四级杆-离子肼质谱介绍
The Whys and Wherefores of Quadrupole Ion Trap Mass Spectrometry
A sensitive and versatile analytical system, capable of identifying both large and small molecules and determining their molecular
structure, is required to address the complex mixtures of molecules found in many types of biological problems. Of fundamental
importance to the biochemist and biologist is the existence of robust, easy-to-use, and inexpensive instrumentation for application
to their studies.
Developments over the last 10 years have made the quadrupole ion trap mass spectrometer (ITMS) an excellent tool for
biomolecular analysis. A quadrupole ion trap is an instrument roughly the size of a tennis ball whose size is inversely proportional to
its versatility. Three hyperbolic electrodes, consisting of a ring and two endcaps, form the core of this instrument. In the early 1950s,
Wolfgang Paul and co-workers invented two instruments that could be used to determine mass -to-charge (m/z) ratios of ions (1).
The first was the quadrupole mass filter that rapidly was applied to a wide range of analytical problems (2). The second was the
quadrupole ion trap. A diagram of the two mass spectrometers is presented in Figure 1. The difficulty of machining the hyperbolic
electrodes, coupled with limited performance, restricted interest in this instrument primarily to the physics community. A user of
note was Hans Dehmelt at the University of Washington who recently won the Nobel Prize for employing the ion trap to investigate
the physical properties of isolated ions (3). The ion trap was operated at that time in the so-called "mass-selective stability" mode of
operation. In this mode, analogous to the operation of a quadrupole mass filter, the amplitudes of rf and dc voltages applied to the
ring electrode were ramped at a constant rf/dc ratio to allow stability, hence storage, of a single (increasing) m/z value in the ion trap.
The chemistry community's interest in the trap was confined to several research groups until 1983 when George Stafford and coworkers
at Finnigan MAT made two major advances. First, they developed the mass-selective instability mode of operation (4). The
fundamental difference between this mode of operation and previous methods is that all ions created over a given time period were
trapped and then sequentially ejected from the ion trap into a conventional electron multiplier detector. Thus, all ions were stored
while mass analysis was performed, unlike the mass-selective stability mode of operation where only one value of m/z at a time was
stored. This new method for operating the ion trap simplified the use of the instrument. Stafford's group's second breakthrough was
finding that a helium gas of about 1 mtorr within the trapping volume greatly improved the mass resolution of the instrument by
contracting the ion trajectories to the center of the trap and reducing the kinetic energy of the ions (5). This allows ions of a given m/
z to form a packet. The ion packet is ejected more quickly and efficiently than a diffuse cloud of ions may be ejected, thus improving
resolution. Both these discoveries led to the successful development of a commercial ion trap mass spectrometer.
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