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 ionization 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
+
→ Li
2+ + e
In physics, the second ionization potential is the energy required to remove 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 radiation Radiation 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 radiation (ultraviolet, X-rays, gammarays). 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 result of the secondary electrons released by ionization. In biological
tissue the effect of ionizing radiation
can be very serious, usually as a consequence of the ejection of an electron 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 indicates an unpaired electron and an *
denotes an excited species.
ion-microprobe analysis A technique for analysing the surface composition of solids. The sample is
bombarded with a narrow beam (as
small as 2 µm diameter) of highenergy ions. Ions ejected from the
surface by sputtering are detected by
mass spectrometry. The technique allows 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 concentrations of speciÜc compounds,
based on the rate at which their ions
migrate through an electric Üeld. The
instrument operates in the gas phase
at atmospheric pressure. The sample
vapour enters an ionizing region,
where ions can be produced by a variety of methods. In compact instruments 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 homogeneous electric Üeld. The rate of movement 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 spectrometers are compact, sensitive, and fastacting. 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 instrument. More sophisticated instruments combine IMS with gas
chromatography or mass spectrometry. The technique is sometimes
291
ion-mobility spectrometry
i
www.AzShimi.ir www.AzShimi.com
ionization potential. Second, third,
and higher ionization potentials can
also be measured, although there is
some ambiguity in terminology.
Thus, in chemistry the second ionization 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
+
→ Li
2+ + e
In physics, the second ionization potential is the energy required to remove 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 radiation Radiation 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 radiation (ultraviolet, X-rays, gammarays). 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 result of the secondary electrons released by ionization. In biological
tissue the effect of ionizing radiation
can be very serious, usually as a consequence of the ejection of an electron 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 indicates an unpaired electron and an *
denotes an excited species.
ion-microprobe analysis A technique for analysing the surface composition of solids. The sample is
bombarded with a narrow beam (as
small as 2 µm diameter) of highenergy ions. Ions ejected from the
surface by sputtering are detected by
mass spectrometry. The technique allows 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 concentrations of speciÜc compounds,
based on the rate at which their ions
migrate through an electric Üeld. The
instrument operates in the gas phase
at atmospheric pressure. The sample
vapour enters an ionizing region,
where ions can be produced by a variety of methods. In compact instruments 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 homogeneous electric Üeld. The rate of movement 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 spectrometers are compact, sensitive, and fastacting. 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 instrument. More sophisticated instruments combine IMS with gas
chromatography or mass spectrometry. The technique is sometimes
291
ion-mobility spectrometry
i
www.AzShimi.ir www.AzShimi.com