Calibrated Raman Systems
In the world of Raman spectroscopy, the game-changing impact of a spectrometer cannot be understated. Across various industries – from pharmaceuticals and bioprocessing to plastics and polymers, these instruments help process environments streamline their decisions, create efficiencies across the supply chain, cut costs, and enhance product quality. Raman spectrometers are no different than any other type of machine. Think of it the same as regularly taking your car to the mechanic for oil changes and inspections. Your Raman spectrometer requires regular maintenance and care to continue at an optimal level. Factors such as temperature, humidity, calibration frequency and proper maintenance impact the quality of your Raman measurements. Create the right environment for your spectrometer, and it will deliver nothing short of superior results in return. This blog post provides an overview on the importance of a calibrated Raman system and offers guidance for Raman users who want to get the best measurement out of their spectroscopic data.
General Overview – Why Calibration is Necessary
Calibration is the major consideration in making sure your instrument remains reliable. Beyond being a regular part of maintenance, calibration is a vital part of any scientific process. All scientific processes and procedures need to have a known reference point, and this is absolutely essential for all instruments.
Several factors may affect the stability, such as temperature, vibrations, and humidity. Raman spectroscopy is commonly used in the investigation of pharmaceutical compounds and its applications have been extensively reviewed. It is highly chemically selective and offers the possibility to monitor the chemical and structural change of ingredients when exposed to harsh environments. Raman spectroscopy can be combined with a temperature and humidity controlled cell to understand the modifications induced by environmental changes. By using high-quality optical components, Tornado Raman spectrometers are rugged systems that allow the user to focus on their process and better handle dust, humidity, and any other manufacturing environment conditions.
Instrument calibration should be verified regularly, regardless of how often the Raman spectrometer is used. Calibration stability can be affected by long-term lack of use, environmental vibration, dust, and temperature swings. Ensuring an accurate calibration is essential to getting reliable and accurate results. On a larger scale, this confirms measurement accuracy and long-term repeatability.
Light intensity
With Raman measurements, the consistency of the intensity of the collected energy is essential to getting an accurate analysis. If the collection throughput is not consistent over time, the measurements can systematically vary. With signal loss on either excitation or collection fiber (even with a stable signal), a much longer collection time is required. A dirty fibre is the most common culprit in optical circuit performance degradation and is easily identifiable.
A Raman system measures a range of wavelengths of light that return from where the laser excites the material to be measured. In modern systems, the light is measured by Charge-Coupled Devices (CCD). In a CCD, an incident photon may add a charge to a capacitor for a specific pixel, and the charge of this capacitor gives the intensity for that wavelength. Each wavelength of light, however, will require a different number of photons to add a unit of charge. This ratio is called the Quantum Efficiency (QE), and by applying a white light source that emits an equal intensity of light at each wavelength, we can calibrate the detector and collection path for the efficiency of collection across the spectrum of measurement.
By regularly calibrating the system to adjust for throughput changes over time, you can ensure that the response is consistent such that the integrity of the spectral measurements will be maintained. As a result, you can verify collection quality using cyclohexane or polystyrene standards to assess the intensity of those standards.
Wavelength Calibration
This is among one of the most common calibration methods in use. In the case of Raman instrumentation that use array detectors, calibration lamps are used to linearize and standardize the detector response. A wavelength standard calibration lamp, typically consisting of an atomic vapor like mercury (Hg), argon (Ar), or neon (Ne) energized by electricity, illuminates the spectrometer at dozens of very specific wavelengths (certified by NIST). By measuring the positions of each of these atomic emission lines on the CCD, the wavelength of each CCD pixel can be determined. This calibration step is a critical part of generating Raman spectral data which is consistent among different Raman devices.
The calibration process of a wavelength standard calibration lamp provides a set of lines that are evenly distributed across the range you would like to calibrate, which is why different types of lamps are often used for different Raman laser wavelengths (532 nm, 633 nm, 785 nm).
Laser Calibration
One of the most important considerations when evaluating a Raman spectrometer is the laser which serves as an “excitation source” illuminating the chemical sample to facilitate the Raman scatter. The chosen laser wavelength will have an impact on Raman intensity and background fluorescence (and in Raman microscopy applications, spatial resolution as well).
With Raman measurements, the integrity of the spectrum is highly dependent on knowing the laser wavelength, i.e., the “zero point” of the spectrum. The entire spectrum is affected if this is inaccurate. Laser calibration is performed using known standards. Preferred materials are cited by ASTM, but among those, the two most common are cyclohexane and polystyrene. Key peaks in the reference spectra for these materials can be used to reveal the critical important information about the wavelength of the laser. For many Raman instruments, laser stability is an issue, and calibration is required once a day or even more often. Fortunately, Tornado’s Hyperflux™ PRO Plus instrumentation has proven extremely stable such that this calibration is required monthly or semi-monthly.
The Importance of Y-Axis Calibration
Calibration transfer approaches can be used to reliably transfer spectral library and chemometric-based Raman methods between instruments. The effects of this are critical, as it makes supporting a fleet of geographically dispersed Raman spectrometers possible. Rather than collect a spectral library or building a chemometric method for each individual instrument, which may require acquisition of hundreds or even thousands of samples on each unit, these approaches serve to streamline method development.
Conclusion
Calibrating your Raman spectrometer is a relatively simple task, but it has significant effects on the results. Ensuring that your tool is properly set prior to use will provide you with more accurate and reliable results.
While the primary purpose of calibrating is to ensure that each test has the most accurate result possible, it can also be a sign of something more seriously wrong with the spectrometer. Make sure that the results are not too varied because that could be an indication that there is something very seriously wrong with the spectrometer.
If you would like to discuss your application and are considering adopting Raman spectroscopy as a process analytic technology, please contact us at [email protected].
