2.1. Chemicals and Reagents

Element reference standard solutions of arsenic (As, GBW(E)080117, 1000 μg/ml), arsenite (As(III), {"type":"entrez-protein","attrs":{"text":"GBW08666","term_id":"1450828289","term_text":"GBW08666"}}GBW08666, 75.7 ± 1.2 μg/g), arsenate (As(V), {"type":"entrez-protein","attrs":{"text":"GBW08667","term_id":"1450828290","term_text":"GBW08667"}}GBW08667, 17.5 ± 0.4 μg/g), monomethylarsonic acid (MMA, {"type":"entrez-protein","attrs":{"text":"GBW08668","term_id":"1450828291","term_text":"GBW08668"}}GBW08668, 25.1 ± 0.8 μg/g), dimethylarsinic acid (DMA, {"type":"entrez-protein","attrs":{"text":"GBW08669","term_id":"1450828292","term_text":"GBW08669"}}GBW08669, 52.9 ± 1.8 μg/g), arsenobetaine (AsB, {"type":"entrez-protein","attrs":{"text":"GBW08670","term_id":"1450828293","term_text":"GBW08670"}}GBW08670, 38.8 ± 1.1 μg/g), arsenocholine (AsC, {"type":"entrez-protein","attrs":{"text":"GBW08671","term_id":"1450828294","term_text":"GBW08671"}}GBW08671, 28.0 ± 1.1 μg/g), and standard reference material {"type":"entrez-protein","attrs":{"text":"GBW09588","term_id":"1450827821","term_text":"GBW09588"}}GBW09588 (Atractylodes macrocephala) were obtained from the National Institute of Metrology (Beijing, China), with their concentrations guaranteed. Working mixture standard solutions of arsenic speciation were prepared daily by diluting the source standard solutions to proper concentrations with pure water. Tune solution and mixed internal standard were provided by PerkinElmer (MA, USA). Na₃AsO₄, NaAsO₂, and As (Cys)₃ were also provided by the National Institute of Metrology (Beijing, China).

Nitric acid (HNO₃, 68%, ultrapure) and hydrogen peroxide (H₂O₂, 30%, ultrapure) were purchased from Suzhou Crystal Clear Chemical Co., Ltd. (Suzhou, China). High purity argon (Ar, 99.999 %) was obtained from Jinxin Gas Co., Ltd. (Xining, China). HPLC grade methanol (Merck, Germany), HPLC grade ammonium dihydrogen phosphate, and aqueous ammonia were obtained from ANPEL Laboratory Inc. (Shanghai, China). Ultrapure water (18.2 MΩ) prepared with a Milli-Q system (Millipore, Co., USA) was used for all solution preparations. The glassware and plasticware used in this experiment were soaked in 20% HNO₃ solution for 24 h prior to use.