Scanning electron microscopy studies on surfaces from electrothermal atomic absorption spectrometry-IV. Total pyrolytic graphite tubes

05848547/L79.503.00+ Alo

Spectrochrmlelr ActoVol 44B,No II, pp 1125-1161, 1989 Printed inGnatBritain.

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Press

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Scanning electron microscopy studies on surfaces from electrothermal atomic absorption spectrometry-IV. Total pyrolytic graphite tubes BERNHARD WELZ*

and

GERHARD SCHLEMMER

Department of Applied Research, Bodenseewerk Perkin-Elmer GmbH, D-7770 Uberlingen, Federal Republic of Germany

HUGO M.

ORTNER

and

WILHELM BIRZER

Metallwerk Plansee GmbH, A-6600 Reutte, Austria (Receioed 2 February 1989; in revised form 16 May 1989)

Ahs~ract-Morphological studies on graphite surfaces by scanning electron microscopy are presented for tubes made from total pyrolytic graphite (TPG). The inner surface of TPG tubes was found to be fairly rough due to adhering residues of the polycrystalline electrographite substrate on which they were deposited. These residues were vaporized on subsequent dry heating of the tube and recondensed on the tube surface in the form of nodules and flakes. The residues disappeared entirely when the tube was heated with an analyte solution. All experiments were continued for about 550 atomization cycles, and all tubes appeared to be in good condition at that point. The sensitivity for vanadium in dilute nitric acid and for molybdenum in iron, however, had decreased by about 50% during the experiment. The signal for phosphorus in the presence of lanthanum as the modifier disappeared almost entirely and that for copper in perchloric acid was very erratic throughout the experiment. However, it was not always possible to correlate a change in analytical performance with a visible corrosion pattern. The absence of any secondary carbon coating on the inner surface of the TPG tubes was considered to be the main reason for the inconsistency of analytical signals. Primary faults caused by irregular crystal growth were found to be the most likely locations for corrosive attack. Delamination, exfoliation and pitting were the types of corrosion found in all tubes, the extent, however, was dependent on the matrix used in a particular experiment.

1.

INTRODUCTION

THE DEMANDS on

tubes for electrothermal atomic absorption spectrometry (ETAAS) are high with respect to lifetime and resistivity to attack by the sample matrix. On the other hand it is well known since the pioneering work of L’vov [l] that the tube material can have a substantial influence on the determination of an element either by direct interaction or via its influence on the composition of the gas phase. Polycrystalline electrographite (EG) was the most widely used material for manufacturing graphite tubes because it is easily machinable and has a number of favorable properties. Using radiotracers L’vov and KHARTSYZOV [2] could demonstrate that, depending on the temperature, significant amounts of metal atoms may diffuse through the wall of an EG tube. L’vov found in later work that tubes made of pyrolytic graphite (PG), or coated with a layer of this material had a much lower porosity. CLYBURN et al. [3] suggested an in situ PG coating of EG tubes by conducting a mixture of methane in argon through the graphite tube at 2000°C. Similar procedures are now used for mass-production of PG coated EG tubes [4]. In earlier years, several authors reported that the properties of EG tubes and PG coating may vary substantially for different suppliers and even for different production batches [S, 61. In the past few years, however, the quality of tubes and coatings have improved significantly [7,8] so that a long lifetime and a reproducible performance could be expected, particularly when the sample is deposited on and vaporized from a platform in the graphite tube [9, lo]. Instead of EG coated with a layer of PG, L’vov proposed the use of total pyrolytic graphite (TPG) as a material for tubes and particularly for platforms [ll, 121. LITTLEJOHNet al. [13, 141, DYMOTT et al. [ 153 and DELOOS-VOLLEBREGT et al. [ 163 investigated tubes made of this material. TPG has a number of properties desirable in ET AAS, such as low gas permeability and high chemical resistance. It is therefore not surprising that attempts have been made to

*Author to whom correspondence should be addressed. 1125

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produce tubes made entirely of this material. Comparison of tensile strength-tormass ratios for a number of materials showed that that of PG is the highest over the entire temperature range up to 3OOO”C,and 5-10 times greater than conventional EG [15, 171. Thus higher heating rates could be achieved by reducing the mass of the tube. To manufacture TPG tubes by vapor deposition the “hot-wall” technique is employed [ 183 where a substrate is heated by a surrounding element while a feed gas flows through the enclosure. The product is known as continuously nucleated pyrolytic graphite. After deposition the enclosure is cooled at a controlled rate to minimize stresses. The deposited material then detaches itself from the pure electrographite substrate due to the difference in coefficients of expansion. Some of the authors who investigated TPG found that this material may be the most beneficial alternative to EG for tube fabrication [ 13-15, 191. One of the reasons for the high hopes associated with TPG, besides the low permeability and the high heating rates, was that “although gradual evaporation of graphite is expected during use, the totally pyrolytic nature of the atomization surface will be consistent throughout the lifetime of the tube” [13]. DE LOOS-VOLLEBREGT et al. [16], however, in a recent study came to the conclusion that, while atomization signals were sharper due to the faster heating rate, the surface of TPG tubes is at best similar to the surface of a PG coated tube, and lifetimes are about the same for both tubes. This contribution is a continuation of a series of reports in which the surfaces of materials used in ET AAS were investigated systematically using scanning electron microscopy (SEM) in order to obtain an insight into their peculiarities [6,8-10,20-221. Endurance experiments were carried out in order to investigate changes in the surface morphology resulting from the thermal stress of repeated heating to high temperatures with and without an added analyte solution. Also investigated was the additional corrosion caused by high concentrations of perchloric acid, iron and lanthanum. An attempt was also made to correlate the changes in morphology with alterations in the analytical signal. 2. EXPERIMENTAL SEM micrographs were obtained with a Jeol JSM-35 CF scanning electron microscope capable of actual magnifications from 10 to 50000 x . In the dark region below the pictures, the following information is displayed from left to right: accelerating voltage in kV, magnification, SEM micrograph number, scale of picture in pm, month and year in which the micrograph was obtained. In some cases topochemical analyses were carried out with the associated energy dispersive X-ray fluorescence analysis system Model 3000 (Princeton Gamma Tech) with Si(Li) detector and a resolution of 150 eV. These are referred to as ED XRF analyses. Endurance and corrosion experiments were carried out using a Perkin-Elmer HGA-500 graphite furnace and programmer in a Perkin-Elmer Model 5000 atomic absorption spectrometer and using a Perkin-Elmer HGA-600 Zeeman graphite furnace in a Perkin-Elmer Zeeman/3030 atomic absorption spectrometer. All temperatures above 900°C were measured with a Keller DP 05 AF 2 disappearing filament pyrometer and an Ircon Modline II i.r. radiation pyrometer at a wavelength of approx. 0.9 pm. The pyrometer was equipped with P-3 optics with a viewing area cross-section of 0.4-0.6mm. The total pyrolytic graphite (TPG) tubes were produced by Ringsdorff-Werke GmbH, Bonn-Bad Godesberg, FRG. A typical temperature program used for the endurance experiments is given in Table 1. Somewhat different temperatures and/or times were used for some of the experiments. Table 1. Typical temperature program for endurance experiments Step Temperature (“C) Ramp time* (s) Hold time (s) Int flow? (ml min-‘)

1

2

3

4

120 20 20 300

1100 10 20 300

2650 0 8 0

2650 1 5 300

* A ramp time of OS means heating with maximum power, corresponding to about 1500”cs-1. t Int flow is argon purge gas flow through the tube. The external argon flow around the tube is permanently turned on.

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All reagents used were of analytical reagent grade or of better quality. Standard solutions were prepared by dilution from stock solutions containing lOOOmgl-’ of the analyte (Merck Titrisol).

3. RESULTSAND

DISCUSSION

As in previous work [&--lo] experiments were carried out under a given set of conditions typically over 500400 atomization cycles. The surface of the inner and outer tube wall was then inspected using SEM. In most experiments the atomization pulses for an analyte element were also recorded and the peak absorbance and integrated absorbance values printed throughout the experiment in order to correlate surface changes with analytical performance. Among the experiments chosen in this work were “dry heating” conditions, determination of a refractory element, such as vanadium, in dilute nitric acid, and application of corrosive and oxidizing matrices, such as iron, lanthanum and perchloric acid. The “dry heating” experiments, in which tubes were heated repeatedly to high temperatures without analyte or solution, were carried out in order to investigate the influence of the thermal stress and possibly of impurities in the purge gas on tube surface and lifetime. The vanadium test was carried out with the same temperature program as the dry heating experiment in order to see if the dilute nitric acid had an influence on tube surface and to find out if morphological changes had an influence on the analyte signal. Perchloric acid with its high oxidation potential is undoubtedly one of the most corrosive acids. This is one of the reasons why it is frequently used for acid digestion of biological and other organic materials. There are several reports in the literature that perchloric acid destroyed graphite tubes almost instantaneously and this may well be true for uncoated EG tubes. PG coated tubes, however, were found to exhibit a surprisingly long lifetime in the presence of this matrix, particularly when the sample was deposited on a PG platform [9]. It was therefore of particular interest to see if the TPG tubes would perform even better in the presence of perchloric acid. Ten microliters of a 5% (w/v) perchloric acid were used and copper (0.5 ng Cu) was chosen as the indicator element. The temperature program given in Table 1 was used with the exception that the pyrolysis temperature was lowered to 1000°C and the atomization temperature to 2300°C. The time for atomization was reduced to 3 s but the clean step was maintained as listed. Iron is encountered at high concentrations when metallurgical samples, such as iron or steel, are to be analyzed. Iron was found to have a vey corrosive effect on EG tubes and also on PG coated tubes when the sample was deposited on and vaporized from the tube wall [8] as well as from a PG platform [9]. The experiments were carried out with 20 pg iron in 10 ~1dilute nitric acid, corresponding to a concentration of 2gl-’ Fe. Molybdenum was chosen as the analyte element in this case in order to also increase the thermal load. The temperature program given in Table 1 was used except for the pyrolysis temperature which was increased to 1600°C. Lanthanum was proposed by MACHATA as a matrix modifier for the determination of lead [25,26] and other heavy metals in biological materials [26,27] as early as 1973. MACHATA reported that matrix effects could be avoided by d&.olving biological materials, after dry ashing, in a 1% lanthanum solution. This was confirmed by ANDERSON [28] who found that lanthanum may also increase the sensitivity for lead due to the formation of a protective coating of lanthanum carbide on the graphite tube surface. Lanthanum was also reported to enhance the absorption signal and to eliminate interferences in the determination of beryllium in biological samples [29]. Soaking graphite tubes with lanthanum solution was found to reduce memory effects and to increase the sensitivity for silicon [30], aluminum, beryllium, chromium and manganese [31]. The most widespread application of lanthanum as a modifier for ET AAS was to the determination of phosphorus [32-371. The addition of lanthanum was reported to enhance the sensitivity for phosphorus by up to a factor of six over that obtained with aqueous solutions and eliminated most interferences observed previously. Phosphorus, for its determination at the 213.6~nm line, must not only be atomized but also excited because this line does not originate from the ground state. This is one of the reasons why the addition of higher and higher amounts of lanthanum continually increased the sensitivity. However, it was also found that increasing amounts of lanthanum may have a very detrimental effect on tubes and platforms [lo, 21, 373. Lanthanum with

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phosphorus as the analyte element was therefore considered to be another useful reagent for the testing of tubes. Masses of 20 pg La (10~1 of 2000mg 1-l La) were used in these experiments because it was found to be analytically most useful [37]. The temperature program given in Table 1 was used with the pyrolysis temperature increased to 1350°C and a clean out temperature of 2700°C. The most interesting and puzzling result of all these experiments was that the morphologies of all the tube surfaces after 500-600 heating cycles were similar, i.e. essentially the same changes had occurred and the same phenomena were observed with surprisingly little dependence on the matrix. The analytical signals, in contrast, and their stability over time were strongly dependent on the analyte element and on the matrix so that the changes in sensitivity and signal shape in most cases could not be easily related to specific changes in surface morphology, at least not macroscopically. This was in contrast to PG coated tubes, particularly when PG platforms were used, where changes in morphology could frequently be used to explain the analytical performance characteristics [8-lo]. Several of the changes observed on the tube surfaces were largely independent of the matrices used in the experiments. These morphological phenomena are therefore discussed separately together with their possible sources and their influence on tube lifetime and analytical performance. The discussion of the different experiments will therefore include only those phenomena which were typical for the particular matrix or analytical condition and their influence on the analyte element. It should be mentioned at this point that all of the tubes used in this set of experiments survived the 500-600 heating cycles without breaking. In addition, all tubes appeared to be in good condition on a preliminary macroscopic inspection and may have survived several hundred additional heating cycles before breaking mechanically.

3.1. Unused TPG tubes The inner surface of an unused TPG tube is depicted in Fig. 1 at two magnifications. The difference between this tube surface and that of a PG coated tube [S] was quite obvious. The comparatively rough structure was caused by the EG substrate on which the TPG was deposited. Particularly when the surface was inspected at the higher magnification of Fig. l(b) it appeared that residues of the EG substrate adhered to the inner surface of the TPG tube. The graphite crystal is an allotropic form of carbon. It is formed by sp2 hybrid orbitals. The Q electrons form strong covalent bonds in the plane, the n electrons form weak van der Waals bonds between the planes. Layer stacking is mostly arranged in a hexagonal system. Industrially produced artificial graphite, however, exhibits “disordered” crystals in which the planes are wrinkled, the layer spacings irregular and atom defects (vacancies) present within the planes [4,23]. All the vacancies are locations for attack by oxidizing agents. Aside from internal lattice defects, the edge atoms of the planes are in a “disordered”, imperfect state as- well and exhibit increased reactivity. Edge carbons do react preferentially with oxidizing agents, and the oxidation rate is approximately ten times higher than that for the basal planes [24]. Edge-carbon planes may also permit easier intercalation of various compounds into the graphitic structure. It may well be assumed that not only the residues of the EG substrate which are left on the TPG surface exhibit high reactivity. It can also be anticipated that after the removal of the EG residues by preferential oxidation edge planes and other defects are exposed on the TPG surface which also exhibit reactivity increased compared to basal planes. Edge planes are also exposed in large quantities in any type of machining of the surface, for example when the sample injection hole is drilled into the tube. Another phenomenon which was frequently observed particularly on the outer surface of TPG tubes is depicted in Fig. 2. Wart-like elevations were found all over the outer surface which, on the other hand, appeared to be much smoother than the inside of the tube. The source for these surface defects became apparent from Fig. 2(b), in which a TPG tube was cut with a turning wheel. The micrograph shows a layer anomaly most probably caused by a dust particle which was incorporated during the deposition process, leading to a wart-like elevation on the outside of the tube.

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Fig. 2. Unused TPG tube with primary faults. (a) wart-like elevations on the outside of the tube, (b) cut through wart-like elevation caused by irregular crystal growth.

The sample injection holes of tubes from the dry heating and the vanadium experiment are depicted in Fig. 3. The most obvious phenomenon in both cases was the pronounced formation of carbon nodules in the injection hole. This was not new and has been observed very frequently in PG coated EG tubes [S-lo]. New, however, was that these nodules were found in the sample injection hole only, but nowhere else in the entire tube in any of the experiments. This must therefore be due to the particularities of the TPG material. In uncoated and PG coated EG tubes it was assumed that an increased resistance at the boundary layers between crystalline grains and binder material may lead to local overheating and, hence, volatilization, predominantly of binder ca&on, which recondenses at the tube surface [9,10,22]. A TPG tube essentially consists of concentric layers of basal planes with low reactivity. Within these basal planes the current can flow with little resistance so

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Fig. 3. Nodular carbon deposits around the sample injection hole of TPG tubes. (a) 560 determinations of vanadium; (b) 545 dry heating cycles.

that overheating is unlikely. The only exception are layer anomalies and defects at which overheating may be possible. In general, hotiever, overheating and thus carbon volatilization and redeposition appears to be less likely in a TPG tube compared to EG. Another aspect is that the basal planes of TPG are very dense so that vaporized carbon would have little chance to penetrate through the layers to the tube surface. It can only migrate between the layers until it eventually reaches an opening such as in the machined sample injection hole where it condenses. Yet another aspect is that edge-carbon planes are exposed in the machining process of the sample injection hole and that these are considerably more reactive than the basal plane. They may therefore also be vaporized more easily.

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Fig. 4. Nodular carbon deposits at the sample injection hole of a TFG tuba after 560 determinations of vanadium.

The morphology of the nodular deposits in the sample injection hole, which are depicted in Fig. 4, was essentially identical with that of similar deposits in PG coated EG tubes [8-IO]. It is also very similar to the morphology of deposists observed on the burner of a fuel-rich nitrous oxide-acetylene flame [21,22] which is in support of the proposed vaporphase deposition mechanism. Another phenomenon which was observed in the sample injection holes of TPG tubes from all experiments was pronounced corrosion and delamination. The extent of the attack, however, was somewhat dependent on the matrix as can be seen in Fig. 5. In the tube from the vanadium experiment, which is depicted in Fig. 5(a), delamination is not very pronounced. However, it appears that a substantial amount of carbon was removed from the injection hole on the inner tube surface. This may well be due to the much higher reactivity of edge carbons which had been exposed in large quantities in the machining process. Preferential oxidation of these carbons by the solvent vapors escaping through the injection hole and subsequent vaporization in the form of CO is the most likely mechanism for this type of attack. The sample injection hole of the tube from the lanthanum experiment exhibited a clearly more pronounced corrosion and a different type of attack as is depicted in Fig. 5(b). Severe delamination, layer swelling and irregularities in the layer structure may be found all around the injection hole. The layer separation which was observed in this experiment has been found in a similar form for PG platforms in earlier work [lo] in the presence of high amounts of lanthanum. It has been attributed to the formation of intercalation compounds of the graphite with lanthanum. The formation of such intercalation (inclusion) compounds results in an increase in graphite volume until, finally, the layers “puff up”. It is no surprise that this reaction apparently originated at the sample injection hole where the largest number of reactive edge-carbon planes is exposed. 3.3. Exfoliation and delamination Delamination of PG layers may be most pronounced in the sample injection hole, but it was not at all limited to this area as can be seen in Fig. 6. Depicted in these micrographs is the outer surface of a TPG tube after 545 “dry” heating cycles. Severe exfoliation was observed at the upper and lower tube parts. The round holes originated from primary faults such as the wart-like elevations which were depicted in Fig. 2(a). These layer anomalies may well be

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Fig. 5. Delamination of layers at the sample injection hole of TPG tubes. (a) 560 determinations of vanadium; (b) 550 determinations of phosphorus with lanthanum modifier.

responsible for the observed exfoliation and delamination because edge-carbon planes were exposed at these faults permitting easier attack and weakening the layer. As this exfoliation was also observed in the dry heating experiment it was apparently caused predominantly by the thermal stress of consecutive rapid heating to high temperatures, possibly supported by trace impurities in the purge gas and/or backdiffusion of air. The influence of the thermal stress on the structure of the entire tube becomes clearly apparent from Fig. 7. The cross-section through the tube wall from the vanadium experiment exhibited the irregularity in the layer structure and beginning delamination which was more pronounced at the tube outside than on its inside. While exfoliation was not yet pronounced in this area of the tube it was obvious that, up to a depth of 50-100 pm, the spacing between layers had increased. The original density of the TPG material was existent

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Fig. 6. Exfoliation of outer layers of a TPG tube 545 after dry heating cycles. (a) tube upper (b) lower tube part.

part;

only in the core of the tube over about half of its original thickness. This in-depth alteration of the TPG structure could only be interpreted as a result of the thermal stress. Any oxidative attack would have removed the outer layers and not have penetrated to such a depth leaving the outer graphite layers in place. Another form of delamination which was typically found at the inner surface of TPG tubes from almost all experiments is depicted in Fig. 8 for two different tubes. It was demonstrated in Fig. 7 that the distance between the PG layers was already widened by the thermal stress. Solvent or matrix may then penetrate through cracks and holes generated by pitting in between the layers and finally cause them to puff up. The visible layers are not, of course, the individual graphite layers, the distance between which is orders of magnitude

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Fig. 7. Delamination of TPG tube layers in the tube wall after 560 determinations of vanadium.

smaller. The layers are rather multi-atomic layers which have been generated during crystal growth by chemical vapor deposition (CVD). 3.4. Primary faults It was apparent from Fig. 6 that primary faults which were caused by irregular growth of the PG during the CVD process may be the weak link where attack could start and proceed most rapidly. Two such primary faults are depicted in more detail in Figs 9(a) and (b). Both faults were found at the outer surface of the TPG tube from the dry heating experiment. The wart-like elevation depicted in Fig. 9(a) remained at the tube surface after exfoliation of a thick layer. Apparently there is little cohesion between this irregular growth cone and the basal plane and exfoliation of the next layer may well begin at this point. This poor cohesion became also apparent in Fig. 9(b) where exfoliation had not yet started but layers had clearly separated. It is obvious that edge-carbon planes were exposed in large quantities around this primary fault and that penetration between the layers was greatly facilitated in this area. While the layer separation in Fig. 9(a) was observed after 545 dry heating cycles, i.e. without any chemical attack, it was not surprising to see more pronounced corrosion in the presence of a sample. A typical corrosive attack at the inner surface of a TPG tube from the vanadium experiment which had started at spots of irregular crystal growth, is depicted in Fig. 10. The extent of corrosion, layer separation and exfoliation becomes quite obvious at the higher magnification of Fig. 10(b). The sample could penetrate deep into the TPG material causing cracks in the basal plane which further facilitated corrosion. Primary faults which are caused by irregular crystal growth, typically due to impurities such as dust particles, are likely to be found on the outside of the tube and at its inside. These primary faults were not detectable on the inside of the unused tube (see Fig. 1) mainly because the inner surface of the tube was fairly well covered by residues from the EG substrate. Another reason may be the direction of crystal growth from the inside to the outside of the tube which would make irregular growth less apparent on the inside. This may also explain the frequently observed difference in morphology of primary faults at the tube inside and outside. This is depicted in Fig. 11 for the tube from the perchloric acid test. Corrosion at the tube inside (Fig: lla) was mainly characterized by pitting and the individual pits were of relatively small dimensions but numerous. This is in agreement with previously made observations in the tube from the vanadium experiment (see Fig. 10). The

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Fig. 8. Delamination of layers on the inside of TPG tubes. (a) after 560 detestation of moly~enum in the presenceof iron; (b) tubeend after 550 d~e~inations of copperin 5% perchloric acid.

primary faults at the tube outside (Fig. 1lb), however, are less numerous but larger and with a wart-like appearance, reflecting their direction of growth. Two primary faults are shown in Figs 12(a) and (b) to stress once again that the general pattern of attack is largely determined by the irregularities in crystal growth and the consequent exposure of edge-carbon planes rather than by the matrix which was applied. The difference in oxidative power and corrosion between dilute nitric acid (Fig. 12a) and 5% (w/v) perchloric acid (Fig. 12b) became apparent essentially only as a difference in the surface roughness of the two tubes. Crater formation and layer separation, however, were very similar and independent of the matrix.

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Fig. 9. Primary faults at the outer surface of a TPG tube after 545 dry heating cycles. (a) wart-like elevation after exfoliation of an outer layer; (b) exposure of edge-carbon pfanes around primary fault.

3.5. Dry heating experiment The inner surface of the tube which had gone through 545 dry heating cycles exhibited a pronounced zone structure which is depicted schematically in Fig. 13. Most of the inner surface had a metallic luster except for two small bands with a dull-black, sooty appearance. The central part of the tube had a metallic appearance but was matt, not shiny. SEM inspection of the shiny parts of the tube exhibited the usual corrosion pattern of moderate pitting, cracks and delamination which was already discussed in previous sections. The “sooty” parts, however, were found to be caused by depositions with an unusual morphology which is shown in Fig. 14. Such depositions were observed only in the tube from the dry heating ex~~rn~t. They gave only the background signal on ED XRF analysis which indicates that they are pure carbon. An explanation for these depositions may be that

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Fig. 10. Primary fault at the inner surface of a TPG tube after 560 determinations of vanadium. (a) 100x;(b) 1000x.

they originated from the EG material which was left on the inner surface from the substrate (see Fig. 1). In the absence of any solvent in the dry heating experiment, these EG residues may not have been removed completely but only vaporized in part and recondensed closeby so that only a kind of rearrangement or recrystallization had taken place. In the presence of water, i.e. in any of the other experiments, the carbon would react to form CO and thus be volatilized and removed from the tube without recondensing. The overview of the sooty area, which is shown in Fig. 14(a), exhibited particles of different structure. The spheres, while of significantly smaller dimension, resembled the nodular depositions in the sample injection hole (see Fig. 4) which are caused by deposition of carbon from the vapor phase. In the dry heating experiment the spheres may have been generated by sublimation of residues from the EG substrate and recondensation in a thermodynamically

Fig 12. ~~~~sian of primary faults (irregular growth cones) at the outer surface of TEG tubas. (a) after 560 datarminations d vanadium; (b) after 560 d~te~ination~ of copper in 5% ~r~b~~~~ acid.

deposits, SW&rrs depicted in Fig 3S(a)~were scattered over thkispart of the tube, As expected, this deposit gave only the background signal on ED XRF analysis, Le. was pure carbon. The overall structure resembled that of the deposits in the sample injection hole and, hence, was formed by deposition of carbon from the vapor phase. FarticularIy interesting, however, was the surface rnor~ho~o~ ufthese nodular deposits found on closer Irrspection as is depicted in Fig. tsfb) AU deposits which fracl heen observed untif now on PC_3 coated EG tubes @I> PG platforms f9, HYjand in the sam@e i@ec&on hale of TIW tubes @If had a rather smooth tuberous sutiace (see Fig. 4). The nodules in’the muddle ofthis tube, in contrast, were covered with Baky de~~$~ts which re~rnb~~ those of Fig, lobs. If the nodules depicted in Fig, 15 were formed by vapo~~hase deposition ofcarbon, and of this there is no doubt, the flakes on top

Total pyrolytic graphite tubes

a

b

a

Fig. 13. Macroscopic appearance. of the inner surface of a TPG tube aRer 545 dry heating cycles. (a) sooty, dull black area; (b) dull, metaitic appearance; aII other parts metal& luster.

Tg. 14. Carbon deposits in the sooty, dull black area of a TPG tube after 545 dry heating cycles. (a) noduiar and flaky deposits I000 x ; (b) flaky deposits 1OooOx .

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gig. 15. Carbon deposit in the center of the lower part of a TPG tube after 545 dry heating cycles. (a) nodular deposit; (b) flakes on top of the nodular deposit.

of these deposits must have been formed in the same way. There remains the question: why did carbon deposit in these two different crystalline forms. The most likely explanation is that the partial pressure of gaseous carbon (PC) in the tube was decreasing during the experiment and that a higher pC resulted in nodular deposits while at a lower pC the formation of flaky deposits was favored. A decreasing pC over the lifetime of the tube could be explained by removal and/or recrystallization of the residues of the EG substrate on the TPG surface. 3.6. Vanadium experiment The inner surface of the tube which had been used for 560 determinations of vanadium, as in the dry heating experiment, exhibited a zone structure which is depicted schematically in Fig. 16 . Most of the inner surface had a metallic luster and the central part was again matt. Even the sooty, dull-blank bands were present, only in the form of two pairs of small rings instead of two bands. The microscopic appearance of the tube, however, was in contrast to

Total pyrolytic graphite tubes

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a

b

a

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a

Fig. 16. Macroscopic appearance of the inner surface of a TPG tube after 560 dete~natio~ vanadi~. (a) and (b) see Fig. 13.

of

Fig. 17. Delamination of graphite layers in a TPG tube after 560 determinations of vanadil um. (a) upper tube part, inside; (b) lower tube part, inside.

the macroscopic appearance, No deposits at ail could be found on the inner tube surface, and there were also no residues of the EG substrate left on the surface. The most apparent change on the surface was a scaly delamination which could be observed all over the upper part of the tube (Fig. 17a) and in the lower part where the sample

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had been deposited (Fig. 17b). From these micrographs it becomes obvious that one or more layers of several 10 pm thickness were removed almost entirely from the surface. This must have been due to oxidative attack of the solvent (dilute nitric acid) via the vapor phase because essentially all parts of the tube were equally affected. The surface also exhibited very little variation over the inside of the TPG tube and is depicted in Fig. 18 at two magnifications. It featured moderate pitting and surface roughening. It should be kept in mind, however, that this was already an “underlying” surface and a layer of substantial thickness had been removed in this experiment. While the overall appearance of the TPG tube from this experiment had not changed very much over the 560 atomization cycles and the “totally pyrolytic nature” of the atomization surface was expected to be consistent throughout the lifetime of the tube [ 131, the sensitivity for vanadium had decreased by 50% during the experiment. The superimposed atomization

Fig. 18. Corrosion of the inner tube surface after 560 determinations of vanadium. (a) lower tube part, area of sample deposition; (b) upper tube part.

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pulses Nos 501-510 are depicted in Fig. 19, and it is apparent that the signals were no longer “sharp” and exhibited significant tailing. Peak height reproducibility also had deteriorated to RSD values of around 10%. In general, however, the shape of the atomization pulses was still acceptable and, with more frequent calibration, the tube could be used for more than the 560 vanadium determinations carried out in this experiment.

The inside surface of the TPG tube which had been used for 560 ato~~~on c-y&es for copper in the presence of 5% fw/v) perchloric acid had a dull, metallic appearance7 but no zone structure such as that in the tubes from the previously discussed experiments. On closer inspection a rather uniform corrosion pattern was found in the lower and the upper parts of the inside of the tube. Several deep pits, such as the one depicted in Fig. 20, were found at

Fig, 20. Prmmnced pitting on the inside near the sampIe injection hole of a TPC tube after 560 determinations of copper in 5% per&fork acid. s&mU:,l-F

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BERNHARDWELZ et al.

Fig. 21. Inner surface of a TPG tube after 560 copper determinations in S% perchforic acid. (a) upper tube part near to the sample injection hole; (b) lower tube partin the sampie deposition area.

various locations on the surface, This type of corrosion presumably originated from primary faults caused by impurities on the EG substrate. The entire inner surface of the tube had become very rough, and it was particularly interesting to note that pitting and roughening were clearly more pronounced in the upper tube part (Fig. 21a) than in the lower part where the sample had been deposited (Fig. 21b). Aside from the depth of the pits, there was actually very little difference between the surface of this tube and that used in the vanadium experiment (see Fig. 18b).The attack of perchloric acid was probably a vapor phase oxidation of the graphite surface. A surface, such as that depicted in Fig. 21(a),must be very reactive due to the large number of edge-carbon planes exposed by oxidation. The change in the surface reactivity of the TPG tube was very clearly reflected by the dramatic change of the absorption pulses for copper depicted in Figs 22(a)and (b). Already

Total pyrolytic graphite tubes 0.5

1

1147

a

Signah No. 2f-326

very carry in the experiment it was impossible to obtain reproducible and reliable signals for capper as can be seen for the superimposed absorption pulses Nos, 21-24. Occasional good signals were followed by very trratie peaks, some of which hardly exceeded the &lank signak This situation did not change very much throughuut the entire expetiment as can be seen for the 300 superimposed absorptiun pulses, starting with signal No. 27. Peak shape did no? change systematic&y over the 300 determinations but was very variable from determinatictn to determination. The Integrated absorbance (peak area) was as inconsistent as the peak height, and there was no extended period of time over which the cunsistency of signals was acceptable far analytical work, This was in sharp contrast to the surprisingly good results which have been obtained with a PG platform in a PG coated EG tube in earlier work t9-j. The variability of the copper signal which is apparent from Fig, 22 may have its cause in the variable number of active carbon sites which are exposed in each heating cycle. Corrosion of the TPG tube an8 pittlxlg was even more pronounced near the tube ends as WI be seen in Figs 23(a) and [b). Another phenomenon which was observed in this experiment was a deposition of particles near the tube ends: Large particles with a diameter ofmorethan 10 JSEIwere found in almost all of the deep pits as is depicted in Fig. 23(a). Must of the partiojes, hosever were evenly distributed over the corroded surface and had a diameter of less than 1 pm%as eau be seen in Fig. B(b)_ The density of the particles tended to increase toward the tube end as is depicted in Figs 24(a) and (b)_On ED XRF analysis these

1148

BERNHARDWELZ et al.

Fig. 23. Corrosion and silicon-containing deposits near to the tube end after 560 determinations of copper in 5% perchloric acid. (a) pits with large sikon spheres; (b) small silicon spheres at the other tube end.

particles gave a strong signal for silicon. The origin of these particles was not quite clear because only copper and perchloric acid had been applied in this tube. It is not impossible, however, that the silicon was an impurity in the tube material which had been vaporized during the oxidative attack and corrosion of the surface and deposited at the cool ends of the tube.

~acroscopically the tube which had been used for 560 dete~inations of moly~en~ looked very much like that from the perchloric acid experiment having a similar dull,

Total pyrolytic graphite tubes

Fig. 24. Silicon deposits of Fig. 23(b) at 10000 x ma~ifi~tion. (b) close to the tube end.

1149

(a) further away from the tube end;

metallic appearance. At low magnification the typical corrosion which has been observed on several previous occasions for this material was seen. Delamination of PG layers and pit corrosion could be found over the entire inner tube surface. From Figs 25(a) and (b), obtained at higher magnifications, it is apparent that pitting was pronounced also in areas with no primary faults, and of different morphology as compared to previous experiments. The pitting depicted in Fig 25 was observed in the upper part of the tube and was therefore caused by an attack via the vapor phase. Pa~c~arly from Fig. 25(b) it might appear that, in spite of the pitting, the surface had not been altered very much and the pyrolytic nature of the underlying surface was consistent with that at the beginning of the experiment.

Fig. 25.

pitting of the inner surface, uppr part of a TPG tube after Sdo d~t~n~ti~ rnoly~~n~

of

in iron matrix. (a} 200 x ; fb) 1000 x .

Pitting was clearly more pronounced in the lower tube part where the sample had been deposited as is depicted in Figs 26(a) and (b). This was in contrast to the perchloric acid experiment where attack via the vapor phase had caused more severe pitting The iron matrix, through the formation of intercalation compounds, and/or catalytic oxidation of the graphite, caused more severe in-depth corrosion on direct contact than perchloric acid. Corrosion at the layer boundaries and separation of layers to an extent not observed for the previou~y discussed matrices, became apparent at the high ma~fi~tion of Fig. 26(b). The atomi~tjon pulses for molybdenum depicted in Fig 27, which were recorded at different times during the experiment, indicate that the analytically useful lifetime of the TPC tube may actually be of the order of several h~dred dete~inations. Toward the end of the experiment, i.e. after more than 500 atomization cycles, however, the analytical signal

Total pyrolytic graphite tubes

1lSl

Fig. 26. Pitting of the inner surface, lower part'ofa TPG tube in the sample deposition area after 560 determinations of molybdenum in iron matrix. (a) 600 x ; (b) 3000 x .

for molybdenum became quite erratic. So, although the appearance of the tube was good and it could have been used much longer before breaking, it had reached the end of its analytically useful lifetime. In addition, the good short-term reproducibility of signals 210-217 must not be over-interpreted because there was a sensitivity loss of more than 50% over 400 determinations. Nevertheless the performance of this tube in the presence of such a corrosive matrix was quite satisfactory, particularly considering the extent of the surface corrosion. Again, as in the experiment with perchloric acid, corrosion was most pronounced toward the end of the tube. In addition there was a significant amount of iron deposited in this area, most of it in the form of an irregular layer, part of it, however, was in the form of regular spheres such as those depicted in Fig. 28. These spheres gave a strong signal for iron on ED XRF analysis and may be iron carbide.

BERNHARD WELZ~~ al.

1152

0

t

%! fm

0.2

9

Signals No. 556-560

I

Ttme (s) --+

Fig. 27. Atomization pulses for 1 ng of molybdenum in iron matrix (20 pg Fe); atomization from the wall of a TPG tuba. Signals Nos. 210-217 and 556460 superimposed.

Fig. 28. Deposition of iron (Fe3C?) nodule near to the tube end after 560 determinations molybdenum in iron matrix.

of

Total pyrolytic graphite tubes

1153

Another corrosion pattern which was typically observed near to the tube end is depicted in Figs 29(a) and (b) at two magnifications. The attack may have started with an iron deposition which had caused pitting and disintegration of the PG through the formation of intercalation compounds. The lamellar structure seen in the left part of Figs 29(a) and (b) may then be much more susceptible to oxidative attack and, hence, be removed more easily than the smoother surface in the right part of these figures.

The tube which had been used for 550 dete~inations of phosphorus in the presence of 2Opg lanthanum modifier, in analogy to the other experiments, had a dull, metallic appearance. However, there was a pronounced macroscopic difference between the lower

Fig. 29. Pitting and transition betweentwo layer mo~holo~s near to the tube end, upper part, inside after 560 dete~inations of molybdenum in iron matrix. (a) 480 x ; (b) 1500 x .

BERNHARDWELZ et al.

1154

b

Fig. 30. Macroscopic appearance of a TPG tube after 550 determinations of phosphorus with lanthanum modifier. (a) upper tube part inside; (b) lower tube part inside.

and the upper parts of the inner tube surface as is depicted schematically in Fig 30. In the lower part, the area where the sample had been deposited was clearly different in appearance from the rest of the tube whereas very long cracks were visible in the upper part, extending almost over the entire length of the tube. These cracks in the tube surface, which all started from the sample injection hole, are shown in more detail in Fig. 31. It is obvious from these micrographs that formation of intercalation compounds followed by layer swelling and exfoliation was the reason for the formation of these cracks. It is also obvious that this attack was most pronounced at, and had started from the injection hole where the edge-carbon planes of the layers were exposed to the greatest extent. Lanthanum has been found in earlier work [lo] to cause, due to vapor phase attack, severe pitting at the surface of PG coated tubes. A very similar corrosion pattern became apparent in Fig. 31(b) which also exhibited significant pitting. However, at the high magnification of Fig 32 this pitting appeared much less dramatic than at low magnification and was not much different from the corrosion found in the tube from the vanadium experiment (see Fig. 18b). The inside surface of the lower tube part exhibited the most severe erosion of all experiments in the area where the sample has been deposited on the tube wall. This zone is depicted in part in Fig. 33(a). The transition between the zone of condensed-phase and vapor-phase attack was very distinct, as was the difference in surface morphology shown in Fig 33(b). The graphite layers were actually disintegrated in the area where they were in direct contact with the sample solution, whereas “only” severe pitting was observed in the other parts of the tube. The zone where the sample had been deposited gave a strong lanthanum signal on ED XRF analysis indicating that large amounts of the modifier were left in the tube even after the atomization and clean stages. This has also been observed in previous work [ 10,373 using PG platforms. It may be evidence for the formation of a strong intercalation compound between lanthanum and graphite which then caused the severe erosion. The close-up (Fig. 34a) shows that the disintegrated area exhibited little similarity to a TPG surface and demonstrates clearly the degree of devastation caused by the lanthanum. Another revealing detail of the corrosion can be seen in the upper right part of Fig. 33(a) where the disintegrated graphite material was apparently removed in a spot of approx. 1 mm diameter. A detail of the transition from the eroded part to the revealed underlaying surface is depicted in Fig. 34(b). The difference in the morphologies of the two layers was striking and seemed to support once again the assumption of LITTLEJOHNet al. [13] that “although

Total pyrolytic graphite tubes

1155

Fig. 31. Injection hole area of a TPG tube from the inside after 550 determinations of phosphorus with lanthanum modifier. (a) injection hole with cracks; (b) detail of (a).

gradual evaporation of graphite is expected during use, the totally pyrolytic nature of the atomization surface will be consistent throughout the lifetime of the tube.” The analytical signals for phosphorus recorded during the experiment, however, did not at all support the high expectations for this material. The shape of the phosphorus signal in a TPG tube changed quite dramatically already during the first 30 determinations as can be seen in Fig. 35. This was in contrast to platform atomization in a PG coated tube where very few changes were observed Over the first 300 determinations [lo]. After only some 50 determinations the phosphorus signal in the TPG tube was rather odd-shaped and degraded more and more with the number of firings. Actually this tube never met the requirements for satisfactory analytical performance at any point during its lifetime. This means that the disintegration and devastation of the surface had started early in the experiment and also influenced the atomization pulse for phosphorus from the very beginning

1156

BERNHARD WELZ et al.

Fig. 32. Upper part of a TPG tube inside near to the injection hole after 550 determinations of phosphorus with lanthanum modifier at high magnification.

As for previous experiments, corrosion was also very pronounced near the tube ends (Fig. 36), and substantial deposition of lanthanum was found on ED XRF analysis in this part of the tube. Lanthanum, much like iron, had apparently recondensed at the cooler tube ends and caused the massive corrosion, most probably by forming intercalation compounds with the graphite, and/or by catalytic oxidation of the graphite. Another phenomenon observed in the tube from this experiment in several places between the area where the sample was deposited and the corroded tube end was the formation of blisters such as those depicted in Figs 37(a) and (b). Shnilar blisters had already been observed in previous work [lo] with phosphorus and lanthanum in PG platforms. These blisters gave only the background signal on ED XRF analysis and were apparently pure carbon. It was apparent, however, that these blisters were a result of the corrosion because each was found to be imbedded in a large pit in the upper PG layer. They appeared to be hollow shells which may have formed during the pyrolysis stage and burst during atomization and clean stages to release their content. While we certainly do not have enough evidence to propose a mechanism for the formation of these blisters it should be mentioned that lanthanum forms a carbide and that hydrogen and short-chain hydrocarbons are formed on hydrolysis,of this carbide with water [39]. L’vov [40] found that the formation of hollow carbon shells was in accordance with the explanation of the reduced vaporization rate of many elements in graphite tubes because of the formation of gaseous carbon containing compounds. 4.

CONCLUSION

The only experiment in which a correlation appeared to become evident between visible corrosion of the TPG surface and degradation of the analytical ‘signal was the determination of phosphorus in the presence of lanthanum as the modifier. In the other experiments the connection between visible change in tube surface and alteration of the absorption pulse of the analyte element was less obvious. The very pronounced pitting caused by the iron matrix in the area where the sample was deposited resulted only in a slow and moderate degradation of molybdenum sensitivity. Essentially the same gradual sensitivity loss was also observed for vanadium in dilute nitric acid, a matrix which caused only little corrosion. Perchloric acid, on the other hand, which also caused little apparent corrosion in the area of

Total pyrolytic graphite tubes

1157

Fig. 33. Area of sample deposition in the tower part of a TPG tube after 550 dete~inations of phosphorus with l~than~ modifier. (a) overview over sample deposition area; (b) detail of transition to sample deposition area.

the tube where the sample was pipetted, had a very dramatic effect on the analytical signal for copper, which was very erratic from the beginning and throughout the entire experiment. For this matrix pitting was much more pronounced on tube surfaces which had come into contact with perchloric acid vapors only. These dramatic changes in the analytical signal were in contrast to the good short- and long-term reproducibility of atomization pulses obtained under comparable conditions using PG platforms and PG coated EG tubes [9, IO, 121. We believe that the main reason for this striking difference in the analytical behavior was the absence of any secondary coating in TPG tubes. Secondary deposits of carbon, p~ferentially in the form of nodules or of dense surface layers, was found to be very pronounced in PG coated EG tubes and on PG platforms

1158

BERNHARDWELZ et al.

Fig. 34. Area of sample deposition after 550 determinations of phosphorus with lanthanum modifier. (a) close-up of the eroded graphite surface; (b) transition from an eroded layer to the underlying surface.

inserted in these tubes. The mechanism is believed to be a vapor phase deposition of carbon vaporized from overheated areas within the EG microstructure. Particularly the surface of PG platforms was found to be “sealed” this way in the course of repeated heating to high temperatures. This apparently resulted in kind of a “self-healing” process which made this tube-platform combination very little susceptible to matrix attack and hence to changes in atomization characteristics. No such deposition of carbon was observed on the inner surface of TPG tubes in any of the experiments except when the tube was heated without any solvent and analyte. This is in part due to the much higher crystalline order which makes overheating effects and, hence, volatilization of carbon much less likely. The absence of any secondary coating is in part also

1159

Total pyrolytic graphite tubes 0.3

0.2

0.1

t

0 4

0

0.2

I

b si

No. 55-104

0.1

0 0

4

TiifSl

--k

Fig. 35. Atomization pulses for 200 ng of phosphorus with 2Ofig lanthanum mod&r; superimposed signals 5-34 and 55-104.

Fig. 36. Corrosion and lanthanum deposits near to the tube end, inside, after 550 determinations

phosphor

with lanthanum modifier.

1160

BERNHARD WELX et al.

Fig. 37. Blister formation in the lower part of a TPG tube, inside, after 550 determinations of phosphorus with lanthanum modifier. (a) near to the sample deposition zone in the less corroded area; (b) near to the tube end.

due to the very dense layer structure of TPG which makes penetration of any vaporized carbon to the tube surface very difficult except in areas where the layers are open such as in the sample injection hole. Absence of secondary carbon coating in TPG tubes apparently leads to an unhindered propagation of oxidation and corrosion. Edge carbons, once exposed, are very susceptible to oxidation, leaving other reactive carbon sites after their volatilization as CO. Edge-carbon planes were found to be exposed in large number at primary faults on the tube surface which had typically been caused by irregular crystal growth due to impurities on the EG substrate on which the TPG was deposited by CVD. Such primary faults were found all over the outer and inner surfaces of all TPG tubes investigated.

Total pyrolytic graphite tubes

1161

The layer structure of the TPG tubes became apparent also in the corrosion pattern. Delamination and exfoliation of graphite layers was a frequently observed phenomenon in almost all experiments. These layers were of course not the individual graphite planes, which are several orders of magnitude thinner, but multilayers which had formed during crystal growth. It also appeared in several experiments as if corrosive attack would remove only one such layer revealing a comparably untouched underlying TPG layer. This has apparently led others [13] to the assumption that “although gradual evaporation of graphite is expected during use, the totally pyrolytic nature of the atomization surface will be consistent throughout the lifetime of the tube”. It was clearly demonstrated in this work, however, that this assumption does not apply in practice. Acknowledgement-This Wirtschaft”, Vienna.

research was supported in part by the “Forschungsfiirderungsfonds

fiir die gewerbliche

REFERENCES Cl] [Z] [3] [4] [S] [6]

[7] [S] [9] [lo] [ll] [12] [13] [14]

[15] [16] [17]

Cl81 [19] [ZO]

[21] [22] [23] [24] [25] [26] [27] [28]

[29] [30] [31]

[32] [33] [34]

[35] [36] [37] [38] [39]

[40]

B. V. L’vov,Atomic Absorption Spectrochemical Analysis, Adam Hilger, London (1970). B. V. L’vov and A. D. Khartsyzov, Zh. Analit. Khim. 25, 1824 (1970). S. A. Clyburn, T. Kantor and C. Veillon, Anal. Chem. 46,2214 (1974). W. Huettner and C. Busche, Fres. Z. Anal. Chem. 323,674 (1986). W. Slavin and D. C. Manning, Prog. Anal. At. Spectrosc. 5, 243 (1982). H. M. Ortner, H. Krabichler and W. Wegscheider. In Fortschritte in der Atomspektrometrischen Spurenanalytik, Ed. B. Welz. Vol. 1, p. 73. Verlag Chemie, Weinheim (1984). W. Slavin, Anal. Chem. 54,685 A (1982). H. M. Ortner, G. Schlemmer, B. Welz and W. Wegscheider, Spectrochim. Acta 4OB, 959 (1985). B. Welz, G. Schlemmer and H. M. Ortner, Spectrochim. Acta 41B, 567 (1986). B. Welz, A. J. Curtius, G. Schlemmer, H. M. Ortner and W. Birzer, Spectrochim. Acta 41B, 1175 (1986). B. V. L’vov, L. A. Pelieva and A. I. Shamopolsky, Zh. Prikl. Spektrosk. 27,395 (1977). B. V. L’vov, Spectrochim. Acta 33B, 153 (1978). D. Littlejohn, I. Duncan, J. Marshall and J. M. Ottaway, Anal. Chim. Acta 157, 291 (1984). D. Littlejohn, I. S. Duncan, J. B. M. Hendry, J. Marshall and J. M. Ottaway, Spectrochim. Acta 4OB, 1677 (1985). T. C. Dymott, M. P. Wassail and P. J. Whiteside, Analyst 110,467 (1985). M. T. C. de Loos-Vollebregt, M. Bol and L. de Galan, J. Anal. At. Spectrom. 3, 151 (1988). W. H. Smith and D. H. Leeds, Modern Materials, Vol. 7, p. 139. Academic Press, London (1970). B. Lersmacher and W. F. Knippenberg, Philips Tech. Reo. 37, 189 (1977). A. A. Brown and M. Lee, Fres. Z. Anal. Chem. 323,697 (1986). H. M. Ortner, G. Schlemmer and B. Welz, in Colloquium Spectroscopicurn Internationale, Book of Abstracts, Vol. 2, p. 340. (1985). H. M. Ortner, W. Birzer, B. Welz, G. Schlemmer, A. J. Curtius, W. Wegscheider and V. Sychra, Fres. Z. Anal. Chem. 323,681 (1986). B. Welz, G. Schlemmer, H. M. Ortner and W. Wegscheider, Prog. Anal. Spectrosc. 12, 111 (1989). J. C. Bokros, Chemistry and Physics of Carbon, Vol. 9, p. 103. Marcel Dekker, New York (1972). T. Nagaoki and Y. Tominaga, Tanso 40, 19 (1964). G. Machata, Wiener Klin. Wochenschr. 85,216 (1973). G. Machata and R. Binder, Z. Rechtsmed. 73, 29 (1973). G. Machata, Wiener Klin. Wochenschr. 87,484 (1975). A. Andersson, At. Absorpt. Newslett. 15, 71 (1976). J. A. Hurlbut, At. Absorpt. Newslett. 17, 121 (1978). D. B. Lo and G. D. Christian, Can. J. Spectrosc. 22,45 (1977). J. H. Runnels, R. Merryfield and H. B. Fisher, Anal. Chem. 47, 1258 (1975). R. D. Ediger, At. Absorpt. Newslett. 15,145 (1976). R. D. Ediger, A. R. Knott, G. E. Peterson and R. D. Beaty, At. Absorpt. Newslett. 17, 28 (1978). A. PrdvSt and M. Gente-Jauniaux, At. Absorpt. Newslett. 17, 1 (1978). F. J. Slikkerveer, A. A. Braad and P. W. Hendrikse, At. Spectrosc. 1, 30 (1980). B. Welz, U. Voellkopf and Z. Grobenski, Anal. Chim. Acta 136,201 (1982). A. J. Curtius, G. Schlemmer and B. Welz, J. Anal. At. Spectrom. 2, 115 (1987). M. Mentser and S. Ergun, Carbon 5,331 (1967). F. Benesovsky, Ullmanns Encyclopiidie der Technischen Chemie, 4th ed. Vol. 9, p. 122. Verlag Chemie, Weinheim (1975). B. V. L’vov, J. Anal. At. Spectrom. 2, 95 (1987).