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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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Spectrum

Sample is treated before introduction

Manual or automatic

Mainly selector SpectrumOptical Electrical signal

Convert the transducer outputinto a form appropriate for readout as numerical values

Human operator is beingreplaced by microcomputers

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Expression of Optical Intensity

• Optical intensities are expressed in two systems:

• Radiometric system

• Photometric system

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Radiometric SystemRadiometric System Basic DefinitionsBasic 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

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

Cumulative radiance

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• Sources that emit narrow spectral lines (typical half widths << 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 broad band 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

• Addition of thermal, electrical or chemical

energy causes nonradiational excitation of the

analyte and emission of radiation in all

directions (isotropic emission)

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.

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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

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 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 radiant power of emission EE 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 hji, 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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a

Absorbance Transmittance

Absorptivity

Molar absorptivity

Absorption pathlength

Conc.

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 bio luminescence

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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• 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 emissionOf photons

Radiationless processes

Photoluminescence methods:Molecular and atomic fluorescence

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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 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

UV visible 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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. The radiant power monitor provides a numerical read out that is related to the radiant power (number of photons per second or watts) impingent on the transducer.

Radiant power monitor

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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 emissionE.

• 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.

• 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

ChemiluminescenceSpectrometry 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:

Analytical luminescence signal Analytical thermal emission signal

Blank

backgrounddark ScatteringBackgroundluminescence

• 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 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

• In practice, the presence of other signals (dark signal, background emission) necessitates a third measurement• The measured transmittance T' is defined by the equation

sample

reference

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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 tothe 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 light source 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.