Accuracy mass determination

The Use of High-Accuracy Mass Measurements in Combination with LC-MS for the Structure Determination of Drug Metabolites... 

Different isotopes differ in their atomic masses. The intensities of the signals from different isotopic ions allow isotopic abundances to be determined with high accuracy. Mass spectrometry reveals that the isotopic abundances in elemental samples from different sources have slightly different values. Isotopic ratios vary because isotopes with different masses have slightly different properties for example, they move at slightly different speeds. These differences have tiny effects at the level of parts per ten thousand (0.0001). The effects are too small to appear as variations In the elemental molar masses. Nevertheless, high-precision mass spectrometry can measure relative abundances of isotopes to around 1 part in 100,000. 

Principles and Characteristics Mass spectrometry can provide the accurate mass determination in a direct measurement mode. For a properly calibrated mass spectrometer the mass accuracy should be expected to be good to at least 0.1 Da. Accurate mass measurements can be made at any resolution (resolution matters only when separating masses). For polymer/additive deformulation the nominal molecular weight of an analyte, as determined with an accuracy of 0.1 Da from the mass spectrum, is generally insufficient to characterise the sample, in view of the small mass differences in commercial additives. With the thousands of additives, it is obvious that the same nominal mass often corresponds to quite a number of possible additive types, e.g. NPG dibenzoate, Tinuvin 312, Uvistat 247, Flexricin P-1, isobutylpalmitate and fumaric acid for m = 312 Da see also Table 6.7 for m = 268 Da. Accurate mass measurements are most often made in El mode, since the sensitivity is high, and reference mass peaks are readily available (using various fluorinated reference materials). Accurate mass measurements can also be made in Cl... 

Li, Y. Mclver, R. T., Jr. Hunter, R. L. High-accuracy molecular mass determination for peptides and proteins by Fourier transform mass spectrometry. Anal. Chem. 1994, 66, 2077-2083. 

The fourth method, astronomical measurement, is (like the Doppler method) based on the observation and measurement of tiny motions of a star which are due to the mass(es) of the orbiting planet(s). A primary goal of the Space Interferometry Mission (SIM) is the discovery of planets similar to Earth, which orbit around stars like the sun. The SIM is due to start in 2009, and the measurements carried out will increase the accuracy of determinations of distances of stars in our galaxy several hundred times. Exact information can be found in the catalogue which forms part... 

MS equipment is evaluated on several performance metrics. Mass accuracy, mass resolution, and mass range are standard parameters frequently assessed to determine the suitability of an instrument. Mass accuracy is defined as the extent to which a mass analyzer reflects true m/z values and is measured in atomic mass units (amu), parts per million (ppm), or percent accuracy. 

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-... 

This important class of mass spectrometers can measure the mass of an ion with sufficient accuracy to determine its atomic composition. 

Masses computed with Equation 22-8 appear in the last column of Table 22-3. A limitation on the accuracy of molecular mass determination is the accuracy of the mlz scale of the mass spectrum. Proteins of known mass can be used to calibrate the instrument. 

Chaif, B. T., and S. B. H. Kent, Weighing naked proteins Practical, high-accuracy mass measurement of peptides and proteins. Science 257 1885-1893, 1992. Proteins with molecular masses of as much as 100 kd or more can be analyzed at picomole sensitivities to give simple mass spectra corresponding to the intact molecule. This development has allowed unprecedented accuracy in the determination of protein molecular weights. 

In view of experimental simplicity and accuracy, viscosity measurements are extremely useful for routine relative molecular mass determinations on a particular polymer-solvent system. K and a for the system are determined by measuring the intrinsic viscosities of polymer fractions for which the relative molecular masses have been determined independently - e.g. by osmotic pressure, sedimentation or light scattering. 

The most widely used mass spectrometric identification procedure is MALDI-Tof analysis of the entire peptide mixture. Gas-phase matrix interaction with peptide ions in MALDI-Tof results in singly charged ions, giving a mass profile that is highly characteristic of the protein from which the peptides are derived. These peptide masses (actually protonated peptide molecular ions, MH+) can be used to search databases (either protein or nucleic acid databases) to identify the proteins. The two most important factors in successfully identifying proteins by this approach are the number of matching peptide masses and the accuracy of the peptide mass determination. 

However, in the great majority of cases the accuracy of determining the position of this curve on the phase diagrams is far from being satisfactory. The data obtained by various authors may differ very considerably, especially in the case of low solubility values (< 1 mass %). Meanwhile, to... 

There are two important categories of mass spectrometers low (unit) resolution and high resolution. Low-resolution instruments can be defined arbitrarily as the instruments that separate unit masses up to mlz 3000 [R = 3000/(3000 - 2999) = 3000]. A high-resolution instrument (e.g., R - 20,000) can distinguish between C16H2602 and C15H24N02 [R = 250.1933/(250.1933 -250.1807) = 19857]. This important class of mass spectrometers, which can have R as large as 100,000, can measure the mass of an ion with sufficient accuracy to determine its atomic composition (molecular formula). 

The difference between the average mass, the nominal mass and the monoisotopic mass can amount to several Da, depending on the number of atoms and their isotopic composition. The type of mass determined by mass spectrometry depends largely on the resolution and accuracy of the analyser. Let us consider CH3C1 as an example. Actually, chlorine atoms are mixtures of two isotopes, whose exact masses are respectively 34.968 852 u... 

Dashed lines experimental peak. Some problems alter the accuracy of the mass determination at the centroid. Actually, the experimental peak may contain several of these alterations. Furthermore, peaks with low noise on the signal are used here. 

Table 8.6 lists the mass differences due to the substitutions of a given amino acid by another in the sequence of a peptide chain [101]. The accuracy of the molecular mass determination is critical for the success of the mutation characterization. The required accuracy depends not only on the determined molecular mass, but also on the detected substitution. Indeed, the characterization of Gln/Lys substitution needs less accurate mass measurement with a 1 kDa peptide than with a 40kDa protein. In the same way, the characterization of Gly/Trp substitution is easier than the characterization of Gln/Lys substitution. 

From the time of the second edition published in 2001 until now, much progress has been achieved. Several techniques have been improved, others have almost disappeared. New atmospheric pressure desorption ionization sources have been discovered and made available commercially. One completely new instrument, the orbitrap, based on a new mass analyser, has been developed and is now also available commercially. Improved accuracy in low-mass determination, even at low resolution, improvements in sensitivity, better detection limits and more efficient tandem mass spectrometry even on high-molecular-mass compounds are some of the main achievements. We have done our best to include them is this new edition. 

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