Fluorescence of amber

Amber is a fossil resin that was usually secreted by prehistoric trees as a protective reaction to injury. It occurs in a variety of colors, the most common being yellow, orange and brown. Amber can vary from almost transparent to completely opaque. The transparency depends on trapped gas bubbles or inclusions. Sometimes there are also fossil inclusions of, for example, insects, arachnids, plant remains (such as leaves or pollen) and occasionally even small vertebrates. Amber has been used for jewelry, amulets and works of art since the Stone Age.

Amber contains organic molecules such as polycyclic aromatic hydrocarbons (PAHs), terpenes and other complex hydrocarbons. These molecules have a special electron structure that makes them fluoresce under UV light.

At an excitation wavelength of e. g. 375 nm (near UV range), electrons in these molecules are raised to a higher energy level. When the electrons fall back to their ground state, they emit the excess energy in the form of light in the visible range—this is fluorescence. The exact distribution and intensity of the fluorescence can vary depending on the origin and composition of the amber.

The fluorescence of amber is not only of scientific importance, but is also used in art and restoration to determine authenticity and origin. It is an important tool for recognizing forgeries and obtaining information about the deposits of the material. The search for amber, e. g. on beaches, is also facilitated by the use of suitable UV lamps.

Recording a spectrum

Baltic amber from the Baltic Sea was used to record the fluorescence spectrum shown here. This was formed around 44 – 54 million years ago in the Eocene and comes from the resin of extinct conifer species (such as the genus Sciadopityaceae), which formed huge forests in northern and central Europe at that time. The resin was transported by rivers to the area of today's Baltic Sea and fossilized there over millions of years. After storms, amber often washes up on beaches along the Baltic Sea, especially in Poland (e. g. in Gdańsk), Lithuania (Klaipėda) and Germany (Usedom, Darß, Rügen).

A DIY spectrometer in Czerny-Turner design with a focal length of 150 mm, a slit size of 10 µm, a grating with 300 lp/mm and a CCD detector with 3648 pixels (line scan camera e9u-LSMD-TCD1304-STD) was used.

For excitation, we used a 375 nm LED to illuminate a piece of amber at an angle of 45 degrees. The emitted light was captured by a light guide at an angle of 45 degrees and fed to the spectrometer for analysis.

As the fluorescence was only weakly pronounced in the existing sample, a relatively long integration time of 10 s was selected. The recorded spectrum shows a clear emission in the range of 450 – 600 nm, which corresponds to the typical blue-green to yellowish glow that Baltic amber typically shows under UV light.

In the range between 400 nm und 425 nm, remnants of the light from the exciting UV LED can still be seen in the fluorescence spectrum. At a central wavelength of 375 nm, this has a half-width of 15 nm (see image). However, if the integration time of the spectrometer is sufficiently high, offshoots up to 420 nm can still be detected.

However, the sensitivity of the CCD sensor used only starts at approx. 400 nm at short wavelengths (partly due to the sensor cover glass), so that only a small and asymmetrical part of the LED spectrum is detected by the spectrometer (see spectrum below).