Enantiomeric purity determination of malic acid in apple juices by multi-beam circular dichroism detection

A multi-beam circular dichroism (CD) detector which is easily constructed b y inserting inexpensive optics into a conventional photo-diode array detect or has an advantage of simultaneous detection of the absorbance and CD. The enantiomeric purity determination of malic acid in beverages was performed by this detection system. Malic acid when complexed with Cu(II) was found to have an absorbance maximum at around 750 nm. The L-malic acid-Cu(II) com plex showed a positive Cotton effect in its absorbance band and its anisotr opy factor (Delta epsilon/epsilon) was relatively large at about 1/170. Thi s complex was retained on a reversed-phase column with the addition of race mic 2-hydroxy-3-methylbutyric acid to the mobile phase as the ligand. A plo t of the relative peak areas between the CD and the absorption (Delta abs/a bs) versus optical purity showed good linearity with a correlation coeffici ent of 0.999, and the precision expressed as the relative standard deviatio n of the errors from the regression line was +/-2.7% (2 sigma). The accurac y of the proposed method was assessed by capillary electrophoresis. Eight c ommercially available juice products were analyzed using this method. Five of them were thought to be adulterated with synthetic malic acid. (C) 2001 Elsevier Science B.V. All rights reserved.