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—
A U G U S T 1 , 2 0 0 2 / A N A LY T I C A L C H E M I S T R Y
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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
