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Near-infrared spectroscopy (also known as near-infrared spectroscopy or NIRS) is a spectroscopy technique that focuses on the near-infrared region of the electromagnetic spectrum-780 to 2500 nm.
Video source: Wantong Middle East FZC
Near-infrared spectroscopy, mid-infrared spectroscopy (mainly FT-IR) and Raman spectroscopy together belong to vibrational spectroscopy technology.
The first near-infrared spectrometer was developed in the 1980s and was originally designed for industrial applications and chemical analysis. There are two main forms of NIR analyzers: dispersive NIR (scanning) and Fourier transform-based (FT-NIR) spectrometers.
Dispersive spectrometers equipped with monochromators and holographic gratings have many key advantages.
These instruments combine visible light (Vis) and near infrared (NIR) spectroscopy, which means they can perform qualitative (identification and identification) and quantitative analysis of a range of parameters in a single run.
Near-infrared spectroscopy is very common in quality control and quality assurance processes, and its main goal is to accurately determine the chemical and physical properties of samples.
The use of NIRS has benefits for many industries; for example, chemical production, cosmetics, pulp and paper, pharmaceuticals, petrochemicals, polymer production, and palm oil.
NIRS direct multi-parameter analysis of the key parameters required for effective monitoring of hydrocarbon refining is usually also suitable for quality control applications using jet fuel, crude oil or diesel blends.
Image source: Wantong Middle East FZC
Since NIRS is a non-destructive secondary analysis method, it is particularly useful for processing potentially hazardous or valuable samples found in the petrochemical and fuel industries.
NIRS is an ideal method for monitoring crude oil distillation; for example, density, API gravity, Reed vapor pressure (RVP), PIANO measurement, and RON/MON/hexadecane index.
It can also be used to treat lubricating oils, especially when monitoring aromatics content, PAH, pour point, distillation analysis, viscosity, density, ignition point or oil content. When used with diesel blends, NIRS can be used to monitor density, FAME, color, cetan index, cloud point, flash point, etc.
Near-infrared spectroscopy has a wide range of advantages, especially when compared to traditional wet chemical techniques.
NIRS is a fast technology that can provide accurate results within one minute. It does not require the use of potentially toxic reagents or solvents, which means that compared to many contemporary technologies, it is a greener and more environmentally friendly technology.
NIRS is also non-destructive, because the sample is not chemically changed, damaged or destroyed during the analysis, so it can be used again.
The NIR spectrometer is a user-friendly instrument, suitable for non-professional operation. The user does not need to have an in-depth understanding of chemical reactions or chemical analysis, and because the user is familiar with the technology for a short time, the staff training speed is very fast.
NIRS provides multi-parameter capabilities, and because multiple parameters can be determined in a single measurement, many NIR users report significant cost and time savings compared to traditional wet chemical techniques.
The break-even point of NIR spectrometer investment is usually reached very quickly, and return on investment (ROI) calculations usually confirm positive results.
Image source: Wantong Middle East FZC
In addition to the scope of the above-mentioned applications and methods, the measurement of water content is still one of the most commonly used methods in laboratories around the world. Moisture determination is essential in almost every industry, for example, when handling food and feed, lubricants or pharmaceuticals.
The moisture content of food and feed must be strictly controlled so that the food does not taste or become dry. It is also important to ensure that the humid environment does not provide a breeding ground for bacteria and fungi, which can lead to spoilage.
It is important to determine the moisture concentration in the lubricant, because too much moisture will accelerate the wear of the machine.
In the pharmaceutical industry, the moisture content of solid dosage forms (tablets) and freeze-dried products must be closely monitored. Regulations stipulate that the moisture content of freeze-dried products should be kept below 2%.
Figure 1. Water droplets on a spider web. Image source: Wantong Middle East FZC
Karl Fischer (KF) titration was first introduced in the 1930s and is still one of the most popular and respected moisture determination methods. The KF titration method is a fast and highly selective method, which means that only water can be measured.
Figure 2. Manual injection of volume KF titration. Image source: Wantong Middle East FZC
KF titration is based on two redox reactions. The first reaction involves the reaction of methanol and sulfur dioxide to form the corresponding ester. By adding iodine, the ester is oxidized to sulfate species in the water consumption reaction. The reaction will end when there is no water.
KF titration is suitable for the determination of moisture content in liquid, solid, slurry and even gas samples. When the concentration is between 0.1% and 100%, volumetric KF titration is the most suitable method, and when working with a lower moisture content between 0.001% and 1%, it is recommended to use Coulometric KF titration.
The sample can be added directly to the titration vessel, or it may need to be dissolved in a suitable solvent beforehand, depending on the type of sample, its water content, and its potential solubility in the KF reagent.
Figure 3. Fully automatic KF titration using Metrohm 874 KF oven sample processor. Image source: Wantong Middle East FZC
The solvent cannot react with the KF reagent, so aldehydes and ketones are not suitable. If the sample is dissolved in a solvent, a pure solvent should be used for blank calibration.
The measurement process involves injecting the sample directly into the titration vessel using a syringe and needle. Use a polarized double Pt needle electrode to detect the end point, and directly calculate the water concentration based on this value.
Gas extraction technology and KF oven can be used to analyze insoluble or hygroscopic samples. The process involves sealing the sample in a vial, heating to evaporate the water, and then bringing the water into the titration cell.
Since the 1980s, near infrared spectroscopy (NIRS) has been widely used in food and feed, polymers and textiles, and later other industries such as pharmaceuticals, personal care and petroleum products have also adopted this technology.
NIRS can be used to detect overtones and combined bands of molecular vibrations. Typical vibrations of functional groups in organic molecules include -CH, -NH, -SH and -OH, among which the -OH part is a particularly strong near-infrared absorber.
Therefore, NIRS plays a central role in the application of moisture quantification.
Near-infrared spectroscopy is suitable for the quantification of water in solids, liquids and slurries. The detection limit of water in solids is about 0.1%, while the detection limit of water in liquids is in the range of 0.02% (200 mg/L).
In some cases, such as water in THF, the moisture detection limit of 40-50 mg/L can be reached.
NIRS does not require sample preparation, which means the sample can be used as is. Solid samples are usually measured in high-quality disposable sample bottles, while liquids are measured in high-quality disposable cuvettes.
These different samples are placed on the analyzer accordingly so that measurements can be taken.
Figure 4: Solid (top) and liquid (bottom) sample positioning for NIR measurement. Image source: Wantong Middle East FZC
Since NIRS is an auxiliary technology, it can only be used for routine moisture quantification after the development of a predictive model. This process can be understood as similar to HPLC, which requires the initial creation of a calibration curve to establish a measurement standard. The same principle applies to NIRS: Before creating a predictive model, a spectrum with a known moisture content must first be measured.
In order to create a calibration set, an average of 30-50 samples must be measured using NIRS and KF titration, and the value obtained by KF titration must then be linked to the NIR spectrum.
Figure 5. NIR method implementation workflow for moisture analysis. Image source: Wantong Middle East FZC
Then complete the model development and validation steps, which are usually simple for moisture analysis. Water is a strong NIR absorber, with peaks usually around 1900-2000 nm (combined band) and 1400-1550 nm (first overtone).
Once the predictive model is created and validated, NIRS can be used for routine moisture determination of the substance. The moisture content determination result will be obtained within one minute, without any sample preparation or chemical use.
Figure 6. The NIR spectrum of a moisturizing cream showing the absorption of H2O at 1400-1550 nm and 1900-2000 nm. Image source: Wantong Middle East FZC
This also means that the analyst performing the measurement does not need to be a chemist—all they need to do is place the sample on the instrument and press the start button.
Both KF titration and NIR spectroscopy have obvious advantages. KF titration is a universal method that provides low-level detection. Its main advantage is that it works consistently, regardless of whether the sample type in question is measured regularly or encountered for the first time.
Table 1. Summary of the characteristics of moisture determination by titration and NIR spectroscopy. Source: Wantong Middle East FZC
NIR spectroscopy does require a method development process, so it is not suitable for commonly varying sample types, such as different types of oils or tablets.
NIRS is an excellent method for always the same sample type, for example, when determining the moisture content of freeze-dried products or chemicals (such as fertilizers).
When implementing the NIR moisture method, you must first use the KF titration method as the main method for model development to measure samples.
During the routine use of the NIR method, it is recommended to continue to use KF titration to periodically confirm (for example, every 50 or 100th sample) that the NIR model is still robust and the error rate has not increased.
If you notice a change in the error rate, you should add additional samples to the predictive model to cover the observed sample change.
In general, both KF titration and NIR spectroscopy are effective techniques that are very suitable for measuring moisture in a range of samples. The best technology to use depends entirely on the application in question and the personal preference of the user.
Metrohm offers a comprehensive NIRS instrument portfolio suitable for laboratory and process analysis.
The Metrohm NIRS laboratory analyzer can perform routine analysis quickly and confidently, without the need for additional reagents or sample preparation.
By effectively combining visible light (Vis) and near infrared (NIR) spectroscopy, these analyzers can perform qualitative analysis of a range of materials and quantitative analysis of a range of physical and chemical parameters in a single run.
Metrohm's compact DS2500 analyzer is very suitable for fast QC in a range of industries, including chemicals, pharmaceuticals, petrochemicals, polymers, polyols, palm oil, personal care, and pulp and paper.
These instruments can also meet many key industry standards, including ASTM E1655, ASTM D6342, ASTM D6122, USP 856 and USP 1856, and ASTM D8340-20 (standard specification for performance-based spectrometer system certification).
Vision Air is a powerful software package that can be customized according to customer specific requirements. The software's intuitive workflow and modern interface simplify daily operations, while providing powerful monitoring and management functions that are very suitable for laboratory managers and supervisors.
Measurements are pre-configured and can be started with the push of a button, and extensive monitoring and filtering functions ensure that users can easily locate and check results.
Provides a series of ready-to-use calibrations to simplify applications in the petroleum fuel, palm oil, polyol and polymer industries. Calibration includes a series of predefined parameters such as RON/MON, hydroxyl value, viscosity, FFA, density and melt flow rate.
The traceable NIST standard is used for reference calibration, which means that Metrohm's Vis-NIR spectrometer can produce comparable results wherever it is used. The calibration model can also be transferred between analyzers as needed.
Metrohm's spectrometer can be used for comprehensive analysis of liquids and solids, and the instrument can perform automatic analysis when needed.
Image source: Wantong Middle East FZC
Metrohm Middle East FZC (MME), located in Sharjah, UAE, is the regional support center of Metrohm AG (Switzerland), responsible for the sales, service and calibration of laboratory and process analysis instruments from the following countries/regions –
UAE, Saudi Arabia, Kuwait, Bahrain, Oman, Qatar, Egypt, Jordan, Lebanon, Iraq, Bangladesh, Pakistan, Sri Lanka, Ethiopia, Ghana, Sudan, Syria, Yemen, Somalia, Iran, Cyprus, Malta, Eritrea, Djibouti and Afghanistan.
This information is derived from materials provided by Metrohm Middle East FZC, and has been reviewed and adapted.
For more information about this source, please visit Metrohm Middle East FZC.
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