Chemistry, Third edition

MASS SPECTROMETER

For many purposes, the errors involved in using the mass numbers instead of the

isotopic masses may be ignored.

Table 3.2 shows that there is only one isotope of fluorine that occurs naturally.

Accordingly, for this element:

m(^197 F)m(F)

Masses of molecules

The masses of molecules may also be expressed in atomic mass units. For example,

use of the isotopic masses in Table 3.2 allows us to calculate the exact molecular

mass of^1 H^35 Cl as follows:

m(^1 H^35 Cl)(1.0078 u) (34.9689 u) 35.9767 u

Commercially available compounds usually contain atoms of isotopes of ele-

ments in the percentages of their natural abundances, i.e. such compounds are not

isotopically pure. Therefore, in calculations of the yield of a chemical reaction, it is

necessary to use the average atomic masses of each element; we cannot use the iso-

topic masses. For example, for HCl we would use m(H)1.008 u and m(Cl)

35.45 u. The average mass of the HCl molecule in nature is then

m(HCl)(1.008 u) (35.45 u) 36.46 u 36.5 u

Mass spectrometer

How a mass spectrometer works and what it is used for

The isotopic masses and abundance of isotopes in a sample of an element are found

using an instrument called a mass spectrometer(Fig. 3.4).

A mass spectrometer causes atoms to lose electrons, a process called ionization.

Loss of an electron means that the positive charge of the protons in the nucleus

is not balanced by the remaining electrons, and the atom is now positively charged

3.3

35

Detector

Mass spectrum

Electron beam

Vacuum

pumps

Sample
injected
here

Vaporizer Electrically charged plates

to accelerate ions

Ion beam

Magnet

H

L

I

Ionization shown by cross

Fig. 3.4Diagram of a mass spectrometer. This type

of spectrometer is said to be ‘magnetic scanning’

because it uses a varying magnetic field to sort the

ions according to their charge and mass. The paths

of ions of light (L), intermediate (I) and heavy (H)

mass are shown.