Tissue analysis of banana is done generally on leaf blades. The analysis will vary based on the phenological stage and leaf position.
If you take 20 leaf samples from the youngest expanded leaf and dry the material which is usually ground the atomic spectrum analysis can give a good idea of the composition on a per atom basis.
The person needs to know the optimized levels for comparison which can be found in the literature or by optimizing growing environment and determining empirically under your own conditions.
It is good to precede the leaf analysis with soil analysis and to track the information including the response to nutrient interventions.
Hope you found this useful.
You may want to read Menon, N., et al. 2001. Banana Nutrition. J. Appl. Sci. 1(4) I am sure there are a lot of more recent references also.