User guide

188 Advanced User Guide
8 Inlets
inlet temperature program exceeds the Use cryo temperature. If
the Use cryo temperature is less than the inlet setpoint, cryogen
will cool the inlet to the initial setpoint and turn off.
If using compressed air as the coolant when configuring the
initial inlet setpoint, Use cryo temperature behaves differently
when in Compressed air mode than it does when in N2 cryo or CO2
cryo mode. In Compressed air mode, the air coolant is used to cool
the inlet regardless of the Use cryo temperature setpoint during
the cooling cycle. If the inlet reaches setpoint, the air coolant is
turned off and stays off. If the oven temperature is high enough
or the inlet was very hot previously, it is possible that the inlet
temperature will rise and the GC will go not ready. For this
reason, it is better to set the instrument cooling configuration
as N2 cryo or CO2 cryo when using compressed air as the coolant.
When using compressed air, the LN
2
hardware cools the inlet
faster than the LCO
2
hardware. Never use LCO
2
or LN
2
when
the instrument is configured in Compressed air mode.
If cryo is turned on, and if the inlet is cooled during a run,
cryogen is used to achieve the setpoint. This may have a
negative impact on the chromatographic performance of the
oven and cause distorted peaks.
When a method ends, the cooling setpoint returns to the initial
state of the method, unless you load another method. To
conserve coolant when the GC is idle, load a method that does
not use a cooling configuration.
See “To configure the MMI coolant” on page 31.
MMI split mode minimum operating pressures
The minimum recommended inlet total flow is 20 mL/minute.
When the inlet is operated in a Split mode, there will be a
minimum pressure at which the inlet can operate. Typically, low
inlet pressures may be required for shorter, wide bore columns.
The minimum pressure is a function of carrier gas type, total
inlet flow, liner design, and possible contamination of the split
vent tube or trap.
A wide bore column requires a much lower inlet pressure than a
typical capillary column to maintain a given flow. Setting the
split ratio (total flow) too high when using a wide bore column
can create an unstable control relationship between the
pressure and flow control loops.