Modern near-infrared spectrometer from the spectral system can be divided into fixed wavelength filter, grating dispersion, fast Fourier transform and acousto-optic tunable filter (AOTF) four types. Grating dispersion type instruments according to the use of detector differences are divided into two types of scanning and fixed optical path.
In various types of instruments, the filter type is mainly used as a special analyzer, in order to improve the accuracy of the results, the filter type instruments are often equipped with multiple filters for users to choose.
Raster scanning type is the most commonly used type of instrument, using holographic grating spectroscopy, PbS or other photosensitive elements as a detector, with a high signal-to-noise ratio. Due to the moving parts (e.g. grating axis) in the instrument may have wear problems in continuous high-intensity operation, thus affecting the reliability of spectral acquisition, less suitable for online analysis.
Fourier transform near infrared spectroscopy is currently the leading product of near infrared spectroscopy, 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.
AOTF is a new class of spectroscopic devices appeared in the early 90's, the use of birefringent crystals, by changing the radio frequency to adjust the wavelength of the scan, the entire instrumentation system has no moving parts, the scanning speed is fast, and has a better stability of the instrument, especially suitable for online analysis [ 6 ~ 8]. However, the resolution of this type of instrument is relatively low, and the price of AOTF is also high.
With the increasing maturity of multi-channel detection device production technology, the use of fixed optical path, grating spectroscopy, multi-channel detector composition of the NIR instrument, with its stable performance, fast scanning speed, high resolution, performance and price ratio of good features are increasingly attracting people's attention. In the fixed optical path to match the multi-channel detector, commonly used diode array (Photodio de-array referred to as PDA) and charge-coupled devices (Charge e Coupled Devices referred to as CCD) two types.
Figure 2 shows the structure of the near-infrared spectrometer schematic.
In the development of new near-infrared spectrometer, improve the performance of the instrument at the same time, in order to be suitable for the analysis of various types of samples, the development of near-infrared spectroscopy sampling device has also attracted more and more attention. In various types of sampling devices, the most notable is the development of various fiber optic sampling devices. Through the fiber optic sampling device, on the one hand, it can facilitate the sampling process, on the other hand, it can make use of the long-distance transmission characteristics of optical fiber, and use the near-infrared spectroscopy technology for online analysis.