Dispersive NIR Spectrometer Advantages in Process Spectroscopy
Theory of Operation
When considering NIR for an application that involves a “clear” liquid or gas, a dispersive NIR Spectrometer (DG-NIR) is the superior choice. By developing NIR analyzers with dual-beam, post-dispersive planar gratings, our engineers and scientists have advanced the state-of-the-art in dispersive NIR technology.
By incorporating these advancements without compromising the dual-beam operation, we can offer a DG-NIR analyzer with superior accuracy and resolution. The DG-NIR advantage is due to the way these analyzers are designed to control light and minimize all forms of aberration. Our DG-NIR system has been carefully optimized to provide exceptional signal-to-noise ratio, excellent long-term photometric and wavelength stability, built-in multiplexing, and ease of maintenance.
Advantages of Dispersive Spectrometers over FT-NIR Analyzers
Spectroscopy is the study of how light and matter interacts. Light can be broken up into several regions as illustrated in the electromagnetic spectrum below.
Spectroscopy is perfectly suited for accurate real-time analysis and monitoring of continuous and batch processes. The near infrared (NIR), as well as the ultraviolet and visible (UV-VIS) regions of the spectrum provide a wealth of information about many chemical and physical properties to help operators maintain control of their processes. No other technology matches the acquisition speed and the range of measurements returned by a fiber optic-based spectrometer. A comprehensive set of data that takes hours of laboratory analyses to acquire can be available in less than a minute.
FT-NIR vs. Dispersive NIR Spectrometer
FT-IR (not NIR) spectrometers are definitely superior to IR grating spectrometers in the energy limited infrared region. However, the near infrared is not energy limited so many of the advantages of FT technology do not apply. This has led to many misconceptions or myths (listed below) associated with NIR spectrometer technologies.
In the NIR region, FT-NIR spectrometers offer no significant advantages over DG-NIR spectrometers, and many times are not as accurate, efficient or economical as DG-NIR multi-channel, dual-beam analyzers.
FT-NIR Misconceptions and Facts
Additional Considerations and Comparisons: Analyzer Validation
An important consideration for successful process monitoring is the ability to continually monitor the accuracy and precision of the system, thus ensuring the analyzer is producing validated spectra for your applications.
With FT-NIR analyzer validation is often done using external fluids (Pentane and Toluene) which is rarely available and is an expensive consumable. Pentane is a wash fluid. Spectroscopic Grade Toluene is required as the validation sample. Industrial grade Toluene cannot be used for this purpose. Validation can be automated or run manually but requires additional plumbing to inject the sample on the probe. Thus, validation reduces the analyzer up-time and adds complexity (potential failure points) to the sample handling system. With a GUIDED WAVE™ DG-NIR analyzer the validation system is simple and requires no maintenance or consumables. Using the optional Stability Monitoring System (SMS) in the analyzer, there is no need to interrupt the other channel operation. It provides automatic and continuous analyzer validation according to ASTM methodology.
Maintenance Considerations for Process Spectrometers
The ongoing costs and ease of use associated with any instrument is an important consideration. Both FT-NIR and DG-NIR use tungsten-halogen lamps as the light source and an InGaAs detector.
For DG-NIR the lamp replacement is typically every six months, and it is the only consumable needed. The replacement of this light source can be completed by any person in a matter of seconds, as lamps are all pre-aligned.
FT-NIRs also require that the lamp be periodically replaced. Furthermore, the laser has a finite lifetime and occasionally needs replacement. Replacing the laser not necessarily simple as its alignment to the white light beam from the lamp is critical. Thus laser replacement is often done by a factory trained service engineer.
Multiplexing Capability
FT-NIR spectrometers can be multiplexed (multi-channel operation) but to do so often requires fiber multiplexers with moving optical elements. It is not possible to move optical elements and not introduce some noise in the system. On-line spectroscopy often requires SNRs > 105 which exceeds the capability of moving optical element multiplexers. The cost of the additional hardware limits the number of channels to between 2 and 8.
An alternate method of multiplexing is stream switching. This involves an extractive sample system with motor operated valves and possible cross contamination in the sample cell. This is a slow, high maintenance approach.
Our DG-NIR analyzers have built in multiplexing with no moving optical elements. Thus, there is no degradation in the SNR. A twelve channel DG-NIR system can switch between samples in seconds.
Moving Parts are used in FT-NIR Process Spectrometers
Process analyzers are expected to operate 24/7 with minimal maintenance. Moving parts in an analyzer are therefore always looked on with suspicion. Fortunately, most modern spectrometers have long mean time between failure (MTBFs) on the order of years. Both FT-NIRs and scanning grating spectrometers have critical moving parts. FT-NIR spectrometers have one or two oscillating mirrors that provide the phase encoding of the spectrum. If these mirrors do not move smoothly or fall out of alignment, faulty spectral results can and do occur. Similarly, scanning grating spectrometer must rotate the grating precisely and measure that rotation with a precision optical encoder. Again, failure of the mechanism can result in bad spectra. However, the reliability of FT mirror mechanisms and grating drives are exceptional with years of trouble-free service expected from both.
MORE TOPICS
- Why you should use NIR spectroscopy
- NIR Benefits for Online Process Measurements
- How does the Flow Rate, Viscosity, and Pipe Diameter, Effect Accurate (good) NIR Measurements?
- What is pathlength and why is it important when selecting a sample interface?
- Double down on double beam
- Why NIR is better than GC
- Difference between FT-NIR and dispersive dual-beam (DG-NIR) process analyzers
- Benefits of FT-NIR for online process management
- How safety standards in explosive or incendive processes apply to our probes and flow cells
- What is Diffraction Grating?
- Chemometric Calibration Development
- Model Maintenance and Customizing a Starter Model to Meet Your Operational Needs