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1. Instruments for Optical
Spectrometry
• Optical spectroscopy encompasses regions
of the electromagnetic spectrum that are not
strictly within the visible range. However,
the components of instruments used in the
IR, visible, and UV are all very similar.

2. Instrument Components
• Spectroscopic instruments are usually constructed
from the following components:
• A stable source of radiant energy
• A wavelength selector that permits the selection and
isolation of a restricted wavelength region
• One or more sample containers
• A radiation detector or transducer that converts
radiant energy to a measurable signal – normally
electrical
• A signal processor and readout

3. Instrument Components
• In emission instruments the source and sample are
combined so that emission occurs from heating or
electrical excitation of the sample.
• In absorption and fluorescence instruments the
analyte is held in a sample container and an
external radiant source is employed.
• In absorption instruments the source, sample, and
detector are located along an axis. In fluorescence
instruments the source is placed at an angle of 90°
to the detector-sample axis.

4. Optical Materials
• The optical components of a spectrometer must be
optically transparent in the wavelength region
being employed for analysis.
• In IR studies, sodium chloride, potassium
chloride, or silver chloride are used.
• In the visible region, borosilicate glass or plastics
such as polystyrene are adequate.
• In the UV, quartz is used because glass absorbs
too much radiation at these wavelengths.

5. Spectroscopic Sources
• The source must provide a beam of sufficient intensity
in the wavelength region of interest to allow detection
and measurement. It must be stable and not flicker so
P and Po can be measured accurately. Some
instruments measure these values simultaneously so
stability is less of a factor.
• Both line sources and continuous sources are used in
optical spectroscopy. The continuous source is made up
of a whole range of emitting wavelengths that overlap
while line sources emit discrete bands at one or two
wavelengths.

6. Tungsten Filament Lamps
• Theses emit in the range 320 – 2500 nm so most
of the emissions are in the IR.
• The lamp is useful for visible spectroscopy but not
for UV because the lamp housing absorbs strongly
at these wavelengths.
• The lamp should have a stable voltage supply so
that its spectral characteristics remain constant – it
tends to vary as its temperature changes.

7. Tungsten Halogen Lamps
• Tungsten halogen lamps are an improvement over
tungsten filament lamps because they have a
longer lifetime and their emission spectra extend
into the UV.
• The lamp contains a small quantity of iodine
within a quartz envelope that houses the filament.
• The lifetime of the tungsten halogen lamp is more
than double that of the tungsten filament lamp.
• In the tungsten filament lamp, the filament
eventually sublimes.

8. Tungsten Halogen Lamps
• In a tungsten iodine lamp the tungsten
sublimes but reacts with the iodine to give
WI2, which diffuses back to the filament
where it decomposes and re-deposits
tungsten.

9. Hydrogen and Deuterium Lamps
• These lamps generate true continua. The process
for hydrogen may be shown as:
• H2 + E → H2* → H' + H'' + hν where E is the
energy required to excite the hydrogen molecule.
• The products, H' and H'' may have a variety of
kinetic energies, which means that the photon
emitted, hν, will also vary in energy – thus the
continuum.

10. Hydrogen and Deuterium Lamps
• The useful range of these lamps is from 160 – 375
nm. Deuterium is preferred to hydrogen because
of its greater intensity.
• The emission is stimulated by generating an
electrical arc in an envelope filled with the gas. A
regulated power supply is necessary for constant
lamp intensity.

11. Wavelength Selectors
• Spectroscopic instruments need a wavelength
selector to provide a range of wavelengths to
impinge on the sample. The narrower the range of
wavelengths chosen by the wavelength selector,
the more likely Beer’s law is to be followed. This
also leads to improved sensitivity and selectivity.
• The closest type of radiation to a purely
monochromatic source is a laser.
• With normal optical instruments the wavelength
selector provides a range of wavelengths centred
at a nominal wavelength.

12. Wavelength Selectors
• The range of wavelengths provided by the
wavelength selector is known as the
effective bandwidth or just bandwidth and
is defined as the width of the band in
wavelength units at half peak height. The
ordinate in this plot is transmittance so the
bandwidth is the range for 0.5T.

13. Wavelength Selectors
• Bandwidths vary enormously from one
wavelength selector to another. For
instance, a high quality monochromator for
the visible region may have a bandwidth of
a few tenths of a nanometre or less. An
absorption filter operating in the same
region may have a bandwidth of 200 nm or
more.

14. Wavelength Selectors
• Generally two types of wavelength selector
are used, filters or monochromators.
• Filters are rugged, inexpensive, and easy to
maintain.
• Monochromators are costly but allow
nominal wavelengths to be changed
continuously during irradiation of the
sample.

15. Radiation Filters
• A filter allows only a restricted band of
wavelengths to pass through it.
• Two types of filter are used in spectroscopic
instrumentation, interference filters and
absorption filters.
• Interference filters allow a greater
transmittance of radiation at the nominal
wavelength than absorption filters.

16. Monochromators
• Two types of monochromators have been used to
disperse multiwavelength radiation into its various
components. These are prisms and diffraction
gratings.
• Prisms are used infrequently nowadays because
gratings can be made more cheaply. Gratings
scratched on the surface of a mirror lead to more
compact instrumentation.

17. Monochromators
• In a typical reflective grating
monochromator, radiation from a source
enters the monochromator by a narrow
opening called a slit. The radiation is then
collimated by a concave mirror so that a
parallel beam strikes the grating. Angular
dispersion from the grating then occurs at
the reflective surface.

18. Monochromators
• Consider two wavelengths of radiation, λ1 and λ2,
being dispersed by this grating. If λ1 < λ2 radiation
of wavelength λ1will be reflected from the grating
at a sharper angle than that of wavelength λ2. This
is known as angular dispersion.
• After dispersion the two beams of radiation are
focused by a concave mirror on to the focal plane
of the instrument where they appear as two
images. By rotating the grating either one of the
images can be focused on the exit slit.

19. Gratings
• Most gratings used in commercial instruments are
replica gratings obtained by making castings from a
master grating. The master grating is prepared by
scoring many parallel and closely spaced grooves on a
flat polished surface with a sharp diamond tool. In UV-
visible work the grating will have from 300-2000
grooves/mm.
• Preparation of the master grating is tedious and
expensive. Replica gratings are prepared from a liquid
resin casting process that preserves the optical
accuracy of the original master grating. The surface of
the replica is made reflective with a coating of
aluminum, gold, or platinum.

20. Radiation Detectors and
Transducers
• A detector is a device that indicates the existence
of some physical phenomenon. Examples are the
human eye, photographic film, the mercury level
in a thermometer, etc.
• A transducer is a special type of detector that
converts a physical phenomenon, e.g. light
intensity, pH, mass, etc., into electrical signals that
can be amplified, manipulated, and eventually
turned into numbers that are related to the
magnitude of the original signal.

21. Properties of Transducers
• The ideal electromagnetic radiation
transducer responds rapidly to low levels of
radiant energy over a broad wavelength
range.
• The electrical signal it produces must be
easily amplified and have a relatively low
noise level.

22. • It is essential that the electrical signal produced by the
transducer is directly proportional to the beam power,
P.
• G = KP + K'
• G is the electrical response of the detector in units of
current, resistance, or potential
• K is a proportionality constant which measures the
sensitivity of the detector in terms of electrical response
per unit of radiant power
• K' represents the dark current. This is a small constant
response, present even when no radiation strikes the
detector. When K' is reduced to a negligible quantity
by electrical subtraction etc., G = KP
Properties of Transducers

23. Types of Transducers
• Two general types of transducers are used
in optical spectroscopy. UV-visible
transducers respond to light or photons

24. Photon Detectors
• Types of photon detectors that are used
commercially are: phototubes,
photomultiplier tubes, silicon photodiodes.
• Phototubes are one of the simpler types to
understand because they work on the
Einstein photo-emissive cathode principle.

26. Photomultiplier Tubes
• This is similar in design to the phototube but is much
more sensitive.
• The surface of the cathode is similar in composition to
that of the phototube. Electrons are emitted from its
surface and accelerated towards the surface of another
electrode called a dynode. Upon striking the dynode,
each electron produces several additional electrons
which are then accelerated towards another dynode.
This process continues at a number of dynodes until 106
– 107
electrons have been produced for each photon.
• The cascade is collected at the anode where the
resulting electric current is further amplified and
measured.

28. Silicon Photodiodes
• These are transistor devices used in diode array
detectors. In these devices, over 1000 can be
fabricated side by side on a small silicon chip –
each diode is about 0.02 mm thick.
• When one or more diode array detectors are
placed along the focal plane of a monochromator,
the intensity of all wavelengths can be measured
simultaneously.

30. Signal Processors and Readouts
• The signal processor is an electronic device that
amplifies the electrical signal from the detector. It
may also subtract unwanted signals.
• Signal processors may display the signal in a
number of ways including derivative and
integrated peak areas, etc.
• Digital readout devices are most frequently found
in modern instruments. However, chart recorders
are used a great deal still.

31. Optical Instrument Design
• Optical spectroscopic instruments vary from
the very sophisticated which are expensive
and used for research, to the simple
inexpensive variety which are used for
routine chemical analysis. It is the nature of
the components that determines the cost and
eventual use.

32. Common Names of Some
Optical Instruments
• A spectroscope is an instrument for visually identifying
species excited in a flame or by other means. The
instrument has a monochromator and an eyepiece in
the focal plane to observe emission lines. It was used in
early spectroscopic studies.
• A colorimeter is an instrument for absorption
measurements in which the eye serves as the detector.
Comparison standards are used to quantify the amount
of analyze. This is a limited use of the term colorimeter
which is now usually applied to any photometer used
for absorption measurements in the visible region of
the spectrum.

33. Common Names of Some
Optical Instruments
• A photometer is a photoelectric instrument
that can be used for absorption, emission, or
fluorescence measurements. Most
colorimeters are photometers because they
use filters for wavelength selection.

34. Common Names of Some
Optical Instruments
• A spectrograph records spectra on a photographic
film or plate, placed along the focal plane of the
monochromator. The spectra appear as a set of
black images of the entrance slit. Spectrographs
are usually used for qualitative analysis.
• A spectrometer is a monochromator equipped with
a fixed slit at the focal plane. When the detection
device is a phototransducer the instrument is
known as a spectrophotometer.

35. Instrument Types
• Three general types of spectroscopic
instruments are in general use.
• These are single beam, double beam in
space, and double beam in time.
• All three types are used in absorption,
emission, and fluorescence studies.

36. Single Beam Instruments
• In a single beam instrument used in
absorption studies, a filter or
monochromator is used to select the
wavelength of radiation incident on the
sample holder.
• Matched cells can be interposed alternately
in the radiation beam which passes on to a
detector, usually a photomultiplier tube.

37. Single Beam Instruments
• To operate a manual instrument the following
adjustments are made:
• (a) The percent transmittance is corrected to zero
by placing a shutter between the source and the
detector and adjusting the meter until it reads zero.
• (b) solvent is placed in the radiation path, and the
shutter opened. The percent transmittance reading
of the meter is adjusted to 100 percent.

38. Single Beam Instruments
• A logarithmic scale can be used to determine
absorbance; a linear scale is used to determine
transmittance.
• Single beam instruments vary in complexity.

39. Spectrometers

40. Double Beam Instruments
• In a double beam instrument, two beams of
radiation are formed from the same source by a V-
shaped mirror called a ‘beam splitter’.
• One beam passes through the reference solution to
a detector while the second simultaneously passes
through a solution containing analyte to a second
matched detector.

41. Double Beam Instruments
• The two outputs are amplified and their ratio (or
log ratio) is determined electronically and
displayed by the readout device.

42. Spectrometers

43. Double Beam Instruments
• A second type of double beam instrument utilizes
a rotating sectored mirror to direct the radiation
from the source, as a pulse, either through a
reference cell or through the sample.
• The pulses of radiation are recombined by another
sectored mirror before detection.
• Pulsed radiation is the basis of many modern
research instruments.

44. Double Beam Instruments
• Double beam instruments compensate for
anything but very short term fluctuations in
radiant output from the source, as well as for any
drift in the detector and amplifier.
• They also compensate for any wide variation in
source intensity with wavelength.
• This design lends itself particularly well to the
continuous recording of spectra. Most modern
UV-visible recording instruments are based on
this design.

45. Spectrometer
Time separated double beam

46. Molecular Absorption
Spectroscopy
• Molecular optical spectroscopy is useful for both
qualitative and quantitative measurements.
• UV-visible spectrophotometry is mainly used for
quantitative work.

47. Ultraviolet and Visible
Absorption Spectroscopy
• Molecules absorb a finite amount of energy to
place the molecule in an upper electronic state.
Various vibrational and rotational levels are
occupied in this excited electronic state. The
absorption spectrum should reflect this.
• In the gas phase, it is possible to observe
absorption of energy to different vibrational and
rotational energy levels. This is a resolved
spectrum with fairly narrow lines.

48. Ultraviolet and Visible
Absorption Spectroscopy
• In solution or in condensed phases the
spectrum loses its resolution and becomes a
broad band.
• This is because the molecules of absorbing
species can bump into one another more
easily, thus creating a variety of energies
that cannot be resolved.

49. Absorption by Organic
Compounds
• The wavelength at which organic molecules
absorb UV-visible radiation depends to a large
extent on how firmly bound the electrons are in
the molecule.
• Electrons in single covalent bonds are tightly held
between the atoms in the bond. Excitation of these
electrons is only possible by using wavelengths of
radiation below 180 nm.
• This is of no use for routine measurements
because quartz and various atmospheric
components absorb at these wavelengths.

50. Absorption by Organic
Compounds
• Electrons in double or triple bonds are more easily
excited because they are more loosely held than
single bond electrons. Unsaturated molecules have
useful absorption spectra.
• The unsaturated organic functional groups that
absorb UV-visible radiation are known as
chromophores. They can serve as a very rough
guide for identification purposes.

51. Absorption by Organic
Compounds
• Peak position is influenced by a number of factors
such as:
• Solvent effects
• Other structural details of the molecule, such as
conjugation
• Molecules containing heteroatoms with lone pairs
of nonbonding electrons may also absorb in the
UV-visible region of the spectrum and provide
qualitative information on the molecule.

52. Absorbing species
• Electronic transitions
π, σ, and n electrons
– d and f electrons
– Charge transfer reactions
∀π, σ, and n (non-bonding) electrons

53. Sigma and Pi orbitals

54. Electron transitions

55. Transitions
∀ σ−>σ∗
– UV photon required, high energy
• Methane at 125 nm
• Ethane at 135 nm
• n-> σ∗
– Saturated compounds with unshared e-
• Absorption between 150 nm to 250 nm
∀ε between 100 and 3000 L cm-1
mol-1
• Shifts to shorter wavelengths with polar solvents
– Minimum accessibility
– Halogens, N, O, S

56. Transitions
• n->π∗, π−>π∗
– Organic compounds, wavelengths 200 to 700
nm
– Requires unsaturated groups
• n->π∗ low ε (10 to 100)
– Shorter wavelengths
∀π−>π∗ higher ε (1000 to 10000)

57. Absorption by Inorganic Species
• Typically visible absorption of visible radiation
occurs with compounds of elements in the first
and second row of the transition series. This is
why these compounds are colored.
• The absorption involves transitions between filled
and unfilled d-orbitals with energies that depend
on the ligands attached to the elements.
• The position of the energy transition depends on
the position of the element in the periodic table, its
oxidation state, and the nature of the attached
ligand.

58. Absorption by Inorganic Species
• The lanthanide elements behave somewhat
differently in that the peaks are fairly
narrow.
• This is because transitions are between
electrons in f-orbitals. They are shielded
from effects of the outer electrons and the
energy levels are thus not distorted.

59. Qualitative Applications of UV-
visible Spectroscopy
• UV-visible qualitative analysis is useful for
determining the presence of certain gross
structural features of molecules, e.g.
unsaturation.
• However it needs to be supplemented by
other spectroscopic methods such as IR and
NMR to obtain detailed information on
molecular structure.

60. Solvents
• The most useful information is derived from
spectra obtained from dilute solutions or from a
vapour of the analyte.
• Solvents should be of high purity and be
transparent throughout the spectral range.
• Generally non-polar solvents distort spectra least.
Polar solvents tend to obscure structure.
• Solvents tend to change the position of maximum
absorbance so in any qualitative measurement of
wavelength, the same solvent must be used for all
samples.

61. Quantitative Applications
• This is one of the most important tools available
for quantitative analysis.
• Important characteristics of the method are as
follows:
• (1) Wide applicability. Most compounds will
either absorb themselves or can be derivatized so
that the derivatives absorb. UV-visible
spectrophotometry is the single most important
analytical technique for quantitative chemical
analysis.

62. Quantitative Applications
• (2) High sensitivity. Typically measurements can
be made with detection limits as low as 10−5
mol/L.
With procedural modifications these detection
limits can be extended even further.
• (3) Moderate to high selectivity. This means that it
is sometimes, but not too often, possible to make
measurements on the analyte without prior
separation from other absorbing and interfering
compounds.

63. Quantitative Applications
• (4) Excellent accuracy. Normally the
precision of any given measurement will be
within 1–5%. Errors are reduced to similar
levels and can be made even lower by using
special measuring techniques.
• (5) Ease and convenience. Spectrometers
are not cheap, but are easy to use and lend
themselves to automation.