Critical examination of the borate fusion technique for spectrochemical trace analysis of geological materials using the d.c. arc

Spectrochimica Acta, 1968,Vol.23B. pp. 739 to 749. Pergamon Press. Printedin NorthernIreland

Critical examination of the borate fusion technique for spectrochemical trace analysis of geological materials using the d.c. arc F. J. M. J. ML~ESSEN and P. W. J. M. BOUMANS* Laboratory

for Analytical Chemistry, University of Amsterdam, Netherlands

Amsterdam

(Received 25 Ii-larch 1908) Abstract-X-ray diffraction analysk permitted a systematic investigation of the influence of fusion temperature, fusion period, sample-to-flux ratio, and particle size on the isoformation of geological materials. Two rational experimental procedures are given. The favourable effect of flux addition on accuracy and precision of spectrochemical results is discussed in terms of a model for t,he exit of the analysis elements from the electrode cavity.

obtaining optimum analysis results in emission spectroscopy, samples and standards must be excited under entirely identical conditions. When analysing non-conducting solid samples using the d.c. carbon arc we have to consider both the chemical composition and the physical and crystallographic state of the material. To eliminate the influence of the latter on spectroanalytical results, samples are pre-treated by borate fusion [l-9], mineralized calcination [lo] or roasting followed by fusion [6, 111. For geological materials-which the present discussion will be chiefly confined to -fusion with alkali metal borates has gained the greatest popularity. Fusion converts samples and standards to the glass state, thus reducing them to a common form (“isoformation” [5, 91). In the application of fusion techniques the following variables must be considered : (i) sample type, (ii) particle size, (iii) fusing agent (flus), (iv) sample-to-flux ratio, (v) fusion temperature, (vi) fusion period. Even when discussion is restricted to the borate fusion of geological materials, literature shows a large diversity in the adopted fusion procedures, particularly FOR

* Present address: Philips’ Research Laboratories, Eindhoven, Netherlands. [l] W. J. PRICE, Spectrochim. Acta 6, 26 (1953). [2] M. F. HASLER, Spectrochim. Acta 6, 69 (1953). [3] S. LANDERGREN and W. MULD, Mikrochim. Acta 245 (1955). [4] W. H. TINGLE and C. K. MATOCHA, Anal. Chem. 30,494 (1958). [5] A. DANIELSSON and G. SUNDKVIST, Spectrochim. Acta 15, 126 (1959). [6] 0. I. JOENSVU and N. H. SUHR, AppE. Spectry 16, 101 (1962). [7] M. S. WANG, Appl. Spectry 16, 141 (1962). [S] C. 0. INGAMELLS and N. H. SUHR, Geochim. Cosnzochim. Acta 27, 897 (1963). [9] A. DANIELSSON, Proc. 13th CoZZ. Spectr. Id., Ottawa 1967, p. 311. Adam Hilger (1968). [lo] P. E. LEMIEUX, AppZ. Spectry 17, 153 (1963). [ll] H. NICKEL and A. PFLUGMACHER, 2. Anal. Chem. 180,401(1961). 739

740

F. J. M. J. MAESSENand P. W. J. M. BOUMANS

with respect to (v) and (vi), e.g. 85O’C for 15 min [6], 1000°C for 8 min [4], 1000°C for 20 min [3], 1200°C for 10 min [7], and 1350°C for 7 min [9]. The confusion in this matter is often due to the entirely empirical approach of the problem. We mention the merely visual inspection of the glass beads for appreciating their quality [7, 121. In the present investigation optimum values for the particle size, sample-toflux ratio, fusion temperature, and fusion period have been established according to rational and objective criteria. Although the background of this study is trace analysis of silicates and related materials, conclusions doubtless have a wider scope. EXPERIMENTAI,

Objective appreciation of bead quality Isoformation with a fusion technique essentially rests on the destruction of the crystal lattice of the original material. Therefore X-ray diffraction analysis gives accurate information about the degree of isoformation. Using such an objective criterion we examined the influence of the variables (ii), (iv), (v), and (vi) on the isoformation process. The flux was LiBO,, obtained by fusing stoichiometric quantities of L&CO, and B,O, at 1000°C. The sample was a mixture of quartz and calcium oxide in the ratio of 9 to 1. Quartz was chosen because it belongs to the most resistive minerals; calcium oxide was added to obtain a raw imitation of the average geological sample with respect to its influence on the excitation conditions in the d.c. arc (see also below). Figure 1 shows the residual quartz content of the beads as a function of the fusion temperature for different particle sizes of the quartz. Figure 2 plots the residual quartz content as a function of the fusion period for different sample-to-flux ratios. Obviously, decomposition occurs the more readily for small particle size, low sample-to-flux ratio, high fusion temperature, and large fusion period. For practical reasons compromises are necessary, however. (i) Rigorous pulverization of a sample is time-consuming, particularly in view of sieving and sieve cleaning (even when done with an ultrasonic cleaner) for particle sizes below 100 ,u. In trace analysis the risk for contamination during these operations also is a major factor. (ii) A small sample-to-flux ratio deteriorates detection limits. (iii) Fusion at high temperature and/or during a long period leads to evaporation losses for volatile elements. Upper limit for fusion temperature The evaporation behaviour of trace elements does not only depend on the chemical composition of the sample and the added agents, but also on the “location” of the trace element [13]. The greatest difference in location is found between a trace element that is mechanically mixed as an oxide with a matrix and a trace element located on a lattice position in the matrix crystals. For establishing the upper limit for the fusion temperature we therefore considered [12] W. FLESSAand [13] Yu. I. BELYAEV

W. KESSLER,Cflastech. Ber. 12, 461 (1963). and A. N. ZAIDEL, Zh. Anal.

Khim.

12, 30 (1957).

Borate fusion technique for spectrochemical

trace analysis of geological materials

100

1 70

8

+- 50

B 04

f

0

-

20

IO -

0

I

800

700

Fusion

900 temperahwe.

I

I IIOJ

Ixc

Oc

Fig. I. Quartz content of beads as a function of the fusion temperature for different particle sizes (microns) of the quartz. Sample-to-flux ratio = 1: 1. Fusion period = 5 min.

80-

I

0

I

I

I

2

1

3

I

4 5 Fusion period,

I 6 min

I

I

I

7

8

9

Fig. 2. Quartz content of beads as a function of the fusion period for different sample-to-flux ratios. Fusion temperature = 900°C. Particle size of the quartz = 105-150 /A 4

741

742

F. J. M. J. MAESSEN and P. W.

J. M. BOUMANS

two test samples, viz. a natural sample and a synthetic standard. The natural sample was a potassium feldspar containing about 60 ppm of lead in the crystal lattice [14]. The synthetic standard was quartz powder to which various elements, mostly of volatile character, were added in concentrations up to 1000 ppm (see Table 1). The test samples were heated at different temperatures in the range soo-1300°C.

In the same experiment we studied the influence of the flux on the evaporation behaviour of the trace elements for when additives that can react chemically with the sample are present. As additives we considered lithium fluoride and graphite, since these agents are used as diluents in the spectrographic method used in our laboratory [ 151. Thus some insight was obtained in the effect of the flux at temperatures up to 1300°C. Test samples with$ux but without further additives. 200 mg of a test sample were mixed with 200 mg flux and heated during 5 min in a graphite crucible placed in a furnace in which the desired temperature was previously established. This temperature was successively adjusted to 800, 900, 1000, 1100, 1200, and 1300°C. Each test concerned a fresh sample ; accordingly a test sample was only once heated for 5 min. Test samples without flux but with other dditives. Test samples were heated with LiF, graphite, and LiF + graphite: 200 mg sample and 100 mg LiF; 200 mg sample and 300 mg graphite ; 200 mg sample, 100 mg LiF, and 300 mg graphite. The ratios link up with those used in electrode charges for spectrographic analysis [ 151. Each mixture was heated for 5 min at the six temperatures stated. Fused samples with additives. Test samples were mixed with flux in the ratio 1: 1 and fused at 1000°C. 200 mg of a pulverized bead were heated with 100 mg LiF This experiment was also conducted at the relevant six and 300 mg graphite. temperatures. Determination of evaporation losses. Trace constituent determinations in each test sample after treatment in the furnace permitted us to establish the maximum temperature at which samples can be safely fused. The highest temperature at which evaporation losses were still absent will be indicated as t*. Numerical values of t* have been summarized in Tables 1 and 2. Results for standards with flux added show that at the low concentration level considered here a fusion temperature of 1000°C is allowed for all elements in concern. Closer inspection of the results reveals that the flux inhibits the chemical reactions of the metal oxides with the additives (graphite, LiF) in the synthetic standard. This is not observed, on the contrary, for lead in the potassium feldspar. The lattice position of lead in the feldspar obviously renders the element already less accessible for the added agents. The difference in location of lead is also brought out by the different t* values for the synthetic standard and the natural sample when heated without any additive, respectively t* = 1200°C and t* > 13OO’C. A crucial point is that for both the [14] K. S. HEIER, Norsk. Geol. Tidsskr. 42, 415 (1962). [15] P. W. J. M. BOUMANS, Proc. 13th CoZZ. S~ectrosc. ITL~., Ottawa 1967, p. 136. Adam Hilger (1968); Proc. 14th CoZZ. Spectr. I&., Debrecen 1967, p. 819. Felsiioktathsi JegyzetellLM VBllatat (1968).

+ graphite + LiF -+-graphite + LiF without additive f flux + flux + graphite -I- LiF 2:3 2:l 2:3:1 1:1 1:1:3:1

Ratio

800 900 800 1200 1100 1100

100

Pb

800 900 800 1100 1000 1000

1000

Sb

800 900 800 1200 1000 1000

500

Bi

> 1300 > 1300 >1300 >1300 > 1300 > 1300

10 800 900 800 1200 1200 1100

500

Test elements Al In

800 900 800 1100 1000 1000

1000

Cd

+ graphite + LiF + graphite -I- LiF without additive -i- flux + flux + graphite -t LiF

2:3 2:l 2:3:1 1:l 1:1:3:1

Ratio

1100 1100 1100 > 1300 1100 1100

Test aIement Pb

for lead in a potassium feldspar sample with different additives?

t t* = highest temperature for which no evaporation losses, that is < 57&, of trace element were observed.

Sample Sample Sample Sample Sampfe Sample

values of t*(T)

Test mixture

Table 2. Experimental

800 900 800 1200 1200 1100

100

Sn Ag 10 1200 1100 1200 > 1300 >1300 >1300

for test elements in the synthetic standard with different additives?

t t* = highest temperature for which no evaporation losses, that is < 57e, of trace element were observed.

Standard Standard Standard Standard Standard Standard

Concentration of test element (ppm)

Test mixture

Table 1. Experiment~al values of t*(V)

1000 900 1000 900 > 1300 >1300 1200

Zn

744

F. J. M. J. MAESSEN

and P. W.

J. M. BOUMANS

synthetic standard and the potassium feldspar the addition of flux leads to the sccme t* value (1100°C for lead). To conclude, as a result of fusing (i) the difference in location for trace elements in natural samples and synthetic standards is removed, (ii) a new matrix is formed which dominantly determines the evaporation behaviour of the trace constituents. Adopted fusion procedures On the basis of the results in Table 1 and Figs. 1 and 2, a multiplicity of rational fusion procedures can be chosen. We adopted the following alternatives for our practice: (i) Sample-to-flux ratio = 1: 2. Fusion temperature = 9OO’C. Fusion period = 5 min. Particle size ~125 p. (ii) Sample-to-flux ratio = 1: 1. Fusion temperature = 1000°C. Fusion period = 5 min. Particles size <125 p. Procedure (ii) follows immediately from Fig. 1. Procedure (i) is derived from Fig. 2 if we inspect the curve for a ratio of 3: 7 (which is close to 1: 2) and consider that the curve for a particle size <125 p will be shifted to the left, in compliance with Fig. 1. For illustrating the efficiency of these procedures Fig. 3 shows the X-ray diffraction spectra of biotite and microcline, prior to and after treatment with flux according to procedure (i). Discussion

Improvement

of analytical results by jlux addition

From both the literature [l, 2, 6, 111 and observations in our laboratorythat will be discussed at length in three subsequent papers[l6]it is evident that a fusion technique enhances the precision (reproducibility) and reduces bias in spectroanalytical results. How is this to be explained? Frequently an explanation is sought in the influence of the flux on the “evaporation curve”. No doubt, additives can exert an appreciable influence on the evaporation pattern [ 11, 17-271. This happens particularly when thermochemical reactions 1161 P. W. J. M. BOUMANS and F. J. M. J. MAESSEN, S~ectrochim.Acta, to be published. 1171 0. LEUCHS, Spectrochim. Acta 4, 273 (1950). P61 E. SCHROLL, 2. Anal. Chem. 198,40 (1963). [I91 E. SCHROLL, Proc.

14th Co.% Spectr. Int., Debrecen 1967, p. 397. FelscoktatBsi Jegyzetell&t6 VBllatat (1968). L~~m~,Zavodsk. Lab. 29,684 (1963): I201 V.L. GINZBUR~, N.P. GLUKHOVETSKAYA andL.A. Ind. Lab. USSR 29, 729 (1963). r_211A. A. FRISHBERG, Zh. Prikl. Spektrosk. 8, 187 (1965); J. Appl. Spectry USSR 3, 137 (1965). [221 R. RAUTSCHKE, Spectrochim. Acta 23B, 55 (1967). Spectrum 1231 A. N. ZAIDEL, N. I. KALITEEVSKII, L. V. LIPIS and M. P. CHAIKA, Emission Analysis of Atomic Mute&&. Moscow (1960). U.S. Transl., U.S.A. Atomic Energy Commission, AEC-tr-5745 (1963). c241 E. SCHROLL and P. HAWK, Mikrochim. Acta 731 (1964). 157 (1966). 1251 J. BRIL and J. LORE, M&h Phys. Ana&e (B.A.M.S.) I261 J. BRIL, Spectrochim. Acta 23B, 375 (1968). r271 H. NICKEL, Spectrochim. Acta 23B, 323 (1968).

Fig. 3. S-my diffrttction spectra for: (it) biotite + fluq not fused; (11) biotite + flux, hsod;

(0) microcline (d) microcline

i_ flux, not fused; _t &IX, fused.

Borate fusion technique for spectrochemical

trace analysis of geological materials

745

convert the original components into compounds with markedly altered physical properties. Depending on the chemical composition of the electrode content, reaction products with substantially different volatilities can be formed as a result of thermochemical reactions (oxidation, reduction, halidation, sulphidation, formation of carbide). The very reaction products that are formed (with a given temperature distribution in the electrode) and the rate at which the reactions proceed do not only depend on the chemical but also on the physical conditions of the electrode content. Accordingly, the evaporation of an element can proceed differently from a natural or a synthetic standard, in spite of closely matched chemical compositions. This difference in behaviour must be attributed to the difference in “chemical contact” between the trace element and its environment. Such an influence of the chemical contact is illustrated, for example, by our t* value of 800°C for zinc oxide in the presence of graphite of 50 fl particle size, whereas ADDINK [28] reports evaporation losses below 600°C when ashing biological samples. The vital role of the chemical contact was also convincingly demonstrated by SCHROLL and HAUK [24] for the fluoridation of boron from B,C samples of different particle size. A fusion technique can remove a difference in the evaporation behaviour of an element in a natural sample and a synthetic standard; this effect is due to the influence of the flux on the thermochemical reactions in the electrode. This influence can be direct (reactions with the flux) and/or indirect (modification of chemical contact). The favourable inJluence of fusion techniques on analytical results can not be explained, however, from the simple fact that the evaporation curves for trace elements become similar for samples and standards; for the enhancement of precision and accuracy is found also when the evaporation patterns for samples and standards are already identical before fusing. We observed this when studying the evaporation of trace elements from the synthetic standard and the potassium feldspar in a LiF-graphite arc with gasstabilization. The flux did not affect the evaporation curves but appreciably improved the precision and accuracy of the analysis results. To conclude, there is an effect that cannot be simply reduced to a modification of the evaporation pattern. Indeed there are no obvious reasons-except for self-absorption and Schwarzschildt effect-to believe that in an ideally buffered arc different evaporation patterns should result in different time-integrated intensities when the electrode charge is burnt to completion [lS, 291. Explanation

of enhanced accuracy by fusion technique

In previous discussions on the transport mechanism of the d.c. arc [29-321 we pointed out the uncertainty in our knowledge of the mechanism that governs [28] N. W. H. ADDINK, Rec. Trav. Chim. Pays-Bus 70, 168 (1951). [29] P. W. J. M. BO~ANS, Theory of Spectrochemical Excitation. Hilger and Watts (1966); Plenum Press (1966). [30] P. W. J. M. Bo UNANS and L. DE GALAN, Anal. Chem. 38, 674 (1966). [31] p. W. J. M. BOUMANS, Proc. 2nd Int. Symp. High-Purity Mate&a& Dresden 1965, Vol. 2, Reinststoffanalytik, p. 103. Akademie-Verlag (1966). [32] P. W. J. M. BOUMANS, Proc. 14th CoZZ. Spectr. I&., Debrecen 1967, p. 23. Felscioktat&si JegyzetellBt6 Vdllatat (1968).

746

F. J. M. J. MAESSEN

and P. W.

J. M. BOTJNANS

the transit of metal vapours from the electrode to the excitation region. It is profitable, in the present context, to specify this problem more precisely. When using emission spectra for studying transport phenomena we must recognize that our information originates only from those particles that have actually entered the excitation region. In other words, results such as evaporation curves derived from racking-plate spectra inform us about only one path along which an element leaves the electrode cavity. More paths should be considered, however. Schematically we have the following (Fig. 4).

lb

Ilb' I

Fig. 4. Schematic representation of possible exit paths for an analysis element to leave the electrode cavity.

(i) Exit through the open end of the electrode crater into the excitation region, where two possibilities must be distinguished depending on the state of the relevant element. (a) The element enters the excitation zone in a completely atomized state or is immediately atomized after its entry. (b) The element is in a bound state for some part of its residence time in the excitation zone. The bound state comprises solid particles, that partially or completely evaporate during their flight through the excitation region, and stable molecules, particularly

Borate fusion &&nique for speetrochemiod trace analysis of geological materials

747

&atomic oxides. The latter effect can be dealt with adequately in terms of thermodynamic dissociation equilibria [29, 32, 331. (ii) Exit through the open end of the electrode crater but with bypass of the excitation region, where again atomic vaponr, larger solid aggregates (sputtering), and molecules must be considered. (iii) Sideward difi%sion through the electrode matis. (iv) Downward diiiusion (i~usion) into the bottom of the crater. The various possibilities are depicted schematioalIy in Fig. 4, Continuous arrows represent the amounts of an element leaving the electrode by different paths. Broken arrows pertain to the corresponding line intensities, constant excitation conditions and total eons~lmpt~on of sample being assumed. In this s~hernat~~ Ic

1 lcI

T4

F

C

a

Fig. 5, Different distributions of oh&ad amount of element over the exit paths (cf. Fig. 4). Diqmtxm A, B, and C: Iink up with experimenta observations for tho oxit of lead from s potassium feldspar with a particle rJizeof 200 p, 100 p, and fused. Dittgrwn D depicts the theoretically ideal situation. representation the ratio of the lengths of arrows la and ia’ depends on the excitation conditions; for eonvenien~e it was taken to be unity. The ratio for arrows lb and Ib’ also depends on the exxcitation conditions and moreover on the rate at which the solid particles volatilize when travelling through the discharge zone. In general, this ratio is smaller than that for la and la’. Evidently, since the sum of the lengths of the continuous arrows represents the total amount af an element, this sum must be kept constant when conditions are varied. For illustration I?&. 5 shows different distributions of a fixed amount of element over the exit paths. Naturally the highest eEicienc?y is attained when the transport completely follows path la. In terms of the model dqgicted by Pig. 4 it is clear that chemical, physical, and

748

F. J. M. J. MAESSEN and P.

W. J. M. BOUMANS

mechanical processes in the electrode cavity exert an in$uence on accuracy of spectrochemical analysis only if these processes, for natural samples and synthetic standards, lead to different distributions of the analysis elements over the exit paths. In other words, for an accurate analysis the chemical, physical, and mechanical processes should be controlled so that the distribution of an element over the exit paths is the same for samples and standards. Suppressing of analytical bias as a result of the application of fusion techniques must be attributed to the thermochemical and physical properties of the new matrix that is formed from the flux and the sample or the standard. This new matrix contains the analysis element from a natural sample and a synthetic standard in a similar form [l l] ; also the chemical contact with further additives is identical because adequate fusion leads to physically similar glass-like substances for samples and standards. The required identity of the distribution of an analysis element over the exit paths is thus secured. Explanation

of enhanced precision by fusion technique

The fuse matrix proves advantageous to the stability of the arc, especially if combined with gas-stabilization [16]. Arc stability rests for some part on the regular supply of an easily-ionized element, which stabilizes the temperature and the electron concentration in the discharge region [29,33]. The low melting point (SOO’C) of the fuse matrix ensures molecular evaporation and this process altogether is better reproducible than, for instance, sputtering. A rational combination of gas-stabilization and the fusion technique ensures a regular evaporation of the buffer element, lithium, along with the bulk of the sample [16]. Some fractional distillation, particularly for highly refractory elements, should be tolerated, however [16]. By itself the controlled volatilization of the analysis elements achieved by fusing contributes essentially to the enhancement of the reproducibility. This is readily understood in terms of the model discussed above. Without fusing there is a marked exit from the electrode by the paths lb and 2b. Inherently, the magnitude of the portions of an element leaving the electrode by these paths depends largely We found, for example, that when samples on the particle size and its distribution. were burnt to completion in a buffered arc, the spectral-line intensities for trace elements in natural silicates decline rapidly with increasing particle size [34]. This must be attributed to the enhanced exit by paths lb and 2b as the particle size grows larger. The influence of the particle size on the distribution over the exit paths is illustrated schematically in Fig. 5. Diagrams A, B, and C link up with experimental observations for the exit of lead from the potassium feldspar with a particle size Diagram D depicts the theoretically ideal of 200 ,u, 100 ,u, and fused respectively. Clearly, reproducibility is largest when made independent of particle situation. size. This is realized by the fusion technique. Figure 5 also demonstrates the extent of the systematic error occurring in a determination of lead in a potassium feldspar of 200 ,U using a 100 p feldspar as a standard. [34] P. W. J. M. BOUMANS, F. J. M. J. MAESSEN, and A. SPEEE, internal report.

Borate

fusion technqiue for spectrochemical trace analysis of geological materials

Detailed experimental material that illustrates presented in three subsequent papers [16].

the foregoing

749

discussion will be

Requirements upon the chemical composition of standards Synthetic standards for spectrographic analysis of geological materials are often composed of oxides so that the average composition of the relevant natural samples is closely matched. Is this necessary! If the object is to create the same temperature and electron concentration in the arc for samples and standards, the purpose is more satisfactorily realized by the application of a buffer of low ionization potential [29, 31, 331. Then, the chemical composition of the sample need not be accurately reflected in that of the standard. If, on the other hand, the thermochemical reactions in the electrode should proceed similarly for samples and standards, accurate matching of the chemical compositions is useful neither. For, as we considered above, the extent of chemical contact is a governing feature. This point, in turn, is adequately settled by applying Consequently a synthetic standard with a simple chemical a fusion technique. composition will in general satisfy. For geological samples such a matrix can be composed of quartz and calcium carbonate. Preliminary

treatment of samples prior to fusing

Sulphides, carbides, free metals, or carbon, when present in a sample, have an unfavourable influence on the properties of the fuse matrix. The fuse tends to become porous and inhomogeneous, its fusing behaviour being seriously modified [6]. It is desirable therefore to roast those samples in a stream of oxygen, prior to fusing, so as to convert the interfering components into oxides [6, 111. Acknowledgements-The authors are greatly indebted to Dr. Th. W. M. LEVELT, Mr. L. W. S. DE GRA_~FFand Mr W. E. OOSTERBAANfrom the Physicogeographical Laboratory of the University of Amsterdam for conducting the X-ray analyses of the tests samples, and to Dr. N. A. I. M BOELRIJK from the Laboratory for Isotopic Geology for putting the minerals at their disposal. Mr. J. W. ELGERS~TAis acknowledged for his valuable assistance in the experimental work