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Файл:Optical methods of analysis. Educational aid
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7 . A T O M I C E M I S S I O N S P E C T R O S C O P Y
7 . 1 . T h e o r e t i c a l f o u n d a t i o n s
Atomic emission spectral analysis (AES) is based on the thermal excitation of atoms or ions that are in a gaseous state, and the registration of optical spectra (qualitative analysis) or the measurement of the intensity of individual spectral lines of the elements being determined (quantitative analysis). Thus, emission spectroscopy is based on measuring the energy of
the emitted radiation.
If sufficient energy is imparted to the substance, then the electrons of
the atoms will go into an excited state. After ~10-8 seconds, the electrons will
spontaneously return to the lower energy levels. In this case, excess energy
will be emitted in the form of discrete and characteristic for each type of atom
electromagnetic oscillations in the visible and UV-regions of the spectrum.
The obtained spectra have a line character. They are used to determine the nature and quantity of individual elements in a sample.
Each element exhibits a set of lines (spectrum) specific to that element.
The higher the atomic number of an element, the more complex the spectrum.
In the spectrum of the simplest element, hydrogen (H), 21 lines are observed,
and the spectrum of iron (Fe) consists of 5000 lines.
Lines are distinguished in the spectra:
– characteristic lines of the spectrum – inherent only to this element.
Characteristic can be not only lines, but entire spectra. When identifying
spectral lines, they are compared with the lines of the spectrum of iron, taken
as a standard – inherent only to this element. Characteristic can be not only
lines, but entire spectra. When identifying spectral lines, they are compared
with the lines of the spectrum of iron, taken as a standard;
– resonant lines of the spectrum – corresponding to resonant transitions; these are the brightest lines in the spectrum;
– “last” lines of the spectrum – disappearing with a decrease in the con-
centration of the element last; they are used to identify elements using spectral line atlases.
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62
For quantitative determinations, the intensity of the spectral line is used,
which is related to the concentration of the element in the sample by
the Lomakin–Scheibe equation:
I = a · Cb ,
where I is the intensity of the spectral line (relative intensity); C – concentra-
tion of the element in the sample (relative concentration C
sub
/ C
ext.st.
;
a – a value depending on the source of excitation and the properties of
the sample; b – the self-absorption coefficient (the phenomenon of self-absorption is associated with the absorption of part of the radiated energy by
unexcited atoms).
The results are usually processed using the internal standard method.
In this case, the intensity of the spectral line of the determined element is
measured relative to the intensity of the line of the comparison element with
a known constant concentration (internal standard). They form a homologous
pair of lines. The necessity of using this method is related to the dependence
of the intensity on a number of uncontrolled processes (conditions of atomization and excitation). The main component of the sample or a component
specially introduced into the sample, the amount of which is known or constant, is used as an internal standard.
The complex nature of the relationship between the intensity of
the spectral line and the concentration of the element in the sample can be
represented graphically (Fig. 7.1). The curve has two straight segments, in
the region of low and high atomic concentrations.
Fig. 7.1. Dependence of the intensity of the spectral line on the number
of atoms in the plasma
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At low contents of the element (region 1), a directly proportional dependence of the intensity of the spectral line is observed. In this region,
the self-absorption coefficient in the Lomakin–Schaibe equation b is equal to
1, and the equation itself has the form:
I = a·C.
In region 2, the intensity increases with increasing concentration more
slowly than in region 1, and b = 1 ÷ 0, or 0 < b < 1. With a further increase
in concentration, the line intensity reaches a limit (region 3). In this region,
b = 0. This region is not suitable for quantitative determinations.
In AES, the following atomization and excitation methods are used for
the analysis of solid samples: electric arc, spark, laser; and for the analysis of
solutions: flame and inductively coupled plasma (ICP).
The radiation emitted by the sample passes through the monochromator
and enters the detector. The analytical signal from the detector is amplified
and recorded by the corresponding device. Thus, if in absorption spectroscopy the radiation of any source passes through the sample, then in emission
spectroscopy the source of radiation is the sample itself.
In flame emission spectroscopy (flame photometry), a flame is used as
an excitation source, resulting from an exothermic reaction between compounds in gaseous form, one of which is combustible (propane, acetylene),
and the other is an oxidizing agent (air – oxygen, nitric oxide N2O).
Due to the low temperature (~ 2000–3000 K), easily ionizing elements
emit in the flame: alkali and alkaline earth metals. The spectra of these elements consist of a small number of resonance lines.
The sample is injected into the burner flame as an aerosol, where it is
desolvated, evaporated, dissociated, and then atomized (Fig. 7.2).
The intensity of radiation by atoms is proportional to their content in
the flame, which in turn is proportional to the concentration of the solution.
For quantitative determinations, the calibration curve method is usually used.
The method is used mainly for the determination of trace amounts of
alkali and alkaline earth metals. For them, the limit of detection is in the range
of 0.001–1 μg/ml.
The advantages of the method include ease of handling, availability,
low cost of fuel and oxidizer.
The main limitations of the method are:
– strong influence of matrix effects;
– the need to take into account the intrinsic radiation of the flame, as
well as the transfer of the sample into a dissolved state.
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Inductively coupled plasma atomic emission spectroscopy (ICP-AES)
uses plasma for atomization, which is a gas containing X atoms ionized by
a powerful high-frequency field.
Fig. 7.2. Physico-chemical processes in the flame
Electrodeless high-frequency discharges at atmospheric pressure are of
analytical interest. In such sources, the processes on the electrodes and on
other surfaces, which limit the discharge volume, are excluded, the spectra
of almost all elements are effectively excited, and the introduction of a substance into the plasma is facilitated. High-frequency inductively coupled argon plasma (ICP) as a light source in spectral analysis has been used relatively recently. To obtain plasma, as a rule, toroidal-shaped discharges are
used, excited by a powerful high-frequency field in a plasma-forming gas
flow moving along the discharge axis. ICP burners differ in shape, size and
configuration, in the way the burner walls are cooled, and in the type of working gas. This atomization source is highly reproducible and eliminates the effect of sample composition on analysis results. This is ensured by reaching
temperatures of the order of 6000–10000 °K, at which the vast majority of
compounds completely dissociate.
The ICP method is now widely used for analysis due to the industrial
production of equipment. Basically, the ICP method was developed for
the analysis of solutions that are injected using special nebulizers. It is known
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methods of introducing solid samples, there are also options for the introduction of gaseous hydrides for the determination of arsenic, tellurium, selenium, tin. One of the significant specific difficulties in carrying out the analysis is the spectral superposition on the analytical lines from the side of
the spectra of the samples.
The main advantages of the method are as follows:
– the possibility of determining all elements in argon plasma, including
those that are difficult to excite, as well as forming heat-resistant compounds;
– linearity of graphs up to five orders, which makes this analysis
method unique among spectral methods;
– low detection limits, which in many cases are 1–3 orders of magnitude lower compared to other sources;
– the possibility of simultaneous analysis of up to 20–40 elements;
– low consumption of the analyzed solution;
– automation, computer control of the analysis process;
– high reproducibility (Sr = 0.01–0.03);
– high accuracy, which is ±1 %. This is due to the low systematic error
due to interfering effects in the source and the high stability of the instrument.
In laser-assisted atomic emission spectroscopy, atomization and excitation of the sample is performed by a laser beam. Sometimes an electric discharge is used as an additional source of energy. A feature of the method is that
the method allows you to explore small surfaces (up to 50 microns in diameter).
The AES method is used to quantify elements from Li to Bi. Moreover,
it is possible to define several elements at the same time.
AES is the basis of the pharmacopoeial method of flame staining in
determining the authenticity of pharmaceutical substances. At the same time,
metal ion salts color the burner flame in different colors: potassium – purple,
sodium – yellow, calcium – brick red, barium – green.
7.2 . F l a m e e m i s s i o n s p e c t r o s c o p y
7.2 .1. E q u i p m e n t a n d m e a s u r e m e n t t e c h n i q u e
Two classes of instruments are used in flame photometry: flame photometers and spectrometers. In any equipment there are a burner and atomizer, a spectral one and measuring devices.
66
Burner and atomizer. The accuracy and sensitivity of the determination
in flame photometry primarily depend on the light source and the excitation
conditions of the spectra. For each type of flame, a burner of a certain design
is used. Most often, solutions are sprayed in a spray chamber using compressed air or oxygen pneumatically. A compressor and pressure gauges are
used to measure the pressure of gas and air. The results of the analysis depend
on the efficiency of spraying: solution supply rate, droplet size and aerosol
monodispersity.
Spectral instruments. Light filters, prism or diffraction monochromators are used to isolate the measured spectral line.
Absorption and interference light filters are the most common. Interference filters are often combined with absorption filters to obtain a narrow
symmetrical bandwidth. One of the disadvantages of interference filters is
that the bright line of the interfering element can give a background and interfere with the definition.
Radiation receivers. Photovoltaic cells and photomultipliers are used as
radiation receivers. In filter photometers, the receiver is usually a semiconductor photocell, the current of which is measured by a microammeter.
By using selective photocells, it is possible to increase the selectivity of
the determination. For example, when determining sodium in the presence of
potassium, a selenium photocell is used, which is insensitive to red potassium
radiation.
The flame photometer (Fig. 7.3) is designed for the quantitative determination of potassium, sodium and calcium in solutions. The consumption
of the test solution does not exceed 5 ml/min. The duration of one measurement is no more than 30 seconds. To isolate the spectral line of the measured
element, interference light filters are used with the following wavelengths at
the maximum transmission: to measure the emission of sodium, potassium
and calcium, λ
max
is 589 ± 5, 768 ± 5 and 622 ± 5 nm, respectively.
Compressed air from the compressor enters the sprayer through a filter,
an adjustment valve and a pressure gauge. In the upper part of the capillary,
the air flow creates a vacuum, due to which the test solution is sucked into
the capillary and sprayed in the mixing chamber in the form of small droplets.
The combustible gas from the network through a filter, a control valve
and a pressure gauge is fed into the mixing chamber, where it mixes with air
and sprayed test solution. Large drops are deposited in the spray chamber,
and small ones, together with gas and air, enter the burner of the device and
burn in a flame.
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Fig. 7.3. Functional diagram of the flame photometer: 1 – a cylinder with
a combustible gas; 2 – glass filters; 3 – pressure gauges; 4 – a mirror;
5 – a compressor; 6 – the analyzed solution; 7 – a mixer with a burner;
8 – a condenser; 9 – light filters; 10 – a photocell; 11 – an amplifier;
12 – a microammeter; 13 – a stabilizer
Turning on the device. Before switching on the device, make sure that:
the device is connected to air and gas sources. The “Gas” valve on the device
must be closed, the range selector knob must be in position 5, the burner
nozzle must be installed so that the long side of the hole rectangle is perpendicular to the optical axis. After that you need:
1. Plug in the electric lighter.
2. Plug the compressor and photometer into the mains.
3. Open the air valve and turn on the compressor. Adjust the air pressure
on the pressure gauge with a valve to 0.6–0.8 atm.
4. Open the gas valve.
5. Bring the electric lighter to the nozzle and open the gais valve on
the photometer until the flame is ignited.
6. It is necessary to regulate the flame with the gas valve, observing
through the viewing window. The device has the best performance with
the size of the internal flame cones of 3–4 mm and their bluish-green color.
7. Place a glass with distilled water under the capillary. The operation
of the atomizer can be judged by the change in the level of liquid in the glass.
8. Turn on the photometer.
9. Set the second measurement range, the arrow of the light filter switch
against the designation of the measured element and the arrow of the device
to the middle of the scale.
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10. Warm up the device for 30 minutes.
11. When the pointer of the microammeter is in a stable position, use
the “Zero Setting” knob to set zero on the scale of the device.
Setting up the device. The measurement of the concentration of sodium,
potassium and calcium in the studied solutions begins with the construction
of a calibration graph for each element according to standard solutions. Then
distilled water is sprayed and the “Zero setting” handle sets the microamme-
ter arrow to the zero division of the scale. When photometrizing distilled water, the microammeter arrow should return to “0” of the scale. If the arrow is
not set at zero division, with the “Zero setting” knob, it is set to “0” of
the scale and then the solution of the maximum concentration is photometrized again. Then, reference solutions of a certain element with a known,
uniformly increasing concentration are photometric. Several standard solutions are taken in order to obtain a sufficient number of points for constructing a calibration graph, which is plotted in current-concentration coordinates.
After every hour of operation on the device, it is necessary to check the position of the extreme points of the working interval by photometry of standard solutions and distilled water. In the process of working on the device, it
is necessary to monitor the pressure of air and gas by pressure gauges.
In the presence of a flame background, a spark slip, instability of
the microammeter readings, it is necessary to clean the nozzle and rinse it
with ethanol and distilled water.
Turning off the device:
1. Rinse the distilled water sample intake tract for 10 minutes.
2. Close the gas valve on the photometer.
3. Close the valve of the gas main.
4. Turn off the device with a toggle switch.
5. Disconnect the compressor from the mains.
7.2 .2. F l a m e p h o t o m e t r i c d e t e r m i n a t i o n o f s o d i u m
i o n s
Instruments and reagents: flame photometer; standard solutions; meas-
uring flasks with a capacity of 50 and 100 ml; graduated pipettes for 1.0 and
5.0 ml; distilled water.
69
The initial standard NaCl solution should contain 200 mg/l of sodium.
It is used to prepare five more dilute solutions. Standard solutions are prepared in measuring flasks with a capacity of 50 ml by dilution with distilled
water (Table 7.1).
Table 7.1
Results of definitions
Standard solution
1 2 3 4 5
Volume of the initial solution, ml
0.25
0.5
1.0
1.5
2.0
Sodium content, mg/l
1 2 4 6 8
The magnitude of the photocurrent, mA
Photometry begins with the solution having the highest concentration
in order to avoid deflection of the microammeter arrow beyond the scale.
Then they consistently move from a lower concentration to a higher one. According to the data obtained, a graph of the dependence i = f (C) is constructed.
Before photometry of the analyzed solutions, the nebulizer and the gas
burner are thoroughly washed with distilled water for 2–3 minutes, the analyzed solutions are photometered and the initial concentrations are found according to the graph i = f (C).
7.2 .3. F l a m e p h o t o m e t r i c d e t e r m i n a t i o n o f p o t a s s i u m
i o n s
Instruments and reagents: flame photometer; standard solutions;
50 and 100 ml volumetric flasks; 1.0 and 5.0 ml graduated pipettes; distilled
water.
The initial KCl standard solution containing 200 mg/l of potassium is
used to prepare five or more dilute solutions. Solutions are prepared in volumetric flasks with a capacity of 50 ml (Table 7.2).
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Table 7.2
Results of determinations
Standard solution
1 2 3 4 5
The volume of the initial solution, ml
0.25
0.5
1.0
1.5
2.0
Potassium content, mg/l
1 2 4 6 8
Photocurrent value, mA
Photometry of standard solutions begins with the solution having
the highest concentration, and continues in the same way as in the case of
sodium determination.
Based on the data obtained, a calibration graph is built i = f (C), the analyzed solutions are photometered and their concentrations are graphically
found.
7.2 .4. F l a m e p h o t o m e t r i c d e t e r m i n a t i o n
of c a l c i u m ions
Instruments and reagents: flame photometer; standard solutions; 50 and
100 ml volumetric flasks; 1.0 and 5.0 ml graduated pipettes; distilled water.
The original calcium chloride standard solution contains 1.0 g/L of calcium. A number of standard solutions are prepared from this solution in volumetric flasks with a capacity of 50 ml (Table 7.3).
Table 7.3
Results of determinations
Standard solution
1 2 3 4 5
The volume of the initial solution, ml
0.25
0.5
1.0
1.5
2.0
Calcium content, mg/l
5
10
20
30
40
Photocurrent value, mA
The method of photometry and processing of the results is the same as
in the case of the determination of sodium and potassium.
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