What is NIR?
The near-infrared region lies between the visible and mid-infrared red light regions. As defined by the ASTM, the wavelength range of the near-infrared spectral region is between 780–2526nm. Moreover, the area is divided into two parts:
- Near-infrared short wave. (780–1100nm)
- Near-infrared long wave. (1100–2526nm)
Working rule of Near-Infrared Spectrometer:
The Near-Infrared Spectra contains extensive information about the structure and compositional features, making it the perfect choice for hydrocarbon organic matter’s composition and property measurement. The near-infrared region is produced when the molecular vibrations shift from the ground to the higher energy state. The working principle of a near-infrared spectrometer is based on the mutual relationship between the spectrum and parameters to be measured. Therefore, the necessary quality parameters are also measured through the spectrum and the mutual relationship if the sample spectrum is measured. However, the NIR exhibits poor absorption intensity and sensitivity followed by wide and overlapped absorption bands, making quantitative analysis by traditional methods untrustworthy.
Advantages and disadvantages of near-infrared spectrometer technology
- It allows non-destructive testing.
- The fast analysis allows sample collection in less than one minute.
- No extensive or rigorous training of the staff is required.
- It’s safe for the environment as it does not promote pretreatment of the sample from analysis to testing.
- Multiple qualitative and quantitative studies can be carried out simultaneously because of the high resolution.
- It is ideal for sample analysis in production processes and hostile settings.Near-infrared optical fibers are readily available, online analysis and monitoring are simple.
- It’s simple and convenient. Multiple sampling devices measure samples directly.
- Not ideal for trace analysis.
- Establishing a related model library is a prerequisite, making it an indirect method.
What You Need to Know Before Using NIR Spectroscopy
- The temperature of the laboratory should be maintained at 15–30°C with relative humidity under 65%. A voltage stabilizer and grounding wire should be attached to the power supply. A dehumidifier in the laboratory room is a must, and because the relative humidity is maintained under strict conditions, the room should not be huge. Only spacious enough for necessary equipment.
- If the single beam Fourier transform Infrared Spectrophotometer is employed, the carbon dioxide content should be strictly monitored and maintained at the cut-off value.
- If the testing sample belongs to the hydrochloride class, potassium chloride should be used instead of potassium bromide for pressing due to the ion exchange phenomenon.
- The infrared laboratory must be kept dry to prevent moisture from affecting the instruments’ service life. If laboratory equipment is not used, the spectrometer must be used twice a week for half a day. For dehumidification, the dehumidifier must be turned on.
- The potassium bromide tableting method is a typical sample preparation procedure for infrared spectroscopy. The primary choice for reagent should be KBr to minimize the effect on the measurement. Before using, it should be well ground, dried at 120°C for at least 4 hours, and stored in a desiccator. In case of lumpiness, they should be dried again.
- The sample amount is usually 1–2mg. Adding it after weighing on a weighing balance is technically impossible, and the consistency of absorption of infrared light is also questionable; therefore, experience outshines everything else. Most absorption peaks are in the range of 10–80% transmittance. In the case of large transmittance of the strongest absorption peak, the sample sent for testing was of inadequate amount. On the other hand, the most substantial absorption peak is close to 0 percent transmittance and is a flat-top peak; the sampling amount is too tiny. It should be re-measured after adjusting the sampling amount if it is too much.
- The sample should be free from moisture. If not, drying under infrared light for a few minutes is preferred. Once the sample is fully ground and installed in the mould, the vacuum pump should be attached and evacuated for 2 minutes. The pressure should be 0.8–1Gpa for 2–5 minutes. If vacuuming is skipped, the transparency of the film will be affected.
- KBr dose for tableting is set at 200mg. The amount should be according to the thickness of the tablet after tableting. Ideally, the thickness must be less than 0.5mm.
- All parts must be washed to remove traces and then dried and placed in a desiccator
- For tableting, the test product should be crushed, followed by adding Kbr, and then crushed again. The agate mortar offers the perfect grinding as the inner surface of the glass mortar is rough, which makes sample adherence easy. The force should always be in the same direction and must be controlled. Excessive force is not required. The goal is to make the particles so small that they are no longer visible to the naked eye. After grinding, the sample should be shifted to the tableting mould and spread evenly. Transparency of the sample influences the determination
Common NIR Spectrometers
- Filter-type miniature near-infrared spectrometer
Filter-type miniature near-infrared spectrometers can be divided based on filter type into:
- Tunable filter-based spectrometers: It’s a Fabry-Pere interferometer, and these contain upper and lower mirrors along with a resonator.
- Liner filter-based spectrometers: It splits the light instantaneously by varying the thickness of the conical cavity layer. The essential components include tunable Fabry-Perot filters and linear gradient filters.
- Dispersive miniature near-infrared spectrometer:
The dispersive micro-NIR spectrometer works by separating light using gratings, including dispersing by altering the grating’s angle. It captures information in different wavelength regions of the spectrum using the unit detector. Array detection type miniature near-infrared spectrometer is used for spectral information of the band. The scanning grating qualifies as the core element of the raster scanning micro-spectrometer. Once the incident light passes and collimates with the optical fibre, the angle changes due to scanning grating dispersing the light for imaging and passing through the slit. The single-tube detector collects it.
- Modulated miniature near-infrared spectrometer
Fourier Transform Miniature Near-Infrared Spectrometer
Fourier transform and light interference principle are the basis of Fourier Transform Miniature Near-Infrared Spectrometer. The core elements include a layered grating interferometer and optical path difference sequence, which is obtained by changes in the spatial position of the grating. It helps analyze the spatial modulation of the interference signal. The core component is the Michelson interferometer. The driver controls the spatial motion of the mirror to achieve the interferometric modulation pattern. A single-tube detector detects interferometric light, and the spectrum is derived using the Fourier transform.
- Other miniature near-infrared spectrometers
Miniature near-infrared spectrometer based on MOEMS and MEMS technology
Micro-Opto-Electro-Mechanical Systems (MOEMS) and Micro-Electro-Mechanical Systems (MEMS) work on the semi-conductor micromachining technology. It evaluates the miniaturization of devices. The micro-spectrometers developed on this particular technology offer accurate measurements, fast scanning, cost-effectivity, and high sensitive research hotspot
- New material miniature near-infrared spectrometer
There are some miniature spectrometers produced using new materials in addition to the above several famous miniature near-infrared spectrometers.
How to choose a near-infrared micro-fibre spectrometer?
- Optical resolution: A micro-spectrometer with a multi-pixel detector is often the user’s choice working with an integrated near-infrared micro fibre spectrometer. Several detectors generally evaluate the optical resolution and are associated with grid line density and slit width. One way to express resolution is through full width at half maximum. Using this technique, the optical performance of multiple spectrometers can be compared.
- Sensitivity: Sensitivity is barely associated with order as current mainstream microfiber spectrometers use linear array detectors. The area array detector is treated as an exception as each area array detector present in the vertical direction will be accumulated. Therefore, all cumulative accumulations can be looked at as relative scales. Analyzing the response curve of the detector when considering the sensitive requirements of a specific application is essential.
- Signal-to-noise ratio: A signal-to-noise ratio exists for CCD photometers. An average of the spectrum can be taken multiple times to optimize the signal-to-noise ratio. Specific applications require higher signal-to-noise ratios which can be achieved by comparing the overall signal-to-noise ratio of the optical table and detector. There’s no practical use for a sensor with a high signal-to-noise ratio if it has a low-performance optical path. Taking 100 measurements and calculating each pixel’s mean and standard deviation is a better technique to compare the signal-to-noise ratio of different detectors and NIR microfiber spectrometers.
- Grid selection: The grating selection is based on:
- Expansion of the wavelength range
- Optical resolution.
Additionally, the grating system can influence the system’s sensitivity due to the speed of wavelength, optical resolution, and complex splitting.
- Gap: An inverse relationship exists between the gap and optical resolution. If the gap is small, the optical illusion will be higher and vice versa.
- Miscellaneous: Upgrading the UV4 detector means replacing the standard BK7 window with a quartz window. For UV wavelengths below 340nm, enhanced ocean optics microfiber spectrometers are employed. A long-pass filter is placed between the SMA905 connector slit and delcaated mode hole to avoid second and third-order diffraction effects. The OFLV instalment can eliminate higher-order diffraction filtering devices.