Hi,

Maybe this have to do with the conversion efficiency of the catalyst your are using. The following paper might be useful:

Watanabe, K., Badr, E. S., Pan, X., & Achterberg, E. P. (2007). Conversion efficiency of the high-temperature combustion technique for dissolved organic carbon and total dissolved nitrogen analysis. International Journal of Environmental and Analytical Chemistry, 87(6), 387-399.

Sincerely,

Claude

Hi,

Just to be sure; You didn't by any chance purge the solution before analysis, as is usually done in the direct method?

If you are quantifying TOC or DOC as non-purgeable organic carbon (NPOC), then for anything that is volatile or semivolatile and hydrophobic, you run the risk of underestimating the concentration because of the loss during purge. It is also easy to lose sparingly soluble hydrophobic analytes during handling and storage from adsorption or partitioning into the headspace of the storage vessel. Rinsing storage vessels prior to filling and avoiding headspace can minimize these losses.

Many thanks Claude, Marcel and John.  My method does not require me to purge the solution and I have made sure there's no head-space to forestall losses due to volatilization, however, there was only but a slight improvement. I appreciate your helpful contributions.

Dear Emmanuel, 

Did you verify that there was no loss of benzene by volatility (the vial containing the test solution was properly sealed?)?

Antonio Eduardo

I think it is most likely that your machine is using a NPOC procedure as suggested by Dr Helms. The machine likely adds acid and purge the sample in order to remove carbonate from the sample before the analysis of the organic carbon. This purge procedure will partially remove semi-volatile organic chemicals. 

Hello Antonio, thanks for your response. Yes the sample bottle was properly sealed.

Hello Henrik, thanks for your response. My machine doesnt use the NPOC principle which requires the acidification step. It makes use of the TC-TIC principle. Both Total Carbon (TC) and Total Inorganic Carbon (TIC) are measured by high temperature oxidation.

It depends on your instrumental set-up, but it is also sometimes possible to underestimate TOC in TC-TIC=TOC mode because you are overestimating TIC (to include volatiles). One thing to try is to replace your acid reservoir for TIC determination with low carbon water. If you generate TIC under neutral conditions, this is likely to be volatiles such as benzene.

Benzene is hard to oxidize. You're using an oxidative catalyst; it doesn't work well with "refractive" aromatics. You would get the same effect with some of the heavier PAH compounds.