From The Editor | September 20, 2012

Raman Spectroscopy: An Overview

By Ron Grunsby, Editor

Raman spectroscopy is a type of vibrational spectroscopy that is used to assess molecular motion. It facilitates the identification and analysis of molecular species and can be used to study liquids, solids, gels, powders, slurries, and aqueous solutions. The technique is based on the Raman effect, the inelastic scattering of a photon discovered by Indian physicist C. V. Raman in 1928. This overview will discuss how Raman spectroscopy works, areas of application, and various solutions.

The detection of scattered light is at the heart of Raman spectroscopy. When monochromatic laser light is shined on a sample that needs to be identified, a small amount of the light excites molecular vibrations in the sample and is scattered at a slightly different wavelength (inelastically scattered). Plotting the intensity of this inelastically scattered light versus frequency results in the sample’s Raman spectrum. Every compound has a unique Raman spectrum, which leads to its identification.

Raman spectroscopy is similar to IR spectroscopy in that both are fast and nondestructive. However, IR spectroscopy falls short when it comes to collecting data from aqueous samples or samples containing a lot of moisture because a big part of the vibrational spectrum is masked by the water signals. This is not the case with Raman signals from water. Raman spectroscopy also can capture information from samples in transparent containers such as glass or plastic. Areas of application for Raman spectroscopy include:

  • Forensics
  • Pharmaceutical processing
  • Food safety and agriculture
  • Incoming raw material identification
  • Counterfeit detection
  • Homeland security
  • Law enforcement

The development of smaller and less-costly lasers and detectors for Raman spectrometers has recently opened some of the application areas above, and with the ever-evolving technology of today’s photonics industry, what’s next? Photonics Online spoke with developers of Raman spectroscopy technology to find out where they believe it is headed.

“The handheld market is out there,” said Yvette Mattley, Ph.D., senior applications scientist at Ocean Optics. “With cloud computing – the ability to store massive amounts of data – and wireless communications, you can have a Raman library of whatever you want, such as narcotics identification to assist law enforcement. I can picture a smartphone-based Raman system that can tell you what you are looking at right there at the scene. Smartphones are getting powerful enough, and spectrometers are getting small enough that you could package them together for a really useful tool.”

“The biggest limitation with traditional Raman is that it only offers part-per-thousand-level detection – not part per million or billion – so it’s not as sensitive as a lot of other techniques,” said Robert V. Chimenti, marketing manager at B&W Tek. “On the plus side, it’s very selective and offers a very high degree of sampling utility. Ideally, we want to get it into the hands of every police officer, firefighter – everyone doing any kind of field analysis – to be used like Mr. Spock’s tricorder.”

“Ultimately, it is going to be used by doctors for diagnostics and the treatment and prevention of diseases,” said Scott Rudder, VP marketing and sales at Innovative Photonic Solutions. “It’s a great solution looking for more problems. That’s the biggest dilemma it has. There are currently small niche problems and a lot of companies trying to find problems in small niches.”

For more information about Raman spectroscopy, please check out the following articles:

Raman Spectroscopy Solutions

Following is a selection of Raman spectroscopy solutions. Click on the product names or related links for additional specifications. For more information on these or other Raman spectroscopy solutions, please contact us.

AvaSpec SensLine

The AvaSpec SensLine family of spectrometers from Avantes is designed for high sensitivity, which is ideal for Raman spectroscopy. Among them are the AvaSpec-ULS TEC models, which can be delivered with two different TE-cooled detector arrays with 2048 or 3648 pixels as 1- or 2-channel instruments. Size is reduced more than 35% from the previous TEC spectrometers, and a new three-stage Peltier cooling device reduces CCD chip temperature by -35°C.

The AvaSpec-HS1024x58/122 spectrometers, also part of the SensLine family, were specifically designed for Raman applications with TE-cooled detectors and optical benches with a numerical aperture (N.A.) of .22 matching the fiber optic cable N.A., giving the optical bench very high throughput. The detector pixels in the AvaSpec-HS1024x58 are 1.392 µm high, and in the AvaSpec-HS1024x122 are 2.928 µm high. The spectrometers are available with a choice of seven standard diffraction gratings to enable applications from 200 to 1160 nm.

“We also have software and laser systems for Raman, and we can put together a custom system,” said Greg Neece, president of Avantes. “If you need a 633 nm laser to do something unique, we just need to find the right grating, build it up, and off you go.

“I have been hearing more about NIR Raman, too. We haven’t done it yet, but we have NIR spectrometers. If someone is looking into it, we can do it.”

C11713CA/C11714CA and S11500-1007 and S11510 Series

The C11713CA and C11714CA miniature spectrometers from Hamamatsu are integrated with optical elements, a back-thinned CCD image sensor, and a driver circuit, and offer a spectral resolution of 0.3 nm. Their dimensions are 120 mm x 60 mm x 70 mm, slit size is 10 µm x 1000 µm, and integration times are between 10 ms and 10 seconds. The C11713CA’s CCD image sensor has 2048 pixels and a spectral response range from 500 to 600 nm, and the C11714CA has 1024 pixels and a spectral response range from 790 to 920 nm. Hamamatsu’s newly developed back-thinned CCDs have improved etaloning characteristics, which reduces the possibility of sensitivity fluctuations.

Both the S11500-1007 and the S11510 series back-thinned CCDs have 40% quantum efficiency (QE) at 1000 nm and spectral response of 200 nm to 1100 nm. Hamamatsu’s laser processing technology was used to form a MEMS structure on the back of these CCDs, which enhances their NIR sensitivity. This is especially helpful for detecting long-wavelength Raman emissions. The S11500-1007 features a pixel size of 24 x 24 µm, while the S11510 series has a pixel size of 14 x 14 µm.

QE65 Pro and Maya2000 Pro-NIR

The QE65 Pro from Ocean Optics is a new version of their flagship spectrometer, the QE65000. The QE65 Pro includes a TE-cooled (down to -15°C) Hamamatsu FFT-CCD detector with 90% maximum QE and low etalon characteristics. Signal-to-noise ratio is 1000:1 at full signal, and dark noise is at 3 RMS counts. Stray light performance is <0.08% at 600 nm and 0.4% at 435 nm, and the spectrometer features an SMA 905 connector assembly with replaceable slit design, which allows the user to change throughput and optical resolution right on his/her bench.

The Maya2000 Pro-NIR offers a back-thinned 2-D FFT-CCD detector with peak QE of 80% at 700 nm and sensitivity out to 1150 nm. It has 15000:1 dynamic range and signal-to-noise ratio of 450:1. Slits are available in widths of 5, 10, 25, 50, 100, and 200 μm. Optional order-sorting filters eliminate second- and third-order effects.

“For customers doing 785 nm Raman, this detector gives them more sensitivity,” Ocean Optics’ Mattley said. “They can use a lower-cost spectrometer like this one instead of a TE-cooled spectrometer.”

In this video, Jada-Star Mains, application sales engineer with Ocean Optics, discusses the Maya2000 Pro-NIR.

LS-2 LabSource

Shifted excitation Raman difference spectroscopy (SERDS) is a unique and highly effective solution to reducing strong fluorescence background from sample spectra from Raman-based spectrometers. SERDS is accomplished by utilizing laser sources commonly available at 785 nm and locking their outputs with a wavelength difference (offset) of approximately 1 nm — 784.5 nm and 785.5 nm, for example. By comparing the spectral outputs of each laser from virtually any commercially available spectrometer then applying a numerical table, one easily derives the sample signature from the differential data. SERDS is a powerful technique for rapidly emerging applications for Raman systems in detecting biological and chemical hazards.

The key to this solution is precise wavelength stability from the laser sources. PD-LD incorporates its patented Volume Bragg Grating (VBG) technology with high-reliability semiconductor lasers to deliver locked-on wavelength performance from a wide variety of lasers at many wavelengths and power levels. VBG-locked lasers are available from PD-LD in the LS-2 LabSource series of laboratory instruments for the researcher. The LS-2 is a turnkey, plug-and-play instrument that gives the user full control over laser performance from front panel controls or through a USB interface. For OEM instrument applications, PD-LD offers specifically designed free-space or fiber-coupled VBG-stabilized lasers and drivers required for portable and benchtop Raman systems.

In the video below, Tom Deberardine, executive VP of sales at PD-LD, discusses the company’s stabilized light sources for use in Raman spectroscopy applications.

NanoRam and i-Raman

The NanoRam from B&W Tek combines a compact Raman handheld spectrometer and integrated computing system for material identification and verification in cGMP-compliant facilities. “What we’ve done is take traditional handheld Raman and put a very ergonomic, user-friendly intuitive graphical interface on it – an iPhone-style design,” B&W Tek’s Chimenti said.

The NanoRam is configured for a 785 nm laser excitation wavelength and achieves a spectral resolution of 9 cm-1. The NanoRam weighs less than 2.2 pounds, is 8.8 x 3.9 x 2 inches in size, and has a temperature-controlled detector to promote data quality and system stability.

Weighing less than 7 pounds and containing a TE-cooled 2048-pixel CCD array, the i-Raman combines high resolution and field portability. It can achieve spectral resolution of 3 cm-1 and can collect data to within 65 cm-1 of the Rayleigh line. Excitation wavelength options include 532 nm, 785 nm, and 830 nm.

“It’s fiber-coupled, which is an advantage of smaller instruments,” Chimenti said of the i-Raman. “The fiber optic probe allows you to couple into any sample configuration you want. It’s not as small as the NanoRam, but with its relatively compact box you can do a wide range of samples with fairly high sensitivity.”

In the video below, Travis Thompson of B&W Tek discusses new Raman spectroscopy solutions.

Wavelength-Stabilized Raman Laser Sources

Innovative Photonic Solutions (IPS) offers a variety of wavelength-stabilized Raman laser sources that use wavelength stabilization technology to “lock” the laser to the desired spectral line and shape the spectral output. As a result, IPS can provide very narrow wavelength sources that stay locked at the desired excitation wavelength regardless of ambient temperature changes, vibrations, back reflections, and time.

Standard wavelengths offered include 532 nm, 635 nm, 647 nm, 785 nm, 808 nm, 830 nm, 976 nm, and 1064 nm. However, IPS can manufacture any of their Raman spectroscopy laser sources at most wavelengths from 635 to 2400 nm.

“The OEMs we work with are on two different paths – multimode and single-mode wavelength-stabilized lasers,” IPS’ Rudder said. “Multimode was the workhorse and probably still is from a numbers perspective. Many of the little handhelds use this. We like to help our OEMs find the best fit for their product vision. Sometimes they choose the multimode product, and sometimes they take a leap of faith and move to a smaller, lower-power-consumption single-mode laser.

“My bet is five years from now almost everyone will go with the single-mode option. You can minimize power consumption, weight, size, and heat generation. You can make them very small and efficient. A lot of people are reluctant to change their view on laser excitation power, but it’s not always power that matters – sometimes it’s power density, and understanding which is important for a given application enables users to better optimize their system for size, cost, weight, and performance.”

Raman-HR-TEC and Ramulaser

The Raman-HR-TEC high-resolution Raman spectrometer from StellarNet has a spectral range of 200 to 2200 cm-1 at 785 nm with 4 cm-1 resolution. It has an integrated TE cooler for signal-to-noise ratio of 1000:1 with detector integration times greater than 3 seconds. The detector is a CCD array with 2048 pixels. The Raman-HR-TEC’s dimensions are 2.5 cm x 7.5 cm x 12.5 cm, and it weighs 400 g.

The Ramulaser is a 785 nm Raman laser with 350/499 mWatt adjustable power. The laser line is 0.2 nm FWHM. Lithium-ion battery powered in a ruggedized metal case, the Ramulaser is only 2 x 4 x 6 inches. With the Ramulaser-Probe, the laser attaches to Raman-Probe-785 using a standard FC/APC connector. The Ramulaser-Vial has a direct-attach ½-inch vial holder.

In addition to making its units compact and rugged for portable applications, StellarNet has designed its instrumentation to be modular. This allows customers to mix and match their Raman spectrometer with the proper accessories, enabling cost-effective customized solutions. StellarNet offers a variety of laser modules, sample probes, and probe holders. It also provides free Raman spectroscopy software that allows for a real-time library search using unknown samples to look for matches.