Determination of Rare Earth Elements in Some Flower Herb Teas and their Infusions

The aim of this study was to determine the sixteen rare earth elements (REEs) in twelve types of popular f﻿﻿﻿﻿lower herb teas and their infusions consumed in China. Microwave-assisted acid digestion was used for all of the samples, and the contents of these elements were determined by inductively coupled plasma mass spectrometry (ICP-MS). The concentrations of REEs varied from 94.6 to 7492 μg kg among these f﻿﻿﻿﻿lower samples. Ce was the most abundant elements in all f﻿﻿﻿﻿lower samples (27.3-3016 μg kg). The leached ratios of REEs for each type of tea ranged from 7.3% (globe amaranth) to 29.8% (carnation). The leach ratios of REEs in these f﻿﻿﻿﻿lower herb teas were at different percentages depending on types of f﻿﻿﻿﻿lowers. The daily intake of rare earth oxides from these f﻿﻿﻿﻿lower herb tea infusions may not produce any health risks.


Introduction
The history of drinking herb tea can be traced back to as early as ancient Egypt, while nowadays herb tea is widely consumed all over the world due to their mild features, relatively low price and low side effects.China is one of the countries with high consumption of herb tea products.Among these products, fl ower herb tea is the most popular drinks consumed by Chinese in daily life.Their consumptions have been recognized to exhibit many health benefits.For example, roselle (Hibiscus sabdariffa L.) can control or lower the blood pressure; 1 honeysuckle (Lonicera japonica Thunb) is extensively used for releasing the fever, and some has natural antiphlogistic effect. 2,3Many flower herb teas can send out pleasing fragrance after brewing, which make people relaxing.The commercialization of fl ower herb tea has increased by many folds in the past few years in China.
Generally, the chemical contaminants in herb tea were classified into toxic metals and non-metals, where the heavy metal is an issue of major concern.][6][7] By comparison with heavy metal, the levels of rare earth elements (REEs) in herb tea receive little concern.Some evidences show that excessive exposure to rare earth oxides (REOs) is harmful to human health such as leukemia. 8,9ietary intake is the main pathway of exposure to REOs by human who are not regularly exposed to REEs polluted environments.3][14] For example, Guo et al. 12 has studied the content of REEs in various types of Oolong tea, varying from 0.62 to 10.1 mg kg -1 , with the leached rated of 9.86-32.6%.The maximum limits of REEs for nuts established by Ministry of Health P.R.China is 2 mg kg -1 for tea and cereal products. 15In addition, the estimated daily intake (EDI) of REEs has been proposed by some Chinese researches based on animal experiments. 16,17o the best of our knowledge, limited number of studies concerned on the concentration of REEs in fl ower herb tea and their infusion.To ensure that REE levels in fl ower herb teas are safe for consumers, the main objectives of this study were: (i) determine the contents of REEs levels in various fl ower herb teas, (ii) study the leaching rate of REEs of various varieties of fl ower herb teas, and (iii) estimate the health risks associated with REO intake by drinking fl ower herb teas.

Reagents and standards
Nitric acid (trace metal grade, 69%, Fisher, Canada) and hydrogen peroxide (guaranteed reagent, 30%, Guanghua Sci-Tech, Tianjin, China) were used for the digestion of fl ower herb tea samples.REEs standard solution including La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y was purchased from PerkinElmer (Shelton, CT, USA).The standard solution of the element Re used as an internal standard to calibrate the matrix interference was provided by the National Analysis Center of Iron and Steel (Beijing, China).

Apparatus
All solutions were prepared using deionized water purified by a Milli-Q system (Millipore, Bedford, MA, USA).A Mars 6 microwave oven (Cem, Matthews, NC, USA) was applied for decomposing the fl ower herb tea samples.A PerkinElmer ICP-MS (NexIon 300D, Shelton, CT, USA) instrument was used for the analysis of rare earth elements.Inductively coupled plasma mass spectrometry (ICP-MS) measurement conditions were optimized on daily basis.

Sample preparation and digestion
Milled flower samples (ca.0.25 g) were accurately weighed into a pre-cleaned Tefl on tube, followed by the addition of 5.0 mL of concentrated HNO 3 and 1.0 mL 30% (m/m) H 2 O 2 .The Tefl on tube was placed in the microwave oven.The temperature was increased to 130 °C in 10 min, and maintained for 10 min, and then further increased to 200 °C in 5 min and maintained for 30 min.After cooling, the solution was transferred into a Tefl on beaker, which was then heated nearly to dryness at 200 °C and diluted to 10 mL with deionized water.For sample preparation, one blank, one reference material (citrus leaf, GBW10020, Institute of Geophysical and Geochemical Exploration of China, Beijing, China), and three replicate for each sample were performed on each batch.
The infusions were prepared as Chinese consumption habit of flower herb tea.Dried flower (2 g, unground) was transferred into a glass beaker, followed by addition of 200 mL of boiling deionized water, and the infusion was left at room temperature for 20 min.The supernatant was filtered with an ashless filter paper.A 20.0 mL volume of this solution was transferred into the Teflon tube, and then evaporated nearly to dryness at 150 °C.After cooling, 1.0 mL of concentrated HNO 3 and 1.0 mL 30% (m/m) H 2 O 2 were added.The solutions were digested by using the microwave method as above described.Finally, the digested solutions were diluted to 10.0 mL with deionized water.

ICP-MS analysis
The details of the ICP-MS parameters information are given in Table 1.Some oxides can generate polyatomic interferences such as the oxides interference of 135 Ba16O + on 151 Eu + measurement, and some isotope mass of REEs are very close.These interferences may cause substantial variations from the actual values, in particular, when the REEs are at trace and ultra trace levels.Therefore, a solution of 1 µg L −1 Li, Be, Mg, Fe, In, Ce, Pb, and U was used to optimize the instrumental parameters, in particular, to minimize the oxide ratio, 156 CeO + /140Ce + < 1.5%, 151 BaO + / 135 Ba + < 0.2%.Additionally, some calibration equations (Table 1) were applied to correct the isotope mass interferences.External calibration with REEs multielement standard solutions (0, 0.01, 0.05, 0.2, 1, 2, 5 µg L −1 ) was used for ICP-MS analysis.The pH of fl ower herb tea infusion was analyzed using Sartorius PB-10 (Sartorius Instruments, Beijing, China).

Quality control
Limits of detection (LOD) were determined as 3 standard deviations (SD) of the 10 consecutive measurements of the independent digestion blank taken into consideration the weight of sample and the final dilution employed in the recommended procedure (solid sample: 0.25/10, infusion: 20/10), which were adequate for the determination of REEs concentrations in fl ower herb tea samples and its tea infusion (Table 2).The precision of the method, expressed as the relative standard deviation (RSD, %) at 0.01 µg L -1 concentration level for the REEs were all < 3%.Accuracy validation of the ICP-MS procedure was made by analyzing a certified reference material (citrus leaf, GBW10020).The results are listed in Table 2 and mean values obtained from this experiment were in good agreement with the reference mean values, thus demonstrating the excellent accuracy of the selected method.

REEs contents of the flower herb tea
The REEs of the fl ower herb tea are listed in Table 3.The values of REEs in various types of fl ower herb tea varied from 94.6 to 7492 µg kg -1 , the highest REEs content was found in Coreopsis tinctoria, and the lowest in peony.The REEs values of honeysuckle are in agreement with those reported by our previous report, 18 and the REEs levels of the rest of the fl owers were not found in literature.The levels of REEs in most measured fl ower samples were similar to the values of green tea leaf (Camellia sinensis L.) 12,19 from China and wild mushroom from Poland, 20 and were much higher than the values of rice and wheat 21 that was consider to be a main source of dietary REEs in China.Despite the limited sampling numbers of each fl ower, the values of REEs in different fl owers indicate that variety is an important factor for the accumulation of REEs.
Of the sixteen measured elements, the content of Ce (27.3-3016 µg kg -1 ) was the most abundant in all flower herb tea sample, followed by the content of La (11.2-1260 µg kg -1 ), Nd (10.2-1100 µg kg -1 ), Y (12.6-699 µg kg -1 ) and Sc (15.3-305 µg kg -1 ), which was similar to the previously reported results in terrestrial plant such as tomato, 22 tea leaf, 19 and navel orange 23 from China.The contents of Ce, La, Sc, Y and Nd accounted for 84.5-89.7% of REEs in all of measured samples.The contents of Lu were the lowest among these REEs in all fl ower herb tea sample, followed by the content of Tm and Tb.The content values of light rare earth elements (LREEs) were much higher than the values of heavy rare earth elements (HREEs) in all measured fl ower samples.The ratios of LREEs to HREEs ranged from 7.99 to 12.5 which were similar to the values of tea leaf in China 12,19 (Table 3).

REEs contents of the flower herb tea infusion
Actual amount of exposure to REEs by drinking these flower herb tea infusion was evaluated by determining the concentrations of infusions in these flowers.The concentration ranges of sixteen REEs determined in 12 types of fl ower herb tea infusions were calculated on the basis of 2 g of herb tea in 200 mL.In this study, the leached ratio (LR) was calculated as follow: (1) Here, C 1 is the REEs concentration in fl ower herb tea infusion (µg L -1 ), C 2 is the REEs content in fl ower herb tea (µg kg -1 ), V is the volume of leaching water (200 mL), and m is the mass of fl ower herb tea for leaching (2 g).As shown in Table 4, the leached ratios of each type of tea ranged from 7.3% (gobe amaranth) to 29.8% (carnation).In this study, the REEs leached ratios of these fl owers were lower than those in most types of tea leaves (Camellia sinensis L.) (25.8-51.0%).The element of La was the most difficult to leach into the infusion in all fl ower samples.The mean leached ratios of each REE in each type of fl ower varied from 0.4 (La) to 72.8% (Sc).The leached ratios of most REEs significantly varied among the different types of fl ower.For example, the leached rates of Ce were 5.0% for globe amaranth, while it was 34.4% for peony.The pH value is considered to be an important factor to affect the solubility and complexation of REEs. 24,25In this study, the pH value of each fl ower herb infusion varied in the range of 2.56-5.97(Table 4).Due to high content of water-soluble organic acids, the pH value of roselle infusion (2.56) was significant lower than values in other fl ower infusions (4.55-5.97)measured in this study.However, in this study, no significant correlation was observed between the pH values and REEs concentration of the fl ower herb tea infusions.Only the concentration of Y and Yb in these infusions exhibited significant correlation with the pH values (p < 0.05).

Health risk assessment
Daily REEs intake depends on both element concentration in fl ower herb tea infusion and the daily drinking volume.Estimated daily intake (EDI) was calculated as: Here, EDI is the estimated daily intake of rare element oxides (REOs) (µg day -1 person -1 ); C REOs is the REOs concentration in flower herb tea infusion; and M rdc is the mass of flower herb tea consumed daily in China (according to Chinese consumption habit of fl ower herb tea, M rdc = 5 g day -1 for adults); LR is the leached ratio of REOs ).EDI was compared with the dietary reference intake (DRI) for REOs (4200 µg day -1 person -1 ) proposed by Zhu et al., 16 who carried out a comprehensive investigation in a REE-rich area in China, and animal experiments.As shown in Table 5, the EDI of each type of flower herb tea infusions ranged from 0.31 (peony) to 2.07 (lily), and the maximum EDI (5.67) value was found in Coreopsis tinctoria.All these values account for 0.01 to 0.05% of DRI.According to the investigation by Zhou et al., 17 the average daily dietary intake of REOs was 250.9 µg day -1 for adults in China, and the main sources of REOs were cereal and vegetable in most parts of China, which accounted for 28.6 and 24.7%, respectively.The above-calculated EDI of REOs through drinking fl ower herb tea is negligible.Thus, the ingestion of REOs by drinking fl ower herb tea is harmless to human health.

Table 2 .
Limits of detection of rare earth elements and concentrations (mean ± standard deviation) in certificated value (GBW10020) and fl ower herb tea samples a LOD 1 : limits of detection of REEs concentrations in fl ower herb tea sample; b LOD 2 : limits of detection of REEs concentrations in fl ower herb tea infusion.

Table 3 .
Rare earth elements (REEs) concentrations in fl ower herb tea samples a Light rare earth elements (LREEs) include La, Ce, Pr, Nd, Sm, and Eu; b heavy rare earth elements (HREEs) include Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

Table 4 .
Leached ratios (%) of rare earth elements (REEs) concentrations of fl ower herb tea by three successive infusions -89.7% of REEs in measured fl ower herb teas.For their infusions, the leached ratio of each type of fl ower ranged from 7.3 (globe amaranth) to 29.8 (carnation).The total content of REEs exhibited no significant correlation with the pH values of the infusion while concentrations of Y and Yb exhibited significant correlation with the pH values (p < 0.05) of the infusion.After calculating the EDI, it confirms that the risk to the human health of REOs by drinking fl ower herb teas is negligible.

Table 5 .
Estimated daily intake (EDI) of rare element oxides (REOs) and percentage of dietary reference intake (DRI)