Transcript Chapter 2 Spectrochemical Meaurements

CHAPTER 2
Spectrochemical Measurements
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COMPLETE SPECTROCHEMICAL
MEASUREMENT
• 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
• O 2, O 3.
• 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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Spectrum
Mainly  selector Spectrum
Optical
Electrical signal
Manual or
automatic
Human operator is being
replaced by microcomputers
Sample is treated
before introduction
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Convert the transducer output
into a form appropriate for readout
as numerical values
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
6 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
Cumulative radiance
radiance in the wavelength interval 2 - 1
• Total radiance, B:
the radiance from a source related to spectral radiance:
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• Sources that emit narrow spectral lines (typical halfwidths << 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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1.
Emission measurements
Emission and chemiluminescence (bioluminescence) methods
The energy changes that
occur during excitation
(dashed lines) or emission
(solid lines)
Typical spectrum
• Addition of thermal, electrical or chemical
energy causes nonradiational excitation of the
analyte and emission of radiation in all
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directions
(isotropic emission)
e.g., sodium atoms are
excited in a flame by
Collisional processes
and emit characteristic
radiation.
• 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
Statistical factor of state i
Excitation energy relative
to the ground state
Partition function
• Na and other alkali metals have excited levels close to the
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ground state levels. Thus their resonance lines occur in the
visible and near IR regions and are readily observed in media
such as flames.
• 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
Absorbance
Absorption
pathlength
Transmittance
Molar absorptivity
Also,
a
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Conc.
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 < 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
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to these optical devices
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
36 spectrochemical methods.
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.
1. 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.
2. The second source is the background signal, EB due to
background radiation that strikes the transducer.
•
•
•
•
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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.
Analytical signal in Emission and
Spectrometry
Chemiluminescence
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
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of the blank signal
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
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monitor
• The total analytical signal EtL is expressed:
Blank
Analytical luminescence signal
dark
Analytical thermal emission
signal
background
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 E’L
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
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the zero position of the readout device.
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:
1. 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
50 absorbance A.