Orbitrap mass analyzer/spectrometer

Orbitrap-based mass spectrometers (Hu et al., 2005 Scigelova and Makarov 2009) have seen rapid uptake into metabolite identification laboratories, mainly in the hybrid iontrap—Orbitrap configuration discussed below. The Orbitrap mass analyzer provides a resolution of up to 100,000 FWHM at m/z 200 at a 1 Hz acquisition rate. Mass accuracy is <2 ppm with internal calibration and is <5 ppm with external cabbration. It is stable over extended periods (days) when using external cabbration. The mass accuracy is also very good for low abundance ions and is reproducible from scan to scan. Dynamic range is >4,000 within a spectrum and acquisition rates of 10 Hz are possible at reduced resolution. 

One of the latest mass analyzer is the linear-trap quadrupole (LTQ) Orbitrap mass spectrometer. In this, the commercial LTQ is coupled with an ion trap, developed by Makarov [73, 74]. Due to the resolving power (between 70000 and 800000) and the high mass accuracy (2-5 ppm), Orbitrap mass analyzers, for example, cab be used for the identification of peptides in protein analysis or for metabolomic studies. In addition, the selectivity of MS/MS experiments can be greatly improved. However, the coupling is not useful with UHPLC for rapid chromatographic pre-separation, as the data acquisition rate is too low for a reproducible integration of the narrow signals produced with UHPLC. 

Figure 3.17 Schematic representation of the orbitrap mass spectrometer. It consists of a QMF and a dual LIT. The C trap pushes ions into the orbitrap mass analyzer which is installed in the ultrahigh vacuum region...

The orbitrap mass analyzer is an ion trap device that provides high accuracy and resolution mass measurement without the need of a magnetic field, (Hu et al. 2005 Zubarev and Makarov 2013) and thus it is more accessible in terms of lab requirements, and initial and operating costs. Some consider it to be the gold standard mass spectrometer for proteomic-based measurements (Mitchell 2010). The orbitrap mass analyzer is usually found in a hybrid configuration interfaced with a linear ion trap mass analyzer and transfer octopoles and C-trap (Fig. 2.5) (Senko et al. 2013). 

Fig. 9.27. Schematic of the Thermo Scientific LTQ Orbitrap Velos hybrid FT mass spectrometer combining dual-LIT and HCD with an Orbitrap mass analyzer. Courtesy of Thermo Fisher Scientific, Bremen.

Recent innovations in mass spectrometry have provided incorporation of two, three, and four analyzers into commercially available tandem instruments. In addition, different mass analyzers may be combined to form a hybrid mass spectrometer such as the quadrupole-TOF (Q-TOF). Various types of tandem mass spectrometers include the quadruopole-TOF, time-of-flight-time-of-hight (TOF-TOF), triple-quadrupole, and Orbitrap-FTICR configurations. 

Mass spectrometers are composed of three main components the ionization source, the mass analyzer, and the detector. With the development of different mass analyzers, the choice of mass spectrometer for a specific analysis becomes critical (53), for example, if the analysis is to quantitatively profile all proteins in a sample or quantify a preselected set of protein targets. One or a combination of mass analyzers, such as time of flight (TOF), quadrupole (Q), ion trap (IT), ion cyclotron resonance, and Orbitrap , can be implemented in MS or tandem MS (MS/MS). 

Commercial LITs were introduced in 2002 as either a stand-alone mass spectrometer (LTQ) [318] or as part of a triple quadrupole (Q-Trap) [319] or in 2005 as part of hybrid tandem mass spectrometers (LTQ-Orbitrap and LTQ-FTICR) [88,90], Application of LTQ-FTICR for metabolism studies has been reviewed by Shipkova et al. [90], In comparison to other mass analyzer types, FTICR-based mass spectrometers are not very popular for metabolite identification studies due to availability of less expensive and more user-friendly LTQ-Orbitrap and Q-TOF-based systems. Another limitation associated with the FTICR-based hybrid mass spectrometers is the TOF effect, which results in efficient trapping of only the high-mass ions [90],... 

Along with advances in various ionization sources, significant improvements have been made in the area of mass analyzers. Mass analyzers can be differentiated based on several attributes such as scan speed, duty cycle, mass resolution, mass range, and cost [126], The most common analyzers used for metabonomics analyses include the quadrupole and TOF-based analyzers [125-127], Some other analyzers that have been reported for use in MS-based metabonomics analyses are the ion traps, Orbitraps, and Fourier transform mass spectrometers [128,129],... 

Various hybrid tandem mass spectrometers, which combine two or more distinct types of mass analyzers, have been developed to maximize analytical performance and functionality. From the standpoint of ion/ion reactions, the incorporation of an electrodynamic ion trap into a hybrid instrument allows for the physical separation of the three basic steps involved in an ion/ion reaction experiment, that is, ionization, ion/ion reaction, and mass analysis of reaction products. The separation of these processes provides for the highest degree of flexibility and minimal compromises in the optimization of each step. To date, three major types of hybrid instrnments have been described for ion/ion reaction studies using an electrodynamic ion trap as the reaction vessel. The three major types of hybrid instruments are (i) quadrupole/TOF tandem mass spectrometer (ii) Orbitrap and (iii) LIT /FT-ICR. 

Analyze by direct infusion using a Q Exactive Hybrid Quadrupole-Orbitrap mass spectrometer (ThermoFisher Scientific). AH analyses are performed with resolving power set to 140,000, four microscans for 30 s. 

Top-down sequencing of the denatured and reduced protein was analyzed by direct infusion using a Exactive Hybrid Quadrupole-Orbitrap mass spectrometer with a resolving power of 70,000. 

Most mass analyzers operate under high-vacuum conditions to ensure that charged particles do not deviate from their trajectories due to collision with residual gas. Mass spectrometers can be grouped into different types of operation mode continuous mode (magnetic sector, quadrupole), pulsed mode (time of flight), and ion trapping mode (quadrupole traps, Fourier transform ion cyclotron, orbitrap). 

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