Derivative spectrophotometric determination of nitrate in plant tissue.

A derivative spectrophotometric method was developed to determine NO3(-)-N in plant tissues. The method is based on measurement of the first-derivative spectrum of nitrosalicylic acid in basic solution. The nitrosalicylic acid was obtained by reaction of samples with salicylic acid in concentrated sulfuric acid and was used by Cataldo et al. in nonderivative spectrophotometry. The main strength of this technique is the lack of matrix background interference, typical of plant extracts in traditional spectrophotometric methods. This method is fast, inexpensive, easy-to-apply, and highly selective. The calibration graph was linear in the range of 0.1 and 1.0 mg/L N as NO3(-). Average recovery in real matrixes (lettuce and spinach) was 102.6%; average standard deviation was 3.3. This method has been applied to leaves of 4 types of lettuce.