## 2 Materials and Methods  
### Method of Selection of the Representative Sample  
The tested materials were approximately 1.5 cm thick high-impact polystyrene (HIPS) and acrylonitrile-butadiene-styrene (ABS) flakes (Figure 2). Plastics were created as a result of the recycling of refrigeration equipment: refrigerators, freezers and refrigerator-freezers for commercial or domestic use.  
In the papers [1; 4], the size of the ABS and PS flakes tested was 2.0-4.0 mm. In the study [2], the ABS and PS tested were sieved into fractions of 0.9-2.0 mm, 2.0-2.5 mm, 2.5-3.2 mm and 3.2-4.0 mm. In the paper [3], selected waste plastics with sizes of 5-10 mm were used for the research. In papers [1; 2; 3; 4], the samples were crushed using laboratory crushers. The material used in this study was already collected in a ground form (and in such a form, it would be subjected to flotation on a larger scale) and was not crushed further.  
The sample was collected by the Polish Recycling Corporation--a plant specializing in processing waste electrical and electronic equipment (WEEE). The unit samples were taken manually from 3 randomly selected collection bags (BigBags) directly from the line (during material processing) from three different diagonal places of the raw material entering into the BigBag and put into a plastic container. Each incremental sample had a similar weight (approximately 0.5 kg). The bulk sample was made by combining and thoroughly mixing the incremental samples as well as homogenizing them. The total sample was reduced by  
Figure 1: Scheme of flotation recycling—fotation tank with air bubbles and the plastics: HIPS and ABS.  
Figure 2: Laboratory sample for the “Pre-identification of HIPS and ABS with the handheld NIR spectrometer” test.  
the quartering method, resulting in the laboratory sample weighing approximately 0.2 kg for laboratory tests.  
The method of quartering consists of making a cone, flattening it and dividing it into 4 equal parts. The next step is to discard the two opposite parts and leave the other two for a further decrease. Then, it is necessary to build a cone again, taking the material from one quarter, and then from the others. The heaped cone is successively flattened and divided into four equal parts, of which two opposite ones should be discarded, and the next two should be left for further reduction. The operations are repeated until a sufficient sample mass is obtained [20].  
As follows from the paper [21], the sampling process should be monitored from the beginning to reduce the total sampling error. It is important to identify and prevent errors soon enough. The basic sampling error is inherent in the properties of the material (among others, the size, shape, grain density, and composition of the materials). It is very important to homogenize the samples thoroughly before each step of their combination and reduction. It is important to get to know the heterogeneity characteristics of the materials. The use of the representative method of mass reduction is recommended, and the quartering method applied in this paper is such a method. In addition, it is recommended to grind the sample (grain size reduction), which would reduce the value of the basic sampling error, but in the case of these tests, this action was undesirable (plastic flakes must be intact for subsequent tests). The larger the sample weight, the more representative the sample is. At the same time, however, the current analytical volumes steadily decreased with increasing precision of the analytical instruments. The analytical results refer to a smaller and smaller volume, but nevertheless, these results are supposedly representative of the entire batch. This is the root cause of any sampling and representativeness issues. This is why it is so important to follow the proper sampling procedure from the very beginning.