**5. Practical aspects of infrared spectrophotometric analysis**

The IR absorption spectrum is the graphical representation of a measure of energy depending on a measure of wave of the involved radiation. IR practice has established the use of the wave number (reciprocal of the wave length and proportional to the frequency of the radiation) and of the percentage transmission (T%) or absorbance (A), as related to the energy of the radiation.

Organic Compounds FT-IR Spectroscopy 155

the sample spectrum in these areas, is repeat the recording spectrum in another solvent that is transparent.omide, allows the identify the crystallization form of the interest substance. Another technical detail is related by the fact that in IR absorption domain the solvents presents absorption bands, sometimes quite strong. Some IR spectrophotometers operating in double beam mode (similar to the double beam spectrophotometers used in the spectral UV-Vis). In the case of UV-Vis spectrophotometers is introduced, in the right reference optical path, a vat filled with pure solvent, and the right of the second optical path is introduced a vat of the same thickness, filled with solution (solvent and solute). The electronics parts of the spectrophotometer compare the absorbances of vats located in the two optical paths and subtract the absorbance of the solvent, located in the reference route, from solution absorbances located in the route of sample. Because the interest substances absorbance is marked in UV-Vis, and the absorbance of solvent is insignificant, the difference between the absorbances associated with two optical route is almost always

In IR domain, where the absorbances of dissolved substances are comparable with those of solvents. It is easily understood that if a certain place in the spectrum (the number of

ε0( v **)** and the solute does not absorb significantly at that wavenumbers (ε**(** v **)** small), then, the same layer thickness "d", the absorbance values measured in the two opticalroute (**Aref(** v **)** for route reference and **A(** v **)** for solution route), are expressed by the relations (9).

> \* 00 0

=⋅ ⋅ +⋅ ⋅

A(v) d ε (v) C d ε (v) C

In the (9) relation, **C0** şi **C**\***<sup>0</sup>** represents molar concentration of pure solvent in the two cells (placed in reference route and in the sample route respectively), and C is the molar concentration of solute in the cells placed in the route sample. It is obvious that **C**\***0** < **C0** because in the sample cells is in addition to solvent, a quantity of solution. The signal recorded by spectrometry, Δ**A(** v **)**, to the number of wave **(** v **)**, is the difference between the

The first term in the right side of the parenthesis right above relationship is negative, because **C**\***0** < **C0** . If ε0( v ).is significantly higher than ε**(** v **)**, then it can happen that Δ**A(** v **)** to have a negative value for the number of wave v . Obviously, such a negative value of absorbance is an artifact, without spectrophotometric real significance. To overcome this problem, manifested in solutions recording spectra, are used in the reference route a cell with variable thickness. The solution is placed in a cell of fixed thickness"**d\***", lower than thickness"**d**", in the reference route. The absorbances **A**\***ref(** v **)** and **A(** v **)**, and the difference of absorbances, Δ**A\*(** v **)**, associated with the two optical route, the new working conditions,

() ()

A vd ε v C

∗ ∗ =⋅ ⋅

() () () ref 0 0

Av d ε vC d ε v C

\* 0 0

=⋅ ⋅ +⋅ ⋅

ref 0 0

=⋅ ⋅

A (v) d ε (v) C

) the solvent has a strong absorption band (molar absorptivity large solvent

(9)

[ . ε0( v )·C\*0 – C0) + ε( v ).

(11)

C] (10)

positive.

wavelengths ΰ

both absorbance of relationship (9)

are given by the relations (11).

ΔA( v )= A( v ) – Aref( v ) = d.

There are significant differences between the UV-Vis and IR absorption spectra. IR spectra, even those of samples in condensed states, are generally characterized by a large number of well defined, sharp bands, with easily localizable positions. Therefore, IR spectra are useful for the fast, non-destructive identification of the chemical substances, and it is extremely unlikely that two substances that are chemically different to have, accidentally, identical IR spectra. Instead, the quantitative determinations in the IR spectral range are more difficult because diffuse radiation in the IR spectrophotometers is greater than in the UV-Vis spectrophotometer, so the error sources affect the results of quantitative determinations more than in the UV or Vis range.

Another difference between the two spectral areas consists in the transparency (and usability) of auxiliary materials (glass, optics, etc.). Spectra of liquid samples are recorded using similar cells to those used in UV-Vis spectrophotometry, except that glass walls are made of specific transparent materials (NaCl, KBr, KCl, ZnSe, As2S3, KRS-5, and others). The thickness of the sample in IR spectrophotometry are usually much smaller (0.05 mm – 1 mm) than those found in the UV-Vis absorption spectrophotometry (1 – 10 mm).

The IR absorption spectra can be recorded for solid, liquid or gaseous samples. The most common presentation state of samples in drug control is solid state. Commonly practiced method for obtaining IR spectra of substances or mixtures of pharmaceutical interest in solid form, consists in incorporate them into a solid, microcrystalline medium (for exemple potassium bromide). This method of sample preparation is called "inclusion in the tablet (or pill) of potassium bromide." To achieve such a compressed is triturated a small amount medium (1-2 mg) of solid interest with 200-250 mg of potassium bromide microcrystalline. Potassium bromide used for this purpose must be high purity (purity "for spectroscopy") and dried before use for several hours at 180 ° C. The triturating of solid mixture containing the substance of interest and potassium bromide, is running in agate mortar medium (glass or porcelain mortar is not appropriate). After the grain sufficiently fine, solid mixture is placed in a special mold will compress high pressure medium (about 10 ton-force) with a hydraulic press. Before applying pressure, air is removed from the stencil to prevent inclusion of air microbubbles in solid mass during pressing, that may produce microfissure in mass tablet at the end pressure . During pressing, the potassium bromide microcrystalline are sinterising forming a solid transparent, optically homogeneous. A compressed pellet carried out in ideal conditions is transparent without opaque area. In the spectral range 4000 - 300 cm-1 potassium bromide shows very good transparency, which is why this mode is used preferentially for sample preparation. Whereas, the included technique of sample in a potassium bromide matrix keeps crystallization form of the sample solid, the IR spectrum obtained by compression in potassium bromide is dependent from the crystallization of the sample. For substances that shows polymorphic, the IR absorption spectrum of solid samples , included in compressed potassium br An essential difference between the UV-Vis spectra registration procedure and the IR field is that in the UV-Vis domain, the solvents absorbtion is insignificant, so their absorption can be compensate, in the case of IR domain all used solvents presents its own band absorption, sometimes even more powerful, that in these areas of the spectrum of energy received by the detector is too small to differentiate the strong absorption of the solvent from the sample absorption that exceeding only in small extent the solvent absorbtion. For this reason, in the IR absorption spectra of the solutions are frequently areas where the IR radiation detector is inactive, and the signal recorded in these areas is irrelevant. To view

There are significant differences between the UV-Vis and IR absorption spectra. IR spectra, even those of samples in condensed states, are generally characterized by a large number of well defined, sharp bands, with easily localizable positions. Therefore, IR spectra are useful for the fast, non-destructive identification of the chemical substances, and it is extremely unlikely that two substances that are chemically different to have, accidentally, identical IR spectra. Instead, the quantitative determinations in the IR spectral range are more difficult because diffuse radiation in the IR spectrophotometers is greater than in the UV-Vis spectrophotometer, so the error sources affect the results of quantitative determinations

Another difference between the two spectral areas consists in the transparency (and usability) of auxiliary materials (glass, optics, etc.). Spectra of liquid samples are recorded using similar cells to those used in UV-Vis spectrophotometry, except that glass walls are made of specific transparent materials (NaCl, KBr, KCl, ZnSe, As2S3, KRS-5, and others). The thickness of the sample in IR spectrophotometry are usually much smaller (0.05 mm – 1

The IR absorption spectra can be recorded for solid, liquid or gaseous samples. The most common presentation state of samples in drug control is solid state. Commonly practiced method for obtaining IR spectra of substances or mixtures of pharmaceutical interest in solid form, consists in incorporate them into a solid, microcrystalline medium (for exemple potassium bromide). This method of sample preparation is called "inclusion in the tablet (or pill) of potassium bromide." To achieve such a compressed is triturated a small amount medium (1-2 mg) of solid interest with 200-250 mg of potassium bromide microcrystalline. Potassium bromide used for this purpose must be high purity (purity "for spectroscopy") and dried before use for several hours at 180 ° C. The triturating of solid mixture containing the substance of interest and potassium bromide, is running in agate mortar medium (glass or porcelain mortar is not appropriate). After the grain sufficiently fine, solid mixture is placed in a special mold will compress high pressure medium (about 10 ton-force) with a hydraulic press. Before applying pressure, air is removed from the stencil to prevent inclusion of air microbubbles in solid mass during pressing, that may produce microfissure in mass tablet at the end pressure . During pressing, the potassium bromide microcrystalline are sinterising forming a solid transparent, optically homogeneous. A compressed pellet carried out in ideal conditions is transparent without opaque area. In the spectral range 4000 - 300 cm-1 potassium bromide shows very good transparency, which is why this mode is used preferentially for sample preparation. Whereas, the included technique of sample in a potassium bromide matrix keeps crystallization form of the sample solid, the IR spectrum obtained by compression in potassium bromide is dependent from the crystallization of the sample. For substances that shows polymorphic, the IR absorption spectrum of solid samples , included in compressed potassium br An essential difference between the UV-Vis spectra registration procedure and the IR field is that in the UV-Vis domain, the solvents absorbtion is insignificant, so their absorption can be compensate, in the case of IR domain all used solvents presents its own band absorption, sometimes even more powerful, that in these areas of the spectrum of energy received by the detector is too small to differentiate the strong absorption of the solvent from the sample absorption that exceeding only in small extent the solvent absorbtion. For this reason, in the IR absorption spectra of the solutions are frequently areas where the IR radiation detector is inactive, and the signal recorded in these areas is irrelevant. To view

mm) than those found in the UV-Vis absorption spectrophotometry (1 – 10 mm).

more than in the UV or Vis range.

the sample spectrum in these areas, is repeat the recording spectrum in another solvent that is transparent.omide, allows the identify the crystallization form of the interest substance. Another technical detail is related by the fact that in IR absorption domain the solvents presents absorption bands, sometimes quite strong. Some IR spectrophotometers operating in double beam mode (similar to the double beam spectrophotometers used in the spectral UV-Vis). In the case of UV-Vis spectrophotometers is introduced, in the right reference optical path, a vat filled with pure solvent, and the right of the second optical path is introduced a vat of the same thickness, filled with solution (solvent and solute). The electronics parts of the spectrophotometer compare the absorbances of vats located in the two optical paths and subtract the absorbance of the solvent, located in the reference route, from solution absorbances located in the route of sample. Because the interest substances absorbance is marked in UV-Vis, and the absorbance of solvent is insignificant, the difference between the absorbances associated with two optical route is almost always positive.

In IR domain, where the absorbances of dissolved substances are comparable with those of solvents. It is easily understood that if a certain place in the spectrum (the number of wavelengths ΰ ) the solvent has a strong absorption band (molar absorptivity large solvent ε0( v **)** and the solute does not absorb significantly at that wavenumbers (ε**(** v **)** small), then, the same layer thickness "d", the absorbance values measured in the two opticalroute (**Aref(** v **)** for route reference and **A(** v **)** for solution route), are expressed by the relations (9).

$$\begin{aligned} \mathbf{A}\_{\text{ref}}(\bar{\mathbf{v}}) &= \mathbf{d} \cdot \boldsymbol{\varepsilon}\_{0}(\bar{\mathbf{v}}) \cdot \mathbf{C}\_{0} \\ \mathbf{A}(\bar{\mathbf{v}}) &= \mathbf{d} \cdot \boldsymbol{\varepsilon}\_{0}(\bar{\mathbf{v}}) \cdot \mathbf{C}\_{0}^{\*} + \mathbf{d} \cdot \boldsymbol{\varepsilon}\_{0}(\bar{\mathbf{v}}) \cdot \mathbf{C} \end{aligned} \tag{9}$$

In the (9) relation, **C0** şi **C**\***<sup>0</sup>** represents molar concentration of pure solvent in the two cells (placed in reference route and in the sample route respectively), and C is the molar concentration of solute in the cells placed in the route sample. It is obvious that **C**\***0** < **C0** because in the sample cells is in addition to solvent, a quantity of solution. The signal recorded by spectrometry, Δ**A(** v **)**, to the number of wave **(** v **)**, is the difference between the both absorbance of relationship (9)

$$\Delta \mathbf{A}(\bar{\mathbf{v}}) = \mathbf{A}(\bar{\mathbf{v}}) - \mathbf{A}\_{\text{ref}}(\bar{\mathbf{v}}) = \mathbf{d} \cdot \left[ -\varepsilon \mathbf{0}(\bar{\mathbf{v}}) \, \mathbf{C}^{\star} \mathbf{e} - \mathbf{C} \mathbf{o} \right] + \varepsilon (\bar{\mathbf{v}}) \cdot \mathbf{C} \, \mathbf{J} \tag{10}$$

The first term in the right side of the parenthesis right above relationship is negative, because **C**\***0** < **C0** . If ε0( v ).is significantly higher than ε**(** v **)**, then it can happen that Δ**A(** v **)** to have a negative value for the number of wave v . Obviously, such a negative value of absorbance is an artifact, without spectrophotometric real significance. To overcome this problem, manifested in solutions recording spectra, are used in the reference route a cell with variable thickness. The solution is placed in a cell of fixed thickness"**d\***", lower than thickness"**d**", in the reference route. The absorbances **A**\***ref(** v **)** and **A(** v **)**, and the difference of absorbances, Δ**A\*(** v **)**, associated with the two optical route, the new working conditions, are given by the relations (11).

$$\begin{aligned} \mathbf{A}\_{\text{ref}}^{\*}\left(\bar{\mathbf{v}}\right) &= \mathbf{d}^{\*} \cdot \boldsymbol{\varepsilon}\_{0}\left(\bar{\mathbf{v}}\right) \cdot \mathbf{C}\_{0} \\ \mathbf{A}\left(\bar{\mathbf{v}}\right) &= \mathbf{d} \cdot \boldsymbol{\varepsilon}\_{0}\left(\bar{\mathbf{v}}\right) \cdot \mathbf{C}\_{0}^{\*} + \mathbf{d} \cdot \boldsymbol{\varepsilon}\left(\bar{\mathbf{v}}\right) \cdot \mathbf{C} \end{aligned} \tag{11}$$

Organic Compounds FT-IR Spectroscopy 157

the ratio of useful signal and noise is more disadvantageous. The new concept of Fourier transform IR spectrophotometers meant an important step in achieving spectra with high quality even for difficult samples where the spectrophotometers with traditional

Construction scheme and specific features of operating mode for a Fourier Transform Infrared Spectrophotometer (FTIR – "F*ourier* **T**ransform **I**nfra**R**ed") are presented in Figure 4. It is noted that the optical assembly has no monochromator, which is replaced by a Michelson interferometer type. The polychromatic radiation from **LS** (light source) source is transmitted through concave mirror **M1** (mirror 1) and radiation divider **BS2** (beam splitter 2) to sample **S** (sample). After crossing the sample, the radiation reach the radiation divider **BS1** (beam splitter 1) that divides the flow of radiation in two tracks: one for the mirrors **M2** (mirror 2) and another for the mirrors **M3** (mirror 3). Mirrors **M2** and **M3** turn back the radiation to the radiation divider **BS1**. Reaching its surface, radiations which had different routes, merge and produce a interference phenomena. The only constructive element moving while recording the spectrum is the set of **M3** mirrors. If the mirrors **M3** are in position **A**, the optical path difference, corresponding to the two optical paths is null, thus the radiations which turn back on the surface of the radiation divider produces an interference maximum. By translation of **M3** mirrors, optical path difference δ between the

Fig. 4 Construction and operating scheme for Fourier Transform Infrared

construction proved to be powerless.

two routes is changed progressive.

Spectrophotometer

If we choose suitable variable thickness of the cell in the reference route, the expression ΔA\*( v ) = A( v ) – A\*ref( v ~) = ε0( v ). (d. C\*0 – d\*. C0) + d. ε( v ). C

If we choose suitable variable thickness of the cell in the reference route, the expression **(d.C**\***0 – d\*.C0)** is null for any value of wavenumber (as the expression does not depend on the number of wavelengths), so the choice of suitable thickness **d\*** resolve the problem reported for the entire spectrum. In this case the absorbance of the solute depends on its concentration, according to the relation BLB.

$$\Delta \mathbf{A}^\*(\bar{\mathbf{v}}) = \mathbf{A}(\bar{\mathbf{v}}) - \mathbf{A}\_{\text{ref}}(\bar{\mathbf{v}}) \tag{12}$$

Because IR absorption spectra are generated by transitions between vibrational states of sample molecules and the frequency of normal modes vibration depends (in addition to the force constants associated with deformations of the molecule) of the masses of atoms, it is expected that the replacement of atoms in molecular structure sample with different isotopes of the respective atoms (isotopic marking of the molecule) to induce dramatic changes of the IR absorption spectrum of the sample.

By isotopic marking in the known positions of molecule and by confronting these changes with changes in IR absorption band positions, significant conclusions can be drawn on whether the different atoms are involved in the normal modes of vibration of the molecule. If an atom of molecule is replaced by its heavier isotope, then IR absorption bands is moving to lower wavenumbers. The most significant movement is found in these absorption bands corresponding to normal vibration modes which involves mostly the replaced atom with heavier isotope.

The biggest relative change in mass of an atom by isotopic substitution is made for replacement of the hydrogen atom (isotope 1H) with deuterium (2H isotope). It follows that by sample deuterating, the IR absorption bands associated with chemical bonds in which one of the atom is hydrogen, suffer very significant movement toward smaller wave numbers.

Isotopic displacement of absorption bands is useful and for choice of suitable solvent in those cases where the IR absorption spectrum should be recorded in solution. Because of own absorption, some solvent (eg chloroform, HCCl3) can not be used except in those domain where this is sufficiently transparent. The spectral regions in which the chosen solvent substantially absorbed are not used. But if using deuterated solvent (e.g. deuterocloroform, DCCl3), this have unusable areas at other wavenumbers. Original solvent (undeuterated) and deuterated solvent presents identical dissolution properties, but are complementary with respect to transparency in the IR spectral range.

### **6. Aspects of construction and specific features of operating mode for Fourier transform spectrophotometers (FTIR)**

Old design spectrophotometers work similar with those for UV-Vis domain, i.e. are composed of radiation source, monochromator designed to select a desired wavelength radiation, the sample chamber and the radiation detector. In IR domain, diffuse radiation presents more serious problems then in ultraviolet and visible domain. Thus, in IR domain,

If we choose suitable variable thickness of the cell in the reference route, the expression

If we choose suitable variable thickness of the cell in the reference route, the expression **(d.C**\***0 – d\*.C0)** is null for any value of wavenumber (as the expression does not depend on the number of wavelengths), so the choice of suitable thickness **d\*** resolve the problem reported for the entire spectrum. In this case the absorbance of the solute depends on its

Because IR absorption spectra are generated by transitions between vibrational states of sample molecules and the frequency of normal modes vibration depends (in addition to the force constants associated with deformations of the molecule) of the masses of atoms, it is expected that the replacement of atoms in molecular structure sample with different isotopes of the respective atoms (isotopic marking of the molecule) to induce dramatic

By isotopic marking in the known positions of molecule and by confronting these changes with changes in IR absorption band positions, significant conclusions can be drawn on whether the different atoms are involved in the normal modes of vibration of the molecule. If an atom of molecule is replaced by its heavier isotope, then IR absorption bands is moving to lower wavenumbers. The most significant movement is found in these absorption bands corresponding to normal vibration modes which involves mostly the replaced atom with

The biggest relative change in mass of an atom by isotopic substitution is made for replacement of the hydrogen atom (isotope 1H) with deuterium (2H isotope). It follows that by sample deuterating, the IR absorption bands associated with chemical bonds in which one of the atom is hydrogen, suffer very significant movement toward smaller wave

Isotopic displacement of absorption bands is useful and for choice of suitable solvent in those cases where the IR absorption spectrum should be recorded in solution. Because of own absorption, some solvent (eg chloroform, HCCl3) can not be used except in those domain where this is sufficiently transparent. The spectral regions in which the chosen solvent substantially absorbed are not used. But if using deuterated solvent (e.g. deuterocloroform, DCCl3), this have unusable areas at other wavenumbers. Original solvent (undeuterated) and deuterated solvent presents identical dissolution properties, but are

complementary with respect to transparency in the IR spectral range.

**Fourier transform spectrophotometers (FTIR)** 

**6. Aspects of construction and specific features of operating mode for** 

Old design spectrophotometers work similar with those for UV-Vis domain, i.e. are composed of radiation source, monochromator designed to select a desired wavelength radiation, the sample chamber and the radiation detector. In IR domain, diffuse radiation presents more serious problems then in ultraviolet and visible domain. Thus, in IR domain,

C0) + d.

ε( v ). C

> ε( v ) .

C (12)

C\*0 – d\*.

. (d.

ΔA\*( v ) = A( v ) – A\*ref( v ~) = ε0( v )

heavier isotope.

numbers.

concentration, according to the relation BLB.

ΔA\*( v ) = A( v ) – Aref( v ) = d.

changes of the IR absorption spectrum of the sample.

the ratio of useful signal and noise is more disadvantageous. The new concept of Fourier transform IR spectrophotometers meant an important step in achieving spectra with high quality even for difficult samples where the spectrophotometers with traditional construction proved to be powerless.

Construction scheme and specific features of operating mode for a Fourier Transform Infrared Spectrophotometer (FTIR – "F*ourier* **T**ransform **I**nfra**R**ed") are presented in Figure 4. It is noted that the optical assembly has no monochromator, which is replaced by a Michelson interferometer type. The polychromatic radiation from **LS** (light source) source is transmitted through concave mirror **M1** (mirror 1) and radiation divider **BS2** (beam splitter 2) to sample **S** (sample). After crossing the sample, the radiation reach the radiation divider **BS1** (beam splitter 1) that divides the flow of radiation in two tracks: one for the mirrors **M2** (mirror 2) and another for the mirrors **M3** (mirror 3). Mirrors **M2** and **M3** turn back the radiation to the radiation divider **BS1**. Reaching its surface, radiations which had different routes, merge and produce a interference phenomena. The only constructive element moving while recording the spectrum is the set of **M3** mirrors. If the mirrors **M3** are in position **A**, the optical path difference, corresponding to the two optical paths is null, thus the radiations which turn back on the surface of the radiation divider produces an interference maximum. By translation of **M3** mirrors, optical path difference δ between the two routes is changed progressive.

Fig. 4 Construction and operating scheme for Fourier Transform Infrared Spectrophotometer

Organic Compounds FT-IR Spectroscopy 159

a. using a spectral interferometer ensures the achievement of a resolution much higher than that offered by spectrophotometers with dispersion or a high signal / noise for a

b. lack of slits in the optical assembly of the Fourier transform spectrophotometers removes a series of disadvantages related to the fact that for spectrophotometers with dispersion the optical image of the input slit is deformed due to dispersive optical

c. the signal / noise ratio achieved in Fourier transform spectrophotometers it is more advantageous with several magnitude orders compared with dispersion

d. because of the signal / noise ratio advantage, recording of an interferogram (a single displacement of mirrors **M3** from position corresponding to δ = 0 to a position with a extreme δ value) can be achieved in a very short time (less one second) reason why, within a reasonable time, interferogram recording operation can be repeated for several

The last aspect is particularly important for difficult samples, which absorb infrared radiation in a very advanced position. For these samples, a single interferogram recording, with all the inherent advantages of Fourier multiplexing technique, signal / noise ratio is often unsatisfactory. In these cases, overlapping a larger number of records (the number can be **N**), followed by calculating the arithmetic average of the records, significantly improves the signal / noise ratio. In theory errors can be demonstrated that the overlap (acquisition) of N records, followed by mediation of the obtained interferogram, the signal / noise ratio is improved by a factor equal to *N* in comparison with a single record case. Thus, and in IR spectra (obtained by the Fourier transform of the interferogram) the signal / noise ratio is

The electrical signal of the detector is digitized with an appropriate electronic interface and data (pairs of wavenumber values vs. absorbance or wavenumbers vs. transmission percentage) are stored in a file created by a computer. The stored data can be later processed in different ways. Thus, you can add or subtract algebraic different spectra, can make corrections of baseline, can reduce noise by techniques different than acquisition (e.g. "signal smoothing") or it can be presented the absorbance derived as a function of scanned size (wavenumber). Derivation of the original spectrum as a function of wavenumer often surprises some details of the IR spectrum which are harder to observe in its original form

Elucidation of the molecular structure is especially important in organic chemistry. An analytical method for the identification of functional groups from organic compounds uses one of the most physical properties of a chemical compound: the infrared absorption spectrum. Compared with other physical properties: melting point, refractive index, or specific gravity which offer only a single point of comparison with other substances, the IR spectrum of a specific compound, gives a multitude of important information (position of bands, band intensity). The intensity is indicative of the number of a particular group

given resolution;

spectrophotometers;

improved by acquisition of spectra.

(absorbance vs. wavenumber).

**7. Organic compounds** 

contributing to absorption.

element (prism or diffractive optical network);

times, followed by the mediation of the obtained signals.

It can be shown that the detector **D**, which measures the intensity of interference as a function of **M3** mirrors position (so depending on the optical path difference δ between the two routes) records an interferogram which depends on inverse Fourier transform of emission spectrum of the source **LS** and on inverse Fourier transform of transparency (transmission) spectrum of the sample **S** (sample). After Fourier transform of detector **D** signal and some additional mathematical operations on detector signal the transmission (or optional absorption spectrum) spectrum of the sample **S** in known form is obtained.

Figure 4 shows the typical interferogram recorded by the detector (representing the light flux, which reached the detector, as a function of the optical path difference δ associated with **BS1 – M2 – BS1** and **BS1 – M2 – BS1**).

To know the exact positions of absorption maxima in the IR spectrum of the sample, the position of mirrors **M3** must be known exactly commensurable with the radiation wavelength in each moment of this whole movement. Therefore, together with the radiation of source **LS**, it is sent another radiation, this time the radiation is monochromatic, coming from a laser emitting in the visible (usually red radiation) or near infrared (often with a wavelength of 1064 nm ) range. The interferogram produced by monochromatic laser radiation is practically a sinusoidal function. This sinusoidal signal, also noted by the detector **D**, is superimposed on the signal generated by the sample. By tracking the interference maximum and minimum (sinusoidal type) of the laser radiation, it can be indicated the current location of the mirrors **M3**, in each moment of the recording operation, with an accuracy comparable to the wavelength of laser radiation.

Fig. 5. The typical interferogram recorded by the detector

Recording of an IR spectrum of a sample based on Fourier Transform method has many advantages:

It can be shown that the detector **D**, which measures the intensity of interference as a function of **M3** mirrors position (so depending on the optical path difference δ between the two routes) records an interferogram which depends on inverse Fourier transform of emission spectrum of the source **LS** and on inverse Fourier transform of transparency (transmission) spectrum of the sample **S** (sample). After Fourier transform of detector **D** signal and some additional mathematical operations on detector signal the transmission (or

Figure 4 shows the typical interferogram recorded by the detector (representing the light flux, which reached the detector, as a function of the optical path difference δ associated

To know the exact positions of absorption maxima in the IR spectrum of the sample, the position of mirrors **M3** must be known exactly commensurable with the radiation wavelength in each moment of this whole movement. Therefore, together with the radiation of source **LS**, it is sent another radiation, this time the radiation is monochromatic, coming from a laser emitting in the visible (usually red radiation) or near infrared (often with a wavelength of 1064 nm ) range. The interferogram produced by monochromatic laser radiation is practically a sinusoidal function. This sinusoidal signal, also noted by the detector **D**, is superimposed on the signal generated by the sample. By tracking the interference maximum and minimum (sinusoidal type) of the laser radiation, it can be indicated the current location of the mirrors **M3**, in each moment of the recording operation,

optional absorption spectrum) spectrum of the sample **S** in known form is obtained.

with an accuracy comparable to the wavelength of laser radiation.

Fig. 5. The typical interferogram recorded by the detector

advantages:

Recording of an IR spectrum of a sample based on Fourier Transform method has many

with **BS1 – M2 – BS1** and **BS1 – M2 – BS1**).


The last aspect is particularly important for difficult samples, which absorb infrared radiation in a very advanced position. For these samples, a single interferogram recording, with all the inherent advantages of Fourier multiplexing technique, signal / noise ratio is often unsatisfactory. In these cases, overlapping a larger number of records (the number can be **N**), followed by calculating the arithmetic average of the records, significantly improves the signal / noise ratio. In theory errors can be demonstrated that the overlap (acquisition) of N records, followed by mediation of the obtained interferogram, the signal / noise ratio is improved by a factor equal to *N* in comparison with a single record case. Thus, and in IR spectra (obtained by the Fourier transform of the interferogram) the signal / noise ratio is improved by acquisition of spectra.

The electrical signal of the detector is digitized with an appropriate electronic interface and data (pairs of wavenumber values vs. absorbance or wavenumbers vs. transmission percentage) are stored in a file created by a computer. The stored data can be later processed in different ways. Thus, you can add or subtract algebraic different spectra, can make corrections of baseline, can reduce noise by techniques different than acquisition (e.g. "signal smoothing") or it can be presented the absorbance derived as a function of scanned size (wavenumber). Derivation of the original spectrum as a function of wavenumer often surprises some details of the IR spectrum which are harder to observe in its original form (absorbance vs. wavenumber).
