RESUMO
We report an efficient sample preparation method (freezing) for onsite fat and meat analysis via a specially designed thermoelectric cooling and temperature-controlling system. This investigation also focused on the effect of phase change on the sensitivity and reproducibility of LIBS emission signals and plasma parameters. The plasma emissions of animal fats (lard) were recorded when the sample was frozen (-2 °C), fluid (15 °C), and in a liquid state (37 °C) with a thermoelectric cooling system. At each temperature, the plasma emissions were acquired at laser pulse energy from 50 to 300 mJ and detector gate delay (DGD) from 0.5 to 5 µs. With increasing sample temperature, the DGD, where the optical emission intensity reached a maximum, decreased. At a laser pulse energy of 200 mJ and a sample temperature of -2 °C, the emission signals increased fourfold, the signal-to-noise ratio (SNR) improved tenfold, and the self-absorption in the emission lines decreased significantly. The repeatability of the emission signals and plasma parameters of frozen and liquid fat samples was determined using the relative standard deviation (RSD) of Se I (473.08 nm) and K I (766.48 nm) emission lines. The RSDs of the emission signals improved from 40 to 18 % and 37 to 16 %, whereas the shot-to-shot RSDs of the electron temperature and electron number density get improved from 11 to 6 % and 12 to 6.8 %, respectively.
