X-Ray Fluorescence Spectroscopy for Laboratory Applications. Jörg FlockЧитать онлайн книгу.
and the pellet is mechanically stable. The various possibilities for producing stable pellets are demonstrated in Figure 3.11.
In the case of hard particles with a narrow grain size distribution (for example, spheres of the same size as in Figure 3.11a), the closest sphere packing is obtained by pressing, but it has only relatively few and small points of contact between the individual sample particles. No stability of the pellet can thereby be expected.
If the sample particles have a broader particle size distribution, smaller particles can be incorporated between the gaps of the larger particles. This increases the number of points of contact and thus the stability of the pellets. This case is illustrated in Figure 3.11b by the inclusion of small spheres in the interstices of the distribution of Figure 3.11a.Figure 3.11 Influence of grain size distribution and different hardness of sample particles on the durability of pressed pellets, (a) spheres of same size, (b) spheres of different sizes, (c) particles of differing hardness, and (d) incorporation of binders.
If the hardness of the sample particles is different, the pressing can deform the softer sample parts. They then adhere to the harder parts so that the contact surfaces are significantly enlarged and thus improve the stability of the pellet (Figure 3.11c). If no soft components are present in the sample itself, a binder can be added to the sample. This has the same effect.On the other hand, these soft components tend to segregate to the edges during pressing, i.e. also on the sample surface (see Figure 3.11d). These soft components, in particular the binder then forms an additional absorption layer, which especially influences the low-energy fluorescence radiation of the sample components and thus the analytical result.
Various binders are available. It is important that the binder does not contain any analyte elements. Binders usually are mixed into the sample during the last grinding process; they are ground with the sample material and thus homogenized. These additives often also support the grinding process by preventing particle aggregation. The binders are available as pellets with a defined weight; therefore, no additional weighing is necessary during portioning binder and sample. This is particularly important for an automation of workflows. Liquid binders are also used; they are often the only way to produce a stable pellet. In this case, it is necessary to check whether these binders dissolve elements from the sample; then the evaporation of the solvent can lead to a depletion of these elements.
The selection of an optimum binder as well as the mixing ratio with the sample for a given analytical problem usually requires tests, since there is a dependence between sample material, grain size distribution, and the available preparation technique. Table 3.8 shows a summary of the most commonly used binders.
Table 3.8 Binders and additives for pressed pellets.
| Binder | Function | Properties/application |
|---|---|---|
| Boron acid Borax | Additive and binder Also advantageous for sample stabilization as sample mold | No longer allowed, slightly toxic |
| Paraffin wax | Mostly binder | Slightly toxic, no influence by moisture |
| Cellulose | Mostly binder | Absorption of moisture |
| Methylmetaacrylate in a solution of acetone | Binder for materials that enlarge their volume due to water absorption | Mixing with the sample and wait for evaporation of the acetone, then pressing |
| Polyvinyl alcohol solution | Additive and binder, avoids aggregation of the grounded material and cools down the mill |
The quality of sample preparation influences the analytical accuracy and the reproducibility of the analyses. The manufacturing of pressed pellets produces a much higher quality measurement sample compared to the simple loose powder samples of small size materials; it improves the analytical accuracy. For mineralogical material it is in the order of 0.5% for main components and <1% for secondary components.
3.4.4 Preparation of the Sample by Fusion Beads
3.4.4.1 Improving the Quality of the Analysis
The manufacturing of pressed pellets is fast and relatively easy; however, there still can be effects that influence the analytical accuracy, for example,
grain size effects
mineralogical effects
preferential orientations
surface roughness and
segregations of the material.
The elimination of these effects is possible by the manufacturing of fusion beads. This type of sample preparation also results in a standardization of the matrix, since a considerable dilution of the sample material by the melting agent takes place. As a result, calibrations can be used for a wider range of sample qualities, which also means less reference samples are required for a standard-based calibration. Further, calibration samples can be produced synthetically from pure substances in the form of fusion beads, so that their traceability is possible. The influence that the preparation of the material has on the analytical accuracy is demonstrated in Figure 3.12, which shows the calibration curve for pressed pellets and fusion beads.
Figure 3.12 Calibration curves for the same powder material prepared as pressed pellet (a) and fusion bead (b).
The following materials are required for the manufacturing of fusion beads:
Crucible (platinum/gold 95/5)
Chill molds (platinum/gold 95/5), nickel discs
Cover for the molds (platinum/gold 95/5)
Stirring bar (e.g. platinum/gold 95/5)
Muffle furnace
Fully or partially automated fusion machine heated inductively or with burners
The disadvantages of manufacturing fusion beads are the relatively long processing times, the higher costs, and the dilution of the sample materials, since this often leads to a reduction in the detection limits.
3.4.4.2 Steps for the Production of Fusion Beads
The production of a fusion bead takes place in several steps. There are different detailed descriptions of the advantages and procedures for the manufacturing of fusion beads (Willis 2010; Claisse 1957):
First,