Chemistry, Third edition

MASS SPECTROMETER

powerful pumps which remove unionized atoms, and which ensure that there are

very few molecules of air present which might collide with the ion beam.

4.The beam of ions would continue to travel in a straight line but, because a

powerful electromagnet surrounds the chamber, the ions are bent downwards

(Fig. 3.6). The degree of bend experienced by an ion depends upon its m/evalue

and upon the strength of the magnetic field. For ions of a particular m/eratio,

there will be only onevalue of magnetic field strength which will bend those

ions so that they strike the detector.

5.By slowly adjusting the strength of a magnetic field (a procedure called scan-

ning), ions of a particular m/eratio are brought down to the ion detector one by

one. The size of the electronic signal produced by the detector increases with the

number of ions arriving per second. Since the atoms of all the neon isotopes are

equally easy to ionize, the signal at each m/evalue is proportional to the number

of atoms of each isotope present in the original injected sample. A plot of signal

intensity against m/e(called a mass spectrum), shows the relative abundance of

isotopes for that element (Fig. 3.7).

Application of mass spectrometry to molecules

If molecules are injected into a mass spectrometer, the molecules become ionized.

For hydrogen chloride (^1 H^35 Cl):

(^1) H (^35) Cle( (^1) H (^35) Cl)ee
m/e 36
The ion obtained by ionizing the starting (i.e. parent) molecule is called the parent
ion. Here, the parent ion is (^1 H^35 Cl).
However, it often happens that the molecular ions produced in mass spectrometers
have so much energy that they break up (i.e. fragment) as well as ionize. For example,
(^1 H^35 Cl)^1 H^35 Cl
m/e 36 m/e 35
As HCl gas consists of both^1 H^35 Cl and^1 H^37 Cl molecules its mass spectrum will
include peaks at m/e36 (due to (^1 H^35 Cl)), at 38 ((^1 H^37 Cl)), at 37 (^37 Cl) and
35 (^35 Cl). Peaks derived from species containing^35 Cl, the most abundant chlorine
37
Fig. 3.7Mass spectrum of
neon.
No magnet
Heavy ions
Light ions
(same magnet)
Fig. 3.6A straight beam of
ions (· · · ·) is deflected by a
magnet. For the same
magnetic field strength, lighter
ions (small m) bend from their
path more than heavier ones.
more about the use
of mass
spectrometry in the
identification of
original molecules
may be found on
the website,
Appendix 3.
Calculation of the atomic mass of neon from its mass spectrum
Use Fig. 3.7 to estimate the atomic mass of the element neon.
Exercise 3F
Mass spectra
(i)iiCalculate the m/evalues of the following ions: (^12 C^16 O 2 ), (^12 C^1 H 3 )and (^14 N 2 )^2 .
(ii)iThe mass spectrum
of HF contains peaks at m/e20 and 19; explain this.
(iii)The mass spectrum of the organic compounds benzene (C 6 H 6 ), nitrobenzene (C 6 H 5 NO 2 )
and phenol (C 6 H 5 OH) all contain an intense peak at m/e77 – why?
Exercise 3G