Be careful, because H you will not be able to detect using EDAX. C, O and N are also very likely not detectable with EDAX, depending on the detector type and system you use (entrance window, excitation energy, ...). Such light elements are not measureable using EDAX in a straightforward way because their fluorescence line energies are very low.

Please note that EDAX is a proprietary brand name from Ametek, it should not be used when referring generally to the technique of Energy Dispersive X-ray analysis. Either EDS or EDX are acceptable. 

Philipp is correct. H is not detectable, therefore not quantifiable by this method. C, O & N can be detected with most modern solid state detectors; even older thin window Si(Li) could detect C. There are many problems quantifying these light elements using any X-ray methods, and while the system may generate precise numbers they may be meaningless. You would need to be scrupulous in carrying out a full calibration routine using your own standards.

Other problems you would face if you try this route relate to sample preparation. The sample needs to be homogeneous within the interaction volume (dependant on accelerating voltage and sample density, but generally around 1 micron), it should be stable within the vacuum, it should be stable under the electron beam (most biomass materials burn easily at the currents required for analysis), it should be polished flat to 1/4 micron grade - this is most important for light element work as ANY surface topography will affect the results due to the ease of absorption of these low energy X-rays by surrounding material.

So lots of problems associated with light element analysis by EDS. 

CHNX analysis will give relative amounts of the light elements and halogens, but it is very likely that your biomass may contain Na, P, S, K, Ca in addition.

Thank you Ian and Phillipp.

The point about H detect-ability is important, especially because the objective is characterization of biomass. While studying combustion characteristics of a biomass, C:H:O ratios are useful to compare with other studies. In that regard, CHNX elemental analyzer looks to be a more suitable method here.

Yes biomass contains Na, P, K, Ca in small fraction and Si in case of few biomass from Paddy. I would certainly determine Na, P, S, K, Ca. However if the purpose is characterization of biomass for thermal or combustion studies, CHNX elemental analyzer looks to be a better suited option from the discussion here.

Thank you very much. I appreciate your response.