To my knowledge the only way is approximate. It involves assumptions, e.g. that for C-free, dry ash, Si is present as SiO2, the cations as carbonates, and that the majority of the N, S and H are lost during ashing.

A simple thing to do is to measure how much acid is required to neutralise the ash. Then digest the ash in xs HCl, wash out the soluble salts, dry and weigh them, and dry and weigh the SiO2.

From your question I suspect that you have a high SiO2 material such as a grass.

Dear Paul Milham Sir,

The biomass I deal with is "Sewage Sludge"from waste water treatment plant. From proximate analysis it was found to be having an ash content of 30 %.

Dear Bijesh Raveendra ,

if you did not dry your sample at 105degC prior to CHNS analysis (= as received), the oxygen content is estimated as follows:

O = 100 % - C % - H % - N% - S % - moisture % - ash %

(ash % needs to be on as received basis).

If you carried out ultimate analysis on oven-dried sample, then the equation is as follows:

O = 100 % - C % - H % - N% - S % - ash %

(ash % on dry basis)

Refer to section 5.8 in ASTM 870 (http://www.astm.org/Standards/E870.htm).

Dear

Robert Thomas Bachmann,

Thank you so much for your reply.

The ultimate analysis was conducted without drying in oven.

The doubt I had was whether the ultimate analysis results for Oxygen and Hydrogen incorporates the Hydrogen and Oxygen in moisture or not.

O = 100 - (C+H+N+S+Ash+M)

M=moisture

If moisture and ash content are present in the sample then use this standard formula

O = 100 - (C+H+N+S+Ash+M)

M=moisture