Fticr-MS – Fourier Transform Ion Cyclotron Resonance – Mass Spectrometry

A highly sensitive, high resolution exact mass determination of compounds can be done by the FTICR-MS method. The routine FTICR-MS method has a resolution of up to 106 with an accurate mass < 1ppm. In theory, the FTICR-MS is able to detect molecules having a concentration of single attomole (10-8 mole). Even though the FTICR-MS is not used as commonly in the forensic sciences, because it is expensive (>$800,000 US) and is huge in size, but its application is very useful for forensic sciences.

The m/z finding is not solved in time or space but rather in frequency. The m/z of an ion in a magnetic field is calculated by calculating its cyclotron frequency (fc). This is the product of magnetic energy (B) and its ion charge (Z) which is divided by 2 pi (2π) multiplied by the mass of the ion (m).

Fc = (zB)/( 2πm)

After re-arranging the equation, we can calculate the m/z ratio by:

m/z = (B)/ 2πfc)

After ionization, the ions which are trapped within the magnetic chamber are oscillated in a circular motion by an axial electrical field. The chamber is then held in high vacuum which is cooled to 0 K. the ions are stopped from leaving the chamber because of the presence of trapping plates at both ends. The detection takes place for all m/z at the same time when they are close to the detection plates. At the beginning, the rotational orbit is small and the current produced isn’t too strong to cause a signal. An RF pulse is then fired across the plates on both sides of the chamber which causes the ions to be excited which propels them in a higher orbit. The ions which orbit in higher planes of the chamber create an alternating current which is elicited between the detecting plates as the oscillation takes place between the plates. The frequency that occurs is then equal to the fc of the particular m/z whose intensity is proportional to the number of ions.

The fc signal is then transformed in to the m/z but Fourier transformation whih can be determined up to 7 figures. This causes the FTICR-MS to be a powerful tool for generating elemental composition of unknown compounds and metabolites in the forensic sciences. A study conducted by Ojanepera e al. used FTICR-MS to identify some unknown compounds by comparing their masses observed with a database having the exact masses record. These compounds included benzodiazepines, beta blockers, sleep agents, anti depressants and narcotic analgesics. They also concluded that the accuracy given by this method within the 3ppm was enough to identify unknown compounds which were present in the data base. This technique is currently, cost prohibitive and limitedly used.

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