Instruments for Optical Spectrometry

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Chapter 25

• Instruments for Optical Spectrometry

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• 25 A Instrument components
• Most spectroscopic instruments in the UV/visible
  and IR regions are made up of five components
• a stable source of radiant energy
• a wavelength selector to isolate a limited region
  of the spectrum for measurement
• one or more sample containers
• a radiation detector, to convert radiant energy
  to a measurable electrical signal and
• a signal-processing and readout unit consisting
  of electronic hardware and in modern instruments
  a computer.

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Spectroscopic sources are of two types 1.
Continuum sources emit radiation that changes in
intensity only slowly as a function of
wavelength. A continuum source provides a broad
distribution of wavelengths within a particular
spectral range. 2. Line sources, which emit a
limited number of spectral lines, each of which
spans a very narrow wavelength range. Sources
can also be classified as continuous sources,
which refer to the fact that they emit radiation
continuously with time, or pulsed sources, which
emit radiation in bursts.
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The continuum sources for IR radiation are
normally heated inert solids. A Globar source
consists of a silicon carbide rod. Infrared
radiation is emitted when the Globar is heated to
about 1500C by passing electricity through
it. A Nernst glower is a cylinder of zirconium
and yttrium oxides that emits IR radiation when
heated to a high temperature by an electric
current.
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Monochromators generally have a diffraction
grating to disperse the radiation into its
component wavelengths. The output wavelength of
a monochromator is thus continuously variable
over a considerable spectral range. The
wavelength range passed by a monochromator,
called the spectral bandpass?or?effective
bandwidth, can be less than 1 nm for moderately
expensive instruments to greater than 20 nm for
inexpensive systems. Other instruments used for
emission spectroscopy contain a device called a
polychromator, which contains multiple exit slits
and multiple detectors. This arrangement allows
many discrete wavelengths to be measured
simultaneously.
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Angular dispersion results from diffraction,
which occurs at the reflective surface. The
radiation entering the monochromator is shown as
consisting of just two wavelengths, ? 1 and ? 2,
where ? 1 is longer than ?2. The two
wavelengths are focused by another concave
mirror onto the focal plane of the
monochromator. A high-quality monochromator
will exhibit an effective bandwidth of a few
tenths of a nanometer or less in the
ultraviolet/visible region.
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Most gratings in modern monochromators are
replica gratings. A grating for the ultraviolet
and visible region typically has from 50 to 6000
grooves/mm, with 1200 to 2400 being most
common. One of the most common types of
reflection gratings is the echellette grating.
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A parallel beam of monochromatic radiation
approaches the grating surface at an angle i
relative to the grating normal. The incident
beam is depicted as consisting of three parallel
beams that make up a wave front labeled 1, 2, 3.
The diffracted beam is reflected at the angle
r, which depends on the wavelength of the
radiation. The angle of reflection r is
related to the wavelength of the incoming
radiation by the equation n? d(sin i
sin r)
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Filters operate by blocking or absorbing all but
a restricted band of radiation. There are two
types of filters used in spectroscopy
interference filters and absorption filters.
Interference filters are typically used for
absorption measurements. These filters
generally transmit a much greater fraction of
radiation at their nominal wavelengths than do
absorption filters.
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Interference filters are used with ultraviolet
and visible radiation, as well as with
wavelengths as long as about 14 mm in the
infrared region. An interference filter relies
on optical interference to provide a relatively
narrow band of radiation, typically 5 to 20 nm in
width.
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The nominal wavelength for an interference filter
lmax is given by the equation ?max
2t?/n where t is the thickness of the central
fluoride layer, h is its refractive index, and
n is an integer called the interference order.
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Absorption filters, which are generally less
expensive and more rugged than interference
filters, are limited in use to the visible
region. Absorption filters have effective
bandwidths that range from perhaps 30 to 250
nm. Filters have the advantages of simplicity,
ruggedness, and low cost. Since one filter can
only isolate a single band of wavelengths, a new
filter must be used for a different wavelength
band. In the IR region of the spectrum, most
modern instruments do not disperse the spectrum
at all. Instead an interferometer?is used, and
the constructive and destructive interference of
electromagnetic waves are used to obtain spectral
information through a technique called Fourier
transformation.
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To obtain spectroscopic information, the radiant
power transmitted, fluoresced or emitted, must be
detected in some manner and converted into a
measurable quantity. A detector is a device
that identifies, records, or indicates a change
in one of the variables in its environment such
as pressure, temperature, or electromagnetic
radiation. In modern instruments, the
information of interest is encoded and processed
as an electrical signal. A transducer converts
nonelectrical quantities, such as light
intensity, pH, mass, and temperature, into
electrical signals that can be subsequently
amplified, manipulated, and finally converted
into numbers proportional to the magnitude of the
original quantity.
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The electrical signal produced by the transducer
should be linearly related to the radiant power P
of the beam. G KP K where G is the
electrical response of the detector in units of
current, voltage, or charge. The
proportionality constant K measures the
sensitivity of the detector in terms of
electrical response per unit of radiant power
input. K?, is a small constant response known
as a dark current, even when no radiation strikes
the surfaces. Under ordinary circumstances, G
KP There are two general types of transducers
one type responds to photons, the other to heat.
All photon detectors are based on the interaction
of radiation with a reactive surface either to
produce electrons (photoemission) or to promote
electrons to energy states in which they can
conduct electricity (photoconduction).
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Photoelectrons are electrons that are ejected
from a photosensitive surface by electromagnetic
radiation. A photocurrent is the current in an
external circuit that is limited by the rate of
ejection of photoelectrons.
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Figure 25-13 Diagram of a photomultiplier tube.
(a) photograph (b) cross-sectional view and
(c) electrical diagram illustrating dynode
polarization and photocurrent measurement.
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Photoconductive transducers consist of a thin
film of a semiconductor material deposited on a
nonconducting glass surface and sealed in an
evacuated envelope. Absorption of radiation by
these materials promotes nonconducting valence
electrons to a higher energy state, which
decreases the electrical resistance of the
semiconductor. A semiconductor is a substance
having a conductivity that lies between that of a
metal and that of a dielectric (an insulator).
Crystalline silicon is a semiconductor, a
material whose electrical conductivity is less
than that of a metal but greater than that of an
electrical insulator. Silicon has four valence
electrons, each of these is combined with
electrons from four other silicon atoms to form
four covalent bonds. The conductivity of silicon
can be greatly enhanced by doping.
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A?pn?junction or a?pn?diode, which is conductive
in one direction and not in the other. In its
conduction mode, the positive terminal of a dc
source is connected to the?p?region and the
negative terminal to the?n?region. The diode
is said to be forward biased under these
conditions.
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Photodiodes are semiconductor pn-junction devices
that respond to incident light by forming
electronhole pairs. When a voltage is applied
to the?pn?diode such that the p-type
semiconductor is negative with respect to the
n-type semiconductor, the diode is said to be
reversed biased. The majority carriers are
drawn away from the junction, leaving a
nonconductive depletion layer. Silicon
photodiode detectors respond extremely rapidly,
usually in nanoseconds. Diode arrays can also
be obtained commercially with front-end devices
called image intensifiers to provide gain and
allow the detection of low light levels. In
charge-transfer detectors individual detector
elements are arranged in rows and columns.
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• In a charge-injection device (CID) detector, the
  voltage change arising from movement of the
  charge from the region under one electrode to the
  region under the other is measured.
• In a charge-coupled device (CCD) detector, the
  charge is moved to a charge-sensing amplifier for
  measurement.

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Four types of thermal transducers are used for
infrared spectroscopy. The most widely used is a
tiny thermocouple or a group of thermocouples
called a thermopile. The bolometer consists of
a conducting element whose electrical resistance
changes as a function of temperature. A
pneumatic detector consists of a small
cylindrical chamber that is filled with xenon and
contains a blackened membrane to absorb infrared
radiation and heat the gas. Pyroelectric
detectors are manufactured from crystals of a
pyroelectric material, such as barium titanate or
deuterated triglycine sulfate.
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A signal processor is an electronic device that
may amplify the electrical signal from the
detector. The signal processor may convert the
signal from dc to ac (or the reverse), change the
phase of the signal, and filter it to remove
unwanted components. The signal processor may
also perform such mathematical operations on the
signal as differentiation, integration, or
conversion to logarithms. Sample containers,
which are usually called cells or cuvettes, must
have windows that are transparent in the
spectral region of interest.
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• 25 B Ultraviolet/visible photometers and
  spectrophotometers
• A spectrometer?is a spectroscopic instrument that
  uses a monochromator or polychromator in
  conjunction with a transducer to convert the
  radiant intensities into electrical signals.
• Spectrophotometers are spectrometers that allow
  measurement of the ratio of the radiant powers of
  two beams, a requirement to measure absorbance.
• Photometers use a filter for wavelength selection
  in conjunction with a suitable radiation
  transducer.
• Most spectrophotometers cover the UV/visible and
  occasionally the near-infrared region, while
  photometers are most often used for the visible
  region.
• Photometers find considerable use as detectors
  for chromatography, electrophoresis,
  immunoassays, or continuous flow analysis.

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Many modern photometers and spectrophotometers
are based on a double-beam Design. A
double-beam-in-space instrument is one in which
two beams are formed by a V-shaped mirror called
a beam-splitter. One beam passes through the
reference solution to a photodetector, and the
second simultaneously passes through the sample
to a second, matched photodetector. In a
double-beam-in-time spectrophotometer, the beams
are separated in time by a rotating sector mirror
that directs the entire beam through the
reference cell and then through the sample cell.

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Figure 25-20 Instrumental designs for UV/visible
photometers or spectrophotometers. (a), a
single-beam instrument and (b), a
double-beam-in-space instrument.
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In the double-beam-in-time instrument (c), the
beam is alternately sent through reference and
sample cells before striking a single
photodetector.
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With multichannel systems, the dispersive system
is a grating spectrograph placed after the sample
or reference cell. The photodiode array or CCD
array is placed in the focal plane of the
spectrograph. These detectors allow the
measurement of an entire spectrum in less than 1
s.
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• 25 C Infrared spectrophotometers
• Two types of spectrometers are used in IR
  spectroscopy the dispersive type and the
• Fourier transform variety.
• In most UV/visible instruments the cell is
  located between the monochromator and the
  detector in order to avoid photodecomposition of
  the sample, which may occur if samples are
  exposed to the full power of the source.
• Fourier transform IR instruments contain no
  dispersing element, and all wavelengths are
  detected and measured simultaneously.
• Instead of a monochromator, an interferometer is
  used to produce interference patterns that
  contain the infrared spectral information.
• Fourier transform spectrometers detect all the IR
  wavelengths all the time. They have greater
  light-gathering power than dispersive instruments
  and consequently better precision.