Melting fluxes for use in automatic sample preparation for inductively coupled plasma atomic emission spectrometry

0584-s547/n6 s3.00 + 0.00 0 w36.Fcq?mcmplruLtd.

&&mhbh, Acta,Vol.418,Nos l/2,pp. 73-79,1986. printed in ChwtBritain.

Melting fluxes for use in automatic sample preparation for inductively coupled plasma atomic emission spectrometry A. A. W~MANN and F. R. KOP Institut de Recherches de la Siderurgie Fran&se,

57210 Maizieres-les-Metz, France

(Received 5 March 1985; in revised form 24 June 1985) Abstract-In previous work, the French Institute for Iron and Steel Research (IRSID) developed an automatic solution preparation apparatus for inductively coupled plasma atomic emission spectrometry (ICP-AES), atomic absorption spcctrometry (AAS) or any other physico-chemical or chemical method. However, the applications were limited to steel manufacturing products and the number of melting fluxes to two or three. Now that “PLASMASOL”, the new device allowing automatic preparation by fusion-dissolution at high temperature of powder samples, is in full operation, it seems interesting to test this device in other fields of application and above all to examine the sample digestion ability of different fluxes. Present work reviews these fluxes, especially their behaviour related to glassy carbon crucibles, and their aptness in the dissolution of various kinds of samples.

1, INTR~DUC~~N IRSID,the

French Iron and Steel Research Institute has developed an apparatus for automatic sample dissolution called PLASMASOL [l]. This device allows for the dissolution of oxide samples in a flux at high temperature and the casting of the mixture into a water filled beaker. Similar devices exist on the market [2-6], but PLASMASOL is the only one using a “composite crucible” as melting pot. The “composite crucible” consists of two containers placed one inside the other. The external container is a crucible made of platinum alloy and the inner one a glassy carbon crucible. The first applications of PLASMASOL were limited to steel manufacturing products and the melting fluxes to two or three. The purpose of the present work is to examine the ability of other fluxes to dissolve various kinds of products and to control their behaviour in relation to the glassy carbon crucible. To perform good preparations with this device, it is indeed necessary to recover the whole mass of the molten mixture, so leaving the crucible perfectly clean. 2. FLUXESUSUALLY EMPLOYED By reviewing publications concerning sample preparation by fusion techniques, the following observations can be made: (1) 33 % of the concerned authors use lithium metaborate [3, 7-181; Cl] A. A. WITTMANNand G. H. WILLAY, Spectrochim. Acta 4OB,253 (1985). [2] C. S. KEMPF, R. R. KACZMARCZYK, A. M. BIANCHI,V. J. SNODGRASS, F. W. P. BOCKMANN and J. A. RITZ, Abstracts Pittsburg Conference 1982, Abstract No. 039. [3] I. B. BRENNER,I. GAL, H. ELDADand D. HOFFER,ICP Information Newslet?. 8,567 (1983). [4] K. GOVINDARAJLJ,Preprints 23rd Coll. Spectrosc. Int. and 10th Int. Conf. Atom. Spectrosc. Amsterdam 1983, Abstract No. 120; and K. GOVINDARAJU and G. MEVELLE, Spectrochim. Acta 3SB, 1447 (1983). [S] M. BORSIERand M. GARCIA, Proc. International Conferenceon Industrial Inorganic Element Analysis, 1981. Philips Science and Industry Division Press Office, p. 137. Eindhoven, The Netherlands. [6] D. HOFFER, I. B. BRENNER and L. HALICZ, ICP Information Newsfett. 9,494 (1984). [TJ T. CATALANO, H. HARRIS and J. SARDONE,ICP Information Newslett. 7,243 (1981). [8] D. S. CHASE,D. D. NYOAARD,D. A. LEIGHT~and M. J. CONLEY,ICP Infirmation Newslett. 9,513 (1984). [9] A. Y. CANTILLO,S. A. SINEXand G. R. HEU, Anal. Chem. 56, 33 (1984). [lo] C. FELDMANN,Anal. Chem. 55,245l (1983). [ll] A. Z. GEDEON,ICP Information Newslett. 9, 418 (1983). [12] C. L. Ho and S. TWEEDY,ICP Information Newslett. 9,659 (1984). [13] M. P. JAVALLANA, I. JAWEDand D. APPLER,Abstracts Pittsburg Conference 1984, Abstract No. 747. [14] G. SCHE~TLER,Z. Angew. Geol. 29,453 (1983). [15] A. A. VERBEEK,M. C. MITCHELLand A. M. URE, Anal. Chim. Acta 135,215 (1982). [16] J. N. WALSH,Spectrochim. Acta 37B, 107 (1980). [17] P. J. WATKINSand M. THOMPSON,Geostand. Newslett. 7,273 (1983). Cl83 G. W~~N~CH and N. CZECH,Fresenius Z. Anal. Chem. 317, 5 (1984). 73

14

A. A. WITTMANN and F. R. KOP (2) 15 % of the concerned authors use sodium peroxide [ 12, 19-23 J;

(3) 13 % of the concerned authors use lithium tetraborate [24-281; (4) 10% of the concerned authors use sodium hydroxide [3, 12, 29, 303; (5) 10% of the concerned authors use a mixture of lithium carbonate and boric acid [31-343; (6) the 19% remaining authors [35-41] use other fluxes such as sodium tetraborate, sodium carbonate, potassium carbonate, potassium hydrogen sulfate, potassium oxide, potassium hydroxide, acid potassium fluoride, different mixtures like sodium and lithium metaborate. It clearly appears that the lithium metaborate fusion described by SUHRand INGAMELS [42] in 1966 is the most widely used method for silicate and rock sample preparations. Often, lithium metaborate is replaced by a mixture of lithium carbonate and boric oxide. Leaching the melt with hydrochloric or nitric acid completes the preparation procedure. The second most employed flux is sodium peroxide. Sintering is performed at a temperature between 400 and 700°C and this flux is used for difficult dissolutions. Addition of 10 y0 sodium carbonate slows down the reaction and graphite crucibles can be used. Ho and co-workers [12] use sodium peroxide for the preparation of chromites, cassiterites and zircons. BENNETTand coworkers [44] use both lithium metaborate and tetraborate for minerals and refractories. For refractory minerals, FELDMAN[lo] uses sodium perborate in association with lithium metaborate. FLOYDand co-workers [29] chose sodium hydroxide for the preparation of geochemical and environmental matrices followed by a hydrochloric acid digestion. NADRARNI [38] prepares coal and fly ash samples by fusion with lithium metaborate for ICP analysis and with sodium carbonate for the determination of fluorine with an ion selective electrode. Taking into account all these observations, the different fluxes which must be tested are the

[19] H. R. MIDDLETON, G. M. RU~~ELand A. E. WATSON,Report 1983 MINTEK-M90. [20] J. 0. RASMUSON, ICP Information Newsletr. 9, 530(1984). [21] J. 0. RASMUSON, J. MEARS,G. F~GLEMAN and F. PITARD,ICP Information News/err. 8, 514(1984). [22] A. E. WATSON,G. M. RUSSELand G. BALAES,Rept. Nat. Inst. Metall. 1976, No. 1815. [23] A. E. WATSONand G. M. RUSSEL,ICP Injormation Newsiett. 4,441 (1979). [24] R. I. BOTTO,ICP Information Newsleft. 5, 223 (1979). [25] R. I. BOITO,ICP Information News/err. 6, 126(1980). [26] J. DEBRAS-GUEDON and N. LIODEC,Science Ceramics 11, 65 (1981). [27] J.C.MILLSand M. L. EDWARDS,ICP Information Newsiett. 9, 315(1983). [28] H. WESTLEY, Abstracts Pit&burg Conference 1984, Abstract No. 484. [29] M. A. FLOYD,A. P. D’SILVA,V. A. F~ss~~and M. TSCHETTER, Abstracts Pittsburg Conference 1979,Abstract No. 351. [30] M. A. FLOYD,V. A. FASSELand A. P. D’SILVA,Anal. Chem 52,2168 (1980). [31] K. GOVINDARAJU, G. MEVELLE and CH. CHOUARD, Anal. Chem. 48,9, 1325 (1976). [32] E. Yu. PANTBLEEVA, N. I. GULKO and V. M. LUR’E,Zad. Lab. 47,33 (1981). [33] M. S. VIGLER,A. W. VARNESand H. A. STRECKER, hr. Lob. Oct. 51 (1981). [34] A. WITTMANN, J. HANCART, W. HUGHESand K. OHLS, Developments in Atomic Plasma Spexrrochemical Analysis, Ed. R. M. BARNES,p. 550. Heyden, London, Philadelphia (1981). [35] J. A. C. BROEKAERT, F. LEISand K. LAQUA,Spectrochim.Acra 34B, 167 (1979). [36] Y. ENDS and N. SAKAO,Transactions ISIJ 23 (1983). [37] C. L. Ho, S. TWEEDY and D. GOWER,ICP Information Newsierr. 8,605 (1983). [38] R. A. NADKARNI, Anal. Chem. 52,929 (1980). [39] H. SAISHO,M. TANAKA,K. SUSHIDA and K. NAKAYAMA, Abstracts Pittsburg Conference 1984, Abstract No. 577. [40] K. SATO,I. TANAKA,M. AKIYAMA and T. OTSUKI,ICP Infirmarion Newslerr. 5,639 (1980). [41] Y. ENXI and N. SAKAO,Tersu-to-Hag& 66, 1395 (1980). [42] N. H. SUHRand C. 0. INGAMELS, Anal. Chem. 38, 730 (1966). [43] Geosrandards Newslerr. Appendix I, 07, July (1984). [44] H. BENNETT and G. J. OLIVER,Analyst 101,803(1976).

Melting fluxes in automaticsample preparation

75

following:

l l l l l l l l l l l l

lithium metaborate lithium tetraborate lithium orthophosphate lithium fluoride lithium carbonate sodium tetraborate sodium carbonate sodium fluoride boric oxide sodium peroxide sodium hydroxide different commercial mixtures

: LiBOz : Li2B40, : Li3P04 : LiF : LizCOJ : Na2B407 : Na2C03 : NaF : BzOo : Na,Oa : NaOH

Fusion temperature (“C) 845 930 837 842 723 741 851 988 460 460 318 near 1000

3. REQUIREMENTS FOR THE PREPARATION BYPLASMASOL The most important parameter which has to be considered is the recovery of the melt after the fusion-dissolution process is achieved. This recovery must be complete. If drops of flux stick to the bottom or the wall of the crucible, then the final dilution of the solution is not exact and the analytical results will be false. Glassy carbon crucibles are used for these preparations, so all fluxes presenting a tendency to stick to the crucible must be avoided. Another important point to note is the reaction of the hot melt with’water. At the end of the fusion, the melt is poured into a water filled beaker. All fluxes giving a violent reaction with water must be discarded. Taking into account these requirements, the following fluxes cannot be used alone for the preparations by PLASMASOL: boric acid lithium carbonate 0 sodium carbonate 0 ammonium carbonate l potassium-sodium carbonate l sodium peroxide l lithium fluoride l sodium fluoride l

l

(sticking to the crucible) (sticking to the crucible) (sticking to the crucible) (sticking to the crucible) (sticking to the crucible) (sticking to the crucible) (violent reaction with water) (violent reaction with water)

It could be possible, for these last two fluxes, to replace the water filled beaker by a platinum dish and to pour the melt into this dish forming a bead as done for X-ray fluorescence. Afterwards the bead is dissolved in diluted acid. With these fluxes, the preparation time is a little bit longer. 4. PREPARATIONS BYPLASMASOL 4.1. Various kinds of fluxes Fluxes can be divided into two groups. The first group contains pure components, the second contains commercially available mixtures. 4.1.1. Pure&es. Qualitative trials showed that the following fluxes can be utilized with PLASMASOL: lithium 0 lithium l lithium l lithium

l

and sodium tetraborates, metaborate, orthophosphate, and sodium fluorides with some precautions.

For some of these fluxes it could be advantageous to add strontium nitrate to increase the

A.A.W~MA~N

76

and E

R.KOP

oxidation efficiency of the melt. The proportions could be 2000 mg of flux and 500 mg of sample or 2000 mg of flux, 500 mg of sample and 500 mg of strontium nitrate. Two preparation programs can be used: program 1 in the case of casting in a dish and program 2 for casting the melt in a beaker. This latter is shown in the sample preparation chart (Fig. 1).

Sample

weight 500 mg

Flux weight 2000 mg

l---------

l I I

wlthout osclllatlon

: 200 sec. 9800~

1 I

"PLASMASOL" t t I ,

Water : 200 ml Heating stirrer

For the preparations with fluorides, the dissolution of the bead in dilute hydrochloric acid (50 : 50 vol) at 90°C takes 25 min with sodium fluoride and is impossible with lithium fluoride. For the other preparations after pouring into a beaker containing 50 ml of water, 150 ml of water and 15 ml of hydrochloric acid are added and the beaker is placed on a heating magnetic stirrer and heated to a temperature of 90°C. Table 1 shows the time necessary for the full dissolution of the melt for three different samples and for the various 5uxes with and without strontium nitrate. Depending on the flux and on the type of sample, the dissolution time varies from 2 to 8 min. It must be noticed that silicon oxide precipitates in the case of refractory samples, and magnetic oxides stick to the magnet in the case of sinter samples when the fusion is performed with lithium metaborate. 4.1.2. Co~rciai~y a~ail~~e @WCmixtures. Table 2 lists different fluxes tested with PLASMASOL. Manufacturer’s name, the composition of the fluxes and the proportions of the components are indicated. All these fluxes behave correctly during the fusion cycle; but flux No. 7 reacts with water leading to some minor projections. With 500 mg of sample and 2000 mg of flux a fusion time of 6.5 min and a r~issolution time of 8 min, the samples are ready for measurement by inductively coupled plasma atomic emission spectrometry (ICP-

Melting fluxes in automatic sample preparation Table 1. Time for full dissolution of the melt (in min)

Fhlx

Iron ore Sinter

Ste&naking s4

Refractory

LiZB407 Li2B40, SWO3b NasB,O, NasB& SW0312 LiBOs LiBOs SrWOd2 LisPO, LisPO* SW0312 Silica precipitates for refractory samples. Magnetic oxides stick to the magnetic bar with the iron ore sinter sample and lithium metaborate.

Table 2. Various commercially available flux mixtures

Company ICPH

JOHNSON-MATTHEY

MERCK

SPEX

SAW

41:1/2-F

Composition lithium tetraborate + lithium metaborate sodium tetraborate + sodium metaborate lithium tetraborate + lanthanum oxide “FLUOSID 21” lithium tetraborate + lanthanum oxide + boric acid “Spectroflux” lithium tetraborate + lithium metaborate lithium tetraborate + lithium metaborate + lanthanum oxide lithium tetraborate +lithium metaborate + lanthanum oxide + lithium fluoride lithium tetraborate + lithium fluoride +strontium oxide sodium tetraborate + lanthanum oxide lithium tetraborate + lithium carbonate lithium tetraborate + lithium carbonate -+lanthanum oxide

Proportions (%I

Test numbers

50-50

1

5&50

2

85-15

3

76-21-3

4

20-80

5

58-29-l 3

6

57-27-12-10

7

85-10-5

8

90-10

9

90-10

10

47-37-16

11

A. A. WITTMANNand F. R. KOP

78

AES) or atomic absorption spectrometry (AAS) or any other method, except in the case of silicon oxide precipitations where a preliminary filtration is necessary. 4.2. Preparation of various kinds of products Commonly used in our laboratory, Fluosid 21 has been chosen for the preparation of various kinds of products. Table 3 lists the products for which normal behaviour was found Table 3. Products for which normal behaviour was found during the preparation process Bauxite G&S Serpentine Disthene Felspar Sillimanite

BX-N VS-N UB-N DT-N FK-N BCS-309

Diorite Phlogopite Biotite Granite Basalt Granite

DR-N MICA-Mg MICA-Fe GH BR GA

Table 4. Certihed reference material, listed values f and standard deviations of the certificate and analytical result c (in %)

Sample BX-N

TiOs

CaO

PsOs

SiOs

54.21 1.16 55.10

2.37 0.23 2.36

0.17 0.14 0.25

0.13

7.40

2.89 2.78

13.44 13.65

1.08 1.07

4.53 4.30

5.83 0.25 5.78

2.90 0.30 2.83

0.11 0.05 0.11

1.20 0.11 (1.35)

0.04 0.04 0.020

39.43 0.53 [25.10]

0.12 0.03 0.129

0.46 0.12 0.44

59.20 1.35 59.07

1.40 0.12 1.42

0.04 0.11 0.13

0.09 0.04 0.097

36.45 0.75 [28.21]

0.008 0.02 0.017

0.04 0.08 0.024

18.61 0.55 (18.00)

0.02 0.04 0.01

0.11

c

0.063 0.03 0.088

0.024 0.035 0.026

65.02 0.71 [49.58]

0.005 0.003 (0.018)

0.010 0.06 0.024

x

1.06

61.10 -

0.22

34.10

(0.03) -

0.17 -

C

1.04

60.12

1.92 1.96

[2&]

0.033

0.162

x

6.78 0.20 6.67

17.52 0.55 17.78

1.09 0.08 1.07

7.05 0.24 7.22

0.25 0.05 0.26

52.85 0.73 [31.27]

0.22 0.02 0.211

4.40 0.27 4.54

6.62 6.81

15.20 16.05

1.63 1.62

0.08 0.10

0.01 -

38.30 -

0.26

20.40 -

0.026

[19.90]

0.248

20.52

17.92 0.5 17.90

19.50 0.65 19.12

2.50 0.20 2.48

0.43 0.11 0.46

0.45 0.12 0.47

34.40 0.68 [19.60-J

0.35 0.04 0.356

4.55 0.37 4.75

0.93 0.14 1.00

12.50 0.35 (13.14)

0.08 0.04 0.09

0.69 0.13 (0.86)

0.01 0.01 0.011

75.80 0.61 [63.32]

0.05 0.008 0.058

0.03 0.02 0.024

9.01 0.25 9.02

10.20 0.40 10.32

2.60 0.17 2.59

13.80 0.35 13.83

1.04

38.20 [l$

0.20 0.023 0.200

13.28 0.42 13.30

1.98 0.15 2.04

14.50 0.31 14.64

0.38 0.04 0.38

2.45 0.11 [2.68]

69.90 0.45 [57.82]

0.09 0.012 0.094

0.95 0.13 0.93

%

Fetot 403

x

16.19 0.58 16.31

s

c

VS-N

x s C

URN

x s C

DT-N

x s c

FK-N

x s

BCS 309

s

DR-N

s C

MICA-I@

2

MICA-Fe

S C x s C

GH

x s C

BR

x s C

GA

f

s C

(zz)

0.30

0.06 0.12 -

-

g, 0.12 0.02 0.11

[E, 55.57 [3Gl]

MnO

MRG

0.05 0.02 0.056

0.11 0.08 0.055

-

( ) values within X f 2 s from the certified value. [ ] abnormal values. All the other values are located within f one standard deviation from the certitied value.

4.51 4.65 35.21 0.62 34.70

79

Meltingfluxesin automaticsamplepreparation

during the preparation process. Only silicon carbide is not included as it has a tendency to stick to the crucible, making casting impossible. But for silicon carbide those standards have been prepared by the Centre de Recherches PBtrographiques et Gtochimiques at Nancy [43]. The certified composition of the samples and the standard deviations given in the certificates are indicated in Table 4. These samples were prepared according to Fig. 1 and the measurements performed by ICP-AES (Table 5). The results obtained by autocalibration are also shown in Table 4. Values in parentheses are within the interval K f 2s of the certificates, all the other values are within the interval X f s of the certificates. Values in square brackets are not in accordance with the certified values. Most of these values concern the silicon determination. It appears that silicon oxide precipitates during the acid dissolution of the melt. Therefore, for the determination of silicon, it is necessary to determine separately the soluble part of silicon in that solution and to determine the part of silicon in the precipitate. This was done on a few samples and Table 6 shows the results obtained. By adding the two partial values, the total silicon oxide determined is in close agreement with the certified values of the samples. If silicon has to be determined directly on the leaching solution it is necessary to change to a hydrofluoric acid medium. Care must be taken for elements which have tendency to coprecipitate with silicon oxide and presenting the risk to be partially or fully lost.

Table 5. Experimentalconditions Spectrometer.ARLQuantoscan35000vacuum Element : Fe Al Ti Ca P Si Mn Mg Wavelength: 259.94 396.15 323.45 393.36 178.3 288.16 257.61 279.56 (nm) ICP

Generator

: DURR 4.5 KW, 50 MHz

Induction coil Plasma torch Argon flow rates

: 6 turns : demountable : plasma gas 12.5lmin-’

Nebuliser Sample uptake

: IRSID cross flow : 0.4lmin-’

Measuring time

: 3x10s

Tuned lines

carrier gas 0.4lmin-’

Table 6. Silicon determination (% silicon oxide)

Sample VS-N URN DT-N BCS 309

SiOz precipitated

SiOl dissolved

Si02 total

SiOz certified

20.0 14.0 8.0 6.0

35.6 25.1 28.2 28.4

55.6 39.1 36.2 34.4

55.5 39.43 36.45 34.11

5. CONCLUSIONS Different fluxes can be used to prepare various kinds of samples by PLASMASOL. The main quality of the flux must be its absence of reaction with the glassy carbon crucible, leaving the crucible free from melt particulates after the melt has been poured. Pure fluxes and commercial mixtures can be used with PLASMASOL. The preparations of reference samples and their analysis by ICP-AES shows a good agreement with the certified values except for silicon if the leaching of the melt occurs in a hydrochloric medium. The addition of the dissolved part of silicon determined by ICP-AES and the precipitated part determined by gravimetry leads to a correct determination of this element.