Fourier transform resolving power

An added consideration is that the TOF instruments are easily and quickly calibrated. As the mass range increases again (m/z 5,000-50,000), magnetic-sector instruments (with added electric sector) and ion cyclotron resonance instruments are very effective, but their prices tend to match the increases in resolving powers. At the top end of these ranges, masses of several million have been analyzed by using Fourier-transform ion cyclotron resonance (FTICR) instruments, but such measurements tend to be isolated rather than targets that can be achieved in everyday use. 

The m/z values of peptide ions are mathematically derived from the sine wave profile by the performance of a fast Fourier transform operation. Thus, the detection of ions by FTICR is distinct from results from other MS approaches because the peptide ions are detected by their oscillation near the detection plate rather than by collision with a detector. Consequently, masses are resolved only by cyclotron frequency and not in space (sector instruments) or time (TOF analyzers). The magnetic field strength measured in Tesla correlates with the performance properties of FTICR. The instruments are very powerful and provide exquisitely high mass accuracy, mass resolution, and sensitivity—desirable properties in the analysis of complex protein mixtures. FTICR instruments are especially compatible with ESI29 but may also be used with MALDI as an ionization source.30 FTICR requires sophisticated expertise. Nevertheless, this technique is increasingly employed successfully in proteomics studies. 

With time of flight instruments, a mass accuracy better than 5 ppm can be achieved, while with Fourier transform ion cyclotron resonance or orbitrap mass spectrometers mass accuracies better than 1 ppm have been reported. It is obvious that, for good mass accuracies, the peaks must be baseline resolved and resolution plays an essential role. For the present example, a mass resolving power of 5000 seems to be quite acceptable. In the case of the [M+H]+ + 1 isotope peak, the situation becomes somewhat more complex for molecules containing nitrogen, sulfur or carbon. Figure 1.5 D illustrates at a mass resolving power of 500000 the contribution of... 

The highest-resolution mass spectra are obtained by Fourier transform ion cyclotron resonance mass spectrometry.34 Molecular ions of two peptides (chains of seven amino acids) differing in mass by 0.000 45 Da were separated with a 10% valley between them. The ions each have a mass of 906.49 Da and a width at half-height of 0.000 27 Da. Compute the resolving power by the 10% valley formula and by the half-width formula. Compare the difference in mass of these two compounds with the mass of an electron. 

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 quadrupole ion trap is seen as a compact and less expensive alternative to FT-ICR mass spectrometers employed for gas-phase metal ion chemical and mass analysis [28,29]. Although lacking the high mass resolving powers available in Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), the quadrupole ion trap has been investigated by using nonselective and resonance-selective ionization of ablated metal samples. 

Figure 20 Ultrahigh mass resolving power mass spectrum of 58Fe+ from a glow discharge (GD) source obtained with a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with a 7-tesla superconducting magnet.

Electrospray ionization with the high resolving power of Fourier transform mass spectrometry (FTMS) makes possible the detection of adducts and subpicomole impurities that confuse lower resolution measurements, as shown in Figure 8.30. This method achieves accurate determination of molecular weights (<0.002 %) and permits the verification of 50-100-mer DNA and RNA sequences. [169,170]... 

Ion-molecular reactions are used to resolve isobaric interferences, as discussed, in ICP-MS with a collision/reaction cell or by utilizing ion traps. The mass spectra of Sr, Y and Zr (Fig. 6.10a) without O2 admitted into the collision cell and (Fig. 6.10b) with 10 Pa Oj are different. By introducing oxygen, selective formation of YO and ZrO, but not SrO, is observed. This behaviour of different oxide formation is relevant for an interference free determination of Sr. Ultrahigh mass resolving power ICP mass spectrometry (at m/Am 260 000) selectively removes unwanted ions prior to transfer to the FTICR analyzer cell by gas-phase chemical reactions, e.g., for separation of Ca from " Ar+ obtained with a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer equipped with a 3 tesla superconducting magnet. ... 

Due to its high resolving power and its abilities to accurate-mass determination, Fourier-transform ion-cyclotron resonance MS (FT-ICR-MS) has become an important tool in proteomics (Ch. 18.3.4-5). Its potential in phosphoprotein... 

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