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2.1.2 Spectroquant

Reagent Tests

The principle behind the reagent tests is that the
reagents necessary for the colour reaction are com-
bined in the form of liquid concentrates or solid-
substance mixtures. In these tests, a few drops of the
respective reagent concentrate are added to, for

example, 5 ml of sample. This means that there is no
need to dilute the sample, which in turn enhances the
sensitivity of the detection. The making up of a
sample in a volumetric flask to a defined volume usual
in conventional photometry can be dispensed with.

2. Photometric Test Kits

The intensity of the colour of a solution, measured as
the absorbance, is proportional to the concentration
of the respective analyte only within a specific range.
This measuring range (effective range) is electro-
nically stored in the photometers for each individual
test kit (see overview table in section 1.3).

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 nonlinearity of the calibration curve,
as shown in the figure below, or else is given by the
limit of detection. The

limit of detection

of a given

analytical method is the lowest concentration that
differs from the zero concentration with a defined
probability (e.g. 99%).

The

upper limit of the measuring range

is charac-

terized by there no longer being any linear correlation
between analyte concentration and absorbance. In
such a case the sample must be diluted accordingly
so that it lies ideally in the middle of the effective
range (least-error measurement).

In photometry it is conventional practice to measure
against the reagent blank value. Here the analysis is
carried out “blind”, i.e. without any analyte added.
Instead of the sample volume, the corresponding
quantity of distilled or DI water is used. This

reagent

blank value is already electronically stored

in the

system photometers, meaning that a separate mea-
surement is not necessary. It is possible, however, to
enhance the accuracy of the determination at the
measuring-range limit by measuring against a self-
prepared reagent blank solution (for adjustment see
Function description, “Blank-value correction”).

There are, however, also cases in which the colour
intensity of the solution and thus the absorbance
drop off again at

very high analyte concentrations

These exemplary cases are listed in the table below.
The values indicated in the display are correct up to
the concentrations specified in the third column, and
false measuring values are obtained above these
concentrations. In such a case it is necessary to con-
duct a plausibility check by running preliminary tests
using test strips or dilution.

2.2 Notes for Practical Use

2.2.1 Measuring Range

Concentration

Abs

orbance

Art.

Method

Correct indica-

Colour change

tion of result up
to sample conc.

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