A combined micro-and semi-micro colorimetric determination of long-chain fatty acids from plant cutin

Summary

Re-examination of the colorimetric fatty acid determination with copper nitrate, followed by complex formation with DIECA has shown that the method is not reliable if applied as described by Duncombe (1962, 1963): The Cu concentration is too high, the DIECA concentration much too low and the wavelength chosen (440 mμ) is suitable only for very low fatty acid concentrations.

According to the results reported here the following alterations have to be adopted: The concentration of the copper nitrate solution should be 3%, a 0.5% solution of DIECA in butanol has to be used and measurements should be done at 492 mμ. The method described here offers the opportunity to determine fatty acid concentrations in the semi-micro range by measuring the filtered chloroform phase directly at 691 mμ, covering a range between 175 μg/ml to 1.2 mg/ml. If the concentration turns out to be lower than 200 μg F. A./ml, the same sample can be used for a micro-determination (up to 200 μg/ml) at 492 mμ, after formation of the yellowish-brown complex by addition of 0.1 ml 0.5% butanolic DIECA solution to 1.0 ml of the chloroform phase.

The method has been applied to determine the amount of free F. A. in cutin layers and cutin powder, revealing that the latter contains 5.6 times more free F. A. than the intact material. The free F. A. within the polymer seem to serve as interconnections for the main units of the cutin polylipid.