The article considers the possibility of contamination of the surface of steel samples with abrasive materials when preparing samples for determining the chemical composition on optical emission spectrometers.The standards for sample preparation methods describe in detail the methods of surface treatment, the materials used, and the requirements for the quality of the analyzed surface. The sample surface can be milled or sanded using various abrasive materials. In practice, the laboratory found that the surface of samples during sample preparation is contaminated with aluminum and calcium.In order to determine how the contamination of analytical surfaces occurs, the chemical composition of all materials used in the preparation of samples was studied, and an experiment was conducted to establish a method for preparing the sample surface that does not lead to contamination of the surface with aluminum and calcium. For the experiment, three standard samples of steel composition were selected with certified values of the mass fraction of aluminum and calcium in different ranges. The surface of each sample was processed in three ways and optical emission spectral analysis was performed on each analytical surface at five points to determine the value of the mass fraction of aluminum and calcium and to estimate the spread of the results obtained. As a result of tests it was found that by grinding the sample surface by using abrasive white corundum and abrasive paper grit P40 is the surface contamination of the analyzed sample in aluminum and calcium, therefore, when determining the mass fraction of aluminium and calcium in steel are required for surface preparation to use the method of milling.