User guide

Inlets 8
Advanced User Guide 211
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.
4 Set up the inlet to perform a 1 uL injection. Use the
split/splitless inlet method conditions, except: