Dealing with difficult samples
“The problem is that users don’t buy refractometers to measure standards,” observes Peter Marriott, product manager at Rudolph Research Analytical (Hackettstown, NJ). “They buy instruments for industrial scale products such as food or chemicals, which exist as complex matrices, and for which measuring refractive index is far more challenging.”
Traditional Abbe refractometers, though dated, are still sometimes used for difficult samples because they provide analogue feedback on measurement quality. Marriott refers to first- and second-generation automatic refractometers as “closed boxes” that generate a number but do not provide the end user with information on how they are reading the sample. “Now, with third-generation systems, for example incorporating Rudolph’s Smart Measure technology, users obtain feedback on what is going on with the sample, which helps improve measurement protocols.”
Another limitation of early automatic refractometers was the measurement of samples that change over time. Evaporation and taking on water from the atmosphere, for example, increase and decrease, respectively, the concentration of target analytes. Both effects may be minimized by using a temperature-controlled sample cover. Another issue is composition change within the sample itself. Regardless of what causes the change, its effects can only be mitigated if the user knows it’s occurring. “Only with current-generation computer-controlled instruments that sample repeatedly at extremely high speed do such changes become easy to detect,” Marriott says.
Temperature control has always plagued refractometer measurements. Despite being “solved” more than a decade ago, it continues to concern present-generation end users. Refractive index is temperature sensitive, and a temperature-corrected refractometer will only work for a limited range of samples, such as those containing sugar, and even then only if the sample temperature is fairly stable to begin with. For nonsugar samples—which include most calibration standards—and for samples that are applied at process temperature, only a temperature-controlled (as opposed to corrected) refractometer provides reliability, according to Marriott.
The standard rule of thumb—match instrument capabilities with measurement needs—is obvious but easy to overlook given the wide diversity of refractometer system capabilities. Neimar da Silva, product specialist at Anton Paar (Ashland, VA), advises purchasers to resist the temptation of over-specifying their instrumentation. “Focus instead on accuracy and simple operation. An advanced refractometer with touchscreen operation and FDA CFR part 11 compliance is a must for pharmaceutical labs, but it is serious overkill for the food industry.”
Similarly, the ability to measure at multiple wavelengths might appeal to a pure research organization but is probably inappropriate for those interested only in a snapshot concentration of one ingredient.
Where high- and low-end applications overlap is in modular instruments that measure two or more parameters. All-in-one instruments that combine refractive index with density, color, polarimetry, pH, or turbidity can save time and instrument costs. Refractive index is usually secondary, as instrument design goes, to the second measurement parameter.
“The flavors and fragrances business is based on purchasing large quantities of raw materials and selling very small quantities in finished products,” da Silva says. “Combined, density plus RI is a quick way to ensure that they’re not receiving adulterated or synthetic materials.”
Daniel Buchmann, product manager for density and refractometry at METTLER TOLEDO (Schwerzenbach, Switzerland), notes that benchtop and more portable refractometers provide different accuracies based not only on the RI measurement but on temperature. Handhelds typically provide three-decimal-point measurements, rarely four, while benchtop units return results accurate to four or five places.
According to Buchmann, such differences matter on the measurement specifications. Quality ranges for raw concentrated juices are broad because such products, and measurement temperatures, vary significantly. “By contrast, final product concentration specs will usually be much tighter. You have more leeway in raw materials,” Buchmann says.
High-accuracy measurements are where temperature control becomes significant, particularly for nonaqueous samples. Water RI changes by 0.0001 units per degree Celsius, but for dodecane the change is four times greater. In pure temperature terms, measuring RI to five-place accuracy requires temperature control to 0.05 °C. “That’s why at the high end purchasers should consider both factors,” Buchmann cautions. An additional factor is evaporation, which not only concentrates analytes but cools the sample.
For additional resources on refractometers, including useful articles and a list of manufacturers, visit www.labmanager.com/refractometers