Modifiers and coatings in graphite furnace atomic absorption spectrometry—mechanisms of action (A tutorial review)

Spectrochimica Acta Part B 57 (2002) 1835–1853

Review

Modifiers and coatings in graphite furnace atomic absorption spectrometry—mechanisms of action (A tutorial review)夞 H.M. Ortnera,*, E. Bulskab, U. Rohra, G. Schlemmerc, S. Weinbruchd, B. Welze a

Department of Chemical Analytics, Institute of Material Science, Darmstadt University of Technology, Petersenstr 23, D-64287 Darmstadt, Germany b Department of Chemistry, University of Warsaw, Warsaw, Poland c ¨ Analytik Jena GmbH, Uberlingen, Germany d Institute of Mineralogy, Darmstadt University of Technology, Darmstadt, Germany e Department of Chemistry, Federal University of Santa Catarina, Florianopolis, Brazil Received 15 May 2002; accepted 1 August 2002

Abstract A multitude of different and often contradictory mechanisms for the effects of modifiers and coatings have been proposed. Many of these proposals lack sufficient experimental evidence. Therefore, a series of statements based on our own investigations is given as ‘facts’. Another series of statements is made as ‘fictions’ related to erroneous proposals on the functioning of modifiers and coatings in the pertinent literature. Two basic concepts are developed for the sequence of processes leading to analyte stabilization for the two most important groups of modifiers: refractory carbide forming elements of the IVa–VIa groups of the periodic system on the one hand and Pt-group metals on the other hand. These concepts are based on the main reactions of graphite with elements and compounds: carbide formation and intercalation. Most important experimental results leading to this understanding are described: Penetration measurements for modifiers and analytes indicated the subsurface zone down to approximately 10 mm as the essential place for graphite–analyte–modifier interactions. The reason for this phenomenon is an open porosity of the pyrocarbon coating of 5–10% (vyv) into which liquids penetrate upon sample application. This also indicates that modifiers are best applied by impregnation or electrolysis whereas dense coatings are not advantageous. It is also shown that graphite tube assemblies are dynamic systems with a limited lifetime and carbon losses are an essential feature of tube corrosion. Most frequently found erroneous statements are discussed: (a) Particles on the tube surface are responsible for analyte stabilization and retention during pyrolysis. (b) Analyte stabilization is taking place by formation of intermetallic compounds or thermally stable alloys. (c) Experiments are performed with unrealistic concentrations of analytes andyor modifiers. (d) Dense coatings are advantageous. Finally, a functional schedule is given for the three steps of graphite furnace atomic absorption spectrometry (GFAAS): sample application and

夞 This paper was presented at the 7th Rio Symposium on Atomic Spectrometry, held in Florianopolis, ´ Brazil, April 2002 and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference. *Corresponding author. Tel.: q49-61-51-166-379; fax: q49-61-51-166-378. E-mail address: [email protected] (H.M. Ortner). 0584-8547/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 2 . 0 0 1 4 0 - 4

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drying; pyrolysis; atomization. Contrary to the vast amount of literature on this topic it tried to provide the analyst working with GFAAS and in an increasing number working with Solid Sampling-GFAAS with a set of most important statements. This might spare the experimentalist a lot of useless optimization procedures but should lead him to a basic understanding of the complex phenomena taking place in his instrument and during his analytical work. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: GFAAS; Graphite atomizer; Chemical modification; Mechanisms of action; Coatings

1. Introduction Mechanisms of action of modifiers in electrothermal atomic absorption spectrometry (ETAAS) is a topic of great scientific and practical interest. Volynsky recently published a respective review with 284 literature citations for the most popular group of modifiers, i.e. the platinum group metals w1x. This demonstrates the worldwide efforts in the search of basic principles of action of modifiers. Coatings can also be considered as permanent modifiers w2,3x, and are, therefore, included in the discussion. The pertinent literature is full of very different and often contradictory proposals for mechanisms of action of modifiers and coatings w4,5x. A series of statements based on our own investigations are presented as ‘facts’. This should not indicate that only our own investigations in this wide field are correct. What should be expressed by this formulation is that the ‘facts’ in this paper are observations based on experimental evidence rather than on mere speculation. On the other hand, some frequently proposed mechanisms of action are shown to be erroneous and only these are summarized under ‘fictions’. This way it is hoped to raise the ongoing discussion on a safer and largely experimentally based level. This concept was already used by us previously w6x but it is now elucidated in much greater detail. Matrix—analyte—and modifier behavior in connection with graphite tube changes with rising temperature of an analysis cycle as well as with a rising number of analysis cycles are very complex processes and it is difficult to keep all these phenomena properly in mind for a sound interpretation of graphite furnace atomic absorption spectrometry (GFAAS)procedures. It therefore, tried to structure this remarkably voluminous knowledge as clearly as

possible in order to facilitate the study of such an overview. An exciting and relatively new version of GFAAS is now approaching maturity—Solid Sampling-GFAAS (SS-GFAAS). Of course, the method has for quite some time been in critical discussion and maybe ten years ago even very reputed analysts did not think that this is a reliable approach to real type samples. However, the obvious advantages have ultimately led to the construction of impressive new instrumentation like the Atomic Absorption Spectrometer (AAS) ZEEnit 60 of Analytik Jena (Germany) which lends itself to relatively simple quality control, e.g. for the determination of traces of Cu, Pb, Cd, Zn and Fe in calcium fluoride w7x, an important new substance for fluoride–phosphate glasses used in laser optics dedicated to nanostructuring processes in the microelectronics industry. The most prominent advantages of this analytical approach for powders and other solid samples are: ● Trace element sensitivity down to the ppb-level in favorable cases. ● Avoidance of contamination by minimum sample preparation with no dissolution procedures. ● Small sample weights in the single mg-range, which also allow heterogeneity investigations on this sample weight level. However, it has been observed in our area that the analysts active in this field in industry have little experience in this very special type of extreme trace analysis and still less knowledge in graphite corrosion phenomena. This is typical for today’s industrial situations caused by over-rationalization w8,9x. Consequently the nice instrumentation which still does bear some instrumental and software bugs, is not handled adequately. Analytical optimization is not properly carried out and

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some basic problems, e.g. related to respective graphite corrosion which often is disastrous in solid sampling into graphite boats is not reduced by adequate measures. It is, therefore, one of the main incentives of this paper to provide the experimentalist with a series of relatively simple statements which maybe can help to avoid respective pitfalls and might lead to a more profound understanding of the essentials of GFAAS which still is one of the most important trace analytical methods available.

TaC, ZrC, NbC) with melting points around and above 3500 8C w13x. That these elements are difficult to atomize in a graphite tube system is obvious. It should also be understood that technical carbonization of these very stable carbide formers is occurring at significantly lower temperatures than their melting points: carburization to WC at 1500–1700 8C, to TaC at 1600 8C, to TiC at 2000–2200 8C w13,14x.

2. Facts

2.2.1. Negative effects: low analyte sensitivity w15x and pronounced tube corrosion ● Lacking sensitivity for thermally stable carbide forming elements, especially lanthanides (actinides), IVa–VIIIa-group elements namely Ti, Zr, Hf, V, Nb, Ta, Mo, W and elements forming diamond like carbides (B, Si). Of course, carbide formation is especially detrimental in GFAAS if the thermal stability of the formed carbides exceeds the pyrolysis temperature and especially if it approaches or also exceeds the atomization temperature. ● Acetylides cause dramatic corrosion due to hydrolysis of respective carbides with water in the sample application step and due to the formation of acetylene. Possibly the formation of gaseous carbides at high temperatures also plays a role w16–18x.

2.1. Fact 1 2.1.1. Reaction of carbon with the elements of the periodic system: carbide formation If one considers the vast variety of inorganic carbon compounds with the elements of the periodic system it is observed that only few elements do not form any compounds with carbon, Table 1. At first sight one even wonders whether graphite should be the proper material for atomizers in atomic absorption spectrometry. However, as it turned out in the long evolution of ETAAS, carbon is the most adequate material for this purpose in spite of all these carbon compounds w10,11x. With almost all metals, carbon is forming carbides of very varying structures and properties. The most stable carbides are the ones of the IVa–VIa-groups of the Periodic System of the Elements (PSE). Many of these metallic carbides belong to the hard materials, which are extensively used in technology w12–14x. Metallic or interstitial carbides are also formed by Mn, the iron metals and by P and As. The diamond-like carbides of B and Si also exhibit a high thermodynamic stability and high melting points. Of the salt-like carbides of the Ia-, IIa- and III-Groups of the PSE including the lanthanides and actinides the latter are the most stable ones. Most of these carbides are decomposed to acetylene by water and are hence called acetylides. Group I-metals form quite a series of intercalation compounds. The elements of group VI and VII and nitrogen form volatile or liquid, covalent carbon compounds. Some of these carbides range amongst the thermally most stable compounds we know (e.g. HfC,

2.2. Consequences in GFAAS

2.2.2. Positive effects: analyte stabilization, matrix decomposition and prolonged tube lifetime ● Some of these elements are good permanent modifiers (impregnants): Especially some of those of groups IV a (Zr, Hf), Va (Nb, Ta), VI a (Mo, W) w19,20x. ● Some matrices are better decomposed during pyrolysis by the catalytic activity of the formed modifier-carbides w21x. As was pointed out by Volynsky w19x, it has been observed by many analysts that the lifetime of carbide-modified graphite tubes is significantly longer than that of standard graphite tubes in many practical applications. This is probably due to the resistance of refractory carbides against the attack of many acids and acid mixtures and their high temperature stability.

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2.2.3. Proposal for the mechanism of action of refractory carbide forming elements as modifiers The question now arises, where such modifier– graphite–analyte interactions take place. Furthermore, the question is why these elements are good modifiers similar in their analyte stabilizing effects to the VIIIa-platinum group elements which do not form carbides at all. It is interesting to observe that especially analytes with a tendency to form oxo-anions are stabilized by IVa–VIa-group modifiers, i.e. the III–VI-group elements. It is known from carbide chemistry that refractory metal (RM) carbides are easily oxidized on their surface. It is also known that many metal oxides form bronzes, i.e. mixed metal oxides with RM-oxides w22x. These exhibit a high thermal stability. Oxoanions are well known to form heteropoly-acids with RM-isopoly acids in acidic aqueous media. These heteropoly acid systems will be dehydrated to thermally quite stable mixed oxides w22x. Although there is no hope presently to investigate analyte–modifier–compound formations at the extreme trace levels of the analytes present in GFAAS it is reasonable to propose such a compound formation as stabilizing effect of the carbide forming modifiers. The breaking of these bonds at high temperatures by a reconversion of the modifier oxides to carbides with CO-formation and subsequent analyte atomization would be a logical process leading to an effective analyte atomization with the analyte atomic vapor being carried into the tube atmosphere by the evolving CO. Subsequent sample introduction will again lead to the necessary surface oxidation of the RM-carbides to form heteropoly acids again with the newly introduced analytes. Fig. 1 should illustrate this novel approach by a new mechanism of action for a large group of permanent modifiers. It is now also understandable why an oxidative treatment of graphite surfaces should improve analyte retention in some cases w24–27x. Here, too, complex formation with surface hydroxyl- and carboxyl-groups will take place with many analytes. Dehydratization will again lead to oxide and carboxide systems, followed by analyte reduction

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and CO-formation as described above. However, the retention will not be as effective as the one by heteropoly-compounds of RM-carbides. It should be mentioned that apart from, e.g. tube lifetime observations for carbide coated tubes, no experimental evidence for the proposed mechanism is yet available. We are, however, planning to study refractory metal carbide oxidation by synchrotron X-ray induced photoelectron spectrometry (XPS). It should be emphasized that the universal interpretation of the retention mechanism of the palladium modifier as dissociative chemisorption w28x is too narrow for the broad variety of high temperature chemical interactions taking place between analytes, modifier and graphite. It is especially the absence of the consideration of the graphite as a highly reactive material at atomization temperatures which underestimates respective reactions with carbon in this context. In order to understand the action of the second large group of modifiers of the platinum metals it is necessary to first consider some essential aspects of graphite interaction with liquid samples. 2.3. Fact 2 2.3.1. Penetration measurements for modifiers and analytes into the depth of graphite tube systems ● Modifiers and analytes penetrate up to 10 mm into the pyrographite (PG)-coating of longitudinally heated graphite atomizer-(LHGA) and transversally heated graphite atomizer (THGA)tube systems already upon sample application and drying w29,30x. This is not a diffusion process but a liquid penetration primarily into the open porosity of the PG. For the Pd–Mgmodifier, there is significantly less MgO than PdO below the surface. Arsenic as analyte also penetrates into the PG to the same depth as the Pd-part of the modifier w29x. ● After pyrolysis the Pd penetrated deeper into the PG-layer (up to 30 mm) whereas the analyte together with the MgO did not change their depth distribution. ● Good modifiers appear to remain in the near

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Fig. 1. Potential permanent modifiers among refractory carbide-forming elements and noble metals (in italics) according to w23x and proposed mechanism of analyte retention for carbide-forming modifiers.

surface region (example: WC from tungstate solutions, noble metals w30x). Similar observations were made by Majidi and Robertson using Rutherford backscattering spectrometry w31x. However, they interpreted their results as pure diffusion because investigations were only performed after pyrolysis. ● Other modifiers or matrices quickly penetrate the whole wall thickness (examples: La, Nb). This might be due to the formation of gaseous carbides and in the case of La also due to extensive corrosion and exfoliation by acetylene

formation during aqueous sample application w16–18,32x. ● Some matrices such as iron do not penetrate into the PG-layer at all (especially covalently bound carbide formers) w18x. 2.4. Fact 3 2.4.1. The pyrographite coating on polycrystalline electrographite: micro- and nanometer-scale-surface morphology and porosity measurements The microstructure of PG-surfaces was exten-

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sively studied by scanning electron microscopy (SEM) w33x. From these studies it was concluded that the PG forms a dense and essentially poreless layer on the EG-base material with a high oxidation resistance and an extremely low permeability as a result of the high crystalline order of the PG. It must be mentioned that no high resolution SEMstudies were performed since at that time (at the end of the 1980s) field emission gun SEMs were still very rare. First investigations by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) did demonstrate great changes in surface nanoroughness during graphite tube lifetime of PG-surfaces w34x. However, detailed studies with very high lateral resolution were not feasable at the very beginning of the fantastic instrumental development of these new methods of surface investigation. Only fairly recently Vandervoort et al. w35x demonstrated the presence of nanometer scale cracks already on virgin PGsurfaces of modern THGA-tube systems. Since the lattice in these domains of the PG-surface is oriented almost perpendicularly to the surface, the resistance toward oxidative attack is much lower than in areas where the lattice is oriented parallel to the surface. Consequently, these domains will be corroded much faster which will give rise to a growing open porosity. Therefore, porosity measurements on LHGA- and THGA-tube systems are of great importance in order to further elucidate this important property of PG-surfaces. Such measurements are, however, not quite easy to perform. The conventional method of porosity measurement by mercury intrusion is not applicable to graphite materials because of the danger of changing the porosity by the breakdown of the graphite pore structure by the high mercury pressures which have to be applied w36x. Therefore, a pycnometric procedure was applied w36,37x which yielded a most surprising result w29x: the open porosity of PG-coated LHGA-systems with platform is in the range of 5–10% (vyv) (compare also with w38x) and that of pyrographite is reported up to 3% (vy v) for well ordered PG w39x. With these results the penetration measurements discussed above become better understandable.

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2.5. Fact 4 2.5.1. The significance of intercalation compounds in GFAAS for matrices, modifiers and analytes Intercalation is the key issue for graphite-interactions w40x with many: ● Analytes. ● Matrices. ● Modifiers. Preferably intercalated elements and compounds w40x: ● Alkali and alkaline earth metals, noble metals. ● All mineral acids except HNO3 (which is rather thermally decomposed). ● Many metal oxides and halides. ● Halogens and halides (especially from organic solvents after thermal decomposition of the latter). One of the reasons of the failing of glassy carbon as a candidate material for GFAAS is its inability to form intercalation compounds! Of course, other important reasons are its becoming fairly reactive at temperatures above 2200 8C and its relatively low temperature resistance: it is rapidly destroyed at temperatures above 2500 8C w41x. 2.5.2. Positive effects of the formation of intercalation compounds in GFAAS: studies on the functioning of modifiers of the platinum-group metals The advantageous use of modifiers of the platinum group metals (PGM) seems to be based on the formation of intercalation compounds and a subsequent activation of these intercalated metal atoms. The such activated metal atoms are proposed to form strong covalent bonds with easily volatile analyte elements which leads to their stabilization to remarkably high appearance temperatures w29,42x. The first step in this process, the intercalation of Pd has been experimentally verified by X-ray emission line shift measurements in the electron microprobe. This line shift of the selected Pd Lb2y15 line for various Pd-compounds is extremely small. This made the development of a special procedure of peak-flank intensity ratio measurements necessary w29x. A comparison of the

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observed line shift for Pd in areas, where no Pdparticles are seen in respective scanning electron micrographs (SEM-micrographs) but Pd is present

Fig. 2.

on or below the surface with samples of prepared PdCl2-graphite intercalation compounds unambiguously demonstrated the presence of intercalated

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Pd in THGA-platforms. Details of this investigation will be published separately. In addition, elution experiments were carried out after the drying step on LHGA-platforms treated with a conventional PdyMg-modifier. It was found that 20% of the Pd could not be eluted any more with 6 M HCl. This also clearly indicates that the retained Pd cannot any more be present as nitrate or oxide but must have reacted with carbon to form compounds which are resistant against the elution. The Mg was recovered to 100% by the same elution procedure w29x. So far the experimental evidence of the Pdmodifier intercalation has been described. We can unfortunately only speculate about the subsequent analyte retention mechanism because we have no experimental possibility to study the bonding of nanogram amounts of analytes with all the instrumental possibilities of today’s impressive range of methods of analysis on solid state systems w43,44x. We propose a mechanism of bonding, e.g. of As to the Pd-atoms which are activated by their intercalation, above a graphite lattice plane in the following way w45x. Fig. 2 should demonstrate the process of Pd-oxide intercalation and the proposed way of bonding of As-analyte atoms. There are chemical bonds between the p-electron system of the graphite lattice on the surface of a lattice plane to the coordinatively unsaturated Pd-atoms on the outer surface of a Pd-cluster adjacent to the graphite lattice. The electrons missing to noble gas configuration for these Pd-atoms are, therefore,

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taken over from the p-electron system of the graphite. This causes an elevated electron density for these Pd-atoms which are, therefore, activated for an interaction with, e.g. As(OH)3 to function as electron donors. This is an analogy to the ferrocenes where the iron atom also exhibits electron donor properties: Fe(C5H5)2qHq™ wFeH(C5H5)2xq w45x. If the Pd, on the other hand, would act as electron acceptor, then analytes in higher oxidation states we.g. As(V)x or metal cations as e.g. Cd2q or Pb2q could not be bound. The bonding between the p-electron system of the graphite lattice and the coordinatively unsaturated Pd atom in the periphery of a cluster is symbolized by a perpendicular line in Fig. 2. The thus activated Pd-atom then reacts with, e.g. arsenic acid in the way shown in Fig. 2. It should again be emphasized that only the Pd-atoms on the phase boundary Pdclusterygraphite are activated. This is in agreement with the observation that the analyte is already bound during the drying step. This bond is not destroyed by a temperature rise in spite of the ongoing reduction of the PdO under the conditions of the pyrolysis. It should also be stressed that the activated domain of the Pd-modifier is the subsurface region to a depth of approximately 10 mm, i.e. the volume of the open porosity and adjacent domains between the graphite layers of the PCstructure. It should also be mentioned that by the strong bonding of the analyte to the Pd its mobility at pyrolysis temperature is practically suppressed.

Fig. 2. (a) Description of the bonding in PdO: the bonding of Pd2q with (4=O2y) 1y4 leads to dsp2—hybridization and not completely filled 5p-orbitals. The oxygen coordination of Pd-atoms is 4-fold and leads to polar covalent bonds. (b) Intercalation of PdO in graphite: from EPMA-investigations it can be concluded that cluster formation with diameters of several micrometers and a height of 1–2 nm takes place when PdO is intercalated near the surface of the PC. With rising temperature up to pyrolysis the reduction of PdO by inner surface carbon atoms with subsequent CO evolution takes place. Maybe some O2 is also formed without reductive action of the carbon. (c) Chemical bonds are formed between the Pd-atoms adjacent to the graphite lattice planes by interaction of the p-electron cloud of the graphite, which fills up the 5p orbitals of the Pd to noble gas configuration (in analogy to ferrocenes). Within the Pd-clusters, the bonds to oxygen prevail and later on metallic bonds between the Pd-atoms which do not have a chance to interact with the p-electron cloud of the graphite. (d) This interaction of Pd-near surface atoms with the graphite lattice seems to activate the Pd to become an electron donor for metalloids. The lower part of the figure should show how such a bonding situation could look like. The ferrocene-like bonding between the graphite and the coordinatively unsaturated Pd-atoms adjacent to the graphite lattice plane is symbolized with a perpendicular bond in a broken line. The two other valency bonds symbolize bonds to the Pd-cluster. Such an activated Pd atom now reacts with, e.g. arsenic acid in the way shown. The formation of a double bond between the Pd and the metalloid is not an exotic situation. It was also observed in other metal-organic compounds, e.g. in (OC)3NisAsR3. It should be emphasized that only Pd-atoms on the phase boundary between the Pd-cluster and the graphite can act in such a manner.

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Hence, the analyte atoms cannot diffuse further into the graphite nor to the inner tube surface. Hence, neither a broadening of the atomization peak occurs nor are analyte losses during the pyrolysis step possible. A more in depth study of the respective atom formation for As, Se, Te can be found in w29x and will be discussed later under Fiction 2. 2.5.3. Negative effects of matrix- or modifierintercalation The intercalation of mineral acids, on the other hand, also leads to a tight binding of many analytes resulting in a pronounced deterioration of their determination w46x. In still other cases, the formation of intercalation compounds leads to PG-layer scrolling and a fast subsequent exfoliation. This was observed, e.g. for the matrices iron and lanthanum although the mode of corrosive attack is quite different for these two matrices w18x. This again results in bad analytical performance and a dramatic decrease of analytical as well as total tube lifetime. 2.6. Fact 5 2.6.1. Permanent modifiers need high temperature stability and are best applied by impregnation or electrolysis Recently, the concept of permanent modifiers became very popular w47x. The elements most commonly used for this purpose are either Ptgroup metals with high melting points (Pt 1772 8C, Rh 1966 8C, Ru 2310 8C, Ir 2410 8C) or very stable carbide forming elements (W, Ta, Zr, etc.). These modifiers may also be used as ‘in situ’ collectors of analyte hydrides w47x. Due to the lower melting point of Pd (1552 8C) as compared to Ir (2410 8C) Pd is inferior to Ir as permanent modifier. This was demonstrated by respective results of Pd- and Ir-determination by instrumental neutron activation analysis after application of 200 mg Pd and 25 mg Ir and analysis under conditions for Sn: after 300 analysis cycles, the modifier is still operative, but 99% of the Pd

are lost. 53% of the Ir are still present mostly below the graphite surface w29x. Tube impregnation is the easiest way to apply permanent modifiers to GFAAS-tube systems w3,48x. Still more effective than impregnation is the deposition of permanent modifiers by electrolysis. The reservoir of, e.g. noble metal modifiers deposited in the elemental form below the surface is thus greater and, hence, available for the total analytical lifetime of the tube system, i.e. for several hundred analysis cycles. This saves reagents and shortens analysis time. Blanks are also kept to a minimum this way w30x. It should have been pointed out already that too high amounts of permanent modifiers can decrease the sensitivity for some elements, and can lead to other negative effects. This is especially true for (dense) coatings applied by Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD). 2.7. Fact 6 2.7.1. Graphite tube assemblies are dynamic systems with a limited lifetime w49,50x For PG-coated tubes three distinct stages of tube system performance can be distinguished w50x: ● A preliminary phase of approximately 20 analysis cycles with greater deviations in sensitivity and large relative standard deviation (R.S.D.). This is caused by high mass losses, a developing smoothing of surface roughness and a reduction in closed porosity due to recrystallization phenomena in the binder phase of the polycrystalline electrographite tube system body. ● A phase of nearly constant and high sensitivity with a minimum in R.S.D. follows the initial phase, caused by minimum mass losses, a minimum in closed porosity and a largely smooth pyrocarbon surface with eventually rising corrosion. This range is often prolonged by secondary carbon coating in the platform region and can last for several hundred analysis cycles. ● A final phase with decreasing sensitivity and increasing R.S.D. The analytically useful lifetime is reached if the sensitivity dropped by

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more than 20% of the maximum sensitivity, and the R.S.D. rose to )2.5%. In this stage, surface roughness is rising due to corrosion, mass losses are also rising. Permeability of carbon for analyte atoms through the tube wall increases. Carbide formation in the gas phase increases. The total lifetime of a tube system is reached when tube breakage occurs. Table 2 gives an overview of respective phenomena in AAS-behavior of the analytes, their underlying phenomena of relevant graphite changes together with important interpretations. 2.8. Fact 7 2.8.1. The significance of carbon losses w46,50x The mass loss and carbon loss of a tube system are identical since the tubes consist of )99.9% carbon. The carbon loss amounts to several percent of the total tube mass over the analytical lifetime of a tube system w29,50x. This loss occurs predominantly during the atomization and the tube scavenging period. The main causes are carbon evaporation and carbon particle emission rather than oxidative processes. However, oxidative processes mainly occurring during pyrolysis are important phenomena leading to activated carbon areas from which carbon evaporation takes place during the high temperature phase of analysis cycles. Carbon losses may amount up to )150 mg per analysis cycle. In-tube losses are around 50% lower than this. They are best characterized by electrothermal vaporizationinductively coupled plasma-mass spectrometry (ETV-ICP-MS) w50x. Of course, corrosive and high temperature tube attack and carbon losses are interrelated. 3. Fictions The most important and experimentally established facts for understanding the action of modifiers have been elucidated. In the following, some

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of the most frequently found erroneous interpretations of observations on the mechanism of action of modifiers will now be discussed as ‘fictions’. 3.1. Fiction 1 3.1.1. Particles on the graphite surface are responsible for analyte stabilization and retention during pyrolysis This is one of the most frequently given explanations for analyte stabilization by modifiers w1x. However, qualified thoughts on how analytes should be stabilized in particles are scarce. Here are our own considerations and experimental results on this topic: ● The particle abundance for the universal modifier (Pd, Mg) is surprisingly low if actual amounts are applied we.g. 15 mg Pd(NO3)2, 10 mg Mg(NO3)2x w29x. Since the particles are decomposing during pyrolysis (at the latest) they become porous. ● Only for amounts used for permanent modification, the particle abundance is initially high (200 mg Pd, 25 mg Ir). However, after 300 analysis cycles (Sn-conditions) the particles have all disappeared, but the modifier is still operative w29x. This was the most convincing observation for us that analyte stabilization cannot be effected by particles. ● There is no reason, why the Pd or Ir in the particles should retain the analytes during pyrolysis due to high diffusion rates and short distances for outward-diffusion in porous particles of 0.1–10 mm in diameter. ● In many cases, an improved modifier action was observed for very fine particles of the modifier with particle diameters in the range of several tens to hundreds of nanometers w51x. This is understandable since a finer and more homogeneous distribution of the modifier will be more effectively intercalated than few larger modifier particles. On the other hand, such small particles cannot possibly retain the analyte by mere diffusional processes. ● Particles or better drops of molten Pd (TMs 1552 8C) cannot retain analytes (e.g. As, Se,

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Table 2 Lifetime profile of PC-coated graphite tubes of polycrystalline electrographite Analytical phenomena Preliminary phase of tube stabilization Greater variations in (a) analyte sensitivity (b) reproducibility (R.S.D. too large)

Range of optimal tube performance Optimal tube performance: (a) Highest and constant sensitivity (b) lowest and constant R.S.D.

Reasons in structural changes of the graphite

Interpretation

Caused by: (a) high mass loss (b) secondary coating (c) decrease of the closed porosity by heating processes especially in the binder of the electrographite

This leads to: (a) high tendency of carbide formation in the gas phase (b) Reduction and unification of PC-surfaceroughness by secondary coating; this leads to a decrease in the activity for carbide formation of the PC coating. (c) Reduction of closed porosity leads to a more homogeneous temperature distribution throughout the tube.

(a) Minimal closed porosity (b) Corrosion of pyrocarbon coating low (c) This range is frequently extended by pronounced secondary coating of platforms.

(a) High analytical performance caused by minimal carbide formation, in the gas phase as well as on the platform and tube surface (b) Minimal analyte diffusion into the graphite

Range of decreasing tube performance terminating analytical tube lifetime and finally total tube lifetime by tube cracking and breakage (a) Loss of sensitivity (a) Rise in closed porosity and increase in R.S.D. due to hysteresis of End of analytical thermal expansion tube lifetime if (b) Rise of surface roughness R.S.D. exceeds 2 due to corrosion to 5% (relative) (c) Rise of mass loss (d) Rise of wall permeability due to thinning or complete destruction of PC-coating

Te) to the observed appearance temperatures of approximately 1900, 1500 and 1600 8C, respectively. Activation energies of diffusion in liquids range from 1 to 30 kJymol only w29x. Contrary,

Loss of sensitivity and reproducibility caused by: (a) Rising tendency to carbide formation in the gas phase (b) Rising carbide formation on the corroded platform and tube surface (c) Pronounced analyte diffusion into the graphite with subsequent carbide formation and subsurface intercalation

activation energies, e.g. for As and Se with the conventional Pd–Mg-modifier were determined to 1300 kJymol and 500 kJymol, respectively, w29x.

H.M. Ortner et al. / Spectrochimica Acta Part B 57 (2002) 1835–1853

3.2. Fiction 2 3.2.1. Analyte stabilization by formation of intermetallic compounds or thermally stable alloys ● In the review by Volynsky w1x, a whole chapter is dedicated to the ‘Application of binary phase diagrams for clarification of the mode of action of platinum group metals (PGM)—modifiers’, stating that binary phase diagrams are a source of important information concerning stabilization of analytes by means of metallic modifiers. This is unfortunately a typical ‘fiction’ for the following reasons: ● The modifier to analyte mass ratio amounts to 1000:1–100 000:1 in all practical cases. On the other hand, all intermetallic compounds or thermally stable alloys are only formed at mass ratios between 1:1–100:1 w52x. Such compounds cannot, therefore, be formed in ETAAS, contrary to the findings of some authors who unfortunately used unrealistic mass ratios between analytes and modifiers andyor between analytes plus modifiers and the graphite tube w53,54x. ● Volynsky stated w1x, ‘phase diagrams cannot be used for an exact description of the characteristics of nanogram quantities of analytes as the amounts are too small to create a phase.’ This is unprobable since the nanogram range and even the picogram range are far above the mass of single atoms or atom clusters (10y22 –10y20 g) where phases would not any more exist. However, the only realistic possibility is the formation of solid solutions of analytes with the modifier present in the metallic form. Of course, grain boundary aggregations or incoherent precipitates could also be formed at low analyte solubility in the metal and in the solid state. There again, however, outward diffusion would occur at pyrolysis temperature and above unless some strong bonds are formed between analyte and the activated metal as described above. 3.2.2. Antithesis: on the atomization mechanism of As, Se, Te—a demonstration of the complexity of such investigations Contrary to a mere speculation on the stability of intermetallic compounds it should now be dem-

1847

Table 3 Appearance temperatures and activation energies (Ea ) for As, Se, Te, measured according to the method proposed by R. Sturgeon in presence of the PdyMg-modifier w55x Element Temperature w8Cx

Appearance wkJymolx

Ea

As Se Te

1900 1500 1600

1300 500 250

onstrated how one can arrive at some experimentally based conclusions on respective atomization mechanisms. This is a tedious process, which also requires special instrumentation not readily available in laboratories dedicated to GFAAS work. The temperature dependence of the rate of atomization and the appearance temperature of the atomization step are influenced by the compound into which the analyte is transformed during pyrolysis. The appearance temperature is defined as the temperature at which the analyte becomes detectable in the atomization step. Sturgeon et al. w55x developed a method for the determination of the temperature dependence of the rate of atom formation and a physical model, which allows conclusions concerning the compound of the analyte before the atomization. In presence of the PdyMgmodifier higher pyrolysis temperatures are feasible for easily volatile elements w56–58x. It is therefore, most likely that the analyte is transferred into a more stable compound. Table 3 shows the appearance temperatures and activation energies which were measured by U. Rohr w29x and which are quite high. The interpretation of these data in terms of compounds, which are atomized, is difficult. Literature data for energies of some selected processes for the atomization of As, Se, Te point towards a combined effect of analyte retention by strong bonding and hindered diffusion, Table 4. Some compounds can be excluded as candidate compounds for the given system: ● Although the enthalpies of formation of the oxides would agree well with the experimentally determined activation energies, the high volatility of these compounds would lead to essentially lower appearance temperatures.

H.M. Ortner et al. / Spectrochimica Acta Part B 57 (2002) 1835–1853

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Table 4 Literature data for energies of some selected processes for the atomization of As, Se, Te Type of process energy

Compound

Value in wkJymolx

Reference

Enthalpy Enthalpy Enthalpy Enthalpy Enthalpy Enthalpy Enthalpy Enthalpy Enthalpy

As2O3 SeO2 TeO2 As Se Te PdSe PdAs PdTe

y654.4 y240.6 y325 32.43 19.33 50.6 y50.24 y24.3 y37.7

Chemiker Kalender Chemiker Kalender Chemiker Kalender Chemiker Kalender Chemiker Kalender Chemiker Kalender B.W. Mountain et al. H. Isper et al. O. Kubashevsky

w59x w59x w59x w59x w59x w59x w60x w61x w62x

in metals in oxides, e.g. S in NiO in liquids

60–300 100–600 370 4–25

R. Freer, J. Mat. Sci. R. Freer, J. Mat. Sci. R. Freer, J. Mat. Sci. ¨ Landolt-Bornstein; Jost

w63x w63x w63x w64,65x

of of of of of of of of of

formation formation formation formation formation formation formation formation formation

Activation energy of diffusion Activation energy of diffusion Activation energy of diffusion

Enthalpy-values are tabulated for 298 K. Correction for the respective appearance temperatures was not possible (no data available). However, corrections would be insignificant (several kJ per 1000 K)

● If the evaporation of the elements would be the process of atom formation, the activation energy should be approximately equivalent to the enthalpy of formation because the activation energy of diffusion in the gas phase is low. Since the enthalpies of formation are quite low, this is not the way the atomization of As, Se, Te is taking place. ● The route over the intermetallic compounds can not be considered due to the concentration ratios of the modifier with the analytes as discussed above. In addition, the respective enthalpies of formation are also too low. ● The high values for the activation energy of the atomization point into the direction of a thermodynamically controlled as well as a diffusion controlled process: the analyte-modifier complex dissociated and the diffusion of the analyte out of the graphite into the gas phase is the process determining the reaction velocity. There is a fair agreement between the determined activation energies and the activation energies for the diffusion in metals and oxides. Only by the existence of a rather stable Pd-analyte compound (with DH of formation around 100 kJy mole) and a fairly high activation energy of diffusion it is possible to understand the high values of the activation energies listed in Table 4. The example for the activation energy of

diffusion of S in NiO was included in Table 4 in analogy to the diffusion of Se in PdO. It can be expected that elements with a rather large ionic radius as compared to the oxygen ion and which form stable compounds with the metal of the crystal lattice of the matrix will exhibit a high activation energy of diffusion. 3.3. Fiction 3 3.3.1. Experiments with unrealistic concentrations of analyte andyor modifier All such experiments result in misleading conclusions. This should be clear from the previous discussion. In the three component system pyrographite coating–modifier–analyte completely different situations will occur if relative concentrations andy or subsurface conditions are changed. Therefore, even experiments with a realistic mass ratio of analyte to modifier but with an unrealistic relation of the applied masses to the mass of the graphite tube system will lead to erroneous results. 3.4. Fiction 4 3.4.1. Dense coatings are advantageous in GFAAS ● All coating procedures which lead to at least initially dense and homogeneous layers in

H.M. Ortner et al. / Spectrochimica Acta Part B 57 (2002) 1835–1853

graphite tube systems are not meaningful. Due to our findings, they block the active subsurface zone of the PG-layer responsible for analyte retention w29,42x. ● All impregnation—or electrolysis procedures— do not lead to such dense layers but to subsurface penetration of the applied modifiers via the open porosity of the PG-coating. Good modifiers appear to be those which do not penetrate too quickly very deep. They concentrate the analyte in the first 10 mm below the surface and do not allow the analyte to be trapped too deep below the surface w3,30,48,49x. ● Incidentally, impregnation and electrolysis are much cheaper and experimentally easier practiced than coating by CVD- or even PVDprocesses. They usually yield longer stability of analytical parameters in routine practice w3,30,48x. ● Furthermore, there is no CVD- or PVD-coating which would not dramatically deteriorate during tube lifetime due to corrosive attack and different thermal expansion of graphite and coating. Hence, tube system performance will change drastically with increasing penetrability of the originally dense layer. Finally, dense layers change the electrical resistance and the emissivity of the tube system. Hence, temperature adjustment by resistance will lead to different temperatures than for mere graphite tube systems for which the calibration has been carried out w10,11,33x. 3.5. Fiction 5 3.5.1. The oblivion of carbide formation in GFAAS In spite of the significance of carbide formation discussed under FACT 1 this important phenomenon remains largely unconsidered in many important studies on phenomena occurring in GFAAS w5,28x. Carbide formation has been experimentally verified by precision peak shift measurements of the C Ka-line with the electron microprobe for Fe and La w18,29x. This is not easy to measure since the much higher peak for the C Ka of the pyrographite is only slightly influenced by the C Ka peak of carbides present especially in surface near regions of the pyrographite. These regions can be

1849

easily located by Fe Ka and La La measurements prior to the C Ka precision measurements with the method of tangents w29x. Respective details will be published in a subsequent paper. It should be noted, however, that the significance of carbide formation in GFAAS has been extensively treated w19,20x by Volynsky and our group w3,6,10,11,29,33,48,66x. 3.6. Fiction 6 3.6.1. The dream of simple approaches to GFAAS atomization mechanisms instead of the use of highly instrumental techniques It has been propagated by B. L’vov, one of the most prominent researchers in GFAAS that ‘‘«... in many respects this approach (of dissociative chemisorption and evaporation) seems to be more preferable (being more efficient and simple) in comparison with different instrumental techniques used nowadays for this purpose: X-ray diffraction, X-ray photoelectron spectroscopy, Rutherford backscattering (spectrometry), «.’’ w28x.

Unfortunately this concept may work for some special cases but it cannot be generalized as an ultimate solution to the elucidation of all atomization mechanisms in GFAAS. We hope to have demonstrated how complex such situations can be and most of these findings are based on highly instrumental investigations, e.g. by line shift precision measurements in electron probe microanalysis (EPMA). Most of the things we know in this field of high temperature chemistry on and in graphite stems from investigations with above mentioned methods and this is the only safe way of collecting such information w18,29–31,33– 35,40,42,43,46,47,50x. As already pointed out, the eventual development of a concept of what happens in GFAAS work is not only a satisfaction of the ever existing strive of mankind for knowledge but an essential help for optimization procedures leading to reliable methods in quality control in industry and many other important institutions in life sciences and elsewhere. 4. Conclusion 4.1. Functional schedule for GFAAS work As a result of the presented discussion a basic concept for the sequence of processes leading to

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thermal stabilization of analytes by the application of modifiers could become visible through the fog of right and wrong interpretations of a vast amount of experimental data. Most of the phenomena which are outlined in the following are, however, based on own experimental work: 4.1.1. Sample application and drying step ● Penetration of analyte (and modifier in case of non-permanent modifiers) into the first 10 mm of the PG-layer of the platform (or tube wall). ● Formation of particles containing modifier and analyte on the surface mainly in the valleys between the dimples of the PG-coating. ● Partial decomposition of nitrates to respective oxides. However, Mg(NO3)2 which is present remains unchanged. ● PGM-modifiers are partially intercalated as oxides and form mixed oxides with analytes. Permanent RM-modifiers which have formed carbides already in the tube pretreatment stage are oxidized on their surface and also form mixed oxides or heteropolyacid-compounds with analytes which form oxoanions in aqueous solutions. ● The retention of analytes from their hydrides is very effective for many analytes by the presence of permanent PGM-modifiers. This is again not an effect of the modifier present in particles on the surface but of activated modifier atoms in the near surface region. Activation is effected by modifier intercalation. Presumably ferrocenelike compounds are formed with analytes also after catalytic hydride decomposition. 4.1.2. Pyrolysis ● PGM-modifiers and other oxides are reduced to the elemental state. A larger percentage of PGmetals is intercalated as compared to the drying stage, if the modifier is applied together with the sample. ● Stable oxide bronzes or heteropoly-compounds are stabilizing analytes which form oxoanions on the surface of carbide forming modifiers. ● In no case a dense layer of a metal or a carbide is formed. ● The analyte usually remains just below the

graphite surface, i.e. its depth distribution stays constant. ● Metal particles with single mm diameters or less on the graphite surface cannot retain analytes to observed appearance temperatures. Molten metal particles cannot at all retain analytes due to enhanced out-diffusion. ● Neither (binary) alloys nor intermetallic compounds can be formed due to the great modifier excess over analyte quantities. Only the formation of solid solutions is possible. 4.1.3. Atomization ● Dissociation of the covalent analyte-metalbonds, out-diffusion of analyte (and modifier followed by partial evaporation, if it exhibits a rather low melting point like Pd) and thus transfer into the gas phase. ● Some analytes like Pb are retained too strongly by PGM-modifiers. The result is a pronounced memory effect. ● Similar effects are known for, e.g. Ti, Zr, Cr, Mo, V which form mixed carbides with RMmodifiers of great thermal stability. These mixed carbides are non-stoichiometric compounds which can be formed at any mass ratio of the components. ● RM-modifier oxo-compounds with analytes are reduced under CO-formation. The evolving CO carries the also formed analyte atom vapors into the gas phase. ● The combined action of the upper layers of PG together with the modifier seems to be operative for most modifier actions on analytes. This is the reason why dense coatings are not advantageous. If they are formed, e.g. by PVDmethods, they are not stable for the analytical tube system lifetime. 5. Nomenclature AAS: Atomic absorption spectrometry AFM: Atomic force microscopy CVD: Chemical vapor deposition EG: (Polycrystalline) electrographite EPMA: Electron probe microanalysis ETAAS:Electrothermal atomic absorption spectrometry

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ETV-ICP-MS: Electrothermal vaporizationinductively coupled plasma-mass spectrometry GFAAS: Graphite furnace atomic absorption spectrometry LHGA: Longitudinally heated graphite atomizer PG: Pyrographite PGM: Platinum group metal(s) PSE: Periodic system of the elements PVD: Physical vapor deposition RM: Refractory metal (with a melting point above 2000 8C) RSD: Relative standard deviation SEM: Scanning electron microscopy SS-GFAAS: Solid sampling-graphite furnace atomic absorption spectrometry STM: Scanning tunneling microscopy THGA: Transversally heated graphite atomizer Melting point TM: XPS: X-ray induced photoelectron spectrometry

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w51x

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w53x

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w55x

w56x

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