Inspection and quality control of incoming goods, identification of hazardous materials, and reaction monitoring in the chemical industry, all these applications and more can employ Raman spectroscopy. The technique allows the identification of unknown substances within seconds and non-destructively. The following article describes the measurement principle as well as example applications of Raman spectroscopy in different industries.
What is Raman spectroscopy?
Raman spectroscopy is an optical measurement technique which analyzes the inelastic scattered light from a sample material. Strong laser light is sent to the sample where it interacts with the material. Most of the light will simply be elastically scattered, meaning that the light changes its direction but not its wavelength. On the other hand, a tiny fraction of the light not only changes its direction but also its color. In this case, energy from the light particle is transferred to the molecules in the material and the remaining energy is emitted as so-called inelastic light scattering. This interaction process is known as the Raman effect. A Raman spectrum is as specific for a sample as a fingerprint and it reflects the chemical bonds present in the material. Thus, in Raman spectroscopy applications the chemical composition can be analyzed and changes within a sample be detected. (For information on molecular vibration, see the wiki article “Basics of Raman spectroscopy”)
Technology of Raman spectrometers
The Raman effect is very weak compared to other optical interaction processes such as absorption, fluorescence, or elastic light scattering. Therefore, Raman spectrometers use a high light intensity to generate a Raman signal from a sample. Generally a Raman spectrometer consists of a light source, a sample, a dispersive element, and a detector.
Raman spectrometers contain a laser as a light source with wavelengths in the visible to near-infrared range (400 nm to 1100 nm). These wavelengths are monochromatic, coherent, and have high intensities. After the interaction of the incident light with the sample, the light is scattered in all directions. A fraction of the scattered light is directed to the dispersive element. Commonly, a diffraction grating in reflection or transmission geometry is used to split up the scattered light beams spatially, thereby separating the different wavelengths. The detector may be composed of a photodiode, which uses the inner photoelectric effect for signal conversion of the different intensity wavelengths into an electronic signal. Older instruments often have single-element detectors, which can only detect one wavelength at a time and need a scanning procedure to record a full spectrum. Newer devices, however, work with CCD detector arrays similar to the ones used in cameras or InGaAs (Indium-Gallium-Arsenide) detectors, which are able to simultaneously detect a specific range of the spectrum. The result of the measurement is called a Raman spectrum and shows a graph on which the intensity (signal strength) is plotted against the reciprocal wavelength, which is proportional to the energy. (For an explanation why the reciprocal wavelength is used, see wiki article “Basics of Raman spectroscopy”)
Advantages of Raman spectroscopy
In a number of applications Raman spectroscopy has technical advantages compared to other methods. It is especially suited for fast identification without direct sample contact:
- Raman spectroscopy is a (usually) non-destructive and non-invasive technique. The measured sample can be re-used for other purposes afterwards.
- The sample can be measured through the packaging (such as glass or thin plastic), which makes it a very safe technique when handling chemicals and hazardous materials.
- There is no or very limited sample preparation: you can easily measure a solid directly or measure powders and solutions by filling them into a small glass vial (Figure 3).
- The analysis is quick: It usually only takes a few seconds to obtain the result.
- The technique is suitable for aqueous solutions: Raman spectroscopy does not suffer from strong absorbance of water which is a common problem for IR spectroscopy.
Limits of Raman spectroscopy
Although there are benefits of Raman spectroscopy there are also some limitations due to the weak Raman effect and the use of the strong laser light:
- The detection limit is usually in the 1 % range which excludes its use for potential trace analysis. However, there are enhancement techniques such as SERS (surface-enhanced Raman scattering) which make trace analysis possible.
- A weak Raman signal may be obscured by concurring fluorescence signals.
- Dark samples are challenging since they absorb most of the light which leads to heating of the sample. This might lead to molecular changes of the structure even to the point of combustion.
- Due to their lattice arrangement metals and purely ionic compounds don’t show molecular vibrations and cannot be characterized.
- Gases cannot be characterized by common benchtop devices and require special instruments.
What are typical Raman spectroscopy applications?
The advantages of Raman technology, especially the measurement through vessels and containers, makes it indispensable when inspecting goods and identifying hazardous material. Moreover, the detection of molecular vibrations is a prerequisite for chemical reaction monitoring. Having those pros and cons in mind, Raman spectroscopy is an essential analytical tool in a number of industrial areas.
Incoming goods inspections
A frequent task in the chemical, cosmetics, and pharmaceutical industry is to verify whether a delivery actually contains what is written on the label. Often such incoming goods inspections are combined with a quality control check so that raw materials of poor quality are detected and prevented from being used in further processing steps. Such tests can easily be performed using a Raman spectrometer as the inspection system. The molecular vibrations present in the sample are as specific as a fingerprint. Product inspection is not only quick but also safe due to the possibility to measure through packaging. For this inspection and quality control, a spectral database comparison to a preselected spectrum in the library is used. The result of the comparison is shown to the user as “pass” or “fail”. This makes it very easy to interpret the result without knowing too much about the technique. In this way, the warehouse personnel can immediately reject incorrect deliveries without the need for time-consuming lab analyses. The results of a product inspection in Figure 4 show the verification of pills that are supposed to consist of acetaminophen (paracetamol). Figure 5 shows the spectra of acetaminophen compared to a substance that failed verification. It is clearly visible how different the measurement results look.
Hazardous material identification
Customs, police, or border control officers often encounter unknown substances and have to evaluate whether these are hazardous, regulated, or harmless. Particularly in hazardous material identification, hazardous waste management, and hazardous waste removal, the specificity of Raman spectroscopy and the speed of the measurement are beneficial. As you can measure through the packaging, investigators and first responders will be in a safe working situation. Figure 6 shows the measurement of an unknown white powder which may be a hazardous substance. Figure 7 shows the results for measurements on two substances: one is a highly regulated narcotic (3,4-Methylenedioxymethamphetamine or MDMA) and one is caffeine (Figure 7). To the human eye, both are white powders and hardly distinguishable. However, for a Raman spectrometer they are completely different, each with a unique chemical fingerprint.
Monitoring the chemical conversion of one substance into another is of key importance in chemical and pharmaceutical production. For example, determining the endpoint of a reaction online saves time and money. Raman spectroscopy measurements are specific to the molecular groups present in a molecule. For example, you can monitor the reaction of an epoxy hardening in-situ. This in-situ reaction monitoring is possible because the scattered light is collected with a fiber probe head which is thermally and chemically resistant. It is placed in the reaction chamber and a Raman spectrum is obtained at user-specified intervals. Since the technique is non-destructive, multiple reaction monitoring does not alter the result of the chemical reaction or process. Figure 8 shows the spectra obtained from the different measurements. Displayed from dark red to light red, you can see the spectra obtained at intervals of 200 s as well as from 200 s to 1400 s after reaction start. Some of the signals are marked with their corresponding molecular vibration. One especially interesting signal is the epoxy ring breathing mode at 1260 cm–1. This signal decreases due to the reaction of the resin. The epoxy ring opens, so that in the end product this specific group is missing, and no epoxy ring mode is present in the spectrum. By evaluating this specific signal, the Raman measurements help to determine when the reaction has ended.
Raman spectroscopy applications include reaction monitoring, and the verification and identification of substances, including illegal and hazardous materials. Measurements through packaging make it a very safe technique for authorities, such as the police, first-responders, and customs but also for warehouse personnel working as incoming goods inspectors. Today’s Raman spectrometers encompass compact design with extremely high technical refinement and make it easy to use with a very intuitive user interface. Anton Paar offers Compact Raman Analyzers (Cora) with a small footprint and state-of-the-art optics.