High resolution mass spectrometers can be used to identify and measure low levels of components in complex mixtures due to their ability to determine, accurately and precisely, small differences in molecular weight.
The Thermo Ltq-Orbitrap installed in our lab can determine mass to charge ratio to within one part per million of the true value, or even better. It is used as detector for an Accela rapid-HPLC, and together they make a high-resolution, high-accuracy LC/MS system..
The heart of this LC/MS is the Orbitrap mass filter. Sample Ions are injected tangentially into this ion-trap where an electric field is set for them between an outer barrel-like electrode and an inner axial electrode. The ions end up moving in ring-like orbits around the axial electrode. Once their attraction to the inner electrode is exactly balanced by the centrifugal forces, the ions are trapped in their orbits.
These rings of ions are now set to move right and left along the axis of the central electrode in harmonic oscillations. An image of these oscillations can be detected on the orbitrap outer electrodes. The detection of this image current from coherent ion packets takes place after the voltages have stabilized.
The complex signal from the electrodes is amplified and transformed into a frequency spectrum by fast Fourier transformation. This is finally converted into a mass spectrum, where the oscillation frequency for each ion is used to calculate its mass to charge ratio - being inversely proportional to the square root of m/z.
The Orbitrap mass-analyzer is actually only one part of a more complex "hybrid FT mass spectrometer". The different compounds separated on the HPLC column are injected in turn into the electrospray ion source. The generated ions are next collected in a Linear Ion Trap and then ejected axially into the C-shaped storage trap. The C-trap s used to store and slow down the ions before injecting them into the orbitrap mass analyzer. The ions transferred from the C-Trap are captured in the orbitrap by a rapid increase in the electric field.