The meter of an inexpensive spectrophotometer had a 5-inch scale scribed in linear units from 0 to 100%T. The scale, which limited the precision of the instrument, could be read to about ± 0.5 %T. Calculate the relative precision of concentration determinations for an absorbance of: (a) 0.020, (b) 0.100, (c) 0.800, and (d) 2.000.
At 510 nm, the iron orthophenanthroline complex has a molar absorptivity of 1.2 x 104. What is the concentration of iron (in ppm) in a solution which gives an absorbance of 0.002 in a 1.00-cm pathlength cell?
The actual limit of detection in a spectrophotometric determination is related to the UNCERTAINTY in the absorbance measurement. Specifically, the detection limit is defined as the concentration of analyte which produces a Signal-to-Noise ratio equal to 3. In the case of absorption spectrophotometry, the "signal" is simply the absorbance and the "noise" is the absolute uncertainty in the absorbance. If the measurement described in question #2 was performed on an instrument having a read-out uncertainty of ± 0.01 %-T, is the concentration of iron determined in question #2 above or below the detection limit? Calculate the actual detection limit.
Created and copyright by Joel M. Goldberg. Last updated: March 6, 2005
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