On the performance of Raman spectroscopy

Why is the horizontal axis of the Raman spectrum displayed in wavenumber?

Although the Raman scattering light itself is light of various wavelengths, the horizontal axis of the Raman spectrum is generally represented by wavenumbers rather than wavelengths. That is because Raman scattering is the conversion of intrinsic vibrational energy of molecules into light. Since the frequency is the reciprocal of the period (wavelength in light), displaying the Raman spectrum with the wave number, which is the reciprocal of the wavelength, makes it possible to understand the natural frequency of the molecule and estimate the molecular species. In addition, it is also useful to be able to compare Raman spectra regardless of the excitation wavelength and to compare with infrared measurement results by displaying the horizontal axis not by wavelength but by wave number.

Can you measure quantitatively with Raman spectroscopy?

Although quantitative measurement can not be performed as an absolute value, it is possible to create a calibration curve using a reference sample.

What is the relationship between spot size, spatial resolution, and pixel size?

The spot size is the spot diameter when the laser beam is narrowed down with the objective lens and is theoretically determined by the laser wavelength and the NA of the objective lens, but the performance depends on whether the optical system for illumination is properly designed Whether or not to change will change. There is no problem in practical use even if spatial resolution is considered the same as spot size. In practice, spot size and spatial resolution are concepts that are strictly different, because the ability of the optical system to detect Raman scattered light generated from the sample is involved. Although microscopic Raman generally uses a confocal (confocal) optical system, the spatial resolution becomes smaller than the spot size as the pinhole diameter and slit width are reduced. However, the amount of light reaching the detector decreases, so it is not a very practical story. On the other hand, the pixel size in the Raman image refers to the scanning pitch of the measurement, which means how finely the spatial data is acquired. From the Nyquist theorem, it is sufficient to select a pixel size that is twice as fine as the resolution.

What is the sensitivity and brightness in Raman spectroscopy?

Raman scattered light may be weak, so some sensitivity, brightness (detected intensity to background), etc. are required. There is a numerical value representing the brightness of the spectrometer called “f value”. The f value decreases as the focal length is shorter and the value becomes brighter, but the optical efficiency does not mean what it should be as an absolute value, but the f value on the side of the spectroscope and the incident light source is matched It is important. For example, if the light source side is too bright compared to the f-number of the spectrometer, the light will spread to the outside of the diffraction grating and become stray light. You Some of the commercially available devices have different f-numbers, but because they have their own appropriate optical system, it can not be said in general that “the f-number of the spectrometer is small = bright, good sensitivity”. So be careful when comparing the specifications.

Is there a problem in supporting UV to near infrared?

Care should be taken when installing multiple wavelength lasers in one device. That’s because there are parts and designs that are appropriate for each wavelength. In particular, the corresponding wavelength range of the lens is fixed, and the performance drops significantly for light outside that range. Therefore, for example, optical microscope manufacturers achieve the required performance in each region by preparing different objective lenses for the ultraviolet region, the visible region, and the near infrared region. The same is true for the optical components inside the Raman spectrometer, but there is also a part that can be shared between lasers with similar wavelengths, but if you try to correspond with the same optical components in all regions from ultraviolet to near infrared Performance will be inadequate even in the We intended to make it an “almighty” device that can measure any wavelength, but in fact nothing can be measured, and it can be that only one wavelength can be used after all. It is necessary to firmly determine whether it is a device that can properly meet the purpose of measurement and the required specifications.

Sample size and FOV story

The size of the sample is about 5 cm x 7 cm x 2.5 cm thick as a guide. The small side can be a powder, as long as it can be found and focused with a microscope. The size of the field of view depends on the objective lens. For example, at 20 ×, the field of view is 400 μm x 600 μm, and the imaging range is up to about 400 μm square. With a 100 × objective lens, the observation field of view is 80 μm × 100 μm, and the imaging range is up to about 80 μm square.

The penetration depth of light and the analysis depth of Raman scattered light

How deep is Raman scattered light collecting information? Although it differs depending on the combination of the substance and the wavelength of light to be irradiated, it can be estimated based on the following concept. First, the light intensity before and after passing through a substance is the inverse of the absorption coefficient according to Lambert-Beer’s law. However, in the case of Raman scattering, both incident and scattered light are absorbed, so the depth of the area actually observed is half that, 1 / (2α). For example, the absorption coefficient at 532 nm of crystalline silicon is about 10 4 cm -1, so if it is measured with this, it will be picking up information to a depth of about 500 nm.

Does the laser beam scan distort the spot when illuminated at the edge of the field of view

This is a common misconception about laser beam scanning. With a well-designed laser beam scanning system, the laser is incident normal to the viewing plane, both when illuminating the center of the microscope field and when illuminating the edge.

About RAMANtouch

Why spatial resolution of RAMANtouch is high?

Designed to be true to optical theory and assembled by specialists in optical design, all aircraft have a stable finish. In addition, because of its compact size, it has a structure that is less likely to cause mechanical displacement due to changes in the surrounding environment.

How do we adjust the optical system when switching the laser?

You do not need. If you can switch in one click, the necessary adjustments will be made automatically.

How is the optical system adjusted when switching the diffraction grating?

You do not need. If you can switch in one click, the necessary adjustments will be made automatically.

How is Raman imaging made?

Each pixel in the imaging area contains Raman spectral data. For example, when colors are assigned to peaks at certain wave number positions, color shading occurs in each pixel according to the intensity of the peak of the Raman spectrum contained in each pixel. That represents the concentration distribution of the component represented by that peak. Similarly, by assigning different colors to different peaks representing different components, it is possible to investigate the distribution of each component even when there are multiple components.

Why use line illumination and laser beam scanning?

Since nanophoton started from the technology of laser microscope, we adopted laser beam scanning as a natural idea. As a result, there is no need to move the motorized stage to fine-tune the measurement position, and there are no problems such as vibration or sample displacement due to the stage movement. Furthermore, the unique line illumination technology enables 400 Raman spectra to be acquired with a single irradiation, enabling high-speed Raman imaging analysis.

What is the uneven intensity of line lighting?

While line lighting is concerned about the unevenness at the center and at both ends, RAMANtouch uses an original technology to substantially eliminate uneven lighting (patented: US7561265).

Does a high power laser cause damage to the sample?

Our device is equipped with an ND filter that can adjust the laser power almost steplessly (256 steps). If the sample does not have a concern for damage, it is possible to obtain Raman signals efficiently with high power, and on the other hand, with samples that are weak to heat, it is possible to lower the laser power with the ND filter and measure.

Why can we measure the fault structure of wrap film nondestructively?

Our laser Raman microscope RAMANtouch guarantees high spatial resolution as high as 1 μm in the Z direction thanks to excellent confocal optics. Therefore, if it is a transparent substance, it is possible to focus on the inside and extract only the Raman signal at that height position. Even for thin wrap films stacked by several hundred nm, it is possible to extract tomographic information by measuring while changing the focal position from the surface.

What is the sample size and the size of the field of view?

The size of the sample is about 5 cm x 7 cm x 2.5 cm thick as a guide. The small side can be a powder, as long as it can be found and focused with a microscope. The size of the field of view depends on the objective lens. For example, at 20 ×, the field of view is 400 μm x 600 μm, and the imaging range is up to about 400 μm square. With a 100 × objective lens, the observation field of view is 80 μm × 100 μm, and the imaging range is up to about 80 μm square.

If it is RAMANdrive, it corresponds to a 300mm square sample.

What is the size of the field of view and the size of the pixel resolution?

The following figures can be used as a reference for the observation magnification, the observation field size, and the maximum imaging range. The pixel resolution has the ability to divide the field of view by 10,000 and the scanning optical system has.

Does surface unevenness affect Raman imaging?

Laser Raman microscopy is not good at imaging sample surfaces with large irregularities. This is due to the use of a confocal optical system that cuts out the information that is not in focus. Irregularities that are not very noticeable when looking at the optical microscope image of the sample do not affect the Raman imaging so much. On the other hand, even if there are asperities that cause problems, the effects of asperities can be reduced by using an objective lens with the same magnification and long working distance. Using the 3D-Build function, it is possible to combine overlapping images of only the in-focus areas of images with different height information, which is useful when measuring samples with large irregularities. Alternatively, our wide-field Raman scope enables imaging analysis with less blurring, even with large irregularities of about 1 mm.

What factors determine the measurement time of Raman imaging?

When imaging with line illumination, data for 400 pixels in the X direction can be obtained with a single exposure. The imaging range is extended by repeating the irradiation position continuously in the Y direction. Therefore, the measurement time is determined by “exposure time / 1 line x number of pixels in Y direction” (strictly speaking, the data transfer time of the CCD also needs to be considered). The required irradiation time varies depending on the sensitivity of the sample to heat damage and the degree of Raman activity.

About involved in maintenance of device

Does the laser have a lifetime?

A typical example is the aging of the diode that oscillates the laser. When current is flowing, the diode will deteriorate rapidly because it becomes particularly hot among the components that make up the laser. In addition to that, there are parts that generate a lot of heat and a short life, so it is generally said that about five years is a guideline for laser replacement.

What is the cost of repair and cost in case of laser failure?

For problems that occur within the one-year warranty period, we will respond free of charge for both business travel and work expenses. For failures beyond the warranty period, costs will be incurred according to the condition. If repair is required, remove the laser and take it home. In that case, please contact us as alternative products may be available. Laser repairs are generally expensive. We have a maintenance contract to help you avoid sudden expenses, so please consider.

What do I need to stock as maintenance parts?

There are no maintenance parts that need to be stocked. Our Raman imaging system has no consumables other than lasers, and you do not have to hold your own inventory of lasers.

What is the response time when a problem occurs?

First of all, please contact us or our sales agent. Even if there is no person in charge of installation, there will be technically capable personnel, so we will first make a phone call to quickly respond to the situation.