Keywords

Introduction

Quality control of industrial products and monitoring of the state of environmental objects are important for both individual enterprises and the state as a whole. Various analytical methods can be used to analyze the chemical composition of substances and materials. The selection of the most appropriate method for the intended purpose is carried out depending on the number of elements to be measured and the number of samples to be analyzed.

As highly rapid and sensitive methods for qualitative and quantitative analysis of elements in various materials and substances, inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) meet the performance requirements of many laboratories. These methods are increasingly being used in diverse fields of science and industry, including the food industry to control the content of various components in food and drinks [1,2,3]; environmental protection to analyze the metal content of various environmental objects [4,5,6]; forensic analysis to identify micro- and toxic elements in biomaterials (hair, nails, epithelial tissue, blood, urine, muscles, etc.) [7,8,9], etc. [10,11,12].

The establishment of a calibration dependence of the output signal of spectrometers on the content of elements under analysis is a necessary step for carrying out measurements using ICP-OES and ICP-MS. Such calibration is only appropriate when reliable reference materials (RMs) are used. RMs certified in the Russian Federation (CRM)—GSO—ensure the metrological traceability of measurements in all laboratories of the country [13, 14]. The calibration of spectrometers based on inductively coupled plasma is carried out using CRMs, which are solutions with the certified value of the mass fraction or mass concentration of a particular element of Mendeleev’s Periodic Table.

The methods of ICP-MS and ICP-OES allow several elements to be measured quickly and simultaneously. This goal can only be achieved when using multi-element RMs, since their use simplifies and accelerates the preparation of a series of working and calibration solutions.

In this research, we aimed to develop a CRM of a multi-element solution of metals (hereinafter ICP-CRM Multi 1), which would enable the calibration and measurement of several elements simultaneously. The CRM under development should be traceable to the State Primary Standard of Unit of Mass Fraction and Unit of Mass (Molar) Concentration of Inorganic Components in Aqueous Solutions Based on Gravimetric and Spectral Methods GET 217–2018 [15].

Materials and Methods

In order to develop a CRM of a multi-element solution of metals, the following metrological characteristics required normalization: the value of a CRM certified characteristic; error and/or uncertainty of a CRM certified value; the expiration date of a CRM.

The values of CRM certified characteristics were normalized by establishing the interval across which the certified values of any RM of a given type must vary. The mass fraction of metals in the developed CRM should range from 900 mg/kg to 1100 mg/kg inclusive; the mass concentration of a metal should range from 900 mg/dm3 to 1100 mg/dm3 inclusive.

The selection of a manufacturing technology was conducted by analyzing possible procedures for preparing an ICP-CRM and approaches to establishing the ICP-CRM certified values. As a result, we decided to certify the developed ICP-CRM according to the gravimetric method of preparation, since the gravimetric preparation of RMs from high-purity starting materials is the most accurate method for reproducing and transferring the units of concentration of metals in a solution. Certification by the gravimetric preparation procedure is an approach to certifying an RM based on the known or specially investigated characteristics and quantitative ratios of the precursor components used for RM preparation by mixing them to obtain the calculated values of the RM metrological characteristics.

The material chosen for preparing the ICP-CRM is a solution of metals or their compounds in dilute nitric acid. The mass fraction of the main component in the starting materials and the solvent was established using GET 217–2018. The ICP-CRM was produced using the VNIIFTRI equipment. In order to carry out preliminary experiments, pure metals were chosen as a starting material for preparing a solution with a certified value of the mass fraction of zinc, cobalt, and cadmium; metal salts were chosen for a solution with a certified value of the mass fraction of lithium, barium, and lead. Bottles made of darkened high-density polyethylene with various capacities (30, 60, and 125 cm3) were selected as containers. They had been previously used in the development of single-element CRMs [16]. The solvent was 5% nitric acid by volume, which ensured complete dissolution of the starting material and stability of the ICP-CRM in accordance with the preparation procedure. Particular attention was paid to the selection and pre-cleaning of polymer ware intended for both laboratory research and further use as a container for the ICP-CRM.

In order to assess the purity of the starting materials, the “100% minus the amount of impurities” method was chosen. Thus, all detected impurities were subtracted from 100%, and elements with a concentration below the detection limit were taken into account by subtracting half of their detection limit from 100%. The mass fraction of the main component (\(\omega\)) was calculated by Eq. (1):

$$ \omega = 100\% - \sum\limits_{i} {w_{i} } - \sum\limits_{j} {\frac{{LOD_{j} }}{2}} $$
(1)

where wi is the mass fraction of detected impurities, %;

\(LOD_{j}\) is the detection limit of undetected impurities, %.

The impurity composition of the solvents after purification was assessed using GET 217–2018.

The mass fraction of a component in the ICP-CRM was estimated by equations [17]:

$$ \overline{A} _{{m_{i} }} \; = \;\sum\limits_{{i = 1}}^{N} {w_{j} } \; \cdot \;a_{{m_{{ij}} }} $$
(2)
$$ \mathop w\nolimits_{j} = \frac{{\mathop m\nolimits_{j} }}{{\sum\limits_{j = 1}^{N} {\mathop m\nolimits_{j} } }} $$
(3)

where \(\overline{{\mathop {A_{m} }\nolimits_{{_{i} }} }}\) is the weighted average estimate of the content of the i-th component in the mixture;

\(a_{{m_{ij} }}\) amis the content of the i-th component in the j-th component in the mixture;

\(\mathop w\nolimits_{j}\) are weight coefficients;

\(\mathop m\nolimits_{j}\) is the mass of the j-th component in the mixture.

A scheme of the ICP-CRM Multi 1 manufacturing technology from carrier materials is shown in Fig. 1. The mass fraction of the components of the carrier materials was preliminarily established according to the “100% minus the ∑amount of impurities” method using GET 217–2018.

Fig. 1
A flowchart of I C P-C R M multi 1 manufacturing technology. Weighing starting materials; weighing solvent dissolution and homogenization, packaging; and homogeneity and stability research.

A scheme of the ICP-CRM Multi 1 manufacturing technology

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The traceability of the certified values of the ICP-CRM to the measurement unit was carried out by establishing the value of the certified characteristics in the starting materials using GET 217–2018.

Results and Discussion

The developed ICP-CRM Multi 1 was packed in bottles with a capacity of 30, 60, and 125 cm3 (Fig. 2).

Fig. 2
Three Multi 1 container bottles in different sizes.

The ICP-CRM Multi 1

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The reliability of the certified values of the mass fraction of metals in the developed ICP-CRM was repeatedly measured using GET 217–2018. The measurement results of the mass fraction of metals in the ICP-CRM obtained using both the gravimetric sample preparation and the measurement method using GET 217–2018 are in good agreement within the limits of their uncertainties, as well as with the results of comparative measurements carried out for the developed ICP-CRM and CRMs of other manufactures. The certified value of the developed ICP-CRM is taken to be that obtained by the gravimetric method of preparation.

The measurement uncertainty of the certified value of the mass fraction of a component in the solution was calculated in accordance with [18]. The uncertainty components for the gravimetric preparation of the solution of the mass fraction of a component are presented in Fig. 3 as a cause-and-effect diagram. The results of the gravimetric preparation and the measurement results using GET 217–2018 are presented in Fig. 4.

Fig. 3
A fishbone diagram the error components of a CRM-certified value. A horizontal rightward arrow A m is at the center. Downward slanting arrows labeled mass of starting material, purity of starting material, and purity of the solvent and upward slanting arrows labeled mass of solution, homogeneity, and storage ability points at A m.

A cause-and-effect diagram for the error components of a CRM certified value

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Fig. 4
Dot plots with error bars plot mass fraction of metals m g forward-slash k g versus elements. Gravimetric sample preparation, (B a, 1000.05), (C d, 999.98), (C o, 999.99), (L I, 999.98), (P b, 1000.01), and (Z n, 1000). G E T 217-218 B a, 999.80), (C d, 999.20), (C o, 1000.20), (L I, 999.30), (P b, 1000.20), and (Z n, 999.70).

The results of the gravimetric preparation and the measurement results using GET 217–2018

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The results of estimating the uncertainty due to the method for determining the certified value of the CRM of a multi-element solution of metals according to the gravimetric method of preparation are presented in Tables 1 and 2.

Table 1 The uncertainty due to the method for determining the certified value of the CRM of a multi-element solution of metals based on the results of the gravimetric sample preparation
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Table 2 The uncertainty budget for gravimetric preparation of the CRM of a multi-element solution of metals on the example of cobalt
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The uncertainty of a certified value includes the uncertainty due to instability and the uncertainty due to homogeneity, in addition to the uncertainty due to the method for determining the certified value. Therefore, future research should investigate the stability and homogeneity of the developed ICP-CRM.

At the current research stage, special attention was paid to the selection of both suitable packaging for storing the developed CRM and its stable composition. However, the mass fraction of any component in a solution is a time function and may change due to possible evaporation losses during storage after unpacking. In this regard, we are planning to conduct additional studies into the long-term stability of the certified value of ICP-CRM Multi 1 and its stability during storage after unpacking. Provided that positive results are obtained in terms of long-term stability, the expanded measurement uncertainty of the certified value of the metal mass fraction in the CRM aqueous solution will not exceed 0.5%.

Conclusion

In this research, we have developed a CRM of a multi-element solution of metals (ICP-CRM Multi 1), which can be used for ensuring the metrological traceability of measurements in inorganic analysis using ICP-MS and ICP-OES according to GET 217–2018. The developed solution will also allow one of the main advantages of these methods to be applied in routine analysis, namely the ability to quickly and simultaneously measure several elements in materials.