A comparison of methods for the measurement of dissolved organic carbon in natural waters

A small suite of natural samples spanning a wide range of DOC concentrations and salinities were analyzed by three high-temperature combustion (HTC) techniques and persulfate oxidation. One of the HTC techniques, sealed-tube combustion (STC), served as a referee method. Using this method, it was possible to obtain absolute DOC concentrations for the natural samples free of any analytical blank offset. Prior to the comparison of samples, an exchange of carbon-free water and calibration standards showed that all methods were equally well calibrated. Linear correlation analysis was used to differentiate whether the differences observed between methods was due to variable oxidation yields or to an artifact of the instrument blank. Agreement among the various methods was quite good, but yields for all methods decreased compared to the STC technique at concentrations of > 400 μM C. Persulfate results were found to be very similar to HTC results. Finally, carbon-free distilled water prepared by UV/H2O2 oxidation or Milli-Q systems had near-zero DOC concentrations and was adequate for blank correction of all the various techniques yielding DOC concentrations in excellent agreement with the referee method.