Spectroscopy and mass spectrometry

Spectroscopy studies how light (and other electromagnetic radiation) interacts with matter. It is used to identify substances, measure coloured compounds, work out structures, follow enzyme reactions and study chemical bonds.

The key law is the Beer-Lambert law: A = ε × l × c. A is the absorbance (how much light the sample absorbs); ε is a constant for that substance (the molar absorption coefficient); l is the path length, the distance light travels through the sample (the width of the cuvette); and c is the concentration. Transmittance T = I/I₀ (light out ÷ light in), and A = -log T = log (I₀/I). If ε and l are known, measuring A gives the concentration. More concentrated solutions, or longer paths, absorb more.

Two instruments:

• A colorimeter uses coloured filters to choose a band of wavelengths. Parts: light source, filter, cuvette (sample holder), photocell.
• A UV-visible spectrophotometer uses a prism or grating (a monochromator) to choose a single wavelength, so it is more precise. Parts: lamps, monochromator, cuvette, detector, display.

Other methods: atomic absorption and emission (for metals, such as sodium and potassium), infrared and Raman (functional groups, minerals), NMR (molecular structure and the surroundings of each atom), X-ray absorption, and fluorescence.

Mass spectrometry turns a sample into gaseous ions, sorts them by their mass-to-charge ratio (m/z), and records how much of each there is. Its three parts are the ion source, the analyser and the detector. Mass is measured in daltons (1 dalton = 1/12 of the mass of a carbon-12 atom).