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Composition of tomatoes and tomato products in antioxidants (WG1) page 83
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electrochemical detection were measured as low as 10 fmol representing a 100- to 1000-fold
increase over conventional UV-Visible detections (Ferruzzi et al. 1998). However because
electrochemical response was shown to be dependent and sensitive to operational conditions,
proper manipulation of the different variables is required. Different carotenoids including cis
and trans isomers can be detected by this method.
We will not here describe specifically all these methods but rather we will try to
present several of the main factors that could influence the separation and the quantification
of the analytes.
Reverse phase analyses with isocratic conditions lead to fast separation of main
plasma carotenoids but they did not allow efficient separation of lutein and zeaxanthin and
trans and cis isomers of lycopene (Aebisher et al. 1999). When normal phases were used, 12
cis isomers of lycopene were resolved from the same plasma sample and they consisted
around 60% of the total lycopene area. These isomers exhibit different coefficient of
absorption (Ε1%/cm) (Table 29). Thus in plasma, quantification as a single peak (reverse
phase) of lycopene would result in an overestimation of its concentration in the sample as
compared with the quantification of both trans-lycopene and cis-lycopene (normal phase)
using their respective Ε1%/cm. However, very few cis-lycopene was detected in tomato and
tomato products suggesting that quantification of one peak of lycopene would be an adequate
measurement.
Table 29. Absorption coefficients of lycopene isomers
Analyte λ max (nm) E(1%/cm) Solvent
all-trans-Lycopene 472 3450 n-Hexane
5-cis-Lycopene 470 3466 n-Hexane
7-cis-Lycopene 470 2901 n-Hexane
15-cis-Lycopene 470 2072 n-Hexane
(Adapted from Aebisher et al.1999)