Miniaturizing Ultraviolet-Visible Spectrometers

Overcoming physical, mechanical barriers

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miniaturizing UV-Vis spectrometersMiniaturization of analytical equipment has escalated dramatically since the first miniature ultraviolet-visible (UV-Vis) spectrometers of the early 1990s. The same principles that made this technology so attractive to early adopters—small size, greater flexibility, excellent performance, and rapid scalability—have been reinforced with refinements from two decades of customer input and advances in information and light processing.

“Today’s challenge is to continue the drive toward smaller, faster, and more powerful spectroscopy platforms, to help people solve measurement challenges that they may not even realize can be solved with spectroscopy,” says David Creasey, VP for sales and marketing at Ocean Optics (Dunedin, FL). “Also, making systems that are scalable to changing communications interfaces—Wi-Fi, GPS, and more—is necessary as the world becomes more connected through its electronic devices.”

Related article: UV-Vis Spectroscopy: Improving Sources and Detectors Expand the Applications

Physical barriers present numerous hurdles to miniaturization. Since the spectral response of standard silicon charge-coupled device (CCD) detectors in spectrometers drops off at wavelengths as long as 400 nm, and because oxygen and water absorb in the UV (especially below 200 nm), maintaining sufficient signal outside a vacuum is not possible. Designing vacuum-compatible miniature spectrometers to address these issues is difficult and expensive. However, coated and dichroic thin-film mirrors are now available that improve spectrometer performance by boosting signal response in the UV, while the accessibility of new CCD detectors with strong inherent UV response has improved spectrometer performance.

Applications

“Without exaggeration, hundreds and even thousands of absorbance, reflectance, and emission applications are made possible through miniature UV-Vis spectroscopy,” Creasey says. He mentions possibilities ranging from anti-counterfeiting applications (such as fuel authentication and spirit identification) to life sciences applications (such as cancer detection and protein analysis) to farm-totable uses (including crop monitoring and process line quality control), and many more.

Although performance barriers between miniature spectrometers and scientific-grade lab instruments have lowered in recent years, traditional instruments perform some functions better and more consistently. “But most typical applications can be handled very capably with today’s miniature spectrometers,” Creasey tells Lab Manager. One of the remaining limitations—the need to connect the spectrometer to a PC—is also eroding as the evolution of microprocessors creates new ways for electronic devices to capture and process spectral data. The next technological frontier will be developing UV-Vis spectrometers as smart sensors with fully adaptive optical components inside the spectrometer. This, Creasey says, will allow the instrument to react to changing experimental conditions and adjust its wavelength range, optical resolution, throughput, and other parameters.

Performance by the pint

Miniaturization has taken the instrument world by storm, to a large degree as a result of shrinking electronic circuitry. Several factors have made miniaturization less accessible to ultraviolet spectroscopy.

“With UV you buy performance by the pint,” says Jonathan Pundt, senior manager of UV-Vis product marketing at Thermo Fisher Scientific (Madison, WI).

It’s all about getting light into the detector. Larger desktop UV spectrometers have larger mirrors and beams, which process more photons than smaller or handheld devices do. More photons mean an improved signal-to-noise ratio. “If you miniaturize UV, you have to be really good at electronics and optics to maximize performance,” Pundt adds.

Related article: UV-VIS Spectroscopy: Withstanding the Test of Time

Broad-spectrum UV also requires a serious light source, typically a xenon or deuterium lamp, which can consume more power than a white light source for visible spectroscopy and can generate more heat. Miniaturized systems must dissipate that heat. LED light sources are cooler and more energyefficient, but their emission spectra are relatively narrow. Xenon lamps have the benefit of requiring less than 5W to deliver broadband light from 190 to 1100, and they offer a single (cooler) source that can be used instead of a 30W deuterium or 10W tungsten source.

Another consideration is moving light from source to sample. Where conventional spectrometers use reflective optics, mini-spectrometers rely on optical fibers whenever possible. Fibers deliver light in situations that normal optics would not allow. However, a fiber’s numerical aperture limits the angle at which a fiber is capable of receiving light. Fibers also suffer connection losses that may be twice as high as losses through reflective (mirror-based) optics.

Sample presentation is a component of miniaturization that is easy to overlook. Thermo Fisher’s NanoDrop™ 2000 UV analyzer, a product within Pundt’s managerial portfolio, is an example. The instrument’s cuvette is a 1-microliter drop joined to an interrogating and reading optical fiber through the fluid’s surface tension. Xenon light passes through, and the spectrum—including absorption at 260 and 280 nm—provides a measure of DNA and protein as well as possible contaminants.

“Seeing” the light

“As you make things smaller, you reach a practical physical limit to how many photons you can get to the detector,” Pundt explains. “Performance is therefore limited by the size of your instrument and by the detector’s physical size.”

Compared with conventional monochromator instruments, for example, in a spectrograph-type instrument, the tininess of a pixel in the array detector limits the signal. No matter how cleverly designers push light through the optical system, in an array-based spectrograph the instrument’s “eye” at the end of the optical circuit is on the order of 14 µm2 compared with 3 mm2 or larger in a monochromator-based system.

Related article: UV-Vis Spectrophotometry: The New Look of a Mature Platform

With miniaturization comes lighter weight, by itself a desirable quality. Low-profile instruments often rely on exotic materials of construction, but optical benches must be sturdy and not deform under environmental conditions or mechanical stress. “The key is making a system lightweight but stable from an optical perspective, so you can trust measurements made half an hour apart,” Pundt explains.

A further limitation on miniaturized, handheld, or otherwise portable UV-Vis spectrometers is their formal qualification for outdoor use—presumably the reason for miniaturization in the first place. Manufacturers of electronic, optical, and other products establish an IP (ingress protection) rating for their products consisting of a two-digit number: the first (on a scale of 0 through 6) refers to the degree of dust protection, and the second (0 through 9) to water protection. Thus an IP value of 69 is the highest level of protection.

Manufacturers of miniaturized UV-Vis instruments for regulated markets must demonstrate a sufficient IP rating or label their devices “for indoor use only.”


For additional resources on ultraviolet-visible spectrometers, including useful articles and a list of manufacturers, visit www.labmanager.com/UV-Vis 

 

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Published: November 13, 2015

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