Spectroscopy Part 2 Flashcards
What is transmittance?
The ratio of the power (P) of the transmitted beam to the incident beam (P0) often expressed as a percentage
What is absorbance?
The negative logarithm of transmittance. Effectively the number of orders of magnitude by which transmittance is decreased
What is a blank measurement?
Account for non-absorption losses of light, measured using only the solvent and the same or a matched cell
What is Achromophore equal to?
Asolution - Asolvent
What is A, Absorbance in the Beer-Lambert Law?
The amount of light absorbed and it is wavelength-dependent
What is ε, Molar absorption coefficient (M⁻¹ cm⁻¹) in the Beer-Lambert Law?
The measure of how strongly a molecule absorbs light. It is wavelength-dependent and is proportional to the absorption cross-section of a molecule (cross-section of photon capture)
What is b, Path length (cm) in the Beer-Lambert Law?
The distance light travels through the sample
What is c, Concentration (M) in the Beer-Lambert Law?
The amount of chromophore per unit volume
What question does the Beer-Lambert Law try to answer?
How much (monochromatic) light is absorbed?
What is the relationship between double bonds and molar absorption coefficient (ε)?
The more conjugated double bonds, the larger the value of the molar absorption coefficient
What do the Molar absorption coefficient (ε) and path length (b) determine?
The slope of a calibration plot (A vs. c), and thus, the sensitivity of the method
What are real deviations?
At high concentrations (>0.01 M), each chromophore affects the charge distribution of nearby chromophores (changes ε). The value of ε depends on the refractive index (dielectric constant), which can change as concentration increases
What are apparent deviations?
Since the spectrophotometer radiation is not perfectly monochromatic, the chemistry is not suitably controlled (e.g.: the chromophore reacts with solvents or solutes). That is, the concentration of chromophore does not vary linearly with the amount added
