Fast and Sensitive Determination of Cadmium and Selenium in Rice by Direct Sampling Electrothermal Vaporization Inductively Coupled Plasma Mass Spectrometry

The direct sampling electrothermal vaporizer (DS-ETV) was made in our laboratory, and its schematic diagram is shown in Figure 1. The actual device picture is displayed in Figure S4. The DS-ETV system mainly consists of a graphite furnace, an automatic sampling device (EXPEC 723, Hangzhou PuYu Technology Development Co., Ltd., Hangzhou, China, 40 positions) with graphite boats, a temperature sensor and control system, a gas line system, an interfacing tube for ICP-MS, and a power supply. Herein, the graphite furnace (28 × 7.4 mm, Φ 5.9 mm, Beijing Xiangchenghe Photoelectric Technology Co., Beijing, China) and sample boat (18 × 4.2 × 3.3 mm, 120 μL, Beijing Xiangchenghe Photoelectric Technology Co., Beijing, China) were both coated with pyrolyzed carbon (methane and ammonia at high temperature to form a pyrolytic coating according to the previous study [39]) were employed to heat (longitudinal heating with graphite electrodes) a slurry aliquot of powdered rice. To monitor the real-time temperature of the graphite furnace, a photoelectric sensor (S1226-44BQ, Beijing Hamamatsu Photon Techniques Inc., Beijing, China) and an infrared thermometer (SA120BSK, Wuxi Shiao Technology Co., Ltd., Wuxi, China) were jointly utilized, and then to control the heating temperature ranging from 0 to 2600 °C via a proportional integral differential (PID) based on a computer and a power supply (AC, 50 Hz). The gas line system is mainly composed of two gas mass flow controllers (GMFC) and a needle valve to control carrier gas, bypass gas, and protective gas, respectively. A cyclonic nebulizer was made of PFA (inner capacity ~40 mL) and interfaced with the ETV outlet with ICP-MS (SUPEC 7000, Hangzhou PuYu Technology Development Co., Ltd., Hangzhou, China). The detailed parameters of DS-ETV-ICP-MS instrumentation are shown in Table 3 and Table 4.

To verify the proposed method, a separate ICP-MS was used to detect Cd and Se in rice samples after the microwave digestion process. The detailed operating conditions of microwave digestion (EXPEC 790S, Hangzhou PuYu Technology Development Co., Ltd., Hangzhou, China) and ICP-MS (SUPEC 7000, Hangzhou PuYu Technology Development Co., Ltd., Hangzhou, China) are listed in Table S1.

The standard stock solutions (1000 mg L⁻¹) of Cd, Se, and other elements were purchased from the National Center for Analysis and Testing of Nonferrous Metals and Electronic Materials (Shanghai, China). The certified reference materials (CRMs) of powered rice (GBW10010a and GBW10045a with certified 53 ± 4, 320 ± 40 ng g⁻¹ Cd and 36 ± 8, 60 ± 10 ng g⁻¹ Se values, respectively) were purchased from the National Research Center for Certified Reference Materials (Beijing, China). Working standards were obtained by stepwise dilution of stock solutions with deionized (DI) water, which was prepared using a Milli-Q Elix Essential water purification system (Millipore, Burlington, MA, USA). HNO₃ and H₂O₂ (guaranteed grade) were used for the microwave digestion of rice samples. Triton X-100 from PerkinElmer (Waltham, MA, USA) was used to prepare slurry samples of powdered rice after sieving by 60-mesh.

The analytical procedures of slurry direct sampling ETV-ICP-MS for Cd and Se in rice sample are summarized as follows: (1) dehydration: a 10 μL slurry sample was pipetted into the sampling boat and then was automatically moved into the graphite furnace for heating under 200 °C for 15 s to complete the dehydration. Here, carrier and bypass argon gases were set as 300 mL min⁻¹ and 100 mL min⁻¹, respectively, to blow the moisture outside via the switching outlet. (2) Ashing: the graphite furnace was heated to 450 °C within 20 s and held for 20 s to ash the rice sample; the above carrier and bypass gases were employed to blow the organic substance outside via the switching outlet. (3) Vaporization: the graphite furnace was heated to 1900 °C within 7 s and then kept for 5 s. The carrier and bypass argon gases were changed to 600 mL min⁻¹ and 500 mL min⁻¹, respectively, when switching off the outlet mentioned above. (4) Detection: the Cd and Se analytes were transported into the ICP to excite them for the following detection using a quadrupole mass spectrometer, of which peak area (cps) was calculated for the quantitative analysis. (5) Cleaning: 500 mL min⁻¹ carrier Ar and 800 mL min⁻¹ bypass Ar under 2100 °C was performed for 5 s to eliminate the potential interference caused by residual matrix in the furnace and transportation.

A 0.1 g rice sample was accurately weighed and placed in a modified tetrafluoroethylene (TFM) digestion vessel with 1 mL HNO₃, 2 mL H₂O₂, and 1 mL DI water. The procedures of the microwave digestion system (EXPEC 790S, Hangzhou PuYu Technology Development Co., Ltd., Hangzhou, China) are shown in Table S1. After microwave digestion, the sample solutions were cooled to 40 °C and then transferred to a 50 mL centrifuge tube to be diluted to 15 mL with 2% HNO₃ (v:v). The digested solutions will be measured by ICP-MS, as shown in Table S2.

Rice samples were purchased online. The rice samples were first dried at 70 °C for 4 h in a constant temperature oven (101-3B, Union Whale Electronic Technology Co., Ltd., Shanghai, China). The dried samples were ground using a grinder (BJ-800A, Deqing Baijie Electric Co., Ltd., Huzhou, China) and then passed through a 60-mesh sieve. According to the previous studies [40,41], 0.1 g rice powder was placed into a crushing tube (pre-placed zirconium beads, Φ 0.15 mm, 1.5 g) and 1 mL of 1:5000 (v:v), and then Triton-100 solution was added. After sealing, the sample was placed in a grinder (DS1000, Hubei Xinzongke Viral Disease Engineering Technology Co., Ltd., Wuhan, China) and ground 3 times at 5500 rpm for 15 s. After grinding, the tube was placed in a shaker (NMYJ-12, Taizhou Nomi Medical Technology Co., Ltd., Taizhou, China) and shaken for 120 s. The above sample preparation can be finished within 7 min.

Statistical analysis of experimental data was performed using Origin 2021 (OriginPro 2021 9.8.0.200, OriginLab Co., Northampton, Massachusetts, USA) and Microsoft Excel^R 2019 (2210 Build 16.0.15726.20070, Microsoft Co., Washington, Redmond, USA). Significant differences were assessed by a t-test, in which p-values lower than 0.05 (p < 0.05) supported statistical significance.