Errors in Spectroscopy Measurements

1
Errors in Spectroscopy Measurements
There can be many sources of error with a
spectroscopic method. Electronic noise in the
detector is the most common. Mistakes with the
cuvet can also be important--improper
positioning, touching with fingers or allowing
dust on the cell all could introduce error.
2
Beers Law and Calibration
Iron can be analyzed spectroscopically as an
iron-ferrozine complex. However, some metals,
especially copper can interfere with the
complexation reaction. If copper is masked, then
the analysis can be done. Theoretically, the
y-intercept should be at 0, but as youve
probably seen in lab, error can cause an
absorbance reading at 0 concentration.
3
The Effects of Absorbing Light
UV-Vis promotes electrons into higher orbitals
IR promotes bending and stretching of bonds
4
Emission of Light
Fluorescence occurs at lower wavelengths (higher
energy) than phosphorescence. Spectroscopy using
emission of light is considerably more sensitive
than absorption of light due to inherently lower
background.
5
Absorption vs. Emission Spectra
Molecules always emit radiation at longer
wavelengths than they absorb at. The absorbance
and emission spectra usually resemble a mirror
image when viewed together.
6
Emission Spectroscopy Experimental Setup
Pretty similar to the absorbance measurements,
except that you add in an extra monochromator
after the sample and before the detector. We
typically hold the excitation (absorption)
wavelength constant and measure the emission
spectrum resulting from it.
7
Chemiluminescence

• When a chemical reaction makes a product that
  emits light, it is chemiluminescence
• This is the type of chemistry that takes place
  when a firefly gives off light or when you have a
  glow stick
• Some biochemical reactions also chemiluminesce

8
Chemiluminescence for Air Quality

• Chemiluminescence is used routinely for qir
  quality measurements of O3 or NOx
• Anytime you hear ozone or nitrogen oxide
  concentrations reported, its from this
  analytical technique