31
IonTrapOperation
Themajor difference between a quadrupoleMS and an ion trapMS are themechanisms of
ion focus and scanning. Three hyperbolic electrodes, a ring and two endcaps, form the core
of an ion trapMS (Figure 30). In electron impact mode the sample is ionized, fragmented,
and introduced into the ion trap through a pulsing electronic gate that opens and closes, con-
trolling the number of ions that enter the trap. Ions that enter the trap are stored in stable
orbits.Adjusting the voltage around the ring electrode pushes some of these ions into unsta-
ble orbits, causing them to exit to the detector. Because all ions entering the trap are stored
temporarily, only a finite amount of sample can be allowed to enter the trap area, otherwise
the systemwould be overloaded.
Interfacing theCapillaryColumn to theMS
The ion source and analyzer of theMS are under vacuum. To enable the pumping system to
maintain this vacuum, the volume of carrier gas entering theMSmust be small. Regardless
of the pumping capacity of theMS vacuum system, the best sensitivity is achieved if the
carrier gas flow rate is approximately 1mL/min. Because a narrow-bore capillary column
routinely is operated at near 1mL/min. flow rates, it can be connected directly to theMS
without overwhelming the pumping system.Wide-bore capillary columns, however, usually
are operated at flow rates that are too high formostMS systems. Consequently, an interface
must be used to reduce the flow to a level that is compatiblewith theMS pumping system.
Figure 31 shows the twomost common interfaces – the open split and the jet separator.
An open split interface (OSI) functions like an inlet splitter system in a chromatograph. It
allows asmuch as 90% of the carrier gas to be vented away from theMS vacuum system.
Correspondingly, this is reflected by a sample loss of up to 90%, which reduces sensitivity
by an order ofmagnitude. Therefore, anOSI is not suitable for trace-level environmental
analysis. Splitting the sample at the injection port, combinedwith analysis on a narrow-bore
column, is favored over using anOSI because a high desorb flow rate can be used to ensure
better sample transfer from the trap.Also, a 0.25mm ID or narrower column increases effi-
ciency and improves resolution of analytes.
Another alternative to anOSI, the jet separator, reduces the carrier gas flowwithout signifi-
cant loss of sensitivity.A jet separator works on the principle ofmomentum.Very small
molecules such as helium (or other carrier gas) do not have sufficient momentum to pass
across a small gap in the jet separator and are routed away from theMS, using a vacuum
pump. Largermolecules, such asmost target components, have the necessarymomentum to
carry them across the gap and into theMS. Using this device, much of the carrier gas can be
eliminatedwithout significant loss of target compounds.Addedmomentum is required to
carry very small analytemolecules, such as gases, across the gap, however. In these situa-
tionswe recommend addingmake-up gas to provide the extramomentum and improve
responses for lowmolecular weight target compounds.
Figure 29.
Electromagnetic fields force the ions into a spiral, three-dimensional
sinewave through the center of the quadrupole arrangement.
AMU
Offset
GammaRays
Dynode
Surface
Electron
Nullifier
Signal
+
+
-
-
Figure 30.
Ring and end caps form the
core of an ion trapMS.
