Mass analyser resonance

Different mass analysers can be combined with the electrospray ionization source to effect analysis. These include magnetic sector analysers, quadrupole filter (Q), quadrupole ion trap (QIT), time of flight (TOF), and more recently the Fourrier transform ion cyclotron resonance (FTICR) mass analysers. Tandem mass spectrometry can also be effected by combining one or more mass analysers in tandem, as in a triple quadrupole or a QTOF. The first analyzer is usually used as a mass filter to select parent ions that can be fragmented and analyzed by subsequent analysers. 

In addition to the diversity of ionisation techniques available, mass spectrometers offer a selection of mass analyser configurations. Of note are single (MS) and triple quadrupole (MS—MS) instruments, ion trap analysers (MS)n, time-of-flight (ToF) analysers, sector field analysers, and Fourier transform-ion cyclotron resonance (FTICR) instruments. 

However, most modem highly accurate time-of-flight (TOF), Fourier-transform ion cyclotron resonance (FT-ICR) and Orbitrap mass analysers [107] have not been reported for TA analysis so far and are thus not discussed in this chapter. 

Gas-phase acid-base studies are usually performed by using one of the following techniques high-pressure mass spectrometry (HPMS), chemical ionization mass spectroscopy (CIMS) with mass-analysed ion kinetic energy spectroscopy/collision induced dissociation (MIKES/CID), flowing afterglow (FA) or ion cyclotron resonance (ICR) spectrometry. For a brief description of all methods, Reference 8 should be consulted. 

Figure 2A3 Schematic diagram of a Fourier Transform-Ion cyclotron resonance mass analyser (Figure used by kind permission of Dr Paul Cates, School of Chemistry, University of Bristol, UK).

Figure 9.2 The basic components of a mass spectrometer. All mass spectrometers consist of an ion source linked to a mass analyser then to a detector. The important ion sources and mass analysers for biological mass spectrometry are listed. There are many other potential ion sources and mass analysers used generally in mass spectrometry, but only the indicated are of use in the analysis of biological macromolecules and amphiphilic lipids, and also in proteomics FAB fast atom bombardment MALDI matrix-assisted laser desorption and ionization ESI electrospray ionization ToF time of flight FTICR fourier transform ion cyclotron resonance MS/MS tandem mass spectrometry.

Fourier transform ion cyclotron resonance (FTICR) mass analysers... 

Figure 9.11 Schematic of a Fourier Transform Ion Cyclotron Resonance (FTICR) Mass Analyser. Ions are constrained in circular orbits by electric and magnet fields generated by superconducting magnet before selective detection. These analysers have the highest sensitivity and accuracy of any mass analysers presently available (Reproduced from Daas, 2001 [Wiley]).

Matrix assisted laser desorption/ionisation (MALDI) For laser desorption methods a pulsed laser is used to desorb species from a target surface. Therefore, a mass analyser compatible with pulsed ionisation methods has to be used. Typically, time-offlight (TOF) analysers are employed, but several hybrid systems (Q-TOF) and, recently, high resolution Fourier transform ion cyclotron resonance (FT-ICR) analysers have been successfully adapted (see Section 10.2.4). Direct laser desorption rehes on the very rapid heating of the sample or sample substrate to vapourise molecules without decomposition. The more recent development of MALDI relies on the absorption of laser energy by a solid, microcrystalline matrix compound such as a-cyano-4-hydroxy ciimamic acid or sinapinic acid [8, 34]. MALDI has become an extremely popular method for the rapid and sensitive analysis of high-molecular-weight compounds [4]. 

Various forms of tandem mass spectroscopy (MS/MS) have also been used in the analysis of biomolecules. Such instruments consist of an ionisation source (ESI or MALDI or other) attached to a first mass analyser followed by a gas-phase collision cell. This collison cell further fragments the selected ions and feeds these ions to a second mass detector. The final mass spectrum represents a ladder of fragment ions. In the case of peptides the collision cell usually cleaves the peptides at the amide bond. The ladder of resulting peptides reveals the sequence directly [496]. Thus, tandem MS instruments, such as the triple quadrupole and ion-trap instruments have been routinely applied in LC-MS/MS or ESI-MS/MS for peptide sequencing and protein identification via database searching. New configurations, which have been moving into this area include the hybrid Q-TOF [498], the MALDI-TOF-TOF [499] and the Fourier transform ion cyclotron resonance instruments [500]. 

UV spectroscopy in molecular beams involves either laser-induced fluorescence (LIP) or resonance-enhanced multiphoton ionization (REMPI) methods. The latter method has the advantage that the resulting ionized molecules can be mass-analysed in a TOP mass spectrometer. The application of either REMPI or LIP spectroscopy requires the molecule of interest to incorporate a UV chromophore, such as an aromatic moiety. The DNA and RNA bases adenine, guanine, cytosine, thymine and uracil are aromatic molecules with well-known UV absorptions. Of the 20 naturally occurring proteinogenic amino acids, 3 - phenylalanine, tyrosine and tryptophan - feature an aromatic side chain, as do many neurotransmitter molecules. To study molecules that lack a UV chromophore, such as peptides without Trp, Tyr and Phe residues and carbohydrates, a UV chromophore needs to be chemically attached [47, 48, 74]. 

In a laser desorption/photoionisation (LD/PI) experiment a number of experimental variables needs to be defined (i) desorption parameters (requirement minimal fragmentation) (ii) photoionisation parameters (resonant or non-resonant near UV, far UV, VUV single or multiple step laser shot laser power) and (Hi) mass analyser. By fine-tuning of the laser diminished fragmentation can be achieved by setting the laser power to produce ions near the threshold value for ionisation ( soft ionisation). 

Relative mass is an intrinsic molecular property which, when measured with high accuracy, becomes a unique and unusually effective parameter for characterization of synthetic or natural biomolecules. Mass spectrometry based methods can be broadly applied not only to unmodified synthetic biomolecules, but also to modified synthetic and natural biomolecules (e.g. glycosylated proteins). The level of mass accuracy one obtains during the measurement will depend on the capabilities of the mass analyser used. Quad-rupole and TOE instruments yield lower mass accuracies than sector or Fourier transform ion cyclotron resonance (FTICR) instruments. High mass accuracy is not only necessary for qualitative analysis of biomolecules present in a sample, but is necessary to provide unambiguous peak identification in a mass spectrum. 

Many techniques for the analysis of anthocyanins have been used for almost a century and are still of importance, along with considerable advances in technologies such as mass spectroscopy (MS) and nuclear magnetic resonance (NMR). This section summarizes the analytical procedures for quantitative and qualitative analyses of anthocyanins, including classical and modem techniques. 

There have been no reports of complexes of " JV-substituted thiosemicarbazones derived from 2-formylpyridine, but 2-acetylpyridine JV-methyl-thiosemicarbazone, 3a, formed [Fe(3a-H)2]C104 and [Fe(3a-H)2]FeCl4 [117]. The nature of these two species was established by partial elemental analyses, molar conductivities, magnetic moments, electronic, infrared, mass and electron spin resonance spectra. A crystal structure of a related selenosemicarbazone complex confirmed the presence of a distorted octahedral iron(III) cation coordinated by two deprotonated anions so that each ligand is essentially planar and the azomethine nitrogens are trans to each other the pyridyl nitrogen and selenium donors are both cis. 

Johnson SG, Fearey BL (1993) Spectroscopic study of thorium using continuous-wave resonance ionization mass-spectrometry with rrltraviolet ionization. Spectrochim Acta Part B 48 1065-1077 Knoll GF (1989) Radiation Detection and Measurement. J. Wiley and Sons, New York Kuss HM (1992) Applications of microwave digestion technique for elemental analyses. Fresenins J Anal Chem 343 788-793... 

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