Highest mass

Tabulate the prominent ion peaks, starting with the highest mass. 

The molecular ion typically is not observed but can be deduced by adding 31 to the highest-mass peak observed (even in the case of methyl cyanoacetate). An intense ion in the mass spectra of methyl esters is m/z 74, formed in a McLafferty rearrangement. Also present is m/z 87, but it is of low intensity. Characteristic losses from the molecular ion are 31, 40 (small), 59, and 73 Daltons. 

The molecular ion is usually not observed, but may be deduced by adding 35 Daltons to the highest-mass ion observed. In the mass... 

The best way to identify the MWs of unknown glycols and glycol ethers is to examine the mass spectra of the TMS derivatives. The TMS derivatives are identified in the chromatogram by plotting masses 73 and 147. The higher of the two high-mass peaks, which are 15 Daltons apart, is the molecular ion of the TMS derivative. If no peaks are separated by 15 Daltons, add 15 to the highest mass peak observed to deduce the MW. 

Molecular weight In general, to deduce the molecular weight of the TMS derivative of sugars, add to 105 the highest mass observed. 

The molecular ion must contain the highest number of atoms of each element present. For example, if a lower-mass ion contains four chlorine atoms, but the highest-mass ion observed contains only three, then at least 35 Daltons should be added to the highest-mass ion observed to deduce the molecular ion. 

The highest mass peaks observed in the mass spectra of alkyl chlorides may correspond to the loss of HX or X (loss of HI is seldom observed), depending on the structure of the molecule. In order to deduce the molecular ion, add the mass of X or HX to the mass at which the highest mass peak is readily observed. (Note that higher mass ions having the isotope pattern of X may be present... 

Compounds such as CKF17CH2CH3 may not show a molecular ion but M—HF as the highest mass ion. 

In proceeding to a consideration of the chemical ionization mass spectra of more highly branched paraffins, it will be most convenient to consider separately the several different classes of alkyl ions found in the spectra—i.e., MW — 1+, MW — 15+, MW — 29 +, etc. We can see from Table II that a considerable amount of variation in the relative intensity of the MW — 1 ions (always the highest mass ion for which an intensity is given in the table) occurs. However, we shall show that the observed MW — 1 intensities can be approximately accounted for in terms of the concept of localized electrophilic attack by the reactant ion. First, however, we must consider the energetics of two processes which may be important in generating the spectra of branched paraffins. One of these is the abstraction of a primary hydrogen by the reactant ion. As a typical example we may write... 

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. 

Unknown 5. Can the ion of the highest mass be the molecular ion if the following series of fragment ions is detected in the high m/z region of the spectrum ... 

It is often neglected that the first step of de novo sequencing is data acquisition. The quality of the spectrum or spectra used for sequencing is the most critical parameter of the entire procedure. First of all, the mass spectrometer should be well calibrated and tuned. If it can operate in different modes, the one with the highest possible mass accuracy and resolution should be applied. If the experimenter has more spectrometers to choose from, the one with the highest mass accuracy and resolution should be used, provided it shows good fragmentation efficiency. 

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