High-resolution double focusing mass

An interface has been connected to a high-resolution double-focusing mass spectrometer applied to the analysis of mixtures of both chemically synthesized peptides and peptides obtained from proteolytic digests of proteins. Detection limits of about 35 fmol were reported. Reinhold et al.73 described the on-line coupling of CE and MS using a CF-FAB system in... 

The molecular ion overlaps due to plasma and solvent species are most severe below mass/charge 82. A high-resolution, double-focusing mass spectrometer was used to identify molecular ions observed in ICP-MS (Table 3.3) [3]. Common molecular ions that produce intense signals from plasma and solvent species include ArO+, ArOH+, ArH+, ArN+, Ar+, Ar2H+, 0+, N2+, NO+, and 0+. Other molecular ions become a problem at lower analyte concentrations. These include CO, CO H+, NOt, ArO+ ions with minor isotopes of Ar or O, ArC+, ArN+, and minor isotopes of Ar as ArJ. 

C. The High-Resolution Double-Focusing Mass Spectrometer. 153... 

This article describes recent advances in the experimental determination of electron impact ionization cross sections for silane (SiH4) its radicals, SiH. (x = 1 to 3) and the Si-organic molecules tetramethylsilane (TMS), Si(CH3)4 tetraethoxysilane (TEOS), Si(0-CH2-CH3)4 and hexamethyldisiloxane (HMDSO), (CH3)3-Si-0-Si-(CH3)3, which is one of the simplest siloxane compounds. These are model substances, and the results obtained for these species may be used in efforts to predict the ionization properties of other, more complex Si-organic molecules. The ionization cross sections of the stable compounds were measured using a high-resolution double-focusing mass spectrometer. The cross-section data for the radicals were obtained in a fast-neutral-beam apparatus. 

In the following sections of this article, we describe the principles of ionization cross-section measurements, including a brief description of the fast-beam apparatus and the high-resolution double-focusing mass spectrometer employed in the present studies. A comprehensive review of semiempirical calculations of total ionization cross sections is given. Comparisons between these calculated cross sections and the experimental results are presented. The decomposition of the various molecules in a low-temperature plasma is discussed on the basis of the measured ionization-cross-section data, and comparisons are made with the results of in situ plasma diagnostics studies using mass spectrometric techniques. 

Fig. 2. Schematic diagram of the high-resolution double-focusing mass spectrometer. The insert shows an enlarged view of the ion extraction optics for the high-extraction-efficiency mode. P, pusher electrode C, collision chamber B, electron beam (z direction) S, collision chamber exit slit S2, penetrating field extraction slit S3, grounded slit S4 and Ss, deflector electrodes.

High-resolution double-focusing mass spectrometers... 

Fig. 3.2. High-resolution double-focusing mass analyser with Nier-Johnson geometry. The electrostatic sector filters ions according to kinetic energy before magnetic sorting according to m/z. The combination of electric and magnet fields produces narrow peaks and high spectral resolution.

Commercial Instruments Because of the proliferation of applications of mass spectrometry in organic and analytical chemistry, there are instruments marketed by numerous companies today. Some of the popular ones are the low resolution, single focusing model 21-490 and the high resolution, double focusing models, 21-492 and 21-110 of Du Pont de Nemours Co, and the double focusing model MS-9 of Associated Electrical Industries... 

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-... 

There are many types of mass spectrometer, from high-resolution double-focusing instruments, which can distinguish molecular and fragment masses to six decimal places, to bench-top machines with a quadrupole mass detector which can resolve masses up to about m/z = 500, but only in whole-number differences. Routinely you are most likely to encounter data from bench-top instruments and therefore only this typie of spectrum will be considered. 

Identifrcation of components in the extracts was conducted by mass spectrometry. The sample was injected onto an HPS890 GC. The chromatographic conditions for the OV-1 column were the same as described for GC analysis. The end of the GC capillary column was inserted directly into the ion source of the mass spectrometer via a heated transfer line maintained at 280°C. The mass spectrometer was a Micromass Prospec high resolution, double-focusing, magnetic sector instrument. The mass spectrometer was operated in the electron ionization mode (El), scanning from ni/z 450 to m/z 33 at 0.3 seconds per decade. 

In essence, any of the mass spectrometer types described in the GC/MS section can be used in combination with LC/MS. Double focusing mass spectrometers provide high resolution, but are very expensive. The most popular choices in LC/MS are quadrupole and ion trap mass spectrometers. TOP mass spectrometers are quickly gaining popularity in LC/MS. Recent advances in the technology make it possible to measure exact masses of the ions, which allows the unequivocal determination of their elemental composition. Thus, results comparable to those produced by high-resolution double focusing instruments can be obtained at a fraction of the cost. 

Double-focusing mass spectrometer A mass spectrometer consisting of electrostatic and magnetic sector analysers capable of achieving high-mass spectral resolution. 

With few exceptions, magnetic sector instruments are comparatively large devices capable of high resolution and accurate mass determination, and suited for a wide variety of ionization methods. Double-focusing sector instruments are the choice of MS laboratories with a large chemical diversity of samples. In recent years, there is a tendency to substitute these machines by TOE or by Fourier transform ion cyclotron resonance (FT-ICR) instruments. 

Exact mass measurement at high resolution is an important tool along with other spectroscopic methods to help confirm the structure of novel flavonoids. It is used as structural proof when elemental analysis is not possible, e.g., when studying minor components. When EI-MS can be used, 1 to 10 pmol samples are required for one measurement however, when FAB-MS is used, 0.1 to 1 nmol is normally required. The use of ESI on a double focusing mass spectrometers and MALDI-TOF-MS requires smaller amounts of sample, and subpi-comole amounts of flavonoids may be adequate. 

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