passed through a filter to remove scattered light before detection. After irradiation,
the buffer gas is pumped away, and the ions in the trap are ejected and analysed
using a dynode/channeltron assembly (not shown).
Note, the use of a CW laser limits the wavelengths which can be studied.
Furthermore, the energy of the ion is not strictly defined as ions can absorb and
fluoresce multiple times. Absorption by daughter ions also affects the results.
3.2.2 Fluorescence Measurements in an FT-ICR
Another instrument in which fluorescence experiments can be performed is a
Fourier-Transform Ion Cyclotron Resonance (FT-ICR) instrument [32]. At the
FT-ICR instrument in Zurich (Fig. 3.20) [33], ions are produced by electrospray
ionisation, mass selected using hexapole filters, and stored in an ICR cell which is
enclosed in a 4.7-T superconducting magnet (Penning trap). Fluorescent light
emitted by the ions after irradiation by a CW laser is detected and the photoproducts
determined: The ICR cell is comprised of two excitation elements, two detection
elements, and two trapping plates for axial trapping. The uniform magnetic field, B,
provided by the superconducting magnet induces ions to move along circular paths
in a perpendicular plane (radial trapping). The frequency of this motion is known as
the cyclotron frequency, ω c , and depends on the mass-to-charge ratio of the ion:
ω c ¼ (q/m) B (q ¼ z e). Ions of a specific m/q are excited to a higher orbit by
applying a uniform electric field oscillating at ω c on the excitation elements, and
can be detected by measuring the image charge induced on the detection plates over
time. Excitation of all the ions can be effected almost simultaneously (by scanning
the excitation voltages), and as the frequency of oscillation is proportional to mass,
by applying a Fourier transform to the data, a mass spectrum of the ions can be
found. Such spectra obtained from FT-ICR instruments have a very high mass
resolution as kinetic energies are not measured. It should be noted that the actual
Fig. 3.19 Fluorescence measurements can be conducted in a quadrupole ion trap coupled to a
fluorescence microscope, such as the instrument in Karlsruhe, Germany [31]
3 Experimental Techniques
39
the buffer gas is pumped away, and the ions in the trap are ejected and analysed
using a dynode/channeltron assembly (not shown).
Note, the use of a CW laser limits the wavelengths which can be studied.
Furthermore, the energy of the ion is not strictly defined as ions can absorb and
fluoresce multiple times. Absorption by daughter ions also affects the results.
3.2.2 Fluorescence Measurements in an FT-ICR
Another instrument in which fluorescence experiments can be performed is a
Fourier-Transform Ion Cyclotron Resonance (FT-ICR) instrument [32]. At the
FT-ICR instrument in Zurich (Fig. 3.20) [33], ions are produced by electrospray
ionisation, mass selected using hexapole filters, and stored in an ICR cell which is
enclosed in a 4.7-T superconducting magnet (Penning trap). Fluorescent light
emitted by the ions after irradiation by a CW laser is detected and the photoproducts
determined: The ICR cell is comprised of two excitation elements, two detection
elements, and two trapping plates for axial trapping. The uniform magnetic field, B,
provided by the superconducting magnet induces ions to move along circular paths
in a perpendicular plane (radial trapping). The frequency of this motion is known as
the cyclotron frequency, ω c , and depends on the mass-to-charge ratio of the ion:
ω c ¼ (q/m) B (q ¼ z e). Ions of a specific m/q are excited to a higher orbit by
applying a uniform electric field oscillating at ω c on the excitation elements, and
can be detected by measuring the image charge induced on the detection plates over
time. Excitation of all the ions can be effected almost simultaneously (by scanning
the excitation voltages), and as the frequency of oscillation is proportional to mass,
by applying a Fourier transform to the data, a mass spectrum of the ions can be
found. Such spectra obtained from FT-ICR instruments have a very high mass
resolution as kinetic energies are not measured. It should be noted that the actual
Fig. 3.19 Fluorescence measurements can be conducted in a quadrupole ion trap coupled to a
fluorescence microscope, such as the instrument in Karlsruhe, Germany [31]
3 Experimental Techniques
39