  We observed a sample of 90 red giant branch (RGB) stars in NGC 2808 using
FLAMES/GIRAFFE and the high resolution grating with the set up HR21. These
stars have previous accurate atmospheric parameters and abundances of light
elements. We derived aluminium abundances for them from the strong doublet Al I
8772-8773 Angstrom as in previous works of our group. In addition, we were able
to estimate the relative CN abundances for 89 of the stars from the strength of
a large number of CN features. When adding self consistent abundances from
previous UVES spectra analysed by our team, we gathered [Al/Fe] ratios for a
total of 108 RGB stars in NGC 2808. The full dataset of proton-capture elements
is used to explore in details the five spectroscopically detected discrete
components in this globular cluster. We found that different classes of
polluters are required to reproduce the (anti)-correlations among all
proton-capture elements in the populations P2, I1, and I2 with intermediate
composition. This is in agreement with the detection of lithium in lower RGB
second generation stars, requiring at least two kind of polluters. To have
chemically homogeneous populations the best subdivision of our sample is into
six components, as derived from statistical cluster analysis. By comparing
different diagrams [element/Fe] vs [element/Fe] we show for the first time that
a simple dilution model is not able to reproduce all the sub-populations in
this cluster. Polluters of different masses are required. NGC 2808 is confirmed
to be a tough challenge to any scenario for globular cluster formation.
