ASTM D6732 Determination of Copper in Jet Fuels by Graphite Furnace Atomic Absorption Spectrometry
4. Summary of Test Method
4.1 The graphite furnace is aligned in the light path of the atomic absorption spectrometer equipped with background correction. An aliquot (typically 10 µL) of the sample is pipetted onto a platform in the furnace. The furnace is heated to low temperature to dry the sample completely without spattering. The furnace is then heated to a moderate temperature to eliminate excess sample matrix. The furnace is further heated very rapidly to a temperature high enough to volatilize the analyte of interest. It is during this step that the amount of light absorbed by the copper atoms is measured by the spectrometer.
4.2 The light absorbed is measured over a specified period. The integrated absorbance Ai produced by the copper in the samples is compared to a calibration curve constructed from measured Ai values for organo-metallic standards.
5. Significance and Use
5.1 At high temperatures aviation turbine fuels can oxidize and produce insoluble deposits that are detrimental to aircraft propulsion systems. Very low copper concentrations (in excess of 50 µg/kg) can significantly accelerate this thermal instability of aviation turbine fuel. Naval shipboard aviation fuel delivery systems contain copper-nickel piping, which can increase copper levels in the fuel. This test method may be used for quality checks of copper levels in aviation fuel samples taken on shipboard, in refineries, and at fuel storage depots.
6.1 Interferences most commonly occur due to light that is absorbed by species other than the atomic species of interest. Generally, this is due to undissociated molecular particles from the sample matrix. The char step in the furnace program is used to eliminate as much of the matrix as possible before the atomization step. Spectrometers are equipped with background correction capabilities to control further possibilities of erroneous results due to molecular absorption.