Modern Spectroscopy

However, in the late 20th century the electronics, optics, and other components inside a spectrometer system improved. Modern dispersive spectrometers use highly engineered gratings to separate the wavelengths of light and reduce stray light. A diffraction grating is essentially an aluminum-coated mirror with thousands of parallel and equally spaced grooves etched into its surface. As such, a grating is one of the most precise objects ever made. How gratings work is described in all good freshman level college physics textbooks.

In a peer-reviewed study, Coates found that both Dispersive Analyzers (DG-NIR) and Fourier Transform Spectrometers (FT-NIR) have equivalent performance (Coates, 1994 NIR news DOI:10.1255/nirn.250). This study shows that with modern technology many of the marketing claims by manufacturers of the advantageous of FT-NIR Spectrometers are no longer valid. How do you decide which spectrometer technology is the best choice for your analysis?

Is FT-NIR or DG-NIR a Better Choice For Process Spectroscopy?

FT-NIR is a powerful technique, especially in a laboratory setting where samples can be introduced to the spectrometer without using optical fiber. FT’s also work well for measuring samples in reflection such as powders or solids. However, when considering NIR for a process application that involves a “clear” liquid or gas using optical fibers, FT-NIR instruments lose many of their advantages.

The bandwidth of spectral features varies with the state of the sample. Gases and vapors have spectral features (lines) that are very narrow and often require high resolution to see. Liquids and solids have very broad spectral features (bands) due to the hindered rotation of the molecule in its matrix. Thus, for clear liquid hydrocarbon samples (the bulk of NIR applications), bandwidths range from 2 nm to 12 nm. Aqueous samples are even broader. Thus for condensed samples, the higher resolution of FT-NIR is not required nor desirable since high resolution always comes at the cost of signal-to-noise ratio (SNR). The latter property for chemometric analysis of complex hydrocarbon mixtures is more important for accurate analysis than resolution.

A scanning grating dual-beam (DG-NIR) spectrometer actually produces a superior signal-to-noise ratio (i.e., sensitivity for lower detection limits) and greater stability to withstand ambient air fluctuations. In addition, the dual-beam operation provides the added benefit of superior long-term stability required of process analyzers that must operate unattended for weeks. Dual-beam operation is easy to implement in a grating instrument but difficult in an FT-NIR analyzer. Most FT-NIR analyzers do not operate in dual-beam mode hence require more frequent referencing to compensate for photometric drift.

Advancements in Process Spectroscopy

Since Coates’ report in 1994, dispersive analyzers have taken on another level of advancement with the development of Guided Wave’s full spectrum post dispersive planar grating, dual-beam (DG-NIR) technology. These advancements applied to NIR process analyzers will ensure accuracy due to the reduction of stray light and excellent signal-to-noise ratio.

Post-Dispersed Design Lowers Black-Body Impact

Post-dispersed design means that ambient or black-body radiation will be dispersed like all other radiation, such that at any given wavelength, its impact will be much smaller.

Plane Grating Improves Efficiency

Most commercial grating spectrometers use concave holographic gratings because the optical systems are very simple. Concave gratings always introduce off-axis aberrations such as astigmatism and coma which rob light from the image. Holographic gratings are hard to blaze, thus not very bright at the angles of use, again robbing light from the image. Instead of using a concave grating Guided Wave uses a highly blazed plane grating which is a more efficient design. Collimation and focusing are provided by a pair of triplet achromatic lenses. These lenses produce a virtually aberration-free image of the source fiber in the focal plane of the spectrometer. The result is a spectrometer of exceptional brightness, i.e., throughput, hence, a very high signal-to-noise ratio.

FT-IR (not NIR) spectrometers are definitely superior to 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 our 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 Facility – The Input Module

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.

Conclusion: DG-NIR has an Advantage for Process Spectroscopy

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, the engineers and scientists at Guided Wave have advanced the state-of-the-art in dispersive NIR technology. By incorporating these advancements without compromising the dual-beam operation, we can offer to the market 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 aberrations 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.

We are a leader in online, process monitoring for over 35 years. Today we have NIR analyzer installations on six continents and in more than 50 countries, with thousands of analytical instruments sold worldwide for the Chemical, Refinery, Pharmaceutical, Polymer, Semiconductor, and Sterilization industries.

We are the only process NIR manufacturer that provides a complete optically matched system, yielding the best throughput efficiency and long-term performance, exceeding industry standards. These workhorse analyzers have been industry-proven for over three decades with individual analyzers in the field typically running 24/7 lasting more than 10 years with >99% uptime