Mass Spectrometry

In the example above involving benzamide (C₇H₇NO), the molecular ion (M+) has a mass-to-charge ratio (m/z) of 121. This value is calculated using the most abundant isotopes of the elements present in the molecule:

 

7	*	12C	=	84
7	*	1H	=	7
1	*	14N	=	14
1	*	16O	=	16
				121

This calculation gives the unit mass molecular ion. Modern high-resolution mass spectrometers are capable of determining very precise and accurate molecular weights for substances. Whereas typical chemical methods (vapor density, cryoscopy, vapor pressure osmometry, and neutralization equivalent) are accurate to within 0.1 to 1% (two or three significant figures), certain mass spectrometers have an accuracy of 0.0005% or better. What does this mean? The accurate mass can yield the chemical formula for a substance directly. To illustrate, a molecule with a nominal molecular weight of 60 could have any of the following chemical formulas: C₃H₈O, C₂H₈N₂, C₂H₄O₂, or CH₄N₂O. These compounds would have accurate masses as follows:

If the accurate mass of the M+ were determined to be 60.02112, then the only possible chemical formula for the substance would be C₂H₄O₂.

These accurate masses can be calculated using the accurate masses of the elements given in Table 2 below. In the table, the column “Atomic Weight” contains the chemical atomic weight of the elements, which is an average of the weights of the naturally occurring isotopes (weighted by their abundance on earth).

Returning to the example of benzamide, the accurate masses for the three most likely formulas ascertained from the molecular ion are: 121.08923 (C₈H₁₁N), 121.06411 (C₆H₇N₃), and 121.05281 (C₇H₇NO). Selection of the correct formula is straightforward if the accurate mass is determined by high resolution mass spectrometry.