52 DECADE user manual, edition 9
The pulse mode is quite different from the DC mode:
•
The output signal is sampled during a fraction of the total pulse cycle. This
requires electronic manipulation of the signal. Therefore, only the recorder
output should be used for detection.
•
During the sampling time (ts) the signal generated at the WE is collected
and this value is sent to the detector output. Further smoothening of the
signal by means of rise time filters is not possible.
•
When the frequency of the data acquisition system (integrator) is higher
than the pulse frequency a typical stepwise pattern may appear in the
chromatogram. This is normal and only visible after considerable
magnification of the chromatogram.
•
The parameters of the pulse mode are set in the ‘PULS SET’ screen of the
DECADE. The potential sequence starts when the ‘PULS STAT’ screen is
selected. In all other screens in the pulse mode, the E1 potential is
applied.
From DECADE software version 3.00 and higher: the potential sequence
is
always
applied if the cell is on.
•
The background or cell current is usually considerably higher (100 - 1000
nA) than the maximum compensation in the nA range (160 nA). Therefore,
only the µA range is available in the pulse mode. If the RS232 option is
installed, the pA and nA range can be used via the ‘DECADE Dialogue’
software.
•
The yellow crocodile clamp from the cell cable is connected to the injection
valve to ensure proper grounding of the LC system. However, when an
external injection valve is used (auto injector) in combination with a PEEK
column (CarboPac) and with PEEK tubing, a grounding problem may
arise. In that case a metal low dead volume connector should be placed
near the flow cell which is connected to the crocodile clamp. However, it is
not
advisable to use metal tubing as inlet tubing for the flow cell!
•
After prolonged use of the flow cell with a gold working electrode (WE) in
the pulse mode, the gold oxide which is generated at the WE, precipitates
on the auxiliary electrode (AUX). This gold oxide coating may electrically
isolate the AUX and result in an increase of the noise. Simply polishing the
working electrode on a regular basis (e.g. every month) prevents this.
•
If a salt bridge Ag/AgCl REF is used, the pH in the salt bridge will increase.
Eventually the colour of the (white) cotton wool frit turns brown.
Maintenance of the REF on a regular basis (e.g. every month) ensures a
good performance.
•
If a mobile phase is used with a high pH (pH>10, carbohydrate analysis),
the standard Vespel rotors from the injection valve should be replaced by
Tefzel rotors which are pH resistant.
•
For carbohydrate analysis, only CO
2
-free sodium hydroxide should be
used since carbonate anions may disturb the ion exchange
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