and two strong cation exchange guard columns (Zorbax SCX 5 m, 4.6 mm i.d. × 12.5 mm long, Agilent Technologies, Santa Clara, CA, USA) connected in series as the analytical column
and two strong cation exchange guard columns (Zorbax SCX 5 m, 4.6 mm i.d. × 12.5 mm long, Agilent Technologies, Santa Clara, CA, USA) connected in series as the analytical column