Spectrochemical Measurements

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CHAPTER 2

• Spectrochemical Measurements

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COMPLETE SPECTROCHEMICALMEASUREMENT

• Steps involved in determination of the
  concentration of the analyte in a sample
• acquisition of the initial sample,
• sample preparation or treatment to produce the
  analytical sample,
• presentation of the analytical sample to the
  instrument, measurement of the optical signals,
• establishment of the calibration function with
  standards and calculations,
• interpretation,
• feedback.

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Spectrochemical measuremennt process

• A sample introduction system presents the sample
  to the encoding
• system, which converts the concentrations c1,
  c2, c3 into optical signals O1
• O2, O3.
• The information selection systems selects the
  desired optical signal O1 for presentation to the
  radiation transducer.
• This device converts the optical signal into an
  electrical signal (current i, voltage e,
  frequency f, etc.) that is processed and read out
  as a number.

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Optical Electrical signal
Spectrum
Mainly ? selector Spectrum
Manual or automatic
Human operator is being replaced by
microcomputers
Sample is treated before introduction
Convert the transducer output into a form
appropriate for readout as numerical values
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Expression of Optical Intensity

• Optical intensities are expressed in two systems
• Radiometric system
• Photometric system

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Radiometric System Basic Definitions

• radiometric system of units is based on the
  actual radiant energy emitted by a source or
  striking a receiver (e.g., optical transducer)
  and is preferred in the International System of
  Units (SI).
• The basic quantity in this system is the radiant
  energy Q in joules (J).
• In the radiometric system there are general
  quantities used to describe radiation sources,
  and radiation receiver.
• radiant intensity, emittance, emissivity, and
  radiance
• refer specifically to radiation from a source
• volumes, areas, and solid angles
• refer to properties related to source
• Irradiance and exposure
• describe the receiver and its area

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• All quantities are in general functions of
  spectral position (wavelength, wave number,
  frequency, etc.) in that they are usually
  employed to represent the magnitude of the
  quantity over some spectral interval.
• In general these values represent the cumulative
  magnitude of the quantity over the wavelength
  interval from 0 to ?.
• If the term "total" is employed, as in total
  radiance, it implies the radiance over the
  wavelength interval from 0 to?.
• Generally, radiometric quantities are considered
  within small spectral intervals.

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• Spectral quantities
• radiometric quantities per unit spectral
  interval and given a subscript ? (for
  wavelength), ? (for the frequency)and
  (for wave number)
• spectral radiance B ?, is the radiance per unit
  wavelength interval (per nm)
• Partial radiance, B??
• radiance in the wavelength interval ?2 - ?1

Cumulative radiance

• Total radiance, B
• the radiance from a source related to spectral
  radiance

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• Sources that emit narrow spectral lines (typical
  halfwidths ltlt 1 Ao) are usually characterized by
  reporting the radiance B of each line which is
  the integrated spectral radiance over the total
  width of the line.
• A broadband source is normally characterized by
  its spectral radiance B? because only part of its
  emitted spectral range is selected or observed as
  determined by a wavelength selector.

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Geometric Factors

• Often, radiometric quantities include the
  geometric factors of solid angle and projected
  area.

• (a) Plane angle and one radian of angle are
  illustrated.
• One radian is the angle at the center of a
  circle that
• intercepts an arc equal in length to the
  radius.

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• (b) Solid angle is defined by the cone generated
  by a line that passes through the vertex O and a
  point moved along the periphery of the surface.
• One steradian is the solid angle at the center of
  a sphere of radius r that subtends an area of r2
  units on the surface

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Examples of Use of radiometric terms

• In most spectroscopic situations one is
  eventually interested in the radiant power that
  is incident on a receptor
• Consider, for example, a point source with
  dimensions that are small compared to the
  distance (d) from the source to the receptor of
  projected area Ap.
• The source could be characterized by the total
  radiant power ? that it emits in all directions.
• In this case, it is more useful to use the
  radiant power per unit solid angle (the radiant
  intensity), which is given by

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A Source of significant area
The radiant power, ?I incident on area A2 of the
receptor is the source radiant times the area
times the solid angle viewed times the area
viewed
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Photometric System(will not be used further)

• It is a relevant system based on the apparent
  intensity of av source as viewed by the average
  bright adapted human eye.
• Quantities in this system have meaning only in
  the visible region
• The basic unit of this system is the lumen
• A source of 1 candela emits 1 limen per steradian
• Photometric and corresponding radiometric
  quantities are given in the following table

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Relationships between the radaint quantities and
the spectrochemical methods
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Emission measurements Emission and
chemiluminescence (bioluminescence) methods
The energy changes that occur during excitation
(dashed lines) or emission (solid lines)
Typical spectrum
e.g., sodium atoms are excited in a flame by
Collisional processes and emit characteristic
radiation.

• Addition of thermal, electrical or chemical
• energy causes nonradiational excitation of the
  
• analyte and emission of radiation in all
• directions (isotropic emission)

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• The frequency of the emitted radiation
  corresponds to the discrete energy differences
  between levels, as shown in the figure
• When thermal equilibrium is maintained, the
  number of atoms per cm3 in level i, ni is related
  to the total number of atoms per cm3, nt, by the
  Boltzmann distribution

Excitation energy relative to the ground state
Statistical factor of state i
Partition function

• Na and other alkali metals have excited levels
  close to the
• ground state levels. Thus their resonance
  lines occur in the
• visible and near IR regions and are readily
  observed in media
• such as flames.

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• The radiant power of emission ?E from state j to
  state i is given by the population density of
  excited atoms nj times the probability Aji (s-1)
  that an excited atom will undergo the transition,
  times the energy per emitted photon h?ji, times
  the volume element observed V (cm3). Or

• The equation shows that the radiant power of
  emission
• is proportional to the excited-state
  population density
• and thus to the analyte concentration through
  the
• previous equation.

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2. Absorption measurement

• For absorption to occur, the frequency of the
  incident radiation must correspond to the energy
  difference between the two states involved in the
  transition as shown in the figure.
• For many conditions the absorption of radiation
  follows Beer's law

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Absorptivity
a
Conc.
Absorption pathlength
Absorbance
Transmittance
Molar absorptivity
Also,
a
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3. Luminescence measurement

• Luminescence is radiation emitted from relatively
  cool bodies.
• There are several classes of luminescence
  spectrochemical methods
• Chemiluminescence and bioluminescence
• excited analyte species are produced by
  chemical reactions, and the resulting emission is
  measured.
• Electroluminescence
• It results from the movement of electrons in a
  sample and may be caused by an electrical
  discharge, by recombination of ions and electrons
  at an electrode, and by interactions of materials
  with accelerated electrons as in a cathode ray
  tube.
• Triboluminescence
• It results from the mechanical separation of
  charges followed by a discharge (e.g., broken
  crystals of sugar).
• Thermoluminescence
• It is the enhancement of other types of
  luminescence by the addition of heat.
• Chemiluminescence and bioluminescence are
  employed in
• analytical procedures. The
  excitation/emission transitions for these were
  illustrated in a previous figure.

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Photoluminescence methods Molecular and atomic
fluorescence

• Methods that utilize an external radiation source
  for excitation (as in absorption methods), but
  the sought-for information is the radiation
  emitted by the sample as shown in the figure

Loss of energy by emission Of photons
Radiationless processes
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Measurement of luminesced radiant power

• When a portion of the incident radiant power ?o
  is absorbed so that the transmitted radiant power
  ? is less than the incident radiant power
• Under many conditions the radiant power
  luminesced (for all wavelengths) ?L is
  proportional to the absorbed radiant power (?o -
  ?). Thus,

The transmitted radiant power is related to the analyte concentration by Beer's law
Thus,
Expansion of the above eq. in a Taylor series
gives,
When the term abc is lt 0.01, higher-order terms in the expansion contribute less than 1 to ?L, and under these conditions,
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Scattering measurement

• Radiation from an external source can also be
  scattered by the sample
• The intensity, frequency, and angular
  distribution of scattered radiation can be used
  in spectrochemical methods.
• In molecular scattering methods, particles
  smaller than the wavelength of the incident
  radiation can scatter that radiation elastically
  without a change in its energy.
• Small-particle scattering is called Rayleigh
  scattering
• it typically occurs with atoms or molecules.
• Rayleigh scattered radiation occurs in all
  directions from the scattering particle.

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• Debye Scattering
• It is the scattering that takes place from larger
• particles with dimensions on the order of the
  wavelength of the incident radiation.
• Here the scattered radiation is of the same
  frequency as the incident radiation, but the
  angular distribution of the scattered radiation,
  unlike Rayleigh scattering, is not uniform.
• Mie scattering
• Scattering from much larger particles
• Large-particle scattering (Debye or Mie) can be
  used to determine particle sizes and is important
  in turbidimetry and nephelometry where suspended
  particles are the scatterers.

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• Brillouin and Raman scattering
• These are forms of inelastic scattering which
  involve a change in the frequency of the incident
  radiation.
• Brillouin scattering results from the reflection
  of radiant energy waves by thermal sound waves
• Raman scattering involves the gain or loss of a
  vibrational quantum of energy by
• molecules.
• The scattering signal is proportional to the
  incident radiant power.

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Selection of Optical Information

• In analytical procedures the selection step
  allows us to separate the analyte optical signal
  from a majority of the potential interfering
  optical signals.
• The vast majority of analytical techniques select
  the desired information based only on its
  wavelength
• Thus, wavelength selection is essential!

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Wavelength Selection
Instrumentation for spatial dispersion and
detection of optical signals

• Some of the radiation from the spectrochemical
  encoder enters the
• entrance slit and strikes the dispersion
  element.
• The dispersion element and image transfer
  system cause each
• wavelength to strike a different position in
  the focal plane where
• different photo detector configurations can be
  used
• According to the phtodetector configuration
  various names were given
• to these optical devices

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Specific names given to optical instruments

• spectrograph, a large aperture in the focal plane
  allows a wide range of wavelengths to strike a
  spatially sensitive detector such as a
  photographic plate.
• In recent years, solid-state video-type detectors
  have become available and are often employed in
  spectrographs in place of film.
• These detectors are actually an array of a large
  number of closely spaced miniature photoelectric
  detectors.
• They have the advantage that the spectrum can be
  obtained immediately without the time required
  for film development, for obtaining the density
  of the lines recorded, and so on.
• A spectroscope is a device that allows a visual
  observation of the spectrum. It is a spectrograph
  that uses a viewing screen for observing the
  spectrum in the focal plane.

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• In a monochromator, an exit slit about the same
  size as the entrance slit is used to isolate a
  small band of wavelengths from all the
  wavelengths that strike the focal plane.
• One wavelength band at a time is isolated and
  different wavelength bands can be selected
  sequentially by rotating the dispersion element
  to bring the new band into the proper orientation
  so that it will pass through the exit slit.
• If the focal plane contains multiple exit slits
  so that several wavelength bands can be isolated
  simultaneously, the wavelength selector is called
  a polychromator.

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• A spectrometer is a spectrochemical instrument
  which employs a monochromator or a polychromator
  in conjunction with photoelectric detection of
  the isolated wavelength band(s).
• The photodetector is placed just outside the exit
  slit.
• If a polychromator is employed with a separate
  photodetector for each exit slit, the instrument
  is often called a direct-reading spectrometer.
• Some spectrometers use optical components to
  sweep the spectrum quite rapidly across a single
  exit slit.
• These rapid-scanning spectrometers can obtain a
  spectrum in a few milliseconds.

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• A spectrophotometer is an instrument similar to a
  spectrometer except that it allows the ratio of
  the radiant power of two beams to be obtained, a
  requirement for absorption spectroscopy.
• A photometer is a spectrochemical instrument
  which uses an optical filter for wavelength
  selection in conjunction with photoelectric
  detection.

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• Interferometers are nondispersive devices in
  which the constructive and destructive
  interference of light waves can be used to obtain
  spectral information.

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Measurement of optical signals

• All spectrochemical techniques that operate in
  the UVvisible and IR regions of the spectrum
  employ similar instrumental components, as
  mentioned before.
• The major instrumental differences between
  emission, photoluminescence, and absorption
  techniques occur in the arrangement and type of
  sample introduction system, encoding system, and
  information selection system.
• All techniques depend upon the measurement of
  radiant power.
• The specific transducers and signal processing
  devices used in various regions of the spectrum
  in specific spectrochemical techniques are
  described later.
• In this section we explore how the analytical
  signal is extracted from the readout data in
  spectrochemical methods.

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Radiant power monitor
. The radiant power monitor provides a numerical readout that is related to the radiant power (number of photons per second or watts) impingent on the transducer.
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Analytical Signal

• The analytical signal is rarely obtained directly
  as a result of one spectrochemical measurement.
• Because of the presence of background and other
  extraneous signals, the analytical signal must be
  extracted from the raw readout data.
• The analytical signal for emission and
  chemiluminescence techniques is defined as the
  signal to be displayed by the readout device due
  only to analyte emission.
• It is given the symbol EE, and we presume that EE
  is directly related to the radiant power of
  emission?E.
• Similarly, the analytical signal in
  photoluminescence techniques,? L, is the measured
  signal due only to radiationally produced
  emission of the analyte.
• In the case of absorption methods, the analytical
  signal is the absorbance A due only to absorption
  of radiation by the analyte species.

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• Because of the presence of extraneous signals,
  such as signals from concomitants, the sample
  cell, and room light, at least two measurements
  are required to obtain the analytical signal.
• The background or extraneous signal that
  registers on the readout device is due to two
  primary sources.
• The first source is the dark signal Ed of the
  radiant power monitor, which is the signal
  present when no radiation is impingent on the
  transducer.
• The second source is the background signal, EB
  due to background radiation that strikes the
  transducer.
• The background radiation is composed of radiation
  from all sources other than the desired optical
  phenomenon from the analyte.
• The transducer can convert this optical signal to
  an electrical current, voltage, or charge.
• Normally, the output of the signal processing
  system to be displayed on the readout device is
  an electrical voltage
• Generally, analyte and background signals will be
  written as voltages E.

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Analytical signal in Emission and
Chemiluminescence Spectrometry
Instrumentation for emission spectrochemical
methods.
Spectrochemical encoder

• The excitation source is the spectrochemical
  encoder
• The emission that results from excitation of the
  analyte species
• by a flame, a plasma, or a chemical reaction
  encodes the
• concentration of the analyte as the radiant
  power of emission ?E.
• In some spectrochemical methods the excitation
  source and sample
• container are an integral unit, as in the
  nebulizer-burner used in
• flame emission and the reaction cell used in
  chemiluminescence

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• The analytical signal of the sample is usually a
  total or composite signal EtE
• This total signal is the sum of analytical
  signal EE the dark signal Ed and the background
  emission signal EbE
• To extract the analytical signal, a second
  measurement is required to obtain the sum of the
  dark signal and the background emission signal.
• This second measurement is normally made by
  replacing the analytical sample with a blank,
  then

Blank signal Eb Ebk

• If desired, the dark signal can be obtained
  separately by blocking all
• radiation from reaching the radiant power
  monitor.
• The background emission signal could then be
  obtained from Ebk - Ed.
• In many instruments the blank solution is used
  to adjust the readout
• device to read zero by suppression of the
  blank signal.
• This establishment of the zero position is
  still, however, a measurement
• of the blank signal

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Analytical signal in Photoluminescence
Spectrometry

• An external source of EMR excites the analyte.
  The analyte concentration is optically encoded as
  the luminescent radiant power ?L, which is
  measured with the radiant power monitor.
• The emission wavelength selector that views the
  luminescence of the sample is typically placed to
  collect radiation at 90 with respect to the
  excitation axis.

Instrumentation for photoluminescence
spectrometry
Specific wavelengths from an external radiation source are isolated by the excitation wavelength selector to excite the analyte in the sample cell. The emission wavelength selector selects the wavelength band where analyte luminescence is concentrated and passes it to the radiant power monitor
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• The total analytical signal EtL is expressed

Blank
Analytical luminescence signal
Analytical thermal emission signal
background
dark
Scattering
Background luminescence

• Analyte and background emission in the
  UV-visible region are
• usually significant only in atomic
  spectroscopy.

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• The analyte luminescence signal EL can be
  obtained with two measurements only if the
  analyte emission signal EE is small compared to
  EL, which is often the case.
• If EE is significant, subtraction of the blank
  signal gives a measured analyte luminescence
  signal EL that differs from EL

• To obtain the true analyte luminescence signal
  EL when EE is significant,
• the excitation source must be turned off.
  Then the two measurements EtE
• and Ebk are made to obtain EE.
• Subtraction of EE from EL gives the true
  analyte luminescence signal.
• In some cases it is possible to eliminate the
  measured contribution from
• analyte emission optically or electronically.
• For example, if the excitation source is
  modulated and alternating-current
• (ac) amplification is used, the ac
  luminescence signal can be distinguished
• from the do emission signal. Often the blank
  measurement is used to set
• the zero position of the readout device.

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Analytical signal in Absorption Spectrometry

• Typical absorption spectrometer

• It is similar to the luminescence spectrometer
  except that all
• components are placed on the same optical axis
• The shutter allows the user to block the
  radiation source in order to
• obtain the dark signal. Usually, only one
  wavelength selector is
• required.
• Absorption measurements can be made as
  transmittance T where
• absorbance A is calculated manually or the
  logarithmic conversion can
• be done electronically or with computer
  software and the absorbance A
• displayed by the readout device.

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1. Transmittance readout

• T values could be obtained by
• 1. measuring the signal ES that results from
  the
• source radiant power passing through the
  
• analytical sample
• 2. measuring the signal Er that results from
  the
• source radiant power passing through the
  
• ideal blank or reference solution
• 3. obtaining the transmittance as in

sample
reference
In practice, the presence of other signals (dark signal, background emission) necessitates a third measurement The measured transmittance T' is defined by the equation
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• ESt is the total sample signal obtained with the
  source shutter
• open and the analytical sample in the sample
  container,
• Eot is the zero percent transmittance (0 T)
  signal obtained with the shutter closed and the
  blank in the sample container,
• Ert is the 100 T signal obtained with the
  shutter open and the blank (reference) in the
  sample container
• The 0 T signal Eot is composed of any background
  emission EbE and dark current Ed
• When the blank is in the sample container and the
  shutter open, the measured total reference signal
  Ert called the 100 T signal, is composed of the
  reference transmission signal Er, the 0 T
  signal, and any background luminescence EbL

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• When the analytical sample is in the sample
  container and the shutter is open, the measured
  signal is Est, the total sample signal. This
  signal is given by

Sample transmission signal
emission signal
luminescence signal

• From the above equations, the measured
  transmittance is

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2. Direct absorbance readout

• Many modern absorption spectrometers can display
  absorbance directly.
• The true absorbance A is given by

Voltage proportional to the analyte absorbance
Log conversion factor in volts per A unit

• The voltage EA and hence A are found from two
  measurements
• A reference logarithmic voltage or zero
  absorbance voltage Elr is
• obtained with the shutter open and the blank
  in the sample container
• 2. Then a sample logarithmic voltage Els is
  obtained with the shutter open and the analytical
  sample in the sample container

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• The voltage is then given by

The voltages Els and Elr are logarithmically related to ES and Er
Constant reference voltage

• Often Elr is set to zero on the readout device
  so that Els is read out
• directly as EA
• Note that in the two-step absorbance
  measurement scheme, a
• measurement is not made with the lightsource
  shutter closed (0 T)
• since A would be infinity.
• Thus (Ed EbE) must be negligible compared to
  ES and Er or
• electronically or optically set to zero by
  other means.
• Also, EE EbL EL must be negligible so
  that ES Est and Er Ert
• otherwise, the measured absorbance A' only
  approximates the true
• absorbance A.