ASTM D2008 for Ultraviolet Absorbance and Absorptivity of Petroleum Products
6. Apparatus
6.1 Spectrophotometer, equipped to handle liquid samples in cells having sample path lengths up to 10 cm and capable of measuring absorbance in the spectral region from 220 to 400 nm with a spectral slit width of 2 nm or less. Wavelength measurement shall be repeatable and known to be accurate within more or less 0.2 nm or less as measured by the mercury emission line at 313.16 nm or the absorption spectrum of either holmium oxide glass at 287.5 nm or holmium oxide solution at 287.1 nm. At the 0.4 absorbance level in the spectral region between 220 and 400 nm, absorbance measurements shall be repeatable within more or less 1.0 %.
6.2 For recommended methods of testing spectrophotometers to be used in this test method, refer to Practice E 275. 6.3 An instrument is considered suitable when it can be operated in a manner to give test results equivalent to those described in 6.1.
6.4 Measurements requiring the use of cells having sample path lengths less than 10 cm can be made on instruments equipped to handle only these cells. It is desirable, but not essential, that the instrument be automatic recording when an extended range of the spectrum must be examined. Manually operated spectrometers are suitable for obtaining absorbance readings at specified analytical wavelengths. If measurements are to be made at temperatures higher than room temperature, the spectrophotometer must be provided with a means for maintaining cells at the selected test temperature.
6.5 One or more pairs of fused silica cells having sample path lengths in the range from 0.1000 to 10.00 cm are required. Sample path lengths must be known to within more or less 0.5 % of nominal sample path length or better. Unless otherwise specified, 1-cm sample path length cells are recommended. Suitable procedures for testing and cleaning cells are described in Practice E 275.
7. Reagents and Materials
7.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents shall conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society, where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
7.2 Purity of Water - Unless otherwise indicated, references to water shall be understood to mean reagent water conforming to Specification D1193, Type III.
7.3 Solvents:
7.3.1 Isooctane - (Warning - Extremely flammable, harmful if inhaled.), for use as the preferred spectroscopic solvent.
7.3.2 Technical isooctane is a satisfactory base stock for the preparation of spectroscopic solvent. Allow about 4 or 5 L of this material to percolate through a column of activated silica gel 50 to 75 mm (2 to 3 in.) in diameter and 0.6 to 0.9 m (2 to 3 ft) in depth. Collect only the portion of the solvent that has an absorbance less than 0.05 over the entire spectral range from 240 to 300 nm in a 1-cm cell when compared to water in a 1-cm cell.
7.3.3 Decahydronaphthalene (Decalin) - (Warning - Combustible, vapors harmful.), for use as the first alternative spectroscopic solvent.
7.3.4 The silica gel percolation described in 7.3.2 is also recommended for the preparation of decahydronaphthalene as a spectroscopic solvent.
7.3.5 Some common, commercially available solvents of "spectroscopic purity" are listed in Practices E 169. One of them can be selected for use in absorptivity measurements but only when indicated in Section 13.
7.4 Holmium Oxide Glass or Holmium Oxide Solution - Used to verify the wavelength accuracy of the spectrophotometer.
8. Sampling
8.1 Precautions must be taken to ensure that a representative sample is obtained since ultraviolet absorption is very sensitive to small amounts of extraneous material contaminating the sample through careless handling. If possible, samples should be obtained from packaged products that have been protected from accidental contamination.
8.2 If the petroleum product to be tested is available as a bulk sample weighing more than 1 kg, a representative sample of approximately 1 kg shall be taken and made homogeneous.
8.3 If the petroleum product to be tested is available as a bulk sample weighing less than 1 kg but more than 100 g, the entire sample shall be taken and made homogeneous.
8.4 In no case shall a sample of a petroleum product be considered representative if it weighs less than 100 g. However, measurements may be made on samples weighing less than 100 g if the origin, sampling procedure, and basis of selection of the sample are recorded and reported as part of the results of this test method.
