Determination of DOC in Chromatographic Fractions.
For DOC determinations in chromatographic fractions, the HPLC/
ICP-IDMS system, schematically shown in Figure 1, with a size
exclusion column is used. All analyzed samples are poor in
carbonate. However, for carbonate-rich samples, a suppressor
column must be connected in series with the separation column
to avoid a breakthrough of carbonate. After optimization of the
ICP-MS sensitivity and after calibration of the spike mass flow,
the sample is injected, and the time-resolved signals of the UV
absorption and of the 13C/12C isotope ratio are followed. A
representative isotope ratio chromatogram, obtained with a
solution of fulvic acids isolated from a brown water sample, is
shown in Figure 4. In the beginning, when no DOC-containing
fraction is eluted from the column, the isotope ratio of the spike
solution is measured. The isotope ratio shifts in the direction of
the natural isotopic composition of carbon when DOC-containing
fractions leave the separation column because the spike flow is
totally mixed with these fractions before entering the ICP-MS.