Molecular formula from accurate mass

MOLECULAR FORMULA FROM ACCURATE MASS VALUES... 

Data Annotation Annotation of components in a data set is a significant challenge and currently represents a major bottleneck in metabo-nomics analysis. Approaches that have been applied toward the routine annotation of components measured in a sample include (1) deriving the molecular formula from accurate mass measurements (2) use of databases to identify biologically relevant structures from molecular formulas (3) the use of libraries of LC—MS/MS fragmentation data (4) traditional structure elucidation approaches that use MS" and NMR on isolated or semi-isolated samples ... 

Basic Rules The assignment of molecular formulas from accurate mass... 

The determination of molecular formulas via accurate mass measurements relies on isotopic masses accurate to at least 1 in 10 [10]. Elemental trace analysis is required for the detection of radioactive nuclides in the environment, of transition metals such as Pt in exhaust fumes from automobiles [11], and in the quality control of low-sulfur fuels for the same. All electronic devices demand for high-purity semiconductors and the properties of alloys are critically influenced by trace elements [12]. Age determinations from isotope ratios are applied in archeology, paleontology, and geology [4,13,14]. More recently, elemental MS and biomedical MS are jointly employed to unveil the presence and preferably location of metals in proteins or DNA as well as their lateral distribution in tissues [15-18], a field of research basically going back to seminal work by Houk in 1980... 

In general terms, the main function of the magnetic/electric-sector section of the hybrid is to be able to resolve m/z values differing by only a few parts per million. Such accuracy allows highly accurate measurement of m/z values and therefore affords excellent elemental compositions of ions if these are molecular ions, the resulting compositions are in fact molecular formulae, which is the usual MS mode. Apart from accurate mass measurement, full mass spectra can also be obtained. The high-resolution separation of ions also allows ions having only small mass differences to be carefully selected for MS/MS studies. 

The unknown gave a molecular ion at m/z 193 with fragment ions at m/zs 174, 148, and 42. From the abundance of the molecular ion, it is probably aromatic, and according to the Nitrogen Rule, contains at least one nitrogen atom. From accurate mass measurement data and an examination of the isotopic abundances in the molecular ion region, the molecular formula was found to be CnH15N02. 

With the advent of accurate mass instruments, access to a reference database or library of compounds with the exact masses of precursor and product ions is becoming increasingly important. Many instrument manufacturers, as well as independent companies or institutions, are developing software that allows chemists to manage this type of information for specific applications such as proteomics or chemical contaminants. Examples of online resources that may be useful for searching for unknown residues based on molecular formulas obtained from accurate mass data inciude Metlin from the Scripps Center for Mass Spectrometry and ChemSpider from the Royal Society of Chemistry. " ... 

Calculation of molecular mass from a molecular formula for several simple substances. Accurate masses are shown in parentheses. 

By high-resolution mass spectrometry, ions of known mass from a standard substance can be separated from ions of unknown mass derived from a sample substance. By measuring the unknown mass relative to the known ones through interpolation or peak matching, the unknown can be measured. An accurate mass can be used to obtain an elemental composition for an ion. If the latter is the molecular ion, the composition is the molecular formula. 

Suppose that 10.0 g of an organic compound used as a component of mothballs is dissolved in 80.0 g of benzene. The freezing point of the solution is 1.20°C. (a) What is an approximate molar mass of the organic compound (b) An elemental analysis of that substance indicated that the empirical formula is C3H2C1. What is its molecular formula (c) Using the atomic molar masses from the periodic table, calculate a more accurate molar mass of the compound. 

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. 

A unique molecular formula (or fragment formula) can often be derived from a sufficiently accurate mass measurement alone (high-resolution mass spectrometry). This is possible because the nuclide masses are not integers (see Table 2.2). For example, we can distinguish at a unit mass of 28 among CO, N2, CH2N, and C2H4. 

Figure 5.4 Mass spectra. The monoterpenes -pinene (A) and limonene (B) both have the molecular formula C10H16 and their mass spectra are similar however, the obvious differences at m/z 68 and m/z 93, coupled with accurate and reproducible retention times from GC, enable an identification for each compound. (C) The oxide eucalyptol (1,8-cineole), with molecular formula C H, produces this characteristic pattern when analyzed by mass spectrometry. MS data supplied by Bill Morden of Analytical Intelligence Ltd.

Mass spectrometry (MS) provides the molecular weight and valuable information about the molecular formula, using a very small sample. High-resolution mass spectrometry (HRMS) can provide an accurate molecular formula, even for an impure sample. The mass spectrum also provides structural information that can confirm a structure derived from NMR and IR spectroscopy. 

Even mass spectroscopists use the terms exact mass and accurate mass interchangeably when they probably should not. An exact molecular mass is the mass calculated from the accepted exact atomic masses of the isotopes for a specified empirical formula. It is the value that one would expect to observe if one could measure a molecular mass exactly. An accurate mass is a value measured carefully, with high precision, on an instrument capable of making such precise measurements, typically expressed to at least four decimal places—the nearest 0.1 mDa. An accurate mass measurement is compared to the exact masses of empirical formulae being considered. Sufficiently accurate measurements can be used to assign empirical formulae to peaks in a mass spectrum. Knowing that these two terms are commonly used interchangeably, but that there may be subtle differences in the way practitioners use them, is sufficient for our discussions here. We will comment more on the utility of accurate mass measurements later. 

For biomarker identification, it is also possible to separate out substances of interest from a complex biofluid sample using techniques such as solid phase extraction or HPLC. For metabolite identification, directly coupled chromatography-NMR spectroscopy methods can be used. The most powerful of these hyphenated approaches is HPLC-NMR-MS [24] in which the eluting HPLC peak is split with parallel analysis by directly coupled NMR and MS techniques. This can be operated in on-flow, stopped-flow, and loop-storage modes and thus can provide the full array of NMR and MS-based molecular identification tools. These include MS-MS for identification of fragment ions and FT-MS or TOF-MS for accurate mass measurement and hence derivation of molecular empirical formulae. 

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