Z. Anal. Chem. 286, 36-40 (1977) - �9 by Springer-Verlag 1977

Remarks on the Sample Preparation of Oxidic Materials for X-Ray Analysis

K. Ohls

Hoesch Hfittenwerke AG, D-4600 Dortmund

Eingegangen am 24. Dezember 1976

Bemerkungen zur Probenvorbereitung oxidischer Materialien fiir die R6ntgenfluorescenzanalyse. Ausgehend von einem kritischen Vergleich verschiedener Techniken zur Herstellung von Schmelzaufschlul3-Tabletten f/Jr die RFA wird auf die in der Praxis bewfihrte Universalmethode mit der besten Reproduzierbarkeit hingewiesen. Durch die Untersuchung von Schmelztabletten mit Hilfe lokalanalytischer Methoden sollen Anwender der Tablettentechnik zur Kritik angeregt werden. Die unterschiedlichen Zielsetzungen analytischer Aufgaben bei der Untersuchung von Oxiden werden diskutiert.

Summary. The universal method which is established in practice and has the best reproducibility is indicated in a critical comparison of different techniques for producing melting fusion tablets for the X-ray analysis. By studying fusion tablets with methods of local analysis, users of the tablet technique shall be made critical. The different objectives of analytical problems with the analysis of oxides are discussed.

Probenvorbereitung f/jr oxidisches Material; R6ntgenfluorescenz-Spektrometrie; Homogenisierung durch Schmelzaufschlul3.

With respect to rapidity and reproducibility, X-ray fluorescence analysis is an established technique for quantitative analysis of oxidic materials, e.g. ores, minerals, slags, refractories, powders or alloys which are easily transferred into oxides. Preconditions for a universal method are a sufficient availability of sample material (> 1 g) and on the other hand a complete dissolution of the structure and bonding of the oxidic material.

It is not intended here to consider the rapid routine methods for the analysis of special oxidic mixtures resulting from the direct sample [2], which require about 20 g of substance for filling the A1 box and a special set of calibration curves or sufficient parts of storage in the computer, respectively, for evaluation after X-ray analysis.

Before the universal method with the X-ray analysis for all oxidic materials can be carried out, there has to be a melting fusion technique which is easily and rapidly performed.

As the real X-ray analysis requires a few seconds only, the timing is essentially dependent on the sample preparation. The timing as well as the tolerable deviations of the results depend on the problem that is considered. One should distinguish between two

important aspects: the high speed analysis of single samples during the production process and the rational analysis of sample series.

Both kinds cannot be treated efficiently with a single melting fusion technique. For high speed analysis of a single sample, a melting fusion has to be per- formed within a very short time, apart from the direct method [4]. Up to now, inductive heating in a Pt or graphite crucible is the most rapid technique suitable for this purpose [4]. Ifa set of samples has to be studied there are different techniques which allow to fuse several samples simultaneously. Working with heating furnaces is very simple. However, a certain organiza- tion of the order of crucibles is required. The use of semi-automats is more complicated [1,3,5]. How- ever, a better similarity (same time, constant temper- ature and shaking) can be achieved.

These fusions most often aim at casting glass beads with a certain diameter and a certain depth. The mould usually consists of polished platinum so that these glass beads get a plane surface. To prevent a splitting of the glass beads, the mould, on the one hand, is heated and slowly cooled and, on the other hand, a certain fluxing mixture (tetraborate for instance) is

K. Ohls: Sample Preparation for X-Ray Analysis 37

chosen. In general, lithium borates are suitable for this purpose. They cannot, however, fuse different oxides equally well. That is why partly oxidizing agents are added. In general, different melting fusion mixtures have to be used for different product species. As it is impossible to fuse all oxidic materials with a single fluxing mixture, you may as well use specific salts instead of mixtures.

The melting fusion of chrome magnesites for instance is difficult with borates, but is easily done with sodium meta- phosphate. However, only the X-ray fluorescence analysis can sensibly be used after such a melting fusion. It is not very successful to dissolve the solidified fusion for chemical analysis. Moreover, this fusion attacks the Pt mould.

That is why it would be important for a universal procedure to be able to get on with a single fluxing mixture. At once the question arises whether there is still any sense in casting glass beads because it is pos- sible that the melting fusions do not occur ideally in all cases. There is often the argument that the glass bead can be placed directly into the X-ray spectrometer. Further steps are grinding of the glass and briquetting of the powder. However, this requires only about 30 s while on the other hand several minutes of melting time can be spared.

Several methods for producing glass beads were checked in order to extend the argumentation for our method [4]. The examples described below mainly refer to melting fusions with a single substance: sodium tetraborate.

Two questions have to be answered: 1. How do element distributions in glass beads

possibly change after a longer period of time, or can they be kept as standards, respectively?

2. How do element distributions change during solidifying out of a homogeneous solution?

First of all, you have to distinguish between sub- stances which can easily be fused, and substances which are very hard to fuse completely, if at all, using the same fluxing mixture. If you assume that in hard routine work it is useful to apply a single universal procedure, you have to orient towards the most difficult case. The fact that the visual judgement about the quality of a melting fusion is subjective must be considered as well.

The experiments were built up accordingly. The melting fusion took place with sodium tetraborate in excess by a factor of 10 in a heating furnace at 1100 ~ C. The melt was poured into a heated Pt mould. The time of melting fusions has been varied in order to produce consciously good and bad fusion tablets.

The resulting glass beads have been studied suc- cessively with microprobe, Laser spectrography and X-ray fluorescence analysis.

Fig. 1. Double layer bead

The concentration profiles and the relative concen- tration differences over the surface of the glass beads were determined using materials which are difficult to fuse, e.g. chrome magnesite and A1 containing fire- brick which are characterized by the fact that they contain more than one typical main component.

The long-term assays included the storage under atmospheric conditions, at 200~ in the drying oven and alternately under X-ray radiation as well. Of course, we could not expect to see measurably the diffusion of certain elements in this way within such a short time. The radiation in the instrument to which standard samples often are submitted and the tempering did not show univocal results either.

There is, however, an interesting course with the S-values. The S-values usually increase at the surface. The S-ions appear to diffuse on the surface in pure sodium tetraborate. If metal borates and alkaline sulphates are present, this process obviously takes place more slowly. You can study diffusion processes with a double tablet (Fig. 1) that was produced from pure sodium tetraborate (right-hand side) and the fluxing mixture.

Laser spectrography allows a direct analysis with glass beads. As there is no calibration basis, only the intensity ratio of e.g. two main components can be determined exactly.

After a bad melting fusion with sodium tetraborate (10 min), there is a relative standard deviation of /8 ~ for chrome magnesite for the measurement of the intensity ratio Mg/Cr over the surface, while a melting fusion considered to be good (20 min) shows 5 ~ only (Table 1).

A similar result for chrome magnesite can be found as well with the much more appropriate fluxing mixture Na(POa)x.

As "good" and "bad" tablets represent subjective judgements, fluctuations in the distribution of Mg and Cr in the fusion tablets must always be taken into account.

The concentration profiles taken with the micro- probe confirm the results stated above.

38 Z. Anal. Chem., Band 286 (1977)

30

25

20 %

15

10

Boraxfusion

good

, . , , . . , . . . . . . . . . . . , , , . , ' . , , , , - - , . \ . . . , .

Cr- inclusion

............................. '" Ng 0

_~--- Cr2 0 3

4 5 - -

bad

I i 35 ~1 I I

I I

~ I I - - ~ _ _

15

I I

I I

5 , ,~I, ^ ' 1 , ~ f i l / \ ^ Mg0 '.....,', t., ,, / \ "\ M 0 . . . . . . , . . . . g

0 / ~ C r 2 0 3 0 ~ ~ C r 2 0 3 Microprobe measuring points ( distance 0,5 mm

Fig.2. Micro probe analysis of Mg and Cr from both sides of the bead surface

Table 1. Comparison of reproducibility of Laser-micro- spectrography (ruby resonator) with good and bad beads (chrome magnesite)

ead

bad good

Fusion with sodiumtetraborate

(10 min) (20 min)

Ay72 0.245 0.212 s 0.043 0.010 st% 18 5

Fusion with (NaPO3)x

(4 min) (10 rain)

A)77_, 0.177 0.192 s 0.023 0.008 S,~o 13 4

Here, two different cases with the same example of chrome magnesite are shown (Fig. 2) :

1. With "good" tablets there is either a homogeneous distribution, or there are oxide particles present which are locally not yet fused or distributed (high Cr/low Mg signal).

2. "Bad" tablets show the portion of unfused original substance which concentrates during solidify-

ing in that part of the tablet that remains liquid for the longest time (high Mg and Cr signals).

With application of a single fluxing mixture, "good" and "bad" depends largely on the material and is difficult to judge visually.

Now you could get the idea that these concentra- tion differences are compensated by integration over the relatively large surface by X-ray fluorescence analysis.

However, routine ope ra t ion-a t which you should always work on the secure s ide-shows the necessity to have a further process of homogenization followed.

So, all melting fusions are ground and briquetted afterwards in our laboratory. The following results (Table 2) show that this may be necessary.

Fire-brick is excellently fused in sodium tetraborate. In our example of an A1 fire-brick dissolution is a bit more difficult, but the relative standard deviations of Ca, Mg, Ti and Fe show the "good" melting fusion. The main components A1 and Si show a con- siderable improvement after measurement at the powder briquette.

Considering e.g. chrome magnesite (Table 3) which cannot be fused just as well with sodium tetraborate, you find a visible improvement of the relative standard deviation after grinding for nearly all elements, which is significant for the main components Mg and Cr.

In both cases a slight deterioration of the Fe values occurs because the grinding vessels applied did not

K. Ohls: Sample Preparation for X-Ray Analysis

Table 2. Comparison of relative standard deviations (in ~o) between bead and powder briquette

Fire-brick Fe Si Ca Mg A1 Ti

X48 ~ 1.1 21.9 0,2 0.2 74.1 2.8

Fusion bead

s~ 4.7 4.0 4.8 5.7 6.0 0.9

Powder briquette

s~ 5.2 0.6 4.5 5.2 0.4 0.3

39

consist of hard metal but of hardened steel, and the possibility of a slight abrasion must be included.

For many reasons it cannot be useful to keep fused tablets of standard samples for a long time. A standard sample should always be fused freshly.

Another possibility leads to a continuously present standard. For calibrating the analytical programme of a fire-brick we alloyed pure nickel with all components so that the measured values of this Ni disc as high sample corresponded to: 50.6 ~ SiO2, 42.0 ~ A1203, 1.65 ~o TiO2, 2.05 ~o Fe203, 0.10 ~ MnO, 0.42 ~ CaO, 0.75 ~o MgO.

Table 3. Comparison of relative standard deviations (in ~o) between bead and powder briquette

Chrome Fe Si Ca Mg Mn A1 Ti Cr magnesite

J)48 ~ 11.9 2.7 1.0 55.1 0.3 7.2 0.1 21.2

Fusion bead

s~ 0.7 2.0 1.6 5.1 2.4 4.2 1.7 12.3

Powder briquette

s~ 1.2 1.3 1.0 0.6 0.4 1.9 0.9 2.7

Table 4. Comparison of standard deviations between bead (1) and powder briquette (2) with ore reference samples (in digits; n = 10)

Compounds Fe P SiO2 CaO MgO Mn A1203 TiO2 S

(wt ~) 36.21 0.70 19.25 5.63 1.64 0.39 4.07 0.22 0.063

21 5924 123 970 281 156 328 362 139 55 sl 16 3 7 1 9 4 6 2 2

Sample E4 22 5923 125 988 286 161 335 366 150 72 s2 9 2 5 2 5 3 5 3 4

(wt ~) 65.63 0.04 5.88 0.10 0.47 0.06 0.88 0.07 0.032

2i 8227 5 278 12 53 110 69 85 33 sl 53 2 3 2 6 2 4 2 4

Sample E5 22 8337 4 290 16 58 114 79 92 39 s2 51 2 4 1 5 2 2 2 4

(wt ~) 1.64 0.17 10.39 2.02 0.58 47.85 1.64 0.079 0.227

21 652 34 501 108 50 7968 136 95 180 sl 1 2 2 1 4 12 2 2 5

Sample E8 22 733 36 515 113 54 7947 145 105 207 s2 2 2 5 2 5 11 5 2 7

40 Z. Anal. Chem., Band 286 (1977)

A Pt metal disc served for calibration as deep sample.

Simultaneous spectrometers for X-ray fluorescence analysis are frequently used for direct process control in large laboratories where they are usually combined with data processing installations. Economically it is only useful to apply suitable instruments, if they can provide as much information as possible within short time. For this it is required that at least 20 elements can be quantitatively determined simul- taneously, that a Rh tube with high power is taken as radiation source, that the fixed installed channels include arrangements for enhancement and discrimi- nation, that the measurement always takes place with internal standards, that additionally a scanner is at hand and that long-term stability of the feeding arrangements is ensured. Because of the rationaliza- tion of analytical tasks and to make full use of the capacity of these instruments, universal procedures gain more and more importance.

The routine procedure used in our laboratory can be applied universally and is performed very easily and without any systematical errors. The substance is fused under inductive heating for 2 min in the mixture ratio of 1 - 1 0 with sodium tetraborate (for X-ray fluorescence analysis) [4], the fusion is poured into a grinding vessel, is ground for 10 s and is bri- quetted in an A1 box with a pressure of 10 t. About 4 min is needed for the preparation. Therefore, the working process in the second part after weighing and melting fusion is practically equivalent to that of the direct rapid method, in which the substance is directly

ground and is briquetted in an A1 box after sieving (< 100 gin). The corresponding time for this is about 1.5 rain.

Since recently it is possible to put a wide, flat Pt cup with plane bottom and a cover including a hole for observation into a correspondingly wide induction coil with ceramic bottom and to heat it inductively within a short time to about 1400 ~ C. Approximately after 2 min the cup is taken out and is slowly cooled during 1 rain. The glass bead jumps from the bottom. If you study beads thus produced from ore samples directly and after the successive grinding and bri- quetting, you do not find any improvement of homo- geneity (Table 4). The time needed for the preparation of a sample is about 4 rain as well. The most decisive step is the careful cooling with closed cover. Only then it is shown whether the tablet is splitting under tension.

There is, therefore, no other universal procedure with the same simplicity and certainty as the method practised in our laboratories.

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Report of IRSID RI 519, Oct. 1974 2. Koch, K. H., Ohls, K., Becker, G. : Arch. Eisenhfittenwes.

41, 87- 89 (1970) 3. Koch, W., Dobner, W., Becker, W.: Arch. Eisenhfitten-

wes. 45, 147-154 (1974) 4. Ohls, K., Becker, G.: Fresenius Z. Anal. Chem. 279,

183-185 (1976) 5. Wittmann, A., Chmeleff, J., Herrmann, H. : X-Ray Spec-

trom. 3, 137-142 (1974)