FT ICR mass spectrometry continues
to be the leader in the resolving
power among all mass spectrometry methods. With the introduction of
the dynamically harmonized FT ICR cell, it has become possible to
achieve the resolving power of more than 10 million at m/q = 1000 with moderate magnetic fields of about
7 T. A further increase in the mass resolving power is desirable mainly
for two reasons: with this we are increasing the number of resolved
components in analyses of complex mixtures (like oil and natural organic
matter (NOM)) and increasing the number of the mass of molecules for
which a fine structure can be resolved. In recent years, some attempts
have been made to further increase the mass resolving power by increasing
the magnetic field and using the multielectrode detection method.
An increase in the magnetic field to 21 T did not show a proportional
increase in the mass resolving power. Likely, the reason for this
is an insufficiently high vacuum to satisfy the requirement of an
increase in the mean free path of ions with the increasing magnetic
field power. We offer a new design for the FT ICR cell and the whole
mass spectrometer, in which an open, dynamically harmonized FT ICR
cell is integrated into a vacuum system with the outer surfaces of
the cell electrodes at atmospheric pressure. In this design, the trap
electrodes are the walls of the vacuum system and have the minimized
active surfaces by combining the vacuum system surface and the cell
surface into one. In this design, the pumping process is accelerated,
and the factor of insufficient vacuum in FT ICR mass spectrometers
with an ultrahigh magnetic field is eliminated.
In Fourier-transform ion cyclotron resonance mass spectrometry, ions are detected by measuring image current induced in the detecting electrodes by trapped ions rotating in a magnetic field at their cyclotron frequencies. The ion trap used for this purpose is called the Penning trap. It can have various configurations of electrodes that are used to create a trapping electric field, to excite cyclotron motion, and to detect the induced signal. The evolution of this type of mass spectrometry is mainly driven by progress in the technology of superconducting magnets and in the constantly improved design of the ion cyclotron resonance (ICR) measuring cell. In this review, we focus on ICR cell designs. We consider that the driving forces of this evolution are the desire to increase resolution, mass accuracy and dynamic range, as well as to adapt new methods for creating and trapping ions.
FT ICR mass spectrometry is the leader in resolving power
among
all mass spectrometry methods. Introduction of the dynamically harmonized
measuring cella closed-cylindrical cell with specifically
shaped electrodeshelps to reach the resolving power of more
than 107 with a magnetic field of about 7 T. From the theory
of FT ICR mass spectrometry it follows that the resolving power of
this type of instrument depends linearly on the magnetic field under
various conditions. However, the results obtained on this type of
mass spectrometer with the maximum magnetic field achievable today
did not show a proportional increase in resolving power. In one of
our previous papers, we assumed that the reason for this was insufficient
vacuum inside the cell, since vacuum quality should be at least proportional
to the magnetic field, since the mean free run time decreases proportionally
to the magnetic field growth. We have presented an open modification
of the dynamically harmonized cell that can help improve the cell
pumping conditions. However, the electric potential distribution inside
this new cell is slightly different from the ideal (harmonic) one,
obtained inside the closed version of the cell, and the resolving
power may have been limited by this difference.
The Fourier transform ion cyclotron resonance method holds the lead in mass accuracy and resolving power among all other mass spectrometry methods. The dynamically harmonized cell is largely responsible for the supremacy. This cell has an ideal hyperbolic trapping potential after averaging over fast cyclotron motion. Recently we have introduced an open modification of the cell (especially useful with ultrahigh magnetic fields) and have found the analytical solution for the averaged potential inside it. The voltage on specific “regularizing” electrodes determines how close a potential is to the hyperbolic one. In this article, we find the optimal voltage on these “regularizing” electrodes analytically. This will assist with both further analysis and tuning of the trap after manufacturing.
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