ASTM D6728 Standard Test Method for Determination of Contaminants in Gas Turbine and Diesel Engine Fuel by Rotating Disc Electrode Atomic Emission Spectrometry
9. Sampling
9.1 The fuel sample taken for the analysis must be representative of the entire system. Good sampling procedures are key to good analyses and samples must be taken in accordance with Practices D4057 or D4177.
10. Preparation of Test Specimen
10.1 Homogenization - Fuel samples may contain particulate matter and free water and, in order to be representative, must always be vigorously shaken prior to pouring a test specimen for analysis.
10.2 Ultrasonic Homogenization - Samples that have been in transit for several days, idle in storage or very viscous, shall be placed in a heated ultrasonic bath to break up clusters of particles and to bring them back into suspension. The samples shall be vigorously shaken with a power mixer after being in the ultrasonic bath and prior to pouring a test specimen for analysis. The bath temperature shall be 60 °C for very viscous fuels and below the flash point of non viscous fuels. The total agitation time for a sample should be at least 2 min.
10.3 Specimen Holders - Fuel samples and oil standards shall be poured into a specimen holder of at least 1 mL capacity prior to analysis. Exercise care to pour the sample consistently to the same level in the specimen holders.
10.4 Specimen Table - The specimen table shall be adjusted so that when it is in the full raised position, at least one-third of the disc electrode diameter is immersed in the oil test specimen.
11. Preparation of Apparatus
11.1 Warm-up Burns - If the instrument has been idle for several hours, it may be necessary to conduct warm-up burns to stabilize the excitation source. The warm-up procedure can be performed with any fuel sample or standard. Consult the manufacturer's instructions for specific warm-up requirements.
11.2 Optical Profile - Perform the normal optical profile procedure called for in the operation manual of the instrument. Optical profile shall also be performed if the instrument has been inoperative for an extended period of time or if the temperature has shifted more than 10 °C since the last calibration check.
11.3 Validation Check - A go/no go standardization check can be performed with one or more check samples or quality control samples to confirm calibration prior to the analysis of routine samples. A calibration standard or fuel sample with known contaminant concentration can be used for this purpose. The optical profile and standardization routine recommended by the instrument manufacturer shall be performed if the validation check fails to meet the accuracy guidelines for each element as described in Appendix X1.
12. Calibration
12.1 Factory Calibration - The operational range for each element is established through the analysis of organometallic standards at known concentrations. A calibration curve for each element is established and correction factors are set to produce a linear response. Analyses of test specimens must be performed within the linear range of response.
12.2 Routine Standardization - A minimum of a two point routine standardization shall be performed if the instrument fails the validation check or at the start of each working shift. A minimum of three analyses shall be made using the blank and working standard.