What analytical methods are used to quantify C, O, N, and H in a sample?
For N:

Total N = N from NO2 + N from NO3 + Total Kjeldahl N

The methods from here are:

Liquid Chromatography: NO2, NO3

Specific electrodes: NO3, Kjeldahl

Spectrophotometry: NO2

This for water including wastewater. I don't know the aplicability for other samples.

Also ICP-MS is worth researching.