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Tech Intelligence
Light-Scattering Detectors From outer space to inner space, new manufacturing
AL AL methods and advanced components change what we can see by Mike May
Maybe more than anything else, scientists explore nature with light. Light-scattering detectors—also known simply as light detectors or photon detectors—provide one useful type of instrument. As Alan Rawle, applications manager at Malvern Instruments (Westborough, Mass.), explains, “They are actually detecting light rather than scattered light per se.” The light that these devices can detect and how they do it help scientists see and quantify things more completely than ever in some applications.
In comparison to a charge-coupled device (CCD) detector in a camera, these light detectors “are extremely diverse and relate to the detection wavelength or wavelength range,” Rawle says. “One could look at any- thing from infrared (IR) detectors used in smoke cameras to devices used to examine the scattered light from the dust in Saturn’s rings, allowing composition and size data to be simultaneously extracted.”
Malvern’s Viscotek GPC/SEC systems and detectors can also be used in “many applications in molecular mass measurement of polymers,” Rawle says.
Evolving equipment When asked about the biggest changes in light-detector tech-
nology, Rawle answers, “Nothing radical—evolutionary rather than revolutionary.” He adds, “For example, now we have blue LEDs, and lower wavelengths give increased scattering.” The detectors also im- prove with advances in semiconductor technology. “Detector areas have become larger and more sensitive, allowing very low light levels to be examined,” Rawle explains.
The detector itself is also evolving. “The biggest move forward in the past 20 years was from conventional photon multiplier tubes—PMTs the size of a baby’s arm with poor quantum efficiency—to avalanche photodiode detectors—APDs, which are smaller and have much higher quantum efficiencies—and photodiode arrays,” Rawle explains.
Moving to APDs allows these detectors to collect adequate signals without increasing the laser power. “More powerful lasers or laser diodes were used in the past to compensate for low signal, such as situations with small particles, low concentrations or poor optical contrast,” Rawle
AMERICAN LABORATORY 20
Light-scattering detectors image a wide range of wavelengths and par- ticles, including the dust in Saturn’s rings. (Image courtesy of NASA/JPL- Caltech/Space Science Institute.)
JANUARY/FEBRUARY 2017
points out. “Indeed, this is the defined space for many biological and protein applications, but more powerful lasers risked heating the sample or damaging the sample.”
The improvement in quantum efficiency with APDs, says Rawle, “has meant smaller systems—less bench space—with just as good or better performance than the antiquated PMT-based equipment.” He adds, “As an example one can measure the size of a small surfactant molecule— say, Triton X-100, which is about 8 nanometers across—in 30 seconds or so.”
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