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Raman Microscope Hits the Spot

Photonics Spectra
Mar 2000
Daniel C. McCarthy

Seeing spots? It may not be your vision. It may be the image quality of your ink-jet printer. Spot size and density determine how sharp printed images appear. To improve both the speed and quality of their product, printer manufacturers such as Hewlett-Packard Co., strive to develop more compact thermal printheads able to fire smaller ink droplets at higher frequencies.

To study these increasingly smaller samples, Hewlett-Packard's analytical services laboratory acquired a Ramascope 2000, a Raman microscope from Renishaw plc, headquartered in Wotton-Under-Edge in Gloucestershire, UK.

"Raman is not diffraction-limited to 10 µm like infrared [spectrometry] is," explained Loma Hammond, who is a product support engineer at the lab. "We can get down to about 2 µm with the Raman instrument."

The microscope integrates dual-mounted lasers that provide 514- and 785-nm beams. Although ultraviolet Raman systems are commercially available, Hammond and her colleagues have found IR wavelengths better suited to their samples.

Because Raman is a scattering technique, it can lessen preparation time, and it allows in situ probing. By reducing the amount of sample micromanipulation, Raman can reduce overall analysis time for some samples by as much as 50 percent, Hammond said.

Also, Renishaw's microscope enables confocal spectrometry, which, in turn, enables depth profiling and the identification of particles within a matrix. In other words, it allows Hewlett-Packard's lab to analyze varying depths within sample volumes as small as 1 µm in diameter by 2 µm in height.

Further, the mercury cadmium telluride detectors used by the lab's infrared microscopes cut off around 650 cm-1, limiting researchers' ability to gather spectral information in the far-infrared. Raman, Hammond reported, provides data down to 100 cm-1, which helps to identify inorganic pigments such as titanium dioxide.

Renishaw's microscope, purchased by Hewlett-Packard two years ago, augments the infrared instruments at its lab. While the Raman technology delivers multiple advantages over the infrared microscopes, there is some room for improvement. "Many samples still fluoresce at 785 nm," Hammond said. "Instrumental improvements which allow these types of samples to be analyzed would be a big help to us." Also, reference materials for interpretation of Raman data are not as readily available as resources for infrared, she said.

Renishaw has since released multi-wavelength models of the Ramascope that excite samples with multiple excitation options, including 244-, 325- and 830-nm light sources, according to Richard Bormett, a business manager at Renishaw Inc., the company's US distributor.

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