Science

Vacuum Ultraviolet Spectroscopy

You’re no doubt familiar with many types of radiation or light that make up the electromagnetic (EM) spectrum, even if you don’t think you are. X-rays, ultraviolet (UV) rays, visible light (ROYGBIV), infrared, microwave, radio waves. See, I knew you knew many of them.

The UV region of the EM spectrum covers wavelengths of light from about 10 to 400 nm. By comparison, the light we visually see spans from 390 to 700 nm. Infrared light has an even longer wavelength, ranging from about 710 nm to 1 mm. UV light is further subdivided into categories like near UV (300-400 nm), or vacuum UV (below 200 nm). And vacuum UV, or VUV light has illuminated some new technology for the cannabis industry.

Spectroscopy is simply the study of how light interacts with matter. When light is shone on a substance, that light can be reflected back from the material, can be transmitted through the material, can scatter in all directions, or can be absorbed. When light is absorbed, the molecule becomes “excited”, as it migrates to a higher energy level from the additional input of energy. Think of what you know about demonic possession…demon in, and the person gets crazy, right. Well, this is sort of like that. Sort of.

Different molecules absorb light at different wavelengths. You can measure specifically where a molecule absorbs UV and visible light using a spectrometer that scans through the wavelengths. Knowing this absorption profile is important in several regards, including being able to determine how much of a substance is present using Beer’s Law.

Many of the detectors used in liquid chromatography are UV detectors. Other types include refractive index (RI), flame ionization (FID), and mass spectrometer detectors. While these all have their varied advantages and disadvantages like anything else, the biggest question at the end of the day is, can they differentiate between really similar molecules being measured in tandem, like cannabinoids or terpenes?

VUV detectors allow nearly every compound to be detected, since most matter absorbs light in this region of the EM spectrum to some extent. This technology has even been touted as the “universal detector”. In an article written for Cannabis Industry Journal, VUV Analytics discussed the limitations of historical UV detectors for gas chromatography (GC). What makes VUV applicable, even to GC, is that each molecule produces its own signature absorption profile. Even isomers can be distinguished. So, quantifying 41 terpenes, for example, all derivatives from that basic isoprene molecule, becomes more practical. [1]A method for the identification of 37 pesticides has also been published. [2]

VUV Analytics recently published a paper demonstrating their GC-VUV technology for the detection and quantification of cannabinoids, and their metabolites. [3] Cannabinoids, like terpenes, have similar structures, and thus, can lead to chromatographic issues like co-elution (when two substances come out of the instrument at the same time), challenging the analyst’s ability to resolve what may be mistaken as one substance. To prevent overlapping peaks, chromatographers typically need to dip into their bags of clever tricks to try to tease apart the co-eluting analytes. With VUV detectors, however, refinement of the chromatographic separation may not be needed, since in addition to the chromatogram, the analyst also has the concomitant VUV data, which can help distinguish overlapping chemical constituents. Thus, VUV can help achieve faster analyses, thereby increasing throughput, without damaging the method’s sensitivity.

References

[1] Qiu, C. et al. “Analysis of terpenes and turpentines using gas chromatography with vacuum ultraviolet detection”, Journal of Separation Science, 2017, Volume 40(4): Pages 869-877. [journal impact factor = 2.415; cited by 25] [2] Fan, H. et al. “Gas chromatography–vacuum ultraviolet spectroscopy for multi class pesticide identification”, Journal of Chromatography A, 2015, Volume 1389(10): Pages 120-127. [journal impact factor = 4.169; cited by 44] [3] Leghissa, A. et al. “Detection of cannabinoids and cannabinoid metabolites using gas chromatography with vacuum ultraviolet spectroscopy”, Sep Sci Plus, 2018, Volume 1: Pages 37-42. [journal impact factor = N/A; cited by 9]

About the author

Jason S. Lupoi, Ph.D.

Jason S. Lupoi, Ph.D.

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