Molecular Formula from Accurate Mass Values

A reference compound is needed to determine the molecular mass. The mass of an unknown is computed by comparing its signal on the mass axis with that of the known mass reference peaks. A reference compound is also required to calibrate the data system and to tune and performance-check the instrument. A calibration standard has the following desirable characteristics (1) it should yield a sufficient number of regularly spaced abundant ions across the entire scan range (2) those reference ions should have negative mass defects to prevent overlap with the nsnal compounds containing C, H, N, and O and (3) it should be readily available, chemically inert, and sufficiently volatile. 

MOLECULAR FORMULA FROM ACCURATE MASS VALUES 

Another piece of information that is critical to structure elucidation of an unknown compound is its molecular formula or elemental composition. One way to obtain this information is the accurate mass of the molecular ion. With that knowledge, the elemental composition can be assigned to compounds with molecular mass up to about 400 u. 

Fortunately, the atomic masses of isotopes are nonintegers (see Table 6.2 and Appendix C). This feature imparts a unique value to the molecular mass of each compound or each formula. A nominal mass may have several combinations of elemental compositions, but the accurate mass can match one composition only. For example, a few of the molecular formulas that can be assigned to a 70-u 

Compound Structure Ionization Mode Mass Range (Da) 

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MOLECULAR FORMULA FROM ACCURATE MASS VALUES... 

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. 

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. 

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. 

In addition we include derivatives that are naturally occurring or have been synthesized. Finally, plant sources (genera, species, family, plant part) for the compound are given. The molecular formula and molecular weight of the alkaloid are given alongside the structure. When an accurate mass from high resolution MS exists it is noted in parentheses. The found, not the calculated, value is presented. 

So far only the magnetic sector has been considered the purpose of the electrostatic sector is to achieve high resolution. Many molecules share the same molecular weight. A spectrum in which all of the masses are accurate to one atomic mass unit (nominal mass) can therefore be very ambiguous, but more specificity can be obtained if the resolution of the instrument can be increased, so that species with the same nominal mass may be resolved on the basis of their exact mass. In addition, accurate mass measurement can be used in conjunction with accurate values of isotopic masses to assign molecular formulae. For example, in a FAB spectrum acquired from a matrix containing caesium iodide (used as an internal standard for mass calibration) a nominal mass peak at m/e = 404 could correspond to either of the following structures ... 

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