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. 

Note The orbitrap is also an ion trap, but there is neither RF nor a magnet to hold ions inside. Instead, moving ions are trapped in an electrostatic field. The electrostatic attraction towards the central electrode is compensated by a centrifugal force that arises from the initial tangential velocity of ions very much like a satellite in orbit [206]. 

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The Orbitrap. The Orbitrap analyzer, [26] invented by Alexander Makarov, has been defined by the company that commercially produces it as the first totally new mass analyzer to be introduced to the market in more than 20 years . Its name recalls the concept of trapping ions. Indeed, ions are trapped in an electrostatic field produced by two electrodes a central spindle-shaped and an outer barrel-like electrode. Ions are moving in harmonic, complex spiral-like movements around the central electrode while shuttling back and forth over its long axis in harmonic motion with frequencies... 

Figure 2.13 The Orbitrap analyzer. From the Thermo web site (http //www.thermo.com)...

Figure 2.18. Schematic of an orbitrap analyzer. The z-direction oscillary motion of the ions induces an image current that is detected by the electrodes.

Trapping-type instrument capable of tandem-in-time experiments and can be linked to ToF analyzer (QIT-ToF). h Trapping-type instrument capable of tandem-in-time experiments and can be linked to Q, FTICFt, or Orbitrap analyzers (e.g., QqLrT-FTICFt, and LIT-Orbitrap). 

The Orbitrap technology is the property of Thermo Fisher Scientific, which has now marketed two different product families with several variants. The Q Exactive instrument, as shown in Figure 2.25, is a bench-top Orbitrap analyzer with a 90° bend of the ion path to limit the footprint of the instrument. Its mass-resolving power is 140000 at m/z 200, and the accessible mass range is up to 6000 Da. 

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. 

Full Fourier transform mass spectrometric (FTMS) data are acquired at various resolving power settings, from 7,500 to 100,000 w/z400 seel and 8 for Orbitrap analyzer operation details). FTMS acquisitions are done using a 30 xm raster. 

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). 

In another work [73], the better analytical performance of an UHPLC—Orbitrap method over a previously published TOF-based method is demonstrated [69] for the determination of more than 100 veterinary drugs in difficult matrices (muscle, kidney, liver, fish, and honey). These improvements are attributed to the combination of a more effective sample preparation with the higher resolution (50,000 versus 12,000 fwhm) and superior mass stability of the Orbitrap analyzer over the previously used TOP instrument. 

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. 

Note Occasionally, FT-ICR-MS is inaccurately referred to as FTMS. Of course, ICR without Fourier transformation would not have become as successful as it has, but Fourier transformation alone cannot separate ions according to m/z. With the advent of the orbitrap analyzer (below), there is a second system that makes use of Fourier transformation. Hence, the acronym FT-MS is proposed for all FT-based methods such as FT-ICR and orbitrap. 

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. 

Therefore, some applications restricted until now to FT-ICR (TOF analyzers have not been used, due to lack of resolving power) can be transferred to orbitrap analyzers, such as proteomics metabolomics, and other omics approaches. The main advantage is that orbitrap does not need such an expensive and delicate maintenance as FT-ICR does. 

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