Operation
4
SCD and NCD User Manual
53
Adjusting the Operating Conditions
on page 47 lists the range of values for each parameter,
as limited by the GC firmware. To provide flexibility during
method development for particular application, the range is
wider than needed for most applications.
However, hydrogen flows in the SCD need particular attention.
Using very high hydrogen flows (both upper and lower), relative
to the oxidizer flow, can permanently damage the ceramic
tubes. This condition may not be recoverable. See
SCD lower hydrogen flow
: Very high flow can damage the
ceramic tubes.
NCD lower hydrogen flow
: The NCD can operate without
hydrogen flow, although this is not recommended. The hydrogen
flame/plasma can help burn off solvent and heavy molecules. If
operating an NCD without hydrogen flow, plumb the 1/16-inch
tubing for the lower hydrogen flow to the oxygen supply.
Otherwise, residual hydrogen in the tubing will continue diffuse
into the burner and affect stability.
1
Disconnect the
Lower H2
line from the back of the detector
and cap off the detector fitting.
2
Install a 1/16-inch Swagelok Tee fitting in the
Oxidizer
output
from the detector body.
3
Connect the
Oxidizer
and
Lower H2
lines to the Tee fitting.
You will typically need to adjust the recommended starting
conditions to create an optimized method for your application.
When optimizing SCD or NCD method parameters, consider the
following:
•
A higher hydrogen to oxidant ratio may initially show higher
response, but later yield a reduced response because of the
accumulation of contaminants that reduce detector
response, such as soot or other active species.
•
Operating the burner at higher temperatures may shorten
the useful lifetimes of the heater, thermocouple, and seal
materials.
In general, when making any parameter change, allow sufficient
time for the system to reach equilibrium. Monitor the baseline
until it stabilizes at its new value.