NIR spectroscopy for biodiesel analysis
Oct 19, 2026
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As renewable energy sources become more mainstream, biodiesel is emerging as an alternative to supplement or even replace fossil fuels. Fast, reliable, nondestructive analysis methods are needed to maintain consistent biodiesel production. Near-infrared (NIR) spectroscopy enables rapid, reagent-free, multiparameter testing. This article explores how NIR spectroscopy facilitates every stage of biodiesel production: raw material control, transesterification monitoring, final product quality control, and blend verification.
This article covers the following topics:
What is near-infrared spectroscopy?
NIR spectroscopy (NIRS) is an analytical technique based on the interaction of near-infrared light (780–2500 nm) with molecular overtones and combination vibrations of chemical bonds (mainly C–H, O–H, and N–H). These interactions produce characteristic spectral fingerprints that can be correlated with chemical or physical properties of a sample through calibration models.
NIRS is fast, nondestructive, and does not require chemicals. In just a few seconds, multiple parameters can be determined simultaneously, making it a powerful solution for process control and routine analysis of biodiesel. NIR spectroscopy is an ideal choice for real-time monitoring of biodiesel throughout the entire production process (Figure 1), like the inspection of raw materials (vegetable oils or animal fats), the precision of the transesterification reaction, and final product certification [1].
This blog article evaluates a practical workflow showing how NIRS can simplify biodiesel quality testing, using the OMNIS NIR Analyzer as a basis for the analyses. More information about how NIRS works and how it compares to infrared (IR) spectroscopy can be found in the articles below.
Control of incoming raw materials for biodiesel production
Every drop of biodiesel starts with an oil—whether it is soybean, rapeseed, palm, animal fat, or even recycled cooking oil. These materials differ widely in composition, and small variations in different quality parameters can significantly influence the yield, reaction efficiency, and quality of the final biodiesel fuel [2].
Quality inspection of incoming raw materials
NIR spectroscopy can quickly measure important properties of the incoming raw materials, assuring biodiesel quality and strategic management of the biodiesel production. All the following parameters can be predicted simultaneously within seconds using a single NIR spectrum (Figure 2).
- Iodine value (IV): The iodine value indicates the level of unsaturation in the oil, which affects oxidative stability and cold flow properties of the final biodiesel.
- Fatty acid composition: The profile of saturated and unsaturated fatty acids determines biodiesel properties like cold flow, oxidation stability, and viscosity.
- Free fatty acids (FFA): High FFA levels can cause soap formation during transesterification and reduce ester yield. Oils with >2% FFA can be quickly identified by NIRS and redirected for the necessary pretreatment.
- Refractive index (RI): Refractive index correlates with molecular structure and oil type. High variations in RI may indicate adulteration or mixtures, which can affect transesterification efficiency.
- Peroxide value (PV): High PV indicates that the oil has started to oxidize, which can lead to the formation of polymers or gums during transesterification.
- Material identification and adulteration checks: With NIR spectroscopy, producers can quickly identify the type of oil and detect possible adulterations or mixtures with lower-quality feedstocks.
To learn more about edible oil measurements, read our related article.
Monitoring the transesterification process
Once the incoming oils have been verified for quality, the next step in the biodiesel production begins: the transesterification reaction. This is the chemical heart of the process, where triglycerides from the oil are converted into fatty acid methyl esters (FAME) through a reaction with methanol and a catalyst (Figure 3) [3].
Inline or atline monitoring of the transesterification reaction
By comparing the NIR spectra of raw oil and finished biodiesel (Figure 4), producers can monitor the progress of the transesterification reaction either atline (e.g., with the OMNIS NIR Analyzer) or inline (e.g., with Metrohm Process Analyzers). Using NIRS leads to faster decisions, less waste, and consistently high-quality biodiesel.
- Reduce reagents, waste, and time: NIRS helps identify the endpoint of the transesterification reaction, preventing overuse of chemicals.
- Avoid reprocessing: Quickly detect unreacted triglycerides, methanol, and water (contamination) with NIRS.
Quality control of final products
Once biodiesel leaves the reactor and purification steps are completed, it is time for a critical final check. Producers must ensure that the fuel meets the specifications of different norms and standards which include key parameters for better storage and engine compatibility.
ASTM D7467 and EN 14214
By meeting the requirements of ASTM D7467 and EN 14214, biodiesel producers safeguard their fuel performance. Key parameters like moisture content and oxidative stability can be assessed quickly using NIR calibration models. These rapid insights make NIR spectroscopy an easy and essential tool for producers aiming to improve quality control, reduce wet chemistry dependency, and confidently release biodiesel to the market.
- Moisture: High moisture levels in biodiesel can lead to microbial growth, poor combustion, and corrosion in engines and storage tanks.
| Figure of merit | Value |
|---|---|
| Range | 141–1162 ppm |
| R2 | 0.999 |
| Standard error of calibration | 10.1 ppm |
| Standard error of cross-validation | 11.4 ppm |
- Oxidative stability: Biodiesel can degrade over time due to oxidation, especially during long-term storage. Poor oxidative stability reduces shelf life and engine compatibility [4].
| Figure of merit | Value |
|---|---|
| Range | 2.51–11.15 h |
| R2 | 0.953 |
| Standard error of calibration | 0.55 h |
| Standard error of cross-validation | 0.63 h |
- Residual methanol: In biodiesel, this can lower the flash point and damage fuel systems. NIR spectroscopy can quickly detect and quantify residual methanol, reducing the need for GC testing in routine quality control.
Learn more about how NIRS complies with the petrochemical industry in our related blog post below.
Blend verification
After biodiesel meets all required quality standards, the next step is to verify its blend ratio with diesel fuels and check for adulteration. Whether it's B0 (pure diesel), B5, B20, or B100 (pure biodiesel), getting the proper blend is essential for engine performance, fuel taxation, regulatory compliance, and emissions control.
Biodiesel blend analysis
Combined with the quality checks for methanol, moisture, and oxidative stability, this last analytical step ensures that only fully compliant and high-quality biodiesel reaches the end user. By checking all critical steps of biodiesel production, producers protect their products, their customers, and the environment.
- Engine compatibility: guarantee that the right mixture will be destined for the right bio/diesel engine.
- Fuel taxation: many countries regulate and subsidize blends based on the declared percentage of biodiesel. Incorrect blend ratios can lead to legal penalties, loss of tax benefits, etc.
- Environmental impact: higher biodiesel content means lower CO₂ and lower particulate emissions compared to diesel.
- Non-compliant mixtures: verifying the biodiesel purity helps prevent adulterated samples and maintain consumer and regulatory trust.
Conclusion
From the moment raw materials arrive at a biodiesel facility to the final fuel blend delivered to market, near-infrared spectroscopy proves to be both the most flexible and the best method for biodiesel analysis.
At the raw material stage, NIRS enables rapid identification of oils—distinguishing between types (e.g., soybean, canola, used cooking oil) and checking important quality parameters of the oils. This ensures that only suitable feedstocks enter the process, protecting product quality from the start.
During the transesterification reaction, NIRS can continuously monitor the chemical transformation, detecting unreacted triglycerides, methanol, or water. With this information, operators can more accurately identify the reaction endpoint. This avoids costly reprocessing and reduces reagent use and waste, boosting process efficiency.
In final product quality control, NIRS replaces other biodiesel analysis methods by measuring parameters like moisture, oxidative stability, and residual methanol content. Results can be compared directly with ASTM D7467 and EN 14214, ensuring that only compliant batches are released. Ultimately, biodiesel and diesel blends can also be verified, and adulteration can be detected to help the biodiesel supply chain.
By consolidating all these analyses with a single, nondestructive, and reagent-free technique, NIR spectroscopy empowers biodiesel producers to streamline operations, cut costs, and respond quickly to quality issues.
Metrohm offers the OMNIS NIR Analyzer for easier, faster, and more efficient biodiesel analysis. For laboratories with high demand, the OMNIS Sample Robot NIR (Figure 9) provides automated sample handling and unattended operation, increasing throughput while reducing manual work. A single software platform controls the complete workflow, simplifying analysis, data processing, and data management.
Biodiesel is just one example of how NIR spectroscopy can support fuel analysis. To learn more about the use of NIR technology for gasoline, diesel, and other fuel products, explore our fuels industry video.
References
[1] Guns, W. NIR Spectroscopy in the Petrochemical and Refinery Industry: The ASTM Compliant Tool for QC and Product Screening – Part 2. Analyze This – The Metrohm Blog, 2021.
[2] Atabani, A. E.; Silitonga, A. S.; Badruddin, I. A.; Mahlia, T. M. I.; Masjuki, H. H.; Mekhilef, S. A Comprehensive Review on Biodiesel as an Alternative Energy Resource and Its Characteristics. Renewable and Sustainable Energy Reviews 2012, 16 (4), 2070–2093. DOI:10.1016/j.rser.2012.01.003
[3] Santori, G.; Di Nicola, G.; Moglie, M.; Polonara, F. A Review Analyzing the Industrial Biodiesel Production Practice Starting from Vegetable Oil Refining. Applied Energy 2012, 92, 109–132. DOI:10.1016/j.apenergy.2011.10.031
[4] Canha, N.; Felizardo, P.; Menezes, J. C.; Joana Neiva Correia, M. Multivariate near Infrared Spectroscopy Models for Predicting the Oxidative Stability of Biodiesel: Effect of Antioxidants Addition. Fuel 2012, 97, 352–357. DOI:10.1016/j.fuel.2012.02.017
Additional resources
Blog post: How to determine if your edible oils are rancid
Application Note: Biodiesel content in diesel with near-infrared spectroscopy
Application Note: Iodine value, FFA, refractive index, and fatty acid composition in edible oils
Application Note: Multiparameter Quality Control of Palm Oil with NIR Spectroscopy
