Dictionary of Chemistry [6th Ed.]

strongly bound electron is the Ürst
ionization potential. Second, third,
and higher ionization potentials can
also be measured, although there is
some ambiguity in terminology.
Thus, in chemistry the second ioniza-
tion potential is often taken to be the
minimum energy required to remove
an electron from the singly charged
ion; the second IP of lithium would
be the energy for the process

Li+→Li2++ e

In physics, the second ionization po-
tential is the energy required to re-
move an electron from the next to
highest energy level in the neutral
atom or molecule; e.g.

Li →Li++ e,

where Li+is an excited singly
charged ion produced by removing
an electron from the K-shell.

A

• A table of values from NIST

ionizing radiationRadiation of
sufÜciently high energy to cause
*ionization in the medium through
which it passes. It may consist of a
stream of high-energy particles (e.g.
electrons, protons, alpha-particles) or
short-wavelength electromagnetic ra-
diation (ultraviolet, X-rays, gamma-
rays). This type of radiation can cause
extensive damage to the molecular
structure of a substance either as a
result of the direct transfer of energy
to its atoms or molecules or as a re-
sult of the secondary electrons re-
leased by ionization. In biological
tissue the effect of ionizing radiation
can be very serious, usually as a con-
sequence of the ejection of an elec-
tron from a water molecule and the
oxidizing or reducing effects of the
resulting highly reactive species:

2H 2 O →e–+ H 2 O + H 2 O*

H 2 O*→.OH + .H

H 2 O++ H 2 O →.OH + H 3 O+

where the dot before a radical indi-

cates an unpaired electron and an *

denotes an excited species.

ion-microprobe analysis A tech-

nique for analysing the surface com-

position of solids. The sample is

bombarded with a narrow beam (as

small as 2 μm diameter) of high-

energy ions. Ions ejected from the

surface by sputtering are detected by

mass spectrometry. The technique al-

lows quantitative analysis of both

chemical and isotopic composition

for concentrations as low as a few

parts per million.

ion-mobility spectrometry (IMS)

A technique for detecting low con-

centrations of speciÜc compounds,

based on the rate at which their ions

migrate through an electricÜeld. The

instrument operates in the gas phase

at atmospheric pressure. The sample

vapour enters an ionizing region,

where ions can be produced by a va-

riety of methods. In compact instru-

ments the source is usually a small

amount of radioactive material. The

ions are allowed in pulses into a drift

tube, where they move to a detector

under the inÛuence of a homoge-

neous electricÜeld. The rate of move-

ment depends on the way the ions

interact with neutral molecules in

the tube and this depends on the

ion’s size and shape. The spectrum is

a plot of detector signal against time,

and is characteristic of the sample

being ionized. Ion-mobility spectrom-

eters are compact, sensitive, and fast-

acting. They are widely used in

screening for drugs and explosives at

airports, border crossings, etc. often

the technique is to wipe a swab over

luggage and place it in the instru-

ment. More sophisticated instru-

ments combine IMS with gas

chromatography or mass spectrome-

try. The technique is sometimes

291 ion-mobility spectrometry

i