D4294_16e1_12

A quality control sample is measured prior to analyzing unknowns to verify that the test method is in control. It is run identically to any unknown sample. If the chosen quality control sample's repeatability varies more than the repeatability value of this test method expected for that concentration (acceptance values obtained from Table 6) then the procedure is deemed to be out of control and the instrument should be recalibrated before running any further analysis. A synthetic solid quality control sample may be used in lieu of liquid samples (see section 15). Fill the cup with the sample to be measured to about 75% of cup capacity. Before filling the cell, it may be necessary to heat viscous samples so that they are easy to pour into the cell. Ensure that no air bubbles are present between the cup window and the liquid sample. Measure each sample once. If the concentration from the first analysis is less than 100 milligrams per kilogram, repeat the measurement using a freshly prepared sample cup and a fresh portion of the sample, and obtain the average of the readings for the sulfur content in the unknown sample. When analyzing more than a single unknown sample, measure a quality control sample at the end of each batch of unknown samples, but no less than 10 unknown samples, to verify that the method is in control. In all situations where the quality control samples vary by more than the repeatability expected for that concentration (Table 6), the analysis must be discontinued and corrective action taken to find the source of air. Use a quality control sample close to the unknown samples' sulfur concentration. Refer to Section 15. For samples containing 100 milligrams per kilogram total sulfur or less, duplicate determinations are required. Each determination must be made on a new portion of sample material and analyzed in accordance with section 12.1 and 12.2. The difference between the duplicate analyses should be equal to or less than the repeatability values indicated in Table 6. If the difference is larger, investigate sample preparation to identify any possible sources of sample contamination, and repeat the analysis. The reason for duplicate measurements is to identify problems associated with sample contamination, leading to improved results precision at lower sulfur levels. The concentrations of ethanol and methanol were calculated assuring a theoretical mixture of hydrocarbons and dibutyl sulfide to which ethanol (or methanol) was added until the sum of the mass coefficients times the mass fractions increased by 5%. In other words, the amount of ethanol (or methanol) that caused a negative 5% error in the sulfur measurement was calculated. This information is included in Table 1 to inform those who wish to use Test Method D4294 for determining sulfur in gasohol (or M85 and M100) of the nature of the error involved.