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Instrumental methods of analysis

June 11, 2024

INSTRUMENTAL METHODS OF ANALYSIS

Analytical chemistry is the study of the separation, identification, and quantification of the chemical components of natural and artificial materials. Qualitative analysis gives an indication of the identity of the chemical species in the sample and quantitative analysis determines the amount of one or more of these components. The separation of components is often performed prior to analysis.

Analytical methods can be separated into classical and instrumental. Classical methods (also known as wet chemistry methods) use separations such as precipitation, extraction, and distillation and qualitative analysis by color, odor, or melting point. Quantitative analysis is achieved by measurement of weight or volume. Instrumental methods use an apparatus to measure physical quantities of the analyte such as light absorption, fluorescence, or conductivity. The separation of materials is accomplished using chromatography, electrophoresis or Field Flow Fractionation methods.

Analytical chemistry is also focused on improvements in experimental design, chemometrics, and the creation of new measurement tools to provide better chemical information. Analytical chemistry has applications in forensics, bioanalysis, clinical analysis, environmental analysis, and materials analysis.

The instruments and techniques developed by the physicist for the determination of physical constants have furnished the chemical analyst with new devices which can be used for the quantitative and qualitative determination of the elementary composition of substances. These new methods have supplemented the classical methods of gravimetric and volumetric analysis which experienced their greatest growth during the nineteenth century. The physical methods have enabled the analyst to broaden the scope of analysis, since in many cases accurate measurements can be made without destruction of the sample. He is also able to analyze complex mixtures quantitatively, which previously would have presented almost unsurmountable difficulties. The analyst now has at his disposal physical methods which enable him to investigate problems of structure in organic chemistry, reaction kinetics, and even the biochemistry of living cells.

Physical and physical-chemical methods of the analysis are based on dependence application between measured physical properties of substances and qualitative (quantitative) composition

PCMA are divided on:

§ Optical methods are based on measurement of optical properties of substances.

§ Chromatographic methods are based on usage of ability of different substances to selective sorption.

§ Electrochemical methods are based on measurement of electrochemical properties of substances.

§ Radiometric methods are based on measurement of radioactive properties of substances.

§ Thermal methods are based on measurement of heat effects of substances.

§ Mass spectrometric methods are based on studying of the ionized fragments (“splinters”) of substances.

§ Kinetic methods are based on measurement of dependence of speed of reaction from concentration of substance

Advantage of PCMA

§ High sensitivity – a low limit of detection (10-9 mg) and definition

§ High selectivity

§ Rapid analysis methods

§ Automation and computerization is possibility

§ Analysis is possibility on distance

§ Possibility of the analysis without destruction of the sample

§ Possibility of the local analysis

Lacks of PCMA

§ Definition error is near ± 5 % (on occasion to 20 %), whereas – 0,01-0,005 % for gravimetry and 0,1-0,05 % for titrimetry

§ Reproducibility of results in separate methods is worse, than in classical methods of the analysis

§ It is necessary of usage of standards and standard solutions, graduation of equipment and plotting of calibration charts

§ Complexity of used equipment, its high cost, high cost of standard substances

Optical methods of the analysis. Classification.

А) On investigated objects

§ The nuclear spectral analysis

§ The molecular spectral analysis

B) On the nature of interaction of electromagnetic radiation with substance

1. Absorption analysis

§ Atomic-absorption analysis – Atomic absorption spectroscopy (AAS) is a spectroanalytical procedure for the quantitative determination of chemical elements employing the absorption of optical radiation (light) by free atoms in the gaseous state.

§ Molecular-absorption analysis – Molecular absorption spectroscopy in the ultraviolet (UV) and visible (VIS) is concerned with the measured absorption of radiation in its passage through a gas, a liquid or a solid.

§ Turbidimetric analysis – Turbidimetry (the name being derived from turbidity) is the process of measuring the loss of intensity of transmitted light due to scattering effect particles suspended in it. Light is passed through a filter creating a light of known wavelength which is then passed through a cuvette containing a solution.

2. The emissive spectral analysis

§ flame photometry – Flame photometry, more properly called flame atomic emission spectrometry, is a fast, simple, and sensitive analytical method for the determination of trace metal ions in solution.

§ fluorescence analysis – Luminescence is emission of light by a substance not resulting from heat; it is thus a form of cold body radiation.

§ The spectral analysis with usage of effect of combinational dispersion of light

3. Other methods

§ nephelometric method – Nephelometry is the measurement of scattered light. This technique requires a special measuring instrument, where the detector is set at an angle to the incident light beam.

§ refractometric analysis – Refractometry is the method of measuring substances’ refractive index (one of their fundamental physical properties) in order to, for example, assess their composition or purity. A refractometer is the instrument used to measure refractive index (“RI”)

§ polarimetric analysis – Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves.

§ interferometric analysis – Interferometry is a family of techniques in which waves, usually electromagnetic, are superimposed in order to extract information about the waves.

C) On electromagnetic spectral range which use in analysis:

§ Spectroscopy (spectrophotometry) in UV and visible spectrum

§ IR – Spectroscopy – Infrared spectroscopy (IR spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light.

§ X-ray spectroscopy – X-ray spectroscopy is a gathering name for several spectroscopic techniques for characterization of materials by using x-ray excitation.

§ Microwave spectroscopy – The interaction of microwaves with matter can be detected by observing the attenuation or phase shift of a microwave field as it passes through matter.

D) By the nature of energy jump

§ Electronic spectrum

§ Vibrational spectrum

§ Rotational spectrum

Spectrum (method)

The characteristic of energy of quantum

Process

Radio-frequency (NMR, EPR)

Microwave

The optical

The visible

Infra-red (IR)

X-ray

Gamma radiation (nuclear-physical)

10¹-10^{–1 meters}

10^-1-10^{–3 meters}

200-400 nm

400-750 nm

10-13000 cm^-1

10^-8-10^{–10 m}

10^-10-10^{–13 m}

Change of electron spin and nuclear spin

Change of rotational conditions

Change of valence electron conditions

Change of vibrational conditions

Change of a condition of internal electrons

Nuclear reactions

Optical methods of analysis

A major part of modern Instrumental Analytical Chemistry, focuses on the study of the energy exchange between electromagnetic radiation and matter. These interactions are visible to the naked eye, when the radiations concerned fall within the visible spectrum.

The nature of electromagnetic radiation

Electromagnetic radiation is represented in two ways, i.e. as an electromagnetic wave (undulating), and as a series of discrete energy packets – the photons – (three-dimensionally). In terms of its wave properties, electromagnetic radiation is a form of energy that propagates even in a vacuum: it is the simultaneous propagation in space of orthogonal oscillations of a magnetic and electric field.

Wave properties of electromagnetic radiation

Frequency→ ν → is the number of vibrations per unit time. It is measured in Hertz (Hz).

Period → T → is the time needed to perform a complete oscillation. It is the reciprocal of the frequency and is measured in seconds.

Wavelength→ λ → is the distance between two points of the same phase. It is measured in Å.

Energy of electromagnetic radiation

Electromagnetic radiation consists of ‘discrete packets’ of energy called Photons,whose energy is related to the frequency via the equation:

E = h*ν

Where:

E: energy;

h: Planck constant: h:=6.63 x 10^-34 J-s;

v: frequency.

Types of radiation

Several types of electromagnetic radiation exist, which differ in wavelength and, consequently, frequency and energy.

Visible light

Although electromagnetic waves account for a number of different phenomena, their nature remains unchanged: what changes is the energy of the wave, and therefore its wavelength. Low-energy electromagnetic waves have a high wavelength, so they don’t interact with living organisms (radio waves). Electromagnetic waves with higher energy levels, and consequently lower wavelength, can interact with animal and vegetable tissue, even molecules and atoms (x-rays, gamma rays, etc.).

Monochromatic light and polychromatic light

When a beam of white light strikes a glass prism, it breaks up into different colours. Dispersion into different colours through a prism can be explained by the following:

white light is, in fact, a mixture of several radiations with different frequencies corresponding to all the colours;

when a ray of light goes from one medium to another it is deviated (refrected), the degree of deviation depending on the wavelength of the incident ray.

A single colour radiation obtained through dispersion, characterised by a specific wavelength is called monochromatic.

Emission and absorption spectra

Spectrochemical methods of analysis are based on analysis of the spectrum of substances, which can be an emission or absorption spectrum.

an emission spectrum is obtained when, in certain given conditions, a substance emits a beam of light;

an absorption spectrum is obtained when a beam of light is analysed after it has passed through a substance.

For the same substance, the emission and absorption spectra could be roughly compared to the positive and negative of a photograph: a radiation found in the emission spectrum will be absent in the absorption spectrum.

Molecular absorption spectroscopy theory

Each molecular analyte can absorb specific electromagnetic radiation wavelengths. In this process, the energy of the radiation is temporarily shifted to the molecule and the intensity of the radiation decreases accordingly.

spectrum analysis makes it possible to identify the nature of the substance examined;

by measuring the intensity of the emitted or absorbed radiations, it is possible to trace the amount of substance analysed.

In energy fields (electric or electromagnetic heaters), atoms or molecules can absorb specific amounts of energy and move up to higher energy states. This phenomenon is called absorption.

spectrum analysis makes it possible to identify the nature of the substance examined;

by measuring the intensity of the emitted or absorbed radiations, it is possible to trace the amount of substance analysed.

In energy fields (electric or electromagnetic heaters), atoms or molecules can absorb specific amounts of energy and move up to higher energy states. This phenomenon is called absorption.

Ultraviolet-visible spectroscopy

Qualitative analysis

Qualitative analysis is performed using polychromatic beams, which are broken up into their various components (monochromatic radiations) by means of monochromators. One by one, the single monochromatic radiations of the beam are passed through the substance under examination, which will absorb the different radiations with differing intensities. The values recorded, reported on a wavelength-absorption diagram, produce the absorption spectrum of the substance examined.

Quantitative analysis

It uses monochromatic beams. Quantitative analysis is based on the fact that when radiation passes through a solution, it is absorbed with a greater or lesser intensity, depending on the concentration. As will be shown later, special devices (detectors) can measure the intensity of the luminous flux; in particular the analysis measures:

I₀: intensity of the luminous flux at the entrance of the cell containing the sample

I: intensity of the luminous flux at the exit of the cell containing the sample.

Absorbance

A = -log I/I₀

Lambert-Beer law

If we take a cell containing a substance in solution, which is crossed by a beam of monochromatic light, it can be demonstrated that:

A = ε x b x C

where:

A = absorbance (it has no measurement unit).

ε= molar absorption coefficient, typical of the substance (mol-1 L cm-1).

b = optical path (cm), that is the thickness of the solution.

C= molar concentration of the substance (mol/L)

Absorbance and concentration

The proportional relationship between absorbance and concentration makes it possible to perform quantitative analysis. The equation A = ε x b x C represents a straight line passing through the origin of the axes and where ε x b is the angle coefficient.

QUALITATIVE ANALYSIS

Reading of the sample at wavelengths typical of the analyte (polyphenols, for example, absorb at wavelengths of 775 nm); alternatively, scan the sample containing the analyte over a fairly broad interval of wavelengths, to obtain the typical absorption spectrum.

QUANTITATIVE ANALYSIS

Preparation of diluted standard solutions of the analyte to be quantified, and construction of the calibration curve indicating concentration on the x axis and absorbance on the y axis. The latter is read at the wavelength corresponding to the maximum absorption of the analyte.

Methods of quantitative analysis:

1. A method of calibration chart

!!! The method can be applied, if:

§ Structure of standard and investigated solutions are similar

§ The interval of concentration on calibration chart should cover of defined concentration

2. Comparison method (a method on one standard)

!! The method can be used if:

§ Dependence structure – property is strictly rectilinear and passes through the beginning of co-ordinates

§ Concentration of standard and investigated solutions values of analytical signals as much as possible similar and minimum different

§ Structure of standard and investigated solutions are as much as possible similar

3. Method of molar or specific (concentration on % w/w) absorptivity

!! The method can be used if:

§ Strict linearity of dependence structure – an analytical signal is observed

§ The analytical device maintains requirements of metrological checking

4. Method of additives

!!! The method can be applied, if:

§ It is necessary to consider stirring influence of extraneous components of sample on analytical signal of defined substance

Multiwave spectrophotometry

The absorbance of any system containing limited number of painted components which chemically one don’t react with another, is equal sum of absorbance of mix components at the same wavelength:

1. Analysis of two componential mix, when light absorption curves both substance bridge along all spectrum, but on it is partite maximums of absorption

2. Analysis of two componential mix, when light absorption curves both substance bridge, but on it is spectral range, where absorption one of substance may neglect

In this case concentration of first substance is calculated on measured absorbance А at wavelength l1:

Concentration of second substance in mix is calculated through concentration С₁

3. Analysis of two componential mix, when it is separate maximums of light absorption for each substance in spectral range

at wavelength l₁

at wavelength l₂

Differential spectrophotometry is used for:

§ Increases of precision of analyses at definition of considerable quantities of substance;

§ For elimination of extraneous influence of another components and an exception of reagent absorption.

Normal spectrophotometry Differential spectrophotometry

Essence of differential spectrophotometry:

Absorbances of investigated and standard solution are measured on ratio to solvent investigated component with concentration С₀ (it is low fidelity equal concentration of investigated solution) instead of ratio to pure solvent (its absorbance is equal practical zero)

The extraction-photometric analysis (EPMA)

it is hybrid method of analysis, in which combine extraction (as method excretion, separation and concentrating) and spectrophotometry