Abundant base peak

Table II. Relative Abundances (% Base Peak) of Some Fragment Ions in the 70 eV Mass Spectra of Bisimides ...

The mass spectrum of benzene is relatively simple and illustrates some of the mfor matron that mass spectrometry provides The most intense peak m the mass spectrum is called the base peak and is assigned a relative intensity of 100 Ion abundances are pro portional to peak intensities and are reported as intensities relative to the base peak The base peak m the mass spectrum of benzene corresponds to the molecular ion (M" ) at miz = 78... 

The formation of a simple El mass spectrum from a number (p) of molecules (M) interacting with electrons (ep. Peak 1 represents M , the molecular ion, the ion of greatest mass (abundance q). Peaks 2, 3 represent A+, B. two fragment ions (abundances r, s). Peak 2 is also the largest and, therefore, the base peak. 

The ion having greatest abundance is said to form the base peak in the spectrum. The base peak may or may not be the same as the molecular ion peak. 

Substituent effects are in some instances predictable and/or easily rationalized, but are more subtle in other cases. iV-Oxides generally show an abundant M—16 peak which is sometimes the base peak, and di-iV-oxides show successive elimination of two atoms of... 

The mass spectrum of a compound is typically presented as a bar graph with masses (m/z values) on the x axis and intensity, or relative abundance of ions of a given m/z striking the detector, on the y axis. The tallest peak, assigned an intensity of 100%, is called the base peak, and the peak that corresponds to the unfragmented cation radical is called the parent peak or the molecular ion (M+). Figure 12.2 shows the mass spectrum of propane. 

Base peak Four next most abundant peaks Compound HighesL in/ peak > l r ... 

Base peak Four next mosi abundant puaks Compound Highest rn/z peak > 1 % ... 

Similarly, a common feature in the mass spectrum of thiirene oxides is the high abundance of the substituted acetylene ion (e.g. [PhC CPh]7) formed by elimination of sulfur monoxide. In fact, this ion constitutes the base peak in the spectrum of 18a whereas the molecular ion has a rather insignificant intensity (0.25% I of M+)91. 

As already stated, the mass spectrum is a two-dimensional graph that reports the m/z ratio of ions (abscissa) and their relative intensity (ordinate). The most abundant ions are assigned as 100%. A mass spectrum can be displayed as peak profiles or as bar graphs corresponding to the peak centroids, i.e. the weighted centre of mass of the peak, or as a table (Figure 2.16). The most abundant ions in a mass spectrum constitute the base peak whose intensity is assumed equal to 100%. 

In the graphic form the abscissa represents the mass of ions (to be more precise, the mass-to-charge ratio, m/z), while the ordinate represents the relative intensity of these ions peaks. Atomic mass units (unified atomic mass unit) or daltons are used as units to measure masses of ions, while intensity is represented in percent relative to the base peak in the spectmm or to the total abundance of all the ions in the spectra. The atomic mass unit (dalton) is equal to the mass of one-twelvth of the mass of a 12C atom (1,661 x 10-27 g) (see Chapter 1). 

Imported data can be sorted so that GC peaks are grouped according to retention time. The user defines the allowable retention time difference, if any, for two peaks to be considered as a match. (We normally use +/ - 0.05 min.) MS data are compared with respect to the number of similar ions, the base peak, and the apparent molecular ion. The ten most abundant ions in each mass spectrum are listed in descending order of abundance from left to right. The user defines how strictly the order of the ten ions must agree among spectra under comparison in order to be deemed a match. An example of the final output is shown in Table 2.2. 

Fig. 11.3. Electron ionization and methane Cl mass spectra of toluene. The key features of the respective mass spectra are labeled. Spectral interpretation is based on recognition and understanding of these key features and how they correlate with structural elements of the analyte molecule of interest. The signal representing the most abundant ion in a mass spectrum is referred to as the base peak, and may or may not be the molecular ion peak (which carries the molecular mass information). Cl spectra provide confirmation of molecular mass in situations where the El signal for the molecular ion (M+ ) is weak or absent. The Cl mass spectrum provides reliable molecular mass information, but relatively little structural information (low abundance of the fragment ions). Compare with Fig. 11.4.

A mass spectrum is a plot of ion abundance against if ratio The highest peak in I the spectrum is called the base peak. 

The electron impact mass spectra of series of 2-aryl-3-nitro-2//-chromenes and 2-aryl-3-nitrochromanes with varying functionalities have been reported101. The molecular ion is always of considerable intensity. Loss of NO2 from M+ is responsible for the base peak in the nitrochromenes101. In contrast, the [M — HONO]+ is apparently the more abundant ion in the nitrochromanes. This unusual loss of HONO is always observed in the metastable time-frame101. 

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