Presentation on theme: "Spectroscopy (continued) Last time we discussed what spectroscopy was, and how we could use the interaction of light with atoms and molecules to measure."— Presentation transcript:
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:
Beer’s Law We can write: –this is the formal statement of Beer’s law where A = absorbance (no units) ε = molar absorptivity (M -1 cm -1, or L µg -1 cm -1 ) b = the pathlength (cm) c = concentration (M, µg L -1 )
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.
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).
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 0.365 Light Source Wavelength Selector (sometimes after the sample) Sample (holder) Light Detector Data Readout Common types are: UV-VIS (ultraviolet – visible) Flame AA (atomic absorption) FTIR (Fourier transform infra-red)
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 From: Willard, Merritt, Dean, and Settle, Instrumental Methods of Analysis 7 th Edition, Wadsworth Publishing, 1988.
A Closer Look at Absorbance and Emission From: Willard, Merritt, Dean, and Settle, Instrumental Methods of Analysis 7 th Edition, Wadsworth Publishing, 1988.
A Closer Look at Absorbance 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 at Absorbance and Emission Fluorescence decay generally occurs 10 -8 – 10 -4 s after absorption. Phosphorescence decay generally occurs 10 -4 – 10 2 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 P o 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 = kP o –and c = concentration of analyte
Reading for Next Time pgs. 501 – 511 Problems to Work on Chap 18 (16)