There are a number application notes that cover this topic, they are available in PDF format for online viewing and downloading. Please see the following Application Notes.
AAA Application Note 49
Analysis of amino acids in wines (grape juice)
Spectrophotometry Application Note 19
Spectral Analyses of Fruit Juices
Spectrophotometry Application Note 36
The Determination of D-Malic Acid using
Ultrospec 2000
Spectrophotometry Application Note 39
The Determination of L-Ascorbic Acid in
Fruit Juices using Ultrospec 2000
If this does not answer your question or solve your problem, then please contact us at support@biochrom.co.uk detailing your question or the problem you are experiencing. Please list if relevant the instrument, software and version numbers you are running, and enclose any relevant data files, etc.