80 Advanced Operation Manual
3 Inlets
The main advantage of the MMI's solvent vent mode is that
you can inject slowly into the inlet, allowing large amounts
of solvent to evaporate in the liner (not in the split vent
line), concentrating the analytes prior to injection. This
requires an injector with variable speed injections, a "large"
syringe, and knowledge of the sample and the solvent. When
developing a solvent vent method, the goal for the injection
is to determine the injection rates and temperatures needed
to evaporate the solvent at the rate it enters the inlet. The
development technique is to gradually scale up to an
injection amount that produces a useful response. The most
significant parameters are:
Inlet temperature. Hold the inlet temperature at or slightly
below the solvent boiling point until after all the sample has
been injected. This is important so that you do not boil away
more volatile analytes, or boil away the solvent in the needle
and trap your analytes there. An additional point to consider
is that the boiling point of the earliest eluting analyte should
be 100 °C > the boiling point of the solvent.
Injection speed. Estimate the evaporation rate of solvent
exiting the needle based on solvent type, inlet temperature,
vent flow, and pressure. Start with about half that number.
Note that you have to make sure that you configure the
syringe properly. If not, you will over- or under- load the
inlet.
Vent time. Make sure the vent time setpoint is greater than
the time the needle spends in the inlet. If the vent time is
too short, you will overload and contaminate the column and
inlet. Change the vent time as you upscale the method.
If using a MMI with an MSD, another tip for method
development is to scan for solvent ions. Detecting the solvent
ions can be useful in troubleshooting residual solvent bleed
onto the column.
To develop a MMI method for large volume injection, try the
following:
1 Determine a small injection volume that works on a
split/splitless inlet in splitless mode. Choose a volume
that does not currently overload the inlet.
2 Start with a 5–10 uL syringe and make sure the syringe
is properly configured in the instrument and data system.
3 Make sure the column is configured.
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