig. 10. The future of spark emission spectroscopy: the missing link left in position and the ebony tower merged into one of the windmills we are fighting?

I am afraid that the perspectives for the spark are not brilliant, for various reasons. It is clear that there are no “push-button answers” to essential questions raised from the side of analytical methodology. To find such answers, one should first define one or more concrete targets, such as achieving better detection limits, or higher precision, or smaller interelement effects, while also the sample types and the analytes should be specified. Once a target has been defined, one may devise a strategy for achieving this target and compare this strategy and the efforts to be made with the perspectives of other approaches, in particular glow discharge emission, absorption, and mass spectrometry, and, perhaps, laser ablation methods using state-of-the art lasers. I fear that, in spite of the availability of an arsenal of fundamental knowledge on the spark, there are many reasons for believing that nobody will be inclined to embark upon methodology development: - The spark as a transient discharge is temporally complex and in fact requires time resolution, also in analysis, at least trace analysis.

Understanding

spectroscopy

725

-

The spark is spatially complex and, even with gas-jet stabilization, requires spatial resolution, at least selective masking. - The electrode processes are complex in many respects. - Whether the instrument manufacturers are satisfied with it or not, they are not interested in a structural improvement of the situation. - Academic scientists believe the spark to be too old and entirely stripped, and therefore cannot find any motivation or see any challenge in embarking upon spark methodology development. This contrasts with glow discharges, which are shifting ever more into the limelight of the analytical community. 6. BASIC METHODOLOGYDEVELOPMENTAND SPECTROCHEMICAL PHYSICS IN THE GLOW DISCHARGE

DOMAIN

If we look at recent reviews [221-2291 and a recent special issue of Spectrochimica Acta, to glow discharges [230], we may be overwhelmed by the steadily expanding number of configurations of glow discharges with planar or cylindrical cathodes or anodes. In some respects, this development might remind us of the 1960s and 197Os, when the catalogues of carbon companies showed many dozens of shapes of electrodes available for dc carbon arc spectrography or spark solution spectroscopy. Is history repeating itself in today’s developments in the field of glow discharges and lending a good deal of topicality to WILLIAM MEGGERS’ aged statement cited above and repeated here [ 111: ‘Even a superficial glance at this literature gives an impression of a superfluity of “methods”, an excessive multiplicity of light sources and operating conditions, and a purely empirical solution of most problems.’ Part B, dedicated

Although this statement is perhaps partially appropriate in the present context, this is certainly not entirely so. Let me attempt to analyze and assess the situation, in general, in terms of a few block diagrams of the same type as used earlier. I shall inspect a few development lines, look at achievements, current trends, gaps in our knowledge, and possible barriers in communication channels. Discussion will be confined to glow discharges (GD) with planar cathodes and not cover hollow cathode or anode discharges. -

The block diagram for the GD will be built up under four headings: Analysis. Methodology development. Innovations. (Spectrochemical) Physics.

The initial focus will be on “Analysis” and “Methodology development” only (Fig. ll), both centred about the Grimm glow discharge [231,232] for AES (see, e.g., [211,221223,233-240] and the literature quoted therein). t This method can be considered as an established technique and is perhaps more widely spread in Europe than in the USA. As the diagram indicates, applications and methodology development have been closely connected. t

GRIMM has been the inventor of the particular configuration of obstructed glow discharge, from then onwards referred to as “Grimm GD” or “Grimm-type GD”. As is evident from the literature, it has been above all KURT LAQUAwho should be considered as the promotor of and engine behind the “Grimm GD”. The popularization of this GD has also been greatly stimulated by the fact that the company RSV in Hechendorf, F.R.G., brought out a commercial version soon after GRIMM had made the invention. I remember that in the early years we made each autumn a “pilgrimage” to Hechendorf, a village south of Munich, to participate in a users’ meeting in “Hotel zur Post”. where we spiced our GD discussions with Bavarian country dishes, beer. and “Enzian”. It was on those occasions that I had the pleasure to become acquainted with GRIMM as a most modest and congenial man.

P. W. J. M. BOUMANS

726

Analysis Spark

Grimm

XRFS

GD-AES

4

4

r

I

Parametric studies

Metals

& alloys:

bulk

Metals

& alloys:

surface

analysis

/

depth

Nonconducting ( pellets

with

profiling materials

Cu.

Ag,

C )

1

I

l

V-i characteristics

0

Sputtering

rates =

f ( p, V,i1

I

0

Line intensities f

=

( p,V,i 1 I

Fig. 11. Evolution of the GD knowledge circuit: start and development planar diode type GDs.

of the Grimm-type

and

The Grimm GD is used for bulk analysis of metals and alloys (e.g., [211,222-223, 233-238]), and ever more frequently for surface analysis and depth profiling (e.g., [222,223,233-235241-2531). Although not yet widely used, it has been shown to be also a viable method for major and minor constituent determinations in nonconducting materials, pelleted with copper, silver, or carbon powder as a conducting binder (e.g., [222,223,233,239,254,255]). GD-AES has not yet replaced the spark for routine metal analysis, probably because the vacuum requirement makes sample changing less convenient and because a fairly long pre-glowing time is usually required before analytical data acquisition can start. Both spark and GD-AES are competitive with X-ray fluorescence spectrometry (XRFS) because the optical methods give easy access to light elements, such as boron and carbon, whereas XRFS does not, or with difficulty. Applications development for GD-AES implied in fact practical methodology development in many respects, but this development has also seen separate parametric studies concerning the dependence of sputtering rates and line intensities on voltage (v), current (i), pressure (p), and sample composition (e.g., [211,222,223,236,240]) as well as fundamental studies of the sputtering process and diffusion in the gaseous [256-2601 or condensed state [261,262]. GD research has not been confined to the Grimm-type GD, but also covered planar diode-type GDs, in particular in the studies of HARRISON and MARCUS and their co-workers (e.g., [226-228,263--2661) while various types of boosted GDs have been explored in the Division of Chemical Physics, CSIRO, Clayton, Australia (e.g., [267-2701). Several parametric studies have led to the formulation of a useful relationship between the sputtering rates of pure metals and the GD parameters, in particular the proportionality of the sputtering rate Q (= mass sputtered in unit time) and the reduced or excess power [240,266]: Q = C(V -

V,,)i,

where C is a proportionality coefficient, V the operating voltage, V. a threshold voltage, and i the current. Coefticient C and threshold voltage V. depend on the GD configuration and are specific for the metal. The threshold voltage has been reported to be independent of the pressure in a Grimm GD [240] but to depend

Understanding

727

spectroscopy

on pressure in a planar diode GD [266]. Although the equation for the sputtering rate looks simple, the actual situtation is rather complex, because we always have three variables: pressure, voltage, and current, two of which can be independently chosen, while the value of the third one is dictated by a set of voltage-current characteristics of the type shown in Refs [240,266].They may be linearized, at least for the planar diode configuration, by plotting voltage against the square of the current [266]. On the whole, one may conclude that the sputtering rate of pure metals is parametrically weI describable and thus experimentally controllable. This, in turn, may facilitate parametric descriptions of the emission in that it gives a handle to separate the effects of metal vapour production and metal vapour excitation and ionization. One may expect that sputtering rates of alloys depend linearly on the sputtering rates of the components and their concentrations. This has been explicitly demonstrated for homogeneous, binary alloys, such as brass [240,265]. This is physically understandable, because sputtering rate is proportional to the sputtered area. If copper is first preferentially sputtered, the surface will show the beginning of “zinc mountains” and thus assume a larger area, which, in turn, will lead to an increased ablation of zinc. Macroscopically, the two components will be apparently sputtered steadily. With complex alloys, however, complications may arise: both the metallurgical history or the presence of inhomogeneities may affect the sputtering rate and thus jeopardize its simple linear dependence on the sputtering rates of the components [233,238,244,248]. The sputtering rate of oxide, carbide, or nitride layers is considerably smaller than would’be predicted from the sputtering rates of metals. The sputtering rate of such layers depends not only on the chemical composition but also on the microstructure, density, and porosity of the layers. This greatly complicates the quantification of depth protile data [241-253,261,262].

7

E[

Innovations

Grimm

k

GD-AES

Applications _ development

Planar

k

Parametric studies

diode

GD with

magnetic

Boosted

GDs:

A

DC boosted

A

RF boosted

I

A

Microwave

GD

Jet-enhanced

field

boosted GD

RF GD

Fig. 12. Evolution of the GD knowledge circuit: the innovations

balloon pulled down.

We now extend the block diagram with “Irmovations” and “(Spectrochemical) physics” and pull down the “innovations balloon” (Fig. 12). It shows developments such as - GD with magnetic field (e.g., [271-275]), - various types of boosted GDs (e.g., [267-270,276280]), - the jet-enhanced GD [e.g., 268,270,281-2931, - the radiofrequency (RF) GD [294-2971. Adding a homogeneous or inhomogeneous magnetic field to a discharge has always challenged scientists, also in arc spectroscopy (see [298] and the references cited therein). The rationale here is not to affect the spectra and to exploit the Zeeman effect, but to improve the discharge stability and to influence the transport of ions, thus increasing the residence time of analytes in the excitation region. However, in spite of many attempts, these studies have hitherto not yet enchained real revolutions in the field of analytical methodology.

728

P. W. J. M. BOUMANS

A boosted GD is a hybrid of a DC GD and a secondary discharge. The purpose is to enhance signal-to-background ratios and to separate sample ablation in the glow discharge from the excitation in the secondary discharge. Boosted GDs are being studied by several groups of authors and may still offer some interesting perspectives. The jet-enhanced GD is a development of earlier ideas originally explored in WALSH’S group in Australia [267,270,28 11. The prime rationale is to increase the sample ablation rate in a modified Grimm GD by argon jets and to transport the sputtered atoms to an optical path where atomic absorption can be efficiently measured. This development has led to a commercial version, called “Atomsource” and an AAS spectrometer using this device for direct analysis of metals and alloys [282-2841. Various groups of workers have embarked upon studies of several versions of jet-enhanced GDs for emission or absorption spectrometry [285-2931. It is still too early, however, to give a definitive assessment of this interesting approach. Also, we must keep in mind that the mechanism of the jetenhancement has still to be clarified, including the role of local pressure gradients (cJ [287,289,290]). The spatial separation of the sputtering and the AAS measurement appears beneficial. Merely observing the emission from the glow of a jet-enhanced GD does not seem to bring advantages, because the effect of enhanced ablation is balanced by the effect of the enhanced transport rate. However, observing the emission from a secondary discharge to which the sputtered atoms are transported, may be an interesting point of research. Further work might also re-open the “hostilities” between absorption and emission protagonists [299], whereby old questions will turn up again: simultaneous against rapid sequential measurement, spectral interferences, dynamic range, and, eventually, precision. In this context, one will also come again across the question as to whether any optical method with its inherently flicker noise dominated atomization will be ever able to match XRFS in precision. Noise power spectra might provide a key to answering this question [96]. Direct comparisons between the performance of XRFS and GD-AES are scarce [300]. The last innovation mentioned in the balloon is the RF GD, which probably is the most crucial of them all in that the relevant research may lead to the development of an optical method for direct analysis of nonconducting solids without dissolution or powdering and pelleting. RF sputtering has been since long extensively applied in IC technology for the production of thin films and semiconductor materials (“plasma sputtering”, “plasma etching”). There is a reservoir of knowledge and know-how available in this field. The introduction of the article of DUCKWORTH and MARCUS [294] on an RF powered GD atomization/ionization source for solids mass spectrometry may serve as a very useful key to the relevant reservoir of literature, which also comprises excellent monographs, e.g., [301,302]. This discussion of GDs has now reached the point where we should look at the further evolution of the block diagram. In Fig. 13, the “innovations balloon” has been contracted to a block to make space for a new block under the heading “(Spectrochemical) Physics”, where the attribute “spectrochemical” has been put purposely in parentheses, because I do not believe that in the field of low-pressure discharges we have already something that may be called “Spectrochemical physics”. I fear that, in this area, spectrochemistry still lives in a vacuum. In spite of some excellent knowledge based on parametric studies, on fundamental investigations of sputtering, or work concerned with excitation mechanisms, *) the available knowledge circuits lie for a substantial part in the physics or physical chemistry domains, viz. - plasma physics, - gas discharge physics, - plasma etching, *) See the literature discussed above, as well as the publications in the recently published special CDL issue [230], in particular [253,280,290,292,297,303-3071 and the references quoted therein.

729

Understanding spectroscopy

r-

( Spectrochemical

I

Analysis

Physics

0 Plasma 0 Gas

1 I

physics

discharge

physics

Applications development

Planar

diode

0

Plasma

etching

0

Plasma chemistry

0

IC technology

GD

Fig. 13. Evolution of the GD knowledge circuit: physics emphasized as the prime source for stimulating innovations and coupling new fundamental knowledge into the spectrochemical field.

- plasma chemistry, - IC technology, rather than in the domain of analytical chemistry. Of course, we know that GDs emit rather simple spectra, that sputtered materials contribute mainly neutral atom lines to these spectra, that the Doppler temperature is low (say, 500 K) so that the lines have small physical widths, that self-absorption is relatively small (but see [290]), and that the sample composition has only a small effect on the excitation and emission characteristics. This last point is the crux, however. There are sound reasons, indeed, for approaching the unravelling of the excitation and emission characteristics of low pressure discharges by the study of separate mechanisms and the measurement of electron number densities and electron energy distributions using diagnostic approaches such as Langmuir probes [308-3101. The problem with low pressure discharges is that the low pressure despoils us of vital armour: the weapon of local thermal equilibrium (LTE). In spectrochemistry, we are accustomed to dealing with LTE plasmas, and even with departures from LTE, to the extent that, for example, gas temperature, electron temperature, and excitation temperatures do not have the same numerical values. But even with such departures, the Boltzmann and Saha equations remain somehow in the picture. This is attractive for chemists: we are born equilibrists! Unfortunately, this handhold gets lost in low-pressure discharges. One of the main features of GDs [311) is that ionization is not balanced by three-body recombination. The electron densities are too small. The loss of electrons and ions is mainly due to ambipolar diffusion to the wall and recombination at the wall. This causes large deviations from LTE and makes it necessary to model low-pressure discharges by accounting for all relevant kinetic processes. Also, radiation losses can be substantial, which can lead to selective depopulation of excited levels and enhance the deviations from equilibrium. For glow discharges, there is usually another dominant recombination process: molecular ion formation followed by dissociative recombination [311]. In summary, the populations of excited levels of atoms and ions in a GD can only be derived by solving a set of rate equations for all relevant processes. In the 197Os, the modelling of GDs has been a hot topic in the group “gas discharges” in Philips Research Laboratories. This group does no more exist. Their publications, in

P. W. J. M. BOUMANS

730

I

Grimm GD-AES

I

I

10

I

0 Plasma physics

Boosted

0

0 Gas discharge

Jet-enhanced

physics ’ 0 Plasma etching

Applications development

*

0 Plasma chemistry

I

Parametric studies

0

IC technology

Fig. 14. Evolution of the GD knowledge circuit: the unlikeliness of a comfortable delivery of the well crated fundamental knowledge at the spectrochemists’ homes.

0 I

Grimm

_

Cm-AFS

l (Gas discharge

l Jet-enhanced -z

1 Applications deveh wment

H

ar

0 Plasma physics

Boosted

1

physics

Parametric studies

ho I

Planar

diode

GD

u-c

Fig. 15. Evolution of the GD knowledge circuit: the role of pioneers in extracting new fundamental knowledge from the domain of physics into the spectrochemical environment.

those of VRIENS and his co-authors ( e.g., [3 1l-313]), may give an idea about the applied approaches using two- or three-electron-group models. These papers may provide further access to a vast domain of literature. Returning to the block diagram (Fig. 14), we observe a few icons, which should not be misunderstood: “physics” does not drop a comfortable theory into the chimney of our homes nor deliver it neatly at the front door. Even if something will be delivered, it will be stiffly packed and firmly crated. I am afraid that actually there is a large barrier (Fig. particular

731

Understandingspectroscopy

15): we need pioneers again to drill holes in the wall of the physics domain and to catch a few puffs of relevant knowledge or to decant a few droplets of that knowledge into our chemical environment. 7. CONCLUSION Understanding spectroscopy with a view to rationalizing spectrochemical analysis: abysmal adventure or a realistic ideal?

an

The answer lies in the way in which we view the various spheres and the communication flows through the connecting channels (Fig. 16), and above all in the way in which we define the goals. We need not understand spectroscopy for spectroscopy’s sake. We need understand it to such an extent that we can develop the analytical methodology by extracting relevant knowledge from physics for creating innovations and building up spectrochemical physics as a rational backing of parametric studies in methodology and instrument development, and, finally, by recasting the knowledge of spectrochemical physics and methodology development in such a form that it can be understood, absorbed, and exploited by the recipients in the sphere of applications development and those who attempt to translate analysts’ behaviour into artificial intelligence. An adventure? Yes! And even realistic! An ideal? Also, but not abysmal!

RATIONALIZING SPECTROCHEMICAL ANALYSIS

III

UNDERSTANDING SPECTROSCOPY

Fig. 16. Spheres and channels symbolizing the rationalization of spectrochemical analysis as a result of (1) an appropriately balanced understanding of spectroscopy in the respective spheres and (2) effective, interactive communication between the various spheres.

RFERENCES [l] G.M. Hieftje, “Toward improved understanding and control in analytical atomic spectrometry”, in: “Future Trends in Spectroscopy”. pp. 113-128. Proceedings of the Symposium held at the Pontifical Academy of Sciences, The Vatican, 27-28 June 1989, to mark the 50th Anniversary of Spectrochimica Acta. 1939-1989. Special Supplement to the 1989 volumes of Spectrochimica Acta. SAW

46:6/7-E

732

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Boumans, Preface of “Slavin Issue”, Specrrochim. Acta 44B, No. 12, p. 1205 (1989). with fhe Carbon Arc Cathode Layer (“Glimmschicht”) Adam Hilger,

[3] L.W. Stro~k, Spectrum An&is London (1936). [4] R. Mannkopff

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[ll]

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Press, New

Societies, 1st

Anal. Chem. 46, 698A (1974). Reprinted in Spectrochim. Acta SSB, 643 (1980). Boumans,

Philips Tech. Rev. 34, 305 (1974).

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Anal. Chem. 46, lllOA,

1155A (1974).

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[18] V.A. Fassel,

Pure Appl. Chem. 49, 1533 (1977).

[19] V.A. Fassel,

Science 282, 183 (1978).

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[22] S. Greenfield,

H.McD. McGeachin

[23] S. Greentield,

Analyst

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[24] P.W.J.M. Boumans, ed., Inductively Coupled Plasma Emission Spectroscopy, Part 1, Methodology, Instrumentation, and Performance. Wiley, New York (1987). [25] P.W.J.M. Boumans, ed., Inductively Coupled Plasma Emission Spectroscopy, Part 2, Applications and Fundamentals. Wiley, New York (1987). [26] A. Montaqr and D.W. Golightly, eds, Inductively Coupled Plasmas in Analytical Atomic Spectrometry. VCH Publishers, Weinheim/New York (1987). [27] P.W.J.M.

Boumans and F.J. de Boer, Spectrochim. Actu 27B, 391 (1972).

[28] P.W.J.M.

Boumans and F.J. de Boer, Spectrochim. Actu 3OB, 309 (1975).

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Spectrochim. Acta 44B. 359 (1989).

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Understanding [4l]

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A.R. Date and A.L. Gray, eds, Applications of Inductively Coupled Plasmu Mass Spectrometry. Blackie, Glasgow/London (1989).

[42] M.A.Vaughan

and G. Horlick,

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