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.
- 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
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
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]
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.
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:
|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|