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Chapter 25 Instruments for Optical

Spectrometry

Contents in Chapter 25

1. Instrument Components2. Basic Components of Spectroscopic

Instrumentation1) Sources of Energy2) Wavelength Selector3) Sample Holder4) Detectors5) Signal Processors

3. UV/visible Photometers and Spectrophotometers

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1. Instrument Components

1) Most spectroscopic instruments in the UV/visible and IR regions are made up of five components:(1) Sources of Energy(2) Wavelength Selector(3) Sample Holder(4) Detectors(5) Signal Processors

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Filter 濾光鏡Interference filter 干涉濾鏡Absorption filter 吸收濾鏡

Monochromator 單光儀 Grating 光柵 Prism 稜鏡Slit 狹縫 Bandwidth 頻寬

Interferometer 干涉計Fourier Transform (FT) 傅立葉轉換Time domain 時域 Frequency domain 頻域

Glossary

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2) Conceptual Block Diagram of Spectrometer

LightSource

Wavelengthselector

Sample Detector Signal processor and readout

(1) For absorption measurement

Wavelengthselector

Sample Detector Signal processor and readout

(2) For emission measurement

Thermal

*****

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LightSource

Wavelengthselector

Sample

Detector Signal processor and readout

(3) For fluorescence measurement

Wavelengthselector

Sample +Reagent

DetectorSignal processor

and readout

(4) For chemiluminescence measurement

Wavelengthselector

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2. Basic Components of Spectroscopic Instrumentation

1) Sources of Energy

• For absorption and scattering spectroscopy: Energy is supplied by photons (electromagnetic radiation).

• For emission and luminescence spectroscopy: Thermal, radiant (photon), or chemical energy to promote the analyte higher energy state.

(1) Light (Electromagnetic Radiation) Sources:

– Continuum source: A source emits radiation over a wide range

of wavelengths.

– Line sources: A source emit radiation at selected wavelength

ranges.

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Electrodeless discharge lamp line source in UV/Vis atomic absorption

(cont’d)

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(cont’d)

(2) Thermal Energy (for emission spectrometer): e.g., flames and

plasmas. Flame: (fuel/oxidant, e.g., acetylene/air), achieve

temperatures of 2000–3400 K. Plasmas: (ionized gases, e.g.,

Ar), temperatures of 6000–10,000 K.

(3) Chemical Sources of Energy: e.g., in chemiluminescence, the

analyte is raised to a higher-energy state by means of a chemical

reaction, emitting characteristic radiation when it returns to a

lower-energy state.

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Effective bandwidth

• For qualitative purpose, narrow slit increase the spectra resolution.

• For quantitative purpose, wilder slit provide greater reproducibility.

• Effective bandwidth: The bandwidth of a monochromator or and interference filter at which the transmittance is 50% of that at nominal wavelength.

• Narrow slit decrease s the effective bandwidth, also diminishes the throughput of radiation.

2) Wavelength Selector

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(1) Filter: A wavelength selector that uses either absorption, or constructive and destructive interference to control the range of selected wavelengths.a) Interference filterb) Absorption filter

Type Interference filter Absorption filter

Theory use constructive and destructive interference to isolate a narrow range of wavelengths.

Colored glass or a dye suspended in gelatin, which removes the complementary color

Effective

bandwidths

10–20 nm 30–250 nm.

Maximum throughputs

> 40%. 10%

(cont’d)

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Interference Filters

• Consists of a transparent dielectric that occupies the space between two semitransparent metallic films. All of this is sandwiched between glass or other transparent materials.

• If the distance traveled by the reflected beam from 1 is some multiple of the wavelength, then reinforcement will occur at point 2.

• The nominal wavelength for an

interference filter λmax is given by the

equation:

max = 2t/nt is the thickness of the central fluoride η is its refractive index n is an integer called the interference order.

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(2) Monochromator: A wavelength selector that uses a diffraction grating or prism, and that allows for a continuous variation of the nominal wavelength.a) Diffraction gratingb) Prism

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Components of a Monochromator

Collimatingmirror

Focusingmirror

A polychromatic radiation is converted to a monochromatic radiation of finite effective bandwidth.A scanning monochromator includes a drive mechanism, allowing successive wavelengths to exit from the monochromator.

prism

grating

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Dispersion of radiation

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Reflection Grating A reflective surface is grooved or blazed giving numerous faces

for reflection to occur, beam 2 travels a longer distance than beam 1. If the extra distance traveled is a multiple of the wavelength, then constructive interference will occur.

• 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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(3) Polychromator: A wavelength selector in which different directions of the dispersed light (by prism or diffraction grating), simultaneously detected by a multi-detector (e.g., photodiode array, PDA)

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(cont’d)

Photodiode array (PDA): A linear array of photodiodes providing the ability to detect simultaneously radiation at several wavelengths.

Polychromator

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(4) Interferometer• Interferometer: A device that allows all wavelengths of light

to be measured simultaneously.• The signal shows a function of the moving mirror’s position.

The result is called an interferogram, or a time domain spectrum.

• The time domain spectrum is converted by Fourier Transform, to the normal spectrum (also called a frequency domain spectrum).

• Fourier transform IR spectrometers have the advantages over dispersive instruments of higher speed and sensitivity, better light-gathering power, more accurate and precise wavelength settings, simpler mechanical design, and elimination of stray light and IR emission.

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3) Sample Holder/Matrix

(1) Feasibility of cuvette materials

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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(2) Feasibility of preparation solvent

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4) Detectors

• Modern detectors use a sensitive transducer to convert a signal

consisting of photons into an easily measured electrical signal.• Ideally the detector’s signal (S):

S = kP + D P: electromagnetic radiation’s power

k: detector’s sensitivity

D is the detector’s dark current

Dark current: The background current present in a photon detector in the absence of radiation from the source.

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Characteristics of transducers for optical spectroscopy

• 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.

• Charge-coupled device (CCD) detector: The charge is moved to a charge-sensing amplifier for measurement.

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5) Signal Processor

Signal processor A device, such as a meter or computer, that

displays the signal from the transducer in a form that is easily

interpreted by the analyst.

• Examples: analog meters, digital meters, recorders, and

computers.

• Additionally, signal processor may be used to calibrate the

detector’s response, to amplify the signal from the detector, to

remove noise by filtering, or to mathematically transform the

signal.

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3. UV/visible Photometers and Spectrophotometers1) Classification

(1) Spectrometer: A spectroscopic instrument that uses a monochromator or polychromator in conjunction with a transducer to convert the radiant intensities into electrical signals.

(2) Spectrophotometer: The spectrometer that allows measurement of the ratio of the radiant powers of two beams, a requirement to measure absorbance.

(3) Photometer: A spectroscopic instrument that uses a filter for wavelength selection in conjunction with a suitable radiation transducer.

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2) Single beam versus Double beam

(1) Single beam instrument

(2) Double beam instrument

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3) Instrumentation for Fluorescence (1) Fluorometer: an

instrument for measuring fluorescence that uses filters to select the excitation and emission wavelengths.

(2) Spectrofluorometer: an instrument for measuring fluorescence that uses a monochromator to select the excitation and emission wavelengths.

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Practice

Q3. Which of followings are used for wavelength selection:

(A) Filter (B) Monochromator

(C) Interferometer (D) Tungsten lamp

A2. (A)

A3. (A)(B)(C)

Q1. The reciprocal of wavelength is called:

(A) Amplitude (B) Wavenumber

(C) Frequency (D) Period

A1. (B)

Q2. Photomultiplier is a:

(A) Photon transducers (B) Thermal transducers

(C) Absorption filter (D) Interferometer

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Homework

Skoog 9th edition, Chapter 25 Questions and Problems25-625-1325-1825-19

End of Chapter 25

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