Double focusing magnetic sector

Resolution Quadrupole instruments are not capable of achieving the high resolution that is common with double-focusing magnetic-sector instruments. In GC/MS analyses, a compromise is struck between sensitivity (ion transmission) and mass resolution. In the quadrupole instrument, the resolution is set to the lowest possible value commensurate with resolving peaks differing by 1 Dalton (unit resolution). 

Identification of unknowns using GC/MS is greatly simplified if accurate mass measurements are made of all the ions in a spectrum so that reasonable elemental compositions of each ion are available. Unfortunately, obtaining a mass measurement that is accurate enough to significantly limit the number of possible elemental compositions requires expensive instrumentation such as a double-focusing magnetic sector or fourier transform ICR MS. 

Quetel CR, Prohaska T, Hamester M, Kerl W, Taylor PDP (2000b) Examination of the performance exhibited by a single detector double focusing magnetic sector ICP-MS instrument for uranium isotope abundance ratio measurements over almost three orders of magnitude and down to pg g-1 concentration levels. J Anal At Spectrom 15 353-358... 

Walder AJ, Freedman PA (1992) Isotopic ratio measurement using a double focusing magnetic sector mass analyser with an inductively coupled plasma as an ion somce. J Anal At Spectrom 7 571-575... 

Table 6.16 summarises the main characteristics of FI-MS. FT uses high voltages and was once restricted to sensitive double-focusing magnetic sector instruments of relatively high cost. Field ionisation is considered to be the softest ionisation mode. The reproducibility of the non-standard techniques, such as FI-MS and FD-MS, is less well assessed than that of EI-MS. A noticeable drop in FI use occurred after the mid-1980s because of the advent of FAB and other desorption/ionisation methods. FI-MS is only used in a few laboratories worldwide. 

The dwelltime of ions within the ion source is defined by the extraction voltages applied to accelerate and focus them into an ion beam and by the dimensions of that ion source. In standard El ion sources the freshly formed ions dwell about 1 ps before they are forced to leave the ionization volume by action of the accelerating potential. [41] As the ions then travel at speeds of some 10 m s they pass the mass analyzer in the order of 10-50 ps (Fig. 2.9). [9] Even though this illustration has been adapted for a double focusing magnetic sector mass spectrometer, an ion of m/z 100, and an acceleration voltage of 8 kV, the effective time scales for other types of instruments (quadrupole, time-of-flight) are very similar under their typical conditions of operation (Table 2.4). 

Fig. 4.27. Photograph of a JEOL JMS-700 double focusing magnetic sector instrument.

Classically, high-resolution work is the domain of double-focusing magnetic sector instruments. More recently, TOP and to a certain degree triple quadrupole instruments are also capable of resolutions up to about 20,000. However, the rapid development of FT-ICR instruments has established those as the systems of choice if ultrahigh-resolution (>100,000) and highest mass accuracy (1 ppm) are required (Chap. 4.6). 

Key Q = Quadmpole IT = ion trap DFMS = Double focusing magnetic sector TOP = Time of flight FTICR = Fourier transform ion cyclotron resonance. 

During the 1960s, high-resolution, double-focusing magnetic sector instruments became available from multiple manufacturers and were widely used in organic chemistry for exact mass measurements and elemental composition analysis. El was used for generating struc-... 

J. Begerow, M. Turfeld, L. Dunemann, Determination of physiological palladium and platinum levels in urine using double focusing magnetic sector held ICP-MS, Frese-nius J. Anal. Chem., 359 (1997), 427D429. 

R. Thomas, A beginner s guide to ICP-MS Part VII Mass separation devices D double-focusing magnetic-sector technology, Spectroscopy, 16 (2001), 22D27. 

Begerov, J., Turfeld, M., Duneman, L. Determination of physiological palladium, platinum, iridium and gold levels in human blood using double focusing magnetic sector field inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom. 12, 1095-1098 (1997)... 

Double focusing magnetic sector analysers use two successive separations, first electrostatic, which separates species on the basis of charge, and then magnetic analysis for mass separation the combined separation is thus made on the basis of the m/z ratio. 

All positive ion DCI mass spectra were obtained from a Finnigan MAT 90 double focusing magnetic sector mass spectrometer. The electron energy was set at 150 eV. The instrument was set to scan from 100-850 amu at 2 seconds per decade with a 1 second InterScan time. Ammonia was used as the reagent gas at a source pressure of lxlO 4Torr. 

Atomic Fluorescence Spectrometry (AFS) (5, 58), quadrupole and double-focusing magnetic sector ICP-MS (59-63) have all been used in the past for the determination of trace element in polar snow and ice. Of those techniques only LEAFS, double-focusing magnetic sector ICP-MS and DPASV have proved capable of direct determination on the analytes in the samples at the required levels. 

The very few laboratories in the world that accepted this challenge have used sophisticated sampling procedures in the field and powerful analytical techniques such as double focusing magnetic sector ICP-MS, LEAFS and TIMS, to produce a wealth of reliable data useful in the difficult task of deciphering snow and ice archives. 

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