Announcements • Take Home part of Exam 2 is due now. • Homework Chapter 15: Problems 11, 12, 13, 16, 17 Due Fri, Nov 21 • Reading Report #3 is due on Wed, Nov 19
Various typical cells (cuvets) used in spectrophotometry. The faces (part in the lightpath) may be silica, quartz, glass, or plastic, but the material must be transparent to the radiation of interest.
Several factors contribute to the error in spectrometric measurements. The minimum error is over the range of 0.4 -0.9 for the value of A, absorance.
The analysis of analytes by spectrophotometry often involves converting the analyte to a light-absorbing species. Pictured here is the chemistry for the analysis of the nitrite ion, NO2-.
A portion of the visible spectrum of the colored complex in the determination of nitrite. The wavelength chosen for the analysis is the max which is 543 nm for the nitrite ion; the greatest sensitivity occurs at the max.
Beer’s Law plot is the calibration curve for the analysis of nitrite. The of 543 nm and b (the pathlength of the cuvet) are kept constant for the analysis. Note that measuresments of known concentrations are made.
Frequently dilutions are made as part of the spectrometric analysis. Each dilution must be accounted for in the final calculation of the amount of analyte in the initial sample.
Most modern instruments are double beam spectrometers. The diagram below shows the light path in such as instrument.
Interaction of light with the diffraction grating to separate polychromic light into monochromatic light. A given wavelength of light is selected by constructive interaction according to the Bragg equation: n = d (sin - sin)
A given wavelength of light is selected by constructive interaction according to the Bragg equation.
The effective bandwidth is the width of the gaussian shaped line at ½ its intensity.
The slit width of the instrument controls the monochromator bandwidth. As the monochromator bandwidth decreases, the peak of a spectral line is better isolated. The intensity of the line decreases as the instrument bandwidth is narrowed.