9/9 Lecture #2: Analytical Chemistry

Spectroscopic Analytical Methods + Electron State concept

measures amount of electromagnetic radiation absorbed/emitted by a molecular or atomic species of interest

Electrons absorb energy from radiation (from a photon of visible light) and move to an excited state. The energy absorbed can come from light, heat, chemical, and electrical energy.

They only stay in the excited state temporarily (~fs-ns) and then return to ground state, emitting photon(s).

Measuring these interactions between light and matter can help us obtain information about a sample/analyte.

Draw a diagram representing an electron moving from ground state to excited state.

The total change of energy from ground to excited state ∆E is the energy of the photon.

An electron that moves from ground state to the first excited state is notated E_1,2.

The electron can move to different excited states. The excited states are drawn closer to each than to the ground state because it takes more energy to move from ground state to excited state than its takes to move from one excited state to another.

Draw a diagram representing an electron moving from excited state to ground state.

The electron can release multiple photons of varying wavelengths/energies based on the level of its excited state.

How does a commercial spectrometer typically work?

Light moves through a prism to split the beam into its individual component wavelengths/colors.

• Different types of light can be used, including UV radiation, visible light, infrared light, and x-rays.

A movable slit allows a specific wavelength of light to pass through.

The light goes through the sample. Some of the light is absorbed and the rest reach the detector which measures the amount of light absorbed as a function of wavelength. This data can be graphed as an absorbance spectrum.

Planck’s Equation

E = hν = hf = hc/λ

E energy: Joules (J)

h Planck’s constant: 6.62607015 × 10^-34 Js

v or f frequency: Hertz (Hz) or s^-1

c speed of light: 3 × 10⁸ m/s

λ wavelength: meters (m)

Transmittance and Percent Transmittance

the fraction of incident radiation transmitted through a solution

T% = I/I₀ x 100%

I is the initial intensity of light and I₀ is the intensity of light after passing through the sample

T% isn’t very useful and instead we use absorbance which is related to transmittance logarithmically.

A = -log(T) = log(I₀/I)

What are the limits of Beer’s law?

The relationships between absorbance, extinction coefficient, concentration, and path length are no longer linear under some conditions:

• Non-monochromatic radiation; multiple types of light passing through the sample will interfere with the data

• Fluorescence and phosphorescence; since light is re-emitted by the sample, I > I₀

• Stray light and scattering light interferes with the data

• At high concentrations, attenuation is too high and all light is absorbed, leaving no EM radiation to be measured. Solute molecules affect each other’s abilities to absorb light. Errors from light leakage in the machine become more significant and make it difficult to collect accurate data when A > 1.0.

When is fluorescence used?

Fluorescence is used instead of absorbance spectroscopy for very dilute concentrations because fluorescence is more sensitive than absorbance.

Unlike absorbance, the color emitted and absorbed is not the complementary of the color observed. Instead, the color absorbed is the color with a wavelength that is longer (less energy) than the color emitted/absorbed, since some energy is lost in fluorescence.

Calibration Curves (how is it graphed, what are the axes, slope, and intercept)

Graphed by plotting the absorbance for each concentration at a constant wavelength λ_max on an absorbance spectrum

The y-axis is absorbance and the x-axis is the concentration.

The slope of a calibration curve is the extinction coefficient ε.

Calibrations curves have an intercept and do not start at zero due to minor absorbance from the baseline when the analyte concentration is zero.