Buyer's Guide: Monochromators for UV/Vis Spectrophotometry

The monochromator is an important component of the UV/Vis spectrophotometer that will allow you to select the appropriate wavelength for your experiment. Certain monochromator features, such as the type of mounting and dispersive element used, can have an impact on resolution and accuracy, so it is important to choose the most suitable options for your application.

Prism vs. Diffraction Grating

The dispersive element of the monochromator is what breaks the beam of light that enters through the entrance slit down into separate wavelengths, and the wavelength that exits through the exit slit can be adjusted by rotation of the dispersive element. The two main types of dispersive elements are prisms and diffraction gratings; diffraction gratings are more commonly used in modern spectrophotometers. This is because gratings are made up of several angled grooves that provide consistent linear dispersion throughout the UV and visible range, while the non-linear dispersion of a prism will cause resolution to diminish as wavelengths approach the red and infrared domain.¹

Prisms do have some advantages, such as high light utilization efficiency, low stray light and no higher-order light, and low polarization.² Prisms can maintain a constant efficiency across a range of wavelengths while each diffraction grating will only have the highest efficiency at its specific blaze wavelength, which is based on its geometry.

More About Diffraction Gratings

Diffraction gratings can be further categorized as reflection or transmission gratings, and ruled or holographic gratings.

Transmission gratings have a lower sensitivity to alignment and polarization interference than reflection gratings, but also have a limited angular range and can suffer absorption and scattering losses at some wavelength regions, and may be suitable in some compact spectrometer designs.³ Ruled gratings are created by mechanically etching a sawtooth-like pattern of triangular grooves onto the grating surface using a ruling engine equipped with a diamond tool, while holographic gratings typically consist of curved (sinusoidal) grooves created in a photolithographic process, in which interference patterns from intersecting laser beams generate the grooves on a photosensitive surface. Ruled gratings are generally considered to provide higher efficiency than holographic gratings, but are also susceptible to having more physical defects than holographically produced gratings, resulting in more stray light and ghost lines.⁴

Selecting a diffraction grating is more than just a “one or the other” decision, however, as other specifications such as the grating’s coating material, groove density, blaze angle, size and shape will further impact efficiency, wavelength range and resolution, among other factors. For example, concave gratings can reduce the need for additional optical components in the system by acting as both a dispersive and focusing element, and may also reduce coma and astigmatism compared to plane gratings.⁵ Aluminum (Al) protected with a layer of magnesium fluoride (MgF₂) is preferable to bare Al when working in the deep UV region to prevent the absorption of UV radiation by aluminum oxide that can form on the grating surface.⁶ Overall, it is most important that you select gratings with the optimal groove density (grooves per mm) and blaze angle to fulfill your dispersion and efficiency needs in the specific spectral range you will be working with.

Products to Consider:

• High Throughput Monochromator from Optical Building Blocks, a Division of HORIBA Scientific
• Omni-Lambda Monochromators and Spectrographs from Zolix Instruments Co., Ltd.

Mounting

Also referred to as aligning elements, the mounting type of a spectrophotometer optical system will affect the configuration of the entrance and exit slits, collimating and camera mirrors and dispersive element within the monochromator, thus determining how light will travel through the device. There are several different mounting types available; here are a few common varieties for UV/Vis spectrophotometry.

Czerny-Turner mountings are among the most popular mountings and employ separate curved collimating and camera (focus) mirrors, producing a flat focal plane that is well suited for CCD detectors.⁷ The light follows a path similar to an M or W shape through the monochromator.

Littrow mounting uses one spherical mirror to act as both a collimating and camera mirror, with the diffracted light traveling back in the direction of the incident light. A blazed diffraction grating in the Littrow configuration will have very high efficiency at the blaze wavelength, however, stray light, internal reflections and multiple dispersions are potential problems that have made this configuration less popular than the Czerny-Turner configuration, which reduces some of these problems.⁸

Rowland circle mounts are mounts that use a fixed concave grating; the Rowland circle is a circle with a diameter equal to the radius of the grating’s radius of curvature. When both the grating and slits are placed along the circumference of the Rowland circle, this is referred to as a Paschen-Runge mount.⁹ There are several different types of Rowland circle configurations based on the exact arrangement of the grating and slits. Rowland circle mounts can suffer from high astigmatism and multiple exit slits with their own detectors are often used.¹⁰

Seya-Namioka mounts are somewhat similar to those fixed on the Rowland circle, but use a rotating concave grating to scan wavelengths. The entrance and exit slits in this design are fixed at a 70°15’ angle. These mounts are very simple and best used for vacuum UV experiments.

Slit Width

The width of the exit slit in a monochromator directly impacts resolution, with a narrower slit providing higher resolution and wider slit resulting in lower resolution. However, a narrower slit is not always better, as there are other conditions that will affect resolution. Narrower slits are susceptible to more data noise that could distort results, while widening the slit reduces both resolution and noise. A wider slit width may be preferred for some applications, such as the measurement of solids, where high resolution may not be needed.²

Products to Consider:

• DS5 Double Beam UV-Vis Spectrophotometer from Edinburgh Instruments Ltd
• V-730 UV-Visible Spectrophotometer from JASCO

Single vs. Double Monochromator

A spectrophotometry system may consist of just one monochromator or more than one monochromator in series; the main benefit of using a double monochromator system is that stray light is significantly reduced due to the use of two dispersion elements. Minimizing or eliminating stray light can improve the accuracy of your spectroscopic measurements, especially for high-concentration or high-absorbance samples where the effects of transmitted stray light may be more pronounced.¹¹

So are two monochromators always better than one? Not necessarily. In a single monochromator system, more light will ultimately make it to the sample, which can be preferable when working with smaller samples or samples that are highly transmissive or reflective. This is why it is important to always tailor your selections to your specific application and situation; if stray light is a major concern, you will want to consider a double monochromator system, while a single monochromator can be the better solution if you are more concerned about light loss.

Products to Consider: 

• UV-3600i Plus UV-VIS-NIR Spectrophotometer from Shimadzu
• Cary 3500 UV-Vis Spectrophotometer from Agilent Technologies

References

1. Wenzel, T. Monochromators https://chem.libretexts.org/@go/page/111327
2. “Monochromators,” Shimadzu. https://www.shimadzu.com/an/service-support/technical-support/analysis-basics/fundamentals-uv/monochromators.html
3. R. Paschotta, article on 'transmission gratings' in the RP Photonics Encyclopedia https://www.rp-photonics.com/transmission_gratings.html
4. “Understanding and selecting diffraction gratings,” Acal BFi. http://5740daf986400cba1fd6-438a983e20136d2f376705dfe1c68aea.r82.cf3.rackcdn.com/Acal_BFi_Diffraction_Gratings_Guide_(EN).pdf
5. “Diffraction Grating Selection Guide,” Newport. https://www.newport.com/g/diffraction-grating-selection-guide
6. “Buyer’s Guide to Diffraction Grating,” Blog, Optometrics. https://www.optometrics.com/blog/diffraction-grating-buyers-guide/
7. “Optical Designs for Popular McPherson Spectrometers, Spectrographs and Monochromators,” McPherson. https://mcphersoninc.com/resources/spectrometerdesign.html
8. Coles, D.; McGhee, K. Spectrometer Working Principles, Uses, and More. Application Note, Ossila. https://www.ossila.com/pages/spectrometer-application-notes
9. Inczedy, J.; Lengyel, T.; Ure, A. M.; International Union of Pure and Applied Chemistry. In Compendium of analytical nomenclature: Definitive rules 1997; Blackwell Science: Osney Mead, Oxford, 1998. https://media.iupac.org/publications/analytical_compendium/Cha10sec3219.pdf
10. Neumann, W. In Fundamentals of Dispersive Optical Spectroscopy Systems; SPIE Press: Bellingham, Washington USA, 2014; pp 27–28. https://spie.org/samples/PM242.pdf
11. “Characteristics of Single and Double Monochromator UV-VIS Spectrophotometers,” Shimadzu. https://www.shimadzu.com/an/service-support/technical-support/analysis-basics/fundamentals-uv/single_double.html

Photo: A diagram of white light being dispersed in a Czerny-Turner monochromator. Credit: Jaeger5432, Wikimedia Commons, CC-BY-SA 3.0