GE Sievers 900 Series Operation And Maintenance Manual page 95

decreases until there is insufficient persulfate to completely oxidize the sample. Somewhere in-
between, the persulfate concentration in the UV reactor is the optimum for the concentration of
TOC in the sample at that moment.
When the persulfate concentration is the optimum, the maximum amount of CO , formed in the UV
2
reactor, permeates through the membrane in the TC channel. If the TOC concentration is 10 ppm or
greater, the conductivity at the TC conductivity cell peaks at a time that corresponds to when the
persulfate concentration was optimum in the UV reactor. The instrument determines when the
maximum conductivity occurs and then calculates the optimum oxidizer flow rate for that sample.
However, samples having lower TOC concentrations produce conductivity plateaus, rather than
peaks. For those samples, the instrument calculates the optimum oxidizer flow rate from the
conductivities and temperatures that exist during the plateau.
The Autoreagent function compensates for the aging of the persulfate reagent and the UV lamp. It
does not compensate for changes in the sample flow rate, such as after the sample pump tubing is
replaced. The sample flow rate increases when new tubing is installed, and the flow rate stabilizes
after a few days of operation. For that reason, the tubing used in the Sievers 900 Series Analyzers
has been "broken in" at the factory so that the flow rate is relatively stable immediately after it is
installed.
Still, the sample flow rate will vary from one set of tubing to another. To compensate for this
variability, Sievers 900 Series Analyzers can calibrate the sample flow rate so that it is optimized
and the same whenever pump tubing is replaced.
The preliminary measurement is necessarily performed differently than a normal analysis because
its purpose is to set the reagent flow rates. Therefore, its results are not reported by the Analyzer. It
also should be noted that the time before the Analyzer reports its first results for the sample is
extended by 14 to 16 minutes. The Autoreagent function is employed most productively when the
concentration of the samples to be analyzed is expected to vary greatly, thus eliminating the need
to run duplicate samples in order to determine optimum flow rates.
When the Analyzer is in Grab mode or operating with the Autosampler, and Autoreagent mode is
activated, the Analyzer initially performs a preliminary measurement on each new sample vial.
Following the preliminary measurement, the Analyzer sets the acid and oxidizer flow rates to be
used on that sample, and the analytical measurements are then made in the normal fashion.
When the Autoreagent function is used in the On-line mode, the Analyzer performs a preliminary
measurement when analysis is started. Then the Analyzer performs analytical measurements of
the sample stream in the normal fashion. The Autoreagent function is not available in Turbo mode.
GE Analytical Instruments ©2009 95 of 236 DLM 90488-01 Rev. A