Kjeldahl method cannot be used for the estimation of nitrogen present in DNA and RNA . This is because the nitrogen in these compounds does not convert into ammonium sulphate under the conditions of this method. To estimate the amount of nitrogen , the ring should be broken first, so that the free nitrogen gets converted to ammonia, which is dissolved in acid and estimated by direct or indirect titration. The difficulty in decomposing ring structures limits this test for DNA and RNA.