Spectrometer
A
spectrometer is an
optical
instrument used to measure properties of
light
over a specific portion of the
electromagnetic spectrum,
typically used in
spectroscopic analysis
to identify materials. The variable measured is most often the light's
intensity
but could also, for instance, be the
polarization
state. The independent variable is usually the
wavelength
of the light, normally expressed as some fraction of a meter, but sometimes
expressed as some unit directly proportional to the
photon
energy, such as
wavenumber
or
electron volts,
which has a reciprocal relationship to wavelength. A spectrometer is used in
spectroscopy
for producing
spectral lines
and measuring their
wavelengths
and intensities. Spectrometer is a term that is applied to instruments that
operate over a very wide range of wavelengths, from
gamma rays
and
X-rays
into the
far infrared.
In
general, any particular instrument will operate over a small portion of this
total range because of the different techniques used to measure different
portions of the spectrum. Below optical frequencies (that is, at
microwave,
radio,
and
audio
frequencies), the
spectrum analyzer
is a closely related electronic device.
Spectroscopes
Spectroscopes are used often in
astronomy
and some branches of
chemistry.
Early spectroscopes were simply a
prism
with graduations marking wavelengths of light. Modern spectroscopes, such as
monochromators,
generally use a
diffraction grating,
a movable slit, and some kind of
photodetector,
all automated and controlled by a
computer.
The spectroscope was invented by
Gustav Robert Georg Kirchhoff
and
Robert Wilhelm Bunsen.
When a
material is heated to
incandescence
it emits
light
that is characteristic of the atomic makeup of the material. Particular light
frequencies give rise to sharply defined bands on the scale which can be thought
of as fingerprints. For example, the element
sodium
has a very characteristic double yellow band known as the Sodium D-lines at
588.9950 and 589.5924 nanometers, the
colour
of which will be familiar to anyone who has seen a low pressure
sodium vapor lamp.
In the
original spectroscope design in the early 19th century, light entered a slit and
a
collimating lens
transformed the light into a thin beam of parallel rays. The light was then
passed through a prism (in hand-held spectroscopes, usually an
Amici prism)
that
refracted
the beam into a spectrum because different wavelengths were refracted different
amounts due to
dispersion.
This image was then viewed through a tube with a scale that was transposed upon
the spectral image, enabling its direct measurement.
With
the development of
photographic film,
the more accurate
spectrograph
was created. It was based on the same principle as the spectroscope, but it had
a camera in place of the viewing tube. In recent years the electronic circuits
built around the
photomultiplier
tube have replaced the camera, allowing real-time spectrographic analysis with
far greater accuracy. Arrays of
photosensors
are also used in place of film in spectrographic systems. Such spectral
analysis, or spectroscopy, has become an important scientific tool for analyzing
the composition of unknown material and for studying astronomical phenomena and
testing astronomical theories.
UV and IR spectrophotometers
The
most common spectrophotometers are used in the
UV and
visible
regions of the spectrum, and some of these instruments also operate into the
near-infrared
region as well. Spectrophotometers designed for the main infrared region are
quite different because of the technical requirements of measurement in that
region. One major factor is the type of
photosensors
that are available for different spectral regions, but infrared measurement is
also challenging because virtually
everything
emits IR
light as thermal radiation, especially at wavelengths beyond about 5 μm.
Historically, spectrophotometers use a
monochromator
to analyze the spectrum, but there are also spectrophotometers that use arrays
of photosensors
and, especially in the IR,
there are spectrophotometers that use a
Fourier transform
technique to acquire the spectral information in a technique called
Fourier Transform InfraRed.
The
spectrophotometer measures quantitatively the fraction of light that passes
through a given solution. In a spectrophotometer, a light from a lamp in a near-IR/VIS/UV
spectrophotometer (typically a deuterium gas discharge lamp) is guided through a
monochromator,
which picks light of one particular wavelength out of the continuous spectrum.
This light passes through the sample that is being measured. After the sample,
the
intensity
of the remaining light is measured with a
photodiode
or other
light sensor,
and the
transmittance
for this wavelength is then calculated.
Visible-region spectrophotometers
Visible region 400-700nm
spectrophotometry
is used extensively in
colorimetry
science. Ink manufacturers, printing companies, textiles vendors, and many more,
need the data provided through
colorimetry. They usually take
readings every 20 nanometers along the visible region, and produce a
spectral reflectance
curve. These curves can be used to test a new batch of colorant to check if it
makes a match to specifications. Traditional visual region spectrophotometers
cannot detect if a colorant has fluorescence. This can make it impossible to
manage color issues if one or more of the printing inks is fluorescent. Where a
colorant contains fluorescence, a
bi-spectral fluorescent spectrophotometer
is used. There are two major setups for visual spectrum spectrophotometers, d/8
(spherical) and 0/45. The names are due to the geometry of the light source,
observer and interior of the measurement chamber. Scientists use this machine to
measure the amount of compounds in a sample. If the compound is more
concentrated more light will be absorbed by the sample.
components: 1. The light source shines through the sample. 2. The sample absorbs
light. 3. The detector detects how much light the sample has absorbed. 4. The
detector then converts how much light the sample absorbed into a number.
Atomic absorption spectroscopy
Atomic
absorption spectroscopy
In
analytical chemistry,
Atomic absorption
spectroscopy is a technique
for determining the concentration of a particular metal
element
in a sample. Atomic absorption spectroscopy can be used to
analyse
the concentration of over 62 different metals in a solution.
Although atomic absorption spectroscopy dates to the nineteenth century, the
modern form was largely developed during the 1950s by a team of Australian
chemists. They were lead by
Alan Walsh
and worked at the
CSIRO
(Commonwealth Science and Industry Research
Organisation)
Division of Chemical Physics in Melbourne, Australia. The technique typically
makes use of a flame to atomize the sample, but other atomizers such as a
graphite furnace are also used. Three steps are involved in turning a liquid
sample into an atomic gas:
-
Desolvation
� the liquid
solvent
is
evaporated,
and the dry sample remains
-
Vaporisation
� the solid sample vaporises
to a gas
-
Volatilisation
� the compounds making up the sample are broken into free
atoms.
The
flame is arranged such that it is laterally long (usually 10cm) and not deep.
The height of the flame must also be monitored by controlling the flow of the
fuel mixture. A beam of
light
passes through this flame at its longest axis (the lateral axis) and hits a
detector.
The
light that is focused into the flame is produced by a
hollow cathode lamp.
Inside the lamp is a cylindrical metal cathode containing the metal for
excitation, and an anode. When a high voltage is applied across the anode and
cathode, the metal atoms in the cathode are excited into producing light with
a certain
emission spectra.
The type of hollow cathode tube depends on the metal being
analysed.
For analysing
the concentration of copper in an ore, a copper cathode tube would be used,
and likewise for any other metal being
analysed.
The
electrons
of the atoms in the flame can be promoted to higher
orbitals
for an instant by absorbing a set quantity of
energy
(a quantum). This amount of energy is specific to a particular electron
transition in a particular element. As the quantity of energy put into the
flame is known, and the quantity remaining at the other side (at the detector)
can be measured, it is possible to calculate how many of these transitions
took place, and thus get a signal that is proportional to the concentration of
the element being measured.
