Raman Spectroscopy

Bruker Senterra Raman Microscope

The Senterra Raman microscope is a laser based microscopic device for Raman based spectroscopic analysis. Commercially available Raman microscopes employ spectrometers that are separate from the microscope leading to problems with alignment and maintenance of these devices and making the use of the equipment time consuming. The Senterra uses a multi laser Raman spectrometer which is integrated into the confocal optical microscope between the base and the binocular of the microscope. As a result of this compact design, the beam path is kept very short, which leads to high stability and sensitivity. The instrument utilizes a permanent calibration method known as the Sure_Cal method which facilitates high wavelength precision and accuracy. 

Principle

In Raman microscopy and Raman spectroscopy the sample is illuminated using monochromatic light (from a laser) which causes a change in the polarizability of the molecule due to the interactions between the light and the molecules. When these energies of transition are plotted to produce a spectrum, a ‘molecular fingerprint’ of a given molecule is produced. As Raman spectroscopy detects fundamental vibrations, the Raman bands have a good signal to noise ratio and are non-overlapping, which makes the technique highly specific. In FT- IR, the presence of water molecules in a given sample can lead to heavy interferences by the OH bands. But as the Raman spectrum for water is weak and unobtrusive, good spectra can be acquired even for species in aqueous solutions. As the measurement intensity of a Raman species is directly proportional to the concentration, a Raman analysis could be performed for quantitative analysis as well. Raman spectroscopy is non-destructive and spectra can be produced in a short time making it a versatile technique for chemical analysis.

Strengths

• Capable of identifying organic functional groups and often specific organic compounds
• Can operate under ambient conditions
• Typically, non-destructive
• Complimentary to FT-IR spectroscopy
• Raman mapping of samples
• Variable temperature Raman analysis Spectrometer is located on a platform of the confocal optical microscope and it has a class I protection from laser radiation
• Raman spectra can be obtained non-invasively, which means that bulk and final products can be tested directly in their packaging
• Spectral range of recording the Raman spectra of 80-4500 cm-1
• Spectral resolution: better than 3 cm-1
• Calibration wavelength: continuous, throughout the process of registration of the spectrum
• Detection: High-sensitive CCD detector with a Peltier-cooling to -70 degrees Сelcius
• 2 diffraction gratings – 400 grooves/mm and 1200 grooves/mm
• Polarizer for measurements in polarized light
• Fiber-optic sensor for recording the spectra of samples in an external sample compartment.

Limitations

• Raman spectroscopy cannot be used for the analysis of metals and alloys.
• The fluorescence of impurities present in the sample or the fluorescence produced by the sample itself can hide the Raman spectrum.
• The intense laser radiation from the laser beam may destroy the sample

Applications

• Pharmaceutical industry - Raman spectroscopy can be used to differentiate polymorphs, differentiation of counterfeit drugs from originals, analyse/detection of degradation products of pharamaceuticals during storage 

• Polymer chemistry - Raman spectroscopy is used in the polymer industry to analyse the chemical strucure of polymer chains and the interactions between polymer chains 

• Material sciences - Raman spectroscopy is used in materials science for the detection of amorphous and microcrystalline silicon and for the characterization of novel 2D materials and Carbon nanotubes.

• Agriculture -  Structural analysis, safety control and classification of crops, fruits and vegetables

• Textile industry - Used to analyse and differentiate types of fibres and different types of dyes used in textiles

Technical Specifications

• Signals detected: Raman scattering (photons)
• Data obtained: Chemical and molecular bonding info
• Depth resolution: ~2μm
• Confocal depth resolution: <0.1 cm-1
• Spectral resolution: <3 cm-1
• Spatial resolution: 1 μm
• Imaging/Mapping: Yes
• Available wavelengths: 785 nm, 633 nm and 532 nm
• The confocal optics with hybrid iris controled by computer.
• Aperture: combined slotted aperture for high luminosity and round aperture for confocal measurements
• Color CCD camera to visualize the viewpoint, with a resolution 1.3 megapixel.
• Trinocular head for optical lenses with different degrees:
     - 20x (working distance – 1.3 mm)
     - 50x (working distance – 0.38 mm)
     - 4x, 10x, 50x (long-focus)
     -100x (operating distance – 0.9 mm).
• Stage: an automated, controlled with joystick and software, movement range – 75 mm x 50 mm, movement accuracy – 0.1 mm