Knauer Azura ECD 2.1, Инструкция по эксплуатации

Страница: 91 / 101

87 Troubleshooting
AZURA® Detector ECD 2.1 Instructions, V6685
13.2 Analytical troubleshooting
Analytical problems like loss of signal, increase in noise level, high cell
current, loss in sensitivity etc. may occur in any (U)HPLC-ECD 2.1 system.
It may be hard to find the cause and several checks need to be perfor-
med to eliminate the source of the problem. The first step is to determine
whether the problem is caused by the ECD 2.1 electrochemical detector
or the rest of the (U)HPLC system. For that purpose two basic checks
should be performed by the end-user:
ƒ
Dummy cell test
ƒ
Stop flow test
Dummy cell test: The outcome of the dummy cell test, which is described
in the next paragraph will give an indication if the problems are caused
by the detector hardware (electronics).
Stop flow test: The stop flow test will eliminate if the problems are caused
by the electrochemical flow cell, or is originating from the rest of the
(U)HPLC system (pump, autosampler, pulse damper, column, mobile
phase etc.).
13.3 Dummy cell test
External dummy cell
An external dummy flow cell is shipped with every ECD 2.1 instrument for
troubleshooting purposes and maintenance checks. The dummy cell test
can be performed standalone via the LCD display in combination with
an A/D converter or via the Dialogue Elite / ClarityChrom® software. A
successful dummy cell test confirms that the controller, including the cell
cable, functions properly. If the result of the noise measurement with the
dummy cell is within specs, the controller is excluded in a troubleshoo-
ting procedure.
Fig. 72 Left: schematics of external dummy flow cell.
Right: photo of the external dummy flow cell.
The dummy flow cell consists of a resistor (R) of 300 MOhm and a capa-
citor (C) of 0.47 µF in parallel. The current is measured over the resistor
according to Ohm’s law (V = I x R), hence with a working potential of 800
mV the current drawn will be about 2.67 nA. Slight differences as to this
(ideal) value are due to the tolerance of the resistor (± 1%). The capacitor
functions as a ‘noise generator’ and in fact resembles the capacitance of a