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Whether you are new to the technique, a seasoned veteran, or merely just curious about near-infrared spectroscopy (NIRS), Metrohm is here to help you to learn all about how to perform the best analysis possible with your instruments.

In this series, we will cover several frequently asked questions regarding both our laboratory NIRS instruments as well as our line of Process Analysis NIRS products.

Did you miss Part 1 in this series? Find it here!

1. What are typical detection limits for liquid samples and for solid samples?

The detection limit varies depending on the substance analyzed, the complexity of the sample matrix, and the sensitivity of both the reference and NIR technology used. NIR spectroscopy systems using dispersive technology are the most sensitive. Using such a system to analyze a simple sample in which the parameter of interest is a strong absorber will allow low detection limits.

For example, water in solvents can be detected down to about 10 mg/L in both offline and online/inline measurements. For more complex matrices (e.g., solids and slurries), detection limits are about 1000 mg/L (0.1%).

For more information about the differences between solid and liquid samples for NIRS analysis, as well as the different methods best suited for such matrices, read our blog post «Benefits of NIR spectroscopy: Part 1» here!

2. What accuracy can I achieve with NIR spectroscopy?

The accuracy of a near-infrared spectroscopic method depends on the accuracy of the reference/primary method. A highly accurate primary method will result in the development of a highly accurate NIR method, while a less accurate primary method lowers the accuracy of the related NIR method. This is because the NIR data and primary data are correlated in the prediction model. A good prediction model will have approximately 1.1x the accuracy of the primary method over the prediction range.

The development of prediction models has been described in detail in our previous blog article: «Benefits of NIR spectroscopy: Part 3».

3. How are instruments calibrated and how often do I need to recalibrate an instrument?

Instruments are calibrated using certified NIST standards. For dispersive systems measuring in reflection mode, NIST SRM 1920 standards are used to calibrate the wavelength / wavenumber axis. Certified reflection standards with a defined reflectance made of ceramic can be used to calibrate the absorbance axis.

In transmission mode, typically NIST SRM 2065 or 2035 are used for the wavelength / wavenumber calibration, and air for the absorbance axis.

A calibration should be performed after each hardware modification (e.g., lamp exchange) and annually as part of a service interval. Ideally, the spectroscopy software guides user through the complete calibration processes.

Find the calibration tools for your Metrohm NIRS instruments here!

Metrohm NIRS reflection standard, set of 2.

4. How do I validate my instrument and how frequently should validation be done?

The Metrohm NIRS DS2500 Solid Analyzer.

NIR spectroscopy software offers different tests to validate the performance of the instrument. The most common one is a basic performance test, which tests some crucial hardware parts as well as the wavelength/wavenumber calibration and the signal to noise (S/N) of the system.

For the regulated environment, further tests according to the USP <856> guidelines are typically implemented, including photometric linearity and noise at high and low light fluxes. Instrument performance tests should be performed on a regular basis, with the frequency depending on risk assessment.

5. What sample types or parameters are not suitable for analysis with NIR spectroscopy?

Samples containing a high amount of carbon black cannot be analyzed by NIR spectroscopy because carbon black absorbs almost all NIR light.

Further, most inorganic substances have no absorbance bands in the NIR spectral region and are therefore not suitable for NIR analysis.

Find out more about the molecules and functional groups which are active in the NIR region of the electromagnetic spectrum in our previous blog post: «Benefits of NIR spectroscopy: Part 2».

Carbon black is not a suitable sample to be measured by NIR technology.

Are you looking for more spectroscopy applications? Check out the Metrohm Application Finder to download free applications across a variety of industries!

6. My industrial process is full of harsh chemicals, so manual sampling is not desirable. Is it possible to perform inline NIR analysis in hazardous areas?

Yes, and we have the right solutions for you. Metrohm not only manufactures instruments for laboratory analysis, but we also cater to the industrial process world! Metrohm Process Analytics offers two versions of process NIRS systems: the NIRS Analyzer Pro and the NIRS XDS Process Analyzer, the latter being the ideal solution for hazardous environments.

Metrohm Process Analytics offers two lines of near-infrared spectroscopic process analyzers: the NIRS Analyzer PRO and the NIRS XDS Process Analyzer.

NIRS is a robust and extremely versatile method, which enables simultaneous, «real-time» monitoring of diverse process parameters with a single measurement. The use of fiber-optics in NIRS means that the process analyzer and measuring point can be spatially separated – even by hundreds of meters if required. In fact, remote monitoring can be achieved at large distances without significant impact to S/N ratios. This is a huge advantage in environments with challenging explosion protection requirements. Fiber-optic probes and flow cells can be placed in very harmful working environments, while the spectrometer and analysis PC remain safe and secure in a shelter. When a shelter is not available, the NIRS XDS Process Analyzer can be directly placed in the hazardous area (ATEX Zone 2 or Class1Div2).

Obtain «real-time» results of your process without the need to take samples, reduce the risks of handling chemicals, and increase your profitability. Download our free brochure here for more information about safe operation of NIRS process analyzers in hazardous areas!

7. How is the maintenance of a NIRS process analyzer performed?

Maintenance is easy, fast, and not necessary to perform very often. NIRS is a reagentless analytical technique, so the only consumable to be replaced is the lamp, which needs replacement once per year.

Compared to other techniques like chromatography  (e.g., GC, IC) or titration, and also because NIR spectroscopic analysis does not degrade samples, there is no chemical waste which is produced. Additionally, thanks to our all-in-one software, automatic performance tests are performed regularly to guarantee that the analyzer is operating according to process specifications. The instrument can be left in the process without any further operator involvement. 

Metrohm Process Analytics NIRS process analyzers are maintenance-free systems that have been designed to guarantee high uptimes and low operational costs.

Are you searching for more process NIRS applications? Check out the Metrohm Application Finder to download them for free!

Want to learn more about NIR spectroscopy and potential applications? Have a look at our free and comprehensive application booklet about NIR spectroscopy.

Download our Monograph

A guide to near-infrared spectroscopic analysis of industrial manufacturing processes

Post written by Dr. Nicolas Rühl (Product Manager Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland) and Dr. Alexandre Olive (Product Manager Process Spectroscopy at Metrohm Applikon, Schiedam, The Netherlands).

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