Thermo Scientific FOCUS GC, Instruction Manual

Page: 105 / 356

Chapter 5
S/SL Injection Techniques Split/Splitless Injector (S/SL)
Instruction Manual 105
For narrow bore columns (< 0.20 mm ID) with inherently lower flows (< 1.0 mL/
min), the transfer is even longer.
You can counter this by using the splitless surge pressure mode. In this mode, the
pressure in the injector temporarily increases during the splitless period to
increase the flow into the column. You set the surge pressure, which activates
during the Prep Run stage.
At the end of the splitless period, the split valve reopens and the split flow flushes
the injector of any remaining solvent vapors. In splitless injection, the absolute
split flow is not important. It need only be sufficient to purge the injector.
Normally 40–50 mL/min is adequate.
Refocusing the Sample
The sample vapors enter the column over an extended period of time and produce
a very broad inlet band. To maintain column efficiency, refocusing must take
place in the column inlet before chromatographic process begins.
To achieve this, keep the oven temperature to a sufficiently low value during the
injection of the sample to trap it at the column head by condensation (thermal
focusing) or solvent effect.
• Thermal Focussing (or Phase Trapping)
The components are refocussed by keeping the oven temperature, during
injection, at about 80-90 °C below their elution temperature. In these
conditions the components are not substantially moving into the column.
They are then related as a sharp band when oven temperature is increased.
Thermal focusing fails for components that elute at temperature below ca.
100°C since this will imply use of sub-ambient temperatures. For these
volatile components refocussing can be obtained by explaining solvent effect.
• Solvent Effect
Refocussing by solvent effect presupposes recondensation of solvent in the
column inlet. Volatile components are refocussed during the solvent
evaporation process into the column. The solvent acts as temporary stationary
phase that retains sample components, as the solvent evaporates the
components are released as a sharp band. Recondensation of the solvent
requires to keep the oven temperature 20-25 °C below the “pressure
connected solvent boiling point”.
As a rule of thumb the boiling point increase by about 2-3 °C every 10 kPa up
to 100 kPa and 1.5-2 °C every 10 kPa from 100 to 300 kPa.