Refractometry is an analytical method based on the phenomenon of light refraction during the transition of rays from one medium to another, which is explained by changes in the speed of light distribution in different environments.
Today, this method of analysis is widely used in many fields: refractometry is often used in pharmaceutical and food analysis, as well as in the study of eyes.
Refractometry in ophthalmology is one of the objective methods for studying the refractive power of the eye - refraction, which is carried out using specialized equipment - an eye refractometer. The refractometry method is used to identify eye diseases such as:
Note! "Before you start reading the article, find out how Albina Guryeva was able to overcome problems with her vision by using...
This research method allows doctors to quickly obtain accurate data about the patient’s eye health. The procedure is possible at any age: both children and adults - this is a definite advantage of the method.
As already mentioned, refractometry is carried out using specialized ophthalmological equipment - refractometers, which come in several types:
Consists of the following parts:
The procedure itself is as follows: a test symbol is introduced into the optical system, which is three vertical and two horizontal stripes. The light beam from the device is directed into the patient's eye under examination and projects onto the retina an image of test symbols, which are referred by the optical system of the eyes to the focal plane of the refractometer. The initial position of the device's optics is a measuring scale with zero values, which are associated with the far points of pure vision of the emmetropic eye. The doctor sees the test symbol through the eyepiece of the device.
With normal refraction of the eye, the two parts of the half-image of vertical and horizontal stripes merge, but in the case of and vice versa, they diverge. The horizontal displacement of the stripes and along the vertical axis indicates.
By turning the device horizontally, the ophthalmologist minimizes the divergence of the bands by installing the device in one of the main meridians. In this way, refraction is measured in a specific meridian. The doctor, by rotating a special ring located near the eyepiece of the device, achieves the merging of the stripes, and the scale of the refractometric device indicates the type and size of the refractive abilities of the eye apparatus. The measurement limit for this type of equipment is from -20.0 to +20.0 diopters, but the accuracy is up to 0.25 diopters.
The most commonly used today are automatic computerized refractometers. The essence of their work is also based on the emission of microscopic beams of infrared rays, which cross the pupil and the refractive medium, are reflected from the fundus of the eye and go in the opposite direction. The device’s sensor reads out the received information, and a special application analyzes the original and newly obtained data, through which the clinical refraction of the eyes is calculated. All results obtained are instantly transferred to the monitor and printed.
The procedure for measuring refraction occurs as follows:
The finished printout contains all the information about the state of refraction of our eyes and their health. And naturally, the results for any patient are of considerable interest. However, not everyone can read a refractogram fluently. How are the indicators deciphered?
The finished printout consists of three columns:
Refractometry values change throughout life. For example, farsightedness is most often detected in a newborn child, but by the age of 20 this anomaly remains in only a third. About 40% of young people have normal refraction, while the rest suffer from myopia. And with age, refraction worsens, which is caused by age-related changes in the lens, at which time patients begin to develop p. Therefore, it is extremely important to undergo periodic examinations in order to promptly prevent the development of eye diseases.
To obtain the most accurate results, before the procedure, the ophthalmologist prescribes a course of atropinization, which the patient undergoes for three days. This procedure consists of daily instillation of an atropine solution twice: in the morning and in the evening. The concentration of the drug is set in accordance with the age group of the subject, but can be changed due to individual factors.
It is strictly forbidden to start using drops on your own, as this can lead not only to false readings, but also worsen eye health. Another important factor in the success of the procedure is avoiding alcohol a few days before refractometry.
If allergic reaction If you experience atropine, you must immediately notify the treating ophthalmologist and stop instilling the drug.
So, what kind of beast is this refractometer? And what do you eat it with?
The review is a logical continuation of the topic about and belongs to the category “but men don’t know!” :)))
Well, let's get started:
First, a lyrical digression. As you already know, my friends, I am the proud owner of a Chinese moonshine still. It came with a set of three alcohol meters along with a thermometer. And everything would be fine, and everything would be wonderful, if not for one BUT. To measure the percentage of alcohol in a liquid, a decent amount of this very liquid is needed. According to the instructions, it comes out to 300 grams! This is the most important inconvenience. You can no longer measure the degree in a glass.
“Is there such a device,” I asked myself, “that can show the percentage of alcohol using a minimum amount of liquid?” He asked and began to search. It turned out that there is such a device! Almost immediately I came across a device with such a mysterious name - “refractometer”. And my eyes were opened, and I saw that this was good :)
With this miracle device you can measure the percentage of alcohol in a liquid using just two or three drops of it! Refractometers, as it turned out, are a great many. Refractometers for beer, wine, honey, milk, antifreeze, electrolyte, aquarium, etc. But, nevertheless, the principle of operation is the same for everyone. They differ only in their scale and the ability to automatically correct the result depending on the temperature. From 0 to 30 degrees. This is just such a function. There is an ATC sticker and it costs more than a device without temperature correction. It also has a calibration screw, closed with a rubber plug and a screwdriver included.
Now let's turn to Wiki:
“A refractometer is a device that measures the refractive index of light in a medium.
Refractometry is a method for studying substances based on determining the index (coefficient) of refraction (refraction) and some of its functions. Refractometry (refractometric method) is used to identify chemical compounds, quantitative and structural analysis, determination of physical and chemical parameters of substances. The refractive index n is the ratio of the speed of light in the adjacent media."
Oh how! Neither more nor less, but we are pulling the speed of light itself by the tail! Cool :)))
Once again I am convinced that after drinking moonshine, you can’t pull anyone’s tail! ha ha ha!!!
So how does it work. We take a refractometer. We look through the hole into the light. We focus on the scale. Open the top glass. We put moonshine (vodka, tequila, etc.) into a pipette, drop two or three drops onto the bottom glass and close the whole thing with a clamp with the top glass. Again we look through the hole into the light. Let's smile :)
The scale with uneven graduations looks exactly the same as on the seller’s website. That is, like this. 
The only caveat. The scale in the picture says ALCOHOL. Mine says ALCOHOLIC. A subtle hint though...
Here's what the complete set looks like. 
Refractometer
Pipette
Screwdriver
Napkin
Instructions
Case
Other photos
Neat box-case 
Everything is packed compactly 
Calibration screw 
Open glass 
Eyepiece 
In hand 
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Ophthalmology is moving forward by leaps and bounds. Over the course of several decades, laser surgery has managed to advance almost from zero to incredible heights, and almost all manual methods studies of the organ of vision were replaced by hardware ones. There is no need to explain that they are much more accurate and reliable than studies that require calculations and measurements from humans. But the eye is not only a sensory organ, but also a highly complex optical system that requires filigree precision. There are diagnostic methods that are rarely used, only in special cases. But there are, on the contrary, routine ones, without which eye examination is no longer possible. These include refractometry - what is it, how and why is it carried out, how should its results be interpreted?
Refractometry is the measurement of the refraction of the eye. However, one cannot talk about refraction without paying attention to how the eye is structured, at least in general terms.
Table. What does the organ of vision consist of?
| Anatomical unit | Characteristic |
|---|---|
| Auxiliary apparatus | Eyelids, eyelashes, eyebrows, eye socket, lacrimal glands and their system, muscle complex. |
| Eyeball | This is the perceptive apparatus of the organ. A light beam penetrates through its transparent elements and is fixed on the retina. In turn, it has several layers in its structure and plays a major role in the processes of refraction and accommodation. |
| Conducting system | Nerves connecting the retina and brain structures. |
| Subcortical elements and higher nerve centers | Areas in the brain responsible for processing visual signals. |
Separately, the structure of the eyeball should be considered. It consists of several structures:
Most people in their lives have come across or at least heard about such concepts as “myopia” and “farsightedness”. These words are not scientific terms and replace the more complex “” and “”, but they describe precisely violations of the refractory ability of the visual analyzer.
The process of refraction is the ability to bend light rays. The eye is a system of optical media, and the main refractive elements are the cornea and lens. All other transparent media are light-conducting. In order for the refraction and transmission of light to take place fully, all media must be perfectly transparent.
In ophthalmology, there is a concept of physical and clinical refraction. This is due to the fact that the eye is not only a set of light-processing structures, but also an organ of the nervous system.
Physical refraction‒ this is directly the ability of the eye to refract light beams; refractive power is described in diopters. Newborn children see practically nothing and the refractive power of their eyes does not exceed 50 diopters. But gradually the clarity of vision increases and eventually increases to 70 diopters.
And at this moment the game comes into play accommodation. This, in turn, is the process of changing the configuration of the lens, which is aimed at increasing image clarity - focusing. In ophthalmology, such concepts as the nearest and further point of clear vision are accepted. The further one is at infinity - at complete relaxation muscles responsible for accommodation. But as soon as a person needs to look at an object located closer to this further point, tension of the eye muscles becomes necessary.
In this regard, two types of clinical refraction are distinguished.
So, based on the above information, the definition of refractometry as the process of measuring refraction becomes more clear. Clinical refraction is examined, since the ability to focus images on the retina is important. Moreover, both the static and dynamic components are studied.
Some time ago, refraction could only be measured manually. For this purpose, special eye diagrams and manual refraction measurement techniques were used. In terms of accuracy, they were largely inferior modern devices Moreover, the possibility of error could not be ruled out.
Today, refractometry is a high-tech procedure that takes no more than five minutes. For this diagnostic method we use special devices- refractometers. The operating principle of this device is infrared radiation. The refractometer is located on the table, is about half a meter in height and has “outputs” on both sides - a screen with a control panel for the doctor and a special device where the patient looks. From a special lens, a beam of rays in the infrared spectrum is directed towards the pupils of the subject, which, penetrating through a hole in the iris, fall on the retina. There is a reflection from the bottom of the eye and return to the sensors of the device. The doctor only needs to direct the rays through the patient's pupil. The devices, in turn, record the received data, and the computer calculates the necessary indicators. The calculations are immediately displayed on the screen and can then be printed.
Despite the simplicity, quick execution and absence of any negative consequences of the procedure, it is irrational to carry it out to everyone. Typically, refractometers are used in specialized ophthalmological centers, where vision is checked before any surgical interventions and other serious procedures, and the technique is also used to clarify the degree of refractive error after the primary diagnosis of visual impairment by a doctor. The use of refractometry as one of the routine dispensary examinations is possible, but not every hospital can afford it.
Clear indications for refractometry:
Contraindications to this technique are very conditional. Of the highly specific ones, only one is a violation of the transparency of the vitreous body, or a disease such as cataracts. From non-specific:
In order for the results to be as reliable as possible, short preliminary preparation is necessary. It consists of instilling a solution of atropine into the eyes in the morning and evening for three days before the intended examination.
The dose of instilled atropine is 1 drop in each eye. Depending on age, the concentration of the solution may vary:
When preparing, you should be careful, since such drops in the eyes can cause transient blurred vision, which is especially dangerous for drivers and people whose activities require maximum eye strain and attention to detail. In addition, atropine is a fairly strong allergen, so allergic conjunctivitis may develop - redness, tearing.
The actual process of performing refractometry is simple.
With hardware refractometry, several types of indicators can be detected on a printed sheet separately for the right (R) and left (L) eyes.
There is no need to be able to decipher these values yourself: the doctor will inform you about any deviations. But it is recommended to save the data after the study so that monitoring can be carried out over time.
The main point of the conclusion voiced by the doctor is the type of refraction and the degree of visual impairment (important for obtaining lenses or glasses). Types of refraction:
Thus, refractometry is a modern informative method for diagnosing the state of vision, which takes a minimum of time, is absolutely safe for both adults and children, and is also very easy to perform.
Modern moonshiners with experience in the process homemade In the case of strong alcohol, they try to use specialized tools to determine the quality of the product and identify the degree of concentration of certain substances in it. The most important assistant of a good moonshiner is a refractometer.
A refractometer is a special optical instrument that measures the concentration of solutions using the phenomenon of light refraction. The term “refraction” itself appeared at the beginning of the 18th century, and Newton introduced it. This metal device with slide and eyepiece. Those people who make homemade moonshine very often use a refractometer to measure the level of sugar or alcohol in the liquid. The device must be calibrated before use.
There are several main types of refractometers.
Some of the most popular refractometers that are well suited for moonshiners are hand-held models with a Brix scale. It is intended for measuring sugar in a liquid solution.
Degree Brix is a measure of the mass ratio of sucrose to the liquid in which it is found. This measure is calculated using a refractometer. For example, 25 °Bx means that there are 25 grams of sugar in 100 grams of liquid.
Many moonshiners are very concerned about the sugar content in the mash they make. It is not always possible to calculate such a share using an ordinary calculator. And there are quite important reasons for this.
In addition, it is precisely by the difference in sugar content that an experienced moonshiner will be able to quickly calculate the percentage of alcohol that the yeast managed to ferment during the time elapsed between measurements. If the proportion of alcohol is in a stable range for a particular product, then everything goes well. And if not, a person will need to aerate his mash, regulate the temperature and feed.
Now on the Aliexpress website you can find many varieties of refractometers for home brewing, which look very similar. The main difference between them is the measurement scales that are printed on the glass. Sometimes it can be Brix (volume fraction of sucrose), sometimes it can be the volume fraction of alcohol, sometimes it can be the ratio of the density of the substance to the density of water, as well as other ratios. In fact, you can purchase anything, and then use recalculation or a special ruler for that. But there is no need for such difficulties. If a moonshiner is going to measure a specific indicator, then it is wiser for him to choose a refractometer whose scale is in the required units of measurement.
Also, certain models of home refractometers can take into account the influence of temperature using the ATC function. Inside these devices there is a bimetallic plate. It can change its size depending on the temperature. In addition, this plate is connected to the optical system of the refractometer, affecting it during temperature fluctuations. Therefore, when purchasing a high-quality refractometer, it is important to pay attention to the presence of this function. If it is not there, the moonshiner will have to use special tables for recalculation. This is why a good refractometer is so important for moonshine brewing.
The principle of operation of a refractometer is based on the fact that only the refraction angle of the liquid under study is determined, and the refractive index of the measuring prism is known.
1 - lighting mirror; 2 - auxiliary folding prism; 3 - main measuring prism; 4 - matte edge of the folding prism; 5 - test liquid; 6 - Amici prisms of the compensator; 7 - telescope lens; 8 - rotary prism; 9 - telescope eyepiece
Figure 2 - Optical diagram of the IRF-22 refractometer.
How to use a refractometer:
1. Before starting work, you need to check the setting of the refractometer zero point. The zero point setting and measurements on the refractometer must be carried out at the same temperature. Checking and setting the zero point is carried out using distilled water. When studying distilled water, the limit of light and shade should be at 1.33299 on the solids scale and 0% on the dry matter scale. Checking and setting the zero point for distilled water is carried out as follows:
Open the upper chamber and rinse the surfaces of the measuring and lighting prisms with distilled water or alcohol and wipe dry with a clean linen napkin;
Using the melted end of the stick, apply one or two drops of distilled water to the plane of the measuring prism and close the upper chamber;
By shifting the illuminator, direct the light beam into the window of the upper chamber;
By moving the handle with the eyepiece up and down along the scale, find the border of light and shade in the field of view;
By moving the handle, the chiaroscuro boundary is aligned with the hairline (if, when aligned with the center of the reticle crosshair, it passes through the scale division 3 = 1.33299 and 0% of the dry matter scale, the zero point is set correctly).
Measuring the refractive index of transparent liquids and the percentage of dry substances for sucrose is carried out similarly to the measurement of distilled water when setting the zero point: after aligning the chiaroscuro boundary with the crosshairs of the grid, make a reading on the scale of refractive indices and the percentage of dry substances for sucrose. Measure three times. The arithmetic mean of three measurements is the final measurement result.
Product Measurement sugar production can be performed at a temperature of 10-30°C, taking into account the correction for temperature according to the table (take the table from the teacher).
For example, if measurements are made at a temperature of 17°C, the scale reading is equal to 37.8% of dry matter. From the table we find the correction equal to 0.22. The refractometer reading will be:
37.80 - 0.22 = 37.58% dry matter.
After taking measurements, it is necessary to open the upper chamber, rinse, wipe dry the surfaces of the upper and lower chambers and smoothly lower the upper chamber of the device.
Calculation of the concentration of a substance based on the refractive indices of the solution is carried out using the following methods: using a calibration graph, using tables, using the refractometric factor, and the method of additives.
According to the calibration graph: a calibration graph is constructed using solutions of a substance of known concentration (concentration - refractive index), the refractive index of the analyzed solution is measured, and the concentration is determined on the graph based on the refractive index.
According to tables: for many substances, tables have been compiled that show the refractive indices of solutions with known concentrations.
By refractometric factor: if the refractometric factor is known, the formula is used to calculate concentrations:
where s 1 is the refractive index of the solution;
z0 is the refractive index of the solvent;
F is a refractometric factor showing an increase in the refractive index with an increase in the concentration of a substance by 1%.
The refractometric factor is determined experimentally or using refractive index tables.
For example, for NaCl, factor F is equal to the difference between the refractive indices of a 4% solution z1 = 1.3397 and a 2% solution z2 = 1.3364, divided by the difference in concentration (equal to 2%):
Automatic refractometers for continuous recording of n in liquid flows are used in production for control technological processes and automatic control of them, as well as in laboratories for control of rectification and as universal detectors of liquid chromatographs.
All currently produced refractometers, regardless of their purpose, are built on the principle of Abbe-type or Pulfrich-type refractometers, but in both of them, measurements are based on determining the value of the limiting refraction angle.
1) The principle of construction of refractometers of the Abbe and Pulfrich type.
The main typical unit of Abbe type refractometers is a complex of measuring and lighting prisms. A thin layer of the liquid under study is located between the tightly pressed edges of both prisms.
The surface of the lighting prism in contact with the liquid being tested is matte, rough, and scatters light entering the liquid layer through it, as a result of which light rays penetrate the liquid in different directions.
The beam, the angle of incidence of which is closest to the direct one (the limiting beam), divides the field visible through the telescope into dark and light halves. Using a special handwheel, the block of prisms can be installed in a position in which the limiting beam will be aligned with the optical axis of the telescope, and the boundary of the light and dark fields with the intersection of two straight lines visible through the tube, through which this imaginary axis passes. Based on the position of the reference line observed in the sighting tube on the scale, the value of the refractive index is determined. The boundary between the dark and light fields would be blurred and colored in all the colors of the rainbow due to the decomposition of white light as it passes through the measuring prism. To prevent this phenomenon, Abbe-type refractometers use special devices - dispersion compensators.
The refractive index of liquids changes significantly under the influence of temperature. Therefore, in refractometers, temperature control is used to increase accuracy. Thermostatic control in Abbe-type refractometers is carried out by circulating water at a certain temperature through the lower and upper chambers of the prism block. The temperature must be maintained with an accuracy of ±0.1-0.5°C.
In Pulfrich-type refractometers there is only one prism, to which a cup is attached, into which the liquid being studied is poured. A light beam directed along the liquid-prism interface is not distorted, so the angle of incidence of this beam is exactly 90°, which determines the greater accuracy of devices of this type.
Submersible refractometers are devices whose measuring prism is immersed in a glass with the liquid being tested. In such refractometers there is no lighting prism and the cut of the measuring prism is in direct contact with the liquid being tested. Modern refractometers are accurate to, and when used special methods Refractometry accuracy can be increased by 10-1000 times.
The domestic industry produces various refractometers, including a universal laboratory refractometer (RLU), a laboratory refractometer, a precision laboratory refractometer, IRF-22 and IRF-23 refractometers.
2) The IRF-23 refractometer is intended for determining the refractive indices of liquid and solids in the range of 1.33-1.78, with an accuracy of 1H The IRF-23 refractometer is the most complex, so its description is given below.
The optical part of the device consists of a measuring prism, a reference system, a telescope and a system for illuminating the object under study. The reading system includes a dial with protective glass, illuminated through a condenser, a light filter with an incandescent lamp, a reading microscope consisting of a lens, reflective prisms and an eyepiece. A red spiral scale with an index is placed in the focal plane of the eyepiece. The reading device is designed to accurately measure the angle of rotation of the telescope along the dial. The dial is covered with a casing. The scale division value is 1°. Coarse rotation of the telescope is done by hand, fine rotation is done with a micrometer screw. The eyepiece of the tube has compensatory aiming for visual acuity.
The telescope consists of a lens, a reflective prism, a crosshair, an illuminating prism and an eyepiece. A spotting scope can operate on the principle of an autocollimator, in which lamp light reflected by two reflective prisms and a collecting lens is used to illuminate the crosshairs.
The object can be illuminated with light from discharge tubes or a sodium lamp.
For accurate measurements, the temperature of the measuring prism and the liquid being tested must be maintained constant within ±0.5°. For this purpose, there are two fittings on the prismatic chamber onto which rubber hoses are put on and connected to the ultra-thermostat. The Carl Zeiss company (GDR) produces numerous models of refractometers, including Abbe refractometers, submersible ones, and for work in the field (manual). One of the latest models (model P) of the Abbe refractometer is not fundamentally different from the domestic RLU refractometer.
The Zeiss submersible refractometer is equipped with thermal prisms, allowing research to be carried out at relatively high temperatures(up to 50°C). An important advantage of thermoprisms is also the ability to use small quantities of substances (on average 0.04 ml) and study volatile substances. In addition to the specified thermal prism, a flow prism is attached to the device, which allows the study of continuously flowing liquids, as well as substances that decompose in air.
The flow prism consists of an immersion prism and a corresponding flow housing mounted on the refractometer. If thermal regulation is necessary, the body of the flow prism can be connected to a thermostat, for which there are fittings on it.
The field (manual) refractometer from this company is of particular interest.
The device is designed to work directly in fields, orchards and vineyards and is used to determine the content of sugary substances in root crops (sugar beets), berries, and grapes.
In addition to the refractometer, the kit contains: a device for taking a sample, a press tongs for squeezing small quantity juice The determination of sugar content is based on a natural relationship between the content of sugary substance in juice and its light refraction. One or two drops of juice are applied to the prism of the refractometer, covered with a lid and looked against the light into the eyepiece, where a scale is visible, the upper part of which is darker than the lower. The dividing line, which coincides with a certain indicator on the scale, corresponds to the content of sugary substances in the juice. The device allows for determination with an accuracy of 0.2%.
3) Refractometer IRF-454 B2M
The IRF-454B2M refractometer is designed to measure the refractive index and average dispersion of non-aggressive liquids and solids.
The IRF-454 B2M refractometer has a number of advantages:
Speed of measurement;
Ease of maintenance;
Minimum consumption of the test substance, which is especially important when working with expensive materials.
Refractometer IRF-454 B2M is used:
1. In medical institutions: to determine protein in urine, blood serum, urine density, analysis of brain and joint fluid, density of subretinal and other eye fluids. The use of a refractometer can significantly reduce time spent during mass examinations of patients.
2. B pharmaceutical industry: refractometer IRF-454b2m can be used to study aqueous solutions of various medicines: calcium chloride (0% and 20%); novocaine (0.5%, 1%, 2%, 10%, 20%, 40%); ephedrine (5%); glucose (5%, 25%, 40%); magnesium sulfate (25%); sodium chloride (10%); cordiamine, etc.
3. In the food industry:
at sugar and bakery factories, confectionery factories for the analysis of products and raw materials, semi-finished products, culinary and flour products, the IRF-454 b2m refractometer determines the moisture content of honey (up to 20%)
to determine the proportion of dry substances in various worts (GOST 5900-73), “wetting”, sugar-agrarian syrup, syrup for marmalade, marshmallows, creams and gingerbreads, “circulation” for gingerbread;
to determine the mass fraction of soluble solids by sucrose (BRIX) in processed products of fruits and vegetables, to determine the percentage of fat in solid food products (gingerbreads, waffles or baked goods) and salt concentrations.
4. When servicing equipment, the IRF-454 B2M refractometer is used to determine with greater accuracy the volumetric concentration of the anti-crystallization liquid "IM", which is added to aviation fuel in an amount of 0.1 to 0.3%. Further processing of the results is carried out according to " Methodological recommendations on analysis of the quality of fuels and lubricants in civil aviation" Part II p. 159. Experience in the use of refractometers has shown that these devices significantly reduce the time and increase the reliability of obtaining analyzes on the percentage of liquid "IM" in aviation fuel.
4) Refractometer ALR-3
Auto laboratory refractometer ALR-3 with microprocessor control is designed to study the concentration of a wide range of liquid media of both low and high viscosity, regardless of transparency and color.
The device automatically measures the refractive index of a solution sample, calculates its concentration and displays the result on a digital LCD indicator. The refractometer has a standard calibration for the concentration of sugar in water (Brix scale), but can be calibrated for the concentration of any solutions at the request of the customer with the corresponding scales being stored in the memory.
The ALR-3 refractometer measures the temperature of the test solution and automatically compensates for its influence on the measurement result.
Refractometric detectors, unlike photometric detectors that react only to substances that absorb light in the ultraviolet, visible and infrared regions of the spectrum, refractometric detectors are universal. They are especially useful when substances do not have strong absorption in UV light, do not fluoresce, and do not have electrochemical activity. Their principle of operation is based on the differential measurement of the refractive index of a pure solvent and a solution of the analyte in this solvent. The contribution of the solute to the change in the refractive index of the solvent is proportional to the volume concentration of this substance, and the solvent is also a detectable substance, since it has a certain refractive index.
These detectors have average sensitivity, their readings are highly dependent on fluctuations in parameters affecting the composition of the mobile phase, such as pressure, temperature and concentration of the analyte. Therefore, a refractometric detector is of little use for gradient chromatography. A painstaking selection of solvent systems with similar refractive indices is required. Only then does it become possible to carry out gradient elution within certain concentration limits of the solvent mixture. The sensitivity of the detector to temperature changes ranges from 5×10-4 to 5×10-5 refractive index units per 1°C for different solvents. As for pressure sensitivity, it is 1×10-4 - 5×10-4 refractive index units per 1 MPa.
The sensitivity of the detector to temperature requires special measures to stabilize the temperature of the detector itself and the mobile phase at the entrance to the detector. In this case, the use of longer connecting tubes at the detector inlet, acting as heat exchangers, leads to high extra-column peak broadening and reduces the separation efficiency achieved in the column. In a chromatograph equipped with a refractometric detector, in order to stabilize the eluent flow and the retention parameters of sorbates in the column, it is advisable to use thermostatting of the column and detector. To achieve the maximum sensitivity of the detector at the level of 10-8 units of the refractive index, the temperature control accuracy should be no more than ±0.01°C. With good temperature control, the detector is little sensitive to changes in the flow rate of the mobile phase. It is simple in design, easy to use, non-destructive and provides high reproducibility of readings. The disadvantage of the detector is its insensitivity to substances that have the same refractive index as the solvent.
Most modern refractometric detectors operate based on three different signal measurement principles: deflection, reflection and interference.
A method based on the law of light reflection (Fresnel's law), according to which the intensity of reflected light incident on the interface between liquid and glass is proportional to the angle of incidence and the difference in the refractive indices of the two media. The advantage of detectors operating on this principle is a smaller cell volume (< 3 мкл), в связи с чем они могут работать при небольших расходах элюента и с высокоэффективными колонками. Однако чувствительность таких детекторов в 50-100 раз ниже чувствительности других типов рефрактометрических детекторов, что, кстати, делает их более пригодными для градиентного элюирования. Так как детектирование происходит на границе раздела жидкости и стекла, для получения стабильной работы детектора необходимо следить за чистотой стекла.
A Fresnel type detector includes a light source, a condenser, a differential cell, glass rods, a lens and a photodetector. It also includes heat exchangers and a diaphragm to regulate the intensity of the light flux. The light source, equipped with an infrared blocking filter, is designed to create a luminous flux in the visible part of the spectrum. The condenser is designed to form a flat beam of light incident on the cell. The refractometer cell is made of of stainless steel, sealed with protective glass, prism and Teflon gaskets. Glass rods and a lens focus the light fluxes passing through the cells onto the photosensitive elements of the photodetector. Focusing eliminates light overlap that can cause chromatographic peaks to differentiate.
The operation of the third type of refractometric detector is based on the interferometric shear principle. Light rays from a visible light source are divided into two parts by a divider, focused by a lens, and passed through a 5-μL working and reference cell. The light rays are then combined using another lens and a splitter and hit the sensing element. The difference in refractive indices between the working and reference eluent flows results in a difference in optical path length, which is measured by the interferometric detector as a change in the wavelengths of light. The readings of this type of detector have sufficient wide range linearity, and the sensitivity is 10 times higher than for other refractometric detectors. Under optimal operating conditions, detection of approximately 3 µg/ml of solute is possible. The detector detects any type of analyte, regardless of structure, molecular weight and others physical and chemical properties. The detection limit for the best refractometric detectors reaches 108 refractive index units. However, the noise level in these detectors is 2 orders of magnitude higher than the noise of a UV detector. They are optimal for applications where high sensitivity is not required, such as preparative LC.
Figure 3 - 1. Sucrose 2. Glucose 3. Fructose 4. Sorbitol Chromatogram apple juice. Column: Rezex RCM-Monosaccharide 300x7.8 mm 8 µm, protective column: SecurityGuard Carbo-Ca2+ 4x3 mm, separation mode: isocratic, mobile phase: water, flow rate: 0.6 ml/min, column temperature: 85°C, volume samples: 20 µl, detector: refractometric.
