## 1 Introduction
There are many indicators determining the level of heavy metal contamination in soils, such as the pollution index [21], geo-accumulation index [22], Nemerow integrated pollution index [23], enrichment factor [24], and potential ecological risk index [25]. There are many methods used for the determination of heavy metal content in dissolved soil samples. For example, pulsed differential stripping voltammetry (DP ASV) is one of the most sensitive instrumental methods that can achieve very low limits of quantification for metal cations [26; 27; 28]. For surface samples, X-ray analysis (XRD) is usedto determine the elemental composition. The percentages of the main mineral components of soils are determined by means of pattern analysis, the number of radiation pulses and the intensity of reflections [5]. Spectroscopy methods based on the interpretation of spectra produced by the interaction of radiation with matter, especially atoms (atomic spectrometry), are widely used. Methods based on atomic spectra can be divided into absorption and emission methods. An example of the former is atomic absorption spectrometry (AAS). The principle of this method is based on determining the relation between absorption of electromagnetic radiation for a particular line of the determined element and its concentration, which is quantitatively expressed by Lambert-Beer law [5; 13; 26]. Atomic emission methods include, among others, the historically first atomic excitation technique: flame photometry. The concentration of elements in solutions after the previous extraction can also be determined using the technique of Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) [29]. The inductively coupled plasma mass spectrometry (ICP-MS) technique was developed by [PERSON] in 1978, and the first instruments to use it was built in 1978 [30; 31]. The first instruments for its application appeared several years later than the ICP-OES technique. ICP-MS spectrometers enable fast multi-element analysis (up to 70 elements within a few minutes). The technique is characterised by high sensitivity, selectivity, low limit of quantification and rapidity of determinations. Inductively coupled plasma analytical instruments are widely used for the determination of trace amounts of metals in environmental samples such as drinking water, wastewater and groundwater. It can also be used for the determination of trace amounts of metals in petrochemical samples, soil samples, or food products. Environmental studies require a wide variety of fieldwork and laboratory determinations. In order for the results to be a basis for further considerations and interpretation they should be characterised by sufficient accuracy and reliability. During both the field and laboratory stages of the study, there are possibilities of errors that may distort the results obtained. In order to control the precision of the determinations it is recommended to take duplicate samples and to assess the precision of laboratory determinations. Unfortunately, even if these recommendations are followed, it may be difficult to obtain fully valid test results since, depending on the preparation and measurement technique/apparatus used, significantly different results may be obtained for the same environmental samples [32]. In Poland, the information on soil contamination is scarce and selective. Usually, the information concerns larger agglomerations, i.e., Warszawa, Lodz, or Wroclaw [33; 34; 35; 36], and mainly characterise concentrations of heavy metals in soils. The information on soil contamination in towns of medium and small size (<100,000 inhabitants) is sporadic. There is also a lack of literature reports on sample preparation and measurement techniques, which may influence the variability of heavy metal determination results in soil. In this context, the methodology of determinations, the accuracy and precision of the measurements performed, and the laboratory equipment used for the studies are important.