In the first part of this series, a brief historical overview was given for both infrared (IR) and near-infrared (NIR) spectroscopy, as well as for Fourier transformation (FT) and dispersive spectroscopy. A few myths were discussed and put to rest, and we showed that Fourier transformation spectroscopy (FT-NIR) is not necessarily the only nor the best way to integrate reproducible spectroscopic measurements into industrial processes. On the contrary—dispersive instruments are a robust possibility with ideal opportunities for model transfer, high resolution, and high light throughput even for sensitive applications. Dispersive NIR is at least as good as FT-NIR.
Now, two more misconceptions will be cleared up. Here we’ll go more into detail comparing the IR and NIR wavelength ranges. Furthermore, we will show that most IR applications can also be realized with NIR spectroscopy, and that this results in many economical benefits for plant operators. In the rest of this article, we will compare NIR and IR spectroscopy directly from a process integration point of view and show a real case study of application development with an IR replacement strategy. With this, we conclude that dispersive NIR is better for process integration than FT-IR.
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Myth 4: Many IR applications cannot be implemented with NIRS due to its lower specificity and higher detection limits
From theory, it is known that just as for UV-VIS spectroscopy, NIR and IR spectroscopy also follow Lambert–Beer's law. Here the measured extinction depends on the optical pathlength, the substance-specific extinction coefficients, and the concentration of the analyte. If you’re interested in seeing Lambert’s original publication, you can find it below.
Due to the high extinction coefficients of organic components in the IR range, even low concentrations can be reliably determined. However, either a strong dilution of the sample is necessary (which is hardly possible in a production process), or the optical pathlength is drastically reduced. Usually 50–200 µm cuvettes are used for the IR wavelength range.
However, this has significant drawbacks within the process: the sample streams might be dirty or form deposits on the optics from time to time, which means cleaning is very difficult and can lead to accidental misalignment. If the optics must be disassembled, a reproducible measurement is hardly possible afterwards since the application has been created for a highly accurate fixed pathlength. This necessitates costly and time-consuming recalibration procedures to readjust the calibration models, with the associated downtime for the instrument(s). Operational reliability is jeopardized because measurements cannot be performed during this time. In this case, the application should be transferred to a sampling solution with higher pathlength, such as to the NIR wavelength range.
Method development: what is it all about? Have a look at our related blog articles on this topic below.
In the NIR wavelength range, immersion probes and flow cells with significantly longer pathlengths (0.5–20 mm) are used. These are either adjusted by spacers or by threaded screws so that an extremely reproducible adjustment can be made. If contamination occurs, cleaning is also much easier.
Another point to consider is that infrared light is blocked through conventional optics. For the IR wavelength range, more expensive materials (e.g., sapphire or chalcogenide) need to be used.
For NIR wavelengths, glass or quartz can be used which transmit almost 100% of the NIR light. NIRS allows low-OH content fiber optics to be used for measuring points at long distances from the analyzer (e.g., >100 m from sample point to the instrument), whereas an IR spectrometer/photometer must be attached as a complete system to each individual measuring point. Therefore IR instruments are not readily adapted for hazardous environments.
NIR fiber optics and flow cell windows (or immersion probes for direct inline measurements) are simpler and quite inexpensive and are therefore very economical.
Download our brochure below for information about the sampling probes and fiber optics from Metrohm Process Analytics.
Most of the time the process analyzer is installed in a safe area away from hazardous conditions, while the light fibers and probes are located in an explosion-proof electrical area. This means the analyzer has received an ATEX, IECEx, or Class I Div2 / Class I Div1 label (depending on the country of installation) which certifies optical intrinsic safety. For plant operators, this kind of setup equates to significant cost savings and less effort because the system does not have to be completely integrated into a hazardous area, as is usually the case with IR photometers.
Read more about our solutions for NIRS Ex-proof analyzers in our free brochure.
Results from NIRS process analyzers are transferred by standard process communication protocols to a Distributed Control System or Programmable Logic Controller (DCS, PLC). This enables real-time control and regulation of the process, even from long distances to the sampling point (Figure 1).
Table 1 shows the most important benefits of NIR spectroscopy compared to IR and summarizes why NIRS is the most economical way of process monitoring without losing any specificity and detection power.
Table 1. Overview comparing NIR and IR spectroscopic techniques
|Vibration types||Combination vibrations and harmonics/overtones||Fundamental vibrations|
|Sample preparation||None||Dilution / none|
|Optical pathlength||0.1 mm to 1000 mm||0.050 mm to 1 mm|
|Spectral information||Overlapping wide bands, chemometrically distinguishable||Fingerprint area, unique bands|
|Evaluation (quantitative)||Calibration model multivariate||Calibration model multivariate or univariate|
|Evaluation (qualitative)||Spectra library with chemometrics||Comparison with spectra library|
|Use of inexpensive fiber optics||Yes||No|
|Acquisition and operating costs||Medium||High|
|Process integration capability||Simple||Difficult|
Now that the hardware comparison is complete, it is time to discuss application possibilities. Can applications be transferred from the IR range to the NIR range?
In our previous blog post, the general advantages of NIR vs. IR spectroscopy were described. Find out more below.
The spectroscopic information in the NIR range overlaps with absorption bands from the fundamental vibrations of the IR range. In addition, there are harmonics/overtones in the NIR range which correspond to multiples of the fundamental vibration with higher frequency but lower intensity (Figure 3).
In a nutshell: the information from the IR range can also be found in the NIR range. So, to answer the question whether applications are transferable from IR to NIR spectroscopy—absolutely!
In addition to the hardware advantages, there are also many application advantages for NIRS. The simultaneous detection of strong combination bands and weak overtone bands in the NIR wavelength range allow a high degree of flexibility regarding the measurement ranges. Concentrations from 100% down to the mg/L (ppm) range can be covered, which brings us to Myth 5.
Myth 5: Only higher concentrations (>500 mg/L) of analytes can be measured with NIRS / NIRS is only used for rough trend analysis
This myth is definitely proven to be false, as seen in both the scientific literature as well as out in the field. There are now numerous NIRS applications developed by Metrohm Process Analytics in which e.g., the residual moisture content in alcohols or solvents are monitored in the final product stream.
Learn more about what makes Metrohm Process Analytics special and find a selection of our free Process NIRS applications here.
Why does it work? Quite simply, it's a combination of the best hardware, which results in an excellent signal-to-noise ratio, and a well-defined sampling point. Users can retrieve a wealth of information out of the obtained spectra by exploiting the entire wavelength range and the associated dynamics in the spectrum. Paying special attention to the details of application development is the final key.
In cases where real-time monitoring of very low concentrations of analytes is required, it is no longer sufficient to simply look for the «right» spectrometer. Rather, precise analysis with a primary detection method carried out with expert know-how plays a decisive role, because every application stands or falls with the primary method.
As an example, the primary method of choice for low water content determination is Karl Fischer (KF) titration. Metrohm Process Analytics offers a unique combination of this high-precision laboratory method as well as the inline near-infrared spectroscopic process analysis. This benefits the user as both the primary method and the secondary (NIRS) method are covered by the same vendor.
Learn more about the power of NIRS and Karl Fischer titration with our free brochure and in our related blog post.
At Metrohm Process Analytics, the application development and system integration into industrial processes always consists of detailed steps combining the synergy of lab and process hardware and expertise from the different product specialists:
- Customized applications: In routine laboratory work, the KF titrator generates results from various samples collected from the process. Meanwhile, spectra of those same samples are recorded in the sample stream for true inline method development.
- Linking of laboratory and process analytics: To determine the water content in a process stream, a robust calibration model is created by linking the spectra from the inline analysis to the results obtained from the primary method (KF titration). Thanks to the availability of the primary method, NIR hardware, and chemometric knowledge from one source, the developed method is precise and optimized for the customer.
- Fully automated process solutions: The continuous, real-time monitoring of residual water shows reliable results. Aside from the communication of results, there are even more details which lead to a robust and long-term stable process application that can withstand the challenges of method development. At Metrohm Process Analytics, we go above and beyond basic process analysis with additional benefits for end-users. For example, signals gathered from a sampling point are transferred to our process software. Information from samples taken for primary analysis can then be matched to the process spectrum. As a result, these methods can be adapted very precisely during process method development and will always provide the correct values at the correct timestamp.
This procedure leads to the successful replacement of error-prone and uneconomical IR process analyzers by a single NIR process analyzer equipped with several measuring points. Figure 4 shows the process trend charts of two IR analyzers and one NIRS XDS Process Analyzer nine channel system equipped with two measuring points.
Find out more about the NIRS XDS Process Analyzer below and download our free Process Application Note to learn more about this application.
Both dynamics and sensitivity are exhibited in the NIR trends so that even the smallest differences are noticeable (Figure 4), which is a desirable trait for fast intervention times in process monitoring.
With proper method development, concentrations of analytes down to <10 mg/L can be determined inline and online with NIR spectroscopy—even for very hygroscopic samples that make reference measurements difficult. With NIRS analysis, the error can be reduced to below 5 mg/L.
Several myths still persist about online NIR spectroscopy, FT-NIR, and FT-IR, though this blog series has put some of them to rest.
Near-infrared spectroscopy is one of the most important analytical tools used in process analytical technology (PAT) and is currently established in almost all branches of industry. The hardware advantages of NIRS process analyzers clearly outweigh IR measurement techniques. NIRS process analyzers can replace historically used IR photometers even for analytes at very low concentrations in the mg/L (ppm) range. In addition to excellent optics in the dispersive spectrometer, this is also due to the good preparation of a representative primary analysis and an intelligent sampling concept.
By using these synergistic effects of highly accurate and sensitive NIR hardware, process understanding, and reference analysis, a customized process solution can be created for each application task in process environments.
The benefits of NIRS are numerous:
Not only do you save on reagent costs (purchase and disposal), but real-time data from the process also helps you quickly intervene and optimize in case of out-of-spec readings.
- low and easy maintenance requirements
- spatial separation of analyzer and measuring point keeps company assets and employees safer
- multiplexing capability up to nine measuring points allows faster return on investment (ROI) and lower costs per measurement