Matrix-type certified reference materials for quality control of metal determination from solid environmental and vegetation samples

In the context of monitoring environmental factors, metals are one of the major analytical components. Applying appropriate determination methods and obtaining accurate results is a requirement imposed on environmental laboratories that perform quality control of water, soil, waste or vegetation. This study presents some examples of certified reference materials for quality control of the results of toxic metal determination from solid environmental and vegetation samples. The analyzed and verified metals were As, Cd, Cr, Cu, Ni, Pb and Zn. The pre - treatment of the samples, the determination methods of metals and the obtained results are also presented. Inductively Coupled Plasma Optical Emission Spectrometry (ICP - EOS) and Inductively Coupled Plasma Mass Spectrometry (ICP - MS) techniques are suitable for low metal concentrations, while ICP - EOS and Flame Atomic Absorption Spectrometry (FAAS) methods can be used at high concentrations.


INTRODUCTION
Due to the advanced industrialization during the last decades of the past century and the first of this century, many areas became contaminated with toxic metals, considerable quantities being found both in soil, sediments, groundwater and surface water. The monitoring and rehabilitation studies have as a starting point the investigation of the quality of environmental factors affected by anthropogenic activities, such as: mining, metallurgical industry, machine building industry, intensive agriculture, etc. Even if some mines have been closed or abandoned, the surrounding areas remain a potential danger to fauna, flora, the aquatic environment and human health [1][2][3][4]. Different types of waste with varied metal content are stored either in specially designed landfills, which over time can produce discharges of toxic substances into the environment, or directly on the ground as long as the content of toxic substances is below certain limits imposed by laws [5,6].
The high metal content in contaminated soils, sediments, waste, biological sludge must be accurately determined using appropriate analytical techniques, so the composition of the complex matrices must not interfere with the determination [7]. In this context, in several international projects, matrix-type Certified Reference Materials (CRMs) have been developed, containing certified values of toxic metal content and the associated uncertainty. Several institutions provide such complex matrices for soils (organic rich soil, sandy soil, road dust, industrial soil), sediments (estuarine, coastal, lake or river sediment), waste (fly ash, sewage sludge amended soil), plant tissue (hay powder, clover, lichen, tomato leaves, apple leaves, plankton, rice, etc.) or animal organs (tuna muscle, cod liver, etc.). Among these, we can mention the best known, namely: European Commission Joint Research Center -JRC (800 CRMs of the BCR-and IRMM-brands as well as the ERM-branded materials that were produced by the JRC) or US Department of Commerce -National Institute of Standards and Technology NIST. Obtaining such materials implies the application of well-established rules, which ensure the homogeneity and stability of the samples, as well as the assigned value and the uncertainty for each element or compound [8].
The international requirements imposed by the EN ISO 17025/2018 standard enforce accredited laboratories around the world to verify their analytical results for the determined pollutants [9]. Thus, the use of matrix-type certified reference materials has become a common practice in such laboratories (Table 1). Another way of testing the obtained results is by participating in proficiency testing (PT) schemes, where the entire applied procedure is tested (pretreatment, determination, reporting results). There are specialized international organizations that organize annually such schemes, these being diversified according to the pollutants and the matrix type (water, soil, waste, sediment, plant tissue). A database for such PTs in which the topics, the type of matrice and the accreditation of the organization that organizes the scheme are presented is EPTIS database [10]. The aim of this study is to present the results obtained by using various CRMs to check the metal content (As, Cd, Cu, Cr, Ni, Pb, Zn) from different environmental matrices (soil, sediment, fly ash, sewage sludge and vegetable tissues) as well as the applied analytical techniques for metal determination.

EXPERIMENTAL PART Methods
In order to determine the metal content in solid environmental samples (soil, sediment, fly ash, sewage sludge), FAAS and ICP-EOS techniques were applied. parameters of the applied ICP-MS method are presented in tables 5 and 6.

Samples preparation
The content of metals extracted in aqua regia solution from the soil, sediment and sewage sludge samples, was determined. To 1 g of solid sample, 9 mL HCl and 3 mL HNO3 were added. For the pretreatment of these solid samples (soil, sediment, and sewage sludge), a digestion program was applied (table 7). After cooling, the solutions were filtered and brought quantitatively to a 50 mL volumetric flask with ultrapure water. were filtered on low porosity filter paper and brought to the mark with ultrapure water in 25 mL volumetric flask. Each experiment was performed in duplicate, the reported values being the average values.

RESULTS AND DISCUSSION
The results obtained for the analyzed metals (As, Cd, Cr, Cu, Ni, Pb, Zn) in the CRMs are presented in this section. Thus, the obtained data for solid environmental samples (ICP-EOS, respectively FAAS results) are compared to the certified values from the quality certificates of the used CRMs (tables 9 ÷ 13).      Table 16 contains the results obtained after participating in an inter-laboratory comparison scheme for a soil sample analysis, the obtained values being compared to the assigned values.
The table also presents the Z scores obtained for the reported data, all values falling within the -2 ÷ 2 accepted range of standard deviation.

CONCLUSIONS
The results obtained in the verification of some methods applied for the determination of toxic metals both from solid environmental (soil, sediment, ash waste, respectively sewage sludge) and vegetation samples indicated that the applied ICP-EOS, FAAS, respectively ICP-MS techniques led to good results both on CRM matrices and in the case of participation in a proficiency test scheme.