ASTM D2008 for Ultraviolet Absorbance and Absorptivity of Petroleum Products
ABSORBANCE OF UNDILUTED LIQUIDS
9. Procedure
9.1 Fill a 1.0-cm reference cell with water. Make sure the cell windows are clean. Position the cells in the cell compartment of the spectrophotometer and obtain absorbance at the wavelengths of interest within the range from 220 to 400 nm. This data gives a cell correction for the 1.0-cm cell. It can be ignored at all wavelengths where the absorbance is between -0.01 and +0.01. After the cell correction data has been determined, the cells shall be designated reference and sample cells and shall be maintained as such.
9.2 Fill a 1.0-cm sample cell with undiluted liquid sample (after complete removal of water) and obtain the absorbance as described in 9.1.
9.3 The absorbance-wavelength curve can be conveniently obtained starting at the long wavelength end of the spectrum. Take readings at successively shorter wavelengths until an absorbance greater than 1.0 is obtained. When using automatic recording instruments (recommended) make the cell correction scan and the sample scan on the same chart. In the longer wavelength region of the spectrum, it may be desirable to use longer path length cells than those recommended to obtain readable absorbances. See the applicable paragraph of Practices E 169. In the shorter wavelength region of the spectrum, absorbances can become too high for accurate measurement in the 0.1-cm cell. Record these values only as greater than 1.0. If numerical values are required it is recommended that absorptivity be measured rather than absorbance.
9.4 Repeat 9.1 and 9.2 using a 0.1-cm cell in place of the 1.0-cm cell (9.3). Record all measurements.
10. Calculation
10.1 Calculate the absorbance of an undiluted liquid sample at each analytical wavelength as follows:
A = AL - AC
where:
A = absorbance of undiluted liquid sample,
AL = chart or absorbance reading of sample-filled sample cell, and
AC = chart or absorbance reading of water-filled sample cell.
10.2 Calculate the absorbance per centimetre path length that is equal to A/b, where b is the sample cell path length in centimetres.
11. Report
11.1 If the numerical value of the absorbance of an undiluted liquid sample is reported, accompany it with a statement of the wavelength of measurement and the sample path length expressed in centimetres.
11.2 If the numerical value of the absorbance per centimetre of an undiluted liquid sample is reported, it must accompany it with a statement of the wavelength of measurement.
