Original researchRapid determination of toxic and rare-earth elements in teas by particle nebulization-ICPMS
Introduction
Tea prepared from the dried leaves of Camellia sinensis is the most popular non-alcoholic beverage worldwide, and two-thirds of the world’s population consume tea daily (Welna et al., 2013). Furthermore, a rapid increase in the consumption of the post-fermented tea Pu’er has been recently observed owing to its many beneficial health effects and medicinal properties (Cao et al., 2010; Lv et al., 2013). However, human intake of pollutants, such as heavy metals (i.e. As, Cd, Cr, Hg, and Pb), and rare-earth elements (REEs), by drinking tea is attracting increasing the public’s concern (Squadrone et al., 2019; Ma et al., 2019; Guo et al., 2015). The maximum REE levels in tea established by the Chinese Government are 2.0 mg kg−1 for As, 1.0 mg kg−1 for Cd, 5.0 mg kg−1 for Cr, 0.3 mg kg−1 for Hg, and 5.0 mg kg−1 for Pb, and that of total rare-earth oxides (REOs) is 2.0 mg kg−1 (NY, 2003; GB, 2762-, 2005). Considering the significant contribution of habitual drinking makes to daily intake of these elements the screening of heavy metals and REEs contents in teas is of a great importance.
Currently, inductively coupled plasma-mass spectrometry (ICP-MS) is the most commonly used analytical technique for multi-element determination of samples with a range of matrix types (Guo et al., 2020; Kilic and Kilic, 2019; Satyanarayanan et al., 2018; Toker et al., 2018; Yuksel and Arica, 2018; Yin et al., 2018; Tel-Cayan et al., 2018; Ward et al., 2018). This predominance is largely because of its’ distinct advantages, which include its high sensitivity, simultaneous multi-element and isotopic measurement capability, and the relatively simple spectra it affords (Nardi et al., 2009; Ma et al., 2016; Liu et al., 2019; Yang et al., 2019; Wang et al., 2019; Zhang et al., 2019). The conventional method for introducing samples into an ICP-MS is by nebulization of dissolved aqueous samples. Prior to analysis by ICP-MS, solid food samples are typically prepared as solutions using dissolution procedures, such as fusion, dry ashing, and microwave-assisted digestion with closed vessels using concentrated acids (HNO3 and H2O2) or even hazardous acids (HF or HClO4) (Richter et al., 2019; Souza et al., 2020). Although these processes are well established, the involved and time-consuming digestion procedures required can lead to loss of volatile elements, contamination, and increased spectral overlaps (Zwierzchowski and Ametj, 2018).
The introduction of solid powders into high-temperature ICP sources by direct laser ablation (LA) can circumvent these difficulties and markedly decrease sample preparation time by combining matrix destruction, analyte atomization, excitation, and ionization in one step (Cakmak et al., 2010; Augusto et al., 2017; Pozebon et al., 2017; Papaslioti et al., 2019; Li et al., 2019). The ablated sample particles are directly introduced into the ICP by an Ar or He carriers, and the ionized species are subsequently detected by MS. However, the lack of available solid-matrix-matched external standards, which are required to overcome nonstoichiometric sampling, aerosol transport, and ionization in LA-ICP-MS analysis, remains a major hindrance to the accuracy and precision of this technique (Günther and Hattendorf, 2005; Miliszkiewicz et al., 2015).
An alternative approach for introducing solid particles involves the nebulization of aqueous suspensions of fine sample particles into the ICP. This process is termed ‘slurry nebulization’ (Ebdon et al., 1997). The most significant advantage over LA sampling is that it can be calibrated using with single aqueous standards by a conventional pneumatic sample introduction system (Ebdon et al., 1990).
There are several reports on the application of slurry nebulization ICP-OES technique for the analysis of the geological and inorganic material samples (Song et al., 2006; Wang et al., 2009; Mujuru et al., 2009; Wang et al., 2006; Wang and Yang, 2014; Wang et al., 2015; Ebdon et al., 1988; Mochizuki et al., 1991; Santos and Nobrega, 2006). Most of these works focused on achieving homogeneous and stable dispersions before slurry-sampling ICP-OES analysis. However, the transportation and ionization behaviors of particle slurries and aqueous standards can be very different in ICP. For instance, Fuller et al. (1981) and Mochizuki et al. (1989) observed that the signal responses for Cr or Ni and REEs in slurry nebulization techniques were 30−50% and 61−83%, respectively, compared to those of aqueous standards. Therefore, accurate quantitation calibration using the simple aqueous standards in particle nebulization ICP-MS technique remains a significant challenge and should be studied in detail.
Accordingly, in the present study, we develop a routine particle nebulization ICP-MS method for direct determination of 21 toxic elements (i.e. As, Cd, Cr, Hg, Pb, and 16 REEs) in tea samples. Our research focused on direct calibrations with aqueous standards by studying the nebulization, transportation, and ionization behavior of particulate tea samples. The optimization of particle size, particle stabilization, and homogenization as well as the thorough validation of method performance by analyzing tea certified reference materials (CRMs) are presented. Furthermore, the practical application of the method to the screening of toxic elements and REEs in Pu’er teas is demonstrated.
Section snippets
Apparatus, reagents and materials
A NexION 350D ICP-MS (PerkinElmer Ltd., Waltham, USA) with a conical U-series nebulizer (Glass Expansion Ltd., Port Melbourne, Vic, Australia) were used in this study. Table 1 lists the optimized operating parameters. A tissue Cell-Destroyer 1000 (Hubei Xinzongke Viral Disease Control Bio-Tech Ltd., Wuhan, China) was used to achieve the desired particle sizes of the food samples by varying the milling time. Particle size distributions were determined by a Mastersizer 3000 laser diffraction
Method development for the determination of toxic elements and REEs
Compared with other techniques for the direct introduction of solid samples for multi-elemental analysis (i.e. laser ablation), the main advantage of particle nebulization method is that it can be calibrated using simple aqueous standards. Four steps are required for the particle nebulization ICP-MS technique: (i) particle size reduction; (ii) particle stabilization and homogenization in the dispersant; (iii) particle introduction and ionization for ICP and (iv) MS measurement and accurate
Conclusions
We develop a rapid method for the direct measurement of 21 toxic elements in teas by particle nebulization ICP-MS. The proposed method has several advantages over conventional acid digestion ICP-MS technique including a lower reagent required, better safety and easy to use, considerably lower loss and contamination, and higher throughput. Screening of the toxic elements and REEs in Pu’er teas was conducted by the proposed method. The results indicated that the levels of toxic elements and Heavy
CRediT authorship contribution statement
Wei Guo: Methodology, Writing - original draft, Supervision. Rong Wang: Data curation, Software. Wuxia Wang: Writing - review & editing. Yue’e Peng: Project administration.
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work.
Acknowledgments
This work was supported by the National Key Research and Development Program of China (No. 2016YFE0203000), the National Natural Science Foundation of China (No. 41873072, and 41521001), and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG170102 and CUG180603).
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