Spectroscopy (continued) Last time we discussed what spectroscopy was, and how we could use the...

Spectroscopy (continued)

• Last time we discussed what spectroscopy was, and how we could use the interaction of light with atoms and molecules to measure their concentrations.

• Today we will expand on this and look at specific types of spectroscopy.

Beer’s Law

• We have already shown that Absorbance is proportional to concentration:

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0.0 0.2 0.4 0.6 0.8 1.0Distance through solution

(Proportional to concentration)

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Beer’s Law

• We can write:

– this is the formal statement of Beer’s law• where A = absorbance (no units)• ε = molar absorptivity (M-1cm-1, or L µg-1 cm-1)• b = the pathlength (cm)• c = concentration (M, µg L-1)

bcA

Beer’s Law• Molar absorptivity (ε) is constant for any one

substance at any one wavelength.– It varies with substance and wavelength.– Gives an indication of how effective a substance is

at absorbing radiation at the specified wavelength.

• The pathlength (b) is the distance that the source radiation passes through the sample (the length of the flame, the width of the cuvette, etc.).

– This should be constant for any experiment or spectrometer.

Molar absorptivity (ε)

Beer’s Law

• Since ε and b are constant, then we can express Beer’s Law as:

– where y = absorbance (A)– m = (εb)– x = concentration (c)– and b is the y-intecept (which should be

close to zero if we analyzed a blank).

bmxy

Beer’s Law• Limitations to applying Beer’s law

– At high concentrations absorbance is no longer proportional to concentration.

• Cannot produce a linear calibration curve.

– Beer’s Law only applies for monochromatic radiation since ε changes with wavelength.

• Important to do quantitative analysis near the peak of molar absorptivity where molar absorptivity does not change much with wavelength.

– Beware of shifts in chemical equilibrium for the analyte of interest.

• Changing concentrations or pH in solution may shift equilibrium, thus shifting concentrations of analytes.

Absorbance Spectroscopy

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(sometimes afterthe sample)

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Common types are:• UV-VIS (ultraviolet – visible)• Flame AA (atomic absorption)• FTIR (Fourier transform infra-red)

UV-VIS Spectroscopy

From: http://icn2.umeche.maine.edu/genchemlabs/uv.html

UV-VIS Spectroscopy• Can be used to identify compounds – but

many compounds’ spectra look alike.• Uses wavelengths from ~180 to 800nm.• Usually look at spectra to determine the best

wavelength for quantification.– Then use the wavelength with the maximum

absorbance (molar absorptivity) for quantitative analysis.

• We will use UV-VIS spectroscopy to analyze Co and Cr later in the semester.

• UV-VIS is also the most common detector for HPLC.

Flame AA Spectroscopy

• Excellent for the quantitative analysis of elements.

• Primarily uses ultraviolet wavelengths for analysis.

• Since we use a monochromatic light source (hollow cathode lamp) there is virtually no interferences from other elements.

– Also cannot look at spectra.

• Flame potentially creates a high background (lowers sensitivity).

– Using a graphite furnace can reduce this effect.

FTIR Spectroscopy

From: Silverstein, Bassler, and Morril, Spectrometric Identification of Organic Compounds, John Wiley and Sons, 1991.

FTIR Spectroscopy

• Usually used for qualitative determination of the identity of a compound.

• Uses infra-red wavelengths (2000 – 25000nm)• Should have a purified sample for analysis.• Quantitative analysis is made difficult do to:

– detector sensitivity,– Thermal noise,– interferences from other compounds.

Emission Spectroscopy

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Emission Spectroscopy

From: Willard, Merritt, Dean, and Settle, Instrumental Methods of Analysis 7 th Edition, Wadsworth Publishing, 1988.

A Closer Look atAbsorbance and Emission

From: Willard, Merritt, Dean, and Settle, Instrumental Methods of Analysis 7 th Edition, Wadsworth Publishing, 1988.

A Closer Look atAbsorbance and Emission

Absorption always occurs at higher energies than emission.

•Due to vibrational transitions to the ground vibrational state within each electronic state

A Closer Look atAbsorbance and Emission

• Fluorescence decay generally occurs 10-8 – 10-4 s after absorption.

• Phosphorescence decay generally occurs 10-4 – 102 seconds after absorption.– Need an efficient chromaphore

• Usually large conjugated organic molecules.

• Fluorescence in molecules is somewhat rare, and phosphorescence is very rare.

Emission Spectroscopy• Emission occurs on a zero background.

– Since emission wavelength is always longer than the excitation wavelength.

– Detection limits can be much lower than for absorption.

• However, instrumentation is more complicated and expensive.

• Limited to analyzing molecules that fluoresce or phosphoresce.– Or molecules that can be derivatized.

Emission Spectroscopy• The emission intensity (I) is proportional to

concentration (c):

• Where Po is the excitation irradiance, and k is a proportionality constant (similar to ε).– Therefore, we can still make a linear

calibration curve y = mx + b– where y = emission intensity– m = kPo– and c = concentration of analyte

ckPI o

Reading for Next Time

• pgs. 501 – 511

Problems to Work on

• Chap 18 (16)