Application of Diode Laser Molecular Absorption Spectroscopy for Studies of Gas Concentrations in Food Packages

University essay from Lunds universitet/Fysiska institutionen; Lunds universitet/Atomfysik

Abstract: When it comes to absorbing and emitting radiation, molecules show similar properties as atoms do. A signicant dierence though, is that a molecular spectrum consists of notably more absorption lines compared to an atomic spectrum. The additional lines appear due to the fact that molecules can not only exist in electronic, but also in vibrational and rotational energy states. Each gas has uniquely dened absorption lines, which makes it possible to associate a ngerprint to the gas. Molecular spectroscopy enables to scan over these lines to determine the existence of a specic gas. With, for example, the help of Tunable Diode Laser Absorption Spectroscopy (TDLAS), which has been used in this work, and by applying the Beer-Lambert law, also the gas concentration can be obtained. Nevertheless, for a precise calculation of the gas concentration the path length travelled by light needs to be available. This especially can cause diculties when turbid materials are examined, in which the light scatters a lot, and therefore the dimension of the sample does not correspond to the path length of the light anymore. This work includes the examination of food packages lled with modied atmosphere. Diode lasers emitting wavelengths around 760 nm and 2054 nm are used to scan over an absorption line of oxygen and carbon dioxide, respectively. First, it is presented how the carbon dioxide concentration of 75 % in a sealed bread rolls package is obtained. Afterwards TDLAS combined with a technique called Wavelength Modulation Spectroscopy (WMS), which allows to detect low absorption signals, is used to perform a time dependent measurement on a package with ageing milk inside. The carbon dioxide and oxygen concentrations are monitored simultaneously over 5 days. With the relation between the two concentrations it is possible to speculate about biological activity in the milk package. Another important aspect is, that all the measurements are done non-intrusively, which leaves the package intact allowing usage after the measurement. 2

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