Measured accurate mass

Even for large molecules, the ability to measure accurate mass means that elemental compositions can be obtained from the accurately measured molecular mass. 

The results for bacterial whole-cell analysis described here establish the utility of MALDI-FTMS for mass spectral analysis of whole-cell bacteria and (potentially) more complex single-celled organisms. The use of MALDI-measured accurate mass values combined with mass defect plots is rapid, accurate, and simpler in sample preparation then conventional liquid chromatographic methods for bacterial lipid analysis. Intact cell MALDI-FTMS bacterial lipid characterization complements the use of proteomics profiling by mass spectrometry because it relies on accurate mass measurements of chemical species that are not subject to posttranslational modification or proteolytic degradation. 

The mass accuracy is defined as the difference between measured accurate mass and calculated exact mass. The mass accuracy can be stated as absolute units of u (or mmu) or as relative mass accuracy in ppm, i.e., absolute mass accuracy divided by the mass it is determined for. As mass spectrometers tend to have similar absolute mass accuracies over a comparatively wide range, absolute mass accuracy represents a more meaningful way of stating mass accuracies than the more trendy use of ppm. 

It has been stated that measured accurate masses when used to assign molecular formulae should always be accompanied by their mass accuracies. [34] Ideally, this can be done by giving the mean mass value and the corresponding error in terms of standard deviation as obtained from several repeated measurements of the same ion. [35] This is definitely not identical to the error which is usually provided with the listing from mass spectrometer data systems, where an error is given as the difference of calculated and measured mass value. 

Example Ethyl loss clearly predominates methyl loss in the El mass spectrum of 2-(l-methylpropyl)-phenol. It proceeds via benzylic bond cleavage, the products of which are detected as the base peak at m/z 121 and m/z 135 (3 %), respectively (Eig. 6.34a). The McLafferty rearrangement does not play a role, as the peak at m/z 122 (8.8 %) is completely due to the isotopic contribution to the peak at m/z 121. From the HR-El spectrum (Fig. 6.34b) the alternative pathway for the formation of a [M-29] peak, i.e., [M-CO-H]", can be excluded, because the measured accurate mass of this singlet peak indicates CgHgO". HR-MS data also reveal that the peak at m/z 107 corresponds to [M-CHs-CO]" and that the one at m/z 103 corresponds to [M-C2H5-H20]. Although perhaps unexpected, the loss of H2O from phenolic fragment ions is not unusual. 

All TOF mass spectrometers measure accurate mass by reference to standard substances, otherwise known as lock mass. This is an internal reference which is introduced via the ion source by means of a second sprayer system (Fig. 4.11) or coinfused with the sample using a single sprayer (Williams et al., 2006). However, the latter method is not preferred due to dilution of analyte with reference compound and issues of suppression of ionization (Herniman et al., 2004). The reference compound... 

Fig. 5.24. Accurate mass result. Measured accurate mass 488.2654 fits tor A C2nH.i,N50i to 1.6 ppm. Measured accurate mass 489.2664 fits lor A C "C HraNsOi to 6 ppm. Measured accurate mass 490.2774 fits for B CssHr D Ns()i to 2.7 ppm. Measured accurate mass 49I.28(K) fits tor B -C CH, D2K5O3 to 4.2 ppm. (Reprinted with permission from 40. )...

Mass accuracy is the difference (AAT) between the measured accurate mass M and the calculated exact mass. It can be stated as absolute units of mass (differences of so many millimass units, mmu, 10-3 u) or as a relative mass accuracy in parts per million. 

Nielen et al. (2001) focused on a related facet of the illegal use of growth promoters in cattle, namely the detection of anabolic steroids in illegal cocktails. They presented a Q-TOF LC-MS-MS method with the aim of measuring accurate mass and calculating elemental composition for identification purposes. 

Accuracy of mass measurement Accurate mass measurements ... 

After the molecular weight has been deduced, the elemental composition of the molecular ion can be determined. The isotope distribution of peaks of the molecular ion may provide a thorough preliminary evaluation of the quantity and type of metal atoms. This analysis is based on the fact that most metal atoms have specific isotope distributions. The next step towards the determination of the elemental composition is to measure accurate mass(es) of the molecular ion(s). These measurements require a mass analyzer with appropriate mass resolution. Sector instruments, TOF mass spectrometers, and ICR spectrometers are the most suitable for accurate mass measurement. 

With the latest ToF-SIMS instruments mass resolutions of 10 to 10 amu are achievable with sampling depths of greater than 1 nm. This has two main advantages. Firstly, the sensitivity is increased, allowing peaks to be resolved where previously there had been overlap (i.e., two peaks at nominally the same mass). This now enables ToF-SIMS to be sensitive to ppm/ppb levels of surface species. Secondly, high mass resolution enables very accurate mass measurement so that empirical formulae can be calculated from the measured accurate mass, making unknown peak identification more certain. 

The absolute mass accuracy, Am/z, is defined as the difference between measured accurate mass and calculated exact mass ... 

Although they are related, the measured accurate mass should be distinguished from the calculated exact mass, which is the mass determined by summing the exact isotope masses of the elements present in a particular ion. For example, the calculated exact mass of (acetone molecular ion) is 58.0419 Da, and... 

The deviation of the measured accurate mass from the calculated exact mass of an ion. It can be expressed as an absolute value in milliDaltons (mDa) or as a relative value in parts-per-million (ppm) error. The absolute accuracy is calculated as accurate mass - exact mass. 

The capacity for high resolution enables double-focusing magnetic-sector mass spectrometers to be used for accurate mass measurement. Accurate mass is always determined by reference to one or more known masses simultaneously introduced into the mass spectrometer, and there are two different methods in common use. 

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