The objective is derived from the need for a reliable absorption spectrum of lipids for component
analysis of in vivo tissue spectra. NIR in vivo spectroscopy enables to derive the concentration of
the key tissue constituents absorbing in the 600-1100 nm range, that is oxy- and deoxyhemoglobin,
water and lipids. Yet, although the first three constituents are already well
characterized in literature, quite few data are available on mammalian lipids. In the present
proceeding we report the absorption spectrum of a clear purified oil obtained from pig lard.
Absorption coefficients were measured with time resolved and spatially resolved diffuse
reflectance spectroscopy techniques. At temperatures of 37°C and higher it is a clear transparent
liquid thus suitable for collimated transmission measurements. In total three independent
measurement techniques were employed the determine the absorption coefficients of mammalian
5 kg of pig lard was divided in pieces of 1 cm3 and placed in water with a temperature of 90°C. After a while a thin
layer of pure oil from the lard formed on top of the water, and was removed by a spoon and placed in a separate
container. This process was continued for 6 hours until the oil secretion process had slowed down. The substance
obtained still contained water and other visible tissue structures. After cooling the oil had solidified into a pure white
grease that could be separated easily from the remaining water and a gelatinous substance. The solid oil was then
heated once more to 80°C and filtered twice in liquid state. After that the oil was heated again to 80°C and poured
onto a filter containing Sodium sulphate to remove the last traces of water. Finally the oil was placed in a centrifuge
for 30 minutes at 1000 rpm at a constant temperature of 70 °C. The bottom of the reaction tubes still contained some
remaining sediment and the pure oil was separated by pipette from the tube, resulting into 250 ml of oil that is
visually clear at temperatures of 37°C and up.
Collimated transmission measurement
Light from a 100 W quartz tungsten halogen lamp is coupled into an optical fiber leading to a cuvette holder and
collimated to a beam of approximately 2mm diameter. Three different cuvettes were used (10, 20 and 50 mm). The
collection of the transmitted light was performed by an integrating sphere with a collection port much larger that the
beam diameter to compensate for differences in beam diameter caused by the divergence of the light beam and the
different cuvette path lengths. The collected transmittance was spectrally projected onto a 16-bit 256-1024 pixel
CCD camera cooled to -30°C Cuvette holder plus integrating sphere were placed in an oven. Measurements were
performed at constant temperatures of 37, 60 and 80°C. For each cuvette path length three sequential transmission
and background measurements were performed.
Figure Absorption coefficient corrected for scattering contribution versus wavelength. The error bars represent the standard
deviation over the 2 temperatures i.e. 37, 60°C
Data is available for the absorption coefficient of fat versus wavelength.
Note that this has been slightly processed (by SAP) so that wavelengths are specified at
whole numbers and so that absorption values have only three digits of accuracy. The original data is also available. Further details
are available in the conference proceedings.
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