Choosing a Refractometer

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Mr. Sapirstein is a Contributing Editor, American Laboratory/Labcompare.

Please check out our Refractometer / Brix Refractometers section for more information or to find manufacturers that sell these products.

First invented by Ernst Karl Abbe in 1869, the refractometer has remained one of the simplest and most accessible tools for quality control and optical analysis. A refractometer measures the angle of refraction as light passes through the interface between two media. The angle of refraction is caused by the difference in the speed with which light is propagated through the two media, and because the speed is due to the density, the refractive index can be used to determine the density—and, by extension, concentration—of the solute. Algorithms are further employed to translate refractive index into concentration, density, or specific gravity.

Most people who are in the market for a refractometer know what their specific needs are, but not everyone who needs a refractometer knows it, according to a representative from Reichert (Depew, NY). An enormous amount of information about a substance can be inferred from knowing its refractive index, such as dry weight; capabilities like these may not show up in a manufacturer's product brochure. By the same token, the versatility of a refractometer can be overestimated. Customers often seek capabilities that aren’t feasible, such as measuring pH, or determining a concentration when there are variable mixtures of solutes.

Before shopping for a refractometer, customers must first consider these questions if they have not done so already:

  • What is my rationale for buying a refractometer?
  • What kinds of materials will I be analyzing? Are my samples homogeneous, or do they contain undissolved elements? 
  • How sensitive must my refractometer be?
  • Which regulatory standards does my workplace adhere to?
  • Will my refractometer be used in harsh environmental conditions?
  • Will the applications I have in mind place restrictions on the size or shape of my system?

To help the customer navigate the complex world of refractometer products, below is a useful chart:

Abbe refractometers

Abbe refractometers are manually operated instruments that can either come in benchtop or handheld form. Samples are sandwiched between a refracting and an illuminating prism. Light illuminates a portion of the sample, and the user aligns a cross-hair with the shadow line, thus deriving a measurement.

Benchtop Abbe refractometers (which are faithful to Herr Abbe’s original configuration) will have a wider measurement range and more precision, at the cost of less portability. Manually operated handheld refractometers more or less employ the same design principles as the benchtop Abbe, even though manufacturers rarely sort them into the same category.

Abbe refractometers will provide high precision in typical, clear, and homogeneous samples so long as the operator is skilled in its manual use. All else being equal, an Abbe refractometer is vastly more cost effective than its automatic siblings. However, if the customer is relying on untrained staff, and also requires a high level of reproducibly, the Abbe type is a bad choice.

Automatic refractometers

Automatic refractometers are digitally controlled and do not require the manual alignment of a sight over a sample, thus allowing use with untrained staff. They also provide temperature control.

Some automatic refractometers can employ extensive algorithms to convert refractive index into degrees Brix or specific gravity. For specific applications such as obtaining sucrose concentrations in food, or urine specific gravity (SG) in patients with questionable hydration, automatic refractometers providing algorithms for displaying results in degrees Brix or urine SG are the best fit, since they obviate the need for deriving these results oneself.

If the customer’s refractometry needs are limited to sucrose or dextrose concentrations, a small, portable system that only displays results in Brix is ideal. The same applies to urine SG. One should be aware that smaller and more targeted refractometers may have a narrower range customized for that application.

On the other hand, refractometers employed for a wide range of substances (such as in the paint and plastics industry) require a family of algorithms in order to assign concentration changes to increments in refractive index/density. Such models are available and can even allow users to define their own algorithm.

Automatic refractometers may not handle particularly novel, viscous, or nonhomogeneous samples well, according to a representative at Anton Paar (Ashland VA), maker of refractometers and similar equipment. Examples include foods such as thick honey, certain types of jam, exotic fruits, or anything that may have extra seeds or pulp. For users with such sample types, a true Abbe model may be the best option, since it allows a thinking and seeing person to realign the cross-hairs and shadow region to avoid individual undissolved constituents that can confuse results.

Clinical refractometers

Those working in hospitals, clinics, and veterinary facilities have targeted uses for refractometry. Examples include urine specific gravity and plasma protein concentration. Handheld clinical refractometers are designed to specifically meet these needs. Part of the requirements for clinical devices is their validation and meeting of regulatory certification.

Some models will have regulatory compliance listed in product descriptions, but not all do. For example, compliance with FDA’s 21 CFR pt. 11 may not be inherently obvious. Moreover, many automated refractometers use reference materials as part of the refractive index calculation validated through agencies such as the National Institute of Standards and Technology (NIST). Refractometers designed for use in pharmacies must adhere to USP<797>, which sets guidelines for reducing infections transmitted to patients via contaminated medicine.

Other considerations

Users must consider whether or not they are working with nontransparent or colored samples. In these instances, a refractometer that offers high sensitivity or multiple-light wavelengths is most useful. This latter feature is especially important with yellow or yellow-green substances, since they will otherwise absorb the typical 589-nm yellow light employed in most standard refractometers.

Another important consideration is the type of environment in which the refractometer will operate. Some products separate the refractometer proper with control, analysis, and memory storage components that can be more sensitive to severe conditions.

Some institutions require refractometers that can be imbedded into piping, mixing devices, or other media through which liquids pass quickly. In this case, an in-line refractometer is the best choice.

Rayleigh scattering refractometers are specifically designed to obtain the refractive indecies of gases.

Lastly, choosing a refractometer requires an appreciation for precision, accuracy, and reproducibility. As we have seen, these are not only defined by model and make, but by the types of material being analyzed; automatic machines will have trouble with nonhomogeneous samples, but will otherwise outperform a manual Abbe machine with respect to precision and reproducibility.

Ultimately, one must have a real understanding of one’s materials and requirements. Know this, and the correct product choice will reveal itself.

Please check out our Refractometer / Brix Refractometers section for more information or to find manufacturers that sell these products.