CHEM Light Measuring Systems Flashcards
Formula for Absorbance
2 - log%T
NOTE: The relationship between absorbance and %T is LOGARITHMIC
What is a bandpass ?
- range of wavelengths that reach the exit slit of a monochromator (max wavelength - min wavelength)
- transmitted at a point equal to half the peak intensity transmitted
How do spectrophotometers measure transmittance ?
By comparing the signal generated by the transmitted light (Is) to the signal of the incident light (Io)
Formula for Absorbance under Beer-lambert’s law
A = εbc
ε = molar absorptivity (L/ mol x cm)
b = light path (cm)
c = concentration of analyte (mol/L)
State the requirements/ limitations of the Beer-Lambert law for measuring Absorbance
- monochromatic light (Io)
- solvent has insignificant absorbance to analyte
- standard and unknown are measured in matched cuvettes
- the concentration range obeys Beer-lambert’s
- absorbance of unknown < absorbance of standard; a dilution can be done
- no optical interferences
- no side reactions between analyte and other molecules
- sides of cuvette are parallel
- no stray light
What are spectral interferences ?
- observed when a compound causes a response similar to that of the analyte
ie. HIL
Explain the principle of reflectance spectrophotometry
- polychromatic light is directed to a diffraction grating or light-emitting diodes to produce monochromatic light
- monochromatic light hits the test surface = light is absorbed and reflected
- color intensity increases with [analyte], absorption increases, and diffuse reflection decreases
- a single wavelength of the diffuse reflected light is selected and directed onto a photodetector
Define reflection density (DR)
A measure of the amount of light absorbed by the chromophore on the test surface:
DR = log (R0 / Rtest)
where,
R0 = light reflected by the white standard
Rtest = light reflected by the surface
In reflectance spectrometry, the relationship between analyte concentration and reflectance is __.
In reflectance spectrometry, the relationship between analyte concentration and reflectance is LOGARITHMIC
What can be used to correct for spectral interferences during analysis ? How ?
-
Bichromatic Analysis:
- two readings are made: one at MAX absorbance (A1) and one where analyte minimally absorbs light (A2)
- the standard curve is based on A1 = A2 or the ratio (A1/A2) -
Allen Correction:
- two wavelengths PLUS the MAX absorbance are monitored to subtract average background absorbance
ie. A(510nm) = 0.100, A(520) = 0.800, and A(530nm) = 0.300
Corrected A = A520 - [(A510 + A530) / 2]
The Beer-lambert law states that the __ of a substance is __ to the amount of radiant energy __
The Beer-lambert law states that the CONCENTRATION of a substance is DIRECTLY PROPORTIONAL to the amount of radiant energy ABSORBED
Define icterus
orange color in a sample because of bilirubin
What is Kinetic Analysis ?
- change in the monitored parameter over time related to concentration
- measurements usually made very early in the rxn period
Define Molar Absorptivity
the absorbance of light, at a specific wavelength, divided by the product of the concentration of moles per litre and the sample path (cm)
Units for Molar Absorptivity
L/(mol x cm)