Composition of a Ternary Compound - Part 3, Mass Spectrometry and Molecular Formula Determination

Greetings,

In this post the basics of mass spectrometry are discussed.  Also, the empirical formula obtained in the last post is used as an example of molecular formula determination from molar mass data.  A mass spectrometry analysis of the hypothetical compound would provide information about the molar mass.  A simple calculation gives a multiplication factor which is then used to get the molecular formula.

The Basics of Mass Spectrometry

Mass spectrometry (MS) is an analytical instrument used to help characterize (confirm identity of) and quantitate (determine amount of) a compound or group of compounds.  Basically, a small sample containing the compound of interest is introduced into a "preparation" chamber (usually by injecting a solution of the sample) where it is "treated" by bombardment with a high energy electron beam (commonly, but other methods exist).  The "treated" compound is immediately broken into charged pieces (aka, ionized).  The pieces then move rapidly through a special chamber where they move at different speeds (Time of Flight - TOF-MS) or are separated in a magnetic field (e.g., Quadrupole MS).  Depending on which kind of MS chamber is used, the compound parts strike a detector at different times (TOF-MS) or different places (Quad.-MS). The detector determines the amu(atomic mass unit)-mass and number of each particular sized compound piece and sends this data to a computer for processing.  A computer application summarizes this data and displays a mass spectrum of the compound.  A mass spectrum is a statistical type of plot with relative abundance on the y-axis and amu-mass (It's actually mass-to-charge ratio, so I'm simplifying a bit.) on the x-axis.

 

Determination of The Molecular Formula From The Empirical Formula and Molar Mass

The empirical formula is converted to the molecular formula by first determining the value of a ratio ('n'), as shown below.  The 'parent molecular peak' in the mass spectrum is the line produced by intact (i.e., unbroken) compound molecules.  The figure below also shows how the molecular formula is obtained.

 

 

The empirical formula from the last post is 'C2H4O', resulting in an empirical formula mass of 44.06 g/mol.  Let's say the MS analysis of our compound produces a molar mass result of approx. 88 g/mol.  Then the value of our 'n' is '2' and our molecular formula is determined to be 'C4H8O2'!

 

That's all for now.  Thank you for reading!