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Diffusion of molecular vapors through heated graphite
Spectrochimica Acta, Vol. 50B, No. 8, pp. 763-780, 1995
~
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0584-8547/95 $9.50 + .00
Pergamon 0584--8547(94)00165-0
Diffusion of molecular vapors through heated graphite DMITRY A. KATSKOV*, R. SCHWARZER,P. J. J. MARAIS and R. I. McCRINDLE Department of Chemistry and Physics, Technikon Pretoria, Private Bag X680, Pretoria, South Africa (Received 13 August 1994; accepted 30 November 1994)
Abstract--Thereasons
for the elimination of interferences in a filter furnace used as an atomizer in electrothermal atomic absorption spectroscopy (ETAAS), are examined theoretically and experimentally. The results of observations of stepped or multipeak background absorption signals for NaCI, NaI, KCi, KBr, KI, CaCI2 and MgSO 4, which are characteristic for each matrix and different types of graphite used, leadsto hypotheses about the formation of molecule-graphite intercalation composites. These are treated as stable compounds of different stoichiometry having substantial specificenthalpy of formation. The validity of the idea is confirmed with the measurements of the rate of NaCI vapor release through a graphite filter as a function of temperature, and comparison with the literature data for similar compounds of alkali metals. The effect of implantation of molecular particles into a crystalline lattice leads to reduction of their diffusion rate through graphite, which in turn provides time-resolved background and atomic absorption signals. Further investigations and development of the idea to understand the analytical effect are proposed.
1. INTRODUCTION THE DESIGN and some analytical characteristics of the filter furnace (FF) as a new atomizer for electrothermal atomic absorption spectroscopy ( E T A A S ) , were recently described in the literature [1-3]. The atomizer (Fig. 1) was a tube of pyrocoated graphite of shape and dimensions typically used in analytical practice, fitted with an insert (filter) in the form of a spool made of porous graphite. An amount of graphite fiber was placed in the ring cavity between the walls of the tube and of the filter. The analyzed liquid was sampled through the dosing hole into the ring cavity; then the FF was heated under a preset program. At the atomization step the sample vapors went into the analytical zone ( A Z ) in the central part of the filter through the graphite partition. It was discovered [3] that among the advantages of the FF, c o m p a r e d to conventional instrumentation, was a significant elimination of chemical and spectral interferences in the determination of Cd, Pb and Bi in the presence of NaCl and CuC12, as a matrix. The observations of atomic background absorption signals gave the result that in the FF molecular v a p o r entered the A Z with some delay relative to atomic vapor. That delay led to partial separation of these signals when the elements were determined under excess of matrices. This tendency provided improved freedom from spectral interferences. H o w e v e r , if it was assumed that the delay mentioned was a function of the specific nature of the sample v a p o r and, probably, of the properties of graphite, the effect on the delay of the amount of matrix dosed remained unclear. Understanding of this issue was important because the magnitude of background signal shift was very specifically dependent on matrix quantity. In comparison with the magnitudes of the atomic signals, which were proportional to the sampled dose, background signals attained their limits when the amount of matrix increased. These limits depended only on the FF temperature. The restriction of background absorption magnitude naturally lead to the same positive analytical effect as mentioned above. Determination of the limit for background signal magnitude p r o v o k e d the idea * Author to whom correspondence should be addressed. 763
D. A. KATSKOVet
764
al. 2
0 0 0 0 0
O
3
O
4
Fig. 1. Filter furnace: 1, pyrocoatedgraphite tube; 2, dosinghole; 3, graphite filter; 4, graphite fiber.
about formation of matrix-saturated vapor pressure in the ring cavity of the FF. In accordance with the theory developed in Ref. [4], the saturation effect could be obtained in any semiclosed ET atomizer for nanogram amounts of matrix if a large evaporation surface and a restriction of vapor exit from the vapor reservoir into the A Z was provided. That meant that the effect was not connected with the nature of the graphite and could be obtained for any high temperature porous material used as a filter in the FF. The problem arising from this preliminary discussion is whether or not the heated graphite possesses any specific properties towards molecular vapors in terms of vapor transport or chemical activity, relative to atomic species. The answer is important for physical chemistry and could help the principle of FF action and the limitations of the new method of atomization to be understood.
2. THEORY The present paper develops the theory proposed in Ref. [4]. The assumption is made in Ref. [4] that some amount of analyzed substance (N~ moles) was distributed as a monolayer on the surface of the graphite fiber in the ring cavity of the FF and occupied an area ~r. Transport of the sample vapors through the graphite partition of area s into the A Z of cross-sectional area S and length L was assumed to be driven by diffusion. To simplify the considerations it was assumed that gas diffusion occurred through the thin partition of effective area (1)
s' = sDc/D ~
where Dg and D c are diffusion constants for the sample vapor in argon and graphite, respectively. The FF temperature was considered to be constant or its increase to be insignificant, relative to the rate of evaporation and vapor transport through the graphite partition and the AZ. Under this condition, the process of vapor release could be considered as stationary. Hence, under the condition s' ~ 2S (which for FF was taken for granted) the concentration of particles in the A Z (n) was described by the equation n = n°/[1 + 2S(1/Gr + 1/s')] where n ° was the concentration of particles for substance evaporates without dissociation, then vapor.) Two cases were examined, cr > s' and large amounts of substance in a ring cavity. For n = n°s'/2S
(2)
thermodynamic equilibrium. (If the n o is the concentration of saturated or < s', corresponding to small and the first case (3)
Diffusion of molecular vapors through heated graphite
765
and, for the second case (4)
n = n°cr/2S
It follows from Eqs. (3) and (4) that the vapor concentration in the A Z is a function of sample quantity only for small amounts of sample. For big samples the vapor concentration is determined by the volatility of the substance and (through the parameter s') by the permeability of the graphite partition. The model [4] is examined as follows. If the process of vapor transport through the A Z of effective volume V = 0 . 5 L S is faster than through the filter, then (5)
n = - (,t/V) d N J d t
where ~" L 2 / 8 D g is the residence time for the evaporated particles in the A Z [5]. From Eqs. (1), (3) and (5) the rate of sample release, while cr(t)> s', can be determined as =
(6)
dNJdt = - 2n°sD¢/L
The flux of particles is kept constant. Considering dm to be the effective diameter of the molecule, NA to be the Avogadro constant and No to be the initial amount of substance, then tr(t) -~ d m E N ~ ( t ) N g , and the duration of the period while Eq. (6) is correct can be estimated for the condition tr -- s' 0 = L(No - s'/NAdm2)/2n°sD ~
(7)
Setting Eq. (4) equal to Eq. (5), the differential equation describing the final part of the evaporation process, when or(t) < s', may be obtained d N J dt = - 2n° dm2 N A Dg N,~ = - k N ~
(8)
k = 2n°dm2NADg/L
(9)
where
To solve Eq. (8), the initial magnitude at the moment 0 when cr = s', that is s'/ dm2NA, is then N ~ = ( s ' / d m 2 N A ) exp [ - k ( t - 0 ) ]
(10)
Following Eqs. (3), (5) and (10) and assuming absorbance A to be determined by the sensitivity factor ~t and effective length of the absorbing vapor layer L / 2 , that is A = etnL/2, the description of the absorbance signal can be obtained. A = om%sD¢/V
for t < 0
(11)
A = (om%sOC/V) exp[-k(t-0)]
for t > 0
(12)
Concerning Eqs. (11) and (12), it should be noted that they are related only to the case when condensed phase exists in a ring cavity of the FF. If the amount of substance is too small to obtain equilibrium at the given temperature T, the general assumption about the stationary character of the vapor release cannot be used. Some parameters of Eqs. (11) and (12), which are strongly dependent on temperature should be defined. The concentration of saturated vapor n o or the saturated vapor pressure pO = nOR T ( R is the gas constant) are described using the fundamental thermodynamic parameters AHv and ASv (the enthalpy and entropy of vapor formation). pO = e x p ( A S J R - A H v / R T )
(13)
766
D . A . KATSKOVet al.
In the more general case, which is important for the results and discussion below, evaporation could be accompanied by thermodissociation of the condensed substance. This reaction is characterized by its own parameters AH~, and AS~. Then, for equilibrium concentration of vapor, it is possible to write pO= exp[(ASr+ASv)/R - (AH~+AHv)/RT]
(14)
The diffusion constant D c is described using the expression [6] D c = D~ exp ( - E~d/RT)
(15)
where D~ and E~ are constants. The last parameter is supposed to be the activation energy of diffusion of vapor in graphite. It follows from Eqs. (11), (13) and (15), that the absorbance magnitude depends on temperature as the function In A(t < 0) = -(AHv + E~)/RT + const., if there is no reaction accompanying evaporation. On the contrary, if such a reaction takes place, then In A(t < O) = -(AHr+AHv+E~)/RT + const, and the diffusion constant can be treated as D . . . . D~'r exp -(AHr+E~)/RT
(16)
This means that the vapor release is supposed to be impeded the same way by the reaction either accompanying evaporation in the ring cavity or transport through graphite. Another source of information about the rate of vapor release is the parameter k. In accordance with Eqs. (9) and (12), the measurements of the function k(T -1) from the exponential decay of absorbance signals at some different but constant temperatures, relates to the enthalpy of vapor formation. The method of determination of that parameter was proposed by Fuller [7] later was used for investigations of Ag vapor diffusion through a porous graphite partition [8]. The very close coincidence of experimental and reference data for AHv for the temperature range 1300-1500 K (consequently, 251 and 266 kJ/mol) [8] confirms the validity of the model. This assumption should be analyzed for the case where two different substances resulting from the processes of condensed thermodissociation and evaporation of the same vapor composition are situated in the ring cavity of FF. They have different equilibrium vapor pressure pO and evaporate independently. In this case the magnitude of absorbance will be represented by superimposed signals from each component. The schematic diagrams, exhibiting the total shape of the signals, for two different initial amounts of evaporating substances, No,1 and No,2, with their corresponding pressures pO > pO, are plotted in Fig. 2 (a,b); this reflects two characteristic cases. When No,1 < No,2 the total signal has a stepped shape. When No.l > No.2 the decay of the total signal represents overlapped exponential curves; the "tail" is determined by the slower of the two processes, that is by pO. Naturally, if the initial amounts of substance are not sufficient to satisfy the condition cr > s' only overlapping exponential curves will be observed. In principle, more than two chemical reactions could be involved in the process of vapor release. These might be, for example, the reactions of disproportionation of the initial compound, or evaporation of metal from carbides of different stoichiometry. Each of the processes could be characterized by its own parameter pO and, consequently, the number of steps or overlapping exponents could be observed in the shape of the absorbance signal when the temperature of the FF is kept constant. In Fig. 2 (c) and (d), the schematic diagram for three components of the absorbance signal are displayed for the case when amounts of each substance are different but large enough to neglect the final decay of the signals. The evidence for chemical reactions that occurred in the ring cavity of the FF can also be observed in the shape of the absorbance signal when the temperature is
Diffusion of molecular vapors through heated graphite
(a)
....
767
(b)
2
"'-.
Time
(c)
(d)
L 2
e~
Time
(f)
(e) 1
/
/ I
3
1//'~
r//I
Temperature Fig. 2. Schematic diagram of overlapping absorbance signals (1-3) from the same vapor component of some different condensed substances of initial masses No,l, No,2, No,3, when the temperature of the FF is constant ( a - d ) or is increased (e,f). Sections a, b: two components, consequently No.l < No,2 and No.1 >No./, for both cases pOt > p° 2. Sections c - f : three components, where p°i/p°Jp° 3 = 4/2/1. Sections c, e: NoJNo,JNo,3 = 1/2/3. Sections d, f: No, I~ No,2/No,3 = 2/8/5.
increasing. For large masses of the sample (when it is possible to neglect the decay of the signal) it should be transformed into a multipeak profile. The schematic diagram for this case is represented in Fig. 2 (e) and (f), for the same distribution Noa-No.3, as in Fig. 2 (c) and (d), using the simplified approach of a linear function of evaporation rate with respect to temperature. It follows from the diagram that the distribution of the peak magnitudes depends on the initial amounts of the components. The revealed theoretical fragments show that the shape of absorbance signal in the FF can yield fundamental information about the chemical processes accompanying vapor release. Practical methods to extract the information and its interpretation are discussed below.
768
D. A. KATSKOVet al. 3. EXPERIMENTAL
3.1. Preliminary comments In the experiments [3] it was discovered that the shape of the background signals from NaCI matrix was very different for the FF and the platform furnace (PF). The set of experiments was carried out with the following aims: to highlight the differences mentioned; to examine their variations under experimental conditions; to find the similarity in behavior of NaCI and another matrices; to discover the role of graphite in the formation of the particular shape or the signal; to use the results and the theoretical approach described above to ascertain the nature of the effects observed. 3.2. Equipment and materials 3.2.1. Commercial instrumentation. A Perkin-Elmer AA spectrometer, model 5000, with graphite furnace HGA-500 accessories was used together with an AS-40 autosampler. The magnitude and the integrated absorbance were registered by the instrument in the conventional mode of operation with continuous and line spectra sources. The shape of signals was recorded with a Perkin-Elmer model 56 recorder. The temperature measurements were performed with a Keller Micropyrometer type PB 06 AF 3. 3.2.2. Furnace. The complete description of the furnace design and its mode of operation is presented in Refs. [3] and [4]. The elements necessary for understanding the FF operation are mentioned in the introduction. FF accessories: Tubes were from Pyrocarbo (South Africa). Graphite (National AGKS) was used as a filter material in general experiments, except in the cases noted in the text. Graphite fiber (Levertex G-868) was used. 3.2.3. Reagents and materials. Aqueous solutions of NaCl, NaI, CaCl2, KCI, KI, KBr, MgSO4 of concentrations from 2 to 20 g/1 were prepared from dry salts of analytical grade. Aqueous solutions of 10 mg/1 Cd and 100 mg/1 Pb were prepared from standard stock solution by addition of 3% HC1. 3.3. General procedure Solutions of 5-20 p~l of the salt or metal under investigation were dosed into the ring cavity of the FF. In all the experiments the temperature at the drying step of the temperature program, as indicated on the HGA-500 power supply, was 250°C, ramp time was 1 s and the hold time was 15 s. At the second step the temperature 500-800°C was chosen with the same ramp and hold times. In the sequential experiments, the background signals from different amounts of matrix or atomic absorption signals from metal were registered at the third step of the temperature program. Two modes were in operation: maximum power heating followed by holding the temperature constant, and heating with increased power. For the first mode, the variations of temperature were in the range 950-13000C. Hold times were chosen in accordance with the durations of the signals (10-500 s). The decay of the signals was registered up to complete removal of molecular vapors from the furnace which was ensured by a 500°C increase of the FF temperature at the cleaning step of the temperature program (1 s ramp time, 5 s hold time). For the second mode the ramp of temperature between 800 and 1500-1800°C during 200-500 s was preset. The experiments did not uncover any significant importance of the value of inner gas flow, displayed by the power supply, at any step of the temperature program. The indicated value of 50 ml/min, chosen for experiments, was quite nominal. In the part of the experiments where the duration of the signals was more than 10 s, the temperature of the filter was determined in situ with the micropyrometer through the dosing hole. The integrated absorbance for each signal was measured automatically with the instrument and the magnitude of the signals was measured from their traces.
Diffusion of molecular vapors through heated graphite
769
(a)
8
o
0
I 5
0
I 5
0
5
5
0
5
0
5
10
(b)
0
Time (s) Fig. 3. Atomic absorption (1) and background (2, 3) signals in the FF (a) and platform furnace (b); atomization temperature (as indicated on the power supply) is 1050°C, wavelength is 283.3 rim; 1-1.2 ng Pb; 2 and 3, consequently, 20 and 200 p,g NaCI.
3.4. Some peculiarities of the experiment 3.4.1. Preparation of the atomizer. In the assembled furnace the thread in the ring cavity was shifted with a needle away from the dosing hole to prevent shielding of radiation of the filter during temperature measurements. The method of FF preconditioning is described in Refs. [3] and [4]. 3.4.2. Optimization of the temperature program. For the long-duration heating mode, a significant difference between the temperature of the filter and that indicated by the power supply was discovered. This difference grew rapidly with increase of power. Temperatures of the filter were, 980, 1120, 1155, 1200 and 1260°C at the indicated temperatures 950, 1050, 1080, 1120 and 1170°C, respectively. Taking into account that the difference was small for low temperatures, the heating rate in the first two steps of the temperature program was chosen as indicated by the power supply. When the third step of the FF heating was programmed, the difference mentioned was taken into account: the limiting temperature for maximal power (MP) operation was set 70-80 degrees lower compared to the indicated temperature for the long duration period (however, overheating of the filter above the tube occurred some seconds after switching on the power).
4. RESULTS The atomic absorption signals from 1.2 ng Pb and the background signals from 20 and 200 ttg NaC1 at the wavelength 283.3 nm are presented in Fig. 3 (a,b) for FF and the ordinary platform-tube furnace, respectively. The MP program was in operation. The stabilized temperature after pulse heating was 1050°C.
770
D . A . KATSKOVet al. 1.0
3
•
'3
8
0
100
200
Amount of NaCl dosed (p.g) Fig. 4. Magnitude (1, 2) and peak area (3, 4) of absorbance signals at wavelength 283.3 nm as a function of amount of NaCl dosed into the FF. MP mode of operation; stabilized temperature 1050°C (1, 3) and 1100°C (2, 4).
Figure 2 shows that the magnitude of the atomic absorption signals increased about three times, when the FF was used because of the decrease of the cross section of the analytical zone [4], but the signal shape remained almost the same for both furnaces. Almost the same results were characteristic for background signals from 20 p,g NaCI. Increasing the amount of sodium chloride dosed led to considerably different signal shapes in the two furnaces: in the ordinary furnace only the magnitude increased whereas in the FF the small increase in magnitude was accompanied by the appearance of a long "tail". The decay in absorbance was not smooth; the small steps were repeated in sequential experiments. The dependence of background signal area and magnitude on the amount of NaCI is presented in Fig. 4. Measurements were made at 283.3 nm after the temperature had stabilized following MP. Two temperatures were used, 1050 and ll00°C. The signal magnitudes approached their limits for large amounts of NaCI; the area increase was almost proportional to the amount of salt dosed. The slight curvature of the plots for signal area could be attributed to incomplete release of vapors in the period of integration. To increase the magnitude and to highlight the peculiarities of the shapes of signals displayed in Fig. 2, the amount of NaCI dosed was increased up to 400 p.g and background absorbance was measured at the wavelength of Cd (228.8 nm). The MP heating mode was used. The shapes of the signals for the same amounts of sodium chloride at the different temperatures are shown in Fig. 5. All the signals have steps in their decay, marked with the arrows and numbers 1-6. These steps grew in magnitude and then disappeared; the new ones then displayed themselves on the signal sequentially together with increase of the FF temperature. The area of signals also increased. It should be noted that before the set of experiments shown in Fig. 5 was carried out, the filter used had several times been exposed to high temperatures. More steps were seen in another set of experiments, when new filters, made of graphite from different sources, were investigated. The signals for the run at ll00°C (as indicated by the power supply) are presented in Fig. 6. All the signals had different areas which indicated variations in the filter permeability. Almost complete vapor release during 250 s occurred for the filters used to obtain the first and the second diagrams (Fig. 6). Only small absorption peaks were observed at the cleaning step after the main signals. For signals 3-5, decreases in the magnitude and area were accompanied by
Diffusion of molecular vapors through heated graphite
1060
1050
1040
1100
1090
1080
1070 *C I
1
1
1
771
1110*C
1
2
2
1120
1130
1160
1170
1140
3
4
1150 *C
0.5 -
< 0-
t
4 6
L 0
1200 *C
i180
J 500
Time (s) Fig. 5. Shape of background signals (wavelength 228.8 nm) in the FF from 400 v-g NaCi. Each sequential diagram is recorded at the marked stabilized temperature after MP heating. Arrows and the identical numbers signify steps attributed to the same nature.
an increase of the signal at the cleaning step. Hence, slow diffusion of NaCI vapor through graphite is characteristic for these materials. Only long duration steps of low magnitude were observed. As was predicted in the theoretical part of the paper, the peculiarities of NaCI vapor release were displayed clearly during FF heating with increasing power (Fig. 7) when that mode of operation was used immediately after the experiments shown in Fig. 5. The steps marked in Fig. 5 have been transformed into the corresponding peaks. The shape of the background signal displayed in Fig. 7 and the positions of each separate peak were repeatable in some sequential firings, under the same experimental conditions.
772
D . A . KATSrOV et al. 1.o 2
o o.~
\1
<
310
Time (s) Fig. 6. Background signals (wavelength 228.8 nm, stabilized temperature after MP heating ll00°C, hold time 300 s; cleaning step: temperature 2000°C, ramp time 1 s, hold time 9 s) from 400 ~,g NaCI in the FF fitted with filters of graphite from different sources. 1, National AGKS; 2, Ringsdorff V-SP; 3, Ultra carbon UF-4S; 4, Ringsdorff EK-906; 5, Ultra carbon UT-6ST.
In contrast, variations in magnitudes of constituents of the signal were observed if the FF had previously been held at high temperature. This is illustrated in Fig. 8 which displays the results of experiments in which NaC1 background signals were sequentially recorded in between high temperature firings when the temperature 2500°C was maintained for different times. The position of each constituent of the signal in sequential records remained almost the same, but the magnitudes changed significantly. Some increase of signal area after a long period of high-temperature exposure was also observed. After the experiments shown in Fig. 8, the same FF was used in a run when the background signals from 200 Ixg of the salts NaI, KBr, KCI, KI, CaCI2 and MgSOa were registered under the same experimental conditions as for NaC1. For KCI only the wavelength was 210 nm in accordance with its absorption maxima. The results are displayed in Fig. 9. Each signal had a specific distribution of components and temperatures corresponding to each constituent. The fine structure of the signals displayed in Fig. 9 was characteristic for the filter which had not been exposed to high temperature for too long. In contrast, for an old filter the structure of signals from all the salts under investigation became similar. The signal was composed of only two peaks but, before, both peaks corresponded to temperatures characteristic of the salt under investigation (Fig. 10). Under simultaneous dosing into the FF of two salts having different vapor release temperatures, the background signal represented superposition of the signals from each one taken separately. To illustrate this point the shapes of signals for NaCI and CaCI2 dosed separately and together are shown in Fig. 11. Visual study of the filter, when it was extracted from the furnace showed that the signals displayed in Figs. 10 and 11 were characteristic for a filter close to destruction: its wall became thin in the center and its material became brittle.
Diffusion of molecular vapors through heated graphite
773
t
0.5
¢0
O
0
800
Temperature ('12)
I
1600 I
Time (400 s) Fig. 7. Shape of background signal for 400 p,g NaCI (wavelength 228.8 nm) dosed into FF, when its temperature increased from 800 to 1600°C over 400 s.
The compound responsible for appearance of some separate component of the signal could be stored in the ring cavity or filter body by cooling the FF. The idea is illustrated in Fig. 12 which displays the results of experiments when at some moment of partial release of NaCI vapors the heating of the FF was interrupted and sequential dosing of new portions of NaCI was performed. The resulting diagram represents the superposition of signals corresponding to the remaining fraction of the signal and to the extra amount dosed.
5. D I s c u s s i o N
Summarizing the results obtained in these experiments, the following must be accentuated. The diagrams in Fig. 4 show that transport of molecular vapor through graphite in the range of temperatures investigated occurs without any significant dissociation. This means that all the peculiarities of the process of vapor release, reflected in the shape of background signals, are related only to variations of evaporation or transport rate. Taking into account that participation of chemical reactions impede evaporation or diffusion rates in the same way, the following discussion will be carried on in terms of transport rate. It follows from Figs. 5-9 that the rate of diffusion of molecular vapor through the filter, depends on the nature of the vapor and the temperature and permeability of graphite. The last index is determined by the grade of graphite used; its initial properties can change due to the effect of high temperature. It is clear from Fig. 3 that decrease of salt content in the FF is accompanied by slowing down of the transport rate for its vapor. Figs. 5-7 show that there are some discrete rates corresponding to the release of the different portions of sample matter. The number of these rates and the distribution of the portions of sample related to each rate depend on the properties of graphite used as a filter material (Fig. 6). These
774
D . A . KATSKOVet al.
/t 5
I I I I I t
o
,.....1
.O
g
7
t 1000
2000
0
i
1000
I
2ooo
I
1000
I
2000
Temperature (*C) Fig. 8. Background signals for 200 t~g NaC1 (increase of the FF temperature from 500 to 2000°C during 300 s) in between firings when 2500°C was held. Before the first signal the FF was held at 2400°C for 140 s. The total hold times at 2500°C before the corresponding signal were, 2, 10 s; 3, 30 s; 4, 70 s; 5, 110 s; 6, 270 s; 7, 430 s; 8, 480 s. The dotted line in the diagram 5 is the atomic absorption signal for 2.5 Ixg Cd, registered for the same conditions as for NaC1 experiment when the background correction mode of the instrument was in operation.
properties could change together with permeability under the effect of high temperature; hence the mentioned distribution could also be altered (Fig. 8). For the same type of graphite, previously exposed to the same high temperature regime, the set of discreet rates of vapor transport through graphite is different but characteristic for each substance under investigation (Fig. 9). For the filter close to destruction, the distribution of these discrete rates is less marked (Fig. 10). A possible reason might be some change in graphite crystalline structure. However, it should be mentioned that the nonisothermal heating for the filter close to destruction (thin in the center) makes this interpretation questionable. Following the idea proposed in the Section 1, the shape of the diagrams in Figs. 5-9 might be the result of formation of chemical compounds in the ring cavity (or in the body of the filter). Upon thermodissociation these compounds give characteristic equilibrium vapor pressures. The data of Fig. 12 provide evidence that they are stable; that is, they keep their structure after the temperature has been lowered. The amount of reagent which is a possible participant in the reactions, is enough to provide independent formation of the compounds for different salts (Fig. 11).
Diffusion of molecular vapors through heated graphite
_
775
t
NaI
NaCI
I.O
LCaCI2 I
.<
0.5
MgS04
500
1000
1500 2000
,Jl
500
I000
I
1500 2000
500
1000 1500' 2000
Temperature(°C) Fig. 9. Background signals from 200 ~,g of different salts for the FF held at 2500°C for 480 s. Increase of the FF temperature from 500 to 2000°C occurred during 300 s. For KCI the wavelength was 210 nm, and for others it was 228.8 nm.
Naturally, only condensed graphite could be such a reagent in the ring cavity of the FF or in the pores of the filter. Let us now suppose that the substance evaporating in the ring cavity of the FF can form some compounds of different stoichiometry with graphite. Following the theory (Section 2), in that case the total absorbance would be characterized by superposition of the separate signals from each compound, differing by p0 and 0. This idea would be correct for alkali metals, which can interact with graphite in the temperature range 600-1800°C forming the set of intercalation compounds MC,. In accordance with the literature [9-13], Li forms the compounds LiEC2 and LiCn, where n = 6,12,18,36,72; Na forms the compounds Na2C2 and NaC,,, where n = 36,64,120; K, Rb and Cs form the compounds MC,, where n -- 8,10,24,36,48,60,64. The enthalpy of formation of these compounds depends on the number of atoms of carbon surrounding each atom of metal, and changes discretely. To illustrate that position, the heats of formation of intercalation compounds of K, Rb, Cs are exhibited in Table 1 [10]. Interaction of alkali metals with graphite resulted in specific shapes of the analytical signals when evaporation of these metals from the surface of porous graphite was studied [14]. The shapes of atomic absorption signals, observed in Ref. [14] (also displayed in Fig. 13) were substantially different from the shapes of a large group of other elements, see Ref. [8]. The same was found in the current investigation for molecular species; the signal depended not only on the furnace temperature but also on the amount of analyte. When the temperature was high enough, the decay of atomic absorption pulses was exponential and could be described with the parameter k from Eq. (9). For lower temperatures some additional exponential curves appeared, which could be determined by a set of parameters k,. In experiments [14] the
776
D . A . KArsgov et al.
NaCI
I~r
1.0
e~
CaCI2
---7 I I 50O 1000 1500 2000
,.O
<
J
l
i i i [ 500 10OO 1500 2000 0.5 -
KCI
I
L
""-f
Temperature (*C) Fig. 10. Background signals from 200 p,g of different salts under the same experimental conditions as for Fig. 9, for the FF after about 20 s exposure at the temperature higher than 2700°C (as indicated by power supply).
exponential curves, where possible, were separated graphically. Functions of log k,,(T -1) were plotted and enthalpies of evaporation of alkali metals were determined from the slopes of the graphs. The most reliable results for Li, Na, K, Rb were consequently 145 kJ/mol, 138 kJ/mol, 148 kJ/mol and 174 kJ/mol, respectively, and for Cs two values were obtained, 182 and 140 kJ/mol. Some of these data are in agreement with theoretical values displayed in Table 1. This verifies the suggestion about the nature of the effect being reflected in the signal shape. Another set of data from the literature [15] should be mentioned in connection with the current results represented in Fig. 7. During the evaporation of K and Cs in a two-step atomizer from a graphite cup under increasing temperature, double peak signals were observed. Just as in the current experiments, the temperatures corresponding to both peaks remained approximately the same but the ratio of their magnitudes was different for a new cup compared to cups exposed to multiple firings. Thus some similarities in the behavior of alkaline metals upon evaporation from the surface of porous grahite and molecular compounds during transport through heated graphite partition take place. Consequently, the effects that occurred in the current investigation could be supposed to be related to formation of similar compounds of molecules with graphite. It should be noted that intercalation compounds are characteristic not only of donors of electrons, such as an alkali metal, but also of acceptors. Among them chlorides
Diffusion of molecular vapors through heated graphite
777
(a)
NaCI +
I
,5-
(b)
u ~J
CaCI 2
O
0-
~ i
~
I
(c)
I 2ooo
65o
Temperature (°C)
Fig. 11. Background signals from 100 ~,g NaCI and 200 ~g CaC1, dosed into FF together (a) and separately (b, c).
1
2
4
3
5
6
8 0.5 O
/
/
,I I
850
1430
Temperature (*(2) Fig. 12. Background signals from 200 ~g NaCI, sequentially dosed into the FF, under increase of temperature from 850 to 1430°C for diagrams 1, 2, 4 and 6. For diagrams 3 and 5 the heating was interruped at the temperature corresponding to the central part of the signals.
D . A . KATSKOVet al.
778
Table 1. The results (in kJ/mol) of calculations of enthalpy of formation for intercalation compounds of K, Rb and Cs with graphite [10] K* Compound MC8 MCIo MC:4 MC36
MC48 MC6o MC~t
Rb*
Cs*
AH, + AH~
AH,
AH~ + AHv
AHr
hHr + AH,,
AHr
119.1 112.9 127.1 132.5 135.0 137.1 143.4
35.7 29.5 43.7 49.1 51.6 53.7 60.0
120.4 115.0 127.9 134.6 138.4 140.5 149.6
44.9 39.5 52.4 59.1 62.9 65.0 74.1
142.9 134.6 145.5 149.6 152.2 153.4 158.8
70.0 61.7 72.6 76.7 79.3 80.5 87.9
* AH~ values taken from Ref. [6], p. D.218, are 83.4 kJ/mol, 75.5 kJ/mol and 72.9 kJ/mol, respectively, for K, Rb and Cs. tThe values of enthalpy found empirically for n > 60.
Temperature al
\ <
Time Fig. 13. Schematic shape of atomic absorption signals for K, Rb, Cs (a) and Na, Li (b) for different stabilized temperatures of a furnace made of porous graphite [14].
and bromides of different metals are mentioned in the literature [9,13]. We found only a qualitative description [9], but the effect of their implantation into the graphite structure is very strong and, consequently, they can remain in graphite up to high temperatures. In accordance with the proposal, the molecules of all investigated substances in the process of transport of vapors through the filter imbed between the layers of graphite crystalline structure. They occupy positions (potential absorbing centers) with different discrete numbers of surrounding carbon atoms. The number of the centers of each type depends on initial crystallinity of the graphite used and on the rate of its destruction at high temperature. The distribution of these potential absorbing centers in the graphite determines, in turn, the corresponding amounts of absorbed molecules. The capacity of graphite for this absorption is high enough so that each type of insertion can be considered as a separate substance. These might be characterized by their own equilibrium vapor pressure and, hence, by their specific enthalpy of formation. The specific of formation of intercalation compounds for each sort of molecule and grade of regularity of graphite structure results in the stepped shape of background signals at constant temperature or in the appearance of some peaks when the temperature of the FF is increasing. Just as in Table 1, the enthalpy of formation for
Diffusion of molecular vapors through heated graphite
779
5
-t
\\
< am O ,d
I 7.0
I 7.5 (T,K) -t 104
Fig. 14. Variation of background absorbance corresponding to the marked steps in the diagrams of Fig. 5, as a function of FF temperature.
molecular intercalation compounds must increase discretely together with the decrease of the magnitude of the steps in Figs. 5 and 6. The data of Fig. 5 are available for estimation of the correctness of the last speculation. Under the consideration that each step on the diagrams A(t) (marked by arrows) corresponded to the specific equilibrium vapor pressure of NaC1 above the compound, pO, the function log (An-DCp °) = f(T-~), was plotted in Fig. 14. It appeared, that for the points marked in Fig. 5 with the same numbers, the diagrams represented straight lines with the slope increasing with increase of appearance temperature of the step. Following Eqs. (11) and (14)-(16), the sum E,~ + AHr + AHv can be calculated from the slopes of the plots in Fig. 14. The data obtained are shown in Table 2. Though the results for the last steps cannot be considered to be precise because of possible variations of temperature close to the front of the signal, they give the basis for the following conclusions. If we neglect the parameter E,~, which in this case can be attributed only to the small value of physical adsorption, the data obtained would characterize the enthalpy of formation of the compounds from the condensed substances. Comparison of the data of Tables 1 and 2 discloses that at least for the two first steps, these parameters are very close, and the same for Cs. As soon as the diameter of the diatomic molecules is closer to the atomic diameter of Cs than to other alkali metals, the particles of these two types can occupy the same absorbing centers of the graphite structure. That is, the similarity mentioned is reasonable. As in our earlier assumption, E,~ ~ AHr, the diffusion constant D c,r (Eq. (16)) is determined by the enthalpy of implantation of molecules into the graphite structure. Table 2. The estimation of enthalpy of formation of NaCl-graphite intercalation compounds
No. of step (Fig. 5) 1 2 3 4 5
Temperature range (K) 1310-1350 1350-1390 1390-1410 1410-1430 1430-1450
E~+AHr+AHv E~+AHr* (kJ/mol)
(kJ/mol)
297 335 358t 439t 698t
62 84 96 305 510
* AH~ = 188 kJ/mol [6] used without temperature correction. tThe data cannot be considered as reliable because of the short time duration of the corresponding steps, some uncertainty of temperature and the short temperature interval used for estimation.
780
D . A . KATSKOVet al.
Although, the results of experiments, the estimations made and comparison with literature data for alkali metals lead to the conclusion that the diffusion of molecular vapor through graphite is accompanied by formation of intercalation compounds of different stoichiometry, this interaction impedes the rate of diffusion.
6. CONCLUSIONS
The discovered effect of the interaction of molecules with graphite, displaying the signs of formation of stable stoichiometric compounds, is of importance for physical chemistry and also for analytical applications. The effect provides differences in the diffusion rates for atomic and molecular vapor that could be used for development of methods for high-temperature separation of these materials. Applied for the purposes of atomic spectral analyses, and realized in the corresponding designs of atomizers, the principle should be useful for minimization of spectral and chemical interferences in the determination of metals. One of the possible versions of such an atomizer is the FF. Experience has confirmed its analytical advantages. For further development of the idea, precise measurements of diffusion rates for atomic and moleculor vapors in graphite of different grades must be performed. The results could provide the information about the thermochemistry of intercalation compounds and help to find a proper selective material for the separation of matrix and atomic vapor. REFERENCES [1] Provisional patent R.S.A., No. 93/4657, 29.6.1993. [2] D. A. Katskov, P. J. J. G. Marais, R. I. McCrindle and R. Schwarzer, 28th Coll. Spectroscopicum Internationale, York, UK, 1993, Abstract No FP 1.64. [3] D. A. Katskov, P. J. J. G. Marais, R. I. McCrindle and R. Schwarzer, J. Anal. At. Spectrom. 9, 431 (1994). [4] D. A. Katskov, P. J. J. G. Marais, R. I. McCrindle and R. Schwarzer, Spectrochim. Acta, Part B, in press. [5] B. V. Lvov, Atomic Absorption Spectrochemical Analysis, Hilger, London (1970). [6] R. C. Weast, (Ed.) Handbook of Chemistry and Physics, 60th Edn., CRC Press, Boca Raton (1979). [7] C. W. Fuller, Analyst 99, 739 (1974). [8] D. A. Katskov, Spectrochim. Acta Rev. 14, 409 (1991). [9] V. P. Sosedov (Ed.), Properties of Constructive Materials on the Basis of Carbon. Reference Book, Metallurgia, Moscow (1975). [10] S. Aronson, F. Y. Salzano and D. Bellafiore, J. Chem. Phys. 49, 434 (1968). [11] M. Begouin, D. Guerard and A. Herold, C.R. Acad. Sci 6, 557 (1966). [12] R. Jusa and V. Wehle, Naturwissenschaften B52, 560 (1965). [13] W. Rudorff, Graphite lntercallation Compounds: Advances in Inorganic Chemistry and Radiochemistry, Academic Press, New York (1959). [14] D. A. Katskov and I. L. Grinstein, Zh. Prikl. Spectrosk. 34, 773 (1981). [15] D. A. Katskov, A. M. Shtepan, R. I. McCrindle and P. J. J. Marais, J. Anal. At. Spectrom. 9, 321 (1994).
~
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0584-8547/95 $9.50 + .00
Pergamon 0584--8547(94)00165-0
Diffusion of molecular vapors through heated graphite DMITRY A. KATSKOV*, R. SCHWARZER,P. J. J. MARAIS and R. I. McCRINDLE Department of Chemistry and Physics, Technikon Pretoria, Private Bag X680, Pretoria, South Africa (Received 13 August 1994; accepted 30 November 1994)
Abstract--Thereasons
for the elimination of interferences in a filter furnace used as an atomizer in electrothermal atomic absorption spectroscopy (ETAAS), are examined theoretically and experimentally. The results of observations of stepped or multipeak background absorption signals for NaCI, NaI, KCi, KBr, KI, CaCI2 and MgSO 4, which are characteristic for each matrix and different types of graphite used, leadsto hypotheses about the formation of molecule-graphite intercalation composites. These are treated as stable compounds of different stoichiometry having substantial specificenthalpy of formation. The validity of the idea is confirmed with the measurements of the rate of NaCI vapor release through a graphite filter as a function of temperature, and comparison with the literature data for similar compounds of alkali metals. The effect of implantation of molecular particles into a crystalline lattice leads to reduction of their diffusion rate through graphite, which in turn provides time-resolved background and atomic absorption signals. Further investigations and development of the idea to understand the analytical effect are proposed.
1. INTRODUCTION THE DESIGN and some analytical characteristics of the filter furnace (FF) as a new atomizer for electrothermal atomic absorption spectroscopy ( E T A A S ) , were recently described in the literature [1-3]. The atomizer (Fig. 1) was a tube of pyrocoated graphite of shape and dimensions typically used in analytical practice, fitted with an insert (filter) in the form of a spool made of porous graphite. An amount of graphite fiber was placed in the ring cavity between the walls of the tube and of the filter. The analyzed liquid was sampled through the dosing hole into the ring cavity; then the FF was heated under a preset program. At the atomization step the sample vapors went into the analytical zone ( A Z ) in the central part of the filter through the graphite partition. It was discovered [3] that among the advantages of the FF, c o m p a r e d to conventional instrumentation, was a significant elimination of chemical and spectral interferences in the determination of Cd, Pb and Bi in the presence of NaCl and CuC12, as a matrix. The observations of atomic background absorption signals gave the result that in the FF molecular v a p o r entered the A Z with some delay relative to atomic vapor. That delay led to partial separation of these signals when the elements were determined under excess of matrices. This tendency provided improved freedom from spectral interferences. H o w e v e r , if it was assumed that the delay mentioned was a function of the specific nature of the sample v a p o r and, probably, of the properties of graphite, the effect on the delay of the amount of matrix dosed remained unclear. Understanding of this issue was important because the magnitude of background signal shift was very specifically dependent on matrix quantity. In comparison with the magnitudes of the atomic signals, which were proportional to the sampled dose, background signals attained their limits when the amount of matrix increased. These limits depended only on the FF temperature. The restriction of background absorption magnitude naturally lead to the same positive analytical effect as mentioned above. Determination of the limit for background signal magnitude p r o v o k e d the idea * Author to whom correspondence should be addressed. 763
D. A. KATSKOVet
764
al. 2
0 0 0 0 0
O
3
O
4
Fig. 1. Filter furnace: 1, pyrocoatedgraphite tube; 2, dosinghole; 3, graphite filter; 4, graphite fiber.
about formation of matrix-saturated vapor pressure in the ring cavity of the FF. In accordance with the theory developed in Ref. [4], the saturation effect could be obtained in any semiclosed ET atomizer for nanogram amounts of matrix if a large evaporation surface and a restriction of vapor exit from the vapor reservoir into the A Z was provided. That meant that the effect was not connected with the nature of the graphite and could be obtained for any high temperature porous material used as a filter in the FF. The problem arising from this preliminary discussion is whether or not the heated graphite possesses any specific properties towards molecular vapors in terms of vapor transport or chemical activity, relative to atomic species. The answer is important for physical chemistry and could help the principle of FF action and the limitations of the new method of atomization to be understood.
2. THEORY The present paper develops the theory proposed in Ref. [4]. The assumption is made in Ref. [4] that some amount of analyzed substance (N~ moles) was distributed as a monolayer on the surface of the graphite fiber in the ring cavity of the FF and occupied an area ~r. Transport of the sample vapors through the graphite partition of area s into the A Z of cross-sectional area S and length L was assumed to be driven by diffusion. To simplify the considerations it was assumed that gas diffusion occurred through the thin partition of effective area (1)
s' = sDc/D ~
where Dg and D c are diffusion constants for the sample vapor in argon and graphite, respectively. The FF temperature was considered to be constant or its increase to be insignificant, relative to the rate of evaporation and vapor transport through the graphite partition and the AZ. Under this condition, the process of vapor release could be considered as stationary. Hence, under the condition s' ~ 2S (which for FF was taken for granted) the concentration of particles in the A Z (n) was described by the equation n = n°/[1 + 2S(1/Gr + 1/s')] where n ° was the concentration of particles for substance evaporates without dissociation, then vapor.) Two cases were examined, cr > s' and large amounts of substance in a ring cavity. For n = n°s'/2S
(2)
thermodynamic equilibrium. (If the n o is the concentration of saturated or < s', corresponding to small and the first case (3)
Diffusion of molecular vapors through heated graphite
765
and, for the second case (4)
n = n°cr/2S
It follows from Eqs. (3) and (4) that the vapor concentration in the A Z is a function of sample quantity only for small amounts of sample. For big samples the vapor concentration is determined by the volatility of the substance and (through the parameter s') by the permeability of the graphite partition. The model [4] is examined as follows. If the process of vapor transport through the A Z of effective volume V = 0 . 5 L S is faster than through the filter, then (5)
n = - (,t/V) d N J d t
where ~" L 2 / 8 D g is the residence time for the evaporated particles in the A Z [5]. From Eqs. (1), (3) and (5) the rate of sample release, while cr(t)> s', can be determined as =
(6)
dNJdt = - 2n°sD¢/L
The flux of particles is kept constant. Considering dm to be the effective diameter of the molecule, NA to be the Avogadro constant and No to be the initial amount of substance, then tr(t) -~ d m E N ~ ( t ) N g , and the duration of the period while Eq. (6) is correct can be estimated for the condition tr -- s' 0 = L(No - s'/NAdm2)/2n°sD ~
(7)
Setting Eq. (4) equal to Eq. (5), the differential equation describing the final part of the evaporation process, when or(t) < s', may be obtained d N J dt = - 2n° dm2 N A Dg N,~ = - k N ~
(8)
k = 2n°dm2NADg/L
(9)
where
To solve Eq. (8), the initial magnitude at the moment 0 when cr = s', that is s'/ dm2NA, is then N ~ = ( s ' / d m 2 N A ) exp [ - k ( t - 0 ) ]
(10)
Following Eqs. (3), (5) and (10) and assuming absorbance A to be determined by the sensitivity factor ~t and effective length of the absorbing vapor layer L / 2 , that is A = etnL/2, the description of the absorbance signal can be obtained. A = om%sD¢/V
for t < 0
(11)
A = (om%sOC/V) exp[-k(t-0)]
for t > 0
(12)
Concerning Eqs. (11) and (12), it should be noted that they are related only to the case when condensed phase exists in a ring cavity of the FF. If the amount of substance is too small to obtain equilibrium at the given temperature T, the general assumption about the stationary character of the vapor release cannot be used. Some parameters of Eqs. (11) and (12), which are strongly dependent on temperature should be defined. The concentration of saturated vapor n o or the saturated vapor pressure pO = nOR T ( R is the gas constant) are described using the fundamental thermodynamic parameters AHv and ASv (the enthalpy and entropy of vapor formation). pO = e x p ( A S J R - A H v / R T )
(13)
766
D . A . KATSKOVet al.
In the more general case, which is important for the results and discussion below, evaporation could be accompanied by thermodissociation of the condensed substance. This reaction is characterized by its own parameters AH~, and AS~. Then, for equilibrium concentration of vapor, it is possible to write pO= exp[(ASr+ASv)/R - (AH~+AHv)/RT]
(14)
The diffusion constant D c is described using the expression [6] D c = D~ exp ( - E~d/RT)
(15)
where D~ and E~ are constants. The last parameter is supposed to be the activation energy of diffusion of vapor in graphite. It follows from Eqs. (11), (13) and (15), that the absorbance magnitude depends on temperature as the function In A(t < 0) = -(AHv + E~)/RT + const., if there is no reaction accompanying evaporation. On the contrary, if such a reaction takes place, then In A(t < O) = -(AHr+AHv+E~)/RT + const, and the diffusion constant can be treated as D . . . . D~'r exp -(AHr+E~)/RT
(16)
This means that the vapor release is supposed to be impeded the same way by the reaction either accompanying evaporation in the ring cavity or transport through graphite. Another source of information about the rate of vapor release is the parameter k. In accordance with Eqs. (9) and (12), the measurements of the function k(T -1) from the exponential decay of absorbance signals at some different but constant temperatures, relates to the enthalpy of vapor formation. The method of determination of that parameter was proposed by Fuller [7] later was used for investigations of Ag vapor diffusion through a porous graphite partition [8]. The very close coincidence of experimental and reference data for AHv for the temperature range 1300-1500 K (consequently, 251 and 266 kJ/mol) [8] confirms the validity of the model. This assumption should be analyzed for the case where two different substances resulting from the processes of condensed thermodissociation and evaporation of the same vapor composition are situated in the ring cavity of FF. They have different equilibrium vapor pressure pO and evaporate independently. In this case the magnitude of absorbance will be represented by superimposed signals from each component. The schematic diagrams, exhibiting the total shape of the signals, for two different initial amounts of evaporating substances, No,1 and No,2, with their corresponding pressures pO > pO, are plotted in Fig. 2 (a,b); this reflects two characteristic cases. When No,1 < No,2 the total signal has a stepped shape. When No.l > No.2 the decay of the total signal represents overlapped exponential curves; the "tail" is determined by the slower of the two processes, that is by pO. Naturally, if the initial amounts of substance are not sufficient to satisfy the condition cr > s' only overlapping exponential curves will be observed. In principle, more than two chemical reactions could be involved in the process of vapor release. These might be, for example, the reactions of disproportionation of the initial compound, or evaporation of metal from carbides of different stoichiometry. Each of the processes could be characterized by its own parameter pO and, consequently, the number of steps or overlapping exponents could be observed in the shape of the absorbance signal when the temperature of the FF is kept constant. In Fig. 2 (c) and (d), the schematic diagram for three components of the absorbance signal are displayed for the case when amounts of each substance are different but large enough to neglect the final decay of the signals. The evidence for chemical reactions that occurred in the ring cavity of the FF can also be observed in the shape of the absorbance signal when the temperature is
Diffusion of molecular vapors through heated graphite
(a)
....
767
(b)
2
"'-.
Time
(c)
(d)
L 2
e~
Time
(f)
(e) 1
/
/ I
3
1//'~
r//I
Temperature Fig. 2. Schematic diagram of overlapping absorbance signals (1-3) from the same vapor component of some different condensed substances of initial masses No,l, No,2, No,3, when the temperature of the FF is constant ( a - d ) or is increased (e,f). Sections a, b: two components, consequently No.l < No,2 and No.1 >No./, for both cases pOt > p° 2. Sections c - f : three components, where p°i/p°Jp° 3 = 4/2/1. Sections c, e: NoJNo,JNo,3 = 1/2/3. Sections d, f: No, I~ No,2/No,3 = 2/8/5.
increasing. For large masses of the sample (when it is possible to neglect the decay of the signal) it should be transformed into a multipeak profile. The schematic diagram for this case is represented in Fig. 2 (e) and (f), for the same distribution Noa-No.3, as in Fig. 2 (c) and (d), using the simplified approach of a linear function of evaporation rate with respect to temperature. It follows from the diagram that the distribution of the peak magnitudes depends on the initial amounts of the components. The revealed theoretical fragments show that the shape of absorbance signal in the FF can yield fundamental information about the chemical processes accompanying vapor release. Practical methods to extract the information and its interpretation are discussed below.
768
D. A. KATSKOVet al. 3. EXPERIMENTAL
3.1. Preliminary comments In the experiments [3] it was discovered that the shape of the background signals from NaCI matrix was very different for the FF and the platform furnace (PF). The set of experiments was carried out with the following aims: to highlight the differences mentioned; to examine their variations under experimental conditions; to find the similarity in behavior of NaCI and another matrices; to discover the role of graphite in the formation of the particular shape or the signal; to use the results and the theoretical approach described above to ascertain the nature of the effects observed. 3.2. Equipment and materials 3.2.1. Commercial instrumentation. A Perkin-Elmer AA spectrometer, model 5000, with graphite furnace HGA-500 accessories was used together with an AS-40 autosampler. The magnitude and the integrated absorbance were registered by the instrument in the conventional mode of operation with continuous and line spectra sources. The shape of signals was recorded with a Perkin-Elmer model 56 recorder. The temperature measurements were performed with a Keller Micropyrometer type PB 06 AF 3. 3.2.2. Furnace. The complete description of the furnace design and its mode of operation is presented in Refs. [3] and [4]. The elements necessary for understanding the FF operation are mentioned in the introduction. FF accessories: Tubes were from Pyrocarbo (South Africa). Graphite (National AGKS) was used as a filter material in general experiments, except in the cases noted in the text. Graphite fiber (Levertex G-868) was used. 3.2.3. Reagents and materials. Aqueous solutions of NaCl, NaI, CaCl2, KCI, KI, KBr, MgSO4 of concentrations from 2 to 20 g/1 were prepared from dry salts of analytical grade. Aqueous solutions of 10 mg/1 Cd and 100 mg/1 Pb were prepared from standard stock solution by addition of 3% HC1. 3.3. General procedure Solutions of 5-20 p~l of the salt or metal under investigation were dosed into the ring cavity of the FF. In all the experiments the temperature at the drying step of the temperature program, as indicated on the HGA-500 power supply, was 250°C, ramp time was 1 s and the hold time was 15 s. At the second step the temperature 500-800°C was chosen with the same ramp and hold times. In the sequential experiments, the background signals from different amounts of matrix or atomic absorption signals from metal were registered at the third step of the temperature program. Two modes were in operation: maximum power heating followed by holding the temperature constant, and heating with increased power. For the first mode, the variations of temperature were in the range 950-13000C. Hold times were chosen in accordance with the durations of the signals (10-500 s). The decay of the signals was registered up to complete removal of molecular vapors from the furnace which was ensured by a 500°C increase of the FF temperature at the cleaning step of the temperature program (1 s ramp time, 5 s hold time). For the second mode the ramp of temperature between 800 and 1500-1800°C during 200-500 s was preset. The experiments did not uncover any significant importance of the value of inner gas flow, displayed by the power supply, at any step of the temperature program. The indicated value of 50 ml/min, chosen for experiments, was quite nominal. In the part of the experiments where the duration of the signals was more than 10 s, the temperature of the filter was determined in situ with the micropyrometer through the dosing hole. The integrated absorbance for each signal was measured automatically with the instrument and the magnitude of the signals was measured from their traces.
Diffusion of molecular vapors through heated graphite
769
(a)
8
o
0
I 5
0
I 5
0
5
5
0
5
0
5
10
(b)
0
Time (s) Fig. 3. Atomic absorption (1) and background (2, 3) signals in the FF (a) and platform furnace (b); atomization temperature (as indicated on the power supply) is 1050°C, wavelength is 283.3 rim; 1-1.2 ng Pb; 2 and 3, consequently, 20 and 200 p,g NaCI.
3.4. Some peculiarities of the experiment 3.4.1. Preparation of the atomizer. In the assembled furnace the thread in the ring cavity was shifted with a needle away from the dosing hole to prevent shielding of radiation of the filter during temperature measurements. The method of FF preconditioning is described in Refs. [3] and [4]. 3.4.2. Optimization of the temperature program. For the long-duration heating mode, a significant difference between the temperature of the filter and that indicated by the power supply was discovered. This difference grew rapidly with increase of power. Temperatures of the filter were, 980, 1120, 1155, 1200 and 1260°C at the indicated temperatures 950, 1050, 1080, 1120 and 1170°C, respectively. Taking into account that the difference was small for low temperatures, the heating rate in the first two steps of the temperature program was chosen as indicated by the power supply. When the third step of the FF heating was programmed, the difference mentioned was taken into account: the limiting temperature for maximal power (MP) operation was set 70-80 degrees lower compared to the indicated temperature for the long duration period (however, overheating of the filter above the tube occurred some seconds after switching on the power).
4. RESULTS The atomic absorption signals from 1.2 ng Pb and the background signals from 20 and 200 ttg NaC1 at the wavelength 283.3 nm are presented in Fig. 3 (a,b) for FF and the ordinary platform-tube furnace, respectively. The MP program was in operation. The stabilized temperature after pulse heating was 1050°C.
770
D . A . KATSKOVet al. 1.0
3
•
'3
8
0
100
200
Amount of NaCl dosed (p.g) Fig. 4. Magnitude (1, 2) and peak area (3, 4) of absorbance signals at wavelength 283.3 nm as a function of amount of NaCl dosed into the FF. MP mode of operation; stabilized temperature 1050°C (1, 3) and 1100°C (2, 4).
Figure 2 shows that the magnitude of the atomic absorption signals increased about three times, when the FF was used because of the decrease of the cross section of the analytical zone [4], but the signal shape remained almost the same for both furnaces. Almost the same results were characteristic for background signals from 20 p,g NaCI. Increasing the amount of sodium chloride dosed led to considerably different signal shapes in the two furnaces: in the ordinary furnace only the magnitude increased whereas in the FF the small increase in magnitude was accompanied by the appearance of a long "tail". The decay in absorbance was not smooth; the small steps were repeated in sequential experiments. The dependence of background signal area and magnitude on the amount of NaCI is presented in Fig. 4. Measurements were made at 283.3 nm after the temperature had stabilized following MP. Two temperatures were used, 1050 and ll00°C. The signal magnitudes approached their limits for large amounts of NaCI; the area increase was almost proportional to the amount of salt dosed. The slight curvature of the plots for signal area could be attributed to incomplete release of vapors in the period of integration. To increase the magnitude and to highlight the peculiarities of the shapes of signals displayed in Fig. 2, the amount of NaCI dosed was increased up to 400 p.g and background absorbance was measured at the wavelength of Cd (228.8 nm). The MP heating mode was used. The shapes of the signals for the same amounts of sodium chloride at the different temperatures are shown in Fig. 5. All the signals have steps in their decay, marked with the arrows and numbers 1-6. These steps grew in magnitude and then disappeared; the new ones then displayed themselves on the signal sequentially together with increase of the FF temperature. The area of signals also increased. It should be noted that before the set of experiments shown in Fig. 5 was carried out, the filter used had several times been exposed to high temperatures. More steps were seen in another set of experiments, when new filters, made of graphite from different sources, were investigated. The signals for the run at ll00°C (as indicated by the power supply) are presented in Fig. 6. All the signals had different areas which indicated variations in the filter permeability. Almost complete vapor release during 250 s occurred for the filters used to obtain the first and the second diagrams (Fig. 6). Only small absorption peaks were observed at the cleaning step after the main signals. For signals 3-5, decreases in the magnitude and area were accompanied by
Diffusion of molecular vapors through heated graphite
1060
1050
1040
1100
1090
1080
1070 *C I
1
1
1
771
1110*C
1
2
2
1120
1130
1160
1170
1140
3
4
1150 *C
0.5 -
< 0-
t
4 6
L 0
1200 *C
i180
J 500
Time (s) Fig. 5. Shape of background signals (wavelength 228.8 nm) in the FF from 400 v-g NaCi. Each sequential diagram is recorded at the marked stabilized temperature after MP heating. Arrows and the identical numbers signify steps attributed to the same nature.
an increase of the signal at the cleaning step. Hence, slow diffusion of NaCI vapor through graphite is characteristic for these materials. Only long duration steps of low magnitude were observed. As was predicted in the theoretical part of the paper, the peculiarities of NaCI vapor release were displayed clearly during FF heating with increasing power (Fig. 7) when that mode of operation was used immediately after the experiments shown in Fig. 5. The steps marked in Fig. 5 have been transformed into the corresponding peaks. The shape of the background signal displayed in Fig. 7 and the positions of each separate peak were repeatable in some sequential firings, under the same experimental conditions.
772
D . A . KATSrOV et al. 1.o 2
o o.~
\1
<
310
Time (s) Fig. 6. Background signals (wavelength 228.8 nm, stabilized temperature after MP heating ll00°C, hold time 300 s; cleaning step: temperature 2000°C, ramp time 1 s, hold time 9 s) from 400 ~,g NaCI in the FF fitted with filters of graphite from different sources. 1, National AGKS; 2, Ringsdorff V-SP; 3, Ultra carbon UF-4S; 4, Ringsdorff EK-906; 5, Ultra carbon UT-6ST.
In contrast, variations in magnitudes of constituents of the signal were observed if the FF had previously been held at high temperature. This is illustrated in Fig. 8 which displays the results of experiments in which NaC1 background signals were sequentially recorded in between high temperature firings when the temperature 2500°C was maintained for different times. The position of each constituent of the signal in sequential records remained almost the same, but the magnitudes changed significantly. Some increase of signal area after a long period of high-temperature exposure was also observed. After the experiments shown in Fig. 8, the same FF was used in a run when the background signals from 200 Ixg of the salts NaI, KBr, KCI, KI, CaCI2 and MgSOa were registered under the same experimental conditions as for NaC1. For KCI only the wavelength was 210 nm in accordance with its absorption maxima. The results are displayed in Fig. 9. Each signal had a specific distribution of components and temperatures corresponding to each constituent. The fine structure of the signals displayed in Fig. 9 was characteristic for the filter which had not been exposed to high temperature for too long. In contrast, for an old filter the structure of signals from all the salts under investigation became similar. The signal was composed of only two peaks but, before, both peaks corresponded to temperatures characteristic of the salt under investigation (Fig. 10). Under simultaneous dosing into the FF of two salts having different vapor release temperatures, the background signal represented superposition of the signals from each one taken separately. To illustrate this point the shapes of signals for NaCI and CaCI2 dosed separately and together are shown in Fig. 11. Visual study of the filter, when it was extracted from the furnace showed that the signals displayed in Figs. 10 and 11 were characteristic for a filter close to destruction: its wall became thin in the center and its material became brittle.
Diffusion of molecular vapors through heated graphite
773
t
0.5
¢0
O
0
800
Temperature ('12)
I
1600 I
Time (400 s) Fig. 7. Shape of background signal for 400 p,g NaCI (wavelength 228.8 nm) dosed into FF, when its temperature increased from 800 to 1600°C over 400 s.
The compound responsible for appearance of some separate component of the signal could be stored in the ring cavity or filter body by cooling the FF. The idea is illustrated in Fig. 12 which displays the results of experiments when at some moment of partial release of NaCI vapors the heating of the FF was interrupted and sequential dosing of new portions of NaCI was performed. The resulting diagram represents the superposition of signals corresponding to the remaining fraction of the signal and to the extra amount dosed.
5. D I s c u s s i o N
Summarizing the results obtained in these experiments, the following must be accentuated. The diagrams in Fig. 4 show that transport of molecular vapor through graphite in the range of temperatures investigated occurs without any significant dissociation. This means that all the peculiarities of the process of vapor release, reflected in the shape of background signals, are related only to variations of evaporation or transport rate. Taking into account that participation of chemical reactions impede evaporation or diffusion rates in the same way, the following discussion will be carried on in terms of transport rate. It follows from Figs. 5-9 that the rate of diffusion of molecular vapor through the filter, depends on the nature of the vapor and the temperature and permeability of graphite. The last index is determined by the grade of graphite used; its initial properties can change due to the effect of high temperature. It is clear from Fig. 3 that decrease of salt content in the FF is accompanied by slowing down of the transport rate for its vapor. Figs. 5-7 show that there are some discrete rates corresponding to the release of the different portions of sample matter. The number of these rates and the distribution of the portions of sample related to each rate depend on the properties of graphite used as a filter material (Fig. 6). These
774
D . A . KATSKOVet al.
/t 5
I I I I I t
o
,.....1
.O
g
7
t 1000
2000
0
i
1000
I
2ooo
I
1000
I
2000
Temperature (*C) Fig. 8. Background signals for 200 t~g NaC1 (increase of the FF temperature from 500 to 2000°C during 300 s) in between firings when 2500°C was held. Before the first signal the FF was held at 2400°C for 140 s. The total hold times at 2500°C before the corresponding signal were, 2, 10 s; 3, 30 s; 4, 70 s; 5, 110 s; 6, 270 s; 7, 430 s; 8, 480 s. The dotted line in the diagram 5 is the atomic absorption signal for 2.5 Ixg Cd, registered for the same conditions as for NaC1 experiment when the background correction mode of the instrument was in operation.
properties could change together with permeability under the effect of high temperature; hence the mentioned distribution could also be altered (Fig. 8). For the same type of graphite, previously exposed to the same high temperature regime, the set of discreet rates of vapor transport through graphite is different but characteristic for each substance under investigation (Fig. 9). For the filter close to destruction, the distribution of these discrete rates is less marked (Fig. 10). A possible reason might be some change in graphite crystalline structure. However, it should be mentioned that the nonisothermal heating for the filter close to destruction (thin in the center) makes this interpretation questionable. Following the idea proposed in the Section 1, the shape of the diagrams in Figs. 5-9 might be the result of formation of chemical compounds in the ring cavity (or in the body of the filter). Upon thermodissociation these compounds give characteristic equilibrium vapor pressures. The data of Fig. 12 provide evidence that they are stable; that is, they keep their structure after the temperature has been lowered. The amount of reagent which is a possible participant in the reactions, is enough to provide independent formation of the compounds for different salts (Fig. 11).
Diffusion of molecular vapors through heated graphite
_
775
t
NaI
NaCI
I.O
LCaCI2 I
.<
0.5
MgS04
500
1000
1500 2000
,Jl
500
I000
I
1500 2000
500
1000 1500' 2000
Temperature(°C) Fig. 9. Background signals from 200 ~,g of different salts for the FF held at 2500°C for 480 s. Increase of the FF temperature from 500 to 2000°C occurred during 300 s. For KCI the wavelength was 210 nm, and for others it was 228.8 nm.
Naturally, only condensed graphite could be such a reagent in the ring cavity of the FF or in the pores of the filter. Let us now suppose that the substance evaporating in the ring cavity of the FF can form some compounds of different stoichiometry with graphite. Following the theory (Section 2), in that case the total absorbance would be characterized by superposition of the separate signals from each compound, differing by p0 and 0. This idea would be correct for alkali metals, which can interact with graphite in the temperature range 600-1800°C forming the set of intercalation compounds MC,. In accordance with the literature [9-13], Li forms the compounds LiEC2 and LiCn, where n = 6,12,18,36,72; Na forms the compounds Na2C2 and NaC,,, where n = 36,64,120; K, Rb and Cs form the compounds MC,, where n -- 8,10,24,36,48,60,64. The enthalpy of formation of these compounds depends on the number of atoms of carbon surrounding each atom of metal, and changes discretely. To illustrate that position, the heats of formation of intercalation compounds of K, Rb, Cs are exhibited in Table 1 [10]. Interaction of alkali metals with graphite resulted in specific shapes of the analytical signals when evaporation of these metals from the surface of porous graphite was studied [14]. The shapes of atomic absorption signals, observed in Ref. [14] (also displayed in Fig. 13) were substantially different from the shapes of a large group of other elements, see Ref. [8]. The same was found in the current investigation for molecular species; the signal depended not only on the furnace temperature but also on the amount of analyte. When the temperature was high enough, the decay of atomic absorption pulses was exponential and could be described with the parameter k from Eq. (9). For lower temperatures some additional exponential curves appeared, which could be determined by a set of parameters k,. In experiments [14] the
776
D . A . KArsgov et al.
NaCI
I~r
1.0
e~
CaCI2
---7 I I 50O 1000 1500 2000
,.O
<
J
l
i i i [ 500 10OO 1500 2000 0.5 -
KCI
I
L
""-f
Temperature (*C) Fig. 10. Background signals from 200 p,g of different salts under the same experimental conditions as for Fig. 9, for the FF after about 20 s exposure at the temperature higher than 2700°C (as indicated by power supply).
exponential curves, where possible, were separated graphically. Functions of log k,,(T -1) were plotted and enthalpies of evaporation of alkali metals were determined from the slopes of the graphs. The most reliable results for Li, Na, K, Rb were consequently 145 kJ/mol, 138 kJ/mol, 148 kJ/mol and 174 kJ/mol, respectively, and for Cs two values were obtained, 182 and 140 kJ/mol. Some of these data are in agreement with theoretical values displayed in Table 1. This verifies the suggestion about the nature of the effect being reflected in the signal shape. Another set of data from the literature [15] should be mentioned in connection with the current results represented in Fig. 7. During the evaporation of K and Cs in a two-step atomizer from a graphite cup under increasing temperature, double peak signals were observed. Just as in the current experiments, the temperatures corresponding to both peaks remained approximately the same but the ratio of their magnitudes was different for a new cup compared to cups exposed to multiple firings. Thus some similarities in the behavior of alkaline metals upon evaporation from the surface of porous grahite and molecular compounds during transport through heated graphite partition take place. Consequently, the effects that occurred in the current investigation could be supposed to be related to formation of similar compounds of molecules with graphite. It should be noted that intercalation compounds are characteristic not only of donors of electrons, such as an alkali metal, but also of acceptors. Among them chlorides
Diffusion of molecular vapors through heated graphite
777
(a)
NaCI +
I
,5-
(b)
u ~J
CaCI 2
O
0-
~ i
~
I
(c)
I 2ooo
65o
Temperature (°C)
Fig. 11. Background signals from 100 ~,g NaCI and 200 ~g CaC1, dosed into FF together (a) and separately (b, c).
1
2
4
3
5
6
8 0.5 O
/
/
,I I
850
1430
Temperature (*(2) Fig. 12. Background signals from 200 ~g NaCI, sequentially dosed into the FF, under increase of temperature from 850 to 1430°C for diagrams 1, 2, 4 and 6. For diagrams 3 and 5 the heating was interruped at the temperature corresponding to the central part of the signals.
D . A . KATSKOVet al.
778
Table 1. The results (in kJ/mol) of calculations of enthalpy of formation for intercalation compounds of K, Rb and Cs with graphite [10] K* Compound MC8 MCIo MC:4 MC36
MC48 MC6o MC~t
Rb*
Cs*
AH, + AH~
AH,
AH~ + AHv
AHr
hHr + AH,,
AHr
119.1 112.9 127.1 132.5 135.0 137.1 143.4
35.7 29.5 43.7 49.1 51.6 53.7 60.0
120.4 115.0 127.9 134.6 138.4 140.5 149.6
44.9 39.5 52.4 59.1 62.9 65.0 74.1
142.9 134.6 145.5 149.6 152.2 153.4 158.8
70.0 61.7 72.6 76.7 79.3 80.5 87.9
* AH~ values taken from Ref. [6], p. D.218, are 83.4 kJ/mol, 75.5 kJ/mol and 72.9 kJ/mol, respectively, for K, Rb and Cs. tThe values of enthalpy found empirically for n > 60.
Temperature al
\ <
Time Fig. 13. Schematic shape of atomic absorption signals for K, Rb, Cs (a) and Na, Li (b) for different stabilized temperatures of a furnace made of porous graphite [14].
and bromides of different metals are mentioned in the literature [9,13]. We found only a qualitative description [9], but the effect of their implantation into the graphite structure is very strong and, consequently, they can remain in graphite up to high temperatures. In accordance with the proposal, the molecules of all investigated substances in the process of transport of vapors through the filter imbed between the layers of graphite crystalline structure. They occupy positions (potential absorbing centers) with different discrete numbers of surrounding carbon atoms. The number of the centers of each type depends on initial crystallinity of the graphite used and on the rate of its destruction at high temperature. The distribution of these potential absorbing centers in the graphite determines, in turn, the corresponding amounts of absorbed molecules. The capacity of graphite for this absorption is high enough so that each type of insertion can be considered as a separate substance. These might be characterized by their own equilibrium vapor pressure and, hence, by their specific enthalpy of formation. The specific of formation of intercalation compounds for each sort of molecule and grade of regularity of graphite structure results in the stepped shape of background signals at constant temperature or in the appearance of some peaks when the temperature of the FF is increasing. Just as in Table 1, the enthalpy of formation for
Diffusion of molecular vapors through heated graphite
779
5
-t
\\
< am O ,d
I 7.0
I 7.5 (T,K) -t 104
Fig. 14. Variation of background absorbance corresponding to the marked steps in the diagrams of Fig. 5, as a function of FF temperature.
molecular intercalation compounds must increase discretely together with the decrease of the magnitude of the steps in Figs. 5 and 6. The data of Fig. 5 are available for estimation of the correctness of the last speculation. Under the consideration that each step on the diagrams A(t) (marked by arrows) corresponded to the specific equilibrium vapor pressure of NaC1 above the compound, pO, the function log (An-DCp °) = f(T-~), was plotted in Fig. 14. It appeared, that for the points marked in Fig. 5 with the same numbers, the diagrams represented straight lines with the slope increasing with increase of appearance temperature of the step. Following Eqs. (11) and (14)-(16), the sum E,~ + AHr + AHv can be calculated from the slopes of the plots in Fig. 14. The data obtained are shown in Table 2. Though the results for the last steps cannot be considered to be precise because of possible variations of temperature close to the front of the signal, they give the basis for the following conclusions. If we neglect the parameter E,~, which in this case can be attributed only to the small value of physical adsorption, the data obtained would characterize the enthalpy of formation of the compounds from the condensed substances. Comparison of the data of Tables 1 and 2 discloses that at least for the two first steps, these parameters are very close, and the same for Cs. As soon as the diameter of the diatomic molecules is closer to the atomic diameter of Cs than to other alkali metals, the particles of these two types can occupy the same absorbing centers of the graphite structure. That is, the similarity mentioned is reasonable. As in our earlier assumption, E,~ ~ AHr, the diffusion constant D c,r (Eq. (16)) is determined by the enthalpy of implantation of molecules into the graphite structure. Table 2. The estimation of enthalpy of formation of NaCl-graphite intercalation compounds
No. of step (Fig. 5) 1 2 3 4 5
Temperature range (K) 1310-1350 1350-1390 1390-1410 1410-1430 1430-1450
E~+AHr+AHv E~+AHr* (kJ/mol)
(kJ/mol)
297 335 358t 439t 698t
62 84 96 305 510
* AH~ = 188 kJ/mol [6] used without temperature correction. tThe data cannot be considered as reliable because of the short time duration of the corresponding steps, some uncertainty of temperature and the short temperature interval used for estimation.
780
D . A . KATSKOVet al.
Although, the results of experiments, the estimations made and comparison with literature data for alkali metals lead to the conclusion that the diffusion of molecular vapor through graphite is accompanied by formation of intercalation compounds of different stoichiometry, this interaction impedes the rate of diffusion.
6. CONCLUSIONS
The discovered effect of the interaction of molecules with graphite, displaying the signs of formation of stable stoichiometric compounds, is of importance for physical chemistry and also for analytical applications. The effect provides differences in the diffusion rates for atomic and molecular vapor that could be used for development of methods for high-temperature separation of these materials. Applied for the purposes of atomic spectral analyses, and realized in the corresponding designs of atomizers, the principle should be useful for minimization of spectral and chemical interferences in the determination of metals. One of the possible versions of such an atomizer is the FF. Experience has confirmed its analytical advantages. For further development of the idea, precise measurements of diffusion rates for atomic and moleculor vapors in graphite of different grades must be performed. The results could provide the information about the thermochemistry of intercalation compounds and help to find a proper selective material for the separation of matrix and atomic vapor. REFERENCES [1] Provisional patent R.S.A., No. 93/4657, 29.6.1993. [2] D. A. Katskov, P. J. J. G. Marais, R. I. McCrindle and R. Schwarzer, 28th Coll. Spectroscopicum Internationale, York, UK, 1993, Abstract No FP 1.64. [3] D. A. Katskov, P. J. J. G. Marais, R. I. McCrindle and R. Schwarzer, J. Anal. At. Spectrom. 9, 431 (1994). [4] D. A. Katskov, P. J. J. G. Marais, R. I. McCrindle and R. Schwarzer, Spectrochim. Acta, Part B, in press. [5] B. V. Lvov, Atomic Absorption Spectrochemical Analysis, Hilger, London (1970). [6] R. C. Weast, (Ed.) Handbook of Chemistry and Physics, 60th Edn., CRC Press, Boca Raton (1979). [7] C. W. Fuller, Analyst 99, 739 (1974). [8] D. A. Katskov, Spectrochim. Acta Rev. 14, 409 (1991). [9] V. P. Sosedov (Ed.), Properties of Constructive Materials on the Basis of Carbon. Reference Book, Metallurgia, Moscow (1975). [10] S. Aronson, F. Y. Salzano and D. Bellafiore, J. Chem. Phys. 49, 434 (1968). [11] M. Begouin, D. Guerard and A. Herold, C.R. Acad. Sci 6, 557 (1966). [12] R. Jusa and V. Wehle, Naturwissenschaften B52, 560 (1965). [13] W. Rudorff, Graphite lntercallation Compounds: Advances in Inorganic Chemistry and Radiochemistry, Academic Press, New York (1959). [14] D. A. Katskov and I. L. Grinstein, Zh. Prikl. Spectrosk. 34, 773 (1981). [15] D. A. Katskov, A. M. Shtepan, R. I. McCrindle and P. J. J. Marais, J. Anal. At. Spectrom. 9, 321 (1994).