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Файл:Optical methods of analysis. Educational aid
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methods of sample preparation are chosen depending on the object under
study and the purpose of the analysis.
Solutions. The advantages of the method are related to the high reproducibility of the results and the ability to obtain correctly rendered contours
and fine structure of all strong bands by varying the concentration.
Disadvantages are associated with the need for a transparent study of
the solvent and solution preparation in this field, as well as with the high
toxicity of the vapors of the most widely used solvents CS2 CHCl3 and СCl4
(Fig. 6.12). The method is suitable for both qualitative and quantitative analysis.
Films. A drop of sample is squeezed between salt plates or “wiped”
with a salt plate on a glass surface. This is the simplest way to analyze liquid
objects. For polymers, resins and varnishes, films can be obtained by evaporating the solvent from a solution applied to a salt plate. The disadvantage is
the inability to strictly observe the thickness of the sample layer.
This method of preparing a substance for analysis is suitable only for
qualitative research.
a
b
c
Fig. 6.12. IR-spectra of solvents: a – CCl4; b – CHCl3; c – CS2
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Suspensions. They are obtained by grinding a solid sample in vaseline
or other oils. The advantages of the method include the simplicity of sample
preparation, the disadvantages are poor reproducibility when the quality of
grinding of a solid substance changes, as well as the presence of a strong
absorption of vaseline oil in the regions of CH vibrations.
Tablets with KBr. Test samples are prepared by pressing a mixture consisting of the analyte and potassium bromide.
In some cases, it is necessary to use different methods of sample preparation to record different parts of the IR-spectrum.
6 . 6 Q u a n t i t a t i v e a n a l y s i s b y I R - s p e c t r a
Quantitative analysis by infrared absorption spectra is based on the application of the combined Bouguer–Lambert–Beer law, which is expressed
by the equations:
,
where I0 is the intensity of the incident radiation; I is the intensity of radiation
transmitted through the substance; ε is the molar coefficient of absorption
(extinction) or extinction; C is the concentration of the absorbing substance,
mol/l; l is the thickness of the absorbing layer.
The molar absorption coefficient is equal to the optical density of a solution with a concentration of 1 mol/l and a light-absorbing layer thickness
of 1 cm.
The value lg I ⁄ I0 = A is called the optical density of the solution.
If transmission is expressed as a percentage, then
, then .
Therefore, if the solution does not absorb radiation, then
.
With complete absorption of radiation:
.
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Application of IR-spectroscopy. For quantitative analysis is difficult
due to the presence of an inclined or uneven zero line (relative to which
the transmission of the sample is measured at various wavelengths), as well
as a large background absorption (in the absence of the analyzed compound).
These difficulties can be overcome by using the absorption intensities
of band I and background I0, measured relative to the baseline rather than
zero, to calculate the transmission (Fig. 6.13).
Fig. 6.13. Measurement of the bandwidth in the IR-spectrum
The baseline is drawn through the base of the measured strip. The optical density is calculated by the formula:
The actual analysis is performed by the calibration graph method in the
coordinates “optical density – concentration”. It is a straight line passing
through the origin and is constructed from at least five points obtained for
solutions of known concentration.
Quantitative analysis with an accuracy of 1 % requires good skills and
high accuracy of work. Sometimes you can limit yourself to semi-quantita-
tive definitions within the accuracy of 10 %.
For the quantitative analysis of suspensions, an internal standard is selected. The measure of the sample concentration in this case is the ratio of
the optical densities of the sample and the reference.
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6 . 7 . F i e l d s o f a p p l i c a t i o n , a d v a n t a g e s a n d
d i s a d v a n t a g e s o f t h e I R - s p e c t r o s c o p y m e t h o d
The main advantages of the method:
– expressiveness;
– versatility: samples can be liquid, solid and gaseous, organic and in-
organic. In some cases, non-volatile mixtures of compounds can be investigated. IR-spectroscopy is widely used for the analysis of high-molecular
compounds;
– simultaneous qualitative and quantitative analysis is possible. The individuality of the spectrum makes it possible to find a band for almost every
component of the mixture that is convenient for quantitative determination,
and to carry out a quantitative assessment of all components at once at
the same time;
– the determination of the qualitative composition of an object is often
accompanied by the identification of individual characteristics of the components (H-bond, dissociation, etc.)
Disadvantages of the method:
– transparency of homonuclear molecules;
– the presence of limitations in the study of inorganic compounds: the-
oretical calculations of the spectra of molecules on a computer for the purpose of their interpretation can be carried out for molecules containing atoms
of only the first four periods of the periodic system;
– the most common solvent H2O strongly absorbs IR-radiation, dis-
solves the windows of the cuvette and therefore is used very limited;
– there is always a danger of accidental overlapping of bands, and when
analyzing spectra using a computer, there are errors in choosing a standard;
– for reliable quantitative determination, it is better to have an individual (pre-purified substance);
– the method is insensitive to impurities contained in an amount of less
than 1 %.
Areas of application of the method:
– IR-spectroscopy is an unsurpassed method for identifying and studying the structure of organic and some inorganic compounds;
– IR-spectroscopy makes it possible to identify spatial and conformational isomers, to study intra- and intermolecular interactions, the nature of
chemical bonds, charge distribution in molecules, chemical equilibria, phase
transformations, kinetics of chemical reactions, to register short-lived
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(lifetime up to 10–6 s.) particles, to measure geometric parameters of molecules, etc.;
– it is possible to analyze complex mixtures of organic substances (for
example, determining the types of soils or coals);
– it is possible to carry out quantitative determinations of the basic substance and trace impurities;
– you can evaluate the results of other analytical methods. For example,
IR-spectroscopy is widely used to identify the components of a mixture obtained as a result of the use of preparative gas-liquid chromatography or
chemical methods.
6.8 . Q u e s t i o n s a n d t a s k s
6.8 .1. Q u e s t i o n s t o p r e p a r e f o r t h e c o l l o q u i u m
1. Basic concepts of infrared spectroscopy: spectroscopy, electromag-
netic spectrum, resolution.
2. In what coordinates is the infrared absorption spectrum constructed?
3. IR-region of the electromagnetic spectrum: ranges of wavelength,
energy, wave number. The relationship of these quantities. Analytically useful IR-area.
4. The nature of bands in the IR-spectrum.
5. The main types of vibrations in the molecule.
6. How are IR-spectra used for qualitative analysis purposes?
7. The main areas of the spectrum for the purposes of qualitative analysis of organic compounds.
8. Qualitative relations between the oscillation frequencies of similar
groups of atoms and bonds.
9. What processes affect the vibrations of atoms and bonds? Their manifestation in the IR-absorption spectra of compounds.
10. Application of IR-spectroscopy in inorganic chemistry.
11. The main components of the IR-spectrometer.
12. Features of sample preparation in IR-spectroscopy.
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13. The basic law of molecular spectroscopy.
14. Features of quantitative analysis in IR-spectroscopy.
15. Possibilities of using IR-spectroscopy for analytical purposes. Advantages and disadvantages of the method.
6 . 8 . 2 . C o n t r o l q u e s t i o n s
1. Calculate the range of wave numbers corresponding to the entire
IR-region of the spectrum.
2. Why is the wavelength usually replaced by a wave number in
IR-spectroscopy?
3. What processes lead to the appearance of bands in the IR-spectra?
4. Why are the absorption bands in the IR-spectra narrower than in
the UV-spectra?
5. Calculate in which range the energy of the entire IR-region of
the spectrum changes.
6. Set what wave number corresponds to the radiation energy of
1 kJ/mol.
7. Describe the types of stretching vibrations of the methyl group –CH3.
8. Describe the types of stretching vibrations of the methylene group
–CH3-.
9. Calculate the number of fundamental vibrations of the C6H6 benzene
molecule. Why is there only 19 bands in the IR-spectrum of this compound?
10. Why do diatomic molecules (H2, N2, Cl2) do not absorb IR-radiation?
11. Why is water very little used as a solvent in IR-spectroscopy?
12. How will the frequency of stretching vibrations of the C-H(D) bond
change upon passing from chloroform CHCl3 to the deuterated analogue
CDCl3?
13. How will the frequency of O-H stretching vibrations of alcohol
change during the formation of O-H ... X hydrogen bonds?
14. What are the difficulties in completely deciphering the IR-absorption spectra?
15. How to distinguish by IR-spectra?
a) sulfate and sulfite;
b) sulfate and hydrosulfate;
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c) phosphate and hydrophosphate;
d) nitrate and nitrite.
16. What characteristic groups can be determined using hexane
CH3(CH2)4CH3 as a solvent?
17. What characteristic groups can be determined when using methyl
alcohol as a solvent?
18. What characteristic groups can be determined using acetone
(CH3)2C=O as a solvent?
19. Why is it sometimes necessary to use different methods of sample
preparation to record different parts of the IR-spectrum?
20. Which solvent (CCl4 or CHCl3) is preferable to use and why?
21. Give a comparative description of the main components of UV- and
IR-spectroscopy devices.
22. What are the similarities and differences in the preparation for sample analysis in vibrational and electron spectroscopy?
23. Propose an IR-spectral study plan for product identification.
24. Analyze the role of reference materials in IR-spectroscopy (qualitative and quantitative analysis).
25. Why is the full interpretation of the IR-spectrum, as a rule, not
given?
26. Why do you think IR-spectroscopy is relatively little used for
the analysis of inorganic compounds?
27. What, in your opinion, are the difficulties of IR-spectral analysis of
macromolecular compounds?
28. What features of the IR-spectroscopy method determine each of its
areas of application?
6.8 . 3 . D e v i c e d i a g r a m
Currently, there is a wide variety of spectrophotometers, the description and operation procedures are given in the instructions for the device.
The general optical scheme of the spectrophotometer is given below
(Fig. 6.14).
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Fig. 6.14. Optical scheme of the spectrophotometer: 1 – radiation source;
2–5, 11, 12, 14, 15, 19, 21–23, 25, 29, 30 – a system of mirrors;
6, 7, 16, 18, 26, 27 – optical slits; 8 – cuvette compartment;
9, 10 – cuvettes; 13 – mirror modulator; 17, 28 – focusing lenses;
20 – diffraction gratings; 24 – prism; 31 – thermoelement
6.8 . 4 . L a b o r a t o r y w o r k 1 . O b t a i n i n g a n I R - s p e c t r u m
a n d i d e n t i f i c a t i o n o f p o l y m e r i c m a t e r i a l s
General procedure for performing work on IR-spectroscopy:
1. Prepare samples of the compounds proposed by the teacher for reg-
istration of IR-spectra.
2. Take IR-spectra of the issued substances.
3. Decipher the spectrum of matter.
4. Make tables of found frequencies.
5. Prepare a report on laboratory work and draw a conclusion in accord-
ance with the assignment.
Identification of the material is carried out by comparing the obtained
spectrum with the reference.
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6.8 . 5 . L a b o r a t o r y w o r k 2 . Q u a n t i t a t i v e a n a l y s i s o f
a m i x t u r e o f x y l e n e i s o m e r s
The position of the absorption bands of substituted benzene derivatives
is very specific for each type of substitution. Thus, disubstituted derivatives
have non-overlapping characteristic absorption regions, which allows
a quantitative analysis of the content of isomers in solution:
Substitution Type
Regions of absorption (cm-1)
1,2-substituted
735–770
1,3-substituted
690–710; 759–810
1,4-substituted
800
Task for work number 2:
1. Prepare 4–5 % solutions (by volume) of each xylene isomer in cyclohexane (C = 0.1 mol/l), select the optimal size of the absorbing layer on the test
spectra; (l = 0.2 mm), take their IR-spectra in the region of 600–1000 cm-1.
Identify absorption bands characteristic of each type of substitution.
2. Prepare reference mixtures of xylenes brought with the following
content of isomers:
Substance
Content in the mixture, %
1 sol
2 sol
3 sol
4 sol
o-xylene
33
65 5 15
m-xylene
33
20
75
5
p-xylene
33
25
20
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3. Take 1 ml of each of the prepared mixtures and dilute with 20 ml of
cyclohexane. Record the IR-spectra of solutions in the range 600–1000 cm-1
at the previously selected thickness of the absorbing layer.
Based on the spectrum of the first solution, select analytical absorption
bands for each of the xylenes, then, using the data for all solutions, build
calibration graphs for each xylene D = f (C). The concentration can be expressed in %.
4. Dilute the mixture of xylenes received from the teacher with cyclohexane in the ratio of 1:20, record the IR-spectrum and, using the calibration
curve, find the concentration of isomers in the mixture.
Report form: IR-spectra of solutions, Tables 6.2 for each of the pre-
Substance
max
I0I D C
o-xylene
m-xylene
p-xylene
pared and studied solutions. Give calibration graphs for each xylene. Give
the found concentrations of isomers in the problem.
Table 6.2
Spectral data
Formulate conclusions on the work.
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