Page 16 - 5_TOC in daily practice

SCA-130-508

■ Blank value considerations Both situations only describe the ratio
When, for instance, the NPOC is calibrated in between the actual blank value and the blank
the lowest concentration ranges, a positive value when the calibration curve has been
area value for the zero value of the x-axis created. To obtain an absolute concentration
generally results, as well as a positive value value, there is the possibility of zero offset.
of the y-axis where it intercepts the x-axis. The zero offset is a parallel offset of the
calibration curve through the zero point.
Through this offset, the absolute term (b) of
the curve is set to zero and the blank value
(positive intercept) of the calibration is taken
into consideration in the sample analysis.

This positive area value reflects the blank
value. This blank value is, however, not
attributable to a specific factor or a specific
cause but appears as the sum of the various
blank value factors. This sum can consist of
the following:

Total blank value = Σ of
+ Blank value of the instrument While the concentration in ultrapure water for
+ Reagent impurity the preparation of standard includes a blank
+ Blank value of the standard value, the carbon concentration in an
(ultrapure water) ultrapure water sample only reflects the
+ Contaminations from the environment actual TOC concentration of the sample.
(dust, contaminations on the
glassware, etc.) In short: dilution water has a blank value, a
sample does not have a blank value.
The calibration is evaluated according to the
general straight-line equation:
y = m • x + b

y = Peak area
x = TOC – concentration
m = slope of calibration
b = intercept

When a blank sample is now measured
against this curve, the concentration 0 mg/L
is obtained or, if the ultrapure water used is
slightly cleaner, even a negative
concentration value.