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Near-infrared Spectrometer

Near-infrared spectrometer used in laboratory

What is near-infrared spectrometer?

Near-infrared spectrometer is an analytical instrument made by the principle of selective absorption of infrared rays by gases or liquids, which has the characteristics of high sensitivity, fast response time, wide analytical range, good selectivity and strong anti-interference ability, etc. It is widely used in petrochemical metallurgy and other industrial production.

What is NIR?

Near-infrared spectroscopy (NIR) is one of the fastest growing and most impressive analytical techniques since the 1990s. With the further application and development of NIR analysis methods, it has gradually gained universal acceptance. In recent years, with the development of chemometrics, optical fiber, and computer technology, online NIR spectroscopy is being applied to many fields including agriculture and animal husbandry, food, chemical, petrochemical, pharmaceutical, tobacco, etc. at an astonishing speed, providing a very wide use space for scientific research, teaching and production process control.

Near Infrared (NIR) is an electromagnetic wave between visible light (VIS) and mid-infrared light (MIR). The wavelength range of the NIR spectral region defined by ASTM is 780~2526nm (12820~3959cm-1). NIR spectra are generated when molecular vibrations jump from the ground state to higher energy levels due to the non-resonant nature of molecular vibrations and record the multiplicity and ensemble absorption of X-H (X = C, N, O) vibrations of hydrogen-containing groups. The NIR absorption wavelengths and intensities of different groups (e.g. methyl, methylene, benzene ring) or the same group in different chemical environments are significantly different. NIR spectra are rich in structural and compositional information and are well suited for the measurement of the composition and properties of hydrocarbon organic substances. However, in the NIR region, the absorption intensity is weak, the sensitivity is relatively low, and the absorption bands are wide and heavily overlapped. Therefore, it is very difficult to rely on the traditional method of establishing a working curve for quantitative analysis, however, the development of chemometrics has laid the mathematical foundation for the solution of this problem. It works on the principle that if the composition of a sample is the same, its spectrum is also the same, and vice versa. If the correspondence between the spectrum and the parameter to be measured is established (called an analytical model), then the desired data on the quality parameters can be quickly obtained by measuring the spectrum of the sample and the above correspondence. The analytical method consists of two processes: calibration and prediction.

In the calibration process, a certain number of representative samples are collected (generally more than 80 samples are required). While measuring their spectrograms, measurements are made using relevant standard analytical methods as needed to obtain various quality parameters of the samples, called reference data. The spectra are processed by chemometrics and correlated with the reference data so that a one-to-one correspondence mapping is established between the spectrograms and their reference data, which is usually called a model. Although the number of samples used for model building is limited, the models obtained by chemometric processing should be highly generalizable. The calibration methods used for model building vary depending on the relationship between the sample spectra and the properties to be analyzed. Commonly used methods include multiple linear regression, principal component regression, partial least squares, artificial neural networks, and topological methods. Obviously, the wider the range to which the model applies, the better. However, the size of the range of the model is related to the calibration method used to build the model, to the property data to be measured, and also to the range of analytical accuracy required to be achieved by the measurement. In practice, model building is achieved with chemometric software and has strict specifications.

In the prediction process, the spectrogram of the sample to be measured is first measured using a near-infrared spectrometer, and then the model library is automatically searched by the software to select the correct model to calculate the mass parameters to be measured.

The main sources of detection errors in NIR spectrometers are as follows.

a. The number of samples used to establish the calibration equation and the test equation.

b. The particle size and distribution of the sample.

c. The testing environment and the temperature of the sample.

d. The error due to chemical analysis.

Feature of near infrared spectrometer

a. Direct measurement of samples without pretreatment

Near-infrared spectroscopy is available in transmission, reflection, and diffuse reflection formats and is suitable for measuring samples in liquid, solid, and slurry forms, making it very versatile. The biggest advantage is that no sample pretreatment is required, such as gasoline can be directly poured into the measuring cup or the fiber optic probe can be directly inserted into the gasoline for measurement, the operation is very convenient and the spectrum scan is completed within seconds.

b. Fiber optic long-distance measurement

NIR light can be transmitted over long distances via fiber optics, allowing for long-distance measurements beyond the spectrometer. Measurement probes or flow cells can be installed directly into the pipeline of the production unit for online measurements or on-site measurements in harsh and dangerous environments.

One online NIR spectrometer can be connected to multiple (2 to 10) fiber optic circuits for simultaneous online measurement of materials at multiple measurement points in a production plant. The online measurement data can be directly transferred to a DCS or advanced control system to provide timely oil quality parameters for production optimization. Compared to the parameters provided by other online measurement instruments (e.g. variables such as pressure, flow, and temperature), the data provided by online NIR analysis (e.g. composition or properties) are direct quality parameters, providing more accurate and useful reference information for production optimization. NIR analysis, when used in conjunction with conventional standard analytical methods, plays a complementary role, not only providing analytical data to the production control department in a timely manner, but also saving a lot of analytical and laboratory costs (including manpower, equipment, and reagents, etc.); online NIR analysis is connected to the DCS, providing data directly to the control system, and the economic benefits of production optimization are enormous; compared to other online Compared with other online instruments, the failure rate and consumption of NIR spectrometer are very low.

Type of near infrared spectrometer

NIR spectrometer can be divided into five types according to the spectroscopic system: fixed wavelength filter, grating dispersion, fast Fourier transform, acousto-optical adjustable filter, and array detection.

The filter type is mainly used as a special analytical instrument, such as a grain moisture meter. Due to the limited number of filters, it is difficult to analyze samples of complex systems.

The grating scanning type has a high signal-to-noise ratio and resolution. It is not suitable for online analysis because the movable parts of the instrument (e.g. grating axis) may have wear problems during continuous high-intensity operation, which affects the reliability of spectral acquisition.

FT-NIR spectrometer with high resolution and scanning speed, the weakness of this type of instrument is also the presence of moving parts in the interferometer and the need for a more stringent working environment.

The acousto-optic tunable filter uses a birefringent crystal to adjust the wavelength of the scan by changing the RF frequency, and the whole instrument system has no moving parts and a fast scanning speed. However, the resolution of such instruments is relatively low and the price is high.

With the increasing maturity of the array detection device production technology, the use of the fixed optical path, grating spectroscopy, array detector NIR instruments, with its stable performance, fast scanning speed, high resolution, high signal-to-noise ratio, and good performance-to-price ratio, and other characteristics are attracting more and more attention.

Precaution of near infrared spectrometer

An important characteristic of NIR analysis technology is the set of technology itself, which means that the following three conditions must be available at the same time.

a. NIR spectrometer with a long-term stable performance of each item is the basic requirement to ensure good reproducibility of data.

b. Full-featured chemometric software, which is an essential tool for model building and analysis.

c. A model that is accurate and has a wide enough range of applications.

The combination of these three conditions can be truly useful for the user. Therefore, it is important to have sufficient knowledge of the usability of the model provided by the instrument when purchasing it.

The fast speed of NIR analysis is due to the fast speed of spectral measurements and the fast speed of computer calculation of the results. However, the efficiency of NIR analysis depends on the number of models the instrument is equipped with. For example, if a spectrogram is measured with only one model, only one data can be obtained. If 10 data models are built, then 10 analytical data can be obtained simultaneously from just one measured spectrum.

How to buy near-infrared spectrometer?

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