High orbitrap analyzer

Tandem mass spectrometry (MS/MS) is very useful for the amino acid sequencing of peptides, and has been used widely in both protein biochemistry and pro-teomics to identify proteins, to deduce the sequence of a peptide, and to detect and locate post-translational modifications. Until around a decade ago, the concept of amino acid sequencing by MS-technologjes was synonymous with ESI-MS/MS, but today MALDI-MS/MS techniques are implemented in high-performance instruments such that the quality of MALDI tandem mass spectra is comparable with that of ESI-MS/MS spectra. Currently, MALDI tandem mass spectrometers exist in a number of geometries, including TOF-TOF, Q-TOF, ion trap and orbitrap analyzers that each provide unique analytical features for the sequencing of peptides and proteins by MS/MS (details of the instrumentation for different types of MS/MS are provided in Chapter 2). 

The same lack of apphcation to trace quantitative analysis appears to be true of a more recent innovation, the Orbitrap analyzer (Makarov 2000) indeed a recent extensive review of this device (Hu 2005) also does not mention quantitation at all. Like the FTICR analyzers, the Orbitrap operates under very high vacuum ( 10 torr) to achieve its ultimate performance it also uses image current detection. At this time neither of these analyzers appears to be suitable for trace level quantitation experiments of the kind discussed in this book. 

Together with the delivery of highly accurate data, a strong point of the Orbitrap analyzer is the availability of all MS" options (both CID and HCD) for peptide sequence confirmation (and for short peptides, there is the potential for de novo sequencing). 

Both ionization methods, MALDI and ESI, are adequate for the analysis of low-and high-molecular-weight molecular ions. Data provided by mass-to-charge ratio (m/z) determination can in some cases directly depict a compositional assignment of the molecule under investigation if the resolution and mass accuracy are sufficient, as in quadrupole TOF (QTOF) analyzers, or high, as in ion cyclotron resonance (ICR) and Orbitrap analyzers. 

The orbitrap analyzer incorporates a completely new concept of m/z analysis [15,205]. Commercialized by Thermo Fisher Scientific in 2005, the Orbitrap delivers high resolving power and accurate mass measurement at a level rivaling FT-ICR to a certain extent [15,206-210]. It is the special charm of the orbitrap to operate without a magnetic field, and therefore, to be available at a much lower price and lower installation room requirements than FT-ICR instruments. Nonetheless, it has one important feature with FT-ICR in common the orbitrap also employs image current detection of ion oscillations and Fourier transformation for the conversion of the transient into the frequency domain. 

The orbitrap analyzer itself does not provide for a mode of tandem MS operation. Instead, the steps of precursor ion isolation and dissociation are performed in a dedicated LIT as MSI (Chap. 4.8.4) prior to high-resolution and accurate mass analysis of the fragments by the orbitrap. Multiple-collision CID (Chap. 9.3) in the LIT of the LTQ-orbitrap instrument is sometimes not hard enough to achieve fragmentation of comparatively stable precursor ions, e.g., it can be insufficient to generate immonium ion fragments from protonated peptides. 

Ion detection is carried out using image current detection with subsequent Fourier transform of the time-domain signal in the same way as for the Fourier transform ion cyclotron resonance (FTICR) analyzer (see Section 2.2.6). Because frequency can be measured very precisely, high m/z separation can be attained. Here, the axial frequency is measured, since it is independent to the first order on energy and spatial spread of the ions. Since the orbitrap, contrary to the other mass analyzers described, is a recent invention, not many variations of the instrument exist. Apart from Thermo Fischer Scientific s commercial instrument, there is the earlier setup described in References 245 to 247. 

Advances in high resolution mass analyzers (TOF, FT-ICR, orbitrap) have greatly improved the detection and identification of metabolites based on accurate mass measurements. In single MS mode accurate mass determination is mainly used to differentiate between isobaric ions. Combined with LC-MS, it allows the detection of predicted metabolites by performing extracted ion current profiles... 

Orbitrap The newest of the major mass analyzers, the Orbitrap is a hybrid MS consisting of a LIT mass analyzer, or transmission quadmpoles connected to the high-resolution Orbitrap mass analyzer. The Orbitrap utilizes electrical fields between sections of a roughly egg-shaped outer electrode and an inner (spindle) electrode (Chapter 5). Ions orbit between the inner and outer electrodes and their oscillation is recorded on detector plates (Hardman and Makarov, 2003 Hu et al., 2005). As with the FTICR, fast Fourier transform of the raw data is used to convert the data for mass analysis, making the Orbitrap the second major type of FTMS instrument. The resolving power of the Orbitrap is intermediate... 

The Orbitrap-based systems have emerged as the newest option for LC-HRMS. When configured as hybrid linear trap-Grbitrap (LTQ-Orbitrap), the systems are conceptually similar to Q-TOF in that mass analyzer 1 is nominally a unit mass analyzer, and mass analyzer 2 is capable of high resolution. These systems are capable of either LC-HRMS or LC-MS/HRMS operation. A new variant on the commercial Orbitrap, the Exactive, is expected to be released in late 2008. This system, which consists only of the single mass analyzer, has shown promising results in early assessment of quantitation by LC-HRMS (Bateman et al., 2008). 

Schmid, D. G., Grosche, P., Bandel, H., and Jung, G. (2000). FTICR-mass spectrometry for high-resolution analysis in combinatorial chemistry. Biotechnol. Bioeng. 71 149-161. Scigelova, M., and Makarov, A. (2006). Orbitrap mass analyzer—Overview and applications in proteomics. Proteomics 6 (Suppl. 2) 16-21. 

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