When You Got the Glow: The Truth About Your Gin and Tonic.

The music is loud, the bass is thumping, people are dancing and laughing, and my gin and tonic is glowing. This Friday night is perfect. This mysterious glowing drink catches the eye of many passersby and is a great conversation starter. At least it was until I learned why my drink was glowing. These days, when someone mentions my glowing drink, I am compelled to explain why. It has become a great conversation ender!

I’m a scientist and naturally curious, but there are some things I choose to not investigate just to hold on to the excitement that comes with wondering why something is the way it is. My desire to keep wondering why my gin and tonic glows, or becomes fluorescent, in the club was thwarted one Friday afternoon (ca. 2001) by Professor David Jameson, University of Hawai’i, in a Biophotonics lecture at the University of Wisconsin – Madison. Professor Jameson, who studied under the late, great Gregorio Weber, is a dynamic teacher with a genuine passion for fluorescence methodology.

Fluorescence is a form of luminescence (the other form is phosphorescence), which occurs when a chemical compound absorbs a particle of light, a.k.a. photon, which excites an electron. What you see is the photon being released as the electron returns to its ground state (in other words, when the electron calms down again). Chemicals that fluoresce are known as fluorophores. Quinine is a model fluorophore used widely as a standard in scientific studies. The molecular structure, shown at the right, contains features that make its electrons particularly susceptible to excitement; specifically the heterocyclic quinolone (two hexagonal rings). The fluorescent light emitted is always a different color than the light absorbed. For example, we cannot see the UV light that excites the quinine, but we can see the blue light that is emitted. This is because some energy following absorption is lost, presumably from all the excitement, which makes the light released a different (longer) wavelength. This phenomenon is known as Stokes Shift. (For more information than you probably want to know, see references 1 – 3 below.)

Quinine fluorescence was first described by Sir John Herschel in 1845 when he observed a solution of quinine sulfate exhibit a faint, blue glow in the sunlight. This observation launched a powerful new field of chemistry. Fluorescent light doesn’t just look pretty; it is a property that has become essential in all areas of molecular and cellular research and drug discovery.  For example, Sir Herschel’s observation stimulated the development of instruments (spectrophotometers) that could measure and quantify fluorescent light output. This allowed scientists to study quinine in more detail for use as an antimalarial drug (4).

You might be asking by now, “What in the world does this have to do with gin and tonic?” Well, it turns out that tonic water contains quinine! Quinine, procured from the bark of the cinchona tree, has been used for centuries as a natural cure for malaria and many other ailments such as fevers, stomach aches or leg cramps. Quinine was generally administered by preparing an extract and mixing with water or wine to make the bitter taste more palatable. Right around the time Sir Herschel and others were describing the science of the quinine molecule, British colonists in India were discovering more interesting ways to take their tonic. Notably, they began taking their tonic mixed with gin! These days tonic water contains considerably less quinine than traditionally therapeutic levels and usually contains some kind of sweetener. (5)

There you have it. Gin and tonic, used as my dad would say, “for medicinal purposes only,” will glow or fluoresce when exposed to UV light.  If you happen to take your ‘medicine’ in a club that happens to have blacklights, you got the glow! (Not to be confused with that of Bruce Leroy as LMFAO may imply.) Just try to avoid telling the whole story during the party.


  1. Lecture slides from Professor Jameson: http://www.fluorescence-foundation.org/lectures/chicago2011/lecture2.pdf
  2. Animation of an excited electron and associated spectra from Olympus: http://www.olympusmicro.com/primer/java/jablonski/solventeffects/index.html
  3. Lecture slides, source unknown: http://homes.esat.kuleuven.be/~bioiuser/bioscenter/doctoralschool/BioSCENTer_Tech_Watch_Nano-bio_tech.pdf
  4. Lakowicz, J. R. (2006). Principles of Fluorescence Spectroscopy, Springer.
  5. Quillen, Columbine. “Tonic Water – a little history” Q Mix-a-lot. May 21, 2010. Accessed May 9, 2012. http://qmixalot.com/tonic-water-the-history-of-tonic
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Karen Reece

Karen served as a Senior Research Scientist in Nucleic Acid Technologies at Promega before switching careers. She has a BS in Biochemistry and MS and PhD in Physiology, all from University of Wisconsin-Madison. Karen was born and raised in Madison, WI.

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