Analytical data on material waste are usually obtained through analysis of samples that represent an insignificant portion of the investigated material mass. Therefore, for the obtained measurement data to be a reliable source of information, and not only analytical disinformation, the collected samples must be representative of the investigated waste material. Waste electric and electronic equipment (WEEE) is considerably heterogeneous material with a high content of copper. The test sample designated for the chemical analysis must truly represent the WEEE from which it was collected. The aim of this study was to develop an acceptable sampling procedure to investigate the composition of heterogeneous printed circuit boards; where the influence of grain size on the precision of chemical analysis of printed circuit boards waste (PCBs) stemming from mobile phones (MPs) has been investigated. Based on the chemical analysis of copper content in the sample (the highest metal content in PCBs) with grain size (d≤200 μm), copper content was 15.5% and dispersion was 1.62, which showed the highest accuracy of chemical analysis.