Lecture 17 Spectrophotometry

Emission Absorption Fluorescence source sample source sample Secondary

Spectrometer Monochromator Detector (filter, wavelength selector) Light Source Detector Sample Spectrometer Data Processing

Spectrometer Monochromator Detector Light Source (filter, wavelength selector) Light Source Detector Sample Spectrometer Data Processing

Light striking a sample can be 1. reflected 2. transmitted 3. absorbed 4. scattered

Absorbing plate

I0 IN <1 Transmittance = I0 I1 I2 I3 We almost never use transmittance! I4 I5 T This is a curve! concentration

dP=  dP P P0 P1 Absorbing plate N=CVolume Volume= 1  dx N = C  dx Incident light Emergent light P P0 P1 dP l dx

Sensitivity is the same for any Absorbing plate absorbance Incident light Emergent light P0 P1 Sensitivity is the same for any power (P) l

Beer’s law Bugert, Lambert and Beer A Straight line! concentration

Least-squares curve fitting. The points (1,2) and (6,5) do not fall exactly on the solid line, but they are too close to the line to show their deviations. The Gaussian curve drawn over the point (3,3) is a schematic indication of the fact that each value of y is normally distributed about the straight line. That is, the most probable value of y will fall on the line, but there is a finite probability of measuring y some distance from the line.

k = Slope = y / x y=kx+b straight line equation b - blank! Y1=kx1 Let us subtract blank: y-b = Y = kx Y1=kx1 Y2=kx2 One standard

If you have several (N) standards, do it several (N) times Procedure: Measure blank. Measure standard. Measure unknown. Subtract blank from standard and from unknown. Calculate concentration of unknown If you have several (N) standards, do it several (N) times