Product Review: Raman Revisited - Analytical Chemistry (ACS

Cheryl M. Harris. Anal. Chem. , 2002, 74 (15), pp 433 A–438 A ... Albert, Todt, and Davis. 2012 89 (11), pp 1432–1435. Abstract: In an effort to m...
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product review

Raman Revisited This spectroscopic tool is on a new course as scientists find ways of expanding it. Cheryl M. Harris

sk Peter Chen just what the word “laser” might mean to people, and the associate professor at Spelman College would perhaps respond with one word: “Cool.” Maybe one can blame it on Star Wars, says Chen, or any other science fiction fantasy. “Our science majors here, when you talk about lasers, the first thing they say is, ‘Oh, that’d be kind of cool to learn about!’” Once relegated strictly to academia, where researchers worked on its intricacies, Raman spectroscopy today is more user-friendly, affordable, efficient, and versatile—changes that have attracted academic and industrial chemists. It has been coupled with other analytical methods, especially microscopy, and researchers are looking to interface it with other techniques, such as chromatography. Some researchers also envision Raman becoming active in nanotechnology. And last but certainly not least, Raman has been brought down to a more convenient size to be used out in the field. “We’re continuing to invest in Raman,” says Ian Lewis of Kaiser Optical Systems. “We know that it’s a growing market.” Analytical Chemistry last reviewed Raman spectroscopy in 1997 (1). The current product review further explores Raman’s transformation into a versatile analytical tool and its future in the academic and industrial laboratory. Table 1 lists some examples of Raman spectrometers and microscopes that are available to prospective buyers.

Raman basics In 1928 in India, C. V. Raman experi-

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mentally confirmed the Raman effect, which he aptly described as “feeble fluorescence” (2). The main problem that plagued—and still plagues—Raman spectroscopy is its weak scattering effect. Raman’s efforts won him a Nobel Prize in Physics in 1930, but Raman spectroscopy didn’t take off until the development of the laser in the late 1960s.

In simple terms, a Raman instrument needs a monochromatic light source, collection optics, optics to filter the Rayleigh scattering, a spectrometer, and a detector. When radiation hits a sample, most of the scattered photons are elastically (or Rayleigh) scattered and have the same frequency as the incident radiation. But a small portion of the scattered photons—

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product review

Table 1. Selected Raman spectrometers and microscopes. Product

FT-Raman Accessory/ System

Handheld Portable Raman Spectrometer

Raman 960

Almega Dispersive Raman

FALCON Raman Chemical Imaging

Company

Digilab U.S.A 68 Mazzeo Dr. Randolph, MA 02368 781-794-6400 www.digilabglobal.com

Digilab U.S.A 68 Mazzeo Dr. Randolph, MA 02368 781-794-6400 www.digilabglobal.com

Thermo Nicolet 5225 Verona Rd. Madison, WI 53711 608-276-6100 www.nicolet.com

Thermo Nicolet 5225 Verona Rd. Madison, WI 53711 608-276-6100 www.nicolet.com

ChemIcon, Inc. 7301 Penn Ave. Pittsburgh, PA 15208 412-241-7335 www.chemimage.com

$25,000+

$90,000

$145,000

$150,000–$300,000

Dispersive, all solid-state spectrometer

FT with dynamic alignment

Dispersive spectrograph with multiple grating turret

Single-stage Czerny–Turnerstyle spectrograph for Raman dispersive spectroscopy; liquid-crystal-tunable imaging spectrometer for Raman chemical imaging

Approximate mid $60,000+ price (U.S.D.) Type

Dynamically aligned, highthroughput Michelson interferometer

Laser (nm)

Diode-pumped Nd:Yag, with 785 at 500 mW delivery up to 1.3 W to sample

1064

532, 633, and 785; additional Standard options: 532 solidsources available state laser diode-pumped Nd:YVO4 or 785 solid-state diode laser; will accommodate other lasers

Rayleigh rejection filters

Dielectric (standard), Rayleigh notch (optional), and holographic notch (optional)

Holographic

Holographic or dielectric

Dielectric

Collection optics

Lens (standard), optional mirror-based

Fiber optic

Gold-coated, reflective

Refractive lenses and microscope objectives

Refractive

Detector

Liquid nitrogen-cooled Ge

TE-cooled CCD array

InGaAs or Ge

TE-cooled CCD, 256  1024 Scientific-grade, slow-scan silicon CCD pixels

Spectral range

150–3500 cm–1 (Stokes) with 200–2700 cm–1 standard dielectric filter; 70–3500 cm–1 (Stokes) with optional holographic notch filter

100–3500 cm–1 Raman shift

400–1050 nm

Dispersive: 350–1100; imaging: 532 nm, 200–4000 cm–1; imaging: 785 nm, 200–3200 cm–1

Spectral resolution

0.5 cm–1

1 cm–1 max

1 cm–1/pixel max

Dispersive: 2 cm–1; imaging: 9 cm–1 (nominal), $90,000

$100,000

$150,000

Approximate price (U.S.D.) Type

Dispersive with single monochromator

Dispersive with triple f/1.4; single-diffraction Confocal Raman and/or FT monochromator; single- grating imaging microscopes monochromator operation mode capability

FT

Laser (nm)

532, 633, and 785; others available

532 , 633, and 785; 532, 633, 785, 830, or other lasers available, use a non-Renishaw from blue to deep red laser from 488 to 830; select up to four excitation wavelengths

532, 633, 785, and 830, 1064-nm and/or or use a non-Renishaw 785-nm Nd:YAG laser from 229 to 830; select up to four excitation wavelengths

Nd:YAG

Rayleigh rejection filters

Rapid-exchange Holo- Required for singlegraphic SuperNotch- monochromator operPlus ation only

Notch, SuperNotch, and edge filters available

229–830 nm; notch, Proprietary design SuperNotch, and edge filters available

Proprietary design

Collection optics

Patented aberrationcorrected optics; true confocal geometry microscope

Mirrors

f/1.4; single-diffraction grating; eight userdefinable digital lines; four input and four open-collector output

Refractive optics; 90º and 180º scattering

Detector

–90 °C class TE CCD

–90 °C class TE CCD

Thermoelectrically cooled CCD: standard, UV-, or NIR-enhanced, deep-depletion or AIMO

Thermoelectrically InGaAs or LN-cooled cooled CCD: standard, Ge UV-, or NIR-enhanced, deep-depletion or AIMO

Spectral range

+8000 to –8000 cm–1

+8000 to –8000 cm–1

50–4000 cm–1

5 cm–1 to >3500 cm–1; laser dependent

70–3600 cm–1 Stokes, 100–2000 cm–1 anti-Stokes

Spectral resolution

1cm–1/pixel resolution