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Remembering the meetings with Paul Boumans
Spectrochimica Acta Part B 64 (2009) 305–308
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Remembering the meetings with Paul Boumans It was in 1967 when I first met with Paul, at the occasion of the XIVth Colloquium Spectroscopicum Internationale, Debrecen, Hungary (August 7–12). It is interesting to recall this event on the occasion that after 42 years the hosting country of the CSI will be again Hungary (August 30–September 3, 2009, Budapest). The first meeting with Paul was one-sided in hearing his plenary lecture and also in trying to follow the discussions he often initiated with several lecturers. Paul, who was 35 years old at that time, was in the communication center of the atomic spectroscopy section. The atmosphere of such an international scientific meeting was a new experience for me, although I was older than Paul by two years (as I learned later). Regarding my presentation (from the Technical University of Budapest), I had a discussion paper on emission spectrographic analysis of industrial alumina using home-made devices to approach to continuous sample introduction into spark discharge (based on spattering of powders) and into arc discharge (based on complete evaporation). Paul, somehow, got interested in my paper, and in the break time he introduced himself and put questions about the devices used (he probably did not want to try my English in an open discussion). He mentioned his planned travel from Debrecen to Budapest and also his stay in the capital for a short time which might be used partly for a short visit in our laboratory to see these devices in operation. That was beyond my best imagination. Paul's lecture material in Debrecen was composed primarily from selected parts of his book, published one year before the CSI meeting (Theory of Spectrochemical Excitation, 1966), which was new and exciting information for most of the participants. It took 40 pages in the CSI Proceedings published in three volumes of in total 1580 pages. This material was important to learn for everybody working with emission spectroscopy in general and in particular for those working with d.c. graphite-arc excitation of powder samples. And, at that time this method was applied in the majority of laboratories world-wide. When reading the conclusions of Paul's comprehensive CSI article, it became clear why he was interested to see our devices. He wrote: “To perform physical measurements in a stable source is not difficult… to produce the stable conditions is more difficult… it is an art, however. Much work remains to be done on the parameters associated with volatilization phenomena, thermochemical reactions in solid and liquid phases.” Paul was not “only” a theoretical spectrochemist, he wished to utilize the theoretical knowledge to improve the spectroscopic source for attaining better analytical performance. From his studies on the main plasma parameters of the arc in local thermal equilibrium (LTE) it became evident that “stable conditions” of the plasma parameters can be attained by a near continuous introduction of sample matrix and/or an ionization buffer. In addition, from the general experience (overviewed and conceptually evaluated in his book) he concluded doi:10.1016/j.sab.2009.03.003
that complete evaporation of sample constituents is mandatory to minimize matrix interference effects from the volatilization phenomenon point of view. How to attain a complete and continuous (non-selective) vaporization with a thermal source, such as the graphite-arc? In Paul's laboratory a workable approach into this direction was under development as described also in his CSI article. The essence was the use of a deep hole in a small diameter graphite rod (as anode) for filling in the sample + graphite powder mixture, which is arced in air (or Ar + O2) atmosphere. Under these conditions, the vaporization is selective only in the initial arcing period during which a steady state vaporization zone is developing in the filling. The upper part of the electrode (close to anode spot) is hot enough to vaporize the nonvolatile components (e.g. Ti, Zr), while from the lower part of the electrode the more volatile components (e.g. Cd, Zn) are vaporized, simultaneously. The vaporization zone is progressing downwards together with the consumption of the electrode tip. So, the resultant vaporization is apparently non-selective which can be held up until the vaporization zone reaches the bottom of the electrode hole. This approach is more successful if the lower part of the electrode (and its filling) is intensively cooled, and this was attained with our “controlled total evaporation” (CTE) device. The electrode is placed in a motor driven spindle (with adjustable forwarding) and the tip protrudes through the central drill of a water-cooled copper “finger”, the extension of the electrode tip is kept constant (4 mm) above the upper end of CTE-device. The movement (readjustment) of the upper graphite cathode could be done by another CTE-device for keeping a constant arc-gap. During the visit of Paul, the “show” was successful with the CTE-arc, and indeed its spatial and temporal stability, especially with the use of a coloring alkali buffer was quite impressive. The simultaneous covaporization of alkali buffers with less volatile sample matrices is analytically an important possibility, which can also be combined with halogenation of nonvolatile carbides (e.g. with the use of LiF buffer). When discussing about future publications, Paul called my attention to earlier work of Dutch authors who described the use of a cooled “copper collar” around the graphite supporting electrode for the same purpose as aimed at by the CTE-device. The use of CTE-device in our following works and publications (with Zakaria Hanna from Egypt) was also connected with Paul's interest and help. It was shown that the plasma thermometric element, Zn, which is relatively volatile, could be simultaneously vaporized with any kind of matrices that was important for a more accurate evaluation of “plasma” matrix effects. Perhaps it is not superfluous to recall the main and somewhat shocking “lesson” from Paul's book about d.c. graphite-arc characteristics regarding matrix effect. In thermal sources, the simultaneous increase of electron pressure with gas temperature is generally expected. However, in an arc discharge, by introducing a substance with more ionizable
306
Remembering the meetings with Paul Boumans
components than the basic discharge components, the electron pressure is increased with a simultaneous decrease of temperature. The latter change is a direct consequence of the decrease of energy dissipation in the arc column for an electrically more conductive plasma (of decreased electric field strength). The main interest of our research group at the Technical University of Budapest (lead and advised by Prof. László Erdey) became, however, the study of the shape of the analytical curves, when applying the CTEdevice (continuous total evaporation) in comparison with the use of continuous spattering of the same powder standards (alumina) into a spark discharge in which a partial evaporation of sample particles took place. In view of the analytical relationship after Lomakin–Scheibe, the relation between emission intensity (I) and the concentration (c) can be expressed by a power function, the exponent (b) of concentration taking the value between 0.8 and 1.0, most generally. When using the CTE-arc excitation the exponent (b) was found consistently equal to unity (proportional function), while with the use of spattering-spark excitation technique b b 1 was found for some of the components using the same analytical lines as with the CTE-arc. The statement from these observations was that the deviation of the (b) exponent from unity must be related to the vaporization pattern of the chemical system, provided that self-absorption is negligible. In a subsequent work the self-absorption could be expressed by multiplication with an exponential term, corresponding to the validity of Beer's law. The manuscript on the above results was submitted to the journal Spectrochimica Acta Part B (SAB) arriving to the Editor, Paul Boumans in the end of 1973 (he became Editor-in-Chief in 1979). The manuscript was sent from the University of Houston (Texas), where I spent a one-year research fellowship, and made a quite ridiculous mistake, not indicating the Department of my laboratory. But, Paul did look after my full postal address in that quite large University. He offered several instructive and helpful suggestions for the revision that became normatives also to my further publication works. The main research interest of Paul underwent a fast change toward inductively coupled plasma atomic emission spectrometry (ICP-AES) after the CSI-1967. Even in his lecture material of this meeting he disputed the features of the ICP source (based on literature information) regarding the LTE concept and wrote: “this type of source has many properties in common with the d.c. arc, and the conditions of the plasma most probably are not far from LTE.” The introduction of the ICP source to atomic spectrometry by the Greenfield' group (1964–65) and the Fassel' group (1965) was quite close to the date of CSI-1967. Nevertheless, four lectures were dealing with the analytical features and applications of (non-commercial) ICP sources. Flame atomic absorption spectrometry (FAAS) was represented at the CSI-1967 with a relatively few (4) lectures, although this method was introduced well before this meeting by Walsh (1955) and independently by Alkemade (1955). The introduction of the acetylene-nitrous oxide flame by Willis (1965, 1966) was, however, quite novel yet. The invention of graphite furnace electrothermal atomization atomic absorption spectrometry (GF-ETAAS) by L'vov (1959), and its publication in Spectrochim. Acta (1961) had no impact apparently on the program of CSI-1967. It was the introduction of the simplified design of the “tube-type” GF-ETA unit by Massmann (1967) that was followed by commercialization in a short time (Perkin-Elmer, 1970). The start of manufacturing the “rod-type” of GF-ETA unit was dated also about to this time (Varian Techtron, Australia). Altogether, these AAS techniques represented the other revolutionary progressing direction within atomic spectrometry, and I myself became involved in these methods, contrasting to the other new direction followed by Paul (ICP-AES). I recall the above sentence of Paul as the concluding remark to d.c. graphite-arc work, saying: “much work remains to be done on the parameters associated with volatilization phenomena, thermochemical reactions in solid and liquid phases.” This kind of work had been
done in part and resulted in valuable comprehensive knowledge to graphite-arc (Schroll in CSI-1967), but the real advent of the research on high temperature condensed phase processes was started with the development of the GF-ETAAS method. It might also be an interesting note that the many efforts made to increase the residence time of sample vapor in the d.c. arc (e.g. by using a magnet around the supporting electrode) were not sufficiently auspicious, while this could be effectively realized in the GF-ETAAS technique. In the next ten years (1967–1977), seven papers were published by Paul on d.c. graphite-arc spectrometry co-authored mostly by his close friend, Frans Maessen, exploring the advantages of gas-stabilization and use of fusion techniques, but also the routes of the analyte loss, as the negative feature. The series of papers was ending with “rigorous statistical approach for establishing accuracy” of d.c. arc procedures. In parallel with these d.c. arc works (of apparently declining interest of continuation) eight papers were published on the different features of ICP-AES, the most frequent co-author being F.J. de Boer. In addition to this research activity and the very active editorial work for SAB, Paul was able to write a comprehensive book chapter on “Excitation of Spectra” (E.L. Grove editor), published in 1972. In my view that was a really heroic performance. This book chapter is starting with the fundamentals of high temperature gas phase and condensed phase processes and continued with the application of this knowledge and concept to the most generally used spectroscopic sources (as below). The sources are described with details of physical principles including electric circuits, and this material is still a unique compilation of emission spectrometry techniques, as the whole. With regards to spark excitation of metals and alloys newer textbooks are barely available up to now, although this analytical technique has an invariably important position in industrial laboratories. The chapter serves also as a comprehensive “source book” of emission spectroscopy, the references are listed in alphabetical order for the topics as follows: Books and Tables (1–64), General (65–158), D.C. Arc (159– 430), A.C. and Intermittent Arc (431–468), Spark and Condensed Arc (468–613), Spark at Reduced Pressure (614–628), Plasma Jet (629–679), Radio-frequency Torch (680–729), HF Discharge at Reduced Pressure (730–753), Hollow Cathode Discharge (754–801), Laser (802–873), Flame 874–892), and additional latest references (893–940). Paul was invited to Hungary in May 1974 as a guest of the Hungarian Academy of Sciences, and a seminar was organized for his honor with presentations by Prof. Tibor Török and Prof. Károly Zimmer in Pécs (the largest South-West city of Hungary). By good luck his visit was just after my return back from Houston (Texas), and after travelling back together from the seminar to Budapest, we, together with his friend from Philips, made a “curvilinear” evening in the Castle of Buda. Paul liked the Hungarian meals and wines and could enjoy the “free atmosphere” being out of the pressure of his many duties. He presented me with a specimen of the above book chapter with the dedication: “To… with the kindest regards of the author, who has an excellent souvenir of his recent visit to Hungary, May 27, 1974”. During the decade in consideration (1967–1977) the elemental analysis of liquid materials became of paramount importance, compared to the formerly dominating analysis of metals and powders. This new requirement was manifested in the large variety of waters and other environmentally important liquid samples. In the analysis of foods and other biological materials chemical destruction and dissolution of relatively large sample masses are also required to ensure reliable sampling accuracy. Until the mid-50s, the flame atomic emission spectrometry (FAES) represented the fastest elemental analytical method for the direct analysis of aqueous solutions, due to the easy-to-use pneumatic nebulization and application of photoelectric detection. The problem was the very limited number of elements that could be determined. For extending the number of analytes, an effective version of “spray-in-spark” method was developed (with the use of a spectrograph) in the Technical University
Remembering the meetings with Paul Boumans
of Budapest, published in 1955 by L. Erdey, E. Gegus and E. Kocsis. The sample spray was introduced through the lower, tubular graphite electrode into the spark gap, the spray droplets collided and deposited in part on the surface of the upper semi-spherical graphite electrode. The high temperature vaporization of the sample components took place dominantly from the surface of the upper electrode, where the layer of the deposited solute was continuously renewed. The same electrode system was also used for the spark excitation of volatile hydrides of As, Sb and Ge, liberated from solutions by nascent hydrogen (Zn + HCl reaction). This method represented the first introduced hydride generation atomic emission spectrometry (HG-AES) method (published in German in a domestically printed periodical) that has not been recorded in the international literature. As known, the HG-AES/AAS methods have made an extremely important career in spectrochemistry. Nevertheless, the outlined solution spectrographic method together with the other known methods in this category could not compete in simplicity and speed with the flame methods. In addition, using acetylene-nitrous oxide flame, the number of determinable elements could be doubled (increased to about 70) relative to that accessible with acetylene-air flame, resulting in a great progress of flame atomic absorption spectrometry (FAAS). Also, to the end of 70s the GF-ETAAS technique underwent a tremendous improvement in temperature programming (stabilized temperature platform furnace atomization) and automation of sampling (solutions and slurries),as well, in providing two orders of magnitude higher detection power than available with the flame (and also with ICP-AES) methods. According to the favored expression of Walter Slavin the GF-AAS was approaching to be a “technology” (rather then to be an “art”, as the arc source was characterized by Paul). So, from the mid-70s, three major elemental analytical challengers were competing in the field of solution analysis; the FAAS, GF-ETAAS and the ICP-AES, all applying photoelectric detection to monochromator type optical systems. Paul, who was playing one of the roles of ICP-AES “apostates” made several experimental studies and evaluations to demonstrate the viability of ICP-AES in this competition. This latter method required a more expensive monochromator of higher resolution power for a similar selectivity as was attainable with the FAAS. However, the ICP-AES had inherently the possibility of multielement analysis, provided that more sophisticated (and more expensive) optical and detection systems are used, which were, however, not available at that time on the level of requirements. On this problematic situation 10 papers were published between 1977 and 1980 by Paul and coworkers, the final paper in 1980 had the title: Is ICP a d.c. arc in a new jacket? Paul explained this title by the intention to express the common problem of the arc and ICP emission methods with respect of “the complexity of the extraction of a true analytical signal from the composite radiation observed in the spectral window of an analyte emission line”. The “arc in new jacket” paradigm could be applied logically also to the subsequent papers (1983–1984), published in cooperation with plasma physicists on the excitation mechanism of ICP that was found “close enough to the LTE concept”. The relatively low matrix (nonspectral) interference effect observed for the ICP source is the consequence of the specific mode of energy dissipation that is dominating in the plasma annulus instead of the plasma axis where to the sample is introduced. The logic might also be extended further in that, due to the discrepancies from the ideal ICP structure, the axial sample channel is not perfectly free from electric field (and energy dissipation) with the consequence of “plasma loading effect” by sample matrix (increase of electron concentration and decrease of temperature) that is relatively small compared with arc situation. After 1974 (Pécs, Hungary) the next occasion to meet Paul was in May 1977, at the International Symposium on Microchemical Techniques (Davos, Switzerland). My “profile” changed in the direction of using FAAS detection combined with newly developed
307
sample introduction methods (as below). Still, I used graphite-arc spectrography applying halocarbon vapors for sample halogenation. In the get-together party answering Paul's question about my work, I outlined the new halogenation method (we communicated mostly in writing), he told me with a smile: “forget arc excitation”. I understood the reason for his advice, however, at that time no other high temperature “plasma” source was at my disposal, and this condition lasted in the further 10 years for me. The new sample introduction methods were based on the production of solid aerosol by graphitearc vaporization (from 1973) and by laser ablation (LA, from 1975), the aerosol was introduced into the flame source (FAES/FAAS). In Davos, the LA-Flame method was the main topic of my presentation and Paul gave me an invitation-card for publication in SAB, which could be realized in 1979. We met again in the same year, in July in Prague at the XXth CSI-1977, and by my good luck also in the next XXIst CSI-1979, Cambridge, UK. In the latter meeting my presentation was about graphite furnace electrothermal vaporization (GF-ETV) as sample introduction to flames using also halocarbon vapor as halogenation reagent, which was again invited by Paul for publication in SAB. Prof. Gordon F. Kirkbright (Chairman of CSI-1979, Imperial College, London) introduced this halogenation method independently in GF-ETV-ICP-AES. The year 1979 became also memorable by another meeting with Paul in Poland (Zaborow, September, 24–27) on the School of Modern Excitation Sources in AES, organized by Jerzy Fijalkowski with the assistance of Alicja Jedlewska under the auspices of the Polish Academy of Sciences. The days were started with tutorial lectures followed by consultative discussions in the afternoons and closed in late evenings with singing and dancing in a Polish manner. Everyone likes to recall that days, Paul asked me later about the pianist Peter Zentay, and also about the other Hungarian participants, Ernö Gegus and Jozsef Posta, he remembered well. The greatest event amongst my meetings with Paul was the unequaled occasion of the XXIIIrd CSI1983 in Amsterdam (June 26–July 1). I received an invitation to hold a keynote lecture and he exactly tailored my contribution by suggesting the title: “New approaches to the separation of vaporization and atomization–excitation in atomic spectrometry”. The manuscript had to be prepared in “camera-ready” form for publication in advance of the CSI meeting (at that time we had no computer), Paul expressed his satisfaction by telling me that “only one paragraph had to be retyped”. It was amazing, how he was able to take care about such small things as to organize the accommodation of four Hungarians in his neighbors' house at Bergeyk (Karola and Wil Maas family). We arrived three days before the conference by driving a car through several Western countries, which was also a new experience. Thus, we could participate in the reception given by the Boumans Family at their house, which was started with a presentation of classical music by Paul's daughters, Christine (flute) and Lidwien (piano). Paul's wife Dorine and son Baptist were engaged with greeting and guiding the many guests. This personal welcome party was on Saturday and was followed on Monday by the official reception at the Rijksmuseum of Amsterdam, and in the same evening by the invitation of Leo de Galan and Lida Schoen to their classical Dutch house. After the highly informative and exciting working days of the CSI meeting we were again spoiled by the unique conference party (with “dancing section”) in the Amsterdam Hilton, thus placing the “cream on the pie”. The coming years for Paul were engaged mostly with high resolution studies of ICP spectra and spectral interference problems (papers co-authored dominantly by J.J.A.M. Vrakking), and simultaneously with editing (and contributing seven chapters) of a two volumes book, Inductively Coupled Plasma Emission Spectroscopy, published in 1987. He presented me with the offprint of the book, the major present being in mid-1987 his proposal to be elected for the membership of the Editorial Advisory Board of SAB. I understood his efforts in “reorganization” of the reviewing process and establishment of “hot” communication lines between authors and editors. This
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Remembering the meetings with Paul Boumans
membership of SAB made it possible to receive the invitation to the celebration of the “50th Anniversary of Spectrochimica Acta 1939– 1989” held in Rome, Pontificial Academy of Sciences, The Vatican, 27– 28 June, 1989. From the several historical and philosophical presentations published in Spectrochim. Acta (1989) Special Supplement, Future Trends in Spectroscopy, let me cite Paul's introductory “thesis” “The aims of scientific publications are: (a) disseminating information to promote scientific and/or technological advancement, (b) establishing the Authors' prestige and reputation. I believe that the latter aim is equally important as the former.” This thesis (the two aims together) could be understood as the basis of a healthy operating society, although its declaration and realization are by far not obvious in several scientific and political administrations. In a subsequent letter Paul expressed his disappointments about the winning position of “inflated big talkers who are not able to do anything useful”… in the conflicts… with those “who prefer and are able to do the work”.
In the “Farewell Issue” of Paul Boumans (SAB-1995), prominent scientists presented highly informative addresses: prefaces by Walter Slavin and Nicoló Omenetto, award greetings by José Broekaert and tributes by Kurt Laqua and James Winefordner, which are right honorable indeed to an outstanding scientist and great man. My personal stories are ending with mentioning my visit to the Boumans' house at Bergeijk, March 28, 2004. It became possible on the way back home, driving from Delft to Budapest, after my one-month work in the laboratory of Margaretha de Loos-Vollebregt (Delft University of Technology, Delft, The Netherlands). Paul copied a map and marked the best route for me and sent it by fax. Dorine invited me for lunch, so I arrived around noon and could touch some topics of our works and lives, including the appearance of Paul's artistic book: “Family Portrait Europe 1400–1800”. Tibor Kántor Geological Institute of Hungary, Stefania road 14, 1143 Budapest, Hungary E-mail address: [email protected].
Contents lists available at ScienceDirect
Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Remembering the meetings with Paul Boumans It was in 1967 when I first met with Paul, at the occasion of the XIVth Colloquium Spectroscopicum Internationale, Debrecen, Hungary (August 7–12). It is interesting to recall this event on the occasion that after 42 years the hosting country of the CSI will be again Hungary (August 30–September 3, 2009, Budapest). The first meeting with Paul was one-sided in hearing his plenary lecture and also in trying to follow the discussions he often initiated with several lecturers. Paul, who was 35 years old at that time, was in the communication center of the atomic spectroscopy section. The atmosphere of such an international scientific meeting was a new experience for me, although I was older than Paul by two years (as I learned later). Regarding my presentation (from the Technical University of Budapest), I had a discussion paper on emission spectrographic analysis of industrial alumina using home-made devices to approach to continuous sample introduction into spark discharge (based on spattering of powders) and into arc discharge (based on complete evaporation). Paul, somehow, got interested in my paper, and in the break time he introduced himself and put questions about the devices used (he probably did not want to try my English in an open discussion). He mentioned his planned travel from Debrecen to Budapest and also his stay in the capital for a short time which might be used partly for a short visit in our laboratory to see these devices in operation. That was beyond my best imagination. Paul's lecture material in Debrecen was composed primarily from selected parts of his book, published one year before the CSI meeting (Theory of Spectrochemical Excitation, 1966), which was new and exciting information for most of the participants. It took 40 pages in the CSI Proceedings published in three volumes of in total 1580 pages. This material was important to learn for everybody working with emission spectroscopy in general and in particular for those working with d.c. graphite-arc excitation of powder samples. And, at that time this method was applied in the majority of laboratories world-wide. When reading the conclusions of Paul's comprehensive CSI article, it became clear why he was interested to see our devices. He wrote: “To perform physical measurements in a stable source is not difficult… to produce the stable conditions is more difficult… it is an art, however. Much work remains to be done on the parameters associated with volatilization phenomena, thermochemical reactions in solid and liquid phases.” Paul was not “only” a theoretical spectrochemist, he wished to utilize the theoretical knowledge to improve the spectroscopic source for attaining better analytical performance. From his studies on the main plasma parameters of the arc in local thermal equilibrium (LTE) it became evident that “stable conditions” of the plasma parameters can be attained by a near continuous introduction of sample matrix and/or an ionization buffer. In addition, from the general experience (overviewed and conceptually evaluated in his book) he concluded doi:10.1016/j.sab.2009.03.003
that complete evaporation of sample constituents is mandatory to minimize matrix interference effects from the volatilization phenomenon point of view. How to attain a complete and continuous (non-selective) vaporization with a thermal source, such as the graphite-arc? In Paul's laboratory a workable approach into this direction was under development as described also in his CSI article. The essence was the use of a deep hole in a small diameter graphite rod (as anode) for filling in the sample + graphite powder mixture, which is arced in air (or Ar + O2) atmosphere. Under these conditions, the vaporization is selective only in the initial arcing period during which a steady state vaporization zone is developing in the filling. The upper part of the electrode (close to anode spot) is hot enough to vaporize the nonvolatile components (e.g. Ti, Zr), while from the lower part of the electrode the more volatile components (e.g. Cd, Zn) are vaporized, simultaneously. The vaporization zone is progressing downwards together with the consumption of the electrode tip. So, the resultant vaporization is apparently non-selective which can be held up until the vaporization zone reaches the bottom of the electrode hole. This approach is more successful if the lower part of the electrode (and its filling) is intensively cooled, and this was attained with our “controlled total evaporation” (CTE) device. The electrode is placed in a motor driven spindle (with adjustable forwarding) and the tip protrudes through the central drill of a water-cooled copper “finger”, the extension of the electrode tip is kept constant (4 mm) above the upper end of CTE-device. The movement (readjustment) of the upper graphite cathode could be done by another CTE-device for keeping a constant arc-gap. During the visit of Paul, the “show” was successful with the CTE-arc, and indeed its spatial and temporal stability, especially with the use of a coloring alkali buffer was quite impressive. The simultaneous covaporization of alkali buffers with less volatile sample matrices is analytically an important possibility, which can also be combined with halogenation of nonvolatile carbides (e.g. with the use of LiF buffer). When discussing about future publications, Paul called my attention to earlier work of Dutch authors who described the use of a cooled “copper collar” around the graphite supporting electrode for the same purpose as aimed at by the CTE-device. The use of CTE-device in our following works and publications (with Zakaria Hanna from Egypt) was also connected with Paul's interest and help. It was shown that the plasma thermometric element, Zn, which is relatively volatile, could be simultaneously vaporized with any kind of matrices that was important for a more accurate evaluation of “plasma” matrix effects. Perhaps it is not superfluous to recall the main and somewhat shocking “lesson” from Paul's book about d.c. graphite-arc characteristics regarding matrix effect. In thermal sources, the simultaneous increase of electron pressure with gas temperature is generally expected. However, in an arc discharge, by introducing a substance with more ionizable
306
Remembering the meetings with Paul Boumans
components than the basic discharge components, the electron pressure is increased with a simultaneous decrease of temperature. The latter change is a direct consequence of the decrease of energy dissipation in the arc column for an electrically more conductive plasma (of decreased electric field strength). The main interest of our research group at the Technical University of Budapest (lead and advised by Prof. László Erdey) became, however, the study of the shape of the analytical curves, when applying the CTEdevice (continuous total evaporation) in comparison with the use of continuous spattering of the same powder standards (alumina) into a spark discharge in which a partial evaporation of sample particles took place. In view of the analytical relationship after Lomakin–Scheibe, the relation between emission intensity (I) and the concentration (c) can be expressed by a power function, the exponent (b) of concentration taking the value between 0.8 and 1.0, most generally. When using the CTE-arc excitation the exponent (b) was found consistently equal to unity (proportional function), while with the use of spattering-spark excitation technique b b 1 was found for some of the components using the same analytical lines as with the CTE-arc. The statement from these observations was that the deviation of the (b) exponent from unity must be related to the vaporization pattern of the chemical system, provided that self-absorption is negligible. In a subsequent work the self-absorption could be expressed by multiplication with an exponential term, corresponding to the validity of Beer's law. The manuscript on the above results was submitted to the journal Spectrochimica Acta Part B (SAB) arriving to the Editor, Paul Boumans in the end of 1973 (he became Editor-in-Chief in 1979). The manuscript was sent from the University of Houston (Texas), where I spent a one-year research fellowship, and made a quite ridiculous mistake, not indicating the Department of my laboratory. But, Paul did look after my full postal address in that quite large University. He offered several instructive and helpful suggestions for the revision that became normatives also to my further publication works. The main research interest of Paul underwent a fast change toward inductively coupled plasma atomic emission spectrometry (ICP-AES) after the CSI-1967. Even in his lecture material of this meeting he disputed the features of the ICP source (based on literature information) regarding the LTE concept and wrote: “this type of source has many properties in common with the d.c. arc, and the conditions of the plasma most probably are not far from LTE.” The introduction of the ICP source to atomic spectrometry by the Greenfield' group (1964–65) and the Fassel' group (1965) was quite close to the date of CSI-1967. Nevertheless, four lectures were dealing with the analytical features and applications of (non-commercial) ICP sources. Flame atomic absorption spectrometry (FAAS) was represented at the CSI-1967 with a relatively few (4) lectures, although this method was introduced well before this meeting by Walsh (1955) and independently by Alkemade (1955). The introduction of the acetylene-nitrous oxide flame by Willis (1965, 1966) was, however, quite novel yet. The invention of graphite furnace electrothermal atomization atomic absorption spectrometry (GF-ETAAS) by L'vov (1959), and its publication in Spectrochim. Acta (1961) had no impact apparently on the program of CSI-1967. It was the introduction of the simplified design of the “tube-type” GF-ETA unit by Massmann (1967) that was followed by commercialization in a short time (Perkin-Elmer, 1970). The start of manufacturing the “rod-type” of GF-ETA unit was dated also about to this time (Varian Techtron, Australia). Altogether, these AAS techniques represented the other revolutionary progressing direction within atomic spectrometry, and I myself became involved in these methods, contrasting to the other new direction followed by Paul (ICP-AES). I recall the above sentence of Paul as the concluding remark to d.c. graphite-arc work, saying: “much work remains to be done on the parameters associated with volatilization phenomena, thermochemical reactions in solid and liquid phases.” This kind of work had been
done in part and resulted in valuable comprehensive knowledge to graphite-arc (Schroll in CSI-1967), but the real advent of the research on high temperature condensed phase processes was started with the development of the GF-ETAAS method. It might also be an interesting note that the many efforts made to increase the residence time of sample vapor in the d.c. arc (e.g. by using a magnet around the supporting electrode) were not sufficiently auspicious, while this could be effectively realized in the GF-ETAAS technique. In the next ten years (1967–1977), seven papers were published by Paul on d.c. graphite-arc spectrometry co-authored mostly by his close friend, Frans Maessen, exploring the advantages of gas-stabilization and use of fusion techniques, but also the routes of the analyte loss, as the negative feature. The series of papers was ending with “rigorous statistical approach for establishing accuracy” of d.c. arc procedures. In parallel with these d.c. arc works (of apparently declining interest of continuation) eight papers were published on the different features of ICP-AES, the most frequent co-author being F.J. de Boer. In addition to this research activity and the very active editorial work for SAB, Paul was able to write a comprehensive book chapter on “Excitation of Spectra” (E.L. Grove editor), published in 1972. In my view that was a really heroic performance. This book chapter is starting with the fundamentals of high temperature gas phase and condensed phase processes and continued with the application of this knowledge and concept to the most generally used spectroscopic sources (as below). The sources are described with details of physical principles including electric circuits, and this material is still a unique compilation of emission spectrometry techniques, as the whole. With regards to spark excitation of metals and alloys newer textbooks are barely available up to now, although this analytical technique has an invariably important position in industrial laboratories. The chapter serves also as a comprehensive “source book” of emission spectroscopy, the references are listed in alphabetical order for the topics as follows: Books and Tables (1–64), General (65–158), D.C. Arc (159– 430), A.C. and Intermittent Arc (431–468), Spark and Condensed Arc (468–613), Spark at Reduced Pressure (614–628), Plasma Jet (629–679), Radio-frequency Torch (680–729), HF Discharge at Reduced Pressure (730–753), Hollow Cathode Discharge (754–801), Laser (802–873), Flame 874–892), and additional latest references (893–940). Paul was invited to Hungary in May 1974 as a guest of the Hungarian Academy of Sciences, and a seminar was organized for his honor with presentations by Prof. Tibor Török and Prof. Károly Zimmer in Pécs (the largest South-West city of Hungary). By good luck his visit was just after my return back from Houston (Texas), and after travelling back together from the seminar to Budapest, we, together with his friend from Philips, made a “curvilinear” evening in the Castle of Buda. Paul liked the Hungarian meals and wines and could enjoy the “free atmosphere” being out of the pressure of his many duties. He presented me with a specimen of the above book chapter with the dedication: “To… with the kindest regards of the author, who has an excellent souvenir of his recent visit to Hungary, May 27, 1974”. During the decade in consideration (1967–1977) the elemental analysis of liquid materials became of paramount importance, compared to the formerly dominating analysis of metals and powders. This new requirement was manifested in the large variety of waters and other environmentally important liquid samples. In the analysis of foods and other biological materials chemical destruction and dissolution of relatively large sample masses are also required to ensure reliable sampling accuracy. Until the mid-50s, the flame atomic emission spectrometry (FAES) represented the fastest elemental analytical method for the direct analysis of aqueous solutions, due to the easy-to-use pneumatic nebulization and application of photoelectric detection. The problem was the very limited number of elements that could be determined. For extending the number of analytes, an effective version of “spray-in-spark” method was developed (with the use of a spectrograph) in the Technical University
Remembering the meetings with Paul Boumans
of Budapest, published in 1955 by L. Erdey, E. Gegus and E. Kocsis. The sample spray was introduced through the lower, tubular graphite electrode into the spark gap, the spray droplets collided and deposited in part on the surface of the upper semi-spherical graphite electrode. The high temperature vaporization of the sample components took place dominantly from the surface of the upper electrode, where the layer of the deposited solute was continuously renewed. The same electrode system was also used for the spark excitation of volatile hydrides of As, Sb and Ge, liberated from solutions by nascent hydrogen (Zn + HCl reaction). This method represented the first introduced hydride generation atomic emission spectrometry (HG-AES) method (published in German in a domestically printed periodical) that has not been recorded in the international literature. As known, the HG-AES/AAS methods have made an extremely important career in spectrochemistry. Nevertheless, the outlined solution spectrographic method together with the other known methods in this category could not compete in simplicity and speed with the flame methods. In addition, using acetylene-nitrous oxide flame, the number of determinable elements could be doubled (increased to about 70) relative to that accessible with acetylene-air flame, resulting in a great progress of flame atomic absorption spectrometry (FAAS). Also, to the end of 70s the GF-ETAAS technique underwent a tremendous improvement in temperature programming (stabilized temperature platform furnace atomization) and automation of sampling (solutions and slurries),as well, in providing two orders of magnitude higher detection power than available with the flame (and also with ICP-AES) methods. According to the favored expression of Walter Slavin the GF-AAS was approaching to be a “technology” (rather then to be an “art”, as the arc source was characterized by Paul). So, from the mid-70s, three major elemental analytical challengers were competing in the field of solution analysis; the FAAS, GF-ETAAS and the ICP-AES, all applying photoelectric detection to monochromator type optical systems. Paul, who was playing one of the roles of ICP-AES “apostates” made several experimental studies and evaluations to demonstrate the viability of ICP-AES in this competition. This latter method required a more expensive monochromator of higher resolution power for a similar selectivity as was attainable with the FAAS. However, the ICP-AES had inherently the possibility of multielement analysis, provided that more sophisticated (and more expensive) optical and detection systems are used, which were, however, not available at that time on the level of requirements. On this problematic situation 10 papers were published between 1977 and 1980 by Paul and coworkers, the final paper in 1980 had the title: Is ICP a d.c. arc in a new jacket? Paul explained this title by the intention to express the common problem of the arc and ICP emission methods with respect of “the complexity of the extraction of a true analytical signal from the composite radiation observed in the spectral window of an analyte emission line”. The “arc in new jacket” paradigm could be applied logically also to the subsequent papers (1983–1984), published in cooperation with plasma physicists on the excitation mechanism of ICP that was found “close enough to the LTE concept”. The relatively low matrix (nonspectral) interference effect observed for the ICP source is the consequence of the specific mode of energy dissipation that is dominating in the plasma annulus instead of the plasma axis where to the sample is introduced. The logic might also be extended further in that, due to the discrepancies from the ideal ICP structure, the axial sample channel is not perfectly free from electric field (and energy dissipation) with the consequence of “plasma loading effect” by sample matrix (increase of electron concentration and decrease of temperature) that is relatively small compared with arc situation. After 1974 (Pécs, Hungary) the next occasion to meet Paul was in May 1977, at the International Symposium on Microchemical Techniques (Davos, Switzerland). My “profile” changed in the direction of using FAAS detection combined with newly developed
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sample introduction methods (as below). Still, I used graphite-arc spectrography applying halocarbon vapors for sample halogenation. In the get-together party answering Paul's question about my work, I outlined the new halogenation method (we communicated mostly in writing), he told me with a smile: “forget arc excitation”. I understood the reason for his advice, however, at that time no other high temperature “plasma” source was at my disposal, and this condition lasted in the further 10 years for me. The new sample introduction methods were based on the production of solid aerosol by graphitearc vaporization (from 1973) and by laser ablation (LA, from 1975), the aerosol was introduced into the flame source (FAES/FAAS). In Davos, the LA-Flame method was the main topic of my presentation and Paul gave me an invitation-card for publication in SAB, which could be realized in 1979. We met again in the same year, in July in Prague at the XXth CSI-1977, and by my good luck also in the next XXIst CSI-1979, Cambridge, UK. In the latter meeting my presentation was about graphite furnace electrothermal vaporization (GF-ETV) as sample introduction to flames using also halocarbon vapor as halogenation reagent, which was again invited by Paul for publication in SAB. Prof. Gordon F. Kirkbright (Chairman of CSI-1979, Imperial College, London) introduced this halogenation method independently in GF-ETV-ICP-AES. The year 1979 became also memorable by another meeting with Paul in Poland (Zaborow, September, 24–27) on the School of Modern Excitation Sources in AES, organized by Jerzy Fijalkowski with the assistance of Alicja Jedlewska under the auspices of the Polish Academy of Sciences. The days were started with tutorial lectures followed by consultative discussions in the afternoons and closed in late evenings with singing and dancing in a Polish manner. Everyone likes to recall that days, Paul asked me later about the pianist Peter Zentay, and also about the other Hungarian participants, Ernö Gegus and Jozsef Posta, he remembered well. The greatest event amongst my meetings with Paul was the unequaled occasion of the XXIIIrd CSI1983 in Amsterdam (June 26–July 1). I received an invitation to hold a keynote lecture and he exactly tailored my contribution by suggesting the title: “New approaches to the separation of vaporization and atomization–excitation in atomic spectrometry”. The manuscript had to be prepared in “camera-ready” form for publication in advance of the CSI meeting (at that time we had no computer), Paul expressed his satisfaction by telling me that “only one paragraph had to be retyped”. It was amazing, how he was able to take care about such small things as to organize the accommodation of four Hungarians in his neighbors' house at Bergeyk (Karola and Wil Maas family). We arrived three days before the conference by driving a car through several Western countries, which was also a new experience. Thus, we could participate in the reception given by the Boumans Family at their house, which was started with a presentation of classical music by Paul's daughters, Christine (flute) and Lidwien (piano). Paul's wife Dorine and son Baptist were engaged with greeting and guiding the many guests. This personal welcome party was on Saturday and was followed on Monday by the official reception at the Rijksmuseum of Amsterdam, and in the same evening by the invitation of Leo de Galan and Lida Schoen to their classical Dutch house. After the highly informative and exciting working days of the CSI meeting we were again spoiled by the unique conference party (with “dancing section”) in the Amsterdam Hilton, thus placing the “cream on the pie”. The coming years for Paul were engaged mostly with high resolution studies of ICP spectra and spectral interference problems (papers co-authored dominantly by J.J.A.M. Vrakking), and simultaneously with editing (and contributing seven chapters) of a two volumes book, Inductively Coupled Plasma Emission Spectroscopy, published in 1987. He presented me with the offprint of the book, the major present being in mid-1987 his proposal to be elected for the membership of the Editorial Advisory Board of SAB. I understood his efforts in “reorganization” of the reviewing process and establishment of “hot” communication lines between authors and editors. This
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membership of SAB made it possible to receive the invitation to the celebration of the “50th Anniversary of Spectrochimica Acta 1939– 1989” held in Rome, Pontificial Academy of Sciences, The Vatican, 27– 28 June, 1989. From the several historical and philosophical presentations published in Spectrochim. Acta (1989) Special Supplement, Future Trends in Spectroscopy, let me cite Paul's introductory “thesis” “The aims of scientific publications are: (a) disseminating information to promote scientific and/or technological advancement, (b) establishing the Authors' prestige and reputation. I believe that the latter aim is equally important as the former.” This thesis (the two aims together) could be understood as the basis of a healthy operating society, although its declaration and realization are by far not obvious in several scientific and political administrations. In a subsequent letter Paul expressed his disappointments about the winning position of “inflated big talkers who are not able to do anything useful”… in the conflicts… with those “who prefer and are able to do the work”.
In the “Farewell Issue” of Paul Boumans (SAB-1995), prominent scientists presented highly informative addresses: prefaces by Walter Slavin and Nicoló Omenetto, award greetings by José Broekaert and tributes by Kurt Laqua and James Winefordner, which are right honorable indeed to an outstanding scientist and great man. My personal stories are ending with mentioning my visit to the Boumans' house at Bergeijk, March 28, 2004. It became possible on the way back home, driving from Delft to Budapest, after my one-month work in the laboratory of Margaretha de Loos-Vollebregt (Delft University of Technology, Delft, The Netherlands). Paul copied a map and marked the best route for me and sent it by fax. Dorine invited me for lunch, so I arrived around noon and could touch some topics of our works and lives, including the appearance of Paul's artistic book: “Family Portrait Europe 1400–1800”. Tibor Kántor Geological Institute of Hungary, Stefania road 14, 1143 Budapest, Hungary E-mail address: [email protected].