ASTM D6258 Determination of Solvent Red 164 Dye Concentration in Diesel Fuels
10. Procedure
10.1 Using a clean 1.0 cm sample cell, scan the sample against air (empty reference sample compartment) from 450 nm to 750 nm using the same instrument settings employed for the working standards and record the absorbance.
10.2 Using derivative analysis software and the same noise level dampening settings used in 9.4, calculate and plot the second derivative spectrum of the sample.
10.3 Measure the amplitude difference between the derivative maximum and minimum specified in 9.5 (see Note 7).
10.3.1 The wavelengths of the derivative minima and maxima for the dye in actual diesel fuel samples normally vary from those observed for the working standards due to variations in the composition of the Solvent Red 164 dye used and sample matrix effects. On instruments that automatically record amplitudes at pre-set wavelengths, adjustments must be made to compensate for this shift to ensure that actual minimum and maximum amplitudes are used in determining amplitude differences.
