Build a spectrometer: see the light

Date:1 February 2013 Tags:, , ,

It takes just a few hours and no more than a couple of hundred rand to build an essential tool of chemical sleuthing. By Adam Hadhazy

Spectrometers fracture light into telltale patterns of coloured bands that reveal the composition of just about any type of matter, whether it’s a plant sample, the dark cloud spewing from a smokestack, or gases swirling in another planet’s atmosphere. The devices can be found in biology and chemistry labs, hooked up to astronomical telescopes, and piggybacking on Mars landers. As I recently discovered, it’s also possible to make one with a few inexpensive parts.

I was inspired by Jeff Warren, cofounder of the citizen-science Public Laboratory for Open Technology and Science (PLOTS). After the Deepwater Horizon disaster of 2010, Warren sought an inexpensive way to test beach-sand samples for oil contamination. He knew that, when struck with ultraviolet light, different kinds of oils fluoresce, generating distinctive light signatures. To capture them Warren crafted a low-cost USB webcam spectrometer that connects to a software program he created, Spectral Workbench. “We started asking, why don’t people have access to these kinds of tools?” he says. Warren posted the plans online, and I used them to build my spectrometer.

I tested two light sources – the Sun and a compact fluorescent lightbulb – and shone light through some olive oil. The results are fascinating, with colours indicating distinct chemical elements.

Colour Decoding

The sun – Chemical elements generate spectral “fingerprints” determined by greater or lesser degrees of light absorption at different wavelengths, measured in nanometres (nm). Sunlight produces prominent lines at 656, 486, 434, and 410 nm, corresponding to hydrogen, and at 762 nm, indicating oxygen.

Olive oil – High-quality olive oil contains a lot of chlorophyll. The substance is green to the naked eye, but its spectra are expressed by concentrations of blue and red, in the ranges of 400 to 500 nm and 600 to 700 nm, respectively.

Fluorescent light – The colour register of a 25-watt CFL includes a bright green line at 546 nm, indicating mercury. Electrified mercury emits UV light, which reacts with the bulb’s phosphor coating to make visible light.

Rough cost: R250 to R500. Parts include a VHS case, a USB webcam, a DVD-R, and black cardboard.

Time commitment: 2 hours to build, additional time for calibration. The software works better on Macs and Linux machines than it does on Windows computers.

Full plans: publiclaboratory.org/wiki/videospectrometer-construction

Online community: spectralworkbench.org; the sources of the spectra are shown here.