Mass Resolution and Accuracy

High resolution and mass accuracy (elemental composition)... 

Balogh, M. P. Debating resolution and mass accuracy in mass spectrometry. Spectroscopy 2004,19, 34-40. 

The orbitrap is the most recently invented mass analyzer. Like with the QIT, ions are trapped and stored in a potential well. However, instead of ejecting the ions for external detection the frequency of the trapped oscillationg ions is measured. This method provides substantially better resolution and mass accuracy in normal operation. 

Performance Parameters. Since the detector is often involved in the separation of isobars, normal mass analyzer resolution and mass accuracy do not really apply. The mass spectrometric resolution would be determined by the magnetic and electric sectors. Only atomic species are analyzed, so that sets the upper m/z... 

The FT-ICR/MS is an ideal instrument for studying ion-molecule reactions over an extended time scale due to the excellent trapping of ions in the cell and the unmatched mass resolution and mass accuracy. Mass resolution is defined as the mass divided by the peak width at half height... 

The sensitivity, compactness, automation, and low prices of ion trap instruments made them very popular in biological MS P Limitations of ion traps include low resolution and mass accuracy at high m/z. In addition, in MS/MS mode, the lower end of the fragment mass range cannot be visualized. Recent developments in the linear geometry of ion traps are aimed at improving on those limitations. 

The Kendrick mass scale is based on the definition M(cr = 14.0000 u. [28] Conversion from the lUPAC mass scale to the Kendrick mass scale is achieved by multiplying each mass by 14.00000/14.01565 = 0.99888. The intention of the Kendrick mass scale is to provide an effective means of data reduction, an issue gaining importance again as a result of the steadily increasing resolution and mass accuracy (next paragraphs) of modem mass spectrometers. 

Note Although resolution and mass accuracy are already high in broadband spectra (R = 3 x 10 -10, Am < 10 u), the performance of FT-ICR instruments improves by at least one order of magnitude by storing only narrow m/z ranges, because fewer ions in the cell mean less distortion by coulombic interactions. 

The limited resolution and mass accuracy of the early MALDI-TOF instruments made the combination of MALDI with magnetic sector instruments (Chap. 4.3) desirable, [148,149] but this set-up suffered from low shot-to-shot reproducibility and poor sensitivity getting a full scan spectrum required thousands of laser shots while scanning the magnet. Even though eutectic matrix mixtures were introduced to circumvent such problems, [90,91] the MALDI-magnetic sector combination never became established. 

TOF High resolution and mass accuracy when operated in reflectron mode or in Q-TOF systems, elemental composition can be obtained Tandem MS available for generation of sub-structure information or quantitative analysis High sensitivity Very fast scan speed Unlimited mass range Initial energy and spatial distribution must be corrected for ions High-performance electronics needed... 

In tandem MS mode, because the product ions are recorded with the same TOF mass analyzers as in full scan mode, the same high resolution and mass accuracy is obtained. Isolation of the precursor ion can be performed either at unit mass resolution or at 2-3 m/z units for multiply charged ions. Accurate mass measurements of the elemental composition of product ions greatly facilitate spectra interpretation and the main applications are peptide analysis and metabolite identification using electrospray iomzation [68]. In TOF mass analyzers accurate mass determination can be affected by various parameters such as (i) ion intensities, (ii) room temperature or (iii) detector dead time. Interestingly, the mass spectrum can be recalibrated post-acquisition using the mass of a known ion (lock mass). The lock mass can be a cluster ion in full scan mode or the residual precursor ion in the product ion mode. For LC-MS analysis a dual spray (LockSpray) source has been described, which allows the continuous introduction of a reference analyte into the mass spectrometer for improved accurate mass measurements [69]. The versatile precursor ion scan, another specific feature of the triple quadrupole, is maintained in the QqTOF instrument. However, in pre-... 

FT-ICR mass spectrometers take advantage of ion-cyclotron resonance to select and detect ions. This analyzer can be used with both ESI and MALDI interfaces. Their particular advantages are their sensitivity, extreme mass resolution, and mass accuracy. The latter allows for the determination of the empirical formulae of compounds under 1000 Da. As far as we know, this analyzer has not been applied to flavonoids. 

The LTQ-Orbitrap has resolution and mass accuracy performance close to that of the LTQ-FTICR. As shown in Table 5.3 (column 4), LTQ-Orbitrap accurate mass measurements, using external calibration, for a set of 30 pharmaceutical compounds resulted in less than 2.3 ppm error. The data were acquired with a 4-min, 1-mL/min-flow-rate, positive-mode LC-ESI-MS method where all measurements were performed within 5h from mass calibration. Mass accuracies below 2-3 ppm, and often below 1 ppm, can be routinely achieved in both the positive- and negative-ion mode (Table 5.3, columns 4 and 5). The long-term mass stability of the LTQ-Orbitrap is not as consistent as observed for the LTQ-FTICR-MS, and the Orbitrap requires more frequent mass calibration however, mass calibration is a routine procedure that can be accomplished within 5-10 min. Figure 5.7 displays a 70-h (external calibration) mass accuracy plot for three negative ions collected with a LTQ-Orbitrap where the observed accuracy is 2.5 ppm or better with little mass drift for each ion. Overall, for routine accurate mass measurements on the Orbitrap, once-a-week calibration (for the desired polarity) is required however, considering the ease of the process, more frequent external calibration is not a burden. 

Although both hybrid LTQ-FTMS instruments find applications in the dmg metabolism area, the LTQ-Qrbitrap, with its simplified maintenance, lower cost, and in most cases better sensitivity at low masses, is generally more applicable to small-molecule analysis. The LTQ-FTICR, with the advantage of higher resolution and mass accuracy, covers a wider range of applications, from small molecules to peptides and proteins, where most advantages are found in the analysis of large biomolecules. 

Figure 7.9 Fourier transformation of the time domain signal (a and b) to the frequency domain signal and corresponding mass spectrum (c and d), showing the effects of signal decay time on spectrum resolution and mass accuracy...

Technological advances of ion-trap mass spectrometers are the ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and the recently released technique, the Orbitrap Fourier transform mass spectrometry (Hu et al., 2005), which enable the determination of molecular formulae with a high mass resolution and mass accuracy in mixtures. Today these ion-trap mass spectrometers are most frequently coupled with atmospheric pressure ionization (API) techniques such as electrospray ionization (ESI) (e.g., Fievre et al., 1997 Qian et al., 2001 Kujawinski et al., 2002 Llewelyn et al., 2002 Stenson et al., 2002,2003 Fard et al., 2003) or matrix-assisted laser desorption/ionization (MALDI) (e.g., Solouki et al.,... 

With respect to MS equipment significant performance improvements are promised by the use of most modem mass analysers (e.g. Orbitrap, FT-ICR, TOF-TOF) that provide highest resolution and mass accuracy important for, e.g. in-depth elucidation of biotransformation. 

If delayed extraction increases the mass resolution without degradation of sensitivity compared with continuous extraction, it also has limitations. Indeed, delayed extraction complicates the mass calibration procedure. It can only be optimized for part of the mass range at a time and is less effective at high mass. Delayed extraction partially decouples ion production from the flight time analysis, thus improving the pulsed beam definition. However, calibration, resolution and mass accuracy are still affected by conditions in the source. For instance, in the usual axial MALDI-TOF experiments, optimum focusing conditions depend on laser pulse width and fluence, the type of sample matrix, the sample preparation method, and even the location of the laser spot on the sample. 

The power and resolution of MS relies on certain inherent features of the MS instrumentation itself. Arguably the two most important factors are mass resolution and mass accuracy. 

In terms of mass spectrometry instrumentation, the currently available instruments such as time-of-flight (TOF) analyzers and hybrid quadrupole-TOF analyzers are able to acquire complete mass spectra at rates compatible with fast CE separations. As CE or ultrafast chromatography replaces conventional, slow HPEC applications, TOF-based mass spectrometers will be needed to replace the less efficient scanning types of instruments such as quadrupoles and ion traps for most high-throughput applications. FTICR mass spectrometry remains unsurpassed in terms of resolution and mass accuracy for both MS and MS-MS applications. However, the throughput of FTICR mass spectrometric... 

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