For GC analysis not pure extracts or substances are used, but solutions at low
concentration levels. Further on, the analytes need to be transferred to the gas phase
prior to separation. Hence, an important part of a gas chromatography is the system
transferring the analytes form a dissolved state in a solution to gaseous state and,
subsequently, towards the capillary column. This function is performed by the
injector. Nowadays the split/splitless injector type is mainly used. A principal
scheme is given in Fig. 4.7. Core of this system is a small glass tube that is heated
to 250–300
C. The carrier gas is flowing through the tube from top to bottom. One
end of the capillary column is placed at the bottom. The carrier gas overpressure
determines its flow rate through the capillary column. Above the top of the glass tube
a septum is installed that allows a penetration by a syringe. For the injection a small
volume (normally 0.2–1 μL) in a syringe, which tip is placed through the septum in
the middle of the tube, is sprayed within the hot tube space. The solvent and the
analytes get rapidly vaporized and are transported by the gas flow onto the column
thereafter. A special feature of this type of injector is the possibility to split the gas
flow. For this purpose, a valve allows a major part of the carrier gas to leave the
injector without passing the column. This induces a split of the gas flow between a
proportion passing the column and a much higher fraction that gets lost. Typically,
the split ratio of column passing fraction and the bypassed fraction is around 1:50.
However, the effect is on the one hand to avoid an overloading of the column
especially by the solvent but more important a discrimination takes place. The very
volatile solvents are preferentially following the bypass, but the less volatile analytes
are more concentrated on the capillary column. Hence, a slight separation of analytes
and solvents is achieved by this technique. If concentrations of analytes are very low,
the split mode hinders a transfer of its total amount reducing the sensitivity of the gas
chromatographic measurement. Here, an alternative spitless mode can be used by
closing the split valve during injection forcing the total injected sample volume to
enter the capillary column. The difference between split and splitless mode injection
is illustrated in Fig. 4.7.
Carrier gas inlet
Split outlet
Column
VaporisaƟon
chamber
Glass liner
(tube)
Heated
metal block
Split valve
Syringe
Syringe
Splitless
Split
Fig. 4.7 Scheme of a split/splitless injector and the two principal injection modes
4.1 High Performance Chromatography: GC, HPLC
45
concentration levels. Further on, the analytes need to be transferred to the gas phase
prior to separation. Hence, an important part of a gas chromatography is the system
transferring the analytes form a dissolved state in a solution to gaseous state and,
subsequently, towards the capillary column. This function is performed by the
injector. Nowadays the split/splitless injector type is mainly used. A principal
scheme is given in Fig. 4.7. Core of this system is a small glass tube that is heated
to 250–300
C. The carrier gas is flowing through the tube from top to bottom. One
end of the capillary column is placed at the bottom. The carrier gas overpressure
determines its flow rate through the capillary column. Above the top of the glass tube
a septum is installed that allows a penetration by a syringe. For the injection a small
volume (normally 0.2–1 μL) in a syringe, which tip is placed through the septum in
the middle of the tube, is sprayed within the hot tube space. The solvent and the
analytes get rapidly vaporized and are transported by the gas flow onto the column
thereafter. A special feature of this type of injector is the possibility to split the gas
flow. For this purpose, a valve allows a major part of the carrier gas to leave the
injector without passing the column. This induces a split of the gas flow between a
proportion passing the column and a much higher fraction that gets lost. Typically,
the split ratio of column passing fraction and the bypassed fraction is around 1:50.
However, the effect is on the one hand to avoid an overloading of the column
especially by the solvent but more important a discrimination takes place. The very
volatile solvents are preferentially following the bypass, but the less volatile analytes
are more concentrated on the capillary column. Hence, a slight separation of analytes
and solvents is achieved by this technique. If concentrations of analytes are very low,
the split mode hinders a transfer of its total amount reducing the sensitivity of the gas
chromatographic measurement. Here, an alternative spitless mode can be used by
closing the split valve during injection forcing the total injected sample volume to
enter the capillary column. The difference between split and splitless mode injection
is illustrated in Fig. 4.7.
Carrier gas inlet
Split outlet
Column
VaporisaƟon
chamber
Glass liner
(tube)
Heated
metal block
Split valve
Syringe
Syringe
Splitless
Split
Fig. 4.7 Scheme of a split/splitless injector and the two principal injection modes
4.1 High Performance Chromatography: GC, HPLC
45