What is a Raman Spectrometer?
Raman’s spectroscopy is an analytical technique based on the interaction between light and chemical bonds. It determines the sample’s morphology, crystallinity, phase, chemical structure, and molecular interactions. Furthermore, a Raman spectrometer is often preferred over other instruments for qualitative and quantitative analysis of various substances.
How Portable and Benchtop Raman Spectrometers Work
Rayleigh scattering, which makes up about 10–6 to 10–10 of the total scattered light, is a phenomenon where the molecules scatter the incident light as it hits the sample surface. The direction of the light deviates, but the frequency still remains the same as excitation light. However, In Raman scattering, the frequency changes and differs from the excitation light. When the frequency is low, it’s called Stokes scattering, and when it’s high, its anti-stokes scattering. Raman spectrometers measure stokes scattering only.
Raman Shift, the difference between scattered light and incident light, has rarely been associated with the frequency of incident light. It only focuses on the structure of the scattering molecule. The primary reason behind the shift is molecular polarizability and depends on the change of molecular vibrational energy. ΔE reflects the change in energy level, so the corresponding Raman shift is also characteristic making Raman spectroscopy useful for molecule structure analysis.
What is the difference between an infrared spectrometer and a Raman spectrometer?
The infrared absorption frequency is equivalent to the Raman shift for any chemical bond. The Raman shift exhibits a striking resemblance with the IR absorption wave number peaks. Both IR absorption wavenumber and Raman shift lay in the IR region, presenting molecular structural information.
For an Infrared Spectrometer, the incident and detection light both come under infrared light, whereas, for the Raman spectrum, the incident and scattered light are in the visible region.
Infrared spectroscopy determines the absorption of light, and the Raman spectrum evaluates the scattering of light.
Raman scattering is based on the temporary depolarization caused by changes in the electron cloud distribution. In contrast, Infrared absorption is induced by vibrational changes in the molecular dipole or charge distribution. Both are different.
The light source for Raman spectrometers is lasers. For infrared spectrometers, it’s Nerst lamp, incandescent lamp lines, or silicon carbide rods.
Sample treatment before Raman spectroscopy is not essential. However, it’s a prerequisite for infrared spectroscopy. Solid samples are used for paste methods and polymer compounds for thin-film methods. Liquid samples are for liquid films.
Raman spectrum presents the molecular skeleton while the infrared spectrum focuses on the functional groups.
The vibrations of Molecules with a center of symmetry are invisible in infrared but visible in the Raman spectrum. Likewise, molecules without a center of symmetry appear in the infrared region but not in Raman.
How to choose infrared spectroscopy and Raman spectroscopy?
Identification of peaks in the Raman spectrum is relatively easier than IR spectra. It makes the mixture identification simple.
Infrared Spectroscopy has a strong hold on identifying organic compounds. The standard database of infrared spectra has more information than Raman spectra.
Raman spectroscopy makes identifying inorganic compounds with spectral information below 400cm-1 simpler than infrared spectroscopy. Hence, the Raman spectrum has more information about inorganic compounds in its database than the infrared spectrum.
Infrared and Raman spectroscopy can be used to complement and prove each other.
How to select the pre-burn time and exposure time for a portable Raman spectrometer?
The portable spectrometer must reach the pre-combustion stage, i.e., when the intensity and relative intensity of the spectral lines of each element is stable. The process takes less time than in air, and the pre-ignition curves for elements of different steel grades are different. The tracing approach and the integration method determine the pre-burning time.
The tracing method is to make the pre-ignition curve of each element, taking into account the time for each element to reach stability and determining the common pre-ignition time. The integration method is to repeatedly excite the sample under different pre-ignition times, observe the reproducibility of the analysis results of each element, and select the appropriate pre-ignition time. The tracing method determines the pre-ignition time by creating a pre-ignition curve on every element and recording the time required to reach stability. The integration method excites the sample repeatedly under different intervals to evaluate the extent of reproducibility and determine the appropriate pre-ignition time.
The pre-ignition time for medium and low alloy steel can be set at 4–6 seconds, 5–8 seconds for high allow steel, and 10–30 seconds for free-cutting steel. It’s essential that the duration of pre-combustion time correlates to the performance of the light source. Therefore, higher energy equates to a short combustion time. The reproducibility extent of elemental analysis is responsible for determining exposure time. The exposure process is defined as the charging mechanism of the photocurrent of the direct reading spectrometer to the integrating capacitor. Therefore, integration results can be regarded as the average photocurrent value. The spark charges should approximately be about 2000–3000 times to ensure accuracy. The iron and analytical elements ratio and light intensity should be within the moderate range. Exposure time of normal analysis should not be compared with portable spectrometer as it is associated directly with light energy.
What wavelength of laser does a Raman spectrometer use?
Lasers that lie in the ultraviolet, visible, and near-infrared regions are used as sources for Raman spectroscopy analysis. The intensity of Raman scattering is equivalent to the fourth power of the laser wavelength. Therefore, the laser is at least 15 times stronger. The formula for calculating the diameter of the laser spot is D=1.22/NA, as per the diffraction limit condition. Here, the wavelength and NA are numerical apertures of the microscopic objective used. The spot diameter of the 532nm laser can theoretically reach 0.72 microns. The theoretical minimal laser spot diameter reaches 1.1 um when the wavelength is 785nm. Therefore, the final spatial resolution is dependent on the selection of the excitation laser. Excitation wavelength can be selected based on sample properties for optimal results; blue-green light for inorganic materials, ultraviolet for bimolecular protein, DNA, RNA, etc., and surface-enhanced tensile testing red and near-infrared lasers for suppressing sample fluorescence.
How to measure transparent organic liquids with a Raman spectrometer?
In determining transparent organic liquid with a Raman spectrometer, the results may be the spectrum of glass when the operation is incorrect. The reason is a faulty focus position and concentration on the glass. A concave glass slide will hold a large amount of liquid. In case of volatility, a cover glass should be used before focusing.
How much is the concentration of the analyte in the organic liquid? Since the Raman spectrometer determines the scattered light, the concentration of the analyte is usually high because the intensity in the solution is relatively low.
Do you have a confocal Raman setup? The focal point should be in the capillary solution. Smudges can make focusing easier.
The amorphous nature of the glass affects the Raman signal, making it weak.
What are the main parts of a laser Raman spectrometer?
The main components of a laser Raman spectrometer are
Laser light source.
Sample cell: Micro capillary, constant liquid cell, gas cell, and tablet sample holder.
Monochromator:. Grating Split and double monochromator are used.
Multi-use continuous gas excitors, including He-Ne Laser with 632.8nm wavelength
Ar-ion laser with main wavelengths 514.5nm and 488.0 nm.
Detection system: Photomultiplier tube is used.
How should Raman spectrometers encounter these problems during testing?
Repeated sharp lines are usually from the emission of fluorescent lamps or the phosphorescence of CRT displays, especially when long working distance objectives are used. The solution can be to turn off the indoor fluorescent lamp or work under a darker incandescent lamp, and the instrument room should be as dark as possible. When none of the above methods solve the problem, and the 514nm laser is used for excitation.
Does the Raman spectrometer get repetitive and sharp peaks during testing?
Especially when long working distance objectives are utilized, repetition of sharp lines is witnessed due to the emission of fluorescent lamps or the phosphorescence of CRT displays. It can be overcome by turning off the fluorescent light and making the instrument room as dark as possible. Use a 514nm laser for excitation if nothing else works.
Why does the vacuum value of the Raman spectrometer drop rapidly during the test?
Analyze the vacuum value to see if it’s flat. In case of an air leak, fix the tightness of the vacuum chamber or replace the sealing ring.
Why does the signal from the spectrum cover the Raman signal when the Raman spectrometer is tested?
Any external light source should be blinded or turned off to avoid stray light from entering the spectrometer.
What is the difference between laser and FT Raman?
The employment of FT Raman and laser microscope Raman are quite different. Essentially, FT Raman is preferred for organic or polymer research, whereas Laser Raman is used for material research.