Mass spectrometers high resolution sector

Johnson and Nier published an ion optic with a 90° electric and 60° magnetic sector, which, because of the outstanding focussing properties, was to become the basis for many high-resolution, organic mass spectrometers (Nier/Johnson analyser). 

The electrospray and continuous flow-FAB experiments were carried out on a Finnigan-MAT TSQ700 triple quadrupole low resolution tandem mass spectrometer and a JEOL four sector HXllO/HXllO magnetic high resolution tandem mass spectrometer, respectively. The electrospray ionization source was manufactured by Analytica of Branford, CT. The MALDI experiments were done on a Vestec-2000 time-of-flight mass spectrometer equipped with a nitrogen laser. Chromatographic purifications of standards were performed on Applied Biosystems HOB microbore HPLC systems. 

Peak matching can be done on quadrupole and magnetic-sector mass spectrometers, but only the latter, particularly as double-focusing instruments, have sufficiently high resolution for the technique to be useful at high mass. 

The four-sector mass spectrometer is the ultimate in MS-MS instrumentation and consists of two high-resolution mass spectrometers in series. The strength of these instruments is in terms of their high-mass and high-resolution capabilities for both precursor-ion selection and product-ion analysis. Their cost, however, precludes their primary use for LC-MS and therefore they will not be considered any further here [12]. 

Specificity is unsurpassed. Traditionally, MS was performed on very large and expensive high-resolution sector instruments operated by experienced specialists. The introduction of low-resolution (1 amu), low-cost, bench-top mass spectrometers in the early 1980s provided analysts with a robust analytical tool with a more universal range of application. Two types of bench-top mass spectrometers have predominated the quadrupole or mass-selective detector (MSD) and the ion-trap detector (ITD). These instruments do not have to be operated by specialists and can be utilized routinely by residue analysts after limited training. The MSD is normally operated in the SIM mode to increase detection sensitivity, whereas the ITD is more suited to operate in the full-scan mode, as little or no increase in sensitivity is gained by using SIM. Both MSDs and ITDs are widely used in many laboratories for pesticide residue analyses, and the preferred choice of instrument can only be made after assessment of the performance for a particular application. 

Applications Sector instruments are applied for niche applications such as high-resolution measurements and fundamental ion chemistry studies. Magnetic sector mass spectrometers remain the instrument of choice in areas of target compound trace analysis, accurate mass measurement and isotope ratio measurement. 

Note The ultimate resolution of a magnetic sector mass spectrometer is reached when the slits are closed to a width of a few micrometers. Often, the slit height is also reduced, e.g., from 5 to 1 mm In daily work, the resolution will be set to fit the actual task, e.g., R = 1000-2000 for low resolution work, R = 3000-5000 if accurate mass determination at high scan rates is needed (GC-MS, Chap. 12) or isotopic patterns of high mass analytes have to be resolved, or R = 7000-15,000 in slow-scanning accurate mass measurements. 

The most common types of MS/MS instruments available to researchers in food chemistry include triple quadrupole mass spectrometers and ion traps. Less common but commercially produced tandem mass spectrometers include magnetic sector instruments, Fourier transform ion cyclotron resonance (FTICR) mass spectrometers, and quadrupole time-of-flight (QTOF) hybrid instruments (Table A.3A.1). Beginning in 2001, TOF-TOF tandem mass spectrometers became available from instrument manufacturers. These instruments have the potential to deliver high-resolution tandem mass spectra with high speed and should be compatible with the chip-based chromatography systems now under development. 

In addition to MS/MS with CID to obtain structural information, it is also useful to use high-resolution exact mass measurements to confirm the elemental compositions of ions. Essentially, exact mass measurements permit the unambiguous composition analysis of low-molecular-weight compounds (mol. wt. <500) through precise and accurate mlz measurements. The types of mass spectrometers capable of exact mass measurements include magnetic sector mass spectrometers, QTOF hybrid... 

A few years ago, we began a research program to develop methods of analysis which would involve the use of FAB and a high performance tandem mass spectrometer. The tandem instrument was the first triple sector mass spectrometer to be designed and built by a commercial instrument company (Kratos of Manchester, U.K.). The first mass spectrometer of the combination is a double focussing Kratos MS-50 which is coupled to a low resolution electrostatic analyzer, which serves as the second mass spectrometer U). This FAB MS-MS combination has been used to verify the structures of an unknown cyclic peptide (2), a new amino acid modified by diphtheria toxin (3), and an ornithine-containing lipid (4). A number of methods have also been worked out which rely on this instrumentation. They Include the structural determination of cyclic peptides (5), nucleosides and nucleotides (6), and unsaturated fatty acids (7) and the analysis of mixtures of both anionic (8) and cationic surfactants (9). 

Particularly for low-mass analytes, sector-based mass spectrometers used in a high-resolution mode can reduce spectral overlaps due to polyatomic ions, thereby improving detection limits compared to those of quadrupole mass spectrometers. However, ion transmission efficiency decreases significantly as the resolution is increased. For example, increasing the resolution provided by the sector mass spectrometer from 300 to 3000 reduces the ion transmission efficiency by about a factor of 14 [127], Further increasing the resolution to 7500 results in another order of magnitude loss in ion transmission efficiency [127]. The use of reaction cells may provide an alternative or complementary means to reduce background due to polyatomic ions. 

The collimated ion beam, having left the source, is separated into groups of ions of different mass to charge ratio m/z) usually by employing a magnetic sector or a quadrupole filter. The ability of the mass spectrometer to separate these ions is termed the resolution, and mass spectrometers may be of low, medium, or high resolution. 

The use of a high-resolution magnetic sector mass spectrometer, which can resolve the small difference in m/z for As" " at 74.922 from that of " °Ar Cl at 74.931, eliminates the chlorine interference. New collision-cell techniques, in which the atomized samples are mixed with a second gas (usually H2) in a reaction cell, also minimize this interference. Arsenic detection limits of a few ng have been reported in matrices containing 1,000 mg NaCl. The chlorine interference can also be avoided by preseparation using HG, GF, or chromatography. 

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