NO - Besoin d'assistance

Composition of tomatoes and tomato products in antioxidants (WG1) page 75
__________________________________________________________________________________________
8.3 Standards
When High Performance Liquid Cromatography (HPLC) is used, identification, then
quantification of the different analytes needs external and internal standards. These standards
must be as pure as possible and must remain stable during standard solution preparation and
storage, during extraction and analysis. Impurities can affect the standard calibration and the
final quantification of the analytes becomes imprecise. There are actually no 100% pure
standards that are commercially available. In consequence when purity is less than 97%, it is
suggested to purify the compounds on open columns or specific cartridges just before their
use. Osberg et al. (1999) have suggested that lycopene from Sigma Chemical was not always
of sufficient quality and that lycopene from Indofine appeared to be better even if more
expensive. It is generally accepted that the purity of standards would be calculated through
HPLC from the specific area of the standard/sum of the areas before preparing calibration
curves.
Unlike tocopherol, carotenoids were shown to be weakly soluble in alcohols and
acetonitrile at room temperature. At –20°C, α-carotene, β-carotene and lycopene in ethanol
precipitated out at concentrations above 1 µg/ml and did not redissolve when returned to room
temperature (Zaman et al. 1993). The more diluted standards were shown to be very stable,
except for lycopene, which had to be diluted to about 0.2 µg/ml. More apolar solvents such as
methylene chloride, hexane, toluene, benzene, or tetrahydrofuran (THF) were thus required to
prepare stock solutions. However, chlorinated solvents were shown to degrade trans-lycopene
and to form several cis-isomers. Hence, Scott (1992) observed that over a 20 day period and
at –20°C, the concentration of lycopene in chloroform declined by 46-48% (Fig. 15-16).