The intensity of the color of a solution, measured as the absorbance, is
proportional to the concentration of the respective analyte only within a
specific range. This mea sur ing range (effective range) is electro nically
stored in the photometers for each in di vidual test kit .
Below the specified measuring range, either a different cell or else another
procedure must be used. The lower limit of the measuring range either
takes the form of non linearity of the calibration curve, as shown in the
figure, or else is given by the method detection limit. The method detec-
tion limit of an analytical method is the lowest concentration of the ana-
lyte in question that can be measured quantitatively with a defined degree
of probability (e. g. 99 %).
The upper limit of the measuring range is the point at which the linear
correlation between the concentration and the absorbance ends. In such a
case the sample must be diluted accordingly so that it lies ideally in the
middle of the ef fective range (least-error measurement).
In photometry it is conventional practice to measure against the reagent
blank va lue. Here the analysis is carried out “blind”, i.e. without any ana-
lyte added. In stead of the sample volume, the corresponding quantity of
distilled or DI water is used. This reagent blank value is prestored in the
photometers belonging to the Spectroquant
®
analysis system, which
means that - due to the high batch reproducibility - it is possible to dis-
pense with a separate measurement of the reagent blank. At the lower
limit of the measuring range, the accuracy of the determination can be
enhanced by performing the measurement against a separately prepared
reagent blank.
In some cases the intensity of the color of the solution and thus the ab-
sorbance can drop again when very high concentrations of the analyte
are present (see package insert).
2 Photometric Test Kits
2.2 Notes for Practicle Use
2.2.1 Measuring range
Concentration
Absorbance
8
Spectroquant
®
photometers
Release 06/2014
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