3.4.2 Preparation by Pouring Loose Powder into a Sample Cup
Powder-like material can be filled directly into sample cups and analyzed there. In the case of excitation from below, the measurement sample rests on a thin polymer film (see Figure 3.17). Since the sample material can only slightly be compacted by pressure, the sample density can vary for the same sample material. The surface roughness of such samples depends essentially on the grain size. This preparation technique is simple and allows for very fast analyses, but usually does not allow for high precision.
The following factors influence the analysis accuracy most:
The film of the sample cup absorbs both the incident and the analyte radiation. This means that low-energy radiation of lighter elements in particular cannot or can only badly be detected. This can lead to significant analytical errors if the analyzed materials have a high content of light elements. In addition, it is possible that contaminations in the film of the sample cup counterfeit elements or their content in the sample.Figure 3.8 Dependence of fluorescence intensity on the morphology of the sample.Source: Courtesy of S. Hanning, FH Münster.
The measurement often takes place in air, on the one hand, in order to save the time pumping down the sample chamber, but on the other hand also to avoid contamination of the spectrometer by swirling sample particles. Air in the beam path acts as an absorber. The contamination of the spectrometer during evacuation with sample particles can be avoided by covering the sample cup with a lid, see Figure 3.17.
The relatively large surface roughness of loose samples influences the analytical accuracy for light elements. Different information depths of the fluorescence radiation result in their varying absorption.
Owing to the filling process the sample density is not uniform. This significantly influences the reproducibility of the analytical result. This is illustrated in Figure 3.8, which shows the measured intensities of Pb in a loose powder of synthetic granules and correspondingly in a pressed pellet of the same material.
This type of sample preparation is usually used for waste or secondary raw materials. High analytical accuracy is not expected here in the first place due to the low reproducibility and representativeness of the sampling. Variations of the analytical results caused by this simple preparation technique are still small compared to the variations generated by the actual sampling and are therefore negligible. Typical uncertainties for the measurement of loose powder samples are in the range of 3–5% relative to the main components; accordingly, for traces they are larger and depend on the element, the matrix, and the grain size.
It needs to be considered that in case of wavelength-dispersive spectrometry (WDS) instruments the film is thermally stressed by the high excitation intensity, i.e. long measurement times should be avoided.
3.4.3 Preparation of the Measurement Sample by Pressing into a Pellet
Higher analytical accuracies are possible by the manufacturing of pressed pellets, since these can produce relatively smooth sample surfaces and uniform sample densities. For this purpose, the ground material is filled into a press mold having a diameter that corresponds to the respective size of the sample holder of the spectrometer. Pressures of 100–200 kN for pellet diameters of 32 mm and of 200–300 kN for pellet diameters of 40 mm have to be typically applied for up to one minute to compress the material. As a result, pellets with a relatively high density are prepared, which are stable and can be stored over a longer period. In order to avoid contamination by the mold, the pressing surfaces can be covered with a thin plastic film, which is removed after pressing the pellet. Figure 3.9 shows a press die. This die allows evacuation of the sample material during the pressing operation. As a result, gas inclusions are avoided, which can destroy the pressed pellets during the pressure changes in the course of vacuum measurements.
Figure 3.9 Press die for to produce pellets from small-sized materials.
Source: Courtesy of Specac Ltd.
Various possibilities exist for the manufacturing of a pressed pellet, which are distinguished, in particular, by the stability of the pressed pellet, but also by the effort for its production; it depends on the starting material as well. Pressed pellets can be made by
direct pressing of the ground raw material without any additives;
mixing the sample with a binder and pressing the mixture into a pellet. The binder increases the strength and thus, if necessary, allows multiple use of the pellet. At the same time, a dilution of the matrix is achieved. The binders used are cellulose, wax, or other soft materials;
pressing the sample material with a binder into solid sample holders such as rings or aluminum molds. The rings are made of steel or plastic. The material of the Al molds must have high purity in order to avoid superpositions of the sample spectrum with that of high-energy fluorescence radiation of impurities. This support of the sample material with molds is particularly useful for often used samples, where particles can flake off and contaminate the spectrometer. The pressing of small-particle material produces stable pellets with very smooth surfaces, as shown in Figure 3.10a;
simultaneous pressing of the sample material into a substrate made of binder or boric acid utilized as a sample vessel. In this case, a test sample can also be produced with a small amount of sample material. Samples with a binder carrier pellet are shown in Figure 3.10b. Calibration samples can also be produced by this preparation method in order to ensure a high analytical accuracy even in the case of small sample quantities. However, an increased spectral background has to be expected for samples prepared in this way because the high-energy incident radiation is scattered on the light matrix of the sample mold;Figure 3.10 Pressed pellet (a) and cross section of a pressed pellet in binder (b).
pressing the sample material at elevated temperatures. Particularly in the case of organic materials, this can lead to a gluing of the individual sample particles and thus to stable pellets; see for example in Section 10.13.2.
In order to avoid the sample thickness influencing the analytical results, always the same sample quantities as well as the same mixing ratios with binders should be used for the production of pressed pellets. Depending on the size of the press die, the usual sample amount is 3–5 g. The binders are added in constant mixing ratios with proportions between 10% and 15%. As a result of the same sample masses, the sample thickness is either greater than the information depth of the analytes or the two parameters are always in the same relation to one another. The addition of binders must be considered for the quantification, both for the matrix interaction and for a possible normalization to 100%.
The durability of pressed pellets depends on the hardness of the sample particles as well as on their grain size distribution. Ultimately, the size of the surfaces of the individual particles contacting each other is important. If this area is large, then the van der Waals forces between the individual sample particles