Organic Chemistry

484 CHAPTER 13 Mass Spectrometry and Infrared Spectroscopy

13.1 Mass Spectrometry

At one time, the molecular weight of a compound was determined by its vapor den-

sity or its freezing-point depression, and molecular formulas were determined by

elemental analysis, a technique that determined the relative proportions of the ele-

ments present in the compound. These were long and tedious techniques that re-

quired relatively large amounts of a very pure sample. Today, molecular weights and

molecular formulas can be rapidly determined by mass spectrometry from a very

small amount of a sample.

In mass spectrometry, a small sample of a compound is introduced into an instru-

ment called a mass spectrometer, where it is vaporized and then ionized as a result of

an electron’s being removed from each molecule. Ionization can be accomplished in

several ways. The most common method bombards the vaporized molecules with a

beam of high-energy electrons. The energy of the electron beam can be varied, but a

beam of about 70 electron volts (eV) is commonly used. When the electron beam hits

a molecule, it knocks out an electron, producing a molecular ion, which is a radical

cation—a species with an unpaired electron and a positive charge.

Loss of an electron from a molecule weakens the molecule’s bonds. Therefore,

many of the molecular ions break apart into cations, radicals, neutral molecules, and

other radical cations. Not surprisingly, the bonds most likely to break are the weakest

ones and those that result in the formation of the most stable products. All the

positively charged fragmentsof the molecule pass between two negatively charged

plates, which accelerate the fragments into an analyzer tube (Figure 13.1). Neutral

M

+

+ e−

molecular ion electron

a radical cation

M

molecule

electron

beam

sample

electron

beam

to vacuum pump

magnet

magnet

negatively

charged

accelerating

and focusing

plates

positively

charged

repeller

plate

neutral

molecules

positively charged ions

(deflected according to m/z)

ion exit slit

collector

analyzer tube

recorder

electron

beam

filament

neutral

molecules,

neutral fragments,

and negative

ions

Figure 13.1

Schematic of a mass spectrometer. A beam of high-energy electrons causes molecules

to ionize and fragment. Positively charged fragments pass through the analyzer tube.

Changing the magnetic field strength allows the separation of fragments of varying

mass-to-charge ratio.