授課教師: Professor 吳逸謨 教授

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Principles of Instrumental Analysis Chapter 16 An introduction to Infrared Spectrometry (This chapter is divided into sections: A-Theroy, B-Instrumentation, C-components.) + pp.15, 26,27 (extra info.) 1

Molecular Spectroscopy Includes: – UV/Visible Absorption (Ch.13 + Ch.14-Applications) – Luminescence Spectrometry (Ch.15) [Note: Luminescence = 1. fluororescence, 2. phosphorescence, and 3. chemiluminescence] – ------------------------------------------------------------------------------------

– IR - Introduction &Theories (Ch.16) – IR - Applications (Ch.17) -----------------------------------------------------------------– Raman Spectroscopy (Ch.18) Not covered – NMR- Ch.19 Not covered – Molecular Mass (Ch.20) Not covered – Surface analysis by Spectroscopy and Microscopy (Ch.21) 2

16-A. Theory of absorption spectroscopy

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•

Table 16-1 gives the rough limits of each of the three regions of IR.

•

Measurements in the near-IR region are often made with photometers and spectrophotometers similar in design and components to the instruments described in earlier chapters for ultraviolet-visible spectrometry. The most important applications of this spectral region (near IR) have been to the quantitative analysis of industrial and agricultural materials and for process control. Applications of near-IR spectrometry are discussed in Section 17D. (Similar to UV-Vis)

•

TABLE 16-1 IR Spectral Regions Mid-IR • Until the early 1980s, instruments for the mid-IR region were largely of the dispersive type and used diffraction gratings. •

Since that time, however, mid-IR instrumentation has dramatically changed so that now the majority of new instruments are of the Fourier transform type.

•

Photometers based on interference filters also find use for measuring the composition of gases and atmospheric contaminants.

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16A THEORY OF IR ABSORPTION SPECTROMETRY

P431

• IR absorption, emission, and reflection spectra for molecular species can be rationalized by assuming that all arise from various changes in energy brought about by transitions of molecules from one vibrational (or rotational) energy state to another. • In this section, we use molecular absorption to illustrate the nature of these transitions. 4

Raman scatterning vs. IR absorption In inelastic scattering, an absorbed photon is re-emitted with lower energy A small fraction of the scattered photons (approximately 1 in 10 million) are scattered by an excitation, with the scattered photons having a frequency different from, and usually lower than, that of the incident photons

In Raman scattering, the difference in energy between the excitation and scattered photons corresponds to the energy required to excite a molecule to a higher vibrational mode.

5

16A-1 Introduction • Figure 16-1 shows a typical output from a commercial IR spectrophotometer. Although the y-axis is shown as linear in transmittance, modern computer-based spectrophotometers can also produce spectra that are linear in absorbance. • The abscissa in this spectrum is linear in wavenumbers with units of reciprocal centimeters (cm-1). A wavelength scale is also shown at the top of the plot. Computer-based spectrophotometers can also produce a variety of other spectral formats such as linear in wavelength, baseline corrected, and derivative and smoothed spectra. FIGURE 16-1 IR absorption spectrum of a thin polystyrene film. Note the scale change on the x-axis at 2000 cm-1.  Note the monotonous multiple peaks at 2800-1600 cm-1, caused by reflection of beam by film surface

6

Cont’d – wavenumber and frequency • A linear wavenumber scale is usually preferred in IR spectroscopy because of the direct proportionality between this quantity and both energy and frequency. The frequency of the absorbed radiation is, in turn, the molecular vibrational frequency actually responsible for the absorption process. • Frequency, however, is seldom if ever used as the abscissa (xaxis) because of the inconvenient size of the unit; that is, a frequency scale of the plot in Figure 16-1 would extend from 1.2×1014 to 2.0×1013 Hz. • Although the axis in terms of wavenumbers is often referred to as a frequency axis, keep in mind that this terminology is not strictly correct because the wavenumber is only proportional to frequency ν. The relationships are given in Equation 16-1.

ν (cm −1 ) =

1

λ ( µm)

× 10 4 ( µm / cm) =

ν ( Hz ) c(cm / s )

7

(16-1)

Dipole Moment Changes during Vibrations and Rotations

• IR radiation is not energetic enough to bring about the kinds of electronic transitions that we have encountered in our discussions of ultraviolet and visible (UV-Vis) radiation. • Absorption of IR radiation is thus confined largely to molecular species that have small energy differences between various vibrational and rotational states. • To absorb IR radiation, a molecule must undergo a net change in dipole moment as it vibrates or rotates. Only under these circumstances can the alternating electric field of the radiation interact with the molecule and cause changes in the amplitude of one of its motions. 8

Cont’d – definition of dipole moment in bonding • For example, the charge distribution around a molecule such as hydrogen chloride is not symmetric because the chlorine has a higher electron density than the hydrogen. Thus, hydrogen chloride has a significant dipole moment and is said to be polar. The dipole moment is determined by the magnitude of the charge difference and the distance between the two centers of charge. •

As a hydrogen chloride (H-Cl) molecule vibrates, a regular fluctuation in its dipole moment occurs, and a field is established that can interact with the electric field associated with radiation. • If the frequency of the radiation exactly matches a natural vibrational frequency of the molecule, absorption of the radiation takes place that produces a change in the amplitude of the molecular vibration. •

Similarly, the rotation of asymmetric molecules around their centers of mass results in periodic dipole moment fluctuations that allow interaction with the radiation field. – at lower 9 frequency.

Cont’d No net change in dipole moment occurs during the vibration or rotation of homonuclear species such as O2, N2, or Cl2.  As a result, such compounds cannot absorb IR radiation. With the exception of a few compounds of this type, all other molecular species absorb IR radiation.

10

IR interactions with molecules – [summary] Dipole changes during vibrations and rotations of molecules UV or visible radiations: causes electronic transition state. IR – not energetic enough to bring about electronic transitions, but causes changes in vibrational or rotational states of molecular dipole moments. IR radiation is absorbed by molecular dipoles ONLY when: The IR frequency matches the frequency of vibrations (or rotation frequency if in microwave ranges).  Quantum states The energy transfer results in changes in the amplitude of vibration. No net changes in dipole moment occur during the vibration or rotation of the following molecules: O2, N2, Cl2, etc.,  No IR absorption for symmetric di-atomic molecules. 11

Vibrations – stretching and bending

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• Vibrations fall into the basic categories of stretching and bending. • Stretching vibration involves a continuous change in the inter-atomic distance along the axis of the bond between two atoms. [symmetric vs. asymmetric types of stretching] • Bending vibrations are characterized by a change in the angle between two bonds and are of four types: scissoring, rocking, wagging, and twisting. These are shown schematically in Figure 16-2 (next page). 12

FIGURE 16-2 Types of molecular vibrations. Note that + indicates motion from the page toward the reader and – indicates motion away from the reader.

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Ch16 An Introduction to Infrared Spectrometry

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FIGURE 16-3 Potential-energy diagrams. (a) harmonic oscillator. (b) Curve 1, harmonic oscillator; Curve 2, anharmonic motion. 15

Ch16 An Introduction to Infrared Spectrometry

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Anharmonic motions and overtones in IR absorption P436

• Refer to Fig. 16-3, Curve-2 (blue) (p. 433) • Anharmonicity leads to deviations of two kinds. • At higher quantum numbers, ∆E (∆E = hνm) becomes small (see curve 2 in Figure 16-3b), and the selection rule is not rigorously followed. • As a result, weaker transitions called overtones (高 頻) are sometimes observed. These transitions correspond to ∆ν = ±2 or ±3. • The frequencies of such overtone transitions are approximately two times that of the fundamental frequency, and the intensities are lower than that of the fundamental. 16

1 νm = 2π

k m

(16-8)

m1m2 µ= m1 + m2

1 νm = 2π

k 1 = m 2π

(16-9)

k (m1 + m2 ) m1m2

(16-10)

17

1 h  E = ν +  2  2π 

k

µ

(16-11)

1  E = ν + hν m 2 

(16-12)

Eradiation = hv = ∆E = hvm =

v = vm =

1 v= 2πc

1 2π

k

µ

h 2π

k

µ

k

(16-14)

µ

= 5.3 × 10

−12

k

(16-15)

µ 18

Force constant in bonds

P435

• IR measurements in conjunction with Equation 16-14 or 1615 permit the evaluation of the force constants for various types of chemical bonds. Generally, k has been found to lie in the range between 3×102 and 8×102 N/m for most single bonds, with 5×102 serving as a reasonable average value. • Double and triple bonds are found by this same means to have force constants of about two and three times this value (1×103 and 1.5×103 N/m, respectively). • With these average experimental values, Equation 16-15 can be used to estimate the wavenumber of the fundamental absorption band, or the absorption due to the transition from the ground state to the first excited state, for a variety of bond types. • The following example (Example 16-1) demonstrates such a calculation. 19

Theoretical wavenumber of C=O bond stretching

20

16A-4. Vibration modes • Number and kinds of vibrations in simple di-atomic or tri-atomic molecules can be estimated. • For complex molecules with many types of atoms and bonds, possible vibrations are difficult to estimate. • For simple molecules of N atoms, 3 coordinates:  3 x N degree of freedom: 3xN - 6

 normal modes

For linear molecules: 3xN - 5

normal modes

Total vibrations:

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16A-5 Vibrational Coupling [偶合]

P437 T

•

The energy of a vibration, and thus the wavelength of the corresponding absorption maximum, may be influenced by (or coupled with) other vibrators in the molecule.

• •

A number of factors influence the extent of such coupling. Strong coupling between stretching vibrations occurs only when there is an atom common to the two vibrations.

•

Interaction between bending vibrations requires a common bond between the vibrating groups.

•

Coupling between a stretching and a bending vibration can occur if the stretching bond forms one side of the angle that varies in the bending vibration.

•

Interaction is greatest when the coupled groups have individual energies that are nearly equal.

•

Little or no interaction is observed between groups separated by two or more bonds.

•

Coupling requires that the vibrations be of the same symmetry species. 22

P438 As an example of coupling effects, let us consider the IR spectrum of carbon dioxide. • If no coupling occurred between the two C=O bonds, an absorption band would be expected at the same wavenumber as that for the C=O stretching vibration in an aliphatic ketone (about 1700 cm-l, or 6 µm; see Example 161). • Experimentally, carbon dioxide exhibits two absorption maxima, one at 2350 cm-1 (4.3 µm), and the other at 667 cm-1 (15 µm). • Carbon dioxide is a linear molecule and thus has (3×3) - 5 = 4 normal modes. Two stretching vibrations are possible; furthermore, interaction between the two can occur because the bonds involved are associated with a common carbon atom. As can be seen, one of the coupled vibrations is symmetric (upper left) and the other is asymmetric (right). 23

IR of carbon dioxide

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Cont’d • The symmetric vibration causes no change in dipole moment, because the two oxygen atoms simultaneously move away from or toward the central carbon atom. Change is one bond is canceled by another in symmetric vibration. • Thus, the symmetric vibration is IR-inactive.  coupling effect. • Note: This can be regarded as: ”Coupling requires that the vibrations be of the same symmetry species.” • In the asymmetric vibration, one oxygen moves away from the carbon atom as the carbon atom moves toward the other oxygen. As a consequence, a net change in charge distribution occurs periodically, producing a change in dipole moment, so absorption at 2350 cm-1 results.

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O=C=O (CO2) • The remaining two vibrational modes of carbon dioxide involve scissoring (bending), as shown here.

These two bending vibrations are the resolved components at 90oC to one another of the bending motion in all possible planes around the bond axis. The two vibrations are identical in energy and thus produce a single absorption band at 667 cm-1. Quantum states that are identical, as these are, are said to be degenerate. 26

Nonlinear molecules: IR active vs. IR non-active

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•

It is of interest to compare the spectrum of carbon dioxide (linear) with that of a nonlinear, triatomic molecule such as water, sulfur dioxide, or nitrogen dioxide. These molecules have (3 × 3) - 6 = 3 vibrational modes that take the following forms:

•

Because the central atom is not in line with the other two, the symmetric stretching vibration produces a change in dipole moment and is thus IR active.

•

For water, for example, stretching peaks at 3657 and 3766 cm-1 (2.74 and 2.66 µm) appear in the IR spectrum for the symmetric and asymmetric stretching vibrations of the water molecule. There is only one component to the scissoring vibration for this nonlinear molecule because motion in the plane of the molecule constitutes a rotational degree of freedom. For water, the bending vibration causes absorption at 1595 cm-1 (6.27 µm).

•

•

 The difference in behavior of linear and nonlinear triatomic molecules with two and three absorption bands, respectively, illustrates how IR absorption spectroscopy can sometimes be used to deduce molecular shapes (linear or 27 nonlinear, etc).

Summary: comparison of Nonlinear, triatomic molecules p. 438 (N=3) with linear ones Nonlinear molecules: such as H2O, SO2, NO2: 3N – 6 = 3x3 - 6 = 3 vibrations, and there are three IR absorptions.

•

•

For water, stretching peaks at 3657 and 3766 cm-1 (2.74 and 2.66 µm) appear in the IR spectrum for the symmetric and asymmetric stretching vibrations of the water molecule. There is only one component to the scissoring vibration for this nonlinear molecule because motion in the plane of the molecule constitutes a rotational degree of freedom. For water, the bending vibration causes absorption at 1595 cm-1 (6.27 µm).

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Linear molecules: such as CO2 [as discussed earlier] 3N - 5 = 3x3 - 5 = 4 vibration modes  Two bending modes are degenerated into one.  Two stretching modes (asymmetric and symmetric). But the symmetric stretching is IR inactive (no dipole moment change).  Thus, in total, there are only two IR absorptions (not four) for linear CO2. 28

The first thing you notice about the background spectrum is there are strong IR peaks near 3800, 2400,1600 cm-1. These peaks are due to the O-H stretch of H2O, the asymmetric stretch of CO2 and the H-O-H bending of H2O, respectively. The H2O and CO2 are present in the air, which fills the spectrometer. The H2O bands consist of many sharp peaks. These peaks are the vibrational transitions between different rotational states of H2O. With a higher resolution instrument, the CO2 band shows similar rotational splitting, but the lines are closer together. Rotational splitting is only observed in the gas phase. Passing a steady stream of nitrogen though 29 the spectrometer can eliminate the H2O and CO2 bands

You can see CO2 as the strong doublet at around 2300 cm-1, and water as the "spiky" peaks (owing to rotational energy differences of water in gas phase) in the 3800 and 1600 cm-1. The "bell curve" shape of the spectrum reflects the output spectrum of the source: strong in the middle, but falling off at the ends. – We will discuss the IR sources later. This background scan (in air) will be subtracted from sample scans.

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Another example of Coupling: examples of C-O bond in different compounds • Coupling of vibrations is a common phenomenon. As a result, the position of an absorption band corresponding to a given organic functional group cannot be specified exactly. • For example, the C-O stretching frequency in methanol is 1034 cm-1 (9.67 µm), in ethanol it is 1053 cm-1 (9.50 µm), and in 2-butanol it is 1105 cm-1 (9.05 µm). [see next page] • These variations result from a coupling of the C-O stretching with adjacent C-C stretching or C-H vibrations [see next page].

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Example for Coupling effects in IR absorption IR absorption peak for a given organic group cannot be always specific exactly, owing to coupling effects from neighboring bands. e.g. –C-O- in methanol, ethanol, and 2-butanol:

C

OH

(C-O) C

C

C

C

ν (C-O stretching) 1034 cm-1

OH

1053 cm-1

C

1105 cm-1

C OH

 Variation (1034, 1053, 1105 cm-1) is owing to “coupling” of C-O 32 stretching with adjacent C-C stretching or C-H stretching bands.

Where is the “C–O peak” for 1-propanol?

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example of coupling by neighboring groups: N-O bond in different compounds. Nitromethane: Asymm. stretch (N-O): 1573. Symm. Stretch (N-O): 1383 cm-1.

Meta-nitrotoluene: Asymm. Stretch (N-O): 1537 cm-1. Symm. Stretch (N-O): 1358 cm-1

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2NO + O2 → 2NO2

Research into NO function led to the 1998 Nobel Prize for discovering the role of nitric oxide as a cardiovascular signaling molecule.

The NO concentration was monitored by IR absorbance in the spectral region of 1960-1780 cm-1 and the NO2 concentration was monitored by IR absorbance in the spectral region of 1660-1550 cm-1 (see Figure 2).

Nitric oxide: N=O double bond Nitrogen dioxide: NO2 single bond

NO2: bond order between one and two. NO test cylinders were prepared to 0.67 ppm (670 ppb) concentration with 35 a target specification for the NO2 of less than 0.03 ppm (30 ppb).

NO2 Asymmetric and symmetric stretching in NO2 For quantitative analyses

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Comparison of bond-lengths in simple structures IR spectroscopy is often used to identify structures because functional groups give rise to characteristic bands both in terms of intensity and position (frequency). The positions of these bands is summarized in correlation tables as shown below. Main article: Infrared spectroscopy correlation table.

Comparison of bond-lengths in simple structures

ethane (1 σ bond)

ethylene (1 σ bond + 1 π bond)

acetylene (1 σ bond + 2 π bonds)

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Questions and Problems *16-7 [Q: Which ones of the following vibrations are IR-active?]

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Ch16 An Introduction to Infrared Spectrometry

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Section B. IR Instrumentation

p. 438

• -To discuss designs and types of IR instruments. • -Comparison of conventional types of IR (Dispersive and non-dispersive) vs. Fourier-transform IR (FTIR).

• -Components in IR instruments: sources and detectors

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16B IR INSTRUMENTATION

p.438

Three types of instruments for IR absorption measurements are commonly available: (1) Dispersive spectrophotometers with a grating monochromator. (All conventional IR instrument) (2) Fourier transform spectrometers employing an interferometer (See Chap. 7-I); (3) Nondispersive photometers using a filter or an absorbing gas that are used for analysis of atmospheric gases at specific wavelengths.

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cont’d p. 438 - Three types of instruments for infrared absorption measurements are available from commercial sources: 1. Dispersive grating spectrophotometers that are used primarily for qualitative work; (Dispersive = conventional IR). Until 1980, this is the most widely used IR instrument. Now, this type of instrument has been largely displaced by FT-IR because of speed, reliability, convenience, and costs. 2. Multiplex instruments (FT-IR), employing the Fourier transform (Section 7-I), that are suited to both qualitative and quantitative infrared measurements. (All modern IR instruments are now FT-IR types) 3. Non-dispersive photometers that have been developed for quantitative determination of a variety of organic species in the atmosphere by absorption, emission, and reflectance 41 spectroscopy.

Cont’d Until the 1980s, the most widely used instruments for IR measurements were dispersive spectrophotometers. Now, however, this type of instrument has been largely displaced for mid- and far-IR measurements by Fourier transform spectrometers because of their speed, reliability, signal-to-noise advantage, and convenience. Dispersive spectrometers are still used in the near-IR where they are often extensions of UV-visible instruments. [Most UV-Vis instruments are not Fouriertransformed]  But many dedicated near-IR instruments are now of the Fourier transform-IR (FTIR) type. 42

Dispersive Infrared Spectrophotometer

Wavelength range 4000 cm-1 to 200 cm-1 Abscissa accuracy: 4000 cm-1 to 2000 cm-1 = +/- 3 cm-1 2000 cm-1 to 600 cm-1 = +/- 1.5 cm-1 Ordinate Accuracy: +/- 1.0% Price: $2,500.00 USD

The "dispersive" or "scanning monochromator" IR method is based on a technique where one wavelength at a time passes through the sample. The dispersive method is more common in UV-Vis spectroscopy, but is less 43 practical in the infrared than the FTIR method

16B-1. Fourier-Transform Spectrometers (FT-IR) Components of FT-IR Instruments: Most are based on Michelson interferometers. (Nobel prize winner) - see Fig.

7-42 earlier in Chap. 7. Interferometer: as following, Fig. 16-4.

p. 440

FIGURE 16-4 Interferometers in an FTIR spectrometer. Subscript 1 defines the radiation path in the IR interferometer. Subscripts 2 and 3 refer to the laser and 44 white-light interferometers, respectively.

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Figure 1. The light path through a Michelson interferometer. The two light rays with a common source combine at the half-silvered mirror to reach the detector. They may either interfere constructively (strengthening in intensity) if their light waves arrive in phase, or interfere destructively (weakening in intensity) if they arrive out of phase, depending on the exact distances between the three mirrors.… Interferometers are widely used in science and industry for the measurement of small displacements, refractive index changes and surface irregularities. In analytical science, interferometers are used in continuous wave Fourier transform spectroscopy to analyze light containing features of absorption or emission associated with a substance or mixture. 46

FT instrumentation Now let's look at an FT instrument.

Now, we send all the source energy through an interferometer and onto the sample. In every scan, all source radiation gets to the sample! The interferometer is a fundamentally different piece of equipment than a monochromator. The light passes through a beamsplitter, which sends the light in two directions at right angles. One beam goes to a stationary mirror then back to the beamsplitter. The other goes to a moving mirror. The motion of the mirror makes the total path length variable versus that taken by the stationarymirror beam. When the two meet up again at the beamsplitter, they recombine, but the difference in path lengths creates constructive and destructive interference:47 an interferogram:

The recombined beam passes through the sample. The sample absorbs all the different wavelengths characteristic of its spectrum, and this subtracts specific wavelengths from the interferogram. The detector now reports variation in energy versus time for all wavelengths simultaneously. A laser beam is superimposed to provide a reference for the instrument operation. Energy versus time is an odd way to record a spectrum, until you recognize the relationship between time and frequency: they are reciprocals! A mathematical function called a Fourier transform allows us to convert an intensity-vs.-time spectrum into an intensity-vs.-frequency spectrum. The Fourier transform:

A(r) and X(k) are the frequency domain and time domain points, respectively, for a spectrum of N points.

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An interferogram

A spectrum (polystyrene) – after Fourier transform

There are several advantages to this design: 1.All of the source energy gets to the sample, improving the inherent signal-to-noise ratio. 2.Resolution is limited by the design of the interferometer. The longer the path of the moving mirror, the higher the resolution. Even the least expensive FT instrument provides better resolution that all but the best CW instruments were capable of. 49 3.The digitization and computer interface allows multiple scans to be collected, also dramatically improving the signal-to-noise ratio.

Procedures of data acquisition: 1. Reference scan: A typical procedure for determining transmittance or absorbance with this type of instrument is to first obtain a reference interferogram by scanning a reference (usually air) 20 or 30 times, coadding the data, and storing the results in the memory of the instrument computer (usually after transforming it to the spectrum). 2. Sample scans: A sample is then inserted in the radiation path and the process repeated. 50

FIGURE 16-5 Timedomain signals for the three interferometers contained in many FTIR instruments. Curve A, IR signal; curve B, white-light signal; curve C, laserfringe reference signal; curve D, square-wave electrical signal formed from the laser signal.

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Ch16 An Introduction to Infrared Spectrometry

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An interferogram from an FTIR measurement. The horizontal axis is the position of the mirror, and the vertical axis is the amount of light detected. This is the "raw data" which can be Fourier transformed to get the actual spectrum.…

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A NEW FTIR spectrometer will be added to the Laboratory 儀分實驗, starting 2013 spring.

FIGURE FTIR spectrometer used in the Laboratory 儀分實驗

FIGURE 16-6 Photo of a basic, benchtop FTIR spectrometer suitable for student use. Spectra are recorded in a few seconds and displayed on the LCD panel for viewing and interpretation. The spectra may be stored in an memory card for later retrieval and analysis, or they may be printed. 53 Ch16 An Introduction to Infrared Spectrometry

P.441

FIGURE 16-7 Diagram of a basic FTIR spectrometer. 1. Radiation of all frequencies from the IR source is reflected into the interferometer where it is modulated by the moving mirror on the left. 2. The modulated radiation is then reflected from the two mirrors on the right through the sample in the compartment at the bottom. After passing through the sample, the radiation falls on the transducer. A data-acquisition system attached to the transducer records the signal and stores it in the memory of a computer as an interferogram.

Ch16 An Introduction to Infrared Spectrometry

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P.442

Question: What is the use of “laser beam” in FTIR? Is laser used for FTIR as a source of IR radiation? Commercial FTIR's used lamps or Nernst cylinder as a IR source. But, manufacturers often install laser in their spectrometers for self calibration of interferometer (not as IR source). The three well-known advantages of FTIR setup ( compared to dispersive) are : 1) The multiplex or Fellgett's advantage. This arises from the fact that information from all wavelengths is collected simultaneously. It results in a higher Signal-to-noise ratio for a given scan-time or a shorter scan-time for a given resolution. 2) The throughput or Jacquinot's advantage. This results from the fact that, in a dispersive instrument, the monochromator has entrance and exit slits which restrict the amount of light that passes through it. The interferometer throughput is determined only by the diameter of the collimated beam coming from the source. 3) The cones advantages. The use a of small HeNe laser (not as a source) 55 allows a very good reproducibility of the wavelength or wavenumbers.

Instrument Designs

P. 441-444

• FT-IR: Usually SINGLE-BEAM  Fig. 16-8 • FTIR spectrometers can be single-beam or double-beam instruments. Figure 16-8 shows the optics of a basic single-beam spectrometer, which sells in the range of $15,000 to $20,000 (USD). • A typical procedure for determining transmittance or absorbance with this type of instrument is to first obtain a reference interferogram by scanning a reference (usually air) twenty or thirty times, co-adding the data, and storing the results in the memory of the instrument computer (usually after transforming it to the spectrum). • A sample is then inserted in the radiation path and the process repeated. The ratio of sample and reference spectral data is then computed to give the transmittance at various frequencies. From this ratio, the absorbance is calculated as a function of wavenumber. • Ordinarily, modern IR sources and detectors are sufficiently stable so that reference spectra need to be obtained only occasionally.

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FIRURE 16-8 Single-beam FTIR spectrometer.

In one arm of the interferometer, the IR source radiation travels through the beam-splitter to the fixed mirror, back to the beam splitter, and through the sample to the IR transducer. In the other arm, the IR source radiation travels to the beam splitter, is reflected to the movable mirror, and travels back through the beam splitter to the sample and to the transducer. When the two beams meet again at the beamsplitter, they can interfere with each other if the phase difference (path difference) is appropriate. A plot of the signal versus mirror displacement is the interferogram. FIRURE 16-8 Single-beam FTIR spectrometer.

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Ch16 An Introduction to Infrared Spectrometry

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Cont’d- Single-beam FTIR spectrometer. The interferogram contains information about all the frequencies present. The spectrum, intensity versus wavenumber, is the FT of the interferogram. It can be calculated with a computer from the signal versus mirror displacement. An empty sample compartment allows the reference spectrum to be calculated. Next, the sample is placed in the sample compartment and the sample spectrum is obtained. The absorbance is then calculated at each wavenumber from the ratio of the sample intensity to the reference intensity.

Double-beam FT-IR spectrometer • A double-beam spectrometer (FT-IR) is illustrated in Figure 16-9. • The mirrors directing the interferometer beam through the sample and reference cells are oscillated rapidly compared to the movement of the interferometer mirror so that sample and reference information can be obtained at each mirror position. • The double-beam design compensates for source and detector drifts. The beam emerging from the interferometer strikes mirror M1, which in one position directs the beam through the reference cell and in the other position directs it through the sample cell. Mirror M2, which is synchronized to M1, alternately directs the reference beam and FIGURE 16-9 Double-beam FTIR the sample beam to the spectrometer. transducer.

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Performance Characteristics of Commercial FTIR Instruments

A number of instrument manufactures offer several models of Fourier transform infrared instruments. The least expensive of these has a range of 7800 to 350 cm-1 (1.3 to 29 μm) with a resolution of 4 cm-1. This performance can be obtained with a scan time as brief as one second.  More expensive instruments with interchangeable beam splitters, sources, and transducers offer expanded frequency ranges and higher resolutions, or higher S/N ratios. 61

Advantages of FT Spectrometers •

Over most of the mid-IR spectral range, FT instruments have signal-to-noise (S/N) ratios that are better than those of a good-quality dispersive instrument, usually by more than an order of magnitude. The enhanced signal-to-noise ratio can, of course, be traded for rapid scanning, with good spectra being attainable in a few seconds in most cases.

•

Interferometric instruments are also characterized by high resolutions ( 50 µm) -Tungsten filament lamp – near IR region (400012,800 cm-1) -Carbon dioxide laser source – for monitoring atmosphere pollutants, or absorbing species in aqueous solutions.

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• 16C-2 Infrared Transducers p.451 Three types of IR transducers: (1) Thermal transducer. (2) Pyroelectric detector (transducer) (which is a specialized “thermal transducer”). (3) Photoconducting detector (transducer). Response characteristics of these three types of detectors increase in this order: (1) < (2) < (3).

76

2. Pyroelectric detectors

p.451

(It can be viewed as a specialized thermal transducers): -Constructed from single wafers of pyroelectric materials, with very special thermal and electrical properties:

 Pyroelectric transducers are constructed from single crystalline wafers of pyroelectric materials, which are insulators (dielectric materials) with very special thermal and electrical properties. Deuterated triglycine sulfate (DTGS) (NH2CH2COOH)3.H2SO4.

Triglycine sulfate (NH2CH2COOH)3．H2S04 (usually deuterated or with a fraction of the glycines replaced with alanine), is the most important pyroelectric material used for IR detection systems.  Pyroelectric detector (DTGS) for FT-IR (Nicolet) shown for class demonstration. 77

Cont’d Mechanism of pyroelectric transducers as an IR sensor: IR radiation  temp. change  elec. current change. Pyroelectric transducers exhibit response times that are fast enough to allow them to track the changes in the time-domain signal from an interferometer. For this reason, many FTIR spectrometers for the mid-IR region employ this type of transducer. Most modern FT-IR instrument uses pyroelectric transducers (DTGS) for faster responses.

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3. Photoconducting transducers:

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Mechanism: Photon (IR Radiation)  electric resistance of semiconductors. Semiconductors suitable for photoconducting detectors: -Lead sulfide (for near IR, operated at room temp.) -Indium antimonide (InSb) -Mercury cadmium telluride (MCT) Liq. N2 (77K) cooling required. For MCT, mercury/cadmium ratios can be varied to suit various wavelength ranges.

Photoconducting transducers provide better sensitivity or responses, and higher S/N ratios.  FT-IR always uses either pyroelectric or photoconducting transducers for faster responses. Pyroelectric transducers (DTGS) are widely used for the mid-infrared region. Where better sensitivity or faster response times are required, liquidnitrogen-cooled mercury/cadmium telluride (MCT) or indium antimonide photoconductive transducers are employed. 79

Photoconducting detector for near, mid, far-IR • Lead sulfide photoconductor is the most widely used transducer for the near-IR region of the spectrum from 10,000 to 333 cm-1 (1 to 3 µm). It can be operated at room temperature. • MCT photoconductor transducers are used for mid- and far-IR radiation. • MCT must be cooled with liquid nitrogen (77 K) to minimize thermal noise. • The long-wavelength cutoff, and many of the other properties of these transducers, depend on the ratio of the mercury telluride to cadmium telluride, which can be varied continuously. • MCT transducer finds widespread use in FTIR spectrometers, particularly in those requiring fast response times, such as spectrometers interfaced to gas chromatographs. 80

Thermal Transducers • Thermal transducers, whose responses depend on the heating effect of radiation, are found in older dispersive spectrometers for detection of all but the shortest IR wavelengths. • With these devices, the radiation is absorbed by a small blackbody and the resultant temperature rise is measured. • The radiant power level from a spectrophotometer beam is minute (10-7 to 10-9 W), so that the heat capacity of the absorbing element must be as small as possible if a detectable temperature change is to be produced. • Under the best of circumstances, temperature changes are confined to a few thousandths of a Kelvin (i.e., 0.00x K).

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Cont’d- Thermal Transducers • The problem of measuring IR radiation by thermal means is compounded by thermal noise from the surroundings. • For this reason, thermal transducers are housed in a vacuum and are carefully shielded from thermal radiation emitted by other nearby objects. • To further minimize the effects of extraneous heat sources, the beam from the source is always chopped [-interrupted intermittently]. In this way, the analyte signal, after transduction, has the frequency of the chopper and can be separated electronically from extraneous noise signals, which are ordinarily broad band or vary only slowly with time.

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Thermal detectors in nature – similar to thermal transducer in IR instrument. - The following is some thermal detectors in biology.

The snake is then able to detect the temperature difference between the two chambers. This system is so accurate that pit vipers are actually able to detect temperature changes as little as 0.002 degrees centigrade. 83

End of Chap. 16 FT-IR spectroscopy

Contents covered were: Theory of molecular vibrations for IR absorption, Instrument design, interferometer, IR sources, detectors. Next class: -Chap. 17: Applications of IR Spectrometry 84