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Studies on the sensitivity of yttrium by electrothermal atomization from metallic and metal-carbide surfaces of a heated graphite atomizer in atomic absorption spectrometry
Spectmhimica Acta, Vol. 36B, No. 5, pp. 463 to 474, Printed in Great Britain.
1981.
Studies on the sensitivity of yttrium by electrothermal atomization from metallic and metal-carbide surfaces of a heated graphite atomizer in atomic absorption spectrometry H. S. WAHAB
and C. L. CHARRABARTI*
Department of Chemistry, Carleton University, Ottawa, Ontario, Canada KlS 5B6 (Receiued
19 November
1980)
Abskx-Atomization
of yttrium in tube-type electrothermal atomizers was studied using various atomization surfaces: pyrocoated graphite surface, carbidized graphite surface and tantalum or tungsten metal surfaces. Carbidizing of pyrocoated graphite tubes with other carbide-forming metals (Ta, Zr or La) produces refractory metal-carbide surfaces thereby preventing the carbide-forming yttrium to come in physical contact with the reactive graphite surface. The result is an enhancement in the analytical sensitivity (peak height absorbance) of yttrium. The atomization of Y from a metal surface (Ta or W) gives better analytical sensitivity, lower atomization temperature, and negligible memory effect compared with those from metalcarbidized surfaces.
1. INTRODUCTION THE CHEMICAL
and physical characteristics of the surface of a graphite tube can influence both the analytical sensitivity and the mechanism of atom formation. L’vov [l] recommended use of pyrolytic graphite (rather than standard graphite) on the basis of its low permeability to gases, low porosity, high purity and a high resistance to oxidation, which gave improved sensititivity and precision in graphite furnace atomic absorption spectrometry. Although pyrocoated graphite tube atomizers were recently used in atomic absorption spectroscopy, these atomizers did not give adequate sensitivity for those elements which formed refractory carbides and/or lamellar compounds of graphite at high temperatures resulting in incomplete vaporization of the elements. The following approaches to deal with the above problem have been reported in the literature [2-61. Graphite tubes were pre-coated with compounds of metals which formed low-volatile carbides. This was accomplished by pre-soaking the graphite tubes in aqueous solution of salts such as ZrO(N03)z, LaC13*6H20 or Ti(S04)2, followed by drying and heating them to high temperature in order to form the metal carbide layer, thereby eliminating physical contact, and hence, reaction between the graphite surface and the analytes. Using the above pre-coated graphite tubes, increase in sensitivity was achieved for Cr, Mn and Al [3], Ba [S], Be [3,6] and Si [4]. Also, a great increase in the useful life of the tantalum-carbidized graphite tubes was reported by ZATKA [2]. Another approach was the use of the metal linings or collars inserted inside the graphite tube. Great care had to be taken to ensure that the sample was contained on the metal surface and did not come in physical contact with the graphite surface. L’vov [l] suggested the use of tantalum-lined graphite tubes for eliminating loss of atomic vapour by diffusion through the walls of porous graphite cuvette, especially at high temperatures. A 20-fold enhancement in the sensitivity of Ba was reported by RBNSHAW [7] using a graphite tube lined with a tantalum foil. The poor sensitivity obtained for elements which formed thermally stable carbides was attributed to carbide formation [7]. RUNNEL~et al. [3] suggested that carbide formation might play a role in the * Author to whom reprint [I] [2] [3] [4] [5] [6] [7]
requests
should
be sent.
B. V. L’VOV, Atomic Absorption Spectrochemical Analysis, p. 206. Adam Hilger, London, V. J. ZATKA, Anal. &em. 50, 538 (1978). J. H. RUNNELS, R. MERRV~ELD and H. B. FISHER, Anal. Chem. 47, 1258 (1975). H. M. ORTNER and E. KANTUSCHER, T&ma 22, 581 (1975). R. CIONI, A. ZAZZUCOTELLI and G. O~~ONELLO, Anal. Chim. Acta 82, 415 (1976). TH. STIEFEL, K. SCHUUE, G. Tou; and H. Z~RN, Anal. Chim. Acta 87, 67 (1976). G. D. RENSHAW, At. Absorption Newslen. 12, 158 (1973). 463
(1970).
464
H. S. WAHAB and C. L. CHAKRAJMRTI
atomization of elements from the surface of a graphite tube atomizer. L’vov and PELIEVA[8] reported a significant increase in the sensitivity of 32 of the 40 elements studied by using a Ta-foil-lined HGA 76B graphite tube atomizer. These authors [8] also reported a considerable reduction in the memory effect and a lower atomization temperature for the majority of the elements studied. More recently, GREGOIRE [9] reported a 59.7-fold enhancement in the sensitivity of uranium atomized from a Ta-foil lining (0.05 mm in thickness) on The Perkin-Elmer graphite tubes using a modified HGA 2100. A significant enhancement in the sensitivity of Ag, Al, Ba, Cu, Mn, Ni, Sn, Sr, Ti and Zr was reported by WALL [lo] using a rigid tungsten collar, 1 cm in length and 0.5 mm in thickness, in a graphite tube atomizer. Other authors used atomizers made entirely of a metal such as Ta [ll-141, Pt [15] and W [16,17]. An enhancement in the sensitivity of Ag, Al, Au, Cr, Cu, and Pb was achieved by ACGETT and SPROTT [12] by using a tantalum surface for atomization. CANTLEand WEST [16] reported a 50% increase in lead signals by using a tungsten filament-the increase was relative to the signals from a carbon filament atomizer. 2. EXPERIMENTAL 2.1 Apparatus The Perkin-Elmer Atomic Absorption Spectrophotometer model 603 fitted with a Heated Graphite Atomizer (HGA) 76B, an Autosampler model AS-l, a Deuterium Arc Background Corrector, and a standard yttrium hollow cathode lamp were used. The hollow cathode lamp was operated at a lamp current of 13 mA. The Y 407.7 nm line and a spectral band-width of 0.7 nm were used. The Perkin-Elmer pyrocoated graphite tubes were used. The atomizer was operated in the gas-stop or mini-flow mode with argon as the purge gas. All electron micrographs were taken with JEOL model JSM-U3 Scanning Electron Microscope. 2.2 Reagents All chemicals used met ACS specifications or were of the highest purity commercially available. A stock solution of 1 x lo-3 kg/dm3 of Y was prepared from yttrium oxide (Y203) dissolved in ULTREX hydrochloric acid and diluted with ultrapure water to make the solution 1% (v/v) HCl. Test solutions were prepared from the stock solution by serial dilution with ultrapure water immediately prior to analysis. Ultrapure water was prepared by passing distilled water through a Milli-Q2 water system (Millipore Corporation, Mississauga, Ontario, Canada). Six per cent (w/u) tantalum soaking solution. Three grams of tantalum of 99.998% purity were weighed into a 100 ml of PTFE beaker and 10 ml of dilute hydrochloric acid (1+ l), 3 g of oxalic acid dihydrate, and 0.5 ml of 30% (v/v) hydrogen peroxide were added. The solution was heated carefully until the metal was dissolved. When the reaction became too slow more hydrogen peroxide was added. After completion of dissolution, 4 g of oxalic acid and 30 ml of ultrapure water were added, the acid was dissolved, the solution was diluted with ultrapure water to 50 ml and stored in a high-density polyethylene bottle. and L. A. PELIEVA, Can. J. Spectrosc. 23, 1 (1978). of Chemistry, Carleton University, Canada (1978). C. D. WALL, Talanta 24, 755 (1977). J. Y. HWANG, C. J. MOKELLER and P. A. ULLUCCI, Anal. Chem. 44,2018 (1972). J. AGGETT and A. J. SPROTT, Anal. Chim. Acta 72, 49 (1974). W. G. SCHFUZNK and R. J. EVERSON, Appl. Spectrosc. 29, 41 (1975). T. TAKEUCHI, M. YANACISAWAand M. SUZUKI, Talanta 19, 465 (1972). S. R. GOODE, A. MONTASERand S. R. CROUCH, Appl. Spectrosc. 27, 355 (1973). J. E. CANTLE and T. S. WEST, Tafanta 20, 459 (1973). H. M. DONEGO and T. E. B~JRGESS,Anal. Chem. 42, 1521 (1970).
[8] B.
V. L’vov
[9] D. C. GREGOIRE, Ph.D. Thesis. Department [lo] [ll] [12] [13] [14] [15] [16] [17]
Ottawa,
Ontario,
Studies on the sensitivity
of yttrium by electrothermal
atomization
465
Six per cent (w/v) lanthanum soaking solution. Lathanum chloride hexahydrate (LaCI,.6H,O) crystals (3.8147 g) were dissolved in a minimum volume of 1% (v/v) ULTBEX hydrochloric acid aqueous solution in a lOOm1 beaker, warmed for a few minutes until the solution became clear, then diluted to 25 ml with 1% (v/v) ULTREX hydrochloric acid aqueous solution so that the test solution contained 600 pg of lanthanum in a 10 ~1 injection volume-the optimum mass was 600 pg of lanthanum
r31.
Six per cent (w/v) zirconium soaking solutidn. Zirconium sulphate (Zr(SO,),*4H,O) crystals (5.8431 g) were dissolved in a minimum volume of cold 10% (v/v) hydrofluoric acid solution in a PTFE beaker, then diluted to 25 ml with the same solution, so that the test solution contained 600 pg of zirconium in a 10 ~1 injection volume-the optimum mass was 600 pg of zirconium [3].
2.3 Gases Argon gas of 99.995% purity (Union Carbide Canada Ltd.) was used. The gas was passed through a gas filter model 6183T (Matheson Co., Whitby, Ontario, Canada) in order to remove all particulates of diameter greater than 3.0~ 10m7m (99.999% efficiency). 2.4 Lining with tantalum and tungsten foils Linings of tantalum or tungsten metal (A. D. Mackay, Darien). The procedure was identical to that described by WAHAB and CHAKRABARTI
[18].
2.5 Surface treatment of the pyrocoated graphite tubes Because of the possibility of formation of low-volatility carbides and/or intercalation compounds [8], the surface of The Perkin-Elmer graphite tubes was pre-treated to prevent or minimize formation of these compounds and thereby to enhance peak absorbance. The pre-treatment of the surface of the pyrocoated graphite tubes consisted of soaking the tubes in the under-mentioned aqueous solutions containing 6% (w/v) of any one of the following metals as their soluble salts: tantalum, lanthanum or zirconium. The following procedure for treating the pyrocoated graphite tubes with a 6% (w/v) aqueous solution of tantalum was that of ZATKA [2]. The graphite tube was immersed vertically in the soaking solution of one of the above metal salts contained in a plastic vial. After immersion for 24 h under atmospheric pressure the tube was removed from the bath and dried, first, at room temperature in the air for 30 min, and then, at 105°C for 1 h. The tube was then mounted in the atomizer unit fitted with new, unused graphite contact rings and, while the flow of argon gas and the cooling water was on, the temperature was raised gradually (over a period of 30 s) to 1000°C and then to 2500°C over a period of 5 s. The procedure was repeated for study of the effect of a second coating. 2.6 General procedure The test solution (10 x 1O-6 dm3) was injected into the HGA 76B using the autosampler. The flow of the external sheath gas to the atomizer was maintained at a rate of 2.2~ lo-* dm3 s-l. The gas-interrupt mode was used for atomization except that a minimum flow rate of 8.3X 10e4dm3 s-l was sometimes used. In order to ensure reproducible experimental conditions, the resistance was measured across the workhead terminals with a model 4328A milliohmmeter (Hewlett-Packard Co. Ltd.) and was maintained (with frequent checking) at a constant value 22 rnti at the ambient temperature. This is necessary because the heating of the graphite tube atomizer is electrical Joule’s heating which depends on both the voltage and the resistance of the [18] H. S. WAHAFIand C. L. CHAKRABARTI,
Spectrochim.
Acta 36B, 475 (1981).
H. S. WAHAB and C. L. WARTI
466
atomizer. Variations in the atomizer temperature caused by changes in the flow rate of the cooling water was eliminated by keeping the flow rate of water passing through the atomizer constant at 1.5 dm3 min-‘. The importance of keeping the flow rate of the cooling water constant has been reported [19]. A decrease in the absorption signals with decreasing rate of water flow was attributed [19] to the resultant, higher atomization temperature causing greater expansion of the purge gas (Ar) which increased loss of analyte atoms from the analysis cell. Blanks were run for each determination. All values reported were the average of Bt least seven replicate determinations done successively. 3. RESULTSAND DISSUSSION 3.1 Atomization
from metal-carbide
surfaces
Table 1 presents the optimum experimental conditions used in this study. Table 2 presents the absolute sensitivity of yttrium atomized from various atomizing surfaces. Figures l-8 present scanning electron micrographs of the interior surfaces and edges of the treated and untreated pyrocoated graphite tubes. It can be seen from Fig. 3 that the interior surface of the pyrocoated graphite tube has porous sponge-like appearance, and the porosity increases with increasing temperature (Fig. 4). The increasing porosity causes increasing penetration of the analyte into pores at the high atomization temperature required for yttrium, resulting in the formation of lamellar compounds in which the analyte atoms, molecules, etc. are inserted or intercalated between the graphite sheets. It can be seen from Fig. 2 that the separation between the pyrolytic graphite layers increases with the increased number of heating cycles (firings) completed by a graphite tube. This effect may be due to a combination of the following factors: increased amounts of lamellar compounds formed by the analyte inside the lamellar structure of the pyrolytic graphite, and increased weakening of the loose, layered structure of the pyrolytic graphite caused by thermal stress. Once the analyte has penetrated between the layers of pyrolytic graphite, atomization becomes difficult and incomplete, resulting in memory effect [20] and a loss in analytical sensitivity. Table 1. Optimum experimental
conditions
Atomization Analytical
parameters
Drying temperature (‘C) Drying time (s) Ashing temperature (“C) Ashing time (s) Atomizing temperature (“C) Atomizing time (s) Integration time (s) Ar-purge gas mode Lamp current (mA) Wavelength (nm) Spectral band pass (nm)
A 110 30 500 30 2700 6 8 GS* 13 407.7 0.7
B 110 30 500 30 2700 6 8 GS 13 407.7 0.7
C 110 30 500 30 2700 6 8 GS 13 407.7 0.7
surfaces D 110 30 500 30 2700 6 8 GS 13 407.7 0.7
A = Pyrocoated graphite tube (PCGT). B = PCGT, carbidized twice with 6% (w/v) Ta solution. C = PCGT, carbidiied twice with 6% (w/v) La solution. D = PCGT, carbidized twice with 6% (w/v) Zr solution. E = PCGT, lined with Ta foil, 0.1 mm in thickness. F = PCGT, lined with W foil, 0.05 mm in thickness. * Gas-stop mode. t Mini-flow mode. cl91 E. AIXCLiENSSENS and P. KNOOP, Anal. C&m. Acta 68, 37 (1973). [20] G. HALL, M. P. BRA-L, JR. and C. L. CHAKRABARTI, Talanta 20, 755 (1973).
E 110 30 500 30 2600 8 10 GS 13 407.7 0.7
F 110 30 500 30 2500 6 8 MF-F 13 407.7 0.7
Studies on the sensitivity of yttrium by electrothermal
Table 2. Absolute
sensitivity*
atomization
467
of yttrium atomized from various surfaces under optimum experimental conditionst
Atomization surface Pyrocoated graphite tube (PCGT) Once-carbidiied PCGT with 6% (w/v) Ta solution Twice-carbidized PCGT with 6% (w/v) Ta solution Twice-carbidized PCGT with 6% (w/v) La solution Twice-carbidized PCGT with 6% (w/v) Zr solution PCGT lined with Ta foil PCGT lined with W foil
Absolute sensitivity, kg
Enhancement of sensitivity*
1.8 x lo-= 1.3 x lo-l2
1.4 fold
9.5 x lo-l3
1.9 fold
8.8 x lo-=
2.1 fold
7.9x 10-1s
2.3 fold
4 x lo-‘” 2.8x lo-l3
4.5 fold 6.4 fold
* The mass of element which gives an absorbance of 0.0044 by the peak absorbance mode. t The experimental conditions used are presented in Table 1. $ Enhancement relative to the pyrocoated graphite tube.
In Table 2, the lower sensitivity of yttrium when it is atomized from pyrocoated graphite surface is probably due to the following causes. The important cause is formation of stable carbides by yttrium with pyrocoated graphite at high temperatures [21,22]. Another cause is probably penetration of the analyte between the layers of pyrolytic graphite which results in a greater loss by diffusion of the analyte atomic vapour through the pores of the pyrolytic graphite tube walls to the outside of the
Fig. 1. Electron micrograph (X 1500) of the fracture edge of an unused pyrocoated graphite tube. [Zl] G. V. SAMSONOV,Handbook of the Physicochemical Propertiesof the Elements, p. 515. Plenum Data Corp., New York, (1968). [22] G. DE MARIA, M. GUIDE, L. MALASPINA and B. PESCE, J. Chem. Phys. 43, 4449 (1965).
468
H.S.WAHABandC.L.
@AlCRAEaRn
Fig. 2. Electron micrograph t x 1500) of the fracture edge of a pyrocoated graphite tube after six atomization cycles at 2700°C.
Fig. 3. Electron micrograph i(x 1000) of the interior surface of an unused pyrocoated graphite tube.
Studies
Fig. 4. Electron
on the sensitivity
micrograph
of yttrium
by electrothermal
469
atomization
(X 1500) of the interior surface of a pyrocoated six atomization cycles at 2700°C.
graphite
tube after
graphite tube. L’vov [l, p. 2051 has given equation (1) for the above difision graphite tube wall L 72=DgrSw.
VW
through
(1)
where 72 is the residence time, t, is the thickness of the tube wall, VWis the volume of the walls, P’ is the diffusion coefficient of the analyte atom for diffusion through graphite pores, and S,,, is the area of the walls of the tube. Equation (1) shows an inverse relationship between the residence time, TV, and the diffision coefficient, P’. Compared with the other atomization surfaces in Table 2, the spongy nature of the pyrocoated graphite (Figs. 3 and 4) will allow more rapid diffusion, and hence, will give a shorter r2 value. Such a decrease in the 72 value will decrease the peak height absorbance as explained below. L’vov [23] has given equation (2) for the accelerated atomization, i.e. the atomization under a linearly increasing temperature of the atomizer, as in the present case A where APeak is the peak height absorbance, N, is the number of analyte atoms in the sample, and or is the atomization time-all other terms have been defined earlier. According to equation (2), AWeakis not sensitive to changes in the TJT~ ratio only if the condition T1/T2<< 1 is maintained. However, in the present case TJT~~ 1; and hence, a decrease in the 72 value (with 71 as constant) will decrease Apeak, as observed experimentally. However, as stated earlier, the major cause for the lower sensitivity of yttrium when atomized from pyrocoated graphite surface is formation of refractory yttrium carbide. Formation of carbides can be prevented or reduced by using a non-carbon surface for atomization. Such a surface can be prepared from the graphite tube itself by forming [23]
B. V. L’vov,
Spectrochim.
Acta
33B,
153 (1978).
470
H. S. WAUAB and C. L.
Fig. 5. Electron
CHAKRABARTI
micrograph (X 1000) of the interior surface of a pyrocoated carbidized once using a 6% (w/v) Ta solution.
graphite
tube
refractory carbides of metals (other than the analytes) on the graphite tube surface and covering the entire interior surface with such refractory metal carbides. This can be accomplished by soaking graphite tubes in appropriate solutions, then drying them and subjecting them to further heat treatment as described earlier. The above cycle can be repeated if desired. Figures 5-8 present electron micrographs of tantalum-carbidized pyrocoated graphite surfaces and fracture edges. The effectiveness of the protective
Fig.
6. Electron
micrograph (X 1000) of the fracture edge of a pyrocoated carbidized once using a 6% (w/v) Ta sohltion.
graphite
tube
Studies on the sensitivity of yttrium by electrothermal
471
atomization
Fig. 7. Electron micrograph (X 1000) of the interior surface of a pyrocoated carbidized twice using a 6% (w/v) Ta solution.
graphite tldbe
lays:r for1ned by the tantalum carbide increases with the number of the treatm’ en t cycles twice-treated surfaces (Figs..7 and 8) rplete d, at least for the initial few cycles-the are more effective than the once-treated surfaces (Figs. 5 and 6) presumabt YtJecause the thick :ness of the metal-carbide protective layer increases its effective ne:ss; the thic:kness of the layer increases with increasing number of the treatme nt cycles con lplete d.
Fig. 8. Electron micrograph (X 1500) of the fracture edge of a pyrocoated graphite tuIbe carbidized twice using a 6% (w/v) Ta solution-after 1.5 atomization cycles at 2700°C
472
H. S. WAHAEI and C. L. CHAKRAFMRTI
The enhancement in sensitivities reported in Table 2 for carbidized surfaces can be explained as follows. First, the metal (Ta, La or Zr) carbide surface prevents the underlying graphite from coming in physical contact and reacting with yttrium to form yttrium carbide. Secondly, the metal (Ta, La or Zr) carbides present an impermeable (non-porous) surface to the analyte solution, thereby preventing the solution from soaking and penetrating into graphite pores. Thirdly, carbide-forming metals, such as, Ta, La and Zr are active catalysts for graphitization [24,25], and graphitization decreases the reactivity of the graphite surface. Moreover, penetration of carbides into the pores of graphite surface dissolves the disordered carbon and precipitates it as nonreactive, small graphite crystallites, without a significant change in the composition of the graphite. This process occurs above 2000 K and produces a more highly ordered graphite surface. The difference in the enhancement of sensitivity reported in Table 2 for the three different carbidizing surfaces-tantalum, lanthanum and zirconium-may be due to the difference in the physicochemical properties of their carbides (reactivity, porosity, density, permeability, etc.), or, to non-uniform distribution of their carbides on the graphite surface, and/or, within the layer structure of the graphite body. 3.2 Atomization
from metal surfaces
Table 2 also shows that the use of tantalum or tungsten foil enhances the absolute sensitivity of yttrium. This can be explained as follows. The metal foil completely prevents the physical contact between the analyte and the graphite surface, thereby preventing penetration of the analyte solution into the pores of the graphite tube and eliminating formation of lamellar compounds, and/or, thermally stable carbides. Also, these metal foils permitted use of lower atomization temperature, which increased the useful life of the graphite tubes. The absolute sensitivity with a W foil lining was better than that obtained with a Ta foil lining. This may be probably due to the following. Because of oxidation by air, the tantalum surface is coated with a coherent, stable film of Ta,O,. SISCO and EPREMIAN[26, p. 3391 have reported that growth of the surface oxide proceeds by a mechanism of diffusion of tantalum metal through the film of Ta,O, to the surface. If the rate of diffusion of tantalum metal through the solid Ta,O, is slow in comparison with the rate of atomization of yttrium, formation of yttrium atoms may be somewhat hindered by the growth of Ta,O,. Table 2 shows that the sensitivity of yttrium is higher from a tungsten surface than from a tantalum surface. In the case of the tungsten surface, solid WO, is formed by air oxidation of the tungsten surface above 5OO”C, but the W03 formed is appreciably volatilized above 1000°C [27, p. 421 and is expected to be completely volatilized at the higher temperature employed for the yttrium atomization. WALL [lo] reported the absence of a film of WO, on tungsten surface at the atomization temperature used. Since the present authors used rapid atomization of yttrium, the above reasoning can be applied to explain the difference between the sensitivity of yttrium at these two metal surfaces. 3.3 Memory effect Formation of metal-graphite lamellar compounds and/or refractory carbides causes a loss of sensitivity and the so-called memory effect. Non-uniform distribution of heat and the resulting thermal gradient from the centre of the graphite tube to its ends are also the cause of the memory effect. FINDLAYet al. [28] reported that on heating the HGA 70 furnace up to 8OO”C, the difference in temperature between the middle and the ends of the graphite tube was 5OO”C, and the difference increased at high [24] A. E. KARU
and M. BEER, J. Appl. Phys. 37, 2179 (1966). [25] W. E. PARKER, R. W. MAREK and E. M. WOODRUFF, Carbon 2, 395 (1965). [26] F. T. SISCO and E. EPREMIAN, Columbium and Tantalum. Wiley, New York (1963). [27] G. D. RIECK, Tungsten and its Compounds. Pergamon Press, Oxford (1967). [28] W. F. FINDLAY, A. ZDROJEWSKI and N. QUICKERT, Spectrosc. Lett. 7,63 (1974).
Studies
on
the sensitivity
of yttrium by electrothermal
473
atomization
Fig. 9. Absorbance of Y as a function of number of firings from different atomizing surfaces. V Pyrocoated graphite tube (PCGT). m PCGT, carbidized twice with 6% (w/v) Ta solution. A PCGT, lined with Ta foil. 0 PCGT, lined with W foil.
temperatures. Because of this large temperature difference between the hot middle part of the graphite tube and its cold ends, the vapour of the sample which evaporates from the middle part of the graphite tube condenses at its cold ends. Thus, there is a serious problem of condensation for elements which have high boiling points such as yttrium (b.p. 3338°C). Memory effect can be reduced if not entirely eliminated by impregnating the graphite surface with metal carbides of low volatility, or, by lining the graphite surface with thin foils of metals such as tantalum or tungsten. This is illustrated in Fig. 9 which shows the memory effect for Y as a function of the number of completed atomization cycles (fkings). For the untreated pyrocoated graphite tube, the effect was large, diminished slowly with successive firings, and the residual absorbance was relatively large even after 16 firings. For the W-lined pyrocoated graphite tube, the effect was smaller to start with, diminished rapidly with increasing number of firings, and became relatively small after three firings. Other two surfaces showed behaviour intermediate between the above two extremes. 3.4 Precision Table 3 presents the precision of yttrium determination with various atomization surfaces. The precision was slightly better with the carbidized pyrocoated graphite surfaces and the metal surfaces. The precision was a complex function of the stability of the emission source (the hollow cathode lamp), the age of the graphite tube, the Table 3. Precision*
Atomization
A B C
surface
% R.S.D.
2.5 2.0 1.8
Atomization
D E F
surface
% R.S.D.
1.9 1.9 2.0
A = Pyrocoated graphite tube (PCGT). B = PCGT, carbidized twice with 6% (w/v) Ta solution. C=PCGT, carbidized twice with 6% (w/v) La solution. D = PCGT, carbidized twice with 6% (w/v) Zr solution. E = PCGT, lined with Ta foil, 0.1 mm in thickness. F=PCGT, lined with W foil, 0.05 mm in thickness. *Precision relates to the final measurement step alone and is expressed as a percentage relative standard deviation of at least seven replicate measurements of the peak height absorbance made successively.
474
H. S. WAHAB and C. L. CHAKRAEMRTI
constancy of the atomizer circuit resistance, the reproducibility of the sample delivery step, and the reproducibility of the metal lining fitting (in the case of the metal foils). No attempt was made to determine the individual contributions of the various factors to the overall precision. 4. CONCLUSIONS The following conclusions can be drawn. 1. Carbidizing of the pyrocoated graphite tubes with metal carbides reduces the porosity of pyrocoated graphite tubes and thereby minimizes the penetration of the analyte into the pores. Carbidizing of pyrocoated graphite tubes prevents formation of analyte metal carbides of low volatility, and/or, lamellar compounds of graphite, resulting in an improvement in the analytical sensitivity. 2. Use of a lining of metals-Ta and W-as foils increases the analytical sensitivity, useful life of graphite tubes, and gives shorter atomization times, lower atomization temperatures, and relative freedom from memory effects. Acknowledgements-The authors are grateful to the Natural Sciences and Engineering Research Council Canada for research grants. One of the authors (H. S. WAHAB) is grateful to the Government of Iraq for a scholarship.
1981.
Studies on the sensitivity of yttrium by electrothermal atomization from metallic and metal-carbide surfaces of a heated graphite atomizer in atomic absorption spectrometry H. S. WAHAB
and C. L. CHARRABARTI*
Department of Chemistry, Carleton University, Ottawa, Ontario, Canada KlS 5B6 (Receiued
19 November
1980)
Abskx-Atomization
of yttrium in tube-type electrothermal atomizers was studied using various atomization surfaces: pyrocoated graphite surface, carbidized graphite surface and tantalum or tungsten metal surfaces. Carbidizing of pyrocoated graphite tubes with other carbide-forming metals (Ta, Zr or La) produces refractory metal-carbide surfaces thereby preventing the carbide-forming yttrium to come in physical contact with the reactive graphite surface. The result is an enhancement in the analytical sensitivity (peak height absorbance) of yttrium. The atomization of Y from a metal surface (Ta or W) gives better analytical sensitivity, lower atomization temperature, and negligible memory effect compared with those from metalcarbidized surfaces.
1. INTRODUCTION THE CHEMICAL
and physical characteristics of the surface of a graphite tube can influence both the analytical sensitivity and the mechanism of atom formation. L’vov [l] recommended use of pyrolytic graphite (rather than standard graphite) on the basis of its low permeability to gases, low porosity, high purity and a high resistance to oxidation, which gave improved sensititivity and precision in graphite furnace atomic absorption spectrometry. Although pyrocoated graphite tube atomizers were recently used in atomic absorption spectroscopy, these atomizers did not give adequate sensitivity for those elements which formed refractory carbides and/or lamellar compounds of graphite at high temperatures resulting in incomplete vaporization of the elements. The following approaches to deal with the above problem have been reported in the literature [2-61. Graphite tubes were pre-coated with compounds of metals which formed low-volatile carbides. This was accomplished by pre-soaking the graphite tubes in aqueous solution of salts such as ZrO(N03)z, LaC13*6H20 or Ti(S04)2, followed by drying and heating them to high temperature in order to form the metal carbide layer, thereby eliminating physical contact, and hence, reaction between the graphite surface and the analytes. Using the above pre-coated graphite tubes, increase in sensitivity was achieved for Cr, Mn and Al [3], Ba [S], Be [3,6] and Si [4]. Also, a great increase in the useful life of the tantalum-carbidized graphite tubes was reported by ZATKA [2]. Another approach was the use of the metal linings or collars inserted inside the graphite tube. Great care had to be taken to ensure that the sample was contained on the metal surface and did not come in physical contact with the graphite surface. L’vov [l] suggested the use of tantalum-lined graphite tubes for eliminating loss of atomic vapour by diffusion through the walls of porous graphite cuvette, especially at high temperatures. A 20-fold enhancement in the sensitivity of Ba was reported by RBNSHAW [7] using a graphite tube lined with a tantalum foil. The poor sensitivity obtained for elements which formed thermally stable carbides was attributed to carbide formation [7]. RUNNEL~et al. [3] suggested that carbide formation might play a role in the * Author to whom reprint [I] [2] [3] [4] [5] [6] [7]
requests
should
be sent.
B. V. L’VOV, Atomic Absorption Spectrochemical Analysis, p. 206. Adam Hilger, London, V. J. ZATKA, Anal. &em. 50, 538 (1978). J. H. RUNNELS, R. MERRV~ELD and H. B. FISHER, Anal. Chem. 47, 1258 (1975). H. M. ORTNER and E. KANTUSCHER, T&ma 22, 581 (1975). R. CIONI, A. ZAZZUCOTELLI and G. O~~ONELLO, Anal. Chim. Acta 82, 415 (1976). TH. STIEFEL, K. SCHUUE, G. Tou; and H. Z~RN, Anal. Chim. Acta 87, 67 (1976). G. D. RENSHAW, At. Absorption Newslen. 12, 158 (1973). 463
(1970).
464
H. S. WAHAB and C. L. CHAKRAJMRTI
atomization of elements from the surface of a graphite tube atomizer. L’vov and PELIEVA[8] reported a significant increase in the sensitivity of 32 of the 40 elements studied by using a Ta-foil-lined HGA 76B graphite tube atomizer. These authors [8] also reported a considerable reduction in the memory effect and a lower atomization temperature for the majority of the elements studied. More recently, GREGOIRE [9] reported a 59.7-fold enhancement in the sensitivity of uranium atomized from a Ta-foil lining (0.05 mm in thickness) on The Perkin-Elmer graphite tubes using a modified HGA 2100. A significant enhancement in the sensitivity of Ag, Al, Ba, Cu, Mn, Ni, Sn, Sr, Ti and Zr was reported by WALL [lo] using a rigid tungsten collar, 1 cm in length and 0.5 mm in thickness, in a graphite tube atomizer. Other authors used atomizers made entirely of a metal such as Ta [ll-141, Pt [15] and W [16,17]. An enhancement in the sensitivity of Ag, Al, Au, Cr, Cu, and Pb was achieved by ACGETT and SPROTT [12] by using a tantalum surface for atomization. CANTLEand WEST [16] reported a 50% increase in lead signals by using a tungsten filament-the increase was relative to the signals from a carbon filament atomizer. 2. EXPERIMENTAL 2.1 Apparatus The Perkin-Elmer Atomic Absorption Spectrophotometer model 603 fitted with a Heated Graphite Atomizer (HGA) 76B, an Autosampler model AS-l, a Deuterium Arc Background Corrector, and a standard yttrium hollow cathode lamp were used. The hollow cathode lamp was operated at a lamp current of 13 mA. The Y 407.7 nm line and a spectral band-width of 0.7 nm were used. The Perkin-Elmer pyrocoated graphite tubes were used. The atomizer was operated in the gas-stop or mini-flow mode with argon as the purge gas. All electron micrographs were taken with JEOL model JSM-U3 Scanning Electron Microscope. 2.2 Reagents All chemicals used met ACS specifications or were of the highest purity commercially available. A stock solution of 1 x lo-3 kg/dm3 of Y was prepared from yttrium oxide (Y203) dissolved in ULTREX hydrochloric acid and diluted with ultrapure water to make the solution 1% (v/v) HCl. Test solutions were prepared from the stock solution by serial dilution with ultrapure water immediately prior to analysis. Ultrapure water was prepared by passing distilled water through a Milli-Q2 water system (Millipore Corporation, Mississauga, Ontario, Canada). Six per cent (w/u) tantalum soaking solution. Three grams of tantalum of 99.998% purity were weighed into a 100 ml of PTFE beaker and 10 ml of dilute hydrochloric acid (1+ l), 3 g of oxalic acid dihydrate, and 0.5 ml of 30% (v/v) hydrogen peroxide were added. The solution was heated carefully until the metal was dissolved. When the reaction became too slow more hydrogen peroxide was added. After completion of dissolution, 4 g of oxalic acid and 30 ml of ultrapure water were added, the acid was dissolved, the solution was diluted with ultrapure water to 50 ml and stored in a high-density polyethylene bottle. and L. A. PELIEVA, Can. J. Spectrosc. 23, 1 (1978). of Chemistry, Carleton University, Canada (1978). C. D. WALL, Talanta 24, 755 (1977). J. Y. HWANG, C. J. MOKELLER and P. A. ULLUCCI, Anal. Chem. 44,2018 (1972). J. AGGETT and A. J. SPROTT, Anal. Chim. Acta 72, 49 (1974). W. G. SCHFUZNK and R. J. EVERSON, Appl. Spectrosc. 29, 41 (1975). T. TAKEUCHI, M. YANACISAWAand M. SUZUKI, Talanta 19, 465 (1972). S. R. GOODE, A. MONTASERand S. R. CROUCH, Appl. Spectrosc. 27, 355 (1973). J. E. CANTLE and T. S. WEST, Tafanta 20, 459 (1973). H. M. DONEGO and T. E. B~JRGESS,Anal. Chem. 42, 1521 (1970).
[8] B.
V. L’vov
[9] D. C. GREGOIRE, Ph.D. Thesis. Department [lo] [ll] [12] [13] [14] [15] [16] [17]
Ottawa,
Ontario,
Studies on the sensitivity
of yttrium by electrothermal
atomization
465
Six per cent (w/v) lanthanum soaking solution. Lathanum chloride hexahydrate (LaCI,.6H,O) crystals (3.8147 g) were dissolved in a minimum volume of 1% (v/v) ULTBEX hydrochloric acid aqueous solution in a lOOm1 beaker, warmed for a few minutes until the solution became clear, then diluted to 25 ml with 1% (v/v) ULTREX hydrochloric acid aqueous solution so that the test solution contained 600 pg of lanthanum in a 10 ~1 injection volume-the optimum mass was 600 pg of lanthanum
r31.
Six per cent (w/v) zirconium soaking solutidn. Zirconium sulphate (Zr(SO,),*4H,O) crystals (5.8431 g) were dissolved in a minimum volume of cold 10% (v/v) hydrofluoric acid solution in a PTFE beaker, then diluted to 25 ml with the same solution, so that the test solution contained 600 pg of zirconium in a 10 ~1 injection volume-the optimum mass was 600 pg of zirconium [3].
2.3 Gases Argon gas of 99.995% purity (Union Carbide Canada Ltd.) was used. The gas was passed through a gas filter model 6183T (Matheson Co., Whitby, Ontario, Canada) in order to remove all particulates of diameter greater than 3.0~ 10m7m (99.999% efficiency). 2.4 Lining with tantalum and tungsten foils Linings of tantalum or tungsten metal (A. D. Mackay, Darien). The procedure was identical to that described by WAHAB and CHAKRABARTI
[18].
2.5 Surface treatment of the pyrocoated graphite tubes Because of the possibility of formation of low-volatility carbides and/or intercalation compounds [8], the surface of The Perkin-Elmer graphite tubes was pre-treated to prevent or minimize formation of these compounds and thereby to enhance peak absorbance. The pre-treatment of the surface of the pyrocoated graphite tubes consisted of soaking the tubes in the under-mentioned aqueous solutions containing 6% (w/v) of any one of the following metals as their soluble salts: tantalum, lanthanum or zirconium. The following procedure for treating the pyrocoated graphite tubes with a 6% (w/v) aqueous solution of tantalum was that of ZATKA [2]. The graphite tube was immersed vertically in the soaking solution of one of the above metal salts contained in a plastic vial. After immersion for 24 h under atmospheric pressure the tube was removed from the bath and dried, first, at room temperature in the air for 30 min, and then, at 105°C for 1 h. The tube was then mounted in the atomizer unit fitted with new, unused graphite contact rings and, while the flow of argon gas and the cooling water was on, the temperature was raised gradually (over a period of 30 s) to 1000°C and then to 2500°C over a period of 5 s. The procedure was repeated for study of the effect of a second coating. 2.6 General procedure The test solution (10 x 1O-6 dm3) was injected into the HGA 76B using the autosampler. The flow of the external sheath gas to the atomizer was maintained at a rate of 2.2~ lo-* dm3 s-l. The gas-interrupt mode was used for atomization except that a minimum flow rate of 8.3X 10e4dm3 s-l was sometimes used. In order to ensure reproducible experimental conditions, the resistance was measured across the workhead terminals with a model 4328A milliohmmeter (Hewlett-Packard Co. Ltd.) and was maintained (with frequent checking) at a constant value 22 rnti at the ambient temperature. This is necessary because the heating of the graphite tube atomizer is electrical Joule’s heating which depends on both the voltage and the resistance of the [18] H. S. WAHAFIand C. L. CHAKRABARTI,
Spectrochim.
Acta 36B, 475 (1981).
H. S. WAHAB and C. L. WARTI
466
atomizer. Variations in the atomizer temperature caused by changes in the flow rate of the cooling water was eliminated by keeping the flow rate of water passing through the atomizer constant at 1.5 dm3 min-‘. The importance of keeping the flow rate of the cooling water constant has been reported [19]. A decrease in the absorption signals with decreasing rate of water flow was attributed [19] to the resultant, higher atomization temperature causing greater expansion of the purge gas (Ar) which increased loss of analyte atoms from the analysis cell. Blanks were run for each determination. All values reported were the average of Bt least seven replicate determinations done successively. 3. RESULTSAND DISSUSSION 3.1 Atomization
from metal-carbide
surfaces
Table 1 presents the optimum experimental conditions used in this study. Table 2 presents the absolute sensitivity of yttrium atomized from various atomizing surfaces. Figures l-8 present scanning electron micrographs of the interior surfaces and edges of the treated and untreated pyrocoated graphite tubes. It can be seen from Fig. 3 that the interior surface of the pyrocoated graphite tube has porous sponge-like appearance, and the porosity increases with increasing temperature (Fig. 4). The increasing porosity causes increasing penetration of the analyte into pores at the high atomization temperature required for yttrium, resulting in the formation of lamellar compounds in which the analyte atoms, molecules, etc. are inserted or intercalated between the graphite sheets. It can be seen from Fig. 2 that the separation between the pyrolytic graphite layers increases with the increased number of heating cycles (firings) completed by a graphite tube. This effect may be due to a combination of the following factors: increased amounts of lamellar compounds formed by the analyte inside the lamellar structure of the pyrolytic graphite, and increased weakening of the loose, layered structure of the pyrolytic graphite caused by thermal stress. Once the analyte has penetrated between the layers of pyrolytic graphite, atomization becomes difficult and incomplete, resulting in memory effect [20] and a loss in analytical sensitivity. Table 1. Optimum experimental
conditions
Atomization Analytical
parameters
Drying temperature (‘C) Drying time (s) Ashing temperature (“C) Ashing time (s) Atomizing temperature (“C) Atomizing time (s) Integration time (s) Ar-purge gas mode Lamp current (mA) Wavelength (nm) Spectral band pass (nm)
A 110 30 500 30 2700 6 8 GS* 13 407.7 0.7
B 110 30 500 30 2700 6 8 GS 13 407.7 0.7
C 110 30 500 30 2700 6 8 GS 13 407.7 0.7
surfaces D 110 30 500 30 2700 6 8 GS 13 407.7 0.7
A = Pyrocoated graphite tube (PCGT). B = PCGT, carbidized twice with 6% (w/v) Ta solution. C = PCGT, carbidiied twice with 6% (w/v) La solution. D = PCGT, carbidized twice with 6% (w/v) Zr solution. E = PCGT, lined with Ta foil, 0.1 mm in thickness. F = PCGT, lined with W foil, 0.05 mm in thickness. * Gas-stop mode. t Mini-flow mode. cl91 E. AIXCLiENSSENS and P. KNOOP, Anal. C&m. Acta 68, 37 (1973). [20] G. HALL, M. P. BRA-L, JR. and C. L. CHAKRABARTI, Talanta 20, 755 (1973).
E 110 30 500 30 2600 8 10 GS 13 407.7 0.7
F 110 30 500 30 2500 6 8 MF-F 13 407.7 0.7
Studies on the sensitivity of yttrium by electrothermal
Table 2. Absolute
sensitivity*
atomization
467
of yttrium atomized from various surfaces under optimum experimental conditionst
Atomization surface Pyrocoated graphite tube (PCGT) Once-carbidiied PCGT with 6% (w/v) Ta solution Twice-carbidized PCGT with 6% (w/v) Ta solution Twice-carbidized PCGT with 6% (w/v) La solution Twice-carbidized PCGT with 6% (w/v) Zr solution PCGT lined with Ta foil PCGT lined with W foil
Absolute sensitivity, kg
Enhancement of sensitivity*
1.8 x lo-= 1.3 x lo-l2
1.4 fold
9.5 x lo-l3
1.9 fold
8.8 x lo-=
2.1 fold
7.9x 10-1s
2.3 fold
4 x lo-‘” 2.8x lo-l3
4.5 fold 6.4 fold
* The mass of element which gives an absorbance of 0.0044 by the peak absorbance mode. t The experimental conditions used are presented in Table 1. $ Enhancement relative to the pyrocoated graphite tube.
In Table 2, the lower sensitivity of yttrium when it is atomized from pyrocoated graphite surface is probably due to the following causes. The important cause is formation of stable carbides by yttrium with pyrocoated graphite at high temperatures [21,22]. Another cause is probably penetration of the analyte between the layers of pyrolytic graphite which results in a greater loss by diffusion of the analyte atomic vapour through the pores of the pyrolytic graphite tube walls to the outside of the
Fig. 1. Electron micrograph (X 1500) of the fracture edge of an unused pyrocoated graphite tube. [Zl] G. V. SAMSONOV,Handbook of the Physicochemical Propertiesof the Elements, p. 515. Plenum Data Corp., New York, (1968). [22] G. DE MARIA, M. GUIDE, L. MALASPINA and B. PESCE, J. Chem. Phys. 43, 4449 (1965).
468
H.S.WAHABandC.L.
@AlCRAEaRn
Fig. 2. Electron micrograph t x 1500) of the fracture edge of a pyrocoated graphite tube after six atomization cycles at 2700°C.
Fig. 3. Electron micrograph i(x 1000) of the interior surface of an unused pyrocoated graphite tube.
Studies
Fig. 4. Electron
on the sensitivity
micrograph
of yttrium
by electrothermal
469
atomization
(X 1500) of the interior surface of a pyrocoated six atomization cycles at 2700°C.
graphite
tube after
graphite tube. L’vov [l, p. 2051 has given equation (1) for the above difision graphite tube wall L 72=DgrSw.
VW
through
(1)
where 72 is the residence time, t, is the thickness of the tube wall, VWis the volume of the walls, P’ is the diffusion coefficient of the analyte atom for diffusion through graphite pores, and S,,, is the area of the walls of the tube. Equation (1) shows an inverse relationship between the residence time, TV, and the diffision coefficient, P’. Compared with the other atomization surfaces in Table 2, the spongy nature of the pyrocoated graphite (Figs. 3 and 4) will allow more rapid diffusion, and hence, will give a shorter r2 value. Such a decrease in the 72 value will decrease the peak height absorbance as explained below. L’vov [23] has given equation (2) for the accelerated atomization, i.e. the atomization under a linearly increasing temperature of the atomizer, as in the present case A where APeak is the peak height absorbance, N, is the number of analyte atoms in the sample, and or is the atomization time-all other terms have been defined earlier. According to equation (2), AWeakis not sensitive to changes in the TJT~ ratio only if the condition T1/T2<< 1 is maintained. However, in the present case TJT~~ 1; and hence, a decrease in the 72 value (with 71 as constant) will decrease Apeak, as observed experimentally. However, as stated earlier, the major cause for the lower sensitivity of yttrium when atomized from pyrocoated graphite surface is formation of refractory yttrium carbide. Formation of carbides can be prevented or reduced by using a non-carbon surface for atomization. Such a surface can be prepared from the graphite tube itself by forming [23]
B. V. L’vov,
Spectrochim.
Acta
33B,
153 (1978).
470
H. S. WAUAB and C. L.
Fig. 5. Electron
CHAKRABARTI
micrograph (X 1000) of the interior surface of a pyrocoated carbidized once using a 6% (w/v) Ta solution.
graphite
tube
refractory carbides of metals (other than the analytes) on the graphite tube surface and covering the entire interior surface with such refractory metal carbides. This can be accomplished by soaking graphite tubes in appropriate solutions, then drying them and subjecting them to further heat treatment as described earlier. The above cycle can be repeated if desired. Figures 5-8 present electron micrographs of tantalum-carbidized pyrocoated graphite surfaces and fracture edges. The effectiveness of the protective
Fig.
6. Electron
micrograph (X 1000) of the fracture edge of a pyrocoated carbidized once using a 6% (w/v) Ta sohltion.
graphite
tube
Studies on the sensitivity of yttrium by electrothermal
471
atomization
Fig. 7. Electron micrograph (X 1000) of the interior surface of a pyrocoated carbidized twice using a 6% (w/v) Ta solution.
graphite tldbe
lays:r for1ned by the tantalum carbide increases with the number of the treatm’ en t cycles twice-treated surfaces (Figs..7 and 8) rplete d, at least for the initial few cycles-the are more effective than the once-treated surfaces (Figs. 5 and 6) presumabt YtJecause the thick :ness of the metal-carbide protective layer increases its effective ne:ss; the thic:kness of the layer increases with increasing number of the treatme nt cycles con lplete d.
Fig. 8. Electron micrograph (X 1500) of the fracture edge of a pyrocoated graphite tuIbe carbidized twice using a 6% (w/v) Ta solution-after 1.5 atomization cycles at 2700°C
472
H. S. WAHAEI and C. L. CHAKRAFMRTI
The enhancement in sensitivities reported in Table 2 for carbidized surfaces can be explained as follows. First, the metal (Ta, La or Zr) carbide surface prevents the underlying graphite from coming in physical contact and reacting with yttrium to form yttrium carbide. Secondly, the metal (Ta, La or Zr) carbides present an impermeable (non-porous) surface to the analyte solution, thereby preventing the solution from soaking and penetrating into graphite pores. Thirdly, carbide-forming metals, such as, Ta, La and Zr are active catalysts for graphitization [24,25], and graphitization decreases the reactivity of the graphite surface. Moreover, penetration of carbides into the pores of graphite surface dissolves the disordered carbon and precipitates it as nonreactive, small graphite crystallites, without a significant change in the composition of the graphite. This process occurs above 2000 K and produces a more highly ordered graphite surface. The difference in the enhancement of sensitivity reported in Table 2 for the three different carbidizing surfaces-tantalum, lanthanum and zirconium-may be due to the difference in the physicochemical properties of their carbides (reactivity, porosity, density, permeability, etc.), or, to non-uniform distribution of their carbides on the graphite surface, and/or, within the layer structure of the graphite body. 3.2 Atomization
from metal surfaces
Table 2 also shows that the use of tantalum or tungsten foil enhances the absolute sensitivity of yttrium. This can be explained as follows. The metal foil completely prevents the physical contact between the analyte and the graphite surface, thereby preventing penetration of the analyte solution into the pores of the graphite tube and eliminating formation of lamellar compounds, and/or, thermally stable carbides. Also, these metal foils permitted use of lower atomization temperature, which increased the useful life of the graphite tubes. The absolute sensitivity with a W foil lining was better than that obtained with a Ta foil lining. This may be probably due to the following. Because of oxidation by air, the tantalum surface is coated with a coherent, stable film of Ta,O,. SISCO and EPREMIAN[26, p. 3391 have reported that growth of the surface oxide proceeds by a mechanism of diffusion of tantalum metal through the film of Ta,O, to the surface. If the rate of diffusion of tantalum metal through the solid Ta,O, is slow in comparison with the rate of atomization of yttrium, formation of yttrium atoms may be somewhat hindered by the growth of Ta,O,. Table 2 shows that the sensitivity of yttrium is higher from a tungsten surface than from a tantalum surface. In the case of the tungsten surface, solid WO, is formed by air oxidation of the tungsten surface above 5OO”C, but the W03 formed is appreciably volatilized above 1000°C [27, p. 421 and is expected to be completely volatilized at the higher temperature employed for the yttrium atomization. WALL [lo] reported the absence of a film of WO, on tungsten surface at the atomization temperature used. Since the present authors used rapid atomization of yttrium, the above reasoning can be applied to explain the difference between the sensitivity of yttrium at these two metal surfaces. 3.3 Memory effect Formation of metal-graphite lamellar compounds and/or refractory carbides causes a loss of sensitivity and the so-called memory effect. Non-uniform distribution of heat and the resulting thermal gradient from the centre of the graphite tube to its ends are also the cause of the memory effect. FINDLAYet al. [28] reported that on heating the HGA 70 furnace up to 8OO”C, the difference in temperature between the middle and the ends of the graphite tube was 5OO”C, and the difference increased at high [24] A. E. KARU
and M. BEER, J. Appl. Phys. 37, 2179 (1966). [25] W. E. PARKER, R. W. MAREK and E. M. WOODRUFF, Carbon 2, 395 (1965). [26] F. T. SISCO and E. EPREMIAN, Columbium and Tantalum. Wiley, New York (1963). [27] G. D. RIECK, Tungsten and its Compounds. Pergamon Press, Oxford (1967). [28] W. F. FINDLAY, A. ZDROJEWSKI and N. QUICKERT, Spectrosc. Lett. 7,63 (1974).
Studies
on
the sensitivity
of yttrium by electrothermal
473
atomization
Fig. 9. Absorbance of Y as a function of number of firings from different atomizing surfaces. V Pyrocoated graphite tube (PCGT). m PCGT, carbidized twice with 6% (w/v) Ta solution. A PCGT, lined with Ta foil. 0 PCGT, lined with W foil.
temperatures. Because of this large temperature difference between the hot middle part of the graphite tube and its cold ends, the vapour of the sample which evaporates from the middle part of the graphite tube condenses at its cold ends. Thus, there is a serious problem of condensation for elements which have high boiling points such as yttrium (b.p. 3338°C). Memory effect can be reduced if not entirely eliminated by impregnating the graphite surface with metal carbides of low volatility, or, by lining the graphite surface with thin foils of metals such as tantalum or tungsten. This is illustrated in Fig. 9 which shows the memory effect for Y as a function of the number of completed atomization cycles (fkings). For the untreated pyrocoated graphite tube, the effect was large, diminished slowly with successive firings, and the residual absorbance was relatively large even after 16 firings. For the W-lined pyrocoated graphite tube, the effect was smaller to start with, diminished rapidly with increasing number of firings, and became relatively small after three firings. Other two surfaces showed behaviour intermediate between the above two extremes. 3.4 Precision Table 3 presents the precision of yttrium determination with various atomization surfaces. The precision was slightly better with the carbidized pyrocoated graphite surfaces and the metal surfaces. The precision was a complex function of the stability of the emission source (the hollow cathode lamp), the age of the graphite tube, the Table 3. Precision*
Atomization
A B C
surface
% R.S.D.
2.5 2.0 1.8
Atomization
D E F
surface
% R.S.D.
1.9 1.9 2.0
A = Pyrocoated graphite tube (PCGT). B = PCGT, carbidized twice with 6% (w/v) Ta solution. C=PCGT, carbidized twice with 6% (w/v) La solution. D = PCGT, carbidized twice with 6% (w/v) Zr solution. E = PCGT, lined with Ta foil, 0.1 mm in thickness. F=PCGT, lined with W foil, 0.05 mm in thickness. *Precision relates to the final measurement step alone and is expressed as a percentage relative standard deviation of at least seven replicate measurements of the peak height absorbance made successively.
474
H. S. WAHAB and C. L. CHAKRAEMRTI
constancy of the atomizer circuit resistance, the reproducibility of the sample delivery step, and the reproducibility of the metal lining fitting (in the case of the metal foils). No attempt was made to determine the individual contributions of the various factors to the overall precision. 4. CONCLUSIONS The following conclusions can be drawn. 1. Carbidizing of the pyrocoated graphite tubes with metal carbides reduces the porosity of pyrocoated graphite tubes and thereby minimizes the penetration of the analyte into the pores. Carbidizing of pyrocoated graphite tubes prevents formation of analyte metal carbides of low volatility, and/or, lamellar compounds of graphite, resulting in an improvement in the analytical sensitivity. 2. Use of a lining of metals-Ta and W-as foils increases the analytical sensitivity, useful life of graphite tubes, and gives shorter atomization times, lower atomization temperatures, and relative freedom from memory effects. Acknowledgements-The authors are grateful to the Natural Sciences and Engineering Research Council Canada for research grants. One of the authors (H. S. WAHAB) is grateful to the Government of Iraq for a scholarship.