Optimizing the Analysis of Volatile Organic Compounds

Figure 29.
Electromagnetic fields force the ions into a spiral, three-dimensional
sine wave through the center of the quadrupole arrangement.
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+
+
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Gamma Rays
Signal
AMU
Offset
Electron
Nullifier
Dynode
Surface
Ion Trap Operation
The major difference between a quadrupole MS and an ion trap MS are the mechanisms of
ion focus and scanning. Three hyperbolic electrodes, a ring and two endcaps, form the core
of an ion trap MS (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, controlling
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 unstable
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 system would be overloaded.
Interfacing the Capillary Column to the MS
The ion source and analyzer of the MS are under vacuum. To enable the pumping system to
maintain this vacuum, the volume of carrier gas entering the MS must be small. Regardless
of the pumping capacity of the MS 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 the MS
without overwhelming the pumping system. Wide-bore capillary columns, however, usually
are operated at flow rates that are too high for most MS systems. Consequently, an interface
must be used to reduce the flow to a level that is compatible with the MS pumping system.
Figure 31 shows the two most 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 as much as 90% of the carrier gas to be vented away from the MS vacuum system.
Correspondingly, this is reflected by a sample loss of up to 90%, which reduces sensitivity
by an order of magnitude. Therefore, an OSI is not suitable for trace-level environmental
analysis. Splitting the sample at the injection port, combined with analysis on a narrow-bore
column, is favored over using an OSI 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 efficiency
and improves resolution of analytes.
Another alternative to an OSI, the jet separator, reduces the carrier gas flow without significant
loss of sensitivity. A jet separator works on the principle of momentum. 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 the MS, using a vacuum
pump. Larger molecules, such as most target components, have the necessary momentum to
carry them across the gap and into the MS. Using this device, much of the carrier gas can be
eliminated without significant loss of target compounds. Added momentum is required to
carry very small analyte molecules, such as gases, across the gap, however. In these situations
we recommend adding make-up gas to provide the extra momentum and improve
responses for low molecular weight target compounds.
Figure 30.
Ring and end caps form the
core of an ion trap MS.
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