Paint samples received by forensic laboratories are usually in the form of small chips or smears. Infrared (IR) spectroscopy is one of the most commonly used tools available for the analysis of these types of samples and serves as a staple comparative technique in the assessment of whether or not a questioned sample could have come from a suspected object
The most direct way to probe the vibrational frequencies of a molecule is through infrared spectroscopy. This is because vibrational transitions typically require an amount of energy that corresponds to the infrared region of the spectrum. Raman spectroscopy, which typically uses visible light, can also be used to directly measure vibration frequencies.
Spectroscopy involves the study of the interaction of electromagnetic radiation with matter. The electromagnetic spectrum is essentially a composite of photons of different wavelengths and frequencies; from the low wavelength gamma rays to the high wavelength radio waves.
The pigments in a painting are organic compounds which can be easily identified via Infrared spectroscopy. When a beam of infrared light (photons in the 700 nm-1000 nm of the electromagnetic spectrum) is passed through the painting, photons of particular wavelengths or energy which are in resonance with the molecular vibrations of the pigment molecules get absorbed whereas the rest of the IR radiation is reflected and directed onto a detector. A plot intensity of the reflected IR light vs wavelength corresponds to the IR spectrum which is unique to a particular chemical substance. Thus, various pigments in a painting that have IR active features can be identified by this method.
The molar mass of Na₂SO₄ - 2 x Na - 2 x23 = 46 1 x S - 1 x 32 = 32 4 x O - 4 x 16 = 64 total = 46 + 32 + 64 = 142 g/mol the molarity of solution - 2.0 M in 1 L of solution , 2.0 moles Therefore in 2.5 L - 2 mol/L x 2.5 L = 5 mol then the mass of Na₂SO₄ required = 142 g/mol x 5 mol = 710 g
