Refractometer

Some types of refractometers

A refractometer is a laboratory or field device for the measurement of an index of refraction (refractometry). The index of refraction is calculated from Snell's law and can be calculated from the composition of the material using the Gladstone–Dale relation.

Types of refractometers

There are four main types of refractometers: traditional handheld refractometers, digital handheld refractometers, laboratory or Abbe refractometers, and inline process refractometers. There is also the Rayleigh Refractometer used (typically) for measuring the refractive indices of gases.

In veterinary medicine, a refractometer is used to measure the total plasma protein in a blood sample and urine specific gravity.

In drug diagnostics, a refractometer is used to measure the specific gravity in human urine.

In gemology, a refractometer is used to help identify gem materials by measuring their refractive index.

In marine aquarium keeping, a refractometer is used to measure the salinity and specific gravity of the water.

In homebrewing, a refractometer is used to measure the specific gravity before and after fermentation to calculate the approximate alcohol by volume.

$Gemology refractometer used to test light bending inside gemstones$

Gemology refractometer used to test light bending inside gemstones
$a wine grower with refractometer$

a wine grower with refractometer
$density evaluation of abdominal fluid of a cat with FIP by a refractometer.$

density evaluation of abdominal fluid of a cat with FIP by a refractometer.
Smart handheld digital refractometer

Smart handheld digital refractometer
$Automatic benchtop refractometer$

Automatic benchtop refractometer

Automatic Refractometers

Automatic refractometers automatically measure the refractive index of a sample. The automatic measurement of the refractive index of the sample is based on the determination of the critical angle of total reflection. A light source, usually a long-life LED, is focused onto a prism surface via a lens system. An interference filter guarantees the specified wavelength. Due to focusing light to a spot at the prism surface, a wide range of different angles is covered. As shown in the figure Schematic setup of an automatic refractometer the measured sample is in direct contact with the measuring prism. Depending on its refractive index, the incoming light below the critical angle of total reflection is partly transmitted into the sample, whereas for higher angles of incidence the light is totally reflected. This dependence of the reflected light intensity from the incident angle is measured with a high-resolution sensor array. From the video signal taken with the CCD sensor the refractive index of the sample can be calculated. This method of detecting the angle of total reflection is independent on the sample properties. It is even possible to measure the refractive index of optical dens strongly absorbing samples or samples containing air bubbles or solid particles . Furthermore, only a few microliters are required and the sample can be recovered. This determination of the refraction angle is independent of vibrations and other environmental disturbances.

Influence of Wavelength

The refractive index of a certain sample varies for nearly all materials for different wavelengths. This dispersion relation is characteristic for every material. In the visible wavelength range a decrease of the refractive index and nearly no absorption is observable. In the infrared wavelength range several absorption maxima and fluctuations in the refractive index appear. To guarantee a high quality measurement with an accuracy of up to 0.00002 in the refractive index the wavelength has to be determined correctly. Therefore, in modern refractometers the wavelength is tuned to a bandwidth of +/-0.2 nm to ensure correct results for samples with different dispersions.

Influence of Temperature

Temperature has a very important influence on the refractive index measurement . Therefore, the temperature of the prism and the temperature of the sample have to be controlled with high precision. With advanced automatic refractometers the temperature is controlled in a double-stabilized system. First the whole optical bench is temperature-stabilized to a constant temperature. This guarantees no distortion of optical parts and a high optical stability without the need to re-adjust any elements. With such a setup precise measurements are possible virtually independent of the ambient temperature. A second, high-precision temperature control stabilizes the prism with the sample on top. For this a temperature sensor is integrated into the prism so it is close to the sample. Prism and sample are temperature-controlled with a Peltier device. External water baths were used in the past but are no longer needed. For precision refractometry refractometers reach an absolute temperature accuracy of up to +/- 0.03 °C and a temperature stability of up to +/- 0.002 °C.

Extended possibilities of automatic refractometers

Automatic refractometers are microprocessor-controlled electronic devices. This means they can have a high degree of automation and also be combined with other measuring devices

Flow cells

There are different types of sample cells available, ranging from a flow cell for a few microliters to sample cells with a filling funnel for fast sample exchange without cleaning the measuring prism in between. The sample cells can also be used for the measurement of poisonous and toxic samples with minimum exposure to the sample. Micro cells require only a few microliters volume, assure good recovery of expensive samples and prevent evaporation of volatile samples or solvents. They can also be used in automated systems for automatic filling of the sample onto the refractometer prism. For convenient filling of the sample through a funnel, flow cells with a filling funnel are available. These are used for fast sample exchange in quality control applications.

Automatic Sample Feeding

Once an automatic refractometer is equipped with a flow cell, the sample can either be filled by means of a syringe or by using a peristaltic pump. Modern refractometers have the option of a built-in peristaltic pump. This is controlled via the instrument‘s software menu. A peristaltic pump opens the way to monitor batch processes in the laboratory or perform multiple measurements on one sample without any user interaction. This eliminates human error and assures a high sample throughput.

If an automated measurement of a large number of samples is required, modern automatic refractometers can be combined with an automatic sample changer. The sample changer is controlled by the refractometer and assures fully automated measurements of the samples placed in the vials of the sample changer for measurements.

Multiparameter Measurements

Today’s laboratories do not only want to measure the refractive index of samples, but several additional parameters like density or viscosity to perform efficient quality control. Due to the microprocessor control and a number of interfaces, automatic refractometers are able to communicate with computers or other measuring devices, e.g. density meters, pH meters or viscosity meters, to store refractive index data and density data (and other parameters) into one database.

Software Features

Automatic refractometers do not only measure the refractive index, but offer a lot of additional software features, like

Instrument settings and configuration via software menu
Automatic data recording into a database
User-configurable data output
Export of measuring data into MS Excel data sheets
Statistical functions
Predefined methods for different kinds of applications
Automatic checks and adjustments
Check if sufficient amount of sample is on the prism
Data recording only if the results are plausible

Pharma Documentation and Validation

Refractometers are often used in pharmaceutical applications for quality control of raw intermediate and final products. The manufacturers of pharmaceuticals have to follow several international regulations like FDA 21 CFR part 11, GMP, Gamp 5, USP<1058>, which require a lot of documentation work. The manufacturers of automatic refractometers support these users providing instrument software fulfills the requirements of 21 CFR part 11, with user levels, electronic signature and audit trail. Furthermore, Pharma Validation and Qualification Packages are available containing

Installation Qualification (IQ)
Operational Qualification (OQ)
Performance Qualification (PQ)

Scales typically used

External links

Refractometer – Gemstone Buzz uses, procedure & limitations.
Rayleigh Refractometer: Operational Principles
Refractometers and refractometry explains how refractometers work.
Refractometer products and functions

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