The Study of UV and VIS Absorption Spectra of the Complexes of Amino Acids with Ninhydrin

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Abstract
The analysis of amino acids with post column derivatization using ninhydrin is usually carried out with automatic analyzers at wavelengths of 570 nm (for the quantitation of α-amino acids) and 440 nm (for the imino acids), or at only one wavelength, 520 nm, for the total acid content. The α-amino acids are reported to give an absorption maximum at 570 nm, after complex formation with ninhydrin; and α-imino acids have a γ at 440 nm, 520 nm is a compromise wavelength. This paper presents a study of the absorption of the complexes of amino acids with ninhydrin between 275 and 700 nm. Two new wavelengths are suggested for use, 290 and 404 nm. In the near UV (290 nm) all the tested acids show a common absorption maximum and good sensitivities. In the visible region at 404 nm, the sensitivities of the α-amino acids are very similar to those observed at 570 nm. At 520 nm the αamino acids have the lowest sensitivities. The α-imino acids (proline and oxyproline) show absorption maxima only at 290 nm; after that their sensitivities decrease with increasing wavelength. The paper offers some practical hints on changing the detector wavelength, and to adapt the HPLC's detector to work at one of these two, newly suggested, bands (290 or 404 nm).