Analyzing Particles to Save the Planet – and for Profit

Hank Hogan, Contributing Editor, hank.hogan@photonics.com

Shane M. Murphy’s research begins with something small but could end with something big. Murphy, a scientist with the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colo., is interested in airborne aerosols. Along with other researchers there, he wants to probe these particles in the air, determine their makeup and then tie that data into what satellites see from space. Doing so would reduce climate change unknowns.

“The aerosol direct effect is one of the bigger uncertainties in climate change,” Murphy said. “It’s just the direct radiative impact of aerosols, how they impact how much sunlight hits the surface of the Earth.”

Understanding that requires particle analysis, which has spurred NOAA researchers to develop specialized photonics-based instruments. Light-driven particle analysis also plays a key role in the manufacturing of semiconductor and microelectromechanical systems (MEMS) as well as for emerging nanotech devices. The same is true for components in the aerospace, automotive, pharmaceutical and other industries. Particle analysis, however, faces challenges, such as how to increase its speed or how to get the goods on a sample of individual particles and for the bulk of a material.

Sampling particles from a construction site near the Rocky Mountain foothills in Colorado revealed that a few were dark and heat-absorbing, something that bulk averaging methods might have missed. Courtesy of NOAA.

In the case of NOAA, the focus on aerosols arises from a need to determine the optical properties of individual particles. In the 1990s, agency researchers developed a portable laser ionization mass spectrometer for in situ measurements of the chemical composition of individual aerosol particles ranging in size from 0.2 to 3 μm. Versions of the device collected data at locations around the world.

NOAA scientist and device developer Daniel M. Murphy (no relation to Shane Murphy) said that the instrument had a compact green laser for particle counting, a compact excimer laser for particle ionization and a miniature photomultiplier tube, all of which enabled particle analysis by laser mass spectrometry (PALMS).

“PALMS used some very state-of-the-art technology when it was developed,” Murphy said. “Much of the laser technology is pretty routine now.”

Although this approach generated the chemical composition of particles, it did not determine their ratio of light scattering to extinction. Such single scattering albedo information is critical to climate models.

NOAA’s Earth System Research Laboratory developed an instrument that measures a single aerosol particle’s light scattering and absorption simultaneously to help resolve climate model uncertainties. Courtesy of NOAA.

For this, the researchers developed their own solution, detailing it in a 2008 Aerosol Science and Technology paper. Deployed in 2010, the system uses a nozzle to inject particles one at a time into a scattering cell. The device is equipped with three high-reflectivity mirrors arranged in a triangle. A 672-nm-wavelength tunable diode laser is locked to create a traveling wave between the mirrors, amplifying the resulting signal. The ratio of forward to wide-angle scattering provides the diameter of the submicron particles to within 50 nm and enables detection of albedo changes of 10 percent or more at a rate of about 20 particles per second.

The device’s performance could be improved by quieter diode lasers, better locking electronics and improvements in the manufacturing of the high-reflectivity mirrors. Shane Murphy noted that future needs include making the device smaller and more portable. There also is a desire to shift to a laser with a shorter wavelength than that of the current red source.

“One of the real big interests scientifically right now is what organics do, and they’re going to absorb down in the blue wavelengths. They’re not going to do anything in the red,” he said.

Advances in laser technology mean that this switch can be accomplished with a change of the laser diode source. Murphy said that a smaller version of the device will be ready for deployment this year in Barbados to measure dust blown from the Sahara Desert.

Commercially available photonics-based particle analysis includes sizing via laser scattering and probing via photoacoustics, which involves the rapid heating of particles and detection of the sounds generated. The technique produces information about particle composition, since both heating and cooling are influenced by particle makeup.

Oil about to be filtered and residual particles analyzed. Courtesy of Olympus Soft Imaging Solutions GmbH.

Other commercial instruments analyze particles by looking at them via optical microscopy. A typical setup involves a microscope, a camera and software. In a common application, an object of interest – such as a fuel injector – is manufactured and rinsed. The liquid is drained and sent through a filter paper, trapping any particles. The particle analyzer is then used to examine the filter paper, allowing particles to be spotted, counted, sized and potentially classified based on shape, reflectivity under various conditions, or other parameters. There often is a specification about the number of allowable particles, their size or other characteristics.

Mario J. Gislao, an industrial marketing and applications specialist with Center Valley, Pa.-based Olympus America Inc., said that this quantifying of cleanliness is done on his company’s products using white light inspection spanning 400 to 700 nm. The image from one field of view is captured, and then the motorized stage moves to the next field of view, with this continuing until the filter is covered.

“It creates essentially a panoramic image of that whole filter. What we’re doing is analyzing each of the independent frames for particles, and the way we do that is based on the gray-scale or color values,” Gislao said.

Panoramic view of a 47-mm filter after an optical scan (left), with examples of the particles found (right). Courtesy of Olympus Soft Imaging Solutions GmbH.

If the filter is white, a black particle will show up readily against it. But because a whiter particle will have a much different gray scale, the system must be trained to a given application. Typically, the particles of interest measure a few microns or larger and are easily visible under the microscope using bright-field imaging. Additional scattering, and therefore composition, information can be gleaned by dark-field inspection, but at a loss of size data.

Tom Calahan, product materials microscopy marketing manager at Carl Zeiss USA of Thornwood, N.Y., noted that the trend has been for analysis to be done on smaller and smaller particles. Beyond a simple count of those above a certain size, the analysis may involve classifying them as to shape, with, for example, long and narrow particles indicating a different problem than would be the case for spherical particles. Another distinction might be to categorize particles as metallic or nonmetallic.

But this push toward analysis of smaller particles comes at a cost. It may require imaging at a higher resolution, which also will force the use of a smaller field of view. Because the area of the filter is not changing, shrinking the field size means that more fields must be scanned, and the scanning time hit can mount rapidly.

“If you double the resolution, you quadruple the time, effectively,” Calahan said.

Vendors are aware of this and are attacking many areas to eliminate bottlenecks. The list includes camera capture, image transfer, autofocus and stage movement times. In shaving time off, however, any gains must be balanced against performance hits.

There has been a tenfold increase in speed over the past few years, and the expectation is that further increases are in the offing, said Wayne A. Buttermore, marketing manager for industrial microscopy at Leica Microsystems AG in Bannockburn, Ill. In part, this is because greater computing power, along with cheaper memory, will make image processing faster.

He also noted that there will be increasing automation, which can show up in a number of ways. One is through wizards, software that guides end users through the steps needed to implement a given task. Another is through taking actions that ensure reproducible results. These go beyond setting illumination, camera gain and other parameters to some predefined value.

“One of the modules that we offer with the microscope allows an individual to select an image that they worked with in the past and actually recall all of the camera and microscope settings to that exacting, reproducible standard, and then be able to put a new sample on there or the sample that they had before and work with it as they had in the past,” Buttermore said.

These instrument speed and capability advances are good news for those interested in particle analysis, but other improvements are needed, said Hugh Gotts, director of research and development for Air Liquide’s Fremont, Calif.-based Balazs Analytical Services, a contract analysis company that services semiconductor, MEMs, advanced photonics and tool vendors.

Gotts noted that there are many optical scanning techniques that can determine the position and shape of particles but that are not as good at determining particle composition. The techniques that are good at composition or molecular analysis, on the other hand, tend not to be good at finding particles and pinpointing their location.

Various methods exist to get around this problem, including the use of spectroscopic libraries against which to compare particle signatures. However, that approach depends upon the library being complete and the entries distinctive enough. Another trick is to locate and perhaps size a particle using an optical scanning method, followed by spectroscopic analysis. Problems may arise, however, in co-registering the two approaches, or one may actually move or otherwise alter the particle being analyzed.

In running through a list of techniques that Balazs uses, Gotts ticked off optical methods, spectroscopy, electron beams and x-rays. Surveying the group, he said, “There’s no one technique that is optimal for locating, sizing and identifying particles.”