When will the Lassaigne's method for detection of nitrogen and Kjeldahl's method for quantitative estimation of nitrogen fail? Lassaigne's method:

The organic compound is fused with dry $Na$ in a fusion tube. The fused mass after extraction with water is boiled and filtered. This filtrate is called sodium extract (S.E.), which is used to detect elements other than $\ce{C}$ and $\ce{H}$ in organic compound.

If the organic compound contains nitrogen then the S.E. will form $\ce{NaCN}$. Further reactions with $\ce{FeSO4 + NaOH}$ and $\ce{FeCl3 + HCl}$ gives blue or green colour, confirming the presence of nitrogen.

This test fails in case of diazo compounds because sodium metal can not extract nitrogen from them. What about the compounds containing nitrogen in the ring or having nitro groups?

Kjeldahl's method:

Nitrogen containing organic compound is heated with conc. $\ce{H2SO4}$. $\ce{N}$ in the compound converted to $\ce{(NH4)2SO4}$. The mixture is then heated with excess of $\ce{NaOH}$. The liberated $\ce{NH3}$ gas is absorbed in excess of standard solution of $\ce{H2SO4}$. From the amount of acid reacted with $\ce{NH3}$, % of $\ce{N}$ can be calculated.

This method is not applicable to diazo, nitro and compounds having nitrogen in the ring. This is because nitrogen of these compounds does not change to ammonium sulphate upon reacting with conc. sulphuric acid.
You are right about the limitations of century old Lassaigne's method. Why are they still teaching it? Modern day CHNSO elemental analysis is done with gas chromatograpy systems. If you are quoting from a book, provide a reference.

This is an excerpt from a 1953 work Sulfur in Organic Compounds, by a Modified. Lassaigne's Method ACTA CHEMICA SCANDINAVICA 7 (1953) 1395


  Objections to Lassaigne's method are: a) low boiling substances evaporate
  before reaction with the sodium, b) diazo-, azo and polynitro compounds evolve nitrogen c) some heterocyclic compounds do not react.