Exam 2- UV-vis/IR/Atomic Spectroscopy

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INCIDENT IRRADIANCE (BACKGROUND TRANSFORM)

P=

TRANSMITTED IRRADIANCE (SAMPLE TRANSFORM)

BANDWIDTH

# OF WAVELENGTHS ALLOWED TO PASS THROUGH

Beer Lambert Law

• light must be monochromatic

• concentration must be low

• sample cannot be fluorescent or heterogeneous

• no photochemical transformation

• sample cannot form complexes with solvent

Spectral interference

refers to the overlap of analyte signal with signals due to other elements or molecules in the sample or with signals due to the flame or furnace

Chemical interference

occurs when a component of the sample decreases the extent of atomization of analyte through some chemical reaction

Isobaric interference

the overlap of different species with nearly the same mass-to-charge ratio in a mass spectrum

Ionization interference

refers to a loss of analyte atoms through ionization

Problem: It’s impossible to get perfectly monochromatic radiation

• uses a small delta wavelength effective (bandwidth)

• do measurements at max wavelength—> best linear relationship

Problem: Differences b/t sample cells, position

Solution: Use carefully-matched cells, use one cell

• position same way

• clamp carefully

• or NEVER MOVE IT!

Problem: Atomic absorption lines are sharp!

Solution: Use a source with a narrower delta wavelength effective (bandwidth) than the absorption linewidth

Problem: Lots of atomic emission lines from hollow cathode lamp

Solution: Use a monochromator to select one line for AAS

Problem: My analyte doesn’t absorb strongly or it absorbs in the same region as other stufff

• increase Intensity

• chemical rxn to convert analyte into something with large extinction coefficient

Problem: Irradiance losses due to reflection, scattering, absorption of cell, absorption by solvent

• run a blank

• wipe down cuvette

• avoid dust

• filter solution if it is cloudy

PMT

Photodiode

conduction in response to light

1. light energizes e- s in depletion layer

2. e-s and hole move toward poles

Advantages of x-ray fluorescence