Dispersive NIR Spectrometers

Theory of Operation

Advantages in Process Spectroscopy

Dispersive NIR Spectrometers

Theory of Operation

The electromagnetic spectrum between 190 nm to 25,000 nm

For applications involving “clear” liquids or gases, a dispersive NIR spectrometer (DG-NIR) is the best choice. Our engineers and scientists have advanced dispersive NIR technology by developing analyzers with dual-beam, post-dispersive planar gratings, which improve accuracy and resolution without sacrificing performance.

These improvements enable the DG-NIR analyzer to control light effectively, thereby reducing aberrations and providing superior performance. As a result, the system offers excellent signal-to-noise ratios, long-term stability, built-in multiplexing, and easy maintenance.

Advantages of Dispersive Spectrometers Over FT-NIR Analyzers

Spectroscopy studies how light interacts with matter, with light divided into various regions, including NIR and UV-VIS. Both NIR and UV-VIS spectrometers provide operators with valuable real-time data about chemical and physical properties, enabling faster analysis compared to traditional methods. Fiber-optic-based spectrometers offer rapid, detailed measurements in under a minute, replacing hours of laboratory analysis.

While FT-IR spectrometers excel in the energy-limited infrared region, they do not offer significant benefits in the NIR range. As such, DG-NIR spectrometers are often more accurate, efficient, and cost-effective compared to FT-NIR analyzers.

FT-NIR Misconceptions and Facts

STATEMENT	FACTS
FALSE: FT-NIR is a newer technology	The fundamental technology of FT systems and dispersive analyzers were both developed in the 1800s.(Michelson interferometer – 1887, Henry Joseph Grayson grating ruling engine – 1899). Both technologies became feasible for process applications with the development modern telecom fibers and detectors, high quality optics, and the advent of the PC. Both use the same high quality optics, detectors, fibers, and light sources.
FALSE: FT-NIR has easier calibration transfer	Both FT-NIR Systems and DG-NIR Analyzers can directly transfer calibrations between channels. The method of light dispersion is not relevant to the success of calibration transfer. Instrument-to-instrument repeatability in terms of the fundamental characteristics (bandwidth, stray light, wavelength axis accuracy) are key in successful calibration transfer. FT-NIR will use their laser source to maintain wavelength accuracy, while DG-NIR instruments use temperature compensated filters with NIST traceability.
FALSE: FT-NIR has lower error in calibrations due to better wavelength resolution	In the near infrared region the small increase in resolution by FT-NIR does not translate into lower error calibrations. (Armstrong, 2006 Applied Engineering in Agriculture. 22. DOI:10.13031/2013.20448)
TRUE Somewhat: Fellgett Advantage – scan time	FT-NIR measure all wavelengths simultaneously while scanning grating systems measure one wavelength at a time. This theoretically gives the FT-NIR a “multiplexed” advantage which improves the SNR. However, since all of the light falls on the FT detector, it is often driven non-linear, and the light must be attenuated. The reality is that grating spectrometers can have a SNR that is equal to or superior to a comparable FT-NIR system.
FALSE: Jacquinot Advantage – higher light throughput	If the FT-NIR system is configured to measure through a fiber optic cable, then the aperture or throughput of light is limited by the diameter of the fiber optic cable which is essentially the same for both types of instruments. This eliminates any potential advantage for FT-NIR online process monitoring.
FALSE: Connes Advantage – wavelength accuracy	FT-NIR systems use a single HeNe laser to verify the wavelength accuracy. FT-NIR wavelength accuracy depends on the precision of the alignment between the laser beam and the white light source and the number of zero crossings measured of the laser fringe, i.e., the resolution at which the spectrum is recorded. Dispersive analyzers use NIST traceable standards to check the accuracy of multiple points along the wavelength range. The wavelength accuracy is limited by the precision of the NIST standards and the reproducibility of the grating drive mechanism. (Armstrong, 2006 Applied Engineering in Agriculture. 22 DOI: 10.13031/2013.20448)

Analyzer Validation

For effective process monitoring, it’s crucial to continually assess the system’s accuracy and precision to ensure the analyzer provides validated spectra for your application.

FT-NIR analyzers often rely on expensive external fluids like Pentane and Toluene for validation. Toluene must be spectroscopic-grade, while Pentane serves as a wash fluid. Validation can be manual or automated but adds complexity, requiring additional plumbing and reducing analyzer uptime.

In contrast, GUIDED WAVE™ DG-NIR analyzers simplify validation. By using the optional Stability Monitoring System (SMS), the system provides continuous, automatic validation according to ASTM standards without interrupting normal operation or requiring consumables or maintenance.

Maintenance Considerations

When choosing an instrument, ongoing costs and ease of use are key factors. Both FT-NIR and DG-NIR use tungsten-halogen lamps and InGaAs detectors.

For DG-NIR, the lamp typically needs replacing every six months. This is the only consumable, and the replacement process is quick and easy, requiring no special training.

In FT-NIR systems, both the lamp and the laser need periodic replacement. The laser requires precise alignment to the white light beam, so replacing it typically requires a factory-trained technician.

Multiplexing Capability

FT-NIR spectrometers can support multiple channels but often require fiber multiplexers with moving optical elements, which introduce noise and limit the number of channels. Another method, stream switching, uses motor-operated valves and is slow and high-maintenance.

In contrast, DG-NIR analyzers feature built-in multiplexing without moving parts, preserving signal quality. A 12-channel DG-NIR system can switch between samples in seconds.

Moving Parts in Process Spectrometers

Process analyzers are expected to operate continuously with minimal maintenance, so moving parts are generally avoided.

Both FT-NIR and scanning grating spectrometers have critical moving parts. FT-NIR uses oscillating mirrors to encode the spectrum. If these mirrors fail or misalign, faulty results can occur. Similarly, scanning grating spectrometers rely on rotating gratings and precise optical encoders. If these mechanisms fail, inaccurate spectra can result. However, both systems are generally reliable and can provide years of trouble-free service.

Dispersive NIR Spectrometers

Theory of Operation

Advantages in Process Spectroscopy