spectrophotometry lecture. interaction of radiation and matter
TRANSCRIPT
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SpectrophotometryLecture
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Interaction of Radiation and Matter
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Absorption and Fluorescence
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Terms
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Interaction of Light with Matter
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Molecules Absorption Wavelengths
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Spectrophotometry
Although a number of different types of spectrophotometers exist all have one thing in common. Utilize light energy to detect molecules in a solution Light energy is reported to the user as wavelengths in
nanometers (nm) Different spec’s utilize wavelengths that fall into different
ranges. Visible (VIS) 350-700 nm Ultraviolet (UV) 200-350 nm
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Absorption Spectrophotometer
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Spectrophotometer
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Spectrophotometer
spectrophotometer measures intensity of a light beam after it is directed through and emerges from a solution Ex: solution of copper sulfate (CuSO4) absorbs light The red part of spectrum has been almost complete
absorbed by CuSO4 and blue light has been transmitted
Gain greater sensitivity by directing red light through the solution because CuSO4 absorbs strongest at the red end of the visible spectrum
But to do this, we have to isolate the red wavelengths
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Spectrum of visible light How do you isolate red wavelengths of light?
In a spectrophotometer, a light source gives off white light which strikes a prism, separating light into its component wavelengths:
Red wavelengths pass through CuSO4 solution and measure amount of red light absorbed
Colored compounds absorb light differently depending on the l of incident light
l = Wavelength, nm=nanometers
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Design of Spectrophotometer
THE BLANK In order to effectively use a spectrophotometer we must first
zero the machine Blank contains everything except compound of interest which
absorbs light. By zeroing machine using "the blank," any measured absorbance is due to the presence of solute of interest
ABSORPTION SPECTRUM Different compounds having dissimilar atomic and molecular
interactions have characteristic absorption phenomena and absorption spectra which differ
The point (wavelength) at which any given solute exhibits maximum absorption of light (the peaks on the curves on the figure below) is defined as that compounds particular lmax
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Design of Spectrophotometer
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Electromagnetic Spectrum
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How is Does a Spectrophotometer Work? Amount of a particular molecule of interest
is measured according to amount of light that is absorbedAbsorbance data is compared to a standard
of a known concentration to determine the concentration of the unknown.
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How is Does a Spectrophotometer Work? All spec’s share
following common features: Lamp
i.e. tungsten or deuterium
Prism or grating Sample holder Display
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Absorption Spectrophotometer
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Absorption Curve
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Background, B
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Detection Limit DL or LOD
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Dynamic or linear range
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Sensitivity
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Calibration Curve
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Calibration Curve Procedures
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Calibration Curve Plot
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Example Nitrite Analysis
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Results
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Plot of Results
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Best Fit Line
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How are Concentrations Obtained Using a Spec? More molec…more to absorb light
Note peaks of absorption curves Lambdamax
Wavelength at which a molecule absorbs the most light
Proteins, like other molecules, interact with certain wavelengths of light Proteins absorption spectrum can
be determined by measuring proteins light absorbance at different wavelengths.
Determine the lambdamax for protein
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How are Concentrations Obtained Using a Spec? Most proteins are colorless
Light in visible range will not workLight in UV range will work for a colorless
solution ~280 nm Does NOT distinguish between different protein
types in a solution
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Using Bradford Reagent
Way to colorize proteins and use white light spectroscopy Solution changes from
brown to blue when proteins present.
Degree of “blueness” of Bradford-protein mixture can be used to determine concentration of protein in a solution
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How are Concentrations Obtained Using a Spec? Calculating protein
concentration in an unknown sample Known standards are
mixed with Bradford reagent and their absorbance values are determined
Standard curve generated. known absorbance values
can be plotted and concentrations determined
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