86
SECTION 10, continued
10.2 Application Specific Calibrations
Nephelometric analysis can yield a nonlinear calibration curve. The 2100AN can
store two ASC curves with up to 8 data pairs in each.
The instrument uses point-to-point interpolation between entered standards for
these calibrations. A series of small, straight lines approximating the nonlinear
curve is generated. Before using ASC measurement modes, the following
important points must be understood and/or implemented:
• At a minimum, ASC curves must contain linear portions because the 2100AN
calculates only point-to-point slopes. The instrument measurements do not
utilize linear regression (least-squares-best-fit) analysis.
• Proper calibration of the instrument for turbidity measurement with
Formazin primary standards is required; the ASC is based on turbidity units in
NTU as the independent variable and the user-defined standards as the
dependent variable.
• The sample must be studied in advance to determine the appropriate Cell
Adapter (when needed) and Filter Assembly (see Section 8.1 on page 77,
Section 9.1 on page 79, and Section 9.1.2 on page 80).
• Prepare a series of standards for the desired ASC unit. Select and install an
appropriate Cell Adapter (when needed) and Filter Assembly. Measure the
turbidity of the standards in NTUs. Record the results, and plot the prepared
standards vs. their corresponding turbidity in NTUs. (See Table 4 as an
example).
• Always measure the NTU value of the known standard in the same size vial to
be used for the ASC unit measurement.
Either ASC can be edited at any time eliminating the need to repeat the
calibration. The sample is under-range if the display flashes 0s; when measuring
Color, Absorbance or Transmittance, re-establish the analytical reference point
and measure again. Additionally, make sure the expected reading is positive when
measuring Absorbance. To measure samples with negative Absorbance, establish
the analytical zero using the sample with the greatest absorbance, and read the
sample with the least absorbance. Report the reading as negative absorbance.
Table 4 Standards
Point Number NTU Concentration (mg/L)
10.0000
2 0.318 10
3 0.542 20
4 0.663 30
5 0.709 40
To Next Page
Loading...
