Choice of technique will depend on the expected concentration levels of Zinc. In case these lie in ppm range then flame version is fine. For lower concentrations you are advised graphite furnace atomization.

Let’s say if I want to determine the Zinc content in fruit juice, should I use flame AAS or graphite furnace AAS, and why so? Is it due to the fruit juice or to the mineral content?

First of all check the remaining life of your hollow cathode lamps. strongly suspect that they are nearing end of shelf life. If shelf life is not over or nearing expiry see if the procedure adopted is a validated one.

Hi Nureni,
It appears that your burner head or burner is having some remaining contamination from previous analysis.Aspirate with 1% nitric acid solution for around 5 minutes and follow with same time with water.The flame should be free of the red glow.Even then if the glow remains check the purity of water as it may be having contamination of alkali/alkaline earth elements

Hi Abdulqawi,
The sensitivitiy is dependent on the number of ground state atoms in the light path.In flame analysis you can rotate the burner or even adjust its height to get the maximum absorbance thereby giving improved sensitivity.