Rapid determination of toxic and rare-earth elements in teas by particle nebulization-ICPMS

Highlights

• Direct nebulization of tea particles into ICP plasma and MS detection.

• Efficient transportation and ionization of tea particles (<3 μm) in ICP.

• Aqueous standard calibration for particle nebulization ICP-MS analysis.

• High levels of LREEs and low levels of toxic elements and HREEs in teas.

• Total REEs in 30% of Pu’er tea exceeds the Chinese national limit.

Abstract

Toxic elements profiling of teas is vital in terms of both quality control as well as a means to generate a comprehensive database for human-health-risk assessment. Accordingly, in the present study, a rapid method using direct nebulization of tea particles for inductively coupled plasma (ICP) ionization and subsequent detection of toxic elements by mass spectrometry (MS) was developed. Dried and well grounded tea particles were stably dispersed in 0.5% polyethylene-imine and the particle slurries were analyzed by ICP-MS using aqueous standard calibration. Monitoring the nebulization, transportation, and ionization behaviors of particles of different sizes revealed that particles with a mean size of 1 μm provide values comparable with those of aqueous standards containing equivalent concentrations of the analyte. The excellent recoveries of the method (90–105%) were verified by analyzing two tea certified reference materials, and the detection limits ranged from 0.03 (for Tm) to 1.2 (for Cr) μg kg⁻¹. Then, we performed screening analysis of five toxic elements (As, Cd, Cr, Hg, and Pb) and 16 rare-earth elements in 20 Pu’er teas, and the results revealed that the contents of all the toxic elements and heavy rare earth oxides were low level, where those of the light rare-earth oxides were high. Furthermore, the total rare-earth oxides content of 30% of the Pu’er teas exceeded the Chinese National limit.

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.