Magnetic resonance and radio frequency mass spectrometers and their application

Abstract

Development of magnetic resonance mass spectrometers (MRMSs) was started at the Leningrad Physico-Technical Institute as far back as the early 1950s, and work on their upgrading and refinement is still in progress. For more than 50 years, the activities in the area of their modification and potential applications have been charged by Boris Aleksandrovich Mamyrin (May 25, 1919–March 05, 2006), assisted by followers and colleagues. Altogether, 7 such instruments have been built, and 5 of them still remain at the Ioffe Institute, with only two of them being actively in use. At about the same time, Lincoln Gilmore Smith (February 12, 1912–December 9, 1972) was developing and building together with colleagues a mass synchrometer at the Brookhaven National Laboratory and, subsequently, a radio frequency mass spectrometer (RFMS) at Princeton University. After the death of L.G. Smith, the unique instrument did not find proper application in the USA and was moved to Delft University of Technology, where it likewise did not enjoy much use for many years. The principle underlying the operation of both instruments, MRMS and RFMS, is based essentially on the cyclotron frequency of ion motion in a circular orbit in a homogeneous magnetic field being dependent on the field strength and the ion mass to charge ratio only. The specific principle of operation employed confers extremely high analytical characteristics to these mass spectrometers. (Fourier-transform ion-cyclotron-resonance mass spectrometers have a radically different design and are not considered here.) Indeed, the resolving power of one of the MRMSs is as high as ∼350,000 at half-maximum of the mass peak, the absolute sensitivities of other instruments amount to ∼30,000 atoms in a mass analyzer (1–3) l in volume under static pumping, with the dynamic range reaching 10 11 (for instance, in studies of the 3He/ 4He isotope ratio). These instruments enjoyed widespread use in studies of the isotopes of helium and of other noble gases in natural and technogenic samples, as well as in precision measurements of fundamental physical constants, in particular, of the magnetic moment of the proton in nuclear magnetons and of the tritium half-life. As for the RFMS, its resolving power reached as high as ∼400,000 in atomic mass measurements. The unique peak matching technique developed by L.G. Smith permitted one to locate the center of a mass line to within 1/2500 fraction of its width, thus offering a possibility of conducting measurements of cyclotron frequencies and, hence, of atomic masses with a relative uncertainty of ∼10 −9. It is the danger of these unique instruments sinking into oblivion that has motivated our writing of this review paper.