CP-3800 GC Operator’s Manual
139
Cell-Current
In order to obtain the optimum linear and dynamic ranges of response, you
must select the proper ECD cell current. Your choice of cell current depends
on the specific carrier gas that is used and the baseline signal (frequency, f
0
)
with only carrier gas present. One of four values may be selected for the cell
current: Ar/CH
4
(1710 pA), CAP (480 pA), N
2
STD (290 pA), and N
2
HIGH
(146 pA). In addition, a Zero cell current setting is provided for convenience
in setting the cell contact potential (see below).
For normal operation with nitrogen carrier gas, use the N
2
STD setting. When
a greater linear range is desired, select the Ar/CH
4
setting and use 90%
Argon/10% Methane as the carrier gas.
Set the switch to CAP when using capillary columns. For capillary columns,
the column bleed is low and the baseline is usually below 1 mV when current
is set to N
2
STD on Range 1.
You may encounter applications in which high column bleed is unavoidable
when using nitrogen as the carrier gas. In such instances, the baseline signal
may be too high. Select the N
2
HIGH setting in these situations.
In general, the baseline signal should never exceed 25 mV when measured
on Range 1. A value above 25 mV indicates that you have selected the
wrong reference current for the chromatographic conditions (column, phase,
sample matrix). A baseline signal above 25 mV decreases the useful linear
range of the ECD detector.
Time
Constant
Switch
The time constant has two settings: SLOW (200 msec) and FAST (50 msec).
For most applications, select the SLOW setting. For capillary applications
with peak widths at half height of less than 2 seconds, select the FAST
setting.
Setting the
Contact
Potential
An extremely clean ECD system can yield a contact potential that must be
nulled for optimum performance. The contact potential may vary with time
and detector temperature for several days after the cell is first put into
operation, therefore you may need to repeat this adjustment procedure daily
until the contact potential stabilizes.
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