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Raman

Raman spectroscopy is like panning for gold. A wealth of information is there, if you can just sift through the rock, dirt, and sand obscuring it. The art to finding the gold in Raman spectra is the instrumentation, which must collect as many photons as possible while rejecting scattered laser light efficiently.

Raman spectroscopy examines materials not through direct absorption, but by scattering of high intensity light in the hopes that one in a million photons scattered will commune with the vibrational and rotational states of a sample molecule and emit light of a slightly different wavelength.

Though Raman spectra are very low in intensity and coded in the most mysterious of energy units (wavenumbers, or cm-1), they probe molecular structure as effectively as IR spectroscopy, but with greater ease of use, more versatility, and lower cost. Raman spectral signatures can play a role in fundamental research, or be matched to a known database for instant identification and quantification of materials.

It’s amazing that C.V. Raman ever discovered this effect, but we’re glad he did. His belief that “success can come to you by courageous devotion to the task lying in front of you” is essential in every new project you undertake, and in every product we develop to make it happen. So come on… let’s dive in together and find the gold.

Advantages

  • Chemical identification: signature can be matched to a known library; no interference from water, so samples can be aqueous or have high moisture content; distinguishes between isomers
  • Ease of use: no sample preparation required, rapid, non-contact, non-destructive, no hazardous byproducts
  • Versatile: applicable to solids, liquids, or powders in bags or vials by altering depth of focus; can test samples in the microliter range or large objects

Applications

Other Common Applications

  • Pharmaceutical QA/QC: incoming raw materials, polymorph analysis, final product testing, through packaging analysis
  • Forensics: currency, art and archeology authentication, counterfeit detection, identification of chemicals, fibers, hairs & inks
  • Law enforcement/ Homeland security: pre-cursor andexplosives identification, illegal drug identification
  • Process control: contaminant analysis, reaction monitoring, product validation, purity analysis
  • Fuel analysis: octane content and density, counterfeit fuel detection, biofuel production quality & process monitoring, identification and characterization
  • Life sciences: clinical diagnostics, tissue biopsy, photosensitive biological samples
  • Materials science: plastic and polymer characterization, study of graphene and carbon nanostructures like fullerenes, organic and inorganic chemistry research, composition of semiconductors

Application Note:

Application Blog Post: Ensuring Food Safety Using SERS
Application Blog Post: Raman and Pharmaceuticals

Technical Note

What Is Raman Spectroscopy and How Does It Work?  

When light is incident on a sample, it can be absorbed, transmitted, or scattered. Almost all of the scattered light is elastically scattered. That is, the photons change direction but keep the same energy and momentum. This process is also called Rayleigh scattering. But something different happens to about one in every million photons. These photons are inelastically scattered by the molecule, resulting in a photon that is redirected at a slightly different energy and therefore wavelength.

What happens to this inelastically scattered photon? There are different ways to describe it. One is that the photon is simultaneously absorbed and re-emitted in a single quantum event, with an energy difference that is equivalent to the difference between two vibrational modes of the molecule. Others say that the light scatters off a virtual state that is temporarily created to facilitate absorption and re-emission. For all intents and purposes, both processes can be considered to be instantaneous. However it is described, this process of energy exchange between scattering molecules and the incident light is known as the Raman effect.

A good way to visualize the Raman effect is to imagine a ball bearing being dropped onto a drum. The drum starts to vibrate at its own frequency, and the ball bearing bounces off with slightly less energy (analogous to Stokes radiation). If the drum is already vibrating and the ball bearing hits at just the right time, the drum acts like a catapult to give energy to the ball bearing and it bounces off with even more energy (analogous to anti-Stokes radiation). The energy difference before and after the ball bearing strikes the drum provides information about the vibrational mode of the drum. [Fundamentals of Molecular Spectroscopy, Banwell and McCash, John Wiley & Sons, Inc., New York, 1988]

Raman Effect

Rayleigh Energy

Now that we know what’s happening with individual molecules and photons, let’s take a step backward to view the big picture. Raman scattering is very weak, so to collect enough photons to make a meaningful measurement, a very strong light source must be used. Lasers serve this purpose well, as they are both intense and monochromatic.

The sample is literally bombarded by so many photons at once that even with a one-in-a-million success rate, Raman scattering occurs often enough to be detected. Since molecules have many different vibrational states, there can be as many as a hundred different Raman scatter wavelengths, all resulting from the same laser excitation wavelength.

The frequency difference or shift between each peak in a Raman spectrum and the laser excitation corresponds to the vibrational frequency of a specific molecular bond, and therefore gives us a clue to the molecule’s structure. These transitions could be probed more easily via direct absorption, but to do so would require mid-infrared light, which is highly susceptible to interference from any water in the sample.

Thanks to the availability of low-cost, portable lasers and relatively sensitive CCD array-based spectrometers, Raman spectroscopy can easily and economically be performed in the lab or in the field. The strength of the Raman signal is directly proportional to the average laser power, so the greatest amount of Raman signal will be generated at the laser’s focus. This can be used to good advantage, and allows a sample inside a clear plastic bag or vial to be probed without opening the packaging. In pharmaceutical samples it allows the system to “look” below the external coating of a tablet to study the medicine within.

Raman spectra consist of distinct peaks based on the chemical formula and structure of the compound, the functional groups attached, and skeletal vibrations and modes. It is ideal for identifying closely related chemical samples, and including isomers and polymorphs. This spectral fingerprint can be compared statistically to a library of known compounds for positive identification using chemometric software. It can also be used for quantitative determinations of mixtures, from 90 – 100% concentrations down to ppt levels (ppb with SERS detection).

Compared to IR or FTIR, Raman spectra typically have fewer and narrower peaks, varying by up to 1000x in intensity within the spectrum, which makes it easier to resolve the compounds in a mixture. IR spectra are easily swamped by water absorption lines, but Raman spectra are not. Water peaks still occur in Raman spectra, but are similar in intensity those of other components in the sample. Raman spectroscopy can thus be used to capture data on aqueous samples or samples with high moisture content, making it particularly useful for biological, pharmaceutical and homeland security applications.

For these reasons, Raman spectroscopy has become the preferred technique for chemical identification. It enables reliable, non-destructive chemical analysis of aqueous solutions, powders, tablets, gels, and surfaces, making it extremely versatile and applicable to a wide range of fields.

Technical Note

What Is the Difference Between Modular and Turnkey Systems?

Modular systems can be lower in total cost, and offer more flexibility for reconfiguration. The excitation laser, fiber optic Raman probe, spectrometer and software are configured by the user, freeing each component for use in other applications.

Turnkey systems are convenient, and are often optimized for a specific application or sample type. Use of free-space optics instead of fiber optic Raman probes improves sensitivity, and enables unique sampling modes like raster orbital scanning (ROS). Some turnkey systems may come with special software for chemometric analysis and libraries customized to the target application.

Turnkey systems for Raman spectroscopy include:

  • Compact handheld units
  • Portable/benchtop systems
  • Raman microscopes

 

Featured Products for Raman:

Raman - Biofuels Analysis

 

QEPRO-RAMAN QE series spectrometer preconfigured for 785 nm Raman analysis; modular options for 532 nm and other wavelengths are also available
LASER-785-LAB-ADJ 785 nm diode laser for Raman excitation; 532 nm and other options are available
RIP-RPB-785-SMA-FC Raman coupled fiber probe for 785 nm with FC Excitation -SMA Collection; 7.5 mm working distance
RIP-PA-SH Raman sample holder
LASER-GL-ML1 Laser safety glasses block 785 nm/808 nm/1064 nm lasers; OD 7+, VLT 45% Green
OceanView Spectrometer operating software
RAM-ANIQ-LAB Single-license chemometrics package from Analyze IQ

What light source can I use for excitation?
What is the best sampling optic?
What spectrometer should I use for detection?
What are my software and data processing options?
Are there handheld Raman systems?
What are my benchtop Raman systems options?
Can I do Raman spectroscopy with a microscope?
How do I read a Raman spectrum?
How do wavenumbers and nanometers (nm) relate?
Should I worry about drift of my laser?
How does SERS enhance sensitivity?
SERS-Substrate

SERS-Substrate

Oberflächenverstärkte Raman-Streuung
SERS Substrates

SERS Substrates

Surface Enhanced Raman Spectroscopy
QE Pro (Custom)

QE Pro (Custom)

High-sensitivity Spectrometer for Low Light Level Applications
Maya2000 Pro (Custom)

Maya2000 Pro (Custom)

High Sensitivity Spectrometer
IDRaman mini 2.0

IDRaman mini 2.0

Handheld Raman System
IDRaman reader

IDRaman reader

Integrated Raman Spectrometer
IDRaman micro

IDRaman micro

Raman Microscope
Maya LSL Spectrometer

Maya LSL Spectrometer

Low Stray Light with High Sensitivity
QE Pro-Raman

QE Pro-Raman

High-sensitivity Spectrometer for Raman
Ventana-785-Raman

Ventana-785-Raman

High-sensitivity Spectrometer for Raman Measurements
Maya2000 Pro-NIR

Maya2000 Pro-NIR

High-sensitivity Spectrometer for Raman and NIR Applications
Ventana-532-Raman

Ventana-532-Raman

High-sensitivity Spectrometer for Raman Measurements
Ventana-785L Raman

Ventana-785L Raman

High-sensitivity Spectrometer for Raman Measurements
HG-1 Calibration Source

HG-1 Calibration Source

Mercury Argon Calibration Source
KR-1

KR-1

Krypton Calibration Source
AR-1

AR-1

Argon Calibration Source
XE-1

XE-1

Xenon Calibration Source
NE-1

NE-1

Neon Calibration Source
Raman Accessories

Raman Accessories

Vials and Other Add-ons for Your Raman System
Raman Laser Safety Glasses

Raman Laser Safety Glasses

Laser Light Protection without Comprising Visibility
Raman Sample Holders

Raman Sample Holders

Accessories for Sampling of Liquids, Powders and More
General Purpose Raman Probes

General Purpose Raman Probes

High Signal Collection and Effective Filtering Design
Raman Immersion Probes

Raman Immersion Probes

Immersible Probes for Lab and Process Applications
Raman Process Probes

Raman Process Probes

Process-ready Probes for Industrial Environments
Turnkey Raman Lasers

Turnkey Raman Lasers

High-power, Spectrum-stabilized Lasers
Multimode Raman Laser Subsystems

Multimode Raman Laser Subsystems

Ideal for Integrating into OEM Packages
OceanView 1.5.2

OceanView 1.5.2

Real-time spectral acquisition and analysis
IDRaman mini Software Libraries

IDRaman mini Software Libraries

Spectral Data for Materials Identification
Analyze IQ Chemistry Software

Analyze IQ Chemistry Software

Chemometric Package is Useful for Raman Analysis