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Chapter 5 Operation modes & Parameters 63
To illustrate how an autozero affects the different detector signals (Ic, ana-
log output Vout and digital cell current), an example is given in figure 19 &
20 using the 5 nA measurement range. In the top screen of figure 19 a cell
current Ic of 2.500 nA is displayed and no compensation is applied. Ic is
the so-called uncompensated cell current and represent the actual current
of the cell. The analog output signal corresponding to a current of 2.5 nA is
Vout = 2.5nA / 5 nA * 1 V (full scale) = 500 mV. The first 4 minutes of the
example run in figure 20 correspond to this condition. At t=4 min an auto-
zero is issued. As can be seen from the bottom screen in Figure 19, the
analog output Voltage, Vout, became 0 and the Compensation state is
now changed to Comp=ON. The Ic remained unchanged (2.500 nA) be-
cause it always shows the actual current of the cell, regardless the com-
pensation state.
It is evident from the plots in figure 20 that using the compensation has an
advantage. Without compensation, one only has half of the range available
for the measurement of an analyte response. So only signals with a peak
height of a maximum of 2.5 nA can be measured in this particular case
(see peak 1 in figure 20). With the compensation ON the full measurement
range of 5 nA is available for the measurement (peak 2).
In case an Autozero, Offset or a change of range is initiated on an instable
or changing current signal (baseline), this may lead to the display of an er-
ratic current value or erratic offset in current. This is expected behavior. An
Autozero, offset or change of measurement range will trigger an internal
calculation procedure in the detector based on the actual cell current. This
is an iterative process and the cell current is measured several times dur-
ing the calculation process. In case the cell current is not stable during the
calculation process this might lead to errors.
Remedy: In the event it does happen, the issue can be simply solved by
setting the Compensation to OFF (Comp = OFF) in the STAT screen or
CDS control software. Subsequently, wait till the background current is sta-
ble/stabilized and execute an Autozero again. The detector should now
display the correct current again.
In case event tables are programmed in LC methods in the CDS software
with an Autozero action, make sure that the autozero is programmed in a
part of the chromatogram were the signal is stable. So at the beginning of
the run, during the hold-up time (so before the unretained/solvent peak
elutes). Or at the end of the run.
Do not execute an autozero, offset or range change when the cell
current is not stabile or the baseline did not stabilize.