High-resolution mass values

Calculated High Resolution Mass Values and Molecular Formulas of... 

Volume 15 of this series features four important reviews of research on alkaloids. Chapter 1 by B. S. Joshi, S. W. Pelletier and S. K. Srivastava is the first comprehensive review of the carbon-13 and proton NMR shift assignments and physical constants of diterpene alkaloids and their derivatives. In addition to the catalogue of spectral and physical data, the chapter includes a table of the occurrences of these alkaloids in various plant species, tables containing molecular formulas versus calculated high-resolution mass values, and calculated high-resolution mass values versus the molecular formulas of diterpenoid alkaloids, as well as seven tables summarizing the carbon-13 chemical shifts of various functional groups in diterpenoid alkaloids. 

A lengthy table of high resolution mass values and formulae indices has been provided by Pelletier et al. (in "Alkaloids Chemical and Biological Perspectives," Vol. 2,... 

If you are lucky, the ion with the highest mass to charge value will be the molecular ion. However, this is often not the case, as textbooks on mass spectrometry make clear. If it is possible to carry out high resolution mass spectrometry on the molecules in question, and the molecular ion is indeed observed, the exact mass can be used in combination with tables to obtain the molecular formula directly. Alternatively, you can use the internet (http //www.sisweb. com/cgi-bin/masslO.pl) to calculate and plot mass distributions for any molecular fragment you think may be present. 

The exact mass of an ion (4 to 6 decimal points) reliably defines its elemental and isotopic composition, while the method is called high resolution mass spectrometry. The measurements are conducted manually or automatically (computerized). Manual measurements are based on the parallel acquisition of the peak of interest with the closest peak of an ion with the known composition. Any compound with an intense ion peak with m/z value in the region +10% may serve as a marker. The most widespread markers are perfluorokerosene, perfluorotributylamine, and other polyfluorinated compounds. The use of these compounds is based on their volatility, as well as on the fact that fluorine is a monoisotopic element. In the spectra of these compounds intense ion peaks randomly cover all the range between m/z 19 and M+. ... 

The Most Intense Peaks. It is not so easy to extract valuable information dealing with the most intense peaks in mass spectra. In contrast to other physicochemical methods (IR, NMR, UV), registration of an ion peak of an integer m/z value does not provide an unequivocal identification of its structure or even composition. Even accurate mass measurements (high resolution mass spectrometry) defining the elemental composition of an ion does not establish its structure, as it could be formed directly from the M+, with minimal distortion of the authentic structure, or as a result of numerous rearrangement processes. 

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

Figure 5. Electron impact mass spectrum of tetraethyleneglycol-di(2-ethylhex-anoate), compound 82. The elemental compositions of m/e 127 and 171 were established by high resolution mass spectrometry the A values indicate the error (in millimass units) between measured and calculated exact masses.

High-resolution mass speedometers which can measure m/e values to four decimal places are capable of confirming the molecular formula of die molecular ions. These so-called exact mass measurements can be used because die atomic weights of die elements are not exactly whole numbers (except for 12C, which is die standard at 12.0000 amu). The exact masses of some elements and their most abundant isotopes are given in Table 11.2. To find the exact mass of a molecule, die atomic mass of the most abundant isotope for each element is used to calculate the exact mass of the compound. This is compared to die exact mass measured on a high-resolution... 

Systematic Errors in Accurate Mass Measurements. 1. Problem Definitions The value of high resolution mass spectrometry is diminished if the mass measurements do not give unambiguous elemental compositions. Accurate mass measurements in FTMS require a precise measurement of ion frequencies and an accurate calibration law for converting ions frequencies to mass. The ion frequencies can be measured to nine significant figures with modern electronics however, the relationship between ion frequencies in the cubic cell and mass still requires further development. 

With the development of high resolution mass spectroscopy the mass of the molecular fragment can be measured to seven significant figures. These very accurate relative atomic masses make it possible to distinguish molecules with very similar molecular mass values. 

PCDTs and PCTAs behave similarly to PCDDs and PCDFs. The exact value of the Ml ion of TeCDTs is 319.8788 and that of TeCDDs 319.8965. An MS response of about 20,000 in high resolution gas chromatography/high resolution mass spectrometry (HRGC/HRMS) is needed for a unique distinction between them. Separation can be achieved based on the longer retention times of the congeneric PCDTs compared to PCDDs in non-polar SE-30 and DB-5 type columns. 

High-resolution mass spectrometers measure miz ratios to four (or more) decimal places. This is valuable because except for carbon-12, whose mass is defined as 12.0000, the masses of all other nuclei are very close to—but not exactly—whole numbers. Table 13.1 lists the exact mass values of a few common nuclei. Using these values it is possible to determine the single molecular formula that gives rise to a molecular ion. 

In high resolution mass spectrometry a mixed spectrum of an internal standard, normally perfluorkerosene, and of the sample is taken. Using the known masses of the internal standard, the accurate mass values of the sample ions are calculated. Their elemental compositions can then be determined by searching for the best fit of combinations of atomic mass... 

Analyzers The mass analyzer separates the ions according to their miz values. The most common analyzers are listed in Table 31-3. The most common analyzers for GC/MS are the quadrupole mass filter and the ion trap. High-resolution mass spectrometers use the double-focusing analyzer, the ion-cyclotron resonance analyzer, or the time-of-flight analyzer. 

With the advent of NMR, new values - in the strongest sense of moral, ethical, and axiological values - came to dominate chemistry, while more traditional values were made redundant and obsolete (Campbell 1960). Take the example of elemental analyses. Before NMR came on the scene, they were the equivalent of a moral obligation. They linked laboratory notebooks to the final publication of the results. Now, elemental analyses became dispensable by the information from the new spectroscopies, high-resolution mass spectrometry (Laidler 2004) even more so than NMR. In spite of this new aspect of laboratory life, journals insisted for a long time (measured not in years but in decades) on the continued insertion of elemental analytical data in the experimental part of manuscripts submitted for publication. 

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