OPTICAL WEDGE EFFECTS IN INSTRUMENTS AND STANDARDS FOR MOLECULAR ABSORPTION SPECTROPHOTOMETRY

The optical wedge of some older, individual solid NIST absorbance filt er standards has been found to cause bias in the indicated absorbance readings of certain instruments. In a collimated-beam spectrophotomete r, the sample beam is deflected by about half of the wedge angle in th e sample. For inverted geometry designs, small deflections can give ri se to large changes in the field of view of the entrance slit of the s pectrometer downstream from the sample. Beam deflection is also found to induce small apparent wavelength shifts in data taken through a wed ged sample, resulting in spectral artifacts derived from spectral feat ures of the system or of an absorbing sample. These spectral artifacts turn out to be robust, and simple cell-reversal difference spectra ca n provide useful diagnostic indicators of optical wedge. Solid photome tric standards are found to be reliable, if manufactured to wedge angl es of less than 0.1 mrad. Despite a wedge tolerance of 0.9 mrad in the sealed cuvettes of a wavelength standard, wavelength shifts are shown to be negligible when compared to the stated uncertainties. For norma l use, accuracy may be achieved by utilizing the same cell in the same orientation for both background and sample spectra or by hand selecti on of cells.