Instant Notes: Analytical Chemistry

Section E – Spectrometric techniques

E14 MASS SPECTROMETRY

Principles Mass spectrometry(MS) is an analytical technique in which gaseous ions formed
from the molecules or atoms of a sample are separated in space or time and
detected according to their mass-to-charge ratio, m/z. The numbers of ions of each
mass detected constitute a mass spectrum, which may be represented graphically
or tabulated as shown for methanol in Figure 1. Peak intensities are expressed as
a percentage of that of the most abundant ion (m/z31 for methanol), which is desig-
nated the base peak. The spectrum provides structural information and often an
accurate relative molecular mass from which an unknown compound can be iden-
tified or a structure confirmed. Quantitative analysis is based on measuring the
numbers of a particular ion present under closely controlled conditions.

A block diagram of a mass spectrometer is shown in Figure 2. It is operated

under a vacuum of 10-^4 to 10-^7 Nm-^2 as the presence of air would swamp the

mass spectra of samples, and damage the ion source and detector.

Mass
spectrometer

Key Notes

Mass spectrometry (MS) is a technique whereby materials are ionized

and dissociated into fragments characteristic of the molecule(s) or

element(s) present in the sample. The numbers of ions of each mass

provide information for qualitative and quantitative analysis.

A mass spectrometer, which is operated under high vacuum,

incorporates a sample inlet and ion source, a mass analyzer, an ion

detector and a data processing system.

Alternative ionization techniques are available differing in energy and

applicability. Some produce a high degree of dissociation of molecules,

while others are used primarily to establish an accurate relative

molecular mass of a compound or to facilitate elemental analysis.

After ionization, molecules may dissociate into fragments of smaller

mass, some carrying a charge. The presence and relative abundances of

the various charged fragments provide structural information and enable

unknown compounds to be identified.

These are peaks in a mass spectrum arising from fragments containing

naturally occurring heavier isotopes of one or more elements.

Spectral data is either tabulated or shown graphically as a plot of the

numbers of ions of each mass detected. For ease of interpretation, these

are presented as line diagrams.

Related topics Inductively coupled plasma Combined techniques (Section F)

spectrometry (E5)

Principles

Mass spectrometer

Ionization

techniques

Fragmentation

Isotope peaks

Mass spectra