H15
Big Instruments
Destructive analysis
The original sample is not recoverable
Non-destructive
testing
The original sample is recoverable. Needed for
 Identification of artworks
 Establishing the authenticity of historical artefacts
 To prove that the metal in jewellery is precious
Examples of instrumental analysis that can be used for small
sample sizes and non-destructive testing include:
 Mass spectroscopy
 Spectroscopy involving electromagnetic radiation
Mass spectroscopy
Form of analysis that separates and identifies substances on
the basis of the masses of the positive ion formed by the
substances when they are bombarded by high-energy particles
(usually electrons) in a high vacuum.
Has been used to:
 Show the existence of isotopes and to measure their
relative masses.
 Identify the presence of a known substance in a sample.
 Deduce considerable information about the structure of
unknown compounds.
Mass spectrum
The record of the ion current (through the detector) as a
function of the mass-to-charge ratio. Since all the ions have a
charge of +1 the mass spectrum records the current produced
by ions of different mass.
Electromagnetic
radiation
Type of wave motion that consists of oscillating electric and
magnetic fields.
The properties of electromagnetic radiation include:
 Transfer energy without transferring mass
 Transfer energy through a vacuum
 Travels with the speed of light
The forms of electromagnetic radiation differ in their
wavelength (or frequency) and include:
 Radio waves
 Microwaves
 Infrared radiation
 Visible light
 Ultraviolet light
 X-rays
 -rays
Spectroscope
Instrument in which the observations of spectra are made by
eye.
Spectrograph
Instrument in which the observations of the spectra are
recorded on photographic plates.
Spectrometer
Instrument in which the observations of the spectra are
produced as a recorder or computer graph.
Dispersion
Separating white light into its spectrum.
Monochromator
An instrument that allows particular wavelengths of light to be
selected.
Emission of light
Light is emitted from an atom when electrons that have been
promoted to a higher energy level jump back to a lower
energy level.
Emission spectra
The series of bright colour light emitted by an atom as its
electrons return from higher energy levels to lower energy
levels as the atom cools.
Atoms of different elements produce unique emission spectra
because the energy of the orbitals occupied by their electrons,
is different.
Emission spectra are used to determine the elements present in
the sample but are unable to give any information about the
nature of their compounds.
Ultraviolet-Visible
spectroscopy
A compound will absorb radiation of the wavelength that will
excite one of its valence electrons into a higher energy level.
A comparison of the intensity of the light passing through the
sample with that of a reference beam gives a measure of the
absorption by the sample at that wavelength.
The u.v.-light spectrum gives useful information about the
structure of the sample without necessarily identifying it.
Infrared spectroscopy
An i.r. spectrum is obtained by scanning through the whole
wavelength range and plotting percentage absorption against
wavelength. Infrared radiation is absorbed at particular
wavelengths that correspond to the vibrational energy
associated with the bending or stretching of particular bonds
in molecules.
The i.r. spectrum gives information about the type of bonds in
a molecule and this information can be used to help identify
the compound.
Analysis of surfaces
The surfaces of objects can be of great interest and there are a
number of techniques that can be used to determine the nature
of stains on surfaces or the composition of the first few
nanometres of an object. These techniques include
 Electron spectroscopy for chemical analysis (ESCA)
 X-ray photoelectron spectroscopy (XPS)
 Scanning tunnelling microscopy
 Atomic-force microscopy
Advantages of spectroscopic methods of analysis
 Fast and easy to perform
 Can be used on very small samples
Disadvantage
 Spectrometers can be very expensive.