Product Intelligence
UV-VIS Spectrophotometers: Measuring the Rainbow
by Josh P. Roberts
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UV-VIS spectrophotometer, as the name implies, measures the in- tensity of light absorbed (or refl ected) by, or transmitted through, a substance at specifi c wavelengths between about 190 and 900 nm—the ultraviolet and visible spectra. In its simplest form, it consists of a light source, a way to separate out the light into its spectrum (like a prism), and a way of detecting the light after it passes through (or bounces off ) the test sample.
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A typical spec (as it’s commonly called) is a box that sits on a bench. For most laboratories a UV-VIS spec “is a commodity, like a pH meter or balance: you use one to measure things in your lab,” says Gordon Bain, who manages the product line for Thermo Fisher Scientifi c (www. thermofi
sher.com). They should be easy to select and buy—most scientifi c equipment catalogs and distributors carry them—and easy to use.
There are varied applications for which a UV-VIS spec will be used, and a host of accessories are available to cater to them, depending on the instrument.
“Then you move into a completely diff erent application, not as well-known unless you’re in the industry, which is the ability to analyze refl ective surfac- es,” allowing for an exact assessment of color, coating thickness, and other parameters, he notes. Specular refl ections like those bouncing off a mirror are relatively easy to directly collect and measure using a simple accessory. Diff use (scattered) refl ections like those coming off cloth, on the other hand, need to be gathered up from all directions with more specialized equipment like inte- grating spheres.
Analysis of DNA, RNA, and protein concentration and quality has largely moved to small, custom-
Applications of UV-VIS spectrophotometers UV-VIS specs are used in a wide range of industries for an assortment of applications, with quality assurance and quality control (QA/QC) account- ing for the largest market segment. “Primarily you’d be doing quantitation, biological applications, color analysis, fi lm thickness analysis, coating analysis…,” says Mark Talbott, Spectroscopy Product Coordinator at Shimadzu Scientifi c Instruments (
www.ssi.shimadzu.com). For example, “it fi nds application in the photovoltaic industry for solar panels, as well as in the semiconductor industry for the analysis of wafers, and any type of lens manufacture.”
About 70% of what people do with specs is look at liquids, Talbott says. Most of this is measuring absorbance (also known as optical density) and—since the quantity of light blocked by a material is inversely pro- portional to the amount it allows to pass through—transmittance as well.
ized specs that rapidly measure high concentrations of very small volumes, without the need for a cuvette (in the case of the Picodrop [Picodrop, www.
picodrop.com], the sample can be measured while in a micropipet tip, al- lowing for full recovery of the sample), and without the need to generate a calibration curve. Such measurements can also be made using a standard UV-VIS spec with a short pathlength cell, but “it’s just not quite as easy or as rapid,” Talbott notes.
Range of instrumentation Bain divides UV-VIS specs into standard (forward) monochromator and
spectrograph (reverse) optical systems. “The optical layout in the two will be diff erent, but they accomplish the same general thing,” he says.
In the former, the source light travels through a monochromator, allowing it to be separated into its spectrum. A narrow portion of that spectrum— selected by turning a grating in the monochromator—is then transmitted to the sample. Light that is not absorbed is transmitted to a detector, which counts the photons hitting it.
In a reverse optical system, the entire “white” source light hits the sample. Light that is not absorbed is then transmitted through a slit to a dispersion
AMERICAN LABORATORY • 8 • MAY 2014
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