In this article we will consider in detail those analytical methods that are based on changing the energy state of individual atoms. These are optical methods of analysis. Let's give a description of each of them, highlight the distinctive features.
Definition
Optical methods of analysis - a set of methods based on changing the energy state of individual atoms. Their second name is atomic spectroscopy.
Optical methods of analysis will differ in the method of obtaining and further recording the signal (required for analysis). The abbreviation OMA is also used to designate them. Optical methods of analysis are used to study the energy flows of valence, external electrons. Common to all their diversity is the need for preliminary decomposition into atoms (atomization) of the analyzed substance.
Types of method
We already know what exactly is an optical method of analysis. Consider now the variety of these methods:
- Refractometricanalysis.
- Polarimetric analysis.
- A set of optical absorption methods.
We will analyze each of the positions of this classification of optical methods of analysis further separately.
Refractometric variety
Where is the refractive index applicable? This type of optical-spectral method of analysis is widely used in the study of food products - fatty, tomato, various juices, jam, jam.
Refractive analysis is based on measuring the refractive index (another name is refraction), which can be used to reliably judge the nature of a particular substance, its purity and percentage in mass solutions.
Refraction of a light beam will always occur at the boundary of two different media, provided that they have different density. The ratio of the sine of the angle of incidence to the sine of the angle of refraction will be the relative refractive index of the second substance to the first. This value is considered constant.
What does the index of refraction depend on? First of all, from the nature of matter. Light wavelength and temperature also matter here.
If the angle of light falls at 90 degrees, this position will be considered the limiting angle of refraction. Its value will depend only on the indicators of those media through which the light passes. What does it give? If the refractive index of the first medium is open to the researcher, then after measuring the limiting angle of refraction of the second, he can determine the refractive index of the medium already of interest to him.
Polarimetric variety
We continue to analyze the basics of optical methods of analysis. Polarimetric is based on the property of certain types of substances to change the vector of light oscillations.
Substances that have this remarkable property, when a polarized beam passes through them, are called optically active. For example, the structural features of the molecules of the entire mass of sugars determine the manifestation of optical activity in various solutions.
A polarized beam is passed through a layer of a solution of such an optically active substance. The direction of oscillation will be changed - the plane of polarization as a result of this will be rotated by a certain angle. It will be called the angle of rotation of the plane of polarization. This position depends on the following number of factors:
- Rotation of the plane of polarization.
- Thickness and concentration of the test layer of the solution.
- The wavelength of the most polarized beam.
- Temperature.
The optical density of the substance in this case will be characterized by specific rotation. What is this value? It is understood as the angle through which the plane of polarization rotates when a polarized beam passes through the solution. The following conditional values are accepted:
- 1 ml solution.
- 1 g of substance dissolved in this volume of solution.
- The thickness of the solution layer (or the length of the polarizing tube) is 1 dm.
Optical absorptionvariety
We continue to get acquainted with optical methods of analysis in analytical chemistry. The next category in the classification is optical absorption.
This includes those methods of analysis that are based on the absorption of electromagnetic radiation by the analyzed substances. They are considered today the most common in research, scientific, certification laboratories.
When light is absorbed, molecules and atoms of absorbing substances will pass into an excited new state. Already, depending on the variety of such substances, as well as the ability to transform the energy absorbed by them, a whole set of absorption optical methods is distinguished. We will present them in more detail in the next subheading.
Classification of optical absorption methods
We bring to your attention the classification of these methods of optical analysis in chemistry. It is represented by four positions:
- Atomic absorption. What is included here? This is an analysis based on the absorption of light energy by the atoms of the substances under study.
- Absorptive molecular. This method is based on the absorption of light by complex ions and molecules of the studied, analyzed substance. Much attention is paid here to the infrared, visible and ultraviolet zones of the spectrum. Accordingly, these are photocolorimetry, spectrophotometry, IR spectroscopy. What is important to highlight here? Spectrophotometry and photocolorimetry are based on the interaction of radiation with a number of homogeneous systems. Therefore, inIn analytical chemistry, they are often combined into one group - photometric methods.
- Nephelometry. This type of analysis is based on the absorption and further scattering of light energy by suspended particles of the substance under study.
- Fluorometric (or luminescent) analysis. The method is based on the measurement of radiation that appears when energy is released by excited molecules of the substance being studied by the researcher. Represented by fluorescence and phosphorescence. We will analyze them separately.
Luminescence
Luminescence in general in the scientific world is called the glow of atoms, molecules, ions and other more complex particles and compounds of matter. It appears as a result of the transition of electrons to the normal state from the excited state.
Thus, in order for a substance to begin to luminesce, a certain amount of energy must be supplied to it from the outside. The particles of the substance under study will absorb energy, passing into an excited state, in which they will remain for a certain period of time. Then return to the previous state of rest, while giving away a share of its own energy in the form of luminescence quanta.
Phosphorescence and fluorescence
Depending on the type of the excited state, as well as the residence time of the substance in it, there are two types of luminescence - phosphorescence and fluorescence. Each of them stands out for its distinctive characteristics:
- Fluorescence. A kind of self-luminescence of a certain substance, whichwill continue only when irradiated. When the researcher removes the source of excitation, the glow will stop either instantly or after 0.001 seconds.
- Phosphorescence. A kind of self-luminescence of a certain substance that will continue even when the light that excites it is turned off.
It is phosphorescence that is used to study food products. The luminescent research method helps to detect a substance in the studied sample at its concentration of 10-11g/g. This method will be good for determining certain types of vitamins, the presence of proteins and fats in dairy products, studying the freshness of meat and fish products, diagnosing damage to fruits, vegetables, and berries. Also, luminescent research is used to detect medicinal inclusions, preservatives, pesticides, and various carcinogenic substances in products.
The entire absorption group is often combined into a spectrochemical (or spectroscopic) category in the classification of optical methods of analysis in analytical chemistry. Despite the fact that the methods are inherently different, they all have one thing in common: they are based on the same laws of light absorption. But at the same time, there are significant differences in the type of absorbing particles, the hardware design of the study, and so on.
Photometric variety
The name of the set of methods of spectral molecular absorption analysis. They are based on selective absorptionelectromagnetic radiation in the visible, ultraviolet, infrared regions by the molecules of the component under study. Its concentration is determined by a specialist according to the Bouguer-Lambert-Beer law.
Photometric analysis includes photometry, spectrophotometry and photocolorimetry.
Photoelectrocolorimetric variety
The photoelectrocolorimetric method is more objective when compared to visual colorimetry. Accordingly, it gives more accurate research results. Various FECs are used here - photoelectric colorimeters.
The luminous flux when passing through a colored liquid is partially absorbed. The rest of it falls on the photocell, where an electric current arises, which registers an ammeter. The more intense the concentration of the solution, the greater its optical density. The greater the degree of absorption of light and the less the strength of the resulting photocurrent.
We examined the entire classification of optical analysis methods used today in analytical chemistry: refractometric, polarimetric, optical absorption. They are united by the need for preliminary atomization of the substance. But at the same time, each of the methods is distinguished by its distinctive characteristics - varieties of receiving and registering a signal for analysis.
