2.2. Reagents and materials
All reagents used were of high purity analytical grade or HPLC grade and the deionized water (specific resistivity 18.1 MΩ cm−1) was produced by a Millipore Milli-Q water purification system (Bedford, MA, USA). Methanesulfonic acid (MSA) and acetonitrile (ACN) were obtained from Acros Organic (Geel, Belgium). MEA (≥99%), DEA (≥99%), TEA (≥99%), DMA (≥33%) and DIEA (≥99%) were purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). Cyclohexane and ether were bought from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
Individual stock standard solutions of each analyte at a con- centration of 2000 mg L−1 were prepared by exact weighing of each compound and diluting with acetonitrile. These solutions were stored in a refrigerator at 4 oC. The accurate concentration of DMA in solution was standardized by titration with hydrochloric acid titrant using phenolphthalein solution as indicator prior to use, while the titrant was standardized with the highest grade sodium carbonate. A series of mixed working solutions containing all analytes were prepared daily by appropriate dilution of each stock standard solution with mobile phase.